RU2354963C1 - Method of detecting organic molecules - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: present invention pertains to gas analysis and can be used for detecting vapours of organic compounds in air. The said method involves recording peaks of ion current on drift-spectra of ions of organic molecules, that is, the relationship between ion collector current I_c and compensation voltage U_c, at different values of amplitude of output voltage of a high-voltage generator. The position of U_c.max of maximum peaks of ion current I_c.max on the compensation voltage scale is determined using the drift-spectrometer of ion mobility, containing series-arranged, in the direction of pumping air through the spectrometer, a device for ionising the probe of organic molecules in air, a device for separating the formed ions on parametres of the linear part of the drift mobility of ions and a device for recording ion current, which comprises at least, an ion collector. One of the electrodes of the device for separating ions is connected to a high-voltage generator with alternating, periodically assymetrical polarity of voltage and a low-voltage source of compensation voltage. A surface-ionisation ion source of organic molecules is used to ionise the sample. Constants ΔU₀, A and B are predetermined for the device calibration relationship between ΔU=ΔU₀+AI_c.max+BU_c.max of the half-width of peaks ΔU of ion current on drift-spectra on half their height with the maximum peak of ion current I_c.max, and the position of the peak U_c.max on the drift-spectrum. After that the drift-spectra of organic molecules are recorded. Taking into account device calibration characteristics, peaks of ion current on the drift-spectrum are separated, and for each, the true position of U_c.max on the scale of compensation voltage is determined.

EFFECT: more accurate detection of organic compounds, through more accurate determination of the position of maximums of peaks of ion current on drift-spectra.

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Description

The invention relates to the field of gas analysis and can be used for the detection and recognition of vapors of organic compounds in air, in particular mixtures of vapors of narcotic, explosive and toxic substances, the molecules of which are effectively ionized on the heated surface of surface-ionized ion thermal emitters.

A known method for the recognition of organic molecules [1] present in the form of vapors in air by measuring the nonlinear part of the drift mobility of ions.

This method is implemented in a device — a transverse drift mobility drift spectrometer, in which organic molecules supplied to the drift spectrometer as part of an air stream through the device’s inlet are ionized using a radioisotope ionizer, and then the non-linear part of the ion drift mobility is determined at a large value electric field strength, by the value of which organic molecules are recognized [1].

The method is based on the well-known relation [1], which determines the value of the drift velocity of ions v _D in an electric field of intensity E,

where μ ₀ is the ion mobility in the limit of low electric field strength, α is the nonlinear part of the ion drift mobility, the value of which is determined in this method. By value α, having a database, and carry out the identification of organic molecules.

The known method has found wide enough use in the technique of gas analysis, for example, in the determination of narcotic, explosive and toxic substances, but it also has serious disadvantages.

As a rule, for reliable recognition of the type of organic molecules, the determination of only one physicochemical parameter of organic molecules is not enough. In addition, the air temperature in the field of ionization of organic molecules and determination of their physicochemical parameters is not more than 150 ÷ 200 ° C. Therefore, with isotopic, as well as laser, corona, and other traditional methods of ionizing organic molecules, depending on the humidity of the air pumped through the device, the formed ions can capture a different number of water molecules, which leads to ambiguity in determining the parameters of the drift mobility of such formed complex ions, that is, to the ambiguity of recognition of organic molecules. In addition, the methods of ionization of organic molecules used to implement this method are not selective for important classes of substances (narcotic, explosive, and poisonous).

Closest to the claimed method is a method for identifying organic molecules, including recording peak ion currents on the drift spectra of ions of organic molecules, that is, on the dependences of the current of the ion collector I _c on the compensation voltage U _c , for various values of the amplitude of the output voltage of the high voltage generator and determining the position U _c.max maxima ion current peaks I _c.max to scale the compensation voltage using the drift of an ion mobility spectrometer comprising in sequence about the direction of pumping air through a spectrometer, a device for ionizing samples of organic molecules in air, a device for separating the formed ions according to the parameters of the nonlinear part of the drift mobility of ions, and a device for detecting ion current, including at least an ion collector, one of the electrodes of a separation device ion is connected to a high voltage generator of alternating periodic asymmetric polarity voltage and a low voltage compensation voltage source [2].

