US2541656A - Method and apparatus for analyzing substance by mass spectrometry - Google Patents

Description

Feb. 13, 1951 R. w. LONG METHOD AND APPARATUS FOR ANALYZING SUBSTANCE BY MASS SPECTROMETRY 2 Sheets-Sheet 1 Filed July 18, 1947 zucmooum 0? Feb. 13, 1951 R. w. LONG 2,541,656 METHOD AND APPARATUS FOR ANALYZING SUBSTANCE BY MASS SPECTROMETRY Filed July 18, 1947 2 Sheets-Sheet 2 'I F F on BEZI: I

T \J T 67 5 g COLLECTOR i ELECTRODE i 50 ELECTRODE l In 'PUSH- PULL A.G. AMPLIFIER qu ymvmvrox. I A @154 Patented Feb. 13, 1951 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR ANALYZING SUBSTANCE BY MASS SPECTROMETRY Robert Warren Long, Winter Park, Fla., assignor,

by mesne assignments, to Standard Oil Development Company, Elizabeth, N. J., a corporation of Delaware Application July 18, 1947, Serial No. 761,789

'7 Claims.

wherein the gaseous material is bombarded with low energy electrons such that a small portion of the molecules are converted into ions of different masses. Each ion usually possessesa unit of positive electricity. The ions are forced into an accelerating field where they acquire equal kinetic energies and then pass into an analyzing portion of the mass spectrometer which usually comprises a curved conduit within anexternal magnetic field.

The ions moving at a high velocity through the analyzer portion of the mass spectrometer constitute an electric current and a magnetic field is produced simultaneously by the moving ions. An external, crossed magnetic field acts upon the magnetic field caused by the moving ions and causes the ions to be deflected in-the curved conduit. The magnitude of the magnetic field and/or the magnitude of the accelerating voltage creating the accelerating field may be controlled such that ions of only one mass pass through an exit slit of the curved conduit of the analyzer and tothe ion collecting means. The ions'impinge on a Faraday cage'or other collecting means which is grounded through a resistance and the neutralization of the charge carried by the ions causes a current to flow through the resistance and set up a potential drop. The potential drop is conventionally amplified in a D. C. amplifier and measured by means of a sensitive galvanometer or recorder. The current set up by neutralization of the ions impinging on the collecting plate is proportional to the number of ions striking the plate per unit of time. By proper manipulation of the external magnetic field and/or the accelerating electric field strengths, ions representing components of different molecular weights or masses can be caused to pass to the collector plate in sequence in order to obtain a record or a mass spectrum representing the composition of the substance.

In the conventional type of mass spectrometer, positive ions produced at the ion source are usually measured. Because of a small potential gradient maintained in the ion source, positive ions are caused to passinto the ion accelerating field and are eventually passed through the analyzer portion of the mass spectrometer, whereas negative ions formed at the ion source are repelled and are thus prevented from passing to the accelerating field. Thus, the ions passing through the analyzer form a non-pulsating or steady positive ion beam which eventually impingesron the collector plate as mentioned above and are neutralized by a fiow of electrons from ground. The electron flow is directly equivalent to the positive ion charge flow to the collector plate, and the I (current) R (resistance) =E (voltage) difference across the ends of the grounded resistor is directly related to the current through the resistor. As previously mentioned, this current is related to the number of ions collected per unit time. Hence the variations of the number of ions collected per unit time are converted into signals represented by voltage changes which are impressed on an electronic tube designed to amplify the magnitude of the original current to a magnitude amenable to standard measuring devices or recorders.

