US3522430A

US3522430A - Method and apparatus for maintaining constant the effective ionization energy in a mass spectrometer

Info

Publication number: US3522430A
Application number: US670475A
Authority: US
Inventors: Ludolf Jenckel
Original assignee: Varian Mat GmbH
Current assignee: Varian Mat GmbH
Priority date: 1967-09-25
Filing date: 1967-09-25
Publication date: 1970-08-04
Anticipated expiration: 1987-08-04

Description

Aug. 4, 1970 3,522,430 EFFECTIVE LQJENCKEL METHOD AND APPARATUS FOR MAINTAINING CONSTANT THE IONIZ ATION ENERGY IN A MASS SPECTROMETER Filed Sept. 25, 1967 zy/ EECOEDl/VG DE 1 C E 5 CHART Fi g2 PE06E4M GENEEATOB United States Patent Ofice 3,522,430 Patented Aug. 4, 1970 3,522,430 METHOD AND APPARATUS FOR MAINTAIN- ING CONSTANT THE EFFECTIVE IONIZA- TION ENERGY IN A MASS SPECTROMETIBR Ludolf Jenckel, Bremen-St. Magnus, Germany, assignor to Varian Mat G.m.b.H., Bremen, Germany, a limited company Filed Sept. 25, 1967, Ser. No. 670,475

Int. Cl. H01j 39/34 US. Cl. 250-413 15 Claims ABSTRACT OF THE DISCLOSURE The present invention overcomes the problem of measuring very small concentrations of substances, such as CO or N with a mass spectrometer in the presence of time fluctuations of the contact potential causing relatively strong fluctuations of the ion current. One of the mixture components, for example CO serves as a standardization component causing measurement peaks which, with two different electron acceleration voltages in an electron beam mass spectrometer, are not appreciably masked by measurement peaks of other substance components. A control signal is provided of magnitude which corresponds to the ratio of the ion currents for the two different electron acceleration voltages and is utilized for controlling at least the smaller of the electron acceleration voltages to main the ratio of the ion currents substantially constant.

The invention relates to a method and arrangement for maintaining constant the effective ionisation energy in the mass spectrometric analysis of mixtures of gaseous substances, with ionisation of the substance mixture by electron bombardment, and measurement of the ion currents of components of the mixture with two differently large electron acceleration voltages, of which at least one lies in the steep commencement region of the ionisation function of the mixture components. The accuracy of the analysis depends essentially on how constant the ionisation conditions remain during the measuring time.

The ionisation probability for a particular kind of molecule is dependent only on the energy of the ionising electrons, at constant tempreature. In order to keep this energy constant, it is not sufficient in all cases to stabilise the externally applied electron acceleration voltage, since the electron energy is not exclusively determined by the ap plied electron acceleration voltage, but additionally is dependent on potentials which cannot readily be dealt with. There are those of the penetration factor of the ion acceleration field in the ionisation zone, and the contact potential which is effective when electrons emerge from the cathode. The latter is at the basis of uncontrollable time fluctuations which are dependent on the surface characteristic of the cathode, and which cause more or less large ion current fluctuations, depending on which point of the ionisation function one uses. The typical course of an ionisation function is to be. seen from FIG. 2 of the accompanying drawings.

Above a certain minimum value of the electron energy, ions occur the number of which increases strongly with increasing electron energy, reaches a maximum value and then decreases again. In order to keep the ion current constant and as far as possible independent of fluctuations of the electron energy, one usually works in the region of the maximum of the ionization function, by using an electron acceleration voltage of for example about 100 volt. The possible time alternation of the contact potential of 1 volt maximum then has only a negligible influence on the ion current. There are however special analysis problems in which this manner of excluding the influence of the contact potential on the ion current cannot be used, since it is required to make use of precisely the characteristic curve of the ionisation function in the steep commencement region for an analytical investigation. An analysis example of this kind is dealt with by J. Cornides in the Zietschrift fiir Analytische Chemie, vol. 171, pages 431-433. It relates to the technically important problem of the analysis of Cor-N mixtures, which cannot be simply solved mass spectrometrically in the usual manner, since the two molecular masses are distinguished by only 1/2500 mass units and therefore cannot be separated with simple devices, and because the occurring fragment ion currents are relatively small. Cornides has proposed to solve the problem by measuring the unresolved molecular ion current consisting of CO ions and N ions at two different electron aceleration voltages U and U for which the ratio of the ionisation probability for both molecular kinds is very different. In the extreme case, the lower electron acceleration voltage U can be so selected that the electron energy has a value which lies above the ionisation threshold of the one, but below the ionisation threshold of the other kind of molecule. In the practical realisation of this extreme case, allowance in any event has to be made for the fact that the absolute ion current yield in this case is also extremely small. With this method, at least one of the electron acceleration voltages has to be so selected that at least one of the molecular kinds is ionised in the steep commencement region of its ionisation function. In this region time fluctuations of the contact potential cause relatively strong fluctuations of the ion current, which reduce the achievable measuring accuracy, so that very small CO or N concentrations cannot be determined in this manner. The exclusion of these time fluctuations, which are of the order of magnitude of seconds, is the problem which underlies the present invention.

