US3787681A

US3787681A - A method for analysis by producing a mass spectrum by mass separation in a magnetic sector field of a mass spectrometer utilizing ionization of a sample substance by electron bombardment

Info

Publication number: US3787681A
Application number: US00133823A
Authority: US
Inventors: C Brunnee; H Bultemann
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-04-14
Filing date: 1971-04-14
Publication date: 1974-01-22
Anticipated expiration: 1991-01-22

Abstract

In an analysis of a sample substance by mass spectroscopy, the sample substance is bombarded in a high vacuum chamber by an electron beam to produce ionized mass particles. To minimize ion residence time in the high vacuum chamber, the bombarding electron beam is interrupted at intervals to permit the ionized mass particles to be quickly drawn out of the high vacuum chamber by a suction voltage.

Description

United States Patent Brunnee et a1.

METHOD FOR ANALYSIS BY PRODUCING A MASS SPECTRUM BY MASS SEPARATION IN A MAGNETIC SECTOR FIELD OF A MASS SPECTROMETER UTILIZING IONIZATION OF A SAMPLE SUBSTANCE BY ELECTRON BOMBARDMENT Inventors: Curt Brunnee, 282 Platjenwerbe uber Vegesach, Birkenweg; Hans-Joachim Bultemann, Walliser Strasse 94, Bremen, both of Germany Filed: Apr. 14, 1971 Appl. No.: 133,823

Related US. Application Data Continuation-in-part of Ser. No. 817,123, April 17, 1969, abandoned.

US. Cl. 250/413 SB, 250/419 ME Int. Cl. H0lj 37/08 Field of Search ..250/41.9 ME, 41.9 G,

[451 Jan. 22, 1974 [56] References Cited UNITED STATES PATENTS 2,810,075 10/1957 Hall 250/419 D 2,694,151 1l/1954 Berry 250/419 ME OTHER PUBLICATIONS Ionization In A Mass Spectrometer By Monoenergetic Electrons, A. E. Fox, Review of Scientific Instr., Vol. 26, No. 12, Dec. 1955, pp. 1101-1107.

Primary Examiner.lames W. Lawrence Assistant Examiner-C. E. Church Attorney, Agent, or FirmWo1f, Greenfield & Sacks [5 7 ABSTRACT In an analysis of a sample substance by mass spectroscopy, the sample substance is bombarded in a .high vacuum chamber by an electron beam to produce ionized mass particles. To minimize ion residence time in the high vacuum chamber, the bombarding electron beam is interrupted at intervals to permit the ionized mass particles to be quickly drawn out of the high vacuum chamber by a suction voltage.

1 Claim, 7 Drawing Figures PATENTED 3.787. 681

SHEET

2 or 3 GRADIENT FROM SUCTION VOLTAOE LOW ELECTRON CURRENT NO TROUOII NO CHANOE OF ORAOIENT I /y/i LARGE ELECTRON CURRENT CHANGE OF ORAOIENT t=O STATIONARY STATE WITH ION CURRENT AND RESIDENCE TIME W g/ACMW QWWWXMQ v PMENTED JAN 2 2 ISM SHEET 3 BF 3 METHOD FOR ANALYSIS BY PRODUCING A MASS'SPECTRUM BY MASS SEPARATION IN A MAGNETIC sECTOR FIELD OF A MASS SPECTROMETER UTILIZING IONIZATION OF A SAMPLE SUBSTANCE BY ELECTRON 5 BOMBARDMENT RELATED APPLICATION This application is a continuation-in-part application of Ser. No. 817,123, filed Apr. 17, 1969 and now abandoned.

FIELD OF THE INVENTION BACKGROUND OF THE INVENTION The known methods and devices of this kind have the disadvantage that if the ion source operates in a socalled space charge working condition, pressure dependent variations of the mass spectrum occur as a result of the pressure dependent ion residence time occuring in this operative condition.

The space charge working condition is present under the following two circumstances: l when a suction voltage is provided to draw ionized particles out of the ionization region; and (2) when an electron bombardment beam of an electron density is used which is so high that a negative space charge will occur which counteracts the suction voltage so that anion intercepting potential trough occurs in the ionization region.

In the space charge working condition the following quantitative relation exists between the ion residence time t, and the operating parameters of the ion source.

