RU2502152C2 - Method of analysing mixtures of chemical compounds based on separation of ions of said compounds in linear radio frequency trap - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: in order to prevent excessive loss of analysed ions, inner walls of input and output diaphragms are divided into segments and alternating or variable voltage is applied to said diaphragms. Mass spectra of product ions during collision-induced dissociation are recorded using a mass analyser which is interfaced with a trap, particularly on a time-of-flight mass analyser with orthogonal ion input.

EFFECT: obtaining quantitative information on analysed compounds and high accuracy of structural chemical analysis.

15 cl, 7 dwg

Description

FIELD OF THE INVENTION

The present invention relates to methods and techniques for the chemical analysis of organic, bioorganic and inorganic compounds based on a combination of the separation of the ions of these compounds according to the mass to charge ratios, mobility, resistance to collisional fragmentation of ions and mass analysis of ion products of this fragmentation. In particular, we are talking about the preliminary separation of ions coming from an electron ionization source or from external ion sources, under the combined action of electric fields and a gas stream in a linear radio-frequency trap directed to its exit, according to the values of charges and masses, collision cross sections and resistance to decay. Subsequent analysis of product ions in terms of mass to charge ratios can be carried out using a time-of-flight mass spectrometer with orthogonal ion input (ortho-VMS) or on some other mass analyzer.

The decay or death of the analyzed ions can be caused both by heating of the rotating ions due to their collisions with atoms or gas molecules, and by collisions of ions with the rods of the trap. The use of such decays or death for the separation and identification of the analyzed compounds is one of the distinguishing features of the present invention, it greatly increases the separation ability of the method.

Among the tasks for which, in addition to sensitivity, both the separation ability and the dynamic range of measurements are important, rapid analysis of microimpurities in the air can be mentioned in relation to their use in security systems, customs and environmental control. The second task, where separation ability and “information performance” are decisive, is the study of the structure of biomolecule ions, which may be important for proteomics, biomedical and biotechnological applications.

BACKGROUND

After the development and creation at our institute of the first time-of-flight mass spectrometers with orthogonal ion injection (ortho-VPMS) [1, 2], devices of this type became widespread in solving various analytical and structural problems [3-5]. The convenience of combining such devices with various devices for preliminary separation of ions, producing a continuous or quasicontinuous ion flux, with pulsed time-of-flight mass analysis, which is record-breaking in speed among all known types of mass analyzers, has led to the high efficiency and attractiveness of such combinations for solving various analytical and structural problems . At the same time, there are important structural and analytical problems for which the separation ability and “information performance” of known instrument systems, including the ortho-VPMS, is insufficient. To overcome these limitations, it is natural to strive to introduce into the mass spectrometric experiment additional measurement dimensions associated with the controlled transformations of the studied ions and the recording of data during these transformations. To carry out such measurements, it is desirable to have a sufficiently large number (or sufficiently intense flux) of the studied ions in the reactor and to separate these ions from other interfering ions.

In the past 20 years, gas-filled radio frequency multipoles, devices containing a set of rods usually parallel to each other, symmetrically located around the axis of the device, have become widespread in mass spectrometry. Radio frequency voltages are most often in antiphase applied to adjacent rods. These devices are usually used as a means of focusing and efficiently transporting ions or for accumulating ions (in this case they are called linear radio frequency traps or linear ion traps) with the possible isolation of selected ions and conducting controlled dissociation and other structural transformations [6-8]. In these devices, the property of high-frequency force fields described in Mechanics of Landau and Lifshitz [9] is used to cause the particles to be ejected in such fields to reduce the strength of these fields. More precisely, the average particle motion in such (electric) fields is described, to a first approximation, by the effective potential, which is directly proportional to the square of the high-frequency field intensity multiplied by the square of the particle’s charge and inversely proportional to the particle mass. For a particular case of an ideal radio-frequency quadrupole, the effective potential depends quadratically on both coordinates (in a rectangular coordinate system in a plane orthogonal to the quadrupole axis), reaching a minimum value on the quadrupole axis, and the average free movement of ions in such a field is independent harmonic oscillations in both coordinates. In these devices, used as ion storage rings and reactors, two important properties in this case - the ability to accumulate ions and the ability to separate these ions can conflict with each other. In order to effectively stop ions inside a multipole, a sufficiently high gas density is usually required, and for high selectivity of isolation of selected ions or excitation of resonant ion oscillations and their heating (for fragmentation and other transformations), the gas density should be relatively low.

One of the possible approaches to overcome this contradiction by creating a slightly diverging supersonic gas flow [16] directed along the multipole axis and creating an increased gas density near this axis is described in our patents in the Russian Federation [10, 11] and in the patent application of the Russian Federation [12] . A second possible method for accumulating selected ions is the subject of the present invention. Its prerequisite is the creation by us of a technique for resonant excitation of the rotation of selected ions around the axis of the radio frequency quadrupole and the fragmentation of these ions due to collisions with buffer gas molecules [13-15]. This technique was new, previously not proposed by anyone. Unlike the present invention, the rotation of the ions in this case is excited during their one-way movement along the quadrupole without accumulation. At the end of the quadrupole, there is no rotating field, and the ions are focused towards the axis of the quadrupole, which requires a relatively high density of the buffer gas. Such a density is also needed in order to have a relatively short time to establish a stationary radius of rotation of ions in the presence of a rotating field. This limits the possibilities for kinetic measurements and provides a limited ability to detun from signals of interfering ions. In addition, this method of resonant rotation imposes very severe restrictions on the manufacturing quality of the quadrupole. Small deviations in the diameter of the rods or in the distances between them leads to significant losses in the resolving power of the method, which, in real measurements, was significantly less than 100 in our case.

In the proposed embodiment, ions rotate at a much lower density of the buffer gas. This density is such that the characteristic relaxation time of the velocity slightly exceeds or becomes comparable with the time the ions travel the distance between the input and output diaphragms. If ions are reflected from the inner surfaces of these diaphragms, rotating ions with their radius of rotation exceeding the effective radii of the openings of the outlet and inlet diaphragms will accumulate in the space between these surfaces, oscillating between them. Thus, the field inhomogeneities are significantly averaged, and their influence on the width of the resonance curves is weakened. In this case, the resolution of the resonant excitation for given ions and a given buffer gas will be mainly determined by the density of this gas in the region of rotation. At a residual pressure of 0.1 mTorr (nitrogen), the expected mass resolution at half maximum peaks for organic ions with a mass of about 500 Da can be about or even more than 1000. For an ideal quadrupole, the resolution of the resonant rotational excitation of ions will increase in proportion to a decrease in the density of the buffer gas.

A theoretical model for heating ions moving in a gas under the influence of an electric field was described by us in [20]. The model is original, its predictions are somewhat different from the known models. In particular, it predicts a slightly higher value of the internal temperature of the ion compared to the temperature of its translational motion, which in the literature is usually called the effective temperature. This model is indirectly confirmed by the available experimental data, and its use to obtain quantitative thermochemical data for the studied ions is possible.

The software for the analysis of experimental data should include software packages that basically implement the original methods developed by us, described in [21–25, 32]. Among these methods, the most important are:

1. The correction method for the effects of saturation and "dead" time when using time-to-digital conversion to register data of the VPMS [23, 32];

2. A method for identifying exponential contributions to a registered signal from an ion ensemble that relaxes to a new stationary state after switching the ion accumulation mode [21, p. 192], with finding the roots of the characteristic polynomial using the procedure described in [25];

3. A method for identifying exponential contributions to a set of ion current curves, developed earlier for the analysis of a set of effusiometric curves [24].

Existing methods for the implementation of collision-induced ion dissociation or for kinetic mass spectrometric measurements usually require preliminary isolation of one type of ion with the loss of all the others, thereby requiring the use of a large volume of the initial sample and a large time-consuming experiment. One of the exceptions is the “multi-reflection” ortho-VPMS A.N. Verenchikova [26], where due to a significant increase in the effective length of the ion drift and, consequently, their flight time, it becomes possible to produce collisional dissociation of not one, but several types of selected ions, which are far enough separated by the exit time (by a time longer than the ion drift time in a secondary time-of-flight mass spectrometer). This approach, which is much more technically complex than in our case, of course, excludes any kinetic measurements and produces primary ions for dissociation only in m / z.