The essence of the known method is that based on the shift of the position of the peaks in the drift spectra when the amplitude of the output voltage of the high-voltage generator changes, it, in principle, allows us to determine the values of the parameters α, β, γ, etc. based on relationship (1), that is, to identify organic molecules at once by several physicochemical parameters. However, for the implementation of this method, it is necessary to accurately determine the positions U _{c.max of the} maximum peaks of the ion current I _c.max on the drift spectra on the compensation voltage scale U _c .

However, the known method for the recognition of organic compounds has significant disadvantages. First, the values of the coefficients α, β, γ, etc. are small, while the accuracy of measuring the compensation voltage is about 3-5% [2]. Therefore, when analyzing mixtures of organic molecules having close values of the parameters α, β, γ, etc., overlapping peaks from various types of ions is possible, which leads to uncertainty in the measurement of the parameters U _c.max peaks in the drift spectra. In addition, the ions of organic compounds formed during isotopic ionization tend to trap a different number of water molecules, which leads to the appearance of additional types of ions. That is, the recognition accuracy of organic molecules in the known method is small.

The aim of the proposed method is to increase the reliability of recognition of organic compounds by increasing the accuracy and reliability of determining the positions of the peaks of ion currents on the drift spectra, primarily in the analysis of mixtures of organic molecules or organic molecules, which upon ionization produce several types of ions.

This goal is achieved by the fact that for the ionization of samples of organic molecules using a surface-ionization source of ions of organic molecules, for the drift spectrometer, the constants ΔU ₀ , A and B are preliminarily determined ΔU = ΔU ₀ + AI _c.max + BU _c.max the half-widths of the peaks ΔU of the ion current in the drift spectra at half their height from the maximum peak of the ion current I _c.max and the position of the peak U _c.max in the drift spectrum, after which the drift spectrum of organic molecules is recorded and taking into account the instrument calibration The dependence of the ion peaks consistently isolated current drift spectrum, for each of which determine its true position on the dial U _c.max compensating voltage.

The constants ΔU ₀ and A are determined in the mode of absence of ion separation in the drift spectrometer, for which, with the high-voltage generator turned off and the compensation voltage source turned on, the ion drift spectra of test organic molecules are recorded at various values of the concentration of organic molecules in the air stream pumped through the spectrometer, and / or ions of the background current of the surface-ionization ion source for various values of the ionization efficiency of the surface-ionization ion source, determines The dependence of ΔU on _{Ic.max of the} peak of the ion current is _determined , while A is found from the slope of the linear part of this dependence, and ΔU _{0 is} found by extrapolating the linear part of this dependence to the zero value of _Ic.max .

The constant B is determined in the mode of ion separation in the drift spectrometer, for which, with the high-voltage generator turned on and the compensation voltage source turned on, the drift spectra of the ions of the test organic molecules and / or background ions of the surface ionization ion source are recorded at a fixed concentration of organic molecules in the flow of air pumped through the spectrometer, and / or with a fixed ionization efficiency of the surface-ionization ion source, but with different x values of the amplitude of the output voltage of the high-voltage generator, for at least one peak i in the drift spectra and for different values of j of the amplitude of the output voltage of the high-voltage generator determine the values of U _{c.max.i, j} , I _{c.max.i, j} and

ΔU _{i, j} , then the constants B _{i, j are} calculated in accordance with the expression

B _{i, j} = (ΔU _{i, j} -ΔU ₀ -AI _{c.max.i, j} ) / U _{c.max.i, j} , and then from the values of the constants B _{i, j} determine the average value of the constant B.

The claimed method is illustrated by the following drawings.

Figure 1 shows the drift spectrum of vapors from a mixture of trinitrotoluene and dinitrotoluene containing a two-component organic binder of substances belonging to the class of amines. The drift spectrum was recorded using a coaxial drift spectrometer with a surface ionization ion source with an output voltage amplitude of the high voltage generator 1700 V.

Figure 2 shows the dependence of the half-width ΔU of the peak of the background current of the drift spectrometer with a surface-ionization ion source on the value

I _c.max peak of the background ion current obtained by processing the drift spectra recorded with the high-voltage generator turned off and the compensation voltage source turned on and using a drift spectrometer with a surface ionizing ion source.

