US2676264A - Automatic control apparatus for mass spectrometers - Google Patents

Description

C. E. BERRY April 20, 1954 AUTOMATIC CONTROL APPARATUS FOR MASS SPECTROMETERS 2 Sheefcs-Sheet 1 Filed Nov.

n NPM A T TOR/VE Y INVENTOR. CLIFFORD E. BERRY om w April zo, 1954 AUTOMATIC CONTROL Filed Nov. 24, 1951 c. E. BERRY 2,676,264

APPARATUS FOR MASS SPECTROMETERS `2 Sheets-Sheet 2 SOURCE 0F POWER FOR T MAGNET y FIG. 2.

IN VEN TOR. CLIFFORD E. BERRY Byawm A T TORNE Y Patented Apr. 20, 1954 A Clifford Berry, Altadena, Calif.,Y assignor to 51"" Consolidated lEngineerin-g 'I'Corpnration, Pasaa)This invention relates to improved *apparatusA lling r the magnitude of if eleratin pot ritial'sffvorv the :Soufie of i p forl @tematica y thee@ The accelerating potentials required for measuringV a.predeternriined'"v series "of "differert ions mayflbe produced. by providing aiseparate" voltage dividexgwfor each type iofi ion tobemeasure'd, and

umer'us typesgof ions, hQPQWQr, drainon the itricf'the' vonage' dividers probe'I mass fspeetromete'r; may be adjusted to se-if-f w rent levels? Qnsfmr may,11e-switchedfromfonefvoitage piller ,immer iomeasurethe series In accordance with ahlzneferihei.-

"IIn'San lalternative embodiment of invenitio'nfit "f--fifactiorialrpart oflthefac'celeratin'gnvolt- `f age' which fispplie'dto thevseirvoi system isfixed,

ing the magnitude of the reference voltage'li f ff fnfbothiembodimentsfofiithefinvention the ac- 'celeratiiiigfvoltagervaiesf infa'ecoidanceswith the -I current' :omhe voltage employedftoproduce the -magn`eticf field, andf'hence the'. acceleratingivoltaigre isfrautomaticallyv fa'd'iusted :sofzasiftoo-correct .partiallyfonfchanges' inthe-'magnetic .iiuxy density. fIf desiiedfadditionalsmeanssmay-obe:.employed to' provide?1 more :complete correctionfofthezmagnitude ofthe'aeceleratingvoltageWhemthe magnetic'fuxdensityhangesx Thevinventionrsliiseexplained with reference to 'thedrwin'gsfai whichfg-: ai; fFignol: isa-schematic diagram of a preferred 1. embodiment fofithinvention; and 21 45 Figi???isfaischematicdiagram of a modification f'theapp ltusshfowiifnFigglyif fffil'im mi' 'f lIrifF'igfY the appaiatus for automatically adjstingtlie imag tide iii-'i'- thei'faczeleating potentials is shown :YconunotionflWithvtheessenf spectrometersoff-the type rillustrated fbut :that it terfft willvbelapparentthatathe appaiat'usY-of the present-'finvention4I isf not? :limited sto aussi in. mass I:nay fsbe-\employedirrfpracticalfly; any type off mass Snetmmter whichif-acqeleratmawotemials 3 are employed to propel the ions into an analyzer.

An ion source I communicates with a conductive analyzer tube I2 which is enclosed within an envelope I4, through the wall of which a gas inlet tube I6 projects. A high degree of vacuum is maintained within the envelope by means of a vacuum pump (not shown) connected to the envelope.

The outlet end of the analyzer tube I2 is provided with a slot through which sorted ions are projected. An electrode I8'servesto collect the ions, and they are discharged through an amplifying and recording apparatus 20 which provides an indication of the quantity of the ions being measured.

The gas inlet tube I6 is composed of insulating material, and it communicates with an inlet electrode 22 in the form of a metallic tube which is partially closed at one end by a closure plate having a transverse slot 24 to accommodate an electron beam. The electron beam (not shown) may be produced and projected through the slot 24 in any suitable manner.

The molecules of a gas sample which are admitted into the ionization chamber through the inlet I6 are bombarded by the electron beam passing through the slot 24 and thus converted into ions. These ions are propelled as an unsorted beam into the analyzer tube I2 by electrical potentials established between the elec` trodes in the ion source I0, lwhich potentials are produced by the Velectrical system which is attached to the electrodes.

A pair of electrodes 26 and 28 serve as accelerating and collimating electrodes for the ion source. The electrodes 26 and 28 have slots which are aligned with the slot 24 in the tube 22 so that the ions are propelled from the outlet 24 lof the tube 22 into the analyzer tube I2 under the influence of the electrical potentials which are established between the tube 22 and the electrode 26, and between the electrodes 26 and 28.