A possible approach that reduces the loss of primary ions is described in US patent A.V. Loboda No. 7,459,679 [27]. This patent proposes, after the accumulation of ions in the quadrupole at a buffer gas pressure of about 0.1 Torr, to carry out dipole excitation of ion vibrations with the selected m / z, so that these ions in the plane of dipole excitation deviate far enough from the axis of the quadrupole on average. During or after this excitation, a quadrupole field that is linearly changing along the quadrupole is created in time. The potentials of this field are chosen so that in the plane of excitation of the vibrations of the selected ions an average electric field is created that moves the ions to the exit of the quadrupole (on the axis of the quadrupole this field is 0, and in the perpendicular plane it moves the ions in the opposite direction). In this case, unexcited ions, having on average a smaller deviation from the axis of the quadrupole in this plane, will be less susceptible to the influence of this pulling field. Thus, the packet of ions of interest can be moved into the collision chamber, and the remaining ions will remain in the storage quadrupole. After completing the first packet in the same way, the next packet can be delivered to the collision chamber. This approach is quite interesting and, apparently, will work. However, its resolution should be low enough (it is unlikely to be more than 10) for several reasons. The main one is the rather high density of the buffer gas needed to trap ions in the trap. Thus, ion packets transferred to the collision chamber will contain many ions in a rather wide mass range, and for the collisional dissociation of “individual” ions, all other ions from this packet must be removed. The relatively high gas density in the radio frequency multipole during the accumulation of ions in existing systems either leads to low selectivity of the ions during their isolation, or requires additional time to download the "excess" gas. Another possible solution is to create complex multi-tambour systems, where the functions of accumulation, isolation, and collisional dissociation are performed in different parts of the system with very different buffer gas densities. Such a design leads to additional ion losses and an increase in the cost of the instrument complex. It is such a construction that is proposed in the just described US patent [27].

Dynamic methods of capturing ions in a quadrupole trap, when the reverse ion exit is blocked by switching on the corresponding potential until the launched ion packet returns from the pivot point, only a small part of the initial ion flux can be used if subsequent manipulations with the ions require a relatively long time. The initial ion flow should be locked at this time, and the corresponding ions are usually lost.

The use of the resonant rotational motion of ions as well as their resonant one-dimensional oscillations in the radio-frequency quadrupole to eliminate unnecessary ions that interfere with the measurement of less intense analytical ions or cause saturation phenomena in the measuring system of a time-of-flight mass spectrometer is described in US patent application No. 20080149825 Kozlovsky V.I. et al. [28]. In our case, similar goals can be achieved by appropriate resonant spin-up of ions oscillating in the storage part of the radio-frequency quadrupole, which will increase the selectivity of this elimination in comparison with the spin-up of ions during a single passage of the quadrupole.

The use of a rotational field for the selective dissociation of ions accumulated in a quadrupole linear trap in a collision with atoms or molecules of a buffer gas is described in US Pat. No. 7,351,965 B2 [29]. It is proposed that the registration of product ions, as well as the removal of unwanted ions, be made through gaps along the vertices of the main electrodes of hyperbolic shape. It is proposed to compensate for the violation of the quadrupole field near these slots using thin electrodes located along the slots in the middle at the exit from them. When conducting dissociation, it is proposed to intentionally distort the quadrupole field by setting potentials on these auxiliary electrodes that are different from the potentials of the main electrodes. This is useful for shifting the resonance frequencies of strongly promoted ions to prevent their death on the quadrupole electrodes. In our case, the use of round rods of a quadrupole (which is technologically much easier to use hyperbolic rods) will lead to the same effect in the dissociation of selected ions. In US patent No. 7,353,965 B2 [29] it is proposed to capture ions dynamically in the trap, raising the voltage at the input diaphragm, because the buffer gas pressure in the quadrupole is not enough to stop the ions reflected from the blocking potential in the last section of the quadrupole. In this case, ions remain in the trap, which have reflected from this potential and did not have time to go back through the output diaphragm of the quadrupole until a blocking voltage is established on it. In the patent [29], to ensure the capture of a sufficiently large number of analyzed ions, the use of a relatively long quadrupole (1000 mm) is assumed. This length not only increases the dimensions of the device, but also makes more stringent requirements for the parallelism of the rods of the quadrupole and other conditions for its manufacture to ensure uniformity of the resonant frequencies of free movements of ions in different places of the quadrupole. The ion accumulation method of the present invention is expected to accumulate a sufficient number of ions in a quadrupole, an order of magnitude shorter, with a comparable residual pressure of the buffer gas.

Apparently, the closest method in its capabilities, which can be considered as one of the analogues of the invention, is claimed in US patent No. 7,507,953 [30]. In general terms, this is the development of the approach described in the previous paragraph, if we do not take into account the compensation of violations of the quadrupole field near the output slits along the rods of the linear quadrupole trap for ion transport to the registration system. In the case under consideration, the selected ions in the form of a “ribbon” beam can also enter a flat collision chamber and then be transported directly or after collisional dissociation to a time-of-flight mass spectrometer with orthogonal ion injection. The possibility of autonomous registration (as a separate system, as in the previous patent) of the initial ions is also provided. In this method, in contrast to generally accepted approaches, the use for collisional dissociation of not only one, but any desired number of species of parent ions is achieved. The same opportunity is realized in the present invention. The patent also provides for the standard use of a linear quadrupole trap for sequential isolation and collisional ion dissociation for the implementation of the MS "method, including at the last stage the registration of product ions using a time-of-flight mass spectrometer. The same possibility can be implemented in the present invention.

In the future, it is planned to use the device described in the patent in combination with a liquid chromatograph for preliminary separation of complex samples of biological origin. In this case, this approach, in comparison with other known ones, seems to be the most effective in terms of the amount of information received, the analysis time, and the amount of sample used.

The disadvantages of the described system include the loss of resolution and sensitivity during the orthogonal introduction of parent ions into the collision cell, including due to deviations of the field near the exit gap from the quadrupole one. In addition, the use of dipole excitation of ions, in contrast to our approach, where rotating fields are used, leads to a stronger effect of the space charge of the remaining accumulated ions on the resolution of the release of parent ions. In our case, these ions are dispersed in different rotation orbits (depending on m / z and ion mobility). In the case of resonant dipole excitation, the remaining ions are concentrated on average near the axis of the quadrupole, and with a sufficiently large number they create a noticeable additional field that distorts the harmonic nature of the effective potential of the quadrupole field near the axis of the quadrupole. For rotating ions, the possibility of exciting additional harmonic oscillations is preserved even if there is a significant effect of the space charge of other ions [11].

To achieve a more efficient use of the source ion stream in one of the options proposed in the patent, it is provided that the linear quadrupole trap is divided into two parts. In the first part, operating at elevated pressure, all ions are accumulated, preferably continuously. In the second part, which carries out the resonant selection of the parent ions under reduced pressure (for better selectivity), the ions in the selected m / z interval are translated by excitation of longitudinal vibrations in the first part to give them enough energy to overcome the potential barrier between the first and second parts. In our case, rather efficient accumulation of ions, the separation of parent ions and their fragmentation is supposed to be carried out in a single quadrupole pumped out by one turbomolecular pump.

In the patent in question [11], kinetic measurements are not provided, and this is hardly possible in the described construction. On the other hand, such measurements as proposed in the present invention, in combination with preliminary chromatographic separation due to time limitations, are possible only for individual chromatographic peaks separated from each other by sufficient time intervals. At the same time, these measurements can provide additional ion separation, which can compensate for the absence or even exceed the separation capabilities of the chromatograph with a shorter total analysis time. A significant increase in the resulting separation is also possible when combined with a chromatograph, because resistance to collisional dissociation is unlikely to strongly correlate with retention times of compounds in chromatographic columns.

A standard method for estimating the cross sections for collisions of ions moving in a gas is one or another type of measurement of the mobility of the ion or the proportionality coefficient between the stationary velocity of the ion and the electric field strength that causes this movement. Often this movement is used for preliminary separation of ions. Since in ordinary versions of the method, the time of movement of ions in a drift tube is relatively small, the most acceptable combination of separation of ions by mobility with a time-of-flight analyzer with orthogonal ion input.

A serious problem of this combination is the provision of high ion transmission through the drift tube in the HPMC. One of the possible solutions was proposed by us in US patent No. 6,992,284 [31], where a fairly detailed review of the work on the separation of ions by mobility is given. Patent 6,992,284 refers to the use of a sequence of alternating sections of a strong and a weak field in a drift tube at a gas pressure of several Torr instead of a uniform electric field. This leads to the focusing of ions to the axis of the quadrupole and allows you to slightly increase the total voltage along the tube, which favorably affects the resolution of ion packets in mobility. Nevertheless, in all realized variants of the separation of ions by mobility, a sufficiently high resolution cannot be obtained. Even for ion drift at atmospheric pressure, a resolution of more than 100 is not achieved.