It is thus seen that direct currents set up by the neutralization ofthe direct ositive ion beam on the collector plate are amplified and measured in the conventional equipment. Direct current amplification of the voltage change, however, hascertain drawbacks. It is known, for example, that A. C. amplifiers are inherently more stable than D. C. amplifiers. Direct current amplifiers also are subject to what is known as zero drift resulting from the fact that aging of filament, minute changes of potentials within the electronic tubes, and similar disturbances upset the balance of conditions previously established causing a new set of operating parameters to exist. These new parameters obviously produce a shift of the apparent balance which is interpreted as a zero drift. A. C. amplifiers respond only to signals of periodic character and have no output for no input signal; hence, in the quiescent period, no output is obtained which means that no shift of balance can appear. Direct current amplifiers are also known to have less usable sensitivity and to respond more slowly to voltage fluctuations than do A. C. amplifiers. It would, therefore, be extremely desirable to have available a means for using A. C. amplification of the ion beam currents in order to eliminate the aforementioned difficulties. The present invention is directed to a novel means of forming a pulsating ion beam 3 signal amenable to amplification in an A. C. amplifier.

It is the principal object of my invention to provide a means for improving the operation of a mass spectrometer.

It is another object of the present invention to provide a magnetic means'for modulating a nonpulsating ion beam such that a pulsating ion beam signal is formed.

It is a further object of the present invention to improve the stability and sensitivity/of amass spectrometer with respect to the amplification and recording of signals produced subsequent to the collection of ion beams of "various'masses.'

Briefly, the present invention involves born-- barding a gasiform material with an electron beam such that ions of various masses are formed, passing the ions through an analyzer portion of a mass spectrometer to segregate an posed on the steady ion beam by the'magnetic The present inventi'outakes' advantage of the fact that the'steady ion beam comprising ions of any one mass leaving the analyzer portion of'the mass spectrometer"constitutes an electric current which produces a magnetic field; By subjecting" the magnetic'field produced by the movingions to an auxiliary magnetic field periodic in character and of one polarity or'to a magneticfield of constantly changing polarity, it is possible to defiect alternately the ion from the collecting portion of a mass spec trometer such that only a portion of the ions reach the collecting means and, consequently, only a portion of the ions are periodically neutralized by a flow of current from the grounded resistance attached to the collecting means; The periodic flow of neutralizing current results in the formation of a pulsating signal which is amenable to amplification in an AC. amplifier.

Likewise, by providing the collecting portion of a mass spectrometer with a plurality of collecting means electrically connected through sepa-' rate channels to a push-pull A.'C. amplifier, the ion beam may be deflected frombnc collecting meansto another ialternately by means of a magnetic field of constantly changingpolar ity which results in producing two pulsating signals which may be amplifiedfand subsequently combined for use as a means of indicating the relative abundance of ions'of any one mass' in the original ion beam. The practice of the present invention is usefuI'in that no matter what types of collecting means are employed, the principle is employed in which a magnetic field,

periodic in character, alters the course of the non-pulsating ion beam to produce a pulsating signal which is a function of the number of ions collected per unit time, The pulsating'signal is more easily controlled and amplified than nonpulsating signals produced in conventional mass spectrometers.

Several embodiments of the device of the present invention will now be described in detail in conjunction with the accompanying drawing in which Figure 1 is an elevation, partly in section, with conventional electric units comprising a side view of theassembly along with conventional symbols,

Figure 2 is a side view of the collecting portion of the mass spectrometer assembly shown in Figure 1,

Figure 3 shows an end view of the collecting portion of the mass's'pectrometer, shown in Figure 1,

Figure 4 shows a side view of another embodiment of the-collection portion of a mass spectrometer,

Figure 5 shows the geometrical relationship between 'the various elements of the collection portion of the mass spectrometer illustrated in Figure 2, and

Figure 6 shows the geometrical relationship of the various elements of the collection portion of the mass spectrometer illustrated in Figure 4.

Turning now to -Figure 1, the numeral H designates the ionization chamber of a mass spectrometer, numeral I 2 designates the analyzer portion of the mass spectrometer and numeral 13 designates the collection portion of the mass spectrometer tube. The particular tube shown is of the type irequently used in the analysis of gaseous mixtures; My invention is not limited, however, to any specific type'of-mass spectrometer tube but is applicable to analysis of gasiform material in the horizontal, 60,

and other types of mass pectrjometer-tubes.