Figure 3 shows the dependence of the half-width ΔU of the peak of the background current on the drift spectra for a drift spectrometer with a surface ionization ion source and the compensation voltage U _c.max peak on the output voltage of the high-voltage generator U _d with the high-voltage generator turned on and the voltage source turned on compensation.

Figure 4 shows the dependence of the half-width ΔU of the peak of the test organic substance - trinitrotoluene on the drift spectra for a drift spectrometer with a surface ionization ion source and the value of the compensation voltage U _c.max peak on the output voltage of the high-voltage generator U _d with the high-voltage generator turned on and compensation voltage source turned on.

Figure 1 shows: 1 - peak current background, 2 and 5 - peaks of the organic ligament, 3 - peak trinitrotoluene, 4 - peak dinitrotoluene.

In Fig. 2, the following are indicated: 1 — linear portion of the dependence, 2 — non-linear portion of the dependence, 3 — extrapolation region of the linear portion to the zero value of the ion current.

None of Figures 3 and 4 are indicated: 1 - dependence of the compensation voltage on the output voltage of the high-voltage generator, 2 - dependence of the peak half-width on the value of the output voltage of the high-voltage generator.

As the active elements of surface ionization sources of ions, materials based on a molybdenum alloy [3], an effectively ionizing compound from the class of amines, or materials based on oxide bronze of an alkali metal [4], effectively an ionizing compound from the class of amines and nitro compounds, were used.

The ionization efficiency of the surface ionization ion source was controlled either by changing the temperature of the active element of the surface ionization ion source, or by changing the potential difference applied between the active element of the surface ionization ion source and the ion source electrode that draws ions from the surface of the active element.

The essence of the proposed method is as follows.

In the drift spectrum shown in FIG. 1, there are 5 peaks, and the maximum values of the ion currents at the peaks are comparable, and the peaks themselves are not completely separated. Therefore, the distributions of ion currents in each peak overlap each other, which leads to a shift in the maximums of the peaks and, accordingly, to an error in determining their positions on the compensation voltage scale, that is, to an error in the identification of organic molecules.

The resolution R of any spectral device - optical, electronic, ionic, X-ray, etc. - calculated as the ratio of the half-width of the image line to the dispersion of the device. On the other hand, it is equal to the experimentally measured resolution equal to the ratio of the half-width of the peak in the spectrum at half its height to the position of the peak on the spectrum scan scale. For electronic and ionic devices, the line width of the image can increase due to the action of the space charge of electrons or ions, various types of aberrations of the device, which are the instrument functions of each device. Accordingly, the peak width in the spectrum will also increase due to these reasons. In this case, for a drift spectrometer, the resolution can be represented as:

where ΔU ₀ is the initial peak width in the spectrum, ΔU ₁ is the peak broadening due to the action of the space charge of ions, ΔU ₂ is the peak broadening due to the action of various aberrations, i.e., nonlinearities in the nature of the motion of ions in the drift spectrometer.

The peak shape in spectral instruments with a high degree of accuracy is usually described by a Gaussian distribution. Therefore, in principle, knowing the position of the peak maximum and its half width, we can distinguish this peak against the background of other peaks. Or, conversely, it is possible to sequentially subtract the most reliable peaks in position and half-width from a signal containing several peaks, thereby sequentially decoding the spectrum containing the not completely separated peaks.

When ionizing the molecules of organic compounds by any known methods, with the exception of surface ionization, both positively charged and negatively charged ions are formed subject to the condition of the general electroneutrality of the air volume. For example, if negatively charged ions of organic molecules are formed during the ionization of organic molecules, then positively charged ions are simultaneously formed from air molecules and water molecules present in the air. In this case, positively charged ions are lighter and, accordingly, have greater mobility. During the formation of positively charged ions of organic molecules, more mobile negatively charged ions are simultaneously formed. When the indicated "mixture" of ions begins to move between the plates of the drift chamber, between the ions with charges of different signs the processes of recharging and recombination of ions occur. In this case, the trajectory of the ion beam, the magnitude of the electric field between the plates of the drift chamber in a complex way change along the direction of movement of the ion beam. Accordingly, the parameters responsible for beam broadening and due to the action of the space charge and aberration also change in a complex way. Moreover, these parameters depend on many factors - air humidity, the concentration of ions of organic molecules in the air, air flow rate, etc.