E 1 2E b e0V2e/mVUe 20 ll e. I 1) E0 /2 zo I 0 e 1 ln the above:

t, ion residence time m i,. ionizing electron current (A) b breadth of the electron bundle (cm) E potential gradient of the ion suction voltage in the ionization chamber, in the absence of a space charge W/ 2 U c c l atiqn1ot sss of h isa zi s sls r n iv P gas pressure in the ionization chamber Tim) s influence constant 8.86 by 1O (AsV" e charge of the electrons m mass of the electrons 0' ionization probability of the gases in the ionization chamber. Dependent on the kind of gas and the electron acceleration voltage U,,. Usual order of magnitude 1 10 (Torr If the constants e, and VZe/m are inserted and if the usual values V and 9 Torr cm" are inserted for U, and 0' for normal operating conditions, then there is obtained [t 2 10 e/ N/1 e/ The space charge working condition exists if i.e., with a potential gradient E of e.g., 5 VLcr r i /b must be greater than 5 "10*" A/ crF:

If e.g., i /b 1 10' A/cm and the pressure p 1 10 Torr, then the ion residence time t, 10" s.

Under otherwise equal conditions, with a pressure of 10" Torr, the ion residence time would be lO s.

lfi /b-E 10 AN, then the ion residence time is independent of the parameter i /b E and is only dependent on the pressure.

Equation (3) then becomes t, l0"/P'(s. Torr) The space charge condition, as will be seen from equation (1), is achieved if operation is done with a large electron current and a small ion suction current. In order to achieve a high detection sensitivity, as large as possible an electron current has to be provided. However, the ion suction voltage cannot be increased in the same ratio without producing ions of an increased energy spread that are unwanted and tend to reduce the resolution of the system. By these reasons operation in the space charge condition cannot be avoided, if maximum sensitivity and resolution are to be obtained.

The ion residence time is particularly large with low pressures, e.g., with residual gas measurements in the UHF region. As a result of this, the disturbing effects of the ion residence time become particularly strong.

The present invention is directed to the problem of providing a method and a device of the kind described above, which enables non-falsified gas analysis with high sensitivity, even in the pressure region below 10' Torr.

In the solution of this problem, the invention is based on the recognition that the observed falsification of the mass spectrum with analysis in ultra high vacuum is due to the unusually large ion residence time in the ionization chamber. It is known, and is utilized in mass spectrometry for investigating collision processes between electrons and ions and between ions and neutral particles, that the enlargement of the ion residence time leads to formation of ion fractions and of multiply-charged ions by electron impact and by association between ions and molecules. Also, it is known that the ion residence time in ultra high vacuum can be very long. From measurements of the formation possibility, of multiply ionized noble gas ions, this residence time has been assessed at an order of magnitude of 100 ms (Redhead, P.A. I4Annual Conference on Mass Spectrometry and Allied Tops 1966, pages 661-667).

These observations indicate that the falsification of the mass spectrum in ultra high vacuum can only be excluded if it is possible to avoid the exceptional elongation of the ion residence time in the ultra high vacuum. It is obvious to shorten the residence time by operating with a higher ion suction voltage. Actually, with a sufficiently high ion suction voltage, the pressure dependent alterations on the residual gas spectrum can be kept within bearable limits. The increasing of the ion suction voltage acts unfavorably however, since the energy spread of the ions is increased and thereby the resolution power is reduced. This disadvantage can be minimized if the mass range is divided over two ion collectors with different path radii. The voltage rise of the ion acceleration voltage is thus many times smaller than with the conventional arrangement with only one ion collector, so that even at the upper end of the mass range, the ion acceleration voltage is still so large that the relative energy spread which determines the resolution power, remains sufficiently small. On the other hand, the division of the mass range over two ion collectors is cumbersome and requires a special construction of the entire mass spectrometer, which cannot be provided or can only be difficultly provided subsequently on existing installations.

In order to arrive at a better solution to the problem, the invention proceeds from the consideration that the larger ion residence times in ultra high vacuum are based on a negative space charge of the electron current in the ionization region. This negative space charge produces, as is shown in FIG. I of the drawing, a potential trough which is superimposed on the potential gradient produced by the ion suction voltage.

This potential trough becomes deeper, the smaller the potential gradient is in the ionization chambers. Ions are intercepted in the trough. The magnitude of the thereby occurring ion space charge is so that the trough just becomes flattened so far that the ions produced can flow away. In the stationary condition, the ion space charge is given by the product of the ion current and the residence time. From this it can be recognized that, as explained above, by a sufficiently large potential gradient the depth of the potential trough and thus the ion residence time can be kept small (see FIG. 1). However, according to the invention, independently of the magnitude of the potential gradient caused in the ionization chamber by the suction' voltage, the ion residence time can be kept practically as short as desired by periodically interrupting the electron current, thus nullifying the electron space charge and in consequence of this destroying the potential trough. This can be realized by use of a pulsed electron current source. The essence of the invention is directed to the use of a pulsed ionizing electron current while operating in a space charge working condition of the ion source, i.e., when an ion intercepting potential trough occurs in the ionization region. It is true that the sensitivity of the measurement is reduced in dependence on the pulsing ratio, but this disadvantage can be compensated for by making the interruption times for the electron current shorter than the current flow times.