In the present invention, the mobility of ions in their separation acts indirectly. The greater the mobility of ions with a given m / z, the greater the speed of movement and a larger radius of rotation of the ions will be under the action of a rotating field and, therefore, a higher internal temperature, the addition of which is proportional to the square of the velocity [20] and the square of the radius of rotation [13] . If the resistance to decay of the ions under consideration is close, then ions with greater mobility will decay faster. Moreover, in our case, it is possible to select m / z ions by setting the frequency of the rotating field, and by choosing the amplitude of the rotating field, you can adjust the decay rate. In the case of the classical separation of ions by mobility, there is a rather weak possibility of influencing the exit time and the width of the packet of ions of interest. By decreasing the field strength along the pipe, the ion exit time can be increased, but the mobility resolution decreases.

SUMMARY OF THE INVENTION

Features of one of the possible implementation of the proposed methods are:

The flow of the studied ions is the result of ionization of compounds entering the gas stream into the ion source of electron ionization, or transporting ions, including multiply charged ions of biomolecules, for example, from an electrospray source to the input of a radio-frequency quadrupole coupled to the ortho-VPMS. Instead of an electrospray source, this ion injection can also use any other ion source operating at atmospheric or relatively high pressure. When using an external ion source, the electron ionization source may be absent.

The accumulation, preliminary separation, controlled fragmentation and focusing of ions are carried out in a radio-frequency quadrupole with a gas stream having a significant velocity component along the axis of the quadrupole, directed towards the exit from the quadrupole. The inner surfaces of the input and output diaphragms are cut into sector fragments, to which are applied opposite in sign and close in absolute value voltages, which are called alternate voltages for brevity. With a relatively large absolute value of these stresses for ions rotating around the axis of the quadrupole, a sufficient repulsive effective potential arises. This is similar to focusing ions moving along a sectioned cylindrical channel with alternating stresses applied to adjacent channel sections, as shown by Marshall [34]. Alternatively, instead of such constant voltages, alternating voltages can be applied to the inner sections of the input and / or output diaphragms if the supply of such voltages can provide, for example, more favorable conditions for the introduction of ions from an ion source into the quadrupole and for the withdrawal of ion products of controlled fragmentation from quadrupole. In this case, the gas flow will ensure the transport of ions located near the axis of the quadrupole (with resonant rotational frequencies different from the spin frequencies) to the output part of the quadrupole and then to the time-of-flight mass analyzer or some other mass analyzer. Including such transport will be carried out for product ions of collision-induced dissociation, if among the rotating fields there is no field with a frequency coinciding with the resonant frequency of any daughter ion. If desired, such fields can be created specifically to accumulate the desired product ions for further research. A similar method for the accumulation of ions is described in our patent application of the Russian Federation [35]. The differences are in the introduction of ions not into a quadrupole, but into a sectioned cylindrical cell, in which, in the accumulation mode, an inhibitory parabolic field averaged over the rotations of the ions is created, and the incoming ions are reflected precisely from this field. The inlet diaphragm also has a partitioned inner surface. However, this sectioning is more complicated than in the case of the present invention, and along with the creation of a repulsive effective potential, the voltage applied to the sections together with the stresses of the sections of the cylindrical surface of the cell creates quadratic potential distributions averaged over ion rotations along the axis and radius of the cell.

By forming the corresponding resonant rotational field (or several fields to isolate several types of selected ions), ions of a given mass to charge ratio begin to rotate and are then held in stable rotation orbits (ions with higher mobility in a higher orbit) in the main part outside the input and output openings aperture. If necessary, alternating or alternating voltage at the output diaphragm can be increased to increase the potential barrier to accumulated ions. A significant decrease in the recorded current of selected ions means the beginning of their accumulation. A return to the previous or slightly lower stationary value (with the possible death of part of the ions inside the quadrupole) corresponds to the achievement of the maximum level of ion accumulation under the given conditions. The achievement of characteristic relaxation times of the detected ion current in the range of seconds or tens of seconds is a desirable condition for convenient measurements for the present invention. Such relaxation after reaching a stationary level of the detected stream of analyzed ions can be caused by a change in the stationary radius of their rotation when setting a new value for the amplitude of the rotating field or by blocking the input ion stream by increasing the voltage on the outer surface of the input diaphragm.

When a sufficient number of rotating ions is reached or after a predetermined accumulation time has passed, the amplitude of the (selected) rotational voltage in the quadrupole increases to the optimum value for holding and resonant spinning with possible fragmentation of the analyzed ions. In this case, the voltage on the outside of the input diaphragm can be increased to stop the flow of ions into the quadrupole. If during accumulation the alternating or alternating voltage of the output diaphragm was increased, then it can be reduced to provide a more free exit of ions moving near the axis of the quadrupole.

Under stationary conditions of heating of individual ions (in the absence of a noticeable loss of ions due to repulsion by a space charge), exponentially decaying signals of product ions should be observed, according to the characteristic time of signal decrease and amplitude ratios, the corresponding decay rate constants can be determined. In the presence of several types of decaying ions with a given value of m / z, it becomes possible, based on the analysis of the peaks of all products, to identify the number of such types and to determine the decay constants for each channel of all these types of ions. The characteristic times of decreasing intensities of the recorded peaks will be determined by the total decay constants of the initial ions, which may allow us to separate the corresponding signals for different initial ions, although their values of m / z and mobilities may coincide.

With increasing amplitudes of rotational stresses, it is also possible to include the decay processes of more stable ions and the elimination of relatively easily decaying ions from the “game”. Thus, all ions whose resonant frequencies are close to the frequencies of the rotating fields can be analyzed, unless a further increase in the radius of rotation of the ions leads to an unacceptable increase in the rate of their death on the rods of the quadrupole. In this case, an alternative may be a periodic short-term increase in the amplitude of the focusing radio frequency voltage by an integer number of times up to the last value not exceeding the stability threshold of the motion of the selected ions in the quadrupole. In this case, it is desirable that other accumulated ions in the quadrupole have large m / z and smaller radii of rotation, so that there is no loss of their stability of motion and death and as a result of collision-induced dissociation. The turn-on time of the increased radio frequency voltage should be close to the period of rotation of the analyzed ions or to a multiple for this period. In this case, these ions, having received multiply increased rotational frequencies for the previous rotation period, will make an integer number of revolutions, and their phase will be close to the phase of the rotational voltage. When you return to the previous amplitude of the radio frequency voltage, the resonant rotational voltage will relatively quickly restore the previous stationary radius of rotation of the selected ions. The time between switching on the increased radio frequency voltage must exceed the time of the exit of the quadrupole ions, not subject to unwinding. In this case, the product ions, even at lower values of m / z than for the initial ions, will be detected by a subsequent mass analyzer and will not die to a large extent due to the loss of motion stability at an increased radio frequency field strength.

Since ions often differ quite a lot in their degree of decay resistance, adequate estimates of resonant frequencies are possible even in cases where these values are close enough and could not be resolved by standard mass analysis methods. This situation is similar to preliminary chromatographic separation of the analyzed compounds or separation based on electrophoresis. The difference is that in the present invention there is the possibility of choosing for kinetic study of ions in a relatively narrow range of m / z due to the selection of frequencies of the corresponding rotating fields.

The theoretical foundations of the method of separating exponential contributions as applied to the analysis of sets of effusiometric curves were developed by us earlier [24]. The essence of the approach is based on a complete analogy between systems of differential and finite-difference linear equations with constant coefficients. This analogy for one equation, for example, was used by us in the development of linear prediction methods for the efficient estimation of the mass numbers of ions from their exit times for a magnetic static mass spectrometer [21, 33]. Having determined the coefficients of such a forecast, in particular, which is accurate for the sum of the exponential curves, the damping factors of the corresponding exponentials can be determined by finding the roots of the characteristic polynomial, as well as for the corresponding differential equation with constant coefficients. In this case, the computational procedure described by us in [25] can be used.