During the surface ionization of organic molecules, only positive ions of organic compounds are formed on the surface of the active element of the ion emitter. They are formed as a result of the capture by an amine of a proton on the surface of one type of ion-emitter material [3] or the capture of an alkali metal ion or proton on the surface of another type of ion-emitter material [4]. Moreover, protons or alkali metal ions can also escape from the surface of the thermal emitter as a result of thermal desorption; these ions can be captured by air or water molecules, but also with the formation of positively charged ions. Therefore, with this ionization method, the processes of ion recharging and recombination in the drift chamber region cannot occur. Therefore, with this method of ionizing organic molecules, the parameters responsible for beam broadening and due to the action of the space charge and aberration are not interconnected. This circumstance makes it possible to experimentally measure the contributions to the broadening of the ion beam due to the action of the space charge and aberration, respectively, and to use these characteristics, which are gauge for each specific design of the drift spectrometer, to determine the true position of the peak of the ion current in the drift spectrum.

The proposed method is implemented as follows.

Figure 2 shows the experimental dependence of the half-width ΔU of the peak of the background current of the drift spectrometer with a surface-ionization ion source on the value of I _{c.max of the} peak of the background ion current. In the region of not too large current values, this dependence is linear and allows you to determine the values of ΔU ₀ and A for this device. In the region of large values of the ion current, there is a deviation of the dependence on the linear one, which is due to the limitation of the current transmission of devices of this type [5]: if the ion current exceeds a certain value determined by the device design, the ion current goes into saturation.

The experimental dependences of the half widths of the peaks shown in Figs. 3 and 4, respectively, for the background current from the thermal emitter and for the test nitro compound on the magnitude of the amplitude of the output voltage of the high voltage generator, which reflect the change in the peak width due to aberration effects, correlate with the dependences of the peak displacement on the voltage amplitude of the high voltage generator. Such a correlation is due to the fact that both the aberration and the shift of the peaks in the drift spectrometer are due to the nonlinear parameters α, β, γ, etc., which are part of the ion motion equation (1). Therefore, using at least one of these correlations, parameter B can be determined for a given device.

After determining the parameters ΔU ₀ , A, and B for a given specific device, for each peak in the complex drift spectrum, the value of its true width can be determined taking into account the broadening of the peak, after which the procedure can be performed to select peaks in the complex drift spectrum and find the true position U _c.max peak on the compensating voltage scale, adjusted for corrections due to overlapping and broadening of the peaks. In this case, it is advisable to begin the decoding of the spectrum from a peak or from peaks, to the left and to the right of which there are other peaks with close values of the maximum ion current.

For example, the spectrum shown in FIG. 1 should be deciphered in the following sequence: determining the positions of the peaks of the peaks with the highest and closest in magnitude intensity, to the left and right of which there are peaks with similar intensity values, in this case peaks 3 and 4; calculation of the true half-width of peaks 3 and 4 in accordance with the relation ΔU = ΔU ₀ + AI _c.max + BU _c.max ; subtraction of the peaks 3 and 4 from the full drift spectrum, taking into account their Gaussian distribution and calculated half-width values; determination of the positions of the peaks of peaks 2 and 5; calculation of the true half-width of peaks 2 and 5; subtraction of the peaks 2 and 5 from the remaining drift spectrum, taking into account their Gaussian distribution and calculated half-width values; determination of the peak position of peak 1; calculation of the true half-width of peak 1; subtracting peak 1 from the remaining drift spectrum, taking into account its Gaussian distribution and the calculated half-width value. Note that after this procedure, small peaks may appear on the remaining drift spectrum, which are not visible on the original drift spectrum due to the effect of overlapping peaks. Their decryption should also be carried out in accordance with the above procedure.

Thus, the proposed method allows to determine the true values of the positions of the peaks in the drift spectra in the case of their partial overlap and taking into account the broadening of the peaks due to the action of the space charge of ions and aberrations of the drift spectrometer.

