DE1918948A1 - Method and device for the analysis of metastable ions in mass spectrometers - Google Patents

Description

Patent application

The P ericin-Elm er Corp., Norwalk, Connecticut, US .A.

Method and device for the analysis of metastable ions in mass spectrometers

The invention relates to mass spectrometry, namely a improved mass spectrometric method and means for identifying components of a sample.

In a mass spectrometer, molecules of a vaporized sample are bombarded with an electron beam, which causes them to die ions formed are accelerated by means of an electric field via an analyzer part of the spectrometer onto a collecting electrode. The accelerated ions pass through a field in the analyzer part that changes over time, its field strength for scanning a mass spectrum by focusing ions of different M / e values on an exit aperture in the The collecting electrode is changed. In one arrangement, the focusing is by mass selection with a changing over time Magnetic field achieved. Ions migrating through the exit aperture excite a particle multiplier and its output signal is strengthened for connection to recording media z.ß. a writing implement. The output signal thus generated contains a spectrum of peaks occurring one after the other, each for the relative frequency of a sample component are characteristic.

In an operating with a magnetic sector-shaped scanning mass spectrometer, the 'device resolution is largely

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of the change in kinetic energy in the scanning field incoming ions dependent. This ion energy is given by

KE = 1/2 mv ² = V ^e , (l)

where e is the charge of the ion, V is the acceleration voltage,

m is the mass of the ion and ν is the speed of the ion. The resolution is increased by additionally switching on an electrical analyzer into the ion path for generation an electrostatic field which, together with the magnetic scanning means, double-focussing the accelerated Ions enables. The ions then first pass through an electrostatic field generated by the electrical sector. in the Operation causes the electric sector field that ions of a given before passing through the magnetic scanning field Mass have an almost constant energy level.

As is known, the ions generated by the spectrometer of a second decomposition are in the ion path between the acceleration electrode and subjected to the electrical sector. In the case of ion decomposition, for example, a precursor decomposes (ABC) into a metastable or transition ion (AB) and an uncharged particle (C). The range of as a result mass peaks generated by this decomposition appear broad and usually occurs with non-whole mass numbers. The identification of metastable ion dissolution is special Importance in the structural determination of organic molecules, and it is desirable to have a mass spectrometer provides the means for such identification.

In a previously known mass spectrometer® is a double focusing Magnetic scanning array used to identify metastable ion transitions. The watched Mass number m = M / e of ions resulting from the metastable transition during magnetic scanning

^is: m ⁺ = M ₁ ² / m _o (2)

where m is the mass of a precursor ion and m is the mass of a

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Daughter ions is. Albeit an infinite number of solutions To satisfy this relationship for the magnetic analyzer, the electric analyzer of the double-focusing one shows Spectrometer features energy differentiators and facilitates the identification. In a previously known arrangement, the Accelerating voltage V changed in order to reduce the kinetic energy

to increase the energy of the transition ion. One between particle mass and Beschieu:
give by:

and acceleration voltage V existing relationship is

vrobei

mV = m-V _ (3)

ο ao 1 al

V ₁ = V + A ^
al ao - a

is. In view of these relationships, the prior art method of identifying metastable junctions is that a mass spectrum at constant electrical analyzer voltage V and constant acceleration voltage V magnetically separated

G ct.

is scanned until a daughter peak of the mass number m is assumed desired peak is detected that the scanning field is kept at a constant magnetic voltage II for focusing

of the peak m and measuring the accelerating voltage V that at X ao

constant V and i'L / magnetic field H the acceleration voltage V around e a

the Gx size / \ V is increased to defocus the first peak and focus a second peak, and that the inclination

voltage V is measured at which the second peak is observed

is tet. From uci \ data m _n , V and V, the mass number of the

1 ao al

Precursor ion m can be calculated from equation (3) above.