The mass spectrometer tube may be constructed of glass and is connected to a sample system It containingdr'awout electrode l5. The tube also is connected to a vacuum pump system It which maintains the entire system-under an extremely low pressuresuch as from 10 to- 10- millimeters of mercury, absolute pressure, and withdraws from the tube unreacted portions of the gasiform material introduced therein. The ionization chamber 1] of the mass spectrometer tube also contains filament" ll, made of tungsten or other'such material, which is connected to an electronically regulated power supply, not

shown. Also contained in -the ionization chamber are electrodes l 8 and IQcQntaining'aIigned I slits 20 and 2i,respectively, and electroncollector electrode 22. Electrodes 23 and '2 placed directly under drawout electrode 15 contain aligned slits 25 and 26, respectively, these two electrodes conveniently being termed the ion source of the mass s ectrometer; H

The analyzerportion I2 of the mass spectrometer tube'containstwo electrodes 27 and 28 placed at opposite ends of the curved conduit and containing slits 29 and 30, respectively. The analyzer electrodes are connectedfthr-ough an internal non-magnetic shield 3|. The curved portion of the mass spectrometer is surrounded by an external crossed magnetic field-BZ, which is produced by a suitable electr o magnet 33, the

magnitude of which is -cont rolled by a power supply, not shown. The ion collection portion of the mass spectrometertube 13 comprises the decelerating field made upof elec'trode 34 containing slit 35 and shielding electrode 36 containing slit 3'! and'the collecting electrode '38. Electrode 3B is connected through lead to grounded resistance 40 and A. C. amplifier 4|. The A. C. amplifier is connected: through'suitable leads represented b numeral 42 to 'a rectifier system 43 which, in turn, is electrically con-' nected through lead 44 to a galvanometer or recorder system 45.

The numerals 4B and itv represent the poles of an external magnet for producing a transverse magnetic field for the collection portion of the mass spectrometer tube. This magnetic field, which will be referred to by the numeral 46, will be described in more detail hereinafter. In Figure 1, it is noted that ion source electrodes 23 and 24 are connected through leads Al, 48 and 39 to ground and that analyzer electrodes 21 and 28 are connected through leads 50 and 5| to a source of high potential at junction 52. Drawout electrode i5 is connected to a source of potential by means, not shown, the magnitude of which is slightly higher than the magnitude of the potential impressed on electrodes 23 and 24.

Figure 2 illustrates a side view of the collection portion of the mass spectrometer tube, showing electrodes 34 and 36 and collector electrode 38. The poles of the electromagnet represented by the numerals 46 and 46' are connected to a suitable electr-omagnet 53.

Figure 3 shows the end View of the collecting portion of the mass spectrometer, the numeral !3 designating the mass spectrometer tube itself, the numerals it and 46 representing the poles of the magnet and the numeral 53 showing the electromagnet supplying energy to the poles 4E and 4.6. Current for magnet 53 is supplied through lead 54 electrically connected to oscillator 55 and lead 55 containing resistor 51 which is electrically connected through lead 58 to oscillator 55. Oscillator 55 determines the frequency of the changes of polarity in electromagnet 53. A suitable Voltage regulator may also be incorporated in the system if desired to regulate the current for the electromagnet.

Figure 4 shows a side View of another embodiment of the collecting portion of the mass spectrometer tube, partl in section, which contains analyzer electrode 28, decelerating electrode 34, shielding electrode 36 and two collecting electrodes 59 and 69, respectively. Electrode 59 is electrically connected through lead 6! to grounded resistor 52 and to one channel of pushpull A. C. amplifier 63. Electrode 55 is likewise connected through lead (54 to grounded resistor 65 and to the other channel of push-pull A. C. amplifier 63.