In accordance with the invention, for one of the mixture components (for example CO serving as a standardisation component, the measurement peaks of which, with the two different electron acceleration voltages are not or are not essentially masked by peaks of other substance components, a control magnitude is formed which corresponds to the ratio of the ion currents of the two electron acceleration voltages, and is applied for controlling at least the smaller of the electron acceleration voltages, in such a manner that the ratio of the ion currents remains constant. By this method, not only is the measurement of the ion current of the standardising component possible with the desired accuracy, but also the ion currents of all other components of the mixture can be measured in the steep commencement region of the ionisation function, i.e. with low ionisation voltage. More particularly, also the procedure proposed by Cornides for separation of substances of similar molecular 'Weight, for example N and CO can be successfully performed.

The method of the invention is not only of advantage where it is a question of separation of substance components with similar molecular weight, but has general significancec for the mass spectrometric analysis of substance mixtures with low ionisation energy (low voltage mass spectrometry), in which the occurrence of fragmentary ions is known, is reduced.

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:

FIG. 1 diagrammatically illustrates an arrangement for performing the method of the invention;

FIG. 2 illustrates the varition of an ionisation function;

FIG. 3 is a graph diagram of the commencement re gion of the ionisation functions for components of a substance mixture to be analysed in the arrangement according to FIG. 1; and

FIG. 4 is a table relating to FIGS. 1 and 2.

The method for maintaining constant the effective ionisation conditions according to the invention is illustrated in the embodiment shown, for the analysis of a substance mixture of CO, N and CO with the aid of a mass spectrometer :which operates with a magnetic sector field for separating the ions, and ionisation of the substance mixture by electron bombardment. In order that no dead time has to be allowed for and in order to be able to form the control circuit as simply as possible, fixed mass setting or constant magnetic field is used, with a plurality of ion interceptors for the ion currents of the various substance components to be measured. The method can of course also be performed with mass spectrometers of a dilferent kind, with a different manner of ion separation.

Only those parts of the sector field mass spectrometer which are essential for understanding the invention are shown in FIG. 1, namely the high vacuum vessel 1, the ion source 2 with a box 4 surrounding the ionisation space 3, the sample inlet 5 to the ionisation chamber, the electrodes 6 and 7 of the electron acceleration device, the 180 deflection magnet 8 and the

ion interceptors

9, 10 and 11.

A heated fialment electrode 6 forming one of the electrodes of the electron path is connected in the usual Way with an emission current control 12, to which also the anode 7 is connected. The box 4 of the ion source 2 is at an ion acceleration potential which is supplied from a high voltage source 13. For producing the desired electron energy, an electron acceleration voltage U is placed between the box 4 and the electrode 6 of the ion source. This voltage is supplied by a direct current source 14 with two fixed voltages U and U and a controllable voltage U3, which is connected with two outputs and a switch 15 for changing over to the two different electron acceleration voltages U =U +U or U :U +U U being smaller than U whereby U =U U The changeover is effected by a relay 6 which is controlled by a programme generator 17 in accordance with a programme described further below. The electron acceleration voltage U is controlled by the measured ion currents in a manner to be further described below, in such a way that the eifective ionisation energy in the ion source 2 is automatically maintained constant at a predetermined value, as long as the voltage U is switched on through the switch 15.