Based on this analogy, for a linear combination of exponentially decaying flows of product ions in the general case, the total intensity vector of these ions for subsequent recordings without taking into account measurement errors can be expressed as the product of a certain transition matrix by a vector of such intensities for the previous registration. The matrix elements of this transition matrix can be found by minimizing the error of such an approximation, which reduces to solving the corresponding systems of linear algebraic equations for each row of this matrix. The eigenvectors of this matrix describe, up to normalization, the intensity vector of product ions of each type of ion present in the mixture being analyzed. The eigenvalues corresponding to these vectors characterize the exponential decay of the number of such ions. The coefficients describing the contributions of the eigenvectors of the transition matrix to the observed intensities of the product ions are found by the least squares method to ensure, on average, the minimum errors of the measured intensities of the mass spectrum peaks for all recordings. The quality of approximation compared with measurement errors is a criterion for the accuracy of the analysis. Knowing the number of types of the initial ions and the characteristic times of their exponential decay, one can find the contributions of each of these exponents to all samples in the recorded mass spectra by solving the corresponding systems of linear equations. Thereby, mass spectra of product ions of each type of starting ions will be obtained, as if they were completely separated before their registration. This is important because will allow to determine the “exact” masses of all product ions, if the corresponding mass spectra were recorded on a high-resolution mass analyzer.

Alternatively, the death and decay of ions can occur when they collide with the surface of the rods of a quadrupole. In this case, the frequency of collisions of sufficiently strongly promoted ions against the surface of these rods will be controlled by the average distance of these ions to the nearest surface, the depth of the potential well created by the effective RF potential and the centrifugal potential of the rotating ions. The effective temperature of the ions, which is the second factor for the collision frequency, will be determined for these ions and the buffer gas by the speed of rotation of these ions or the average radius of rotation. A small part of the ions, depending on their charge, whose energy is sufficient to overcome the difference in potential energies between the bottom of the well and the conditional boundary of the stable motion of ions in the quadrupole, will collide with the rods and die as a result of such a collision with a probability that is probably close to 1 With the contact of multiply charged ions of biomolecules with the rod of the quadrupole due to electron capture, dissociation processes are possible, leading to the formation of product ions with m / z leading them out of resonant rotation. Moreover, the differences in the frequency of all these processes can be many times higher than the differences in the effective temperature of the ions (up to 10 or more times at reasonable measurement times), because The ions on the high-energy “tail” of the Maxwell ion energy distribution or those most remote on the outer side from the average rotation trajectory will collide with the surface of the rods.

If the efficiency of ion death in collisions with the surface of the rods for ions of similar mass and mobility is little different, then the characteristic times of decrease in the number of rotating ions will be determined by the frequencies of these collisions with the surface, which will ultimately be determined by the mobility of these ions. If the difference of 10% in the collision frequencies is sufficient for reliable separation of signals, then this means that differences in the mobility of the initial ions at the level of 1% may well be enough to separate the signals from the corresponding ions. It is possible to combine both of the described methods for organizing the death and decay of ions, when one of them is not enough for reliable separation of signals from close ions.

When studying the ion mixture in the manner described above, carrying out such an analysis at different amplitudes of the resonant rotating field or at different levels of periodically increasing the amplitude of the radio frequency voltage can provide an additional criterion for the adequacy of the results. This can be indicated by the coincident number of components found for different rotation conditions and the close to Arrhenius character of changing the ratios of the formation constants of the corresponding product ions under the assumption that the internal temperature of the ions linearly depends on the average kinetic energy of the ions [20].

After conducting an ion study for a given m / z and turning off the corresponding rotational voltage according to the method described above, all the ions of interest among the accumulated ones can be examined in turn. Unlike many other existing methods, there is no loss in each measurement cycle of other accumulated ions other than those selected in this cycle. The inclusion of appropriate resonant rotations, if necessary, can ensure the preservation of certain product ions in stable rotation orbits around the axis of the quadrupole. In this case, after the accumulation of a sufficient amount, they can undergo collisional dissociation and study the kinetics of this dissociation along with the parent ions.

Isolation of the exponential components in the recorded relaxation curves for ions with selected m / z values with a fast change in the energy of ionizing electrons in the electron ionization source, along with the determination of the characteristic decay times of the found exponents of the stationary levels of the corresponding ion currents, can result in ionization efficiency curves (CEI) for different ions with coinciding m / z values, but differing in their characteristic relaxation times, respectively vuyuschih ionic currents.

Available experimental data, both ours and known from the literature, indicate that a change in the internal energy of ionized compounds usually leads to a shift in the corresponding curves of ionization efficiency over the energy of ionizing electrons. This shift when the temperature of the introduced mixture changes will be specific for various compounds. Thus, stabilization and controlled setting of the temperature of the poured mixture into an ion source with a variable ionization energy can provide an additional criterion for the correspondence of the recorded curves of ionization efficiency to individual compounds. This also applies to obtaining CEI ions based on the selection of exponential components in the corresponding relaxation curves of ion currents with fast switching of the energy of ionizing electrons.

Thermostating and temperature control of the radio-frequency quadrupole will make it possible to obtain quantitative thermochemical data during collision-induced dissociation of the studied ions in the quadrupole.

BRIEF DESCRIPTION OF ILLUSTRATIONS

For a more complete understanding of the present invention, the following description is related to the corresponding illustrations, in which:

Fig. 1. General scheme of the ortho-VPMS interface for preliminary separation of ions and the formation of their ion products of collision-induced dissociation.

Fig. 2. The calculated resonance curves for rotating ions in a quadrupole with m / z = 400 and 401 for a buffer gas of argon at a density corresponding to 0.3 mTorr at room temperature.

Fig. 3. Illustration for the averaged movement of ions in a quadrupole with a multiple increase in the radio frequency voltage for the period of the rotating field, close to the resonance for the selected ions.

Fig. 4. The cross-sectional diagram (AA, Fig. 1) of the ion rotation region.

Fig. 5: Flowchart of the procedure for calculating the exponential contributions to the kinetic curve of decaying ions

Fig. 6. Illustration for the method of separation of exponentially decaying superimposed peaks of ion-products of collisional fragmentation of the initial isobaric ions.

Fig. 7. Illustration for the processes of the death of rotating ions in collisions with the surface of the rods of a quadrupole.

All these illustrations are explanatory in nature and do not impose any restrictions on the possible implementation of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A new approach for transporting ions from the high-pressure region at the exit of the mobility cell to the vacuum part of the mass spectrometer using a supersonic gas flow is described in our US patent No. 7,482,582 of January 27, 2009 [17]. It was further developed to provide additional analytical capabilities due to the resonant excitation of ion rotation around a supersonic flow in our next US patent No. 7.547.878 of June 16, 2009 [19]. A specific development of these approaches to ensure effective quantitative determination of the presence of impurities in gas mixtures and structural analysis of multiply charged ions of biomolecules is described in our patents in the Russian Federation [10, 11], and in the patent application of the Russian Federation [12].

A feature of the proposed method, unlike the mentioned patents and applications, is the implementation of an alternative approach to the previously proposed approach to the accumulation and isolation of the analyzed ions in the radio-frequency quadrupole at a relatively low density of the buffer gas and without the obligatory presence of a supersonic gas flow along the axis of the quadrupole. Its meaning is that instead of “stopping” and focusing the selected ions around the axis of the quadrupole at a relatively high density of the buffer gas or in a slightly diverging supersonic stream, the resonantly rotating ions are dynamically captured between the input and output diaphragms. Its main advantage over conventional widespread approaches is the extraction of the desired ions (possibly not one, but several m / z) from the flow of all ions, and not the removal of all excess ions from those accumulated in the quadrupole, or the use of a separate mass analyzer to isolate necessary ions for accumulation.

Compared to the removal of ions from a supersonic flow by a non-resonant rotating field or an electrostatic repulsive field of buffer gas ions, in this case, to extract and accumulate the necessary ions, it is only enough to create the corresponding resonant rotating fields. However, the use of non-resonant ion spin-up can be useful in recording survey mass spectra to prevent oscillations in the recorded intensities of ion fluxes characteristic of quadrupoles with a low buffer gas density. An advantage of the proposed implementation of the present invention is also the possibility of coaxially interfacing the interface with the subsequent mass analyzer, whereas in the presence of a relatively dense supersonic gas stream, it is necessary to have an electrostatic [12] or some other mirror to remove ions from the gas stream and direct them to the masses -analyzer.