The method according to the invention can be used in a simple manner with existing mass spectrometers by providing a pulse device which is insertable in the form of an adapter in the supply circuit of the ion source.

By such a pulse operation of the electron current, falsifications of the mass spectrum due to the residence effect can be practically completely avoided in a simple manner, so that naturally true spectra can be obtained.

In order to make the invention clearly understood, reference will now be made to the accompanying drawings which are given by way of example and in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the trough superimposed on the potential gradient by the negative space charge within the ionization region.

FIGS. lA-ID illustrate potential gradients associated with different electron-ion conditions of the system, and helpful in understanding the theory of the present invention;

FIG. 2 illustrates a mass spectrometer with a pulse device, according to the invention, in a diagrammatic form; and

FIG. 3 illustrates a mass spectrum with unpulsed electron current with two different suction voltages, and the mass spectrum of the same gas mixture with pulsed electron current according to the invention.

DETAILED DESCRIPTION The drawing illustrates the use of the invention in mass spectrometers to produce a mass spectrum by mass separation in a magnetic sectoral field. Ion formation and ion separation take place in a chamber 1, which is kept at a pressure of preferably less than 10 Torr by a high vacuum pump 2. From a recipient 3, the gas sample to be analyzed is passed into the chamber 1 through an inlet system not shown in the drawing. In the head la of the chamber, the sample substance is ionized by a pulsed electron beam ionization source 4. The ions formed are drawn by a suction electrode 5 from the ionization region and are accelerated by an ion optical system 6 consisting of a number of electrodes. The ions are then shot in the form of a beam or bundle 7 through an entry slit 8 into the sectoral field of a magnet 9. In the sectoral field, a separation of the ions takes place by mass dependent deflection, the magnitude of separation being variable by variation of the field strength of the magnet 9 or by variation of the acceleration voltage of the ion optical system 6. The entire mass scale can be passed through ion collector 10, with an ion current measuring device 11 connected thereto, at the outlet side of the magnetic field. Therefore, the various masses contained in the sample can be successively measured at collector 10 by varying the magnetic field strength or the acceleration voltage.

In chamber 4, ionizing takes place by electron bombardment. For this purpose, electrons from a thermionic cathode 12 are drawn into the entry window 13 of a metal box 14, which surrounds the ionization chamber 4, by a potential difference of e.g., volts. The electrons flow in the form of a narrowly limited beam through the chamber 4, emerge through a window 16 of the chamber and pass to an electron collector 17. By means of a magnet l8, 19 a magnetic field extending in the direction of the electron beam 15 is produced. This field causes a sharp bundling of the electron beam and thus causes a high electron density in the electron beam.

The ions formed in the chamber 4 by electron bombardment are drawn out by a suction voltage U between the box 14 and the suction diaphragm 5 closing the box at one side, of e.g., 5 volts, and are guided through the ion optical system in the already described manner and the analysis is performed by forming the mass spectrum.

A potential gradient is caused by the potential difference between the suction diaphragm 5 and the rear wall of the box 14. This potential gradient is shown in FIG. IA. This potential gradient is sufficient to instantaneously draw out of the chamber 4 the ions formed by electron impact, and the time which is necessary to withdraw the ions of highest mass number from the chamber 4, is in the order of micro-seconds. This procedure is however disturbed by alterations of the potential gradient, which are produced by the space charge caused by the electrons in the ionization chamber as is shown in FIG. 1C.

The problem solved by the present invention may be better understood from a consideration of FIGS. lA-lD, which show the potential gradient between

electrodes

14 and 5 of the ion source under different conditions as set forthbelow. The electrodes I4 and 5 are depicted in FIG. 2. The electrode 5 is a suction electrode, whereas the electrode I4 forms part of the rear wall of the box shown in FIG. 2.