The components of the mixture to be analyzed are ionised in known manner in the ionisation source and are drawn out of the ion source through an ion-optical system not illustrated in the drawing, in the form of a sharply bundled beam into the field of the sector magnet 8. There, according to the magnitude of their mass numbers, they are more or less strongly deflected, as illustrated by broken lines in the drawing, and are separately intercepted by the

interceptors

9, 10 and 11 provided at the output, these being arranged in accordance with the track paths. These track paths depend on the mass number of the molecules, the field strength of the deflecting magnet and on the ion energy, which is adjusted by alteration of the ion acceleration voltage at the voltage source 13.

Whereas the CO+ and 0+ ions separately pass to the interceptors 11 and 10, the 00+ and N+ ions, due to their equally large masses to a first approximation, experience an equally large defiection and pass commonly to the interceptor 9. The relevant ion currents are indicated by 1' and 1' in accordance with the magnitude of the ion masses. These currents are amplified in amplifiers 18, 19 and 20 and after a control procedure which will be subsequently described is performed, they are recorded in chart recording devices 21, 22 and 23.

In order to measure the concentration of the CO and N 2 components of the gas mixture, the ion measurement is performed successively with two differently large ionisation energies, of which one lies in the steep commencement region of the ionisation function of at least one of the two components and of which the other lies in the maximum region of the ionisation functions. In FIG. 2, an ionisation function, namely the magnitude of the ionisation probability in dependence on the electron energy, is illustrated. The path of such functions is known from measurements.

The two electron acceleration voltages U and U are so selected that the components of the ion currents i and if for the two ionisation energies are very different relative to each other, for the ion current i measured for the mass number 28. If by C and C the molar percent proportions of the components N and CO in the gas mixture to be analysed are designated, and by and 6" respectively, the specific indication sensitivities for N with U and U are designated, with 6' and 6' the specific indication sensitivities for CO with U and U respectively are designated, and with I 6 c0 -co and II 6 co co respectively the specific indication sensitivities are desi nated for CO from CO with U and U respectively, then the ion currents measured at the ion interceptor 9 can be described by the following equations:

as far as possible deviate strongly from each other, which is achieved by suitable selection of the electron acceleration voltage U and U In the illustrated example (FIGS. 3 and 4) U =15.5 volt and U =10O volt. The molar proportions C C and C can then be very easily calculated if the specific indication sensitivities e are determined by previous standardisation measurements. Preferably, the electron acceleration voltages are so selected that the lower ionisation energy, which is allocated to the ionisation voltage U is smaller than the minimum ionisation energy of the nitrogen molecule and of the proportion of CO at mass 28, but greater however than the minimum ionisation energy of the carbon monoxide molecule, so that the current proportion C 'e' and the current proportion C 6 'co co in the measured ion current (i U is equal to zero and this is exclusively based on the component CO. This preferred case is considered as a given case in our example. It is however only practicable if the mass spectrometer used is sufficiently sensitive to supply a sufficiently large ion current only for CO, with the required measurement accuracy.

In order to enable accurate measurement, in each case it is necessary to maintain the ionisation energy accurately constant in the steep commencement region, i.e. with the small electron acceleration voltage U '=U |U This is the case if the ratio of the ion currents measured with the large electron acceleration voltage U U -l-U and with the small electron acceleration voltage U =U +U is to be maintained constant. In order to achieve this, the mixture component CO is used as standardising component. The concentration of this standardising component does not have to be known, but the standardising component must satisfy the following requirements:

(at) Its molecular peak or another characteristic peak of its mass spectrum must supply an accurately measurmeasured with the low electron acceleration voltage U must be essentially smaller than the ion current measured with the large voltage U and the ion current measured with the large voltage must lie in the maximum region of the ionisation function of the standardising component, and

(c) The observed peak with the two voltages must be covered as far as possible exclusively by the standardisation component and not by other components of the mixture.

The control of the electron acceleration voltage U must take place in such a manner that for the standardisation component CO the ratio of the ion currents measured for the mass number 44 remains constant. In order to achieve this, use is made at the output of the amplifier of a precision potentiometer 24 which is so adjustedthat the two output voltages U taken from it with use of the electron acceleration voltage U and U with use of the electron acceleration voltage U are equal to each other.