Returning now to Figure 1, the operation of the mass spectrometer will be described in more detail. A small amount of gasiform substance to be analyzed is introduced from drawout electrode 55 into the dissociation region existing between electrodes l8 and l9. An electron beam E58 originating from heated filament il flows in a path perpendicular to the ilow of the gas from electrode l5, this electron beam being maintained in a horizontal fiat ribbon through the dissociation region by means of an externally applied magnetic field, not shown, such that electrons impinge on electron collector 22. A. portion of gaseous molecules entering the dissociation region are bombarded by the electrons and form ions of various masses. A small potential difference of proper polarity is maintained between drawout electrode i5 and electrode 23 such that ions (either positive or negative) are slowly moved t wards accelerating field entrance slit 25 in elec trode 23. The ions then pass, through focusing electrode slit 2E and are accelerated in the field existing between electrodes 2 and 274 due to the large potential difference existing between these two electrodes.

A portion or the ions then passes through i alyzer entrance slit ZQan-d into magnetic field 32. By controlling the magnitude of magnetic field 32 and/or the strength of the accelerating field existing between electrodes 2 and 23, ions of any one selected mass may be made to focus on analyzer exit slit 3d, the remainder of the ions impinge on shield 3i whereon they are neutralized and pumped out of the system by means of vacuum pump 26. The ions focused on exit slit 30 designated by the numeral 575' constitute ions of any one mass in a steady or direct beam. The ion beam 61 then enters the decelerating portion of the collector end of the mass spectrometer tube, the decelerating field existing between electrode 28 and grounded electrodes 3 and 35. Since electrodes 34 and 35 are maintained at substantially the same potential as electrodes 23 and 24, the ions are slowed down in velocity to the same extent that they were speeded up in accelerating field existing between electrodes 24 and 2?. The ion beam reaching electrode 38 is, therefore, slowed down to a few volts energy approximately being equival nt to the potential difierence between drawout electrode and electrodes 23 and 24'.

Ion beam 6'! is subjected to magnetic field of periodically changing polarity such that only a portion of the ions impinge on collector plate 38. The method of ieflecting the ion" beam ofi and on the plate 38 will be described in more detail hereinafter. The portion of ions impinging on collector plate 38 is neutralized by a fiow of current received from ground which flows through resistor ii? to the collector plate. The neutralization of the charge carried by the ions set up a potential drop which is applied to A. C. amplifier 4! containing amplifier tubes such as 68. The signal formed at collector plate 38 and resistor :38 is periodic in character, A. C. amplifier 3!, which may be tuned to the deflecting frequency imposed on ion bear-1 ii'E, will amplify the signal, rectifier 33 will change amplified A. C. signal into a D. C. signal which may be recorded by conventional methods in recorder d5.

Turning now to Figure 2, it is seen that ion beam 67 passing in the magnetic field 3? is alternately deflected from one side of collector plate 38 to the other. Magnetic field of constantly changing polarity has a strength just sufficient to cause ion beam ill to be deflected from either side of the collector plate, the portion of the ion beam sweeping across the collector plate being neutralized by a flow of current through the grounded resistance as hereinbefore described and the portion of the ions deflected off or" bh collector plate impinging on a suitable internal shield Within collecting means !3 and being neutralized and pumped out of the system.

Turning now to Figure 3, the method of controlling the polarity and strength or" magnetic field 46 is shown. Voltage is supplied to oscillator 55 through leads 59 and Ed, oscillator 55 controlling the frequency of the polarity change of the magnetic field. Oscillator is directly connected through lead 54 to eleetrornagnet 53 and indirectly through resistor 53 and lead 55. By changing resistor 5?, it is possible to change the strength of the magnetic field of poles and 46.