A possible organization scheme of the accumulation and isolation of ions proposed in the present invention is shown in Fig. 1. Another difference between the described construction and the previously declared ones is that instead of coating the inner surfaces of the input and output diaphragms (9) with a thin dielectric film with its charging by buffer gas ions to reflect ions [18], it is assumed that these surfaces are cut into sector fragments (26) and (27). Applying constant alternating or alternating voltages to these sectors will create an effective repulsive potential for rotating ions. This option for creating an ion trap is due to the fact that, in the absence of a relatively dense gas flow, ionization of the buffer gas ceases to be a convenient way to charge the internal surfaces of the diaphragms inside the quadrupole. In addition, it becomes possible to control the efficiency of reflection of ions from diaphragms, including the organization, if necessary, of partial death of ions on the surfaces of these diaphragms, if, for example, the influence of the space charge of ions on the measurement results becomes unacceptable. Since the density of the buffer gas inside the quadrupole is assumed to be quite low, there is no need to create longitudinal electric fields in the quadrupole; therefore, quadrupole (10) in this case, unlike our previous constructions, is not assumed to be partitioned.

The ion flow of the analyzed compounds (1) from an external source is transported to the input of the radio-frequency quadrupole (10) by a system of focusing electrodes, not shown in Fig. 1, which can be used instead of a source with electronic ionization (6). When analyzing a neutral gas mixture (2), the effusion stream (4) from the inlet system (3) is introduced into the ion source (6), where the formed ions (5), together with the gas stream residues (1), enter the quadrupole through the inlet diaphragm (8) (10). The ion source (6) has an indirect heating cathode (18) (22), which allows a fairly narrow energy distribution of ionizing electrons. This makes it possible, if necessary, to cut off the formation of ions with a relatively high ionization energy, for example, the main components of air. The pumping of gases entering the stream (1) is carried out mainly by a pump (15). A small part of the flow through the output diaphragm (17) enters the next stage of the differential pumping of the ortho-VPMS or other mass analyzer.

When analyzing mixtures of unknown composition or at the beginning of work, calibration of the measuring system may be required. In the absence of other electric fields than the radio frequency inside the quadrupole (10), registration of the survey mass spectrum can lead to a modulation of the recorded ion flux of various m / z at a sufficiently low gas density (7) inside the quadrupole. This is a consequence of ion vibrations around the axis of the quadrupole in the effective potential as they move along the quadrupole. To obtain a mass spectrum that is more or less adequate for the composition of the mixture under study, excitation of nonresonant rotation with a frequency ω _rot much lower than the resonant frequency ω _res (40) for ions with the maximum possible m / z in the recorded range and equilibrium radius can be used

,

,

not exceeding the radius of the opening of the output diaphragm (16). Here e is an elementary charge; V _rot - rotational stress; V _rf is the radio frequency voltage; ω _rf is the frequency of the radio frequency voltage; r ₀ - inscribed radius of the quadrupole - the minimum distance from the axis of the quadrupole to its rods. In this case, ions (23) that go beyond the outlet of the diaphragm (16) will be reflected by the effective potential created by sections of type (26) and (27) with alternating or alternating voltages. Having reached the inner surface of the inlet diaphragm (8), part of the ions (12), moving along the trajectories (23), (24), outside its holes will be reflected again and will circulate between the diaphragms (8) and (16) until any cycle will not leave the quadrupole in one direction or another. Losses of such ions for registration can be reduced to an acceptable level by increasing the size of the effective aperture of the diaphragm (16) compared to the diaphragm (8), creating a field of both pulling ions from the quadrupole inside the hole (16) and inhibiting the outgoing ions inside the hole (8) . In addition, the presence of a noticeable component of the gas flow velocity in the direction from the diaphragm (8) to the diaphragm (16) will also reduce the return yield of ions when they lose most of the initial energy of the longitudinal motion.

, вблизи резонансной частоты для выбранных ионов газа, которая может быть вычислена по формуле:The high-frequency potentials of neighboring quadrupole rods shifted in phase by π / 2 (10), with an angular frequency

, near the resonant frequency for the selected gas ions, which can be calculated by the formula:

вблизи ω_res добиваемся максимального падения стационарного регистрируемого тока выбранных ионов, тем самым достигая достаточно хорошего приближения к реальной резонансной частоте рассматриваемых ионов. Под действием этого поля (при выключенном нерезонансном вращающем поле) ионы (13) с «резонансным» отношением массы к заряду m/z будут вращаться в стационарных условиях со средним радиусом:a rotational field close to resonance is created (11). The amplitude of this field gradually increases until the stationary recorded current of the selected ions becomes noticeably and stably less than their ion current in the absence of this rotating field due to the death of untwisted ions on the rods of the quadrupole. Varying the frequency of the rotating field

near ω _{res, we} achieve the maximum drop in the stationary recorded current of the selected ions, thereby achieving a fairly good approximation to the real resonant frequency of the ions in question. Under the influence of this field (with the non-resonant rotating field turned off), ions (13) with a “resonant” mass to charge ratio m / z will rotate under stationary conditions with an average radius:

sufficient to prevent unhindered passage through the central opening of the exit diaphragm (16), but insufficient to effectively kill these ions on the rods of the quadrupole (10). Here, τ _v is the characteristic relaxation time of the ion velocity. Such a radius of rotation corresponds to the maxima of the curves (50) and (49) in Figure 2. Here are shown the resonance curves for the relative average ion spin radii, based on formula (43) taken from our work [13]:

Curves (50), (49) are given for hypothetical ions with m / z 400 and 401 for the expected density of residual gases. The widths of these curves at half maximum are predicted by the formula (44):

The characteristic relaxation time of the ion velocity τ _v was calculated by the formula (45):

based on the mass of the buffer gas atoms M = 40 Da, the average thermal velocity V of their motion (ion velocities were neglected) at room temperature and a reasonable value of the cross section for collisions of ions with gas atoms σ = 200Å ² . The gas density n was calculated from the equation of state of an ideal gas at a pressure of 0.3 mTorr. The linear velocity of the rotating ions is determined by the rotational voltage V _rot and their mobility:

When the angular frequency is shifted from resonance, the average rotation radius changes for other pressures and gases in the same way as shown in Fig. 2. This means that the use of round quadrupole rods in our case may be preferable in comparison with ideal hyperbolic rods. Since when approaching circular rods, the resonant frequencies of rotation of ions will change, the radius of rotation of ions with increasing V _rot , starting from a certain value of V _rot and within some reasonable range of this value, will stop growing, and excessive loss of ions on the rods of the quadrupole in this case will be prevented. If necessary, increase the likelihood of ion death, and in such a situation, the ions can be subjected to additional nonresonant rotation.

In formulas (40) and (42), V _rf , ω _rf , V _rot , and ω _rot are the amplitudes and frequencies of the radio frequency and rotating fields. Figure 3 shows the cross section of a quadrupole (A-A). The applied stresses (102) and (103) are given inside the sections of the rods (101). Shown are resonantly rotating ions (106) and (105), the instantaneous direction of the resonant rotating field (108), and the direction of non-resonant rotation (124). When leaving the conditional boundary (107), the ions die on the rods of the quadrupole. When moving inside the effective boundary (104) of the outlet, ions (109) are detected.

The intensity of the focusing RF field, proportional to the amplitude V _rf , should be relatively small,

in order to ensure the stability of the motion of possible ion products of collisional dissociation, m / z of which can be several times smaller than m / z of the initial ions. In this case, the excitation of rotation of ions with an acceptable radius may not lead to their noticeable dissociation. This radius, or the value of V _rot proportional to it, should be such that the steady-state current settling time when the input ion flow is blocked through the diaphragm (8) and V _rot changes is convenient for measuring it in the range of seconds or tens of seconds. If necessary, an increase in this time can be achieved by a certain increase in the amplitude of the radio frequency voltage while maintaining the same radius of rotation of the ions (with corresponding changes in the frequency and amplitude of the rotating field), if there is no noticeable dissociation of ions. In this case, ion focusing improves, and ion clouds are less likely to diffusionly reach both the quadrupole rods and the exit region from the quadrupole near the opening of the exit diaphragm. An increase in the amplitude of the alternating or alternating voltage at the sections (27), (26) of the output diaphragm can also increase this time, since it should reduce the likelihood of rotating ions exit the output diaphragm. A decrease in the radius of rotation, as well as a slight decrease in alternating or alternating voltage, can lead to a decrease in the characteristic time of establishing a new level of the recorded ion current of these ions and to an increase in the initial value of this current.