In FIG. 1A the straight line represents the potential gradient in absence of the electron beam. When a beam of ionizing electrons is caused to flow in the space between

electrodes

14 and 5, the potential gradient is altered as indicated in FIGS. 1B and 1C by the negative space charge produced by the electrons. Now, if the electron current is high as is the case in FIG. 1C, a trough, in the negative direction, is formed in the line representing the potential gradient. The positive space charge of the ions flattens this trough as much until the reversion of the potential gradient is abolished and per time unit as many ions can flow off as are produced (FIG. ID). For producing this positive space charge the ions must remain a certain time in the potential trough. This residence time is inversely proportional to the ion current. When the electron beam is interrupted, the negative space charge is removed, the potential gradient is then as indicated in FIG. IA, and the ions are instantaneously drawn out of the ionization region by the voltage of electrode 5. If, however, the electron current is small as is the case in FIG. 1B, no trough by gradient reversion but only a small deviation from the straight line is formed and consequently no residence time of ions can result from the electron space charge.

FIG. 3A shows the mass spectrum of a gas sample using a suction voltage of 5 volts and a constant electron current of 1.3 mA. Relative to the unfalsified spectrogram, there is an increase in the proportion of ion fractions as a result of splitting of the molecular ions by electron impact: e.g., the

mass numbers

12 and 16 as fractions of the CO molecule (Mass number 28).

There is furthermore an increase in the proportion of multiply ionized mass particles by repeated ionization of an ion by electron impact and multiply charged ions occur already with electron energies which lie below the ionization potential for direct multiple ionization: e.g., occurrence of triply or quadruply ionized argon ions with high intensity. (Mass numbers 13 A; and 10).

Finally, the mass spectrum is falsified by the occurrence of association products by collision between ions and molecules. There occurs e.g.,

mass numbers

29 and 33 as association products of CO+ and H or 0 and H- The spectrum FIG. 3B was taken with an increased ion suction voltage of 15 volts, and, shows a reduction of the disturbing effect caused by the ion residence time, so that more particularly the fraction distribution deviates less from the customary pattern.

In FIG. 3C, the mass spectrum is illustrated which is taken with a suction voltage of 5 volts and pulsed electron current on the order of 2 milliamps.

For the pulsed operation, in the illustrated example a pulse generator 22 is provided which with the aid of a switch 23 can be selectively connected between the thermionic cathode 12 and the diaphragm 24 provided between the thermionic cathode and the entry window 13 of the ion source. The pulse generator produces a rectangular voltage having the pulse ratio 1 l, which is applied as a negative blocking voltage of e.g., 50 volts relative to the cathode 12. This voltage is applied periodically to the diaphragm 24, and in turn the electron current in the ion source is interrupted. For exam ple, triply and quadruply charged ions (mass numbers 13 /3 and 10) and the association product (COH) (mass number 29) are no longer present.

The period of the current flow times and current interruption time amounts to 50 microseconds. Since the ions are instantaneously drawn from the ionization zone on disappearance of the potential trough after interruption of the electron current, there exists an ion residence time of 50 microseconds maximum. Because of the short residence time, the above described disturbance effects cannot noticeably occur.

Within the scope of the invention, many modifications and other embodiments are possible. More particularly, a pulse ratio for the blocking voltage U, of other than 1 i can be selected, so that the blocking time can be restricted to the smallest value possible in order to limit the reduction in sensitivity caused by the pulsing.

Furthermore it is possible to make the duration of the electron current periods larger or smaller than 50 microseconds. According to experience, the electron current periods and thus the ion residence time should not be greater than 1 ms.

If the ion residence time is smaller than the on time of the electron current, a part of the ions can leave the ionization chamber already during the on time of the electron current. Since the starting potentials of the ions which start during the on and off times are different, there is an undesired peak separation. The pulsing frequency must accordingly be selected high enough so that the on time of the electron current is always smaller than the ion residence time given by the operating parameters. If this cannot be achieved, then a changeover must be made to continuous electron current.

be controlled.

The magnitude of the blocking voltage U, is adaptable to particular conditions. The blocking voltage should not be made any larger than; is sufficient for counteracting the potential trough in the ionization chamber, because otherwise a large part of the ions are drawn to the cathode chamber, which would result in a reduction of sensitivity. This action can moreover be reduced if an additional diaphragm 26 lying at a positive potential is arranged between the diaphragm 24 and the box 14.

No requirements are placed on the frequency and amplitude constancy of the blocking voltage, since its values are uncritical. The operation with increased suction voltage is only an emergency aid compared with pulse operation, since with increasing pressure and increasing electron currents the residence times without pulsed operation again become unbearably large. With the pulsed operation it is possible with large electron currents and small ion suction voltages, to overcome the space charge influences on the fraction spectrum completely, even with lowest pressures.