The voltages U and U are fed through a changeover relay in the sequence of the changeover of the relay 16, to a servo-amplifier 24 the output of which controls a servo-control 27 for the variable voltage constituent U The programme generator 17 operates in such a way that in a first period a control of the voltage U, is effected, namely in such a manner that the output voltage U with U =U +U is equal to the output voltage U with U =U +U During this compensation period, the

relays

16 and 25 are changed over in synchronism, with a frequency of 1 to 10 cycles per second. The changeover frequency must be so high that the fluctuations of the ionisation conditions during the duration of the con- 'trol procedure, including the measurement connected therewith, are negligibly small. If the measurement is not effected as in the exemplary embodiment with simultaneous interception of the substance component ion currents by separate interceptors, but with transition through the masses, for example by alteration of the magnet current of a deflecting magnet, then transition through the masses has to be eflected so rapidly that also the fluctuations of the ionsiation conditions are sufliciently small during the transition time.

After this compensation period, the recording of the ion currents i 1' and 11 is efiected, firstly with the electron acceleration voltage U U +U and then with the electron acceleration voltage U =U +U The recording of the ion currents with the two different electron energies is performed when necessary, with changeover of sensitivity at the potentiometers 28 29 and 30, which are changde over synchronously with the

relays

16 and 25 by a relay 31.

The table in FIG. 4 shows the magnitude of the concentrations of the various mixture components in dependence on the measured ion currents. The constants evaluation of this analysis is accordingly simple. If the i011 Currents 10100, azhoo, zshoo, 9 229100, 2s)15.5 are recorded, then the desired concentrations are found By introducing the expressions for C and C into the expression for C after rearrangement one rece1ves:

In the described manner, the eifective ionisation energy can thus be maintained constant also in the steeply rising commencement region of the ionisation functions and analyses may be performed selectively 'with normal and with low ionisation voltages.

Many modifications are possible. As standardising substance, a component present in the mixture may be used, which satisfies the above given requirements. If such a component is not present, then a suitable standardising substance can be added. In the analysis of converter waste gases and blastfurnace gases, the problem exists of continuously analysing a gas mixture which is composed of the components N CO CO and 0 the component CO being present in sufficient concentration so that for this mixture the preliminary requirements that are placed on the standardising substance are satisfied. With compensation of the electron acceleration voltage, the procedure can also be such that the above described relative value for which a suitable value can be derived from the ionisation function of the st-andardising component used for the control, gives the fixed ratio of the measuring times the two electron acceleration voltages. The entire changeover period is in this case not divided into two equal parts, but is divided into the unequal measurement intervals t and t During the measurement interval t the electron acceleration voltage U U +U is switched in and during the measurement interval t the electron acceleration voltage U =U +U is switched in. Similarly as above described, 'with this method, U is controlled so that the commencement voltages U and U at the ion current amplifier for the standardising component, multiplied by the relevant measurement time interval t and t gives the same product U -z =U -t This kind of control is more expensive to provide but it has however, the principal advantage that the measurement time for the low ion currents is essentially larger with the smaller ion acceleration voltage, and in the border case is twice as large as with the control method with equal measurement time intervals described above in detail. This extension of the measurement time for the small ion currents naturally favours the measurement accuracy.

In the embodiments described, the larger electron acceleration voltage U is subjected to the same increase or reduction as the smaller voltage U It would lead to essentially the same result if only the smaller voltage U were controlled, provided that the larger voltage U lies in the maximum region of the ionisation functions, Where the related alterations of the electron acceleration voltages would not cause any noticeable alteration of the ion currents.

What we claim is:

1. A method for maintaining constant the eifective ionisation energy in the mass spectrometric analysis of mixtures of substances, with ionisation of the substance mixture in the gaseous condition by electron bombardment, and measurement of the ion currents of mixture components with two differently large electron acceleration voltages, of which at least one lies in the steep commencement region of the ionisation functions of the mixture components, wherein for a component serving as standardising component, the measurement peaks of which with the two electron acceleration voltages are not or are not essentially masked by peaks of other components of the mixture, which method includes the steps of selectively establishing said two differently large electron acceleration voltages, providing a control signal magnitude representative of the ratio of said ion currents with both said electron acceleration voltages and controlling at least the smaller of said electron acceleration voltages with said control signal so as to maintain said ratio of the ion currents substantially constant.