In order to increase the fragmentation of rotating ions without increasing the radius of rotation, it is possible to periodically multiply the amplitude of the focusing radio frequency voltage during one or more rotation periods of the selected ions. The easiest way to do this is when the radio frequency voltage has a rectangular shape, digitally generated from a computer [36]. A multiple increase in the amplitude of this voltage by the same number of times will increase the natural frequency of rotation of the ions (40). Rather, it will be the movement of ions along an almost elliptical trajectory with decreasing dimensions due to collisions with buffer gas molecules, as shown in Fig. 4. During the period of the rotating field, the ions will make a multiple number of revolutions (two turns (133) are shown in Fig. 4, and their phase will almost coincide with the phase of the rotating field (132). Returning to the previous radio frequency field will return the previous resonant frequency for the ions under consideration, which coincides with the frequency of the rotating field, which, when their phases coincide, quickly enough (134) restores the previous stationary orbit of ion rotation (131).

Since the collision energy of the ion with the buffer gas molecules increases with the amplitude of the radio frequency field approximately in proportion to its square [13], the probability of ion dissociation will increase, and with a certain value of this amplitude, the desired degree of fragmentation of the studied ions can be achieved. Since with an increase in the number of such energetic collisions, the probability of dissociation will increase and reach a maximum by the end of the period of increased amplitude of the radio frequency voltage, the bulk of the ion-products of dissociation will be formed at the end of this period and immediately after it. These product ions, even if their m / z is less than the stability limit at an increased radio frequency voltage, will generally not have time to die before the end of this voltage and will be focused to the axis of the quadrupole and removed from it by the gas stream and then transferred to the mass analyzer, if the next inclusion of high radio frequency voltage occurs after such an exit of the product ions from the quadrupole.

By changing the general speed of ion motion using one of the above methods, one can change the characteristic ion fragmentation time and the ratio of the intensities of the fluxes of ion fragments. As follows from our [20] and other models of heating ions in a gas under the influence of an electric field while maintaining the mean square of the ion velocity and maintaining other experimental conditions, the internal temperature of the ions does not change, and the decay rate and the relationship between the ion fragmentation channels do not change.

If the decrease of these ions occurs only due to the processes of monomolecular decay (as well as any independent death processes described by a certain constant probability of death), then it will be exponentially decaying in time with a characteristic time specific for given conditions for each compound.

As you know, the sum of the exponential curves satisfies the finite-difference equation with constant coefficients, similarly to the differential equation with constant coefficients, or, in other words, there is an accurate linear forecast of the subsequent value of such curves according to the previous measured with some (constant in this case) time step. Having determined the coefficients of such a forecast, for example, by the least squares method, the attenuation factors of the corresponding exponentials can be determined by finding the roots of the characteristic polynomial, as for the corresponding differential equation with constant coefficients. In this case, the computational procedure described by us in [25] can be used, the modified block diagram of which is shown in Fig. 5. The initial data of this procedure are the number and values of the linear prediction coefficients. If we write down the condition for accurate prediction for one exponent exp (-t / τ) for equally spaced measurements with a step Δt, we obtain a characteristic equation if we introduce the notation w = exp (-Δt / τ):

.

.

Its roots (positive numbers less than 1) are equal to the damping factors of all the exponentials included in the measured data. In the flowchart of Fig. 5, these sought roots are called exponential factors. It was the roots of the polynomial with constraints that were found using the procedure described by us in [25]. The procedure takes into account the errors of measurement or calculation of the coefficients of the polynomial, reducing the problem to finding the minimum of the functions of many variables, which describes the sum of the squares of the discrepancies of these coefficients with their calculated values through the desired roots. To increase the accuracy of finding such a minimum, a random choice of the initial descent point to a minimum is made, and the obtained search results are then averaged.

Apparently, one can use standard procedures for finding the roots of a polynomial. However, inaccuracies in setting polynomial coefficients can lead to the appearance of roots that are outside the allowable range, or even complex. What to do in such cases is not entirely clear. It is better to use a method where restrictions on the sought quantities are laid down from the very beginning. Moreover, the accuracy of the approximation of the coefficients of the polynomial through the found roots in the presence of estimates of the error in the calculation of these coefficients will be a criterion for the adequacy of the obtained exponential attenuation factors.

Knowing these factors or the characteristic decay times of all the exponentials included in the signal measured at the ion rotation frequency close to the resonant one, by solving the corresponding system of normal equations, one can find the contributions of these exponentials.

Another possibility of identifying target compounds or separating signals from ions with the same m / z, but differing in the characteristic times of their fragmentation or death, is obtained by analyzing the mass spectra of product ions recorded at ortho-VPMS or in other subsequent mass analyzers.

Figure 6 illustrates the idea of such an analysis of ions. Here, the change in time t of the intensities J of three hypothetical mass spectrum peaks is shown. One of them (150) with a minimum m / z, shown by thick lines, decays relatively slowly exponentially (151) with a characteristic time τ ₁ . The other, (153) with maximum m / z, shown by thin lines, exponentially disappears much faster (154) with characteristic time τ ₂ . The average peak (155) is the sum of the first two peaks and is composed respectively of two lines - thick and thin. These three peaks correspond to two hypothetical starting ions, disappearing with characteristic times τ ₁ and τ ₂ , giving two equally probable product ions during fragmentation, shown by thick and thin lines, respectively. These three peaks at consecutive equally spaced time instants with an interval Δt (152) can be expressed through each other using linear expressions, which are written out in matrix form in (156):

In a more general case, the time-following intensities of the peaks of the mass spectrum are written as the product of the transition matrix A _kl by the vector of previous intensities - equation (157):

и собственные числа (159):Here l numbers the measured mass spectral peaks (the total number is n), k - sets the number of initial ions (no more than n), decaying with different characteristic times and producing product ions, the intensities of which form a linearly independent system of vectors. The matrix elements of the transition matrix can be calculated by the least squares method from the condition of the best in the mean-square fulfillment of equality (157) for all moments of measurement. For this, the number of recorded mass spectra (measurement times) should be no less than the number of peaks of the mass spectrum (n). The eigenvectors are found to solve the eigenvalue problem (158) for the transition matrix

and eigenvalues (159):

The eigenvectors describe the intensity distributions of the product ions for each source ion, which corresponds to the eigenvalue (159), which is an exponential decay factor of the number of these ions. So, for example, for the transition matrix in expression (156), vectors with components (1, 1, 0) and (0, 1, 1) are eigenvalues with eigenvalues equal to the first exp (-Δt / τ ₁ ) and the second exp ( −Δt / τ ₂ ) exponential decay factors. After the number of initial ions and the exponential decay or relaxation factors of the corresponding signals are found, the contributions of the mass spectra of product ions from these initial ions in all recorded samples of the initial mass spectrum to successive equally spaced time instants can be calculated by solving the corresponding systems of linear algebraic equations based on the requirement of a minimum of the standard error of the approximation. Thus, mass spectra can be obtained for each source ion, as in the case of their complete separation before measurements. With previously known m / z, some product ions can be left in stable orbits of resonant rotation. This will make it possible, if necessary, after their accumulation, to carry out a more detailed analysis of these ions, including their separation according to characteristic times of death.

Product ions, in order not to interfere with an adequate analysis of other accumulated ions, must have m / z different from other ions rotating around the axis of the quadrupole. In the case of ordinary singly charged ions, this means that the most convenient are measurements of the decay of ions with the smallest m / z among all rotating ions.

The second possibility of organizing controlled death or perhaps fragmentation of multiply charged ions is to use their collisions with the surface of the rods of the quadrupole. This case is illustrated in Fig. 7. The potential well (215) near the boundary of stable motion in the quadrupole (10) is qualitatively shown for δR in arbitrary units of the minimum position. It is the result of a superposition of the rotating field and the effective potential of the radio frequency field. The density distribution of rotating ions in a potential well (215) near the boundary (107), Fig. 3, in accordance with the Boltzmann principle under stationary conditions, is given by expression (221):

Two parameters of this distribution are determined by the nature of the rotating ions: the effective temperature T _eff and the ion charge ez. For given m / z ions and a rotating field, the effective temperature in the first approximation is mainly determined by the mobility of the κ ions - formulas (222) and (223), however, the proportionality coefficient α, in different models of ion heating, slightly depends on the properties of the ion and atoms or gas molecules can also affect the value of the effective temperature:

The phase shift of the rotating ions φ arises due to the deviation of the frequency of the rotating field from the resonant frequency of the ions. The graphs (216), Fig. 7, show the dependences of the relative density of three hypothetical types of ions rotating with the same average radius, i.e. for a given rotation frequency with the same m / z and mobilities for an ideal quadratic well (215) from the displacement δR along the radius of the quadrupole. Let the cloud of target ions (210) have an increased effective temperature. It is maximally extended among other ions, and it is precisely the ions from the far right side of the cloud that have a chance to recombine or die when they collide with the rod (101). Apparently, the probability of such a death, at least for singly charged complex organic and bioorganic ions, is practically equal to 1. To bounce back and “move away” from the wall upon impact with the conductive surface, an ion that gained additional energy near the surface due to attraction to the counterion image , elastic collision is required, and this is extremely unlikely for a complex ion with its increased energy.