Although this method of identifying metastable junctions satisfactoryiü & de delivers results, it shows numerous Disadvantages that limit the usefulness of the process. For example, the method described changes the Ioneiibesch.1 equilibrium voltage, which is disadvantageous

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is associated with changes in ion drift time between Ion source and electrostatic sector. The procedure leads to a loss of static resolution and can lead to a reduced sensitivity in the defocused State lead. In addition, there is a practical upper limit "on which the accelerating voltage can be increased so that the investigable mass range is substantially limited.

The invention is therefore based on the object of an improved Method for the identification of metastable ion transitions a mass spectrometer.

The invention is also based on the object of providing a mass spectrometer arrangement to create that avoids one or more of the aforementioned disadvantages.

The method according to the invention is characterized by the following Procedure steps:

Generation of a magnetic analyzer field H and an electrical analyzer field V ₁ to focus ions of mass m = Bi ₁ / m on the exit aperture, where ra is a precursor ion and oi is a daughter ion in a metastable ion transition,

Changing the electrical analyzer with constant values

from II and V to a second value V ,, at which the ion a e2

m, on which the ion image parts are focused, and

+ Calculate the mass In ₁ in the relationship m ₁ = V / V

In accordance with the general features of the invention, the identification of metastable ion transitions in a double focusing Mass spectrometer with an electric and

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a magnetic analyzer achieved by first identifying an ion m and then the electrical The analyzer is modified in such a way that transition ions of the Mass m focused on an object of the magnetic analyzer will. Then the daughter ion can be identified, and the precursor ion m is given by the relationship m =

m / m. This measurement of m 'and conservation of the ion energy a ο

stages in the identification of nu result in more advantageous Way that sensitivity and resolution are maintained at normal operating levels.

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A method for identifying metastable lonenübergäng © in a döppelfokussierenden mass spectrometer is j that a peak of mass m äwreh change © IAES "Mag = iietfeldes at constant electric Äaalysatorspanaung V is roughly focused at constant acceleration voltage ¥ _{1 η} ^ electrical Änaiysatorspasmimg ¥ measured electrical Analyzer span is reduced to constant magnetic field H and Bssclileunigungsspanauag V until <sia, aa. ~

whose metastable peak is observed? a, Ed the Aaalysatos = voltage V _{2 is} measured ^ at T? siolier the other peak is observed. The metastable transition can then be determined from the relationship

7 _el m ⁺ - V ₀₂ -Si ₁ . ■ . (4)

According to another feature of the invention, a double focusing mass spectrometer has means for changing * lsr electrical sector voltage over a relatively large range _# with the accelerating voltage and the magnetic field being kept at a constant value _β

An Aüsführraigsbeispiel the invention is in the figures shown and described below * -

Figure 1 is a circuit diagram, partly in block form, of one in accordance with features of the invention constructed mass spectrometer.

Fig. 2 is a block diagram of an electrical power supply for changing the electrical analyzer voltage of the mass spectrometer of Fig

Figure 3 is a detailed circuit diagram of the electrical power supply of Figures 2, and

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FIG. 4 is a view of the control panel for use with the voltage source of FIG. 3.

In Fig. 1 is a double focusing sector-shaped mass spectrometer Represented by Nier-Johnson Geometry »A Nier-Johnson mass spectrometer embodies this general type for example the Perkin-Elmer model MS-27o. The spectrometer 1 comprises an ion source Io and means 12 for introducing a sample to be examined into the ion source. The autosampler means 12 are designed to vaporize a liquid or solid sample and leak the vaporized sample into the ion source at a desired leak rate. Numerous arrangements for sample application are known Although the sample dispenser is shown coupled to the ion source Io via a line 14 in FIG. 1, it can be connected to the ion source and be in close physical contact with it. The ion source Io usually consists of a directional source Electrons to bombard the sample molecules in order to generate positively and negatively charged ion particles «

The ions formed by the ion source Io are placed on a target electrode 16 accelerated and focused on this. A source 18 of an acceleration voltage V is provided and between

Cl

an acceleration electrode 2o with an ion object aperture arranged therein, and the ion source Io is switched. The accelerated ions travel through an enclosed evacuated path defined by a tube 22 and run one after the other through electrical and magnetic scanning fields which are designed in such a way that they focus ions of a specific mass onto an exit aperture 23 of the target electrode 16. In general, the electrical analyzer has a pair of cylindrical plates 24 _t between which a voltage V 'is applied. This tension is from

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a source 26 and applied via lead-in isolator members 27. The electrical analyzer forms an ion lens, through which the ion object aperture 21 onto an imaging point 29 is focused, which is at an intermediate position on the ion path; Point 29 also forms for the magnetic analyzer an ion object. Ions of the same mass are effective at the same energy level in the Focus '29 brought before its entry into a magnetic one Scanning field H. A magnetic analyzer has an electromagnet 28, which is arranged to generate the magnetic scanning field in the path of the accelerated ions. The magnet 28 is energized by a generally ramp waveform sample current obtained from a power source 3o will. An alternative power source for the electromagnet is an adjustable DC power source 32 and a switch 33 which is adapted to supply alternating current to the electromagnet 28 from the source 32 or from the sensing current source 3o there. A magnetic field H with an amplitude varying with time is generated by the magnet 28 when it is excited by current from the source 3o, and ions of different mass numbers at the ion object point 29 are sequentially focused on the exit aperture 23. The electromagnet 28 creates a constant amplitude H magnetic field when current flows therein from source 32.

Those ions which migrate through the aperture 23 of the collecting electrode 16 hit a particle multiplier 34, for example a conventional electron multiplier, the output of which goes to a voltage amplifier 35. That Output signal from amplifier 35 has a plurality in sequence occurring peaks, the amplitude of which is characteristic of the relative frequency of ions of an assigned Mass number. This output signal is recorded on

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switching means, for example the writing instrument 36, the measure of displaying the mass number on an axis of abscissa of a recording strip and the relative abundance of ions may have on an axis of ordinate of the recording strip. These are characteristics of a mass spectrometer known, and there is therefore no need for a further description.

The voltage source 26 is designed so that it supplies an adjustable electrical analyzer voltage. The electrical analyzer acts as an energy discriminator and, with a constant acceleration voltage V _a, focuses ions on point 29 which have an energy corresponding to a specific analyzer voltage V 1. With a constant acceleration voltage V_ and a generated electrical analyzer voltage V ₁ , the kinetic energy of a precursor ion m is given by:

KE - T ₀ - ev _a - 1/2 m _o v _o ²

where ν is the velocity of the precursor ion. If the precursor ion is on the part of the ion path between the ion source and the electrical analyzer to m ~> m. + m, decomposed, then the kinetic energy of the transition ion m _{1 is} given by

1/2 (m _o -m ₂ ) v _o ² - 1/2 In ₁ v _Q ² <T _Q.

According to a feature of the invention, the magnitude of the electrical analyzer voltage is changed to a value V ₂ so that transition ions of the energy T _{1 run} through the electrical analyzer and are focused on the intermediate point 29. / "The focusing of the energy of the ions by the electrical /" These zones are then properly focused for entry into the field of the magnetic analyzer with a mass

+ 2 /
m ■ m. / hl, ·

r 9 -

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Sector delivers a peak width m-, which is the peak width of the Ions m. Is almost the same for a normal spectrum. These Property enables a resolution for metastable ions in the defocused state that is almost the same as the static resolution of the mass spectrometer in the focused state.

When operating the apparatus, an operator observes a reproduced one produced with a sampling current from the source 3o Spectrum at constant acceleration voltage V and con-

el

constant electrical sector voltage V ... When observing a peak with properties that indicate a metastable transition (for example with a relatively wide base), the operator sets by switching off the power source 3o and connection of the adjustable direct current source 32 to the electromagnet 23 via a switch 33 for a direct field H Focusing this particular peak m her. For focusing, the direct current amplitude is set in the electromagnet 28, while at the same time the output of the meter 38 is observed. The voltmeter 38 provides a maximum reading when the peak m is focused. In this way, a voltage V 1 and a ground value m are determined. The voltage source

becomes H = »k. and V «k _o adjusted by reducing the χ a δ.

Voltage V until the original peak m is defocused and a new peak is focused. The electrical analyzer _{voltage V q2 is then} observed at the measuring device 39. The daughter massem. can now be determined from equation (4) above, and the precursor mass m can be determined from equation (2).

Figures 2 and 3 show a variable voltage source for the electrical analyzer. The arrangement is particularly advantageous in that the factor ^ν _θ } / ^ν _θ 2 ^of equation (4), that is,

is given directly, and the mass m by multiplying m and this factor is easy to calculate. The tension

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source 26 contains a voltage supply circuit 5o and an operating mode selection circuit 52 for selecting an overview mode or, alternatively, a measuring mode. The overview circuit 54 is used by the operator to display the general voltage range of the electrical analyzer voltages within which the new peak is focused. The measuring circuit 56 then ensures that the voltage V ₂ ^ n is applied to the electrical analyzer plates as well as a relatively accurate reading of the factor V _el / V _e2 ..

In Fig. 3, the power supply circuit shown includes 50 DC batteries 58 and 6o connected in series which provide relatively positive and relatively negative output voltages for the electrical analyzer. The sum of the battery voltages forms V · ,, the electrostatic analyzer voltage for which the mass spectrometer is normally calculated. The operating mode selector 52 consists of a three-pole triple changeover switch 61 which is arranged to couple the battery supply to the electrostatic plates 24 via overhead potentiometers 62 and 64 or alternatively via the measuring voltage divider circuit arrangement 56. This measuring divider circuit arrangement comprises voltage dividing means having first and second voltage dividing sections connected in series. The voltage divider means are in parallel with the battery source. A first voltage divider section, indicated generally at 66, is connected to an electrostatic plate 24a, and the second voltage divider section, indicated generally at 68, is connected to an electrostatic plate 24b. The voltage divider section 66 has three measuring range rotary switches with eight positions To, 7A and 72 au> f ₄ two groups of precision resistors 76 and 78 and a precision interpolation potentiometer or helipot 8o, which is connected between adjustable contact members of the switches 72 and 74.

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The rotary switch members are switched in the same direction, and one Series connection is provided between the positive terminal of battery 58 and a common connection of the resistor group 78 via the switch 61, the switch 7o, a resistor of the group 76, the "switch 72, the potentiometer 8o, the switch 74 and a resistor of group 78. The common Junction of group 78 is connected in series with the negative terminal of battery 6o via a similar arrangement, which a resistor of a Widastandsgruppe 9o, a switch 86, an interpolation potentiometer 92, a Switch 84, a resistor of the resistor group 88, a switch 82 and the mode selection switch 61. Tensions of these voltage divider sections at adjustable contacts 94 and 96, the potentiometers 8o and 92, respectively tapped and placed on the sector plates 24. The ohmic resistances of resistor groups 76 and 78 are chosen so that clockwise rotation of the controls of switches 7o, 72 and 74 results in a less positive voltage on the terminal 97 of the potentiometer 9o causes. The ohmic resistances of resistor groups 88 and 9o are chosen to be similar a less negative voltage at terminal 99 of potentiometer 92 when the contacts of switches 82, 84 and 86 are rotated to effect counterclockwise. The rotor links of the switches 82, 84 and 86 are connected in parallel with the rotor members of the switches 7o, 72 and 74, while the sliding contact arm 94 and 96 are simultaneously adjustable.

The potentiometers 8o and 92 effect an interpolation between selected measuring switch scale positions (FIG. 4). A voltage V, composed of the total battery voltages, is present between the plates 24 when the switch rotors are arranged in position 1 and the sliding contacts 94 and 96 at the terminals

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97 and 99 of the potentiometers 8o and 92, respectively, are arranged. In other selected positions of the measuring selector switch and the potentiometer contacts, part of the voltage V _e , / V _{2 is applied} ^to the analyzer electrodes 24. The potentiometers 8o and 92 are precision potentiometers constructed with one turn and have a common calibrated scale that shows the part of the resistance in the circuit . The resistance group switches are operated simultaneously and therefore enable the attachment of a calibrated scale showing the resistance in the circuit. A measuring selector switch indicator and interpolation potentiometer indicator produce coarse and fine displays of the factor V, / V o ·

To approximate the measured value at which a new peak is detected, the selector switch 61 alternately couples the batteries 58 and 6o to the overview circuit 54. The overview circuit has a voltage divider connected in parallel with the battery voltage sources. Grinding arms 98 and loo of the overview circle are coupled to sector plates 24b and 24a, respectively. These grinders are switched in parallel, and a calibrated scale shows the coarse setting for the new peak. The overview scale is assigned to the eight measuring switch positions. The potentiometers 62 and 64 are logarithmic to provide decreasing proportions between dial positions on the control panel (Fig. 4) to provide a linear dial reading. A limit switch lo2 and resistors Io4 and lo6 are provided to limit the range of the overview potentiometer to that of the measuring circuit, which is of the order of magnitude of ^ν _θ ι / ^ν _β 2 "9. In general, it is relatively unlikely that metastable ions with less than 1/9 of the mass of the precursor can be detected. However, a second switch position, enables the circumvention of these resistors and expands the range of V ₉₁ A ₀₂ optionally up to infinity ·

In general, the device is _{focused for a certain ratio of V ft} SU-V · When selecting the measuring mode, the

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Sector voltage fluctuations occurring change this ratio and the focus of the device · As it is out of the above For reasons it is desirable to keep the acceleration voltage V, at a constant value, it is adjustable

Voltage control circuit provided with which the electrical sector voltage can be adjusted to the desired ratio maintain. This control circuit shown in Fig. 3 contains a battery I06 and a potentiometer I08, whose sliding arm is connected to the connection point of the batteries 58 and 60 via an earthed circuit. Voltage settings for focus control are performed when the switch 61 selects a survey or measurement mode. Of the The sliding arm of the potentiometer I08 is adjusted so that on Battery connection point a voltage occurs with which the desired ratio is maintained.

4 shows a control panel for use in the voltage source of the electrical analyzer of FIG. 3 in the metastable ion examination. In an example of a sample examination, an observed m ⁺ on the writing instrument 36 of FIG. 1 shows a value of 39.7. The overview mode is then selected and the setting of the overview potentiometer provides a peak indication on the meter 38 when the overview indicator is between the overview dial positions 2 and 3. Then a measuring mode is selected 1 the measuring x switch is turned to 2 and the interpolation y potentiometer is turned to a value 0.14 at which a peak on the measuring device 38 is observed. The factor ^ν _θ ι / ν _β 2 * 2.14 is read directly from the χ and y scales of the desk of FIG. In ₁ - 2.14 χ 39.7 - 84.958 and

^m o " ^m x ² / m ⁺ ■ (84.958) ² / 39.7 - 182.

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An improved method and apparatus for analyzing metastable transitions has thus been described. This method and apparatus are particularly advantageous in that the ion accelerating voltage V remains constant during the analysis,

the sensitivity at a relatively high value is held.

Although a special embodiment of the invention is described, it should be understood that various modifications can be made without would depart from the spirit of the invention and the scope of the claims.