2. A method as claimed in claim 1, wherein alteration between both electron acceleration voltages is produced by switching over between two fixedly provided partial voltages which differ by a fixed difference voltage and controlling at least said smaller voltage so that the ratio of the two allocated ion current values for the standardising component has the said prescribed substantially constant magnitude.

3. A method as claimed in claim 1, wherein the predetermined magnitude of the ion current relationship is established by adjusting a voltage divider at the output of the mass spectrometer so that the ratio of the partial voltage taken off at the voltage divider to the total voltage thereon corresponds to the predetermined ratio of the lower ion current with smailler electron energy to the larger ion current with greater electron energy, and further including the step of controlling at least the absolute value of the smaller electron acceleration voltage so that the said total voltage measured with the lower electron energy is equal to the said partial voltage taken off at the voltage divider, for the larger electron energy.

4. A method as claimed in claim 1, wherein the predetermined magnitude of the ion current ratio is set by adjusting a time generator for two unequally long measurement intervals for the two electron acceleration voltages, respectively, so that the ratio of the lengths of the smaller interval, during which the larger ion current 1s measured with the larger electron energy, relative to the length of the larger interval, during which the smaller ion current is measured with the smaller electron energy, corresponds to the predetermined ratio of the smaller ion current to the larger ion current, and controlling at least the absolute value of the smaller electron acceleration voltage so that the product of smaller ion current with larger measurement interval length is substantially equal to the product of larger ion current with smaller measurement interval length.

5. A method as claimed in claim 1 which method includes mass spectrometically analyzing substance mixtures with low ionisation energy, in the steep commencement region of the ionisation function, and suppressing fragmentary ions.

6. A method as claimed in any one of claims 1 to 4 which method includes mass spectrometically analyzing substance mixtures with components the mass spectra of which contain the strongest ion contributions at equal 8, mass numbers, to a first approximation, the ionisation energies of which are however noticeably different.

7. A method as claimed in claim 6, wherein the smaller ionisation energy lies in the steep commencement region of the ionisation functions of the standardising component, and also one of the mixture components to be measured, but below the minimum ionisation energy of the other mixture component or components to be measured.

8. Apparatus for practicing the method claimed in claim 1, comprising a mass spectrometer with an electron impact ion source, an electron acceleration voltage source which is switchable to said two differently large acceleration voltages and having a fine adjustment device for at least the smaller of said acceleration voltages.

9. An arrangement as claimed in claim 8, wherein said fine adjustment device comprises a precision potentiometer connected to an interceptor system of the mass spectrometer.

10. An arrangement as claimed in claim 9, wherein said precision potentiometer is coupled through a changeover switch to means comprising a servo-device, for automatic fine adjustment of the acceleration voltage source.

11. An arrangement as claimed in claim 8 and further comprising, program controller means for in each individual mass spectrometric measurement performing in the comparison interval the adjustment of the acceleration voltages to the prescribed ion current ratio for the standardising component and in a following measurement interval initiates the indication or the recording of the mass spectrum.

12. An arrangement as claimed in claim 11, and further comprising a changeover switch for controlling the sensitivity of the mass recording, which operates in sequence with the changeover of the acceleration voltage.

13. An arrangement as claimed in claim 8, wherein the precision potentiometer at which the ion current ratio of the standardising component determined by the acceleration voltages is adjustable, is connected to a changeover switch coupled with the acceleration voltage changeover switch, which for the smaller acceleratiton voltage switches on the entire voltage at the potentiometer and for the larger acceleration voltage switches on the voltage lying at the slider of the potentiometer.

14. An arrangement as claimed in claim 9, wherein said precision potentiometer is connected with an interceptor provided solely -for the standardising component.

15. An arrangement as claimed in claim 14, and further comprising means for switching the mass spectrom eter during the compensation interval for recording of the ions of the standardising component, onto the interceptor connected to said precision potentiometer.

References Cited UNITED STATES PATENTS 4/1966 Herzog et al.

OTHER REFERENCES Zeitschrift fur Anallytische Chemie, vol. 171, pp. 431- 433, 1959, Cornicles.