In the mass spectrometer of FIG. 2 the following potentials may be applied:

suction electrode 5: 5 to l5 volts against 14 cathode 12: -70 to 100 volts against 14 box 14: some 100 volts to some 1000 volts against ground electron collector l7: 0 to volts against 14 diaphragm 24: 0 volt against 12 during on time of switch 23 diaphragm 24: 30 to 50 volts against 12 during off time of switch 23 The operation with pulsed electron current in an ion source is known, but has hitherto been used for another purpose and with another result. Thus, in the so-called Fox-ion source (R. E. Fox et al in'Review Scientific Instruments 26 (1955), 1101-1107) an alternating pulsing of the electron current and pusher voltage is used in order to prevent the energy of the ionizing electrons from being influenced by the pusher voltage. The aim is thus the production of an extremely monoenergetic beam of the ionizing electrons. The purpose of the pulsing electron source in the present invention is to decrease the ion residence time, which time is not inherent in the Fox device by virtue of the absence of a negative space charge problem in the ionization region. Stated in another way, the Fox source is designed to obtain ionization probability curves. To obtain exact values of the electron acceleration voltage, it is important to avoid producing a beam of multi-energetic electrons. As is pointed out in the Fox reference, the probability curves are measured with essentially monodence time, in the present ionization chamber. The appearance potential defining the limit of ionization probability is a function of the electron emission from the filament. Thus at increased electron currents, even those in the microamp region but generally above 4 microamps, the space charge at the retarding slit changes the energy distribution across the slit space in such a manner that those electrons passing through the center of the slit will receive a different energy than those passing through the edge of the slit, so that the energy of the electrons passing the slit is no longer mono-energetic.

It should also be pointed out that the space charge that could occur is extremely small and does not produce any trough or gradient reversion as shown in FIG. 1B because the Fox source generally operates at an electron current level a thousandfold less than the elec tron current level in the present ion source which preferably is in the region of 1,3 milliamperes.

On the other hand in the present invention an extremely mono-energetic electron current is notnecessary and therefore the space charges at the entrance of the electron current into the ionization chamber,

, which are troublesome with respect to the purpose of the Fox ion source, are not so with the present invention. While the small space charges within the ionization zone in the Fox device cannot cause an ion resiinvention, which uses a thousandfold higher electron current, the ion residence time was a problem.

In the Bendix aviation mass spectrometer there is likewise an alternating keying of the electron current and ion suction impulse with a delay time therebetween during which the spatial distribution of ions with different commencement energy is so adjusted that with suitable source parameters the influences of ion energy and ion starting point on the passage time are compensated. Accordingly, an improvement of the resolution is obtained by compensation of the influences of the commencement energy and the commencement region of the ions.

In contrast to this, in the present invention a pulsing of the electron current has been used to overcome the pressure-dependent alterations of the mass spectrum, which occur in the space charge condition as a result of the ion residence time. Thus, the method of the present invention is particularly applicable to partial pressure analyzers, for example, wherein a high detection sensitivity is achieved operating with a large electron current.

energetic electrons. This is of course possible only at lower energy levels and moreover with only a very' What is claimed is:

l. A method for analysis with high detection sensitivity of a sample substance by producing a mass spec trum by mass separation in a magnetic sector field of a mass spectrometer, comprising the steps of;

1. bombarding the sample substance in a high vacuum chamber with a high density electrom beam 'of a current greater than 0.5 miliamperes to produce ionized mass particles, said electron beam characterized by a negative space charge potential 7 having an ion intercepting potential trough 2. intermittently interrupting the electron beam to reduce ion residence time resulting from the ion intercepting potential trough,

3. providing a suction voltage at least during the interuption of the electron beam to draw ionized mass particles out of the ionization region of the chamher,

4. and separating said ionized mass particles in the magnetic sector field.

Claims

1. A method for analysis with high detection sensitivity of a sample substance by producing a mass spectrum by mass separation in a magnetic sector field of a mass spectrometer, comprising the steps of; 1. bombarding the sample substance in a high vacuum chamber with a high density electrom beam of a current greater than 0.5 miliamperes to produce ionized mass particles, said electron beam characterized by a negative space charge potential having an ion intercepting potential trough 2. intermittently interrupting the electron beam to reduce ion residence time resulting from the ion intercepting potential trough, 3. providing a suction voltage at least during the interuption of the electron beam to draw ionized mass particles out of the ionization region of the chamber, 4. and separating said ionized mass particles in the magnetic sector field.

2. intermittently interrupting the electron beam to reduce ion residence time resulting from the ion intercepting potential trough,

3. providing a suction voltage at least during the interuption of the electron beam to draw ionized mass particles out of the ionization region of the chamber,

4. and separating said ionized mass particles in the magnetic sector field.