The foregoing shows that in the scientific, technical and patent literature there are no technical solutions to achieve the indicated technical results using the above methods and means, which allows us to conclude that the claimed invention meets the patentability conditions of “novelty” and “inventive step”. The claimed method can be implemented in industry, which allows us to conclude that the claimed invention meets the patentability condition “industrial applicability”.

LITERATURE

1. US patent No. 5420424 from 05/30/1995, (analog).

2. Buryakov I.A. Experimental determination of the dependence of the mobility coefficients of ions in a gas on the electric field strength. ZhTF, 2002, volume 72, issue 11, pp. 109-113 (prototype).

3. RF patent No. 2260869. Material of a thermal emitter for surface ionization of organic compounds in air and a method for activating a thermal emitter. Priority dated April 12, 2004

4. RF patent No. 2265835. The method of analysis of organic compounds in the atmosphere. Priority 04/19/2004

5. Bannykh OA, Povarova KB, Kapustin V.I. A new approach to surface ionization and drift spectroscopy of organic molecules. ZhTF, 2002, volume 72, issue 12, pp. 88-93.

Claims (3)

1. A method for identifying organic molecules, including recording peaks of ion current on the drift spectra of ions of organic molecules, that is, on the dependences of the current of the ion collector I _c on the compensation voltage U _c , at various values of the amplitude of the output voltage of the high-voltage generator and determining the position U _c.max maxima ion current peaks I _c.max on scale voltage drift compensation using an ion mobility spectrometer, comprising successively arranged in the direction of air pumping through spectrum a device for ionizing samples of organic molecules in air, a device for separating the formed ions according to the parameters of the nonlinear part of the ion drift mobility, and a device for detecting the ion current, including at least an ion collector, one of the electrodes of the device for separating ions being connected to a high-voltage alternating periodic generator asymmetric in polarity of voltage and low-voltage source of voltage compensation, characterized in that for the ionization of samples of organic molecules is surface ionization ion source of organic molecules, for drift spectrometer pre-determined constants ΔU _0, A and instrument calibration curve
ΔU = ΔU ₀ + AI _c.max + BU _{c.max the} half-width of the peaks of ΔU ion current in the drift spectra at half their height from the peak value of the ion current peak I _c.max and the position of the peak U _c.max in the drift spectrum and then the drift spectrum of organic molecules is recorded and, taking into account the instrument calibration dependence, the peaks of the ion current in the drift spectrum are sequentially identified, for each of which its true position U _c.max on the scale of the compensating voltage is determined. 2. The method according to claim 1, characterized in that the constants ΔU ₀ and A are determined in the mode of absence of ion separation in the drift spectrometer, for which, with the high-voltage generator turned off and the compensation voltage source turned on, the ion drift spectra of test organic molecules are recorded at different values the concentration of organic molecules in the flow of air pumped through the spectrometer and / or the background current ion of the surface ionization ion source at various values of the ionization efficiency of the surface ion Discount ion source values determine the dependence of the magnitude of ΔU I _c.max peak ion current, wherein the value of A is found by linear part of this dependence of the slope, and the magnitude of ΔU ₀ are extrapolating the linear part of this dependence to zero value
I _c.max . 3. The method according to claim 1 or 2, characterized in that the constant B is determined in the mode of ion separation in the drift spectrometer, for which, with the high-voltage generator turned on and the compensation voltage source turned on, the drift spectra of ions of test organic molecules and / or ions are recorded background current of a surface ionization ion source with a fixed concentration of organic molecules in the air flow pumped through the spectrometer and / or with a fixed ionization efficiency of surface ionization ion source, but for different values of the amplitude of the output voltage of the high voltage generator, for at least one peak i in the drift spectra and for different values of j of the amplitude of the output voltage of the high voltage generator, determine the values of U _{c.max.i, j} , I _{c.max .i, j} and ΔU _{i, j} , then the constants B _{i, j are} calculated in accordance with the expression B _{i, j} = (ΔU _{i, j} -ΔU ₀ -AI _{c.max.i, j} ) / U _{c.max. i, j} , and then from the values of the constants B _{i, j} determine the average value of the constant B.

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