The operation of the collection portion of a mass spectrometer shown in Figure 4 is similar to that previously described. Collecting elec trodes 59 and 66 having equal areas are separated by a very small space and the strength or 7 magnetic field 46 is adjusted such that ionbeams 61 will be alternately deflected from one collector plate to the other. During any portion of the time that the ion beam is on one or the other collector plate, the ions are neutralized and. a potential drop is set up with the grounded resistance connected to each collector plate. During the portion of the time that the ion beam. is passing from one collector plate to the other, it impinges on an internal shield, not shown, which may suitably be placed behind the collector plates and the ions are neutralized and pumped out of the system. Therefore, two pulsating signals are set up in leads SI and 64, each being carried to its respective channel in a conventional push-pull A. C. amplifier E3. The-separate signals may subsequently be combined, rectified, and recorded.

Turning now to Figure 5, the mathematical derivation of the magnetic field strengths of alternating polarity required to deflect an ion beam from a single electrode will be discussed in detail. When the axis of the external coil mag netic system is perpendicular to the axis of the ion collector system, a magnetic field perpendicular to the path of ion beam 6'! may be determined by the following equation:

Equation 1 2 H ev where H=magnetic field strength in oersteds, e=charge in E. M. U. (electromagnetic units) on m mass having v=velocity and r=radius of circle of resulting path.

It is seen that for any one mass, e, v, and m are constant; therefore, the curvature of the path,

is proportional to H, the magnetic field strength. Equation 1 may be suitably written in terms of voltages by use of the following equation:

Equation 2 V010 mo where V=accelerating voltage, e=charge in E. M. U. on m=mass of particle, where o=velocity resulting therefrom.

By combining Equations 1 and 2, the following is obtained:

Equation 3 H er 1 -=2 1 l and when mn=mass hydrogen and M=mass number, Equation 3 becomes:

Equation 4 H r =2MV-10 It is seen that the magnitude of H may be suitably adjusted to control the extent to which ions of any one mass are deflected. Use may be madeof Equation 4 to determine the minimum H necessary to deflect ions having a mass 925 away from a collector electrode having a diameter of 0.6 centimeter. In Figure 5, it is seen that a magnetic field H has been imposed on ion beam 6': causing the ion beam to have a radius of cur vaturer and to be deflected the distance ac causng it to miss the collector electrode 38. D-repreand. since a: is approximately equal to which, when substituted into Equation 4, gives:

EqaationS zflvaor 1 e/m D (2 Whr H is the minimum magnetic field strength to cause the ion beam to be deflected off of the collector plate. When M :100 mass number, V=l9 volts, e/mn=0.96 10 E. M. U./gram, D=2 centimeters, and

%=O.l centimeter the minimum H as determined from Equation 5 is:

HEZIO oersteds If a smaller collector plate is used than is shown above, the H may be decreased. Specifically, for a collector plate having a radius of 0.7 mm., H oersteds. Obviously, for ions hav ing masses below the magnetic field'strength requirements will be lower than those shown above.

Turning now to Figure 6, a difierent situation obtained to that previously described in conjunction with Figure 5; that is, the ion beam is deflected from one of a plurality of collector plates to another. The distance d in this case is the space between electrodes 59 and ac and a: is the distance the magnetic field H deflects the ion beam from its usual path to cause impingement on one of the electrodes' It is readily seen that if d, one obtains the minimum distance the ion beam must travel to contact one of the electrodes. By employing the reasoning used in connection with Figure 5, and assuming that d=0.02 cm. with other values the same as previously used substituted in Equation 5, HE22 oersteds or less.

It is seen from the foregoing discussion that the magnetic fields required to produce deflection of ions having masses as high as 100 are easily produced with currents available from conventional electronic apparatus. Since the frequency of the deflecting magnetic field imposed on the ion beam controls the wave form of the resulting signal produced, it is possible to tune the particular type of A. C. amplifier used to the interruption frequency to amplify the pulsating signal. The method of the present invention is effective in that the magnitude of the collecting currents are very easily controlled. Suitable magnetic field strengths may be employed which will cause the ion beam to avoid the collector plate over any portion of the modulation cycle; that is the. pulse to signal ratio may be varied from pulse time cycle time The system illustrated in Figure 4 is more effective than the system illustrated in Figures 1 and 2 in that a greater portion of the energy of the ion beam in the system is utilized than in the latter system. This is because a smaller quantity of the ion beam is deflected from the collector plates when using two or more collector plates than is the case when using only one collector plate.