The ions enter the analyzer tube I2 as a homogeneous, unsortecl beam. In the analyzer tube I2, the unsorted beam is separated into a plurality of diverging homogeneous ion beams B1, B2, Ba by means of a transverse magnetic eld produced in the analyzer tube by an electromagnet.

The coil for the electromagnet is shown at SIL A source of power 32 is connected through a resistor 34 to the coil. 30. An adjustable tap 36 is provided on the resistor for producing a reference voltage between the tap and ground having a magnitude which is proportional-to the current which ows through the coil 30. Thus, the magnitude of the reference voltage varies vwith variations in the power supplied to 'the coil 38 and hence with variations in the density of the magnetic field which is applied to the analyzer tube I2.V

The reference voltage produced at the tap 36 is compared in a servo system with a second reference voltage which is a fractional part of the accelerating' voltage, and the servo system serves to control theaccelerating voltage so that the magnitude of the fractional part thereofl isvmaintained equalto the magnitude of the reference voltage produced at `the tap 36. f

A source of potential 40 having resistors 42, -44 and 46A connected in series across its output provides the accelerating voltages for the electrodes of the ion'source I0. The tube 22 is connected to the junction betweenthe resistors 42 and 44, the accelerating electrode'26 is connected to a-tap on-the resistor 44, and the accelerating electrode 4 28 is connected to the grounded terminal of the source of potential 40.

The accelerating voltage which is applied to the ion source I0 is developed across the resistors 44 and 46, and a plurality of taps 48 on the resistor 46 provide voltages between the taps and ground which are fractional parts of the accelerating voltage. The voltages produced at the taps 48 are selectively applied to the servo system by means of a switch 58.

The magnitude of the accelerating voltage which is produced across the resistors 44 and 46 is controlled by varying the voltage drop across the resistor 42. A triode Vacuum tube 54 having its plate connected to the junction between the resistors 42 and 44 and having its cathode connected to ground serves to control the voltage drop across the resistor 42 by means of the current which the tube 54 draws through the resistor 42.

The magnitude of the current which the tube 54 draws is controlled by the voltage which is applied to the grid of the tube, which voltage is produced by a battery 56 and a potentiometer 58. The position of the adjustable tap on the potentiometer 58 is controlled by a servomotor 62. Thus, the tube 54 serves as a variable impedance connected in series with the resistor 42, and the magnitude of the impedance is controlled by the servomotor 62.

The motor 62 is a reversible, two-phase type having one of its windings connected to a source of alternating current power 64 and having its other winding connected to the output oi an amplifier 66.

The secondary winding of a transformer 68 is connected to the input circuit of the amplier. The primary winding of the transformer is connected to a chopper 'I8 which is energized by the source of alternating current power 64.

The selector arm of the switch 50 is connected to the armature of the chopper 10, and the adjustable tap 36 is connected to a center tap on the primary winding of the transformer 68.

A correction circuit may be inserted in the lead between the tap 36 and. the transformer 68 if desired. The circuit 80 is not essential and the operation of the apparatus will be discussed first as if the circuit 80 were omitted.

The direction of the flow of current in the primary winding of the transformer 68 is Idetermined by the relative magnitudes of the reference voltage produced at the tap 36 and the fractional part of the accerelating voltage which is produced at the selector arm of the switch 50. Thus, the phase of the signal produced by the amplier 66 with respect to the alternating voltage produced by the source 64 is determined by the relative magnitudes of the reference voltage and the fractional part of the accelerating voltage which are compared in the chopper.

The servo system serves to compare the relative magnitudes of the reference voltage and the fractional part of the accelerating voltage and to control the magnitude of the current which flows through the tube 54 so that the reference 'voltage and the fractional part of the accelerating voltage are maintained substantially equal. Hence, the magnitude of the accelerating voltage is determined by the setting o-f the switch 58 and it varies with variations in the reference voltage.

Sincegthe magnitude of the accelerating voltage is proportional to the magnitude of the reference voltage, the magnitude of the accelerating voltage is Svmaintained f constant even though Ythe voltagcproducedibythe source 4iidrifts orfluctuates.

v'Iliefratio of the square-of .the Adensity of the magnetic eld to thefmagnitude of the-.accelerating-vo1-tage`shou1d Abe a constant for each setting'of thevswitch 58. Since-the lmagnitude of the accelerating voltage Avaries with variations in thermagnitudeof the reference voltage, and since the 'magnitude of vthe Lreference voltageis proportional to :the density of the 'magnetic eld produced vby theicoil of the electromagnet, variations inthe .densityof the magnetic eld are partially corrected ,by a corresponding change in theion accelerating voltage.

