US2370631A - Mass spectrometry - Google Patents

Description

March 6, 1945. C. E, BERRY MASS SPECTROMETRY Filed Aug. 19. 1943 @frau/va ATTORNEYJ Patented Mar. 6, 1945 m8 SPEUI'IOMITIY Clifford tlderry, Pasadena,

denaCallf.,acorperatlenofCalUe|-nh Calihalilnorto Pala Application Allnet 19, 1m, Serial No. 498,287

(u 'II-lll ICIIIBI.

This invention is concerned with mass spettrometers and provides improved mass markers for such instruments.

A mass spectrometer is an apparatus i'or sorting ions according to the ratio of the mass of the ion to the charge that it carries (this ratio being referred to hereinafter as speciilc mass) and can be employed for various purposes, including qualitative and quantitative analyses of mixtures containing ditlerent kinds of molecules, determination of isotope ratios, etc.

A common form of mass spectrometer includes an ionization chamber in which molecules oi' the mixture to be analyzed are converted into ions,

an analyzer tube in which a heterogeneoi beam of the ions is separated by an electric or magnetic field into a plurality of diverging homogeneous beams of ions (the ions o the same specic mass being included in a single beam), an ion collector, means (referred'to here as a beam sweeper) for bringing the several beams successively into contact with the collector and means for indicating separately the currents collected from the several beams.

A record of the separately collected currents is a mass spectrogram, which may take any one of several forms. For example, it may be a graph on which the magnitudes of the currents corresponding to the several collected ion beams are represented by the heights of a series of peaks projecting from an abscissa axis. Such a record can be produced on a laterally moving sheet of photographic paper, or the like, by a recording galvanometer. For some purposes, such a mass spectrogram is entirely adequate, but in many cases, for example in the quantitative analysis of a mixture of hydrocarbons containing a number of different kinds of molecules some of which crack in the ionization chamber to fonn a plurality of dlierent kinds of ions, it is necessary to determine and identify the specific mass of the ions to which each peak corresponds.

In accordance with the invention described and claimed in the copending application orf Robert V. Langmuir Serial No. 494,807, nled July 15. 1943, the interpretation oi' a mass spectrogram is facilitated by indicating in a coordinated nunner and preferably simultaneously with the indication of the peaks (ion currents), the specic masses of the ions to which the respective peaks (currents) correspond. Thus Langmuir contemplated marking on the mass spectrogram adjacent the respective peaks and during the recording thereof. the specific masses of the ions to which the peaks correspond.

Inoneformcfthe Lnngmuirapparatuatbe spcciiic mass indicator is operatively connected wlththebeamsweeperandisactuatedbuit. In auch apparatus. the beam sweeper automatically and gradually alters the potential impressed upontheionapassing through the analyser and thuachangesthepathoi'theionlsothatthe beams are swept successively over the collector. Aspeciiicmassindicatorisconnectedina circuitwlththebeamsweeperandachangein the potential actuatea the indicator. For example, the mass indicator may be a galvanometer calibrated in terms of ion masse or a vacuum tube circuit which is tripped at intervals at successive values o( ion accelerating potential. means being provided to mark the spectrogram each time the circuit trips. In this way. the beam sweeper and the mass indicator are synchronised andthepeaksonthespectrogramarecorrelated withthespeciiic of theionstowhich they correspond.

Apparatus described in the aforementioned- Iangmuir application includes a beam sweeper which automatically decreases the ion accelerating potentials applied to electrodes in the mass spectrometer, and is connected to these electrodes through a potential divider. This in tum is connected in series with a rotary stepping relay, the contact arm of which is connected to the slider of a potentiometer in a direct current amplifier circuit. The output of this D. C. ampiiiler is employed to mark the mass spectrogram at points corresponding to the peaks thereon.

As the result of my investigations. I have developed a mass marker which correlates a spci cinc mass and the current oi' the beam to which the mass corresponds with great accuracy. In general, my invention contemplates in a mass indicator of a mass spectrometer having a resistance, means for impressing across the resistance a voltage which is a function or the specinc mass being detected, and means for selecting successive fractions of this voltage during a period in which the voltage is changing, the combination which comprises means connected to the voltage selector for producing an alternating current corresponding in amplitude to the selected fraction of the voltage. and means responsive to a deviation in the amplitude of the alternating current from a threshold value for indicating the specific mass of ions detected when the threshold is reached and lfor causing the voltage selector to select the next fraction of voltage. In its preferred form. my invention contemplates. in combination with the voltage selector, a balanced network. means for couvert-` lng the output of the balanced network yinto alternating current. a high gain A. C. ampliner. and means ior rectifying the ampliner output and applying this rectiiled output to a trip circuit which marks the spectrogram.