The magnetic field employed to deflect the ion beam from the collector plate, when employing only one collector plate, may be of any one polarity but made periodic in character by periodically interrupting the direct current to the magnet. In other words, only one polarity of the magnet is required in which the axis of the external coil system is placed perpendicular to the axis of the ion collector system such that the magnetic field is perpendicular to the path of the ions. By employing the magnetic field in this manner, the ion beam may be deflected from the center portion of the collector electrode to just beyond one end of the electrode which sets up pulsating signals amenable to amplification in an A. C. amplifier.

When employing two or more collector elec trodes, it is preferred to change the polarity of the magnetic field while making the change periodic in character such that the ion beam may be deflected in two different directions as previously described. Regardless of whether the periodically varying magnetic field is of constantly changing polarity or of one polarity and periodic in character, the pole pieces or coils producing the field should be of such size as to cause deilection of the ion beam without either affecting the focusing of the ion beam or changing the character of the signal resulting from the collected ion beam. The rate at which the magnetic field deflecting the ion beam is varied will depend to a large extent on the type of A. C. amplifier employed. Signals pulsating at a rate of from about 60 to 380 cycles per second are generally preferred; however, the deflecting magnetic field may be varied to produce signals pulsating at lower or higher rates than the range heretofore mentioned under certain conditions.

Another method of carrying out the deflecting system comprises employing shielding electrode 36 as the means for cutting off the deflected ion beam. For example, in Figure 2 the slit .3! in shielding electrode 36 may be made of such size that, as the ion beam is deflected in one direction,

.the beam will contact a portion of the electrode and the ions will be neutralized. When the magnetic field changes in polarity, the ions are deflected across the collector electrode in another direction and are interrupted by another portion of the shielding electrode There are other methods of cutting off the deflected ion beam which may be preferred under some circumstances. but it is to be understood that any method of employing a magnetic field to obtain the desired deflection of the ion beam is within the scope of the present invention.

the d cription of the present invention, I ha e shown [.1149 method of carrying out the magnetic modulation of the ion beam in which the ion source comprises electrodes 23 and 2 and grounded and analyzer electrodes 2'! and 2%! are at high potential while electrodes in the collectof the magnetic field that would be required if the ion source were at high potential and the ion beam passing to the collecting plate had extremely high velocities and high energies.

It is also obvious to one skilled in the art that for ions of any given mass, a specific magnetic field strength will be required to deflect the ions from a collector plate of a given size. As ions of various masses are focus-ed on the collector sys tem, it may be necessary to change the strength of magnetic field 35 in order to obtain the desired deflection of the ion beam. In order to accomplish a change of the collector magnetic field strength, it may be desired to coordinate changes in resistance El shown in Figure 3 with any change imposed on the magnetic field 32 or the accelerating potential imposed at junction 52 shown in Figure 1 when various ion masses are being analyzed. It may not be necessary under some circumstances to change the strength of magnetic field it each time a different mass is focused on exit slit 33 particularly if the slit cpeningtl in shielding electrode 38 is of such size that the ion beam may be cut off by this electrode when it is deflected from one portion of collecting electrode 38 to another portion of this electrode. When analyzing for masses ranging from 2 to 25, approximately the same magnetic field 55 may be employed for any particular mass. In the range of masses from 25 to 50, the strength of the magnetic field 4% may be changed in order to obtain the desired deflection of the ion beam. Similarly, for masses ranging from 50 to another change may be made on the magnetic field 26 to obtain the desired deflection of ion beam.