IfV .the .resistor 34 is. non-linear so that .i-ts resistance/is .proportional :to ythe current which flows `.through it, a vcomplete correction is obtained, .becauselthe magnitude of the acceleratingf'voltage is then maintainedproportional to theasquareof thezmagnetcurrent andhence is maintained proportional 'to the square of the y density of the magnetic eld.

The correction vcircuit ,80 is .an .alternative ar-V rangement for correcting for variations in the density tof themagneticeld. A source .of potential 82 is v.connected across a potentiometer 84;..andthe .voltage Vdeveloped across thepotentiometer is applied in series to thecircuitbetween: the tap I,3.6 .and thecenter-tap on the primary-windingrof :thetransformer 68. Preferably, the .magnitude of the voltage of the source 82 should'be regulated so that it will not drift and introduce-terrors.

.The-voltage .introduced :to the circuit by the potentiometer 84 is adjusted .to be equal to onehalfathejreference voltage developed by the resistorsag. Ther-1a y1% changein the current through theresistor 34 causes a corresponding 2% change inthe accelerating voltage produced across the resistors 44 and 46. Tol a first approximation, this is the same as keepingthe ratio of the square of :the :density of the magnetic iield to the acceler-ating voltage constant.

.The -locations .of the taps 4.8 on the resistor 46 are -determined by the magnitudes of the accelerating .voltages which are required for the series of ions to be measured, and the magnitudes of the accelerating voltages are determined by Specic masses of theions to be measured.

The apparatus disclosed herein is particularly suitable for measuring the quantity of a large series of-diferent types of ions (such as 40 types, for example) because a large number of taps 48 may be employed on the resistor 46 without causing excessive .current drain on the source 4i).

In operation, the gas to be analyzed is admitted through 'the inlet tube i6, and the switch 50 is setvsosas to contact one of the taps 48. Then the tap 36 is adjusted on the resistor 34 so that the recording apparatus 20 indicates amaximum for the type of ions which corresponds to that setting of the switch 58. Then the movable arm of the switch 50 may be connected to any of the taps 48, and the recording apparatus will indicate the quantity of the ions which correspond to the various settings of the switch 50.

With the taps 48 on the resistor 46 arranged to cause the appropriate accelerating voltages to be applied to the ion source IIJ, the quantities of a predetermined series of different ions may be recorded in the recording apparatus 20 merely by causing the movable arm of the switch 50 to contact the taps y118 in sequence.

Ii. desired, the switch 5I] may be a stepping 6. typewliich is :actuated electrically, infwhich Icase the switch will cause the apparatus vto select the series ofy ions sequence-*and the quantity. of each type tof ion will be recorded `during `a predetermined interval of iti-me.

It will be apparentthat the reference voltage, which in :Fig 1 is produced by the magnetgcurrent which -nows through Vthe .resistor 234, maybe produced -by--a potentiometer connectedacross the .source =3=2 of :power .for the. magnet. With suchvan arrangement, rthefreference voltage "varies with vthe L.magnet voltage and .hence `with Lthe magnet a current as before.

In lthe Aembodiment fof .the .invention shown .in Fig..2, .the .locations of the resistors 34 .and 46 are interchanged. .This v.embodiment ofthe apparatus functonsin iessentially the same .manner yas the embodiment shown in Fig. 1.

I claim.:

.1. In anlass-spectrometer having an ion'source including at least `tulo electrodes for accelerating the ionsand Aalso;having .an electromagnet for producing .a..magnetic field which acts upon the accelerated ions, -the combination comprising adjustable means for providing an accelerating voltage for said electrodes, .means for producing a rst reference'voltagehaving va predetermined magnitude, .means lfor producing .a second. reference voltage having ama'gnitude whicnis aipre.- determined fraction of theacceleratng voltage, and -a servo system .responsive to the i'irst and second reference voltages `for .controlling said adjustable :means and causing the relative magnitudes of the first and second reference voltages to .be maintained in a Vpredetermined relationship.

2. The apparatus o'f lclaim 1in which one of the means .for 'producing a reference voltage is provided with a fsmvitch` for selectively causing the'magnitude of 'thereference voltage to vbe one of a plurality of predetermined values.

3; In a massspectrometer having an ion source including `atlleasttwo electrodes for accelerating the ions and also rhaving 1an electromagnet for e producing a magnetic field which acts upon the acceleratedisions'the combination comprising ad'- justable Ameans .for providing an accelerating voltage for ,said electrodes, means for lproducing a rst reference voltage having a magnitude which varies with variations in the density of said magnetic ;eld, means for producing a second reference voltage having a magnitude which is a predetermined Vfraction of the accelerating voltage, and a servo system responsive .to the rst..andsecond reference voltages for controlling said .adjustable means and causing the magnitude of the second reference voltage to be maintained in a .iixedrelationship 'with respect to the magnitude of .the Vfirst v reference voltage.