These and other features o! my invention will be more thoroughly understood in the light of the following detailed description, taken in conjunction with the accompanying drawing, which is a wiring diagram of a preferred form of my ma marker.

Referring now to the drawing. it will be observed that a mass spectrometer l is provided with an ionisation chamber l. an analyser tube l. an ion collector I, and an inlet conduit l for the material to be analysed in the spectrometer. A high voltage from a direct current source is applied to the mass spectrometer through a beam sweeper i. Thus the beam sweeper impresses a high but gradually decaying potential between electrodes in the mass spectrometer. Gas moleeules that are ionized in the ionisation chamber by bombardment with an electron beam 1 are propelled to a slit Si in an accelerating electrode l by a potential impressed between pusher electrodes P1, P: and the electrode l. A heterogene ous ion beam is thus formed and this is further accelerated into the analyser tube by a potential impressed between the accelerating electrode t and a second accelerating electrode l. which has a slit Sa that matches the slit B1 in the rst accelerating electrode. The potential applied. respectively, to the pusher electrodes Pi. Pa may be adjusted -by means oi' a potential divider I I which comprises two potentiometers in parallel and connected. respectively. to the two pusher electrodes. Ihe potenliometers in parallel are followed by a large resistance IIA in series. 'I'he sise oi' this resistance. which is connected to ground through a tapped resistor i 1A of a rotary stepping relay l2, controls the proportion of the potential applied across the gap P1. Pa-Si and the gap Si-S1. An ammeter I3 is connected in series with the resistor IIA oi' the rotary stepping relay and the potential divider and may be calibrated in terms of speclilc mass.

The maior portions oi the apparatus, in addition to those already described. comprise a balanced network. a high gain A. C. amplier, an oscillator coupled to a vibrator, a trip circuit. and appropriate power supplies.

The rotary stepping relay conveniently is the type designated by Guardian Electric Manufacturing Company as "Series R steoplne relay. It comprises a rotary contact arm Il which upon rotation may touch any one oi a series of equally spaced circumferentially positioned ilxed taps T1. Ts. Ts Tas on the resistor. A tap indicating disc or a cylinder 8l rigidly secured to the same shaft as the arm has numbers thereon which are moved beneath the pointer and become illuminated by a lamp 81 when the arm contacts the corresponding tap. Thus, as shown. the numeral I is illuminated on the tap indicator 38 when the rotary arm touches tap Ts. The indicatorllmayalsocarryspecicmassnumbers in addition to tap numbers.

'Iherotarysteppingrclayalsoincludesasteppingil llandaresetcoilll. Whenanelectrical impulse is applied to the stepping coil. the rotary contact arm moves to the next tap in sequence. Whencurrentisappliedtothereset wiLtberotaryarmreturnstothenrsttappositlm Tx.

The contact arm ofthe rotary stepping 'relay isconnectedtoabalancingnetwork ofcouventional design. This balancing network balancesthepotentlalbetweentherotaryarmand ground against a standard potential, i. e.. the standard potential and that between ground andtherotaryarmareopposedtoproducea differential potential. The balancing network is connected to a vibrator li which is coupled to an oscillator l2 and these convert the difference between the standard potential and that impressed from the rotary arm to ground into an alternating current. This alternating current is ampliiled by a high gain A. C. amplifier Il. As described hereinafter, the output oi' the ampliner is rectified and the voltage proportional to the amplitude of the alternating current is applied to a trip circuit Il, which controls the operation of a control relay It which in turn actuates a mass marker lamp Il that marks a mass spectrogram poduced on a photographic i'ilm 41 of a recorder I Currents generated through discharge of ion beams at the ion collector are amplliled and recorded as a spectrogram on the moving photographic film l1 by means of a galvanometer il. The mass marker is synchronised with the galvanometer by the mass marker control Just described.

The control relay, in addition to marking the spectrogram. steps up the rotary arm Il to the next tap. say T4 of the rotary stepping relay.

'lhe oscillator and vibrator are coupled together because a vibrator coil il is connected in the output oi the oscillator. A condenser Bl connected in parallel with this coil forms with the coil a parallel resonant network which may be electrically tuned to the same frequency as the resonant frequency of the mechanical elements of the vibrator. These elements include one normally closed pair l1 of contacts and another set of contacts comprising an armature contact 0I, a signal contact Il and a ground contact Il. The amature normally is in the neutral position and contacts the signal contact and the ground contact alternately when alternating current is applied to the vibrator coil.

The high gain A. C. amplifier has a signal grid Il connected to the armature contact l0 through a shielded conductor 0B. The Signal contact Il is connected through a second shielded conductor 61 to the slider 8l of a potentiometer l! in the balancing network. The positive terminal of the potentiometer is connected, as indicated hereinbellore, to the rotary arm of the rotary stepping re ay.

'I'he amplier Il includes a resonant circuit tuned to the same frequency as the oscillator. so as to render the amplifier most sensitive to input signal components having this frequency. The output oi' the ampliiler is applied through the normally closed contacts II of the vibrator to the signal grid 10 of a relay tube 1i included in the trip circuit The output of the relay tube is applied to a coil 13 which actuates the control relay Il.

A pulse relay 1l is connected in the balancing network Il. It has a pair oi normally open contacts 1| connected between the positive terminal of the potentiometer Il and its slider Il.

A conventional regulated power supply Il supplies power to a panel light li, the marker light A large capacity condenser Il is connected bc- I tween the potentiometer slider and ground. A condenser 02 of small capacity is included in the high gain amplifier and ls connected between ground and the signal grid Il of the first ampliller tube.

The operation oi the mass marker control described hereinbefore is as follows:

'Ihe beam sweeper reduces the voltage across the potential dividing network in order to shift the separated lon beams across the collector at the end of the analyzer tube. As the voltage across the Potential dividing network decreases. the voltage present at any time between the rotary arm and ground is balanced against a negative voltage supplied to the `balancing network from the potentiometer The difference between these two voltages is applied to the large condenser ll.

The amature contact of the vibrator alternately, and at the frequency of the oscillator.

comes in contact with the signal contact 6i and the grounded contact 62. so that part of the charge on the large condenser is alternately applied to the small condenser and then discharged to ground. In consequence, a voltage equal to that appearing across the large condenser is periodically applied to the control or signal grid oi the amplifier. producing on this grid pulsations having a substantially rectangular, unidirectional wave form. Thus, in effect. the vibrator functions as an inverter. The first amplifier tube has a plate circuit with a resonant LC network and this filters from the amplified signal all components other than the fundamental. The latter is thereupon applied to the second amplifier tube itil and thence appears at the output of the ampliiier.

The normally closed pair of contacts 51 of' the vibrator are open when the amplifier output signal voltage is positive and are closed near the negative peak of the amplifier output voltage. Consequently, negative pulses are applied periodically to a parallel resistance condenser network ii which constitutes the grid circuit of the relay tube 1i. The time constant of this grid circuit is long compared to the period of the vibrator, so that there is applied to the relay tube a negative bias having an amplitude corresponding to the excess of the rotary arm voltage over the potentiometer voltage, i. e., corresponding to the differential-potential.

When the bias on the relay tube grid lil falls below a threshold value inherent to the tube. the tube passes current and closes the normally open contacts lin of the control relay 45. When this occurs the panel light is illuminated and the marker light 46 marks the spectrograrn, At the same time the rotary arm is stepped up to the next tap and the normally open contacts of the pulse relay are closed momentarily. thereby connecting the positive terminal of the potentiometer Gil to its sliding contact and impressing on the signal grid of the first amplifier tube a large positive potential. This in turn causes a large nega,- tive bias to be applied to the grid oi' the relay tube. thereby opening the contact of the control relay and the pulse relay. In this condition, the apparatus is ready to make another mark on the mass spectrogram when the trip circuit again closes the control relay.

By maintaining a proper relationship of resistances between the taps of the rotary stepping relay. as described in detail in the aforementioned copending application of Langmuir. the action of the mass marker is synchronised perfectly with the recording galvanometer. In consequence. as each peak is recorded on the photographic paper a beam of iight marks the paper adjacent that 'Ihe mass marker described and claimed herein fires more accurately than one provided with a direct current amplifier. This advantage is achieved through the use of the high gain alternating current amplier in conjunction with the means for converting the output of the balance network into alternating curret which is applied to the amplifier. and the means for rectifying the output of the amplifier and applying the rectified output to the trip circuit. The use of the alternating current amplifier obviates fluctuations in the firing threshold voltage due to drift. as occurs in certain direct current amplifiers. Moreover, since the alternating current amplifier has high gain 'any fluctuations in trip-circuit relay tube characteristics become relatively unimportant All in all, the apparatus described is an extremely accurate one for correlating specic mass of ions to ion abundance.

The mass marker illustrated includes a plurality of coupled three-positioned switches III. 12|, ill. Position i on each switch is o position. Switch III is connected in the power line when on positions 2 and l. Switch III is ol! in both positions i and 2, but in position I completes the circuit of the lower contacts of the control relay. In positions 2 and I the switch |22 connects the battery to the resistance of the potentiometer I9 The three coupled switches may be thrown simultaneously from a common control lever, respectively, to positions i. 2 or l. When the switches are in position i. the circuits, including the switches are open. In position l, power is supplied to the potentiometer B9 and to the regulated power supply. In position I, al1 three circuits controlled by the switches are completed.

I claim:

1. In a mass indicator of a mass spectrometer adapted to indicate the specific masses of ions being detected by the spectrometer and having a resistance, means for impressing across the resistance a voltage which is a function of the specinc mass being detected and changes in accordance therewith, and means for selecting successive fractions of said voltage during a period in which the voltage is changing, the combination which comprises means connected to the voltage selector for producing an alternating current corresponding in amplitude to the selected fraction of the voltage, and means responsive to a deviation in the amplitude of the alternating current above a threshold value for indicating the specific mass of ions detected when that threshold is reached and for causing the voltage selector to select the next fraction of voltage.

2. In a mass indicator of a mass spectrometer adapted to indicate the specic masses of ions being detected by the spectrometer and having a resistance, means for impressing across the resistance a voltage which is a function of the speclflc mass being detected and changes in accordance therewith, and means for selecting successive fractions of said voltage during a period in which the voltage is changing, the combination which comprises means connected to the voltage selector for producing an alternating current corresponding in amplitude to the selected fraction of the voltamsaidmeansineludingabalancingnetmanoeciliatorandavihratorcwnledtothr 3. In a mass indicator of a mass spectrometer adapted to indicate the specinc masses of ions acrossthenetworkavoltagewhichisanmction of the specinc mass being detected and changes voltage selector for producing an alternating cur. rent corresponding in amplitude to the selected traction of the voltage. a high gain AC amplier tor amplifying current, and means responsive to a deviation in the amplitude of the alternating current above a threshold value for indicating the speciiic mass of ions detected when that threshold is reached and for causing the voltage selector to select the next traction oi voltage.

4. In a speciiic mass indicator for a mass spectrometer provided with a circuit and means for impressing across the circuit a voltage which is a function ot the specific mass oi the ions being detected at a given instant, the combination which comprises a stepping relay connected in said circuit and provided with a movable arm making contact with successive points in said circuit and a lead through which the arm is connested to a terminal. means for opposing the potential between the lead and an end of said circuit against a standard potential, means for converting the resulting dierential potential into an alternating current, means for rectifying the a1- ternating current, a mass marker, a control relay which actuates the mass marker and the stepping relay. a trip circuit connected to the control relay, and means for applying to the trip circuit avoltlgemticnalto the amplitude et the rectiedalternatingcurrent. Y

llnamassmarteri'oramassspectrometer having a potential dividing network the voltage acroeswhichisatunction o! thespecinc-mas ofionsbeingdetectedandmeansforselecting progressively diilerent fractions of the voltage acrossthenetworkasthevoltageacrossitdecreases. the combination which comprises means for converting the selected traction into altemating current corresponding in amplitude to the selectedtraction.andmeansresponsive toadeviatlon of the amplitude oi' the alternating current from a threshold value tor indicating the specific mass o! ions which may be detected when that threshold is reached and for selecting s. new frantion ot the voltage across the network.

6. In a mass marker i'or a mass spectrometer having a conductor. means for impressing across the conductor a voltage which is a function oi' the speciilc mass oi ions being detected in the spectrometer. and means for successively selecting diilerent fractions oi the voltage applied across the conductor as that voltage changes, the combination which comprises a balancing network adapted to oppose the selected fraction oi' the voltage with a standard potential to produce a diilerential potential, an A. C. amplifier, circuit means for applying the diilerential potential to the ampliiier as a series of pulses, a relay tube having a grid and an anode, means for applying rectiiied pulses from the A. C. ampliiier output to the grid of the relay tube,-means connected to the anode of the relay tube for altering the selected fraction of the voltage, said means being operative only when the grid voltage of the relay tube is greater than a threshold value, and a vibrator having one set of contacts for alternately connecting the A. C. amplier to the balancing network and to ground and another pair of contacts operating to apply half cycles of the A. C. amblliiler output voltages to the grid of the relay tu CLIFFORD E. BERRY.