In the configuration shown in Fig. 7, ions (214), which have an approximately 10% lower effective temperature than ions (210), will have a more than 2 times lower frequency of collisions with the rod (101) than ions (210) , since in this case the right tails of the distributions “work” - the “cut-off” levels of the dying ions (218) and (220). If the probabilities of the death of these two types of ions in a collision with a wall are close to 1, then such a difference in the collision frequency will lead to approximately twice the relaxation times of the corresponding ion currents and will make it quite easy to separate the signals from such ions using the method described above for finding exponential contributions to experimental dependencies. If the rotation speed of these ions is large enough, and they begin to fragment with appreciable efficiency, forming a spectrum of fragment ions, then using the methods described above, you can also separate the signals from different ions. In this case, the characteristic decay times of these signals will be determined both by the processes of the death of the initial ions on the rods (101) and by the processes of their fragmentation. It is interesting to note that the fragmentation rate of the initial ions in the presence of one dominant decay channel with a variation in the average radius of rotation or the speed of rotation of the ions should follow the Arrhenius dependence on the internal temperature of the ions. This internal temperature, as well as the effective temperature (222), should quadratically depend on the average ion velocity, however, the values of the coefficient a for these temperatures can differ. For example, in our model of heating of ions during their movement in a gas [20], such a coefficient for the internal temperature of an ion, in comparison with its effective (translational) temperature, increases, but no more than 2 times. Moreover, the first term in (222) increases proportionally. Carrying out such measurements, while varying the rotational voltage for ions, which can be comparable dying by two mechanisms, can provide information on the degree of adequacy of our model [20].

A remarkable property of the separation of signals from different ions upon their death on rods (101) is the possibility of almost complete separation of ions of different charge in those cases when they cannot be distinguished by m / z and resistance to collision fragmentation. The cloud of 4 charged ions (213) shown in Fig. 7, which coincide in m / z and are close in mobility to ions (210), is concentrated in a region remote from the surface (101), so that these ions will practically not be with it collide. This approach to identification distinguishes the proposed method from other mass spectrometric methods, where there is no direct separation of ions according to the absolute value of the charge, and ions are separated only by m / z.

Of great importance in the processes of interaction of ions with the surface of the rods of a quadrupole can be the directions and intensities of electric fields acting on ions during their passage near the surface. So, for example, if a radio frequency field at this time attracts ions to the surface, then the probability of collision of an ion with the surface, all other things being equal, will be greater than in the case of the opposite direction of this field. For multiply charged bioions containing ionic groups with charges of opposite signs, such a change in the direction of the local field at sufficiently large ion sizes can lead to the appearance of various product ions. The attraction of positive charges to the quadrupole rod can lead to the loss of one (or several) positive charges by the ion, and in the opposite direction of the field, negative charges may be more likely to be lost. Therefore, interesting structurally dependent results of measurements of the interactions of ions with the surface of the quadrupole rods can be expected if the amplitudes, frequencies and phases of the fields causing the rotation of the ions and their possible collision with the rods of the quadrupole are changed in a consistent manner.

With the computer generation of such fields, such a task does not seem too complicated. So, for example, if the frequency of non-resonant rotation of the ions is five times lower than the "resonant" frequency of rotation of the selected ions, then with this combined movement, the ions, when making a complete revolution with a quarter of the "resonant" frequency, will shift in phase of the non-resonant rotation by π / 2. Thus, rotating ions relative to neighboring quadrupole rods will have the same location, and the probability of their collision with the surface at the same radial and non-resonant rotation radii will be determined by their phase of resonant rotation relative to the quadrupole rod. Moreover, if the 5/4 period of the “resonant” rotation is an odd number of half-periods of the radio frequency voltage, then when matching the initial phases of all three voltages, the ions moving near the surface of the quadrupole rods will be at approximately the same phase distribution of the fields acting on the ions. To achieve such a ratio of the periods of the considered fields, for example, by selecting the appropriate amplitude of the radio frequency voltage. By changing the amplitudes, frequencies, and phases of the fields acting on the ions in concert, one can change the conditions of collisions of ions with the surface, the probabilities of their recombination or death, and, accordingly, the characteristic times of establishment of new levels of recorded ion currents with a relatively rapid change in the mentioned conditions of collisions of ions with the surface.

When performing the measurements described above, it is important to limit the number of accumulated ions. Intentional release from unwanted or excessive ions can also be important in the accumulation of trace impurity ions, which otherwise could be impossible or difficult due to the influence of the space charge. The corresponding resonant spin in the radio-frequency quadrupole allows one to obtain a sufficiently high selectivity of such elimination.

Additional possibilities for separating ions and obtaining information about their structure and thermochemistry during their accumulation in a quadrupole (10) arise when the energy of ionizing electrons in the electron ionization source changes (6) - Fig. 1. With a quick switch from the current value of the electron energy to another, a relaxation transition will begin to new levels of accumulated ions in the radio-frequency quadrupole. The separation of the exponential components in the recorded relaxation curves for ions with selected m / z values by the methods described above can result in obtaining ionization efficiency curves (CEI) of compounds or appearance curves for different ions with coincident m / z values, but differing in characteristic relaxation times corresponding ion currents.

The available experimental data of both our [37] and other researchers indicate that a change in the internal energy of ionized compounds usually leads to a shift in the corresponding curves of ionization efficiency over the energy of ionizing electrons. This shift occurs in the opposite direction with respect to the direction of the change in the internal energy of the compounds and by an amount close to the average value of such a change, if the corresponding distribution over this internal energy is sufficiently narrow. Thus, stabilization and controlled setting of the temperature of the mixture being poured into the ion source (6) with a variable ionization energy can provide an additional criterion for the correspondence of the recorded curves of ionization efficiency to individual compounds, since the heat capacities of complex compounds can vary significantly, even if their molecular weights coincide. The CEI of individual compounds taken at different temperatures should be obtained from each other by some shift on the electron energy scale. This also applies to obtaining CEI ions based on the just described extraction of exponential components in the corresponding relaxation curves of ion currents with fast switching of the energy of ionizing electrons. Since the appearance curves of product ions from these parent ions during their formation in the ion source will also shift by the amount of change in the average energy of the parent ions, an additional criterion for assigning the observed ions to product ions obtained from the parent ion data also appears.

Thermostating and temperature control of the radio-frequency quadrupole is also important for obtaining quantitative thermochemical data during collision-induced dissociation of the studied ions both in collisions with residual gas molecules and in interaction with the quadrupole rods. Registration of relaxation curves for product ions during dissociation of the studied ions at different temperatures of the buffer gas in the quadrupole can provide estimates of the activation energies of the corresponding processes and the energies of broken bonds.

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15. Sulimenkov IV, Kozlovsky VI, Pikhtelev AR, Brusov B.C., Raznikov VV, Dodonov AF. A new method for studying the kinetics of ion decay in a gas-filled radio-frequency quadrupole with resonant rotational excitation. 1. Principles and testing of the method using the example of the complex ion Cs [H2O] +. Mass Spectrometry, 2 (3), 2005.

16. Raznikov VV, Ivashin SV, Zelenev VV, Aparina EV, Savenkov G.N. Ion cooling in a supersonic gas stream as the basis for creating compact efficient instruments for monitoring environmental pollution. Proceedings of the Academy of Sciences "Energy" No. 5, pp. 140-155, 2007.

17. Raznikov V.V., Schultz J.A., Egan T.F., Ugarov M.V., Tempez A., Savenkov G.N., Zeienov V.V. United States Patent 7,482,582, January 27,2009. Multi-Beam Ion Mobility Time-of-Flight Mass Spectrometry with Multi-Channel Data Recording.

18. Raznikov V.V., Zeienov V.V., Aparina E.V., Ivashin S.V., Savenkov G.N., Ugarov M., Tempez A., Schultz J.A. Ion cooling in adiabatic gas flow and their reflecting from coated charged conducting surface as a new way for inserting mobility separated ions into orthogonal injection TOFMS. 17th International Conference on Mass Spectrometry, Prague, Czech Republic, August 27 - September 1, 2006.