4. Ina mass spectrometer havingzan'ion source including atleast two electrodes for accelerating the ions and also having means for producing a magnetic field which acts upon the accelerated ions, the combination comprising means for producing a rst reference voltage having a magnitude which varies with variations in the flux density of said magnetic eld, a source of potential coupled to the accelerating electrodes for providing an accelerating voltage, means for selectively producing a second reference voltage having a magnitude which is a predetermined fraction of the magnitude of the accelerating voltage, adjustable means coupled to the source of potential for varying the magnitude of the accelerating voltage, and a servo system responsive to said first and second reference voltages for controlling said adjustable means and causing the magnitude of the second reference voltage to be maintained substantially equal to the magnitude of the iirst reference voltage.

5. In a mass spectrometer having an ion source including at least two electrodes for accelerating the ions and also having means for producing a magnetic field which acts upon the accelerated ions, the combination comprising means for producing a first reference voltage having a magnitude which varies with variations in the `flux density of said magnetic field, a Source of ptential coupled to the accelerating electrodes for providing an accelerating voltage, means for producing a second reference voltage having a magnitude which is a predetermined fraction of the magnitude of the accelerating voltage, adjustable means coupled to the accelerating electrodes for varying the magnitude of the accelerating voltage, and a servo system responsive to said first and second reference voltages for controlling said adjustable means and causing the magnitude of the second reference voltage to have a predetermined relationship with respect to the magnitude of the rst reference voltage.

6. In a mass spectrometer having an ion source including at least two electrodes for accelerating the ions and also having means for producing a magnetic field which acts upon the accelerated ions, the combination comprising means for producing a first reference voltage having a magnitude which varies in accordance with variations in the flux density of said magnetic field, a source of potential, a voltage divided coupled across the output of the source of potential and connected to the accelerating electrodes for providing an accelerating voltage, means for selectively producing second reference voltages having magnitudes which are predetermined fractions of the magnitude of the accelerating Voltage produced by the voltage divider, adjustable means coupled to the voltage divider for varying the magnitude of the accelerating voltage, and a servo system responsive to said rst and second reference voltages for controlling said adjustable means and causing the magnitudes of the two reference voltages to have a predetermined relationship.

7. In a mass spectrometer having an ion source including at least two electrodes for accelerating the ions and also having means for producing a magnetic field which acts upon the accelerated ions, the combination comprising control means for producing a first reference voltage having a magnitude which varies with the flux density of said magnetic field, a source of potential, first and second resistances connected in series across the output of the source of potential, means connecting the accelerating electrodes to the rst resistance for providing an accelerating voltage between the electrodes having a magnitude proportional to the voltage drop across the resistance, an adjustable impedance connected across the first resistance for varying the magnitude of the accelerating voltage, adjustable means coupled to the iirst resistance for selectively producing second reference voltages having magnitudes which are predetermined fractions of the magnitude of the accelerating voltage, and a servo system connected to said control means and tc said adjustable means for controlling the impedance of the adjustable impedance and causing the magnitudes of the first and second voltages to have a fixed relationship.

8. In a mass spectrometer having an ion source including at least two electrodes for accelerating the ions, the combination comprising adjustable means for providing an accelerating voltage for said electrodes, means for producing a first reference voltage having a predetermined magnitude, means for producing a second reference voltage having a magnitude which is a predetermined fraction of the accelerating voltage, and means operable responsive to the algebraic sum of the first and second reference voltages for controlling the adjustable means and causing relative magnitudes of the rst and second reference voltages to be maintained in a predetermined relationship.

9. In a mass spectrometer having an ion source including at least two electrodes for accelerating the ions and an electromagnet for producing a magnetic field which acts upon the accelerated ions, the combination comprising adjustable means for providing an accelerating voltage for the electrodes, means for producing a rst reference voltage having a magnitude which varies with variations in the density of the magnetic field, means for producing a second reference voltage having a magnitude which is a predetermined fraction of the accelerating Voltage, and means operable responsive to the algebraic sum of the rst and second reference voltages for controlling the adjustable means and causing i the magnitude of the second reference voltage to be maintained in a xed relationship with respect to the magnitude of the first reference voltage.

References Cited in the le of this patent UNITED STATES PATENTS Name Date Berry Mar. 6, 1945 Number OTHER REFERENCES

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