4 0 A P P L I E D SPECTROSCOPY

A Digital Recorder for Mass Spectra V. J. Caldecourt

Spectroscopy Laboratory, The Dow Chemical Company, Midland, Mich.

Abstract As a mass spec t rum is scanned, the m a x i m u m value of each peak

is stored in a capaci tor Then the difference between the stored voltage and the lesser voltage seen af te r the scan has proceeded down the side of the peak operates a peak detector, wh ich in t u r n initiates measure- ment of the stored peak height by a digital vol tmeter A t the same time the mass is determined by measurmg the ion accelerat ing voltage wi th a modified self ba lancing potent iometer equipped wi th a shaf t position digitizer.

Introduction The peaks in a mass spectrum are part icular ly well

suited for &gxtal recording. The readings are needed at discrete, relatively well separated intervals, and the useful recording range xs wide so that a five figure presentation xs worth whde. Each compound can give rise to many peaks and consequently a large volume of data must be handled. In many cases automatm data handling may be desired. This is part icularly true in the field of high-mass spectrometry.

Digital recording of a mass spectrum involves digitizing the mass number and the peak height of each mass peak which is large enough to be reliably detected. A continuous record of the basehne and very small peaks is still drawn. Digit izing the mass is the easier part of the problem since the mass xs a slowly changing variