US3769513A

US3769513A - Ion kinetic energy spectrometer

Info

Publication number: US3769513A
Application number: US00315117A
Authority: US
Inventors: E Delany
Original assignee: Perkin Elmer Corp
Current assignee: Applied Biosystems Inc
Priority date: 1972-12-14
Filing date: 1972-12-14
Publication date: 1973-10-30
Anticipated expiration: 1990-10-30

Abstract

A spectrometer is provided for analyzing metastable decompositions which occur in certain ions. Metastable decompositions result in a precursor ion decomposing into a daughter ion and an uncharged particle of mass. The presence of such ions determine a unique spectra for certain compounds which are difficult to distinguish using conventional mass spectrometer techniques. The spectrometer utilizes a wedge-shaped beam afforded by the use of a relatively inexpensive electrostatic lens assembly. A drift space is provided to allow precursor ions to decompose and thence be analyzed.

Description

United States Patent [1 1 Delany Oct. 30, 1973 ION KINETIC ENERGY SPECTROMETER [75] Inventor: Edward B. Delany, Ridgefield,

Conn.

[73] Assignee: The Perkin-Elmer Corporation,

Norwalk, Conn.

[22] Filed: Dec. 14, 1972 211 Appl. No.: 315,117

[52] US. Cl. 250/290, 250/283, 250/293, 250/294, 250/427 [51] Int. Cl. H01] 39/34, Bold 59/44 v [58] Field of Search 250/281, 282-, 283, 1

[56] References Cited UNITED STATES PATENTS 3,475,604 10/1969 Noda et al. 250/281 X 3,610,921 10/1971 Major, Jr. 250/283 3,673,404 6/1972 Major, Jr. 250/282 Primary Examiner-William F. Lindquist Attorney-John K. Conant [57] ABSTRACT 10 Claims, 3 Drawing Figures I Mimi/7151.5

ION KINETIC ENERGY SFECTROMETER This invention relates to spectrometers and more particularly to a spectrometer for performing metastable anaylsis.

BACKGROUND OF INVENTION tation of the mass spectra.

In any event, a common characteristic of many of these instruments is that ions enter the deflection fields with substantially the same energy or' velocity. After the ionization process of the sample material, a well known phenomenon occurs which is generally referred to as a metastable reaction. Certain sample ions produced, exhibit metastable characteristics. In this manner, a parent or precursor metastable ion decomposes into a charged daughter ion and an uncharged particle of mass. The kinetic energy of an accelerated precursor ion then becomes distributed among these resultant particles in accordance with the mass of the particles. The decomposition of the precursor ion may occur at different points along the trajectory of the ion. In a mass spectrometer, when a decomposition occurs at a location between the ion source and the analyzing field of the spectrometer, the division of the precursor energy between particles of different masses results in a daughter ion having less kinetic energy then is necessary for focusing at the appropriate sector focal point. Accordingly, it has been found that a double focused mass spectrometer, when operated at a pre-established electric field potential, will not indicate daughter ions as they will be defocused. However, the analysis of metastable characteristics can be afforded by the use of a double focused mass spectrometer which is altered in structure and'operation.

The analysis of such ion decompositions is useful to the analytical chemist since it contributes to an understanding of the molecular structure of many sample materials. Therefore, some analytical chemists have modified double focusing mass spectrometers to perform ion kinetic energy analysis to determine the existence and identity of daughter and precursor ions. Such spectrometers are extremely expensive and the modifications to perform such analysis are relatively difficult and inconvenient to implement. The fact remains that metastable analysis is important in that certain materials cannot be conveniently detected with the use of conventional and expensive mass spectrometer techniques. For example, in regard to certain isomers, it is extremely difficult to determine which isomer of a plurality of such isomers is present by a typical mass spectrometer analysis. This is so because of the fact that these isomers exhibit almost identical peaks and defy simple detection. However, their characteristics can be accurately ascertained by the use of metastable analysis as will be further explained.

It would therefore be desirable to provide an economical apparatus to specifically analyze metastable decompositions in order to provide information in regard to the constituents of such samples.

BRIEF DESCRIPTION OF FIGURES FIG. 1 is a partial schematic and block diagram of an ion kinetic energy spectrometer.

FIG. 2is a schematic diagram ofa beam path used in a spectrometer using a 30 energy analyzer section.

FIG. 3 is a schematic diagram of another arrangethem using a 30 energy analyzer section.

DETAILED DESCRIPTION OF DRAWINGS Referring to FIG. 1 there is shown a block diagram of anion kinetic energy spectrometer. Such a spectrometer can also be utilized in conjunction with a gas chromatograph to provide analysis of gaseous samples. Briefly, the system comrises an analyzer portion which includes a scanner 50, various power supplies and an electrometer 60. In order to clarify explanation and description, the optical system and the electronic system will be separately discussed.

TI-IE OPTICAL SYSTEM Before proceeding with a detailed description of the optical system, a few general points should be made. In regard to the selection of an optical system for an ion kinetic spectrometer, one is concerned with simplicity and sensitivity rather than high resolution. These criterion involve consideration of the inherent spectral line widths of many samples. It would be reasonable to fabricate an analyzer which could provide a resolution of 200 to 300 lines with reasonably sized apertures or slits. An energy analyzer 20 using parallel plates with 45 incidence is shown as part of the optical system. Both the entrance S1, 16 and collector slit S 17 can be fixed prior to installation or, alternatively, can be adjustable depending upon particular system requirements. The ion source 10 is of a generally open construction to allow increased helium flow and uses a large area Pierce-type filament to enhance sensitivity. There are included in the ion a series of repeller plates 15 to enhance metastable ion production in the drift space. The lens system is designed to match the source of ion formation to the entrance slit 16 of the energy analyzer 20. In one instance, the lens allowed a 10 inch drift path in a field-free space while providing a very small beam divergence.

relatively non-critical tolerances of lens parts and spacing thus achieving an economical, advantage in the overall system characteristics. A quadrapole doublet lens is a very efficient way to get a wedge-shaped beam together with a long drift length which is necessary to give the precursor ions time to decompose into a daughter ion and a charged particle. This therefore provides an efficient and economical lens assembly for such a spectrometer.

A wedge-shaped beam can also be produced by a magnetic quadrapole lens assembly. However, magnetic quadrapoles are expensive and one does not require a magnetic lens to form a wedge-shaped beam. In this manner, the use of the electrostatic quadrapole assembly provides a convenient way of producing an optimum beam shape resulting in relatively sharp focusing of the beam. The sharper the focusing of the beam, the more improved the sensitivity, as this provides a larger current density across the entrance slit 16 of the energy analyzer section 20.

THE ELECTRONIC SYSTEM The ion source includes a cathode or filament and an anode electrode or target. The filament geometry is a Pierce-type filament which is a large filament and hence gives greater sensitivity and as such is sometimes referred to as a space charged limited ion source. Electrons are emitted from the cathode and drawn to the anode which is biased at a higher positive potential then the cathode via the target power supply 40. The electrons that are used to ionize the sample vapor or gas are confined to a relatively small volume at a ion exit slit 4 of the apparatus. In this manner, the sample which is injected via sample injector 42 and introduced via the sample line 31 is directed to the ion exit slit where the electrons emitted from the ion source bombard the sample molecules thus creating charged ions. The ions at the ion exit slit 41 are passed through a linear accelerator 18 which comprises a plurality of plates. The plates are biased via a voltage dividing network 43. The ion linear accelerator 18 serves to pro vide an essentially parallel beam at its exit plane. This.

is the optimum beam which the electrostatic quadrapole lens requires at its input fiducial plane. In this beam, all ions possess relatively constant energy. The monoenergetic beam passes through the electrostatic quadrapole lens assembly which is biased by means of the voltage divider shown in module 45. As indicated, the electrostatic quadrapole is arranged to provide a wedge-shaped beam. This shape serves to provide a match at the entrance slit 16 of the energy analyzer and because of the wedge-shaped configuration serves to provide an optimum current density at the s1itl6 with non-critical focusing requirements. The electrostatic quadrapole doublet enables one to obtain a wedgeshaped beam with a very long drift length. The drift length is shown on the diagram and this is the area where ions are accelerated along a sufficient length to give the precursor ions time to decompose.

Basically, the formation of daughter ions involve the following operation. As indicated, certain ions will exhibit metastable decompositions. The probability of such decomposition is enhanced when the drift time of an ion along its trajectory is increased. The drift time, of course, is dependent upon the length of the drift path. A metastable decomposition results in a charged daughter particle and an uncharged particle. The ions enter the entrance slit 16 of the energy analyzer and due to theelectric field provided by the scanner 50 are caused to focusat the output slit or aperture 17. The scanner 50 provides a linear ramp up to 4000 volts to the' energy analyzer. This allows scanning of daughter ions to 2E with a 2,000 ev beam or to IE with a 4,000 ev beam. The scanner is a closed loop system employing a vacuum tube or transistor arranged in a sawtooth oscillator configuration. The scanner is synchronized via a marker generator which also controls or synchronizes the display means 70, so that the abscissa of the displayed energy scan is ascertained. It was found that the separation between the entrance slit l6 and the collector slit 17 could be about 4 inches to provide optimum collection of daughter ions. The choice of the slit width isprimarily a function of the desired resolution. Resolution is determined by the effective width of the entrance and collector or exit slits as well as the beam energy. For purposes of ion kinetic energy analysis, it has been found that with a beam energy of about 3,000 electron volts, one can adequately obtain resolutions of 200 to 300 with normal size apertures. Since the spectra, unlike mass spectra, display a wide variation of peak widths, one does not require very high resolution as it has been found that the plurality of peaks result in a unique fingerprint of the sample to be detected. For example, in a system using the apparatus shown above, it was possible to pick out and distinquish pure neopentane iso-pentane, normal pentane and cyclopentane. The differences betweeen these pentanes cannot be easily obtained by anaylsis through a mass spectrometer while the ion kinetic energy spectrometer distinquishes the same relatively easily.

It is, of course, known that the higher the beam energy, the more resolution one can obtain. In regard to certain isomers, such as N-butane, resolutions greater than 100 were obtained for beam energies of 900 electron volts. Experimental results indicate that for most samples of interest, the sharpest line width will range from 2.0 to 10.0 electron volts with many wider peaks also apparent. Unlike a typical expensive mass spectrometer, the output daughter ion beam available at the aperture 17 is directed to the input of an electronic amplifier 60 or an electrometer. The collecting electrode 62 is referred to as a Faraday cup. The input of the Faraday cup .is coupled to the input electrode of the electrometer 60. Bascially, the electrometer. 60 comprises two high gain operational amplifiers arranged in a cascade configuration with a shielded feedback resistor 61 arranged from input to output. The input stage for the electrometer comprises a Mosfet stage having a bootstrap circuit configuration between the source and the drain electrode. It has been found that the amplifier noise which should be low is a function of input capacity. Therefore, this factor should be acknowledged in the design of the amplifier section. Input capacitance is substantially reduced by the use of conventional boot strapping and capacitance neutralization techniques. Such techniques are known in the art. The output of the amplifier 60 is coupled to a suitable display means which may be a recorder or an oscilloscope to provide the operator with a spectrum characteristic of metastable decompositions.

The above description is concentrated on the use of an energy analyzer 20 having a 45 beam incident angle. This type of analyzer is easy to fabricate as the entrance and exit slits are located on the ground plane and hence form part of the analyzer. In any event, it is also possible to use an energy analyzer section with a beam incident angle of 30. This analyzer offers certain advantages as will be seen.

As shown in FIG. 1, only a converging beam 30 can be used in the drift space because of the fact that the entrance slit (8,) l6 and the exit slit (S 17 are located on the ground plane 76 of the analyzer 20.

When one uses an energy analyzer 80 (FIG. 2) with a 30 angle between the beam and the analyzer, the entrance slit (S,) 81 and the exit slit (S 82 can be located in a field-free space and remote from the ground plane of the analyzer. This allows the slits (S and S to be positioned perpendicular to the beam axis which is the optimum theoretical position as this position minimizes ion scattering. The plates including the slits are mounted by means of support brackets or holders within the beam path at a desired location.

In FIG. 2 the location of the drift space is shown between an entrance slit (5,) and the analyzer, and the beam is diverging before it enters the analyzer and within the drift space. The slot in the analyzer is large and hence this diverging beam enhances the probability of acceptance of more metastable ions by the optical system. It is seen that this is so as the aperture in the analyzer of FIG. 1 (45 analyzer) is also the entrance slit 1)- In FIG. 3 there is shown a converging beam in the drift space. This is similar to the 45 case, with the exception that the aperture in the analyzer can again be larger. Here the drift space is located between the lens and the entrance slit (5,).

Thus, the advantages of the 30 analyzer include:

I. Location of analyzing slits, entrance and exit slits, in a field-free space, allowing the slits to be positioned perpendicular to the beam axis.

2. The object distance can be adjusted to be either large to inches) or small (less than 1 inch) so that the drift space can accommodate either a converging or diverging beam.

This enables one to select a drift space either:

1. Between the quadrapole lens and the entrance slit (8,) as in FIG. 3, or

2. Between the entrance slit (S and the energy analyzer (FlG. 2).

The increased width, due to the energy analyzer input arrangement (FIG. 2), also allows the analyzer to collect or accept more metastable ions considering the random probability of energy change and direction change which is inherent in the production of metastable. This energyand direction change is the result of the decomposition of a precursor cursor ion. The decomposition or explosion causes an impluse of momentum to be imparted'to the daughter ion which can then proceed in any direction according to a random probability at a new energy.

I claim:

ll. Apparatus for analyzing metastable decompositions caused by the ionization of certain sample ions which decompose into daughter ions and uncharged particles comprising:

a. an ion source for bombarding an injected sample material to form a plurality of ions including metastable ions,

b. lens means responsive to said ions for forming them into a relatively monoenergetic ion beam having a wedge-shaped configuration,

c. a scanable energy analyzer separated from said means by a given distance selected to permit the decomposition of said metastable ions into said daughter ions and uncharged particles, said analyzer including an input aperture for receiving said ion beam and an output aperture for discharging selected ions, and

d. means coupled to said analyzer for collection of daughter ions at said output aperture.

2. The apparatus according to claim 1 wherein a. said lens means includes an electrostatic quadrapole doublet lens assembly.

3. The apparatus according to claim 1 wherein said ion source includes a Pierce-type cathode for emitting electrons and a target electrode for directing said emitted electrons toward an ion exit slit and means located proximate said exit slit and adapted to receive a sample material to cause said material to be bombarded by said electrons.

4. The apparatus according to claim 3 further including a plurality of biased accelerating plates disposed about said ion exit slit and directed along said ion beam path for forming a parallel beam.

5. The apparatus according to claim 1 wherein said given distance is greater than 8 inches.

6. Apparatus for analyzing metastable decompositions, comprising,

a. an ion source adapted to receive a sample material for bombarding the same with electrons to provide a plurality of ions at an output of said ion source,

b. means including an electrostatic quadrapole lens assembly positioned proximate to said ion source output to form said ions into a beam having a wedge-shaped configuration,

0. an energy analyzer having an input slit and an output slit and positioned a given length from said means, said length selected to permit certain precursor ions to decompose into daughter ions and uncharged particles, said analyzer positioned so that said ion beam is directed through said input slit,

d. biasing means coupled to said analyzer for causing daughter ions to propagate through said output slit, and

e. means responsive to said propagated daughter ions to provide an indication of their presence.

7. The apparatus according to claim 6 wherein said energy analyzer is a parallel plate energy analyzer disposed at an acute angle with respect to said ion beam.

8. The apparatus according to claim 6 wherein said energy analyzer is a parallel plate analyzer disposed at an angle of 30 with respect to said ion beam.

9. The apparatus according to claim 6 wherein said means responsive to said propagated daughter ion, includes,

a. a Faraday cup coupled to the input of a high gain operational amplifier for collecting and amplifying said propagated daughter ions.

10. The apparatus according to claim 6 wherein said means including an electrostatic quadrapole lens assembly further includes a linear plate accelerator for focusing said beam prior to said energy analyzer.

Claims

1. Apparatus for analyzing metastable decompositions caused by the ionization of certain sample ions which decompose into daughter ions and uncharged particles comprising: a. an ion source for bombarding an injected sample material to form a plurality of ions including metastable ions, b. lens means responsive to said ions for forming them into a relatively monoenergetic ion beam having a wedge-shaped configuration, c. a scanable energy analyzer separated from said means by a given distance selected to permit the decomposition of said metastable ions into said daughter ions and uncharged particles, said analyzer including an input aperture for receiving said ion beam and an output aperture for discharging selected ions, and d. means coupled to said analyzer for collection of daughter ions at said output aperture.

6. Apparatus for analyzing metastable decompositions, comprising, a. an ion source adapted to receive a sample material for bombarding the same with electrons to provide a plurality of ions at an output of said ion source, b. means including an electrostatic quadrapole lens assembly positioned proximate to said ion source output to form said ions into a beam having a wedge-shaped configuration, c. an Energy analyzer having an input slit and an output slit and positioned a given length from said means, said length selected to permit certain precursor ions to decompose into daughter ions and uncharged particles, said analyzer positioned so that said ion beam is directed through said input slit, d. biasing means coupled to said analyzer for causing daughter ions to propagate through said output slit, and e. means responsive to said propagated daughter ions to provide an indication of their presence.

8. The apparatus according to claim 6 wherein said energy analyzer is a parallel plate analyzer disposed at an angle of 30* with respect to said ion beam.

9. The apparatus according to claim 6 wherein said means responsive to said propagated daughter ion, includes, a. a Faraday cup coupled to the input of a high gain operational amplifier for collecting and amplifying said propagated daughter ions.