19. Schultz J.A., Raznikov V.V., Egan T.F., Ugarov M.V., Tempez A., Raznikova M.O., Zeienov V.V., Pikhtelev A.R., Vaughn V. United States Patent 7,547,878, June 22,2009. Neutral / Ion Reactor in Adiabatic Supersonic Gas Flow for Ion Mobility Time-Of-Flight Mass Spectrometry. (analog)

20. Raznikov V.V., Kozlovsky V.I., Dodonov A.F., Raznikova M.O. "Heating of Ions Moving in a Gas Under the Influence of a Uniform and Constant Electric Field", Rapid Commun. Mass Spectrom. 13, 370-375, 1999.

21. Raznikov VV, Raznikova M.O. Information-analytical mass spectrometry "Science", M., 1992

22. Raznikov V.V., Pikhtelev A.R., Raznikova M.O., Loboda A.V. New approaches to the conversion and analysis of mass spectrometric and chromato-mass spectrometric information. Proceedings of the Academy of Sciences, Energy, N1.1997, pp. 87-106.

23. Raznikov VV, Pikhtelev A.R., Raznikova M.O. Analysis of incompletely resolved mass spectrometric data. Mass Spectrometry 3 (2), 2006, pp. 113-130.

24. Raznikov V.V., Dodonov A.F., Egorov V.A., Raznikova M.O. Mass-effusiometric gas mixture analysis method. All-Union Scientific and Technical meeting "Development and application of specialized mass spectrometric installations", Moscow, 1983, Abstracts, p.100-101.

25. Raznikov V.V., Raznikova M.O. Decomposition of charge-state distributions for better understanding ofelectrospray mass spectra of bioorganic compounds. Part 1. Basic formalism. // European Journal of Mass Spectrometry. - 2009, v.l5, pp. 367-375.

26. Verenchikov A.N. Time-of-flight mass spectrometry of biopolymers based on planar multi-reflective analyzers. The dissertation for the degree of Doctor of Physics and Mathematics, IAP RAS St. Petersburg 2006

27. Loboda A. United States Patent 7,459,679, December 2, 2008. Method and apparatus for mass selective axial transport using pulsed axial field. (analog)

28. Kozlovski V.A .; Fuhrer R .; Gonin M. United States Patent Application 200080149825. June 26, 2008. Apparatus for mass analysis of ions (analog)

29. Wells G.J. United States Patent 7,353,965 B2, April 1, 2008. Rotating excitation field in linear ion processing apparatus (analog)

30. Makarov A.A., Syka J.E.P. United States Patent 7,507,953, March 24, 2009. Obtaining tandem mass spectrometry data for multiple parent ions in an ion population (analog)

31. Schultz J.A .; Raznikov V. United States Patent 6.992.284 January 28, 2006. Ion mobility TOF / MALDI / MS using drift cell alternating high and low electric field.

32. Gonin M. Raznikov V., Fuhrer K., Schultz J.A., McCully M.I. United States Patent 7,291,834, November 6, 2007. Multi-anode detector with increased dynamic range for time-of-flight mass spectrometers with counting data acquisition.

33. Raznikov V.V., Raznikova M.O. Assignment of the mass number to peaks in high resolution mass spectra acquired via counting of ion Int.J. Mass Spectrom.Ion Proc. v. 85, 1988, p. 1-21.

34. S. Guan, A.G. Marshall, Stacked-ring electrostatic ion guide. J. Am. Soc. Mass Spectrom. 1996, 7,101-106. (analog)

35. V.V. Raznikov, V.I. Kozlovsky, I.V. Sulimenkov. A method for separating ions of organic and bioorganic compounds in the electric field averaged over ion rotations of a sectioned cylindrical cell. Application for a patent of the Russian Federation No. 2011123281 dated 06/09/2011. (analog)

36. Nikolaev E., Sudakov M. Ion Motion Stability Diagram For Distorted Square Waveform Trapping Voltage. Eur. J. Mass Spectrom. 8, 191-199 (2002).

37. V.V. Raznikov, A.F. Dodonov, V.V. Zelenov Disentangling the fine structure of ionization efficiency curves. Int.J. Mass Spectrom. Ion Proc. v. 71 (1986) p. 1-27

Claims (15)

1. The method of analysis of mixtures of chemical compounds based on the accumulation and / or sequential selective fragmentation of a variety of different parent ions in a linear radio frequency trap, including one or more diaphragms with holes along the axis of the trap, characterized in that the surfaces of the diaphragms inside the mentioned trap are partitioned, and to neighboring constant and / or variable controlled voltages are applied to the sections to create a repulsive effective potential for the accumulated ions, and fragmentation induced by by collisions of accumulated ions with atoms or molecules of a buffer gas, it is excited by the periodic movement of ions with selected m / z application to the rods of the trap rods with alternating voltages with a frequency close to the resonance for these parent ions, if necessary with a multiple increase of one or several periods of this periodic the movement of the amplitude of the radio frequency voltage on the rods of the trap with the possible repetition of such an increase after times exceeding the time of exit of the axial ions from the trap; in this case, the time-varying currents of the fragmentation ion products and / or the currents of the selected parent ions are recorded, which makes it possible to further separate these parent ions. 2. The method according to claim 1, characterized in that the linear radio frequency trap is a radio frequency quadrupole. 3. The method according to claim 1, characterized in that the analyzed gas mixture in the form of a molecular beam is passed through an electron ionization source located in front of the input diaphragm of the linear radio frequency trap. 4. The method according to claim 3, characterized in that the electron ionization source of claim 3 is a variable ionization energy source, and the transition from one energy of ionizing electrons to another can be used to trigger relaxation of recorded ion currents with the selection of exponential components of relaxation curves, which can lead to obtaining efficiency curves for the appearance of individual ions even when they coincide in m / z with other ions. 5. The method according to claim 1, characterized in that a gas mixture containing ions of the analyzed compounds from an external ion source, for example, multiply charged ions of biomolecules from an electrospray ion source, is added to the gas stream entering a linear radio-frequency trap. 6. The method according to claim 2, characterized in that the ions are removed from the axial zone of the radio-frequency quadrupole for a relatively large range of mass to charge ratios (m / z) by excitation of non-resonant rotation of the ions by applying π / 2 periodic phase-shifted quadrupole rods voltages with a frequency several times different from the resonant frequency of the mentioned m / z range, which is closest to this non-resonant frequency. 7. The method according to claim 2, characterized in that the selective removal of ions from the axial zone of the radio-frequency quadrupole is carried out by excitation of ions close to the resonant rotation with the selected m / z value by applying periodic voltage voltages with a frequency of π / 2 to neighboring quadrupole rods, close to the frequency of natural vibrations or rotations of these ions in a given field of the radio-frequency quadrupole. 8. The method according to claim 2, characterized in that these rods have a circular cross section, providing near the rods a significant deviation from the ideal quadrupole field and, accordingly, a shift in the resonant frequencies of the ions. 9. The method according to claim 1, characterized in that the linear radio frequency trap is coupled to a time-of-flight mass analyzer with orthogonal ion input. 10. The method according to claim 1, characterized in that the excitation of rotation close to resonant or including such rotation, ions with selected values of m / z and mobility are periodically displayed in the zone of possible diffusion contact with the rods of the quadrupole; by changing the amplitudes and / or frequencies and / or phases of the components of the electric field acting on the ions, the characteristic time of the death of the ions changes, which is determined under the given conditions of stationary rotation by the mobility of the ions, their charge and mass. 11. The method according to claim 9, characterized in that with a temporary increase in the amplitude of the radio frequency voltage, this increase is made periodically, in the same way and synchronized with the periodic start of the measurement cycle during the time-of-flight mass spectrometer during the entire time these curves are measured and / or synchronized with periodic movement of selected ions in a quadrupole. 12. The method according to claim 1, characterized in that when registering successive mass spectra of ion-products of collisional fragmentation, separation of the contributions to these mass spectra of product ions from the initial ions, differing in characteristic times of death, is based on the calculation of a statistically stable matrix transition between time equidistant sets of intensities of the peaks of mass spectra and estimates of its eigenvalues. 13. The method according to claim 1, characterized in that in the presence of fragmenting or dying ions with different characteristic death times in a linear radio-frequency trap, the signals from them are separated based on known methods for identifying exponential components in experimental data. 14. The method according to claim 1, characterized in that in the linear radio-frequency trap the input ion stream is blocked by the corresponding potential on the outer surface of the input diaphragm for the duration of the measurements. 15. The method according to claim 4, characterized in that the temperature of the inlet system and the linear radio frequency trap is stabilized and controlled.

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