CERTIFICATE OF CORRECTION.

Patent No. 2,570,651.

March 6, 1914.5.

CLIFFORD E. BERRY.

It is hmy certified that error appears in the printed specification of the above'nmbered patent requiring creotion as follows: Page 5, second coln, lineylii, for' "curret" read --currentng page il, first column, line 1'?, caiinjl-for "'in" sec occurrence, read -is; and that the Said Letters Pant should be read with this correction therein that the me may conform to the .record of the case in the Patent Office.

signes and .enea this 20th day of november, A. D. 19u15.

(Seal) Leslie Frazer First Assistant Camissioner of'. Patents.

voltamsaidmeansineludingabalancingnetmanoeciliatorandavihratorcwnledtothr 3. In a mass indicator of a mass spectrometer adapted to indicate the specinc masses of ions acrossthenetworkavoltagewhichisanmction of the specinc mass being detected and changes voltage selector for producing an alternating cur. rent corresponding in amplitude to the selected traction of the voltage. a high gain AC amplier tor amplifying current, and means responsive to a deviation in the amplitude of the alternating current above a threshold value for indicating the speciiic mass of ions detected when that threshold is reached and for causing the voltage selector to select the next traction oi voltage.

4. In a speciiic mass indicator for a mass spectrometer provided with a circuit and means for impressing across the circuit a voltage which is a function ot the specific mass oi the ions being detected at a given instant, the combination which comprises a stepping relay connected in said circuit and provided with a movable arm making contact with successive points in said circuit and a lead through which the arm is connested to a terminal. means for opposing the potential between the lead and an end of said circuit against a standard potential, means for converting the resulting dierential potential into an alternating current, means for rectifying the a1- ternating current, a mass marker, a control relay which actuates the mass marker and the stepping relay. a trip circuit connected to the control relay, and means for applying to the trip circuit avoltlgemticnalto the amplitude et the rectiedalternatingcurrent. Y

llnamassmarteri'oramassspectrometer having a potential dividing network the voltage acroeswhichisatunction o! thespecinc-mas ofionsbeingdetectedandmeansforselecting progressively diilerent fractions of the voltage acrossthenetworkasthevoltageacrossitdecreases. the combination which comprises means for converting the selected traction into altemating current corresponding in amplitude to the selectedtraction.andmeansresponsive toadeviatlon of the amplitude oi' the alternating current from a threshold value tor indicating the specific mass o! ions which may be detected when that threshold is reached and for selecting s. new frantion ot the voltage across the network.

6. In a mass marker i'or a mass spectrometer having a conductor. means for impressing across the conductor a voltage which is a function oi' the speciilc mass oi ions being detected in the spectrometer. and means for successively selecting diilerent fractions oi the voltage applied across the conductor as that voltage changes, the combination which comprises a balancing network adapted to oppose the selected fraction oi' the voltage with a standard potential to produce a diilerential potential, an A. C. amplifier, circuit means for applying the diilerential potential to the ampliiier as a series of pulses, a relay tube having a grid and an anode, means for applying rectiiied pulses from the A. C. ampliiier output to the grid of the relay tube,-means connected to the anode of the relay tube for altering the selected fraction of the voltage, said means being operative only when the grid voltage of the relay tube is greater than a threshold value, and a vibrator having one set of contacts for alternately connecting the A. C. amplier to the balancing network and to ground and another pair of contacts operating to apply half cycles of the A. C. amblliiler output voltages to the grid of the relay tu CLIFFORD E. BERRY.

CERTIFICATE OF CORRECTION.

Patent No. 2,570,651.

March 6, 1914.5.

CLIFFORD E. BERRY.

It is hmy certified that error appears in the printed specification of the above'nmbered patent requiring creotion as follows: Page 5, second coln, lineylii, for' "curret" read --currentng page il, first column, line 1'?, caiinjl-for "'in" sec occurrence, read -is; and that the Said Letters Pant should be read with this correction therein that the me may conform to the .record of the case in the Patent Office.

signes and .enea this 20th day of november, A. D. 19u15.

(Seal) Leslie Frazer First Assistant Camissioner of'. Patents.

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