It is to be understood that the present invention is applicable to the analysis of an ion beam comprising negative ions as well as positive ions. If it is desired to analyze for negative ions, proper changes must b made throughout the mass spec trometer." with respect to polarity in order that only negative ions formed by bombardment of gasiform material with electrons are accelerated through the analyzer portion of the mass spectrometer.

The'method and apparatus described in the present invention is applicable for testing a wide variety of solid, liquid or gaseous substances. The procedure is only limited in that the substance be vaporizable under the low pressure conditions existing in the mass spectrometer.

The nature and objects of the present invention having been fully described and illustrated, what I wish to claim as new and useful and to secure by Letters Patent is:

1. In a mass spectrometer including a means for focusing a non-pulsating beam of ions having a selected mass on a collecting means, means for producing a periodically varying transverse magnetic field following said focusing means and ahead of said collecting means whereby said beam is periodically deflected from said collecting means.

2. In a mass spectrometer including means for producing a non-pulsating beam of ions of a particular mass, means for collecting said beam and means for focusing said beam on said collecting means, the improvement which comprises means for producing a periodically varying transverse magnetic field arranged following said focusing means and ahead of said collecting means whereby said non-pulsating beam is periodically deflected from said collecting means.

3. A mass spectrometer including a mean for forming a beam of ions having a selected mass which com-prises, in combination, a plurality of ion collecting means, amplifying means electrically connected to each of said collecting means, means for producing a periodically varying transverse magnetic field arranged external to and in front of said collecting means whereby said beam -of .ions is periodically deflected from one to another of said plurality of collecting means.

4. A mass spectrometer apparatus including means for forming a homogeneous beam of ions corresponding to a selected mass, means for focusing said homogeneous beam upon ion collecting means, a collecting means comprising a pair of ion-collecting members arranged adjacent each other and spaced onopposite sides of the focus path of said beam, means for producing a periodically varying transverse magnetic field arranged external to and spaced in front of said collecting'means Wherebysaid focused beam is periodically deflected from oneto the other of said pair of collecting members, and a push-pull amplifying means having the input terminals thereof electricallyrconnected inshunt to said collecting members.

5. In a method for operating a mass spectrometer wherein a beam of ions corresponding to a selected mass is formed and is focused upon a collecting. member to produce an'electric signal, the improvement which comprises subjecting the focused beam of ions to a transverse field force of periodically changing polarity and of such strength that said focused beam is periodically deflected from said collecting member, and col- 12 lecting a portion of the resulting-deflected ion beam whereby a pulsating electric signal is pro duced.

6. In a method for operating a mass spectrom eter having a collecting means, including at least one ion collecting member, and having means for focusing a beam of ions corresponding to a selected mass upon-said collecting means to produce an electric signal, the improvement which comprises the steps of subjecting the focused ion beam to a transverse magnetic field of periodically changing polarity and of such strength that the ion beam is periodically deflected off of said collecting member, collecting a portion of the resulting deflected ion beam whereby a pulsating electric signal is produced, and amplifyin said pulsating electric signal.

7. In a method for operating a mass spectrometer having a collecting means, including a space pair of collecting members, and having means for focusing a beam of ions corresponding to a selected mass upon said collecting means to produce an electric signaLthe improvement which comprises the steps of subjecting the focused ion beam to a transverse magnetic field of periodically changing polarity and of such strength that the ion beam is periodically deflected from one collecting member, to the other of said pair, collecting portions of the resulting deflected ion beam whereby a pulsating electric signal is produced, and amplifying said pulsating electric signal.

ROBERT- WARREN LONG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,096,653 Soller Oct. 19, 1937 2,314,302 Ziebolz Mar. 16, 1943 2,331,189 Hipple Oct. 5, 1943 2,341,551 Hoover, Jr Feb. 15, 1944 2,457,162 Langmuir Dec. 28, 19-13

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