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Claims (1)

-Jt- 1 91 89A8 Claims (1.! Method for metastable ion analysis with a double-V - "" ^ focusing mass spectrometer with an ion source, Means for generating an ion acceleration voltage V, an electrical analyzer, a magnetic analyzer, an ion object aperture, one between the electrical and magnetic analyzer on an ion trajectory lying ion image and an exit aperture, characterized by the following process stepsχ Generating a magnetic analyzer field H and an electrical analyzer field V ₁ for focusing ions + 2
the mass m = m / m on the exit aperture, where m is a precursor ion and m. a daughter ion in a metastable ion transition, Changing the electrical analyzer with constant values from H and V to a second value V ₀ , at which the a 6 « Ions are focused on the ion image point, and the mass nu is calculated in the relationship nu « ^V _e l / ^V e2 ^m * 2. The method according to claim 1,
characterized, that the value of the electrical analyzer field V of V, is lowered to a smaller amplitude V-. 009845/1647 3. The method according to claim 2,
characterized by the process steps, that the value m and V, at the value of V. and the diminished egg egg The value of the electric field voltage V ~ can be measured 4. Double focusing mass spectrometer with magnetic Scanning analyzer means and an electrostatic analyzer, marked by Means for generating a magnetic field of relatively constant amplitude H for focusing ions on an exit aperture of the spectrometer and a source of adjustable voltage V. which are used to generate a adjustable analyzer voltage on said electrostatic Analyzer is connected. 5. Apparatus according to claim 4,
characterized, that the adjustable voltage source is designed so that the electrostatic analyzer _{voltage V Q is} varied over a voltage range. 6 device according to claim 2, characterized, that the spectrometer has a zone acceleration electrode, means sum generating a constant voltage V at said acceleration electrode, and the adjustable analyzer voltage source is designed so that the Sector voltage ν against the acceleration voltage V. is constantly reduced. - 3 009845/1547 7. Apparatus according to claim 5,
characterized, that the spectrometer has an ion accelerating electrode, means for generating a constant voltage V across the has said accelerating electrode / the electrical analyzer has an electrode to which a variable voltage is to be applied, and the adjustable analyzer voltage source is a voltage source, one connected in parallel with the voltage source Voltage divider circuit arrangement and means for connecting the voltage divider to the 'electrode. 8 device according to claim 7,
characterized,
that the voltage source supplies a voltage V, and the voltage divider circuit arrangement includes means which are adapted to be a part of the at the electrode show the applied voltage V. 9. Apparatus according to claim 5,
characterized, that the electrical analyzer has a first and second electrical analyzer electrode, between which one variable voltage should be, and the adjustable voltage source of the electrical analyzer is a DC voltage source, one with the DC voltage source voltage divider connected in parallel and means for connecting a first and second tap to the voltage divider with different voltage to the first and second electrode. 009845/1847 19189-8 Ιο. Device according to claim 9, characterized in that that the voltage divider has a first and second potentiometer, each of which has an adjustable contact, and the adjustable contacts on the first or second electrode are connected. 11. Apparatus according to claim lo, characterized in that that the voltage divider has a variety of range switching resistors and has associated switching means. 12. Apparatus according to claim 11, characterized in that that the voltage divider is a first group of range switching resistors and associated range switching means, a second group of range switching resistors, and associated range switching means has, and the first and second potentiometers, respectively, the first and second range switching means and resistor groups for interpolation assigned in a selected area. 13. Apparatus according to claim 12, characterized in that that means to the / range switching means and said potentiometers for indicating the part of the between the electrodes applied voltage V are connected. / "* said 0098A5 / 1B47 - 5 - 14. Apparatus according to claim 13, characterized by a second voltage divider and switching means comprising said first voltage divider and said second Alternately connect the voltage divider to the DC voltage source in parallel, Means for connecting the first and second taps to the second voltage divider to the first or second electrode. 15. Device according to claim 14, characterized in that that means on said second voltage divider for displaying the portion of the voltage between the first and second electrodes applied voltage V are connected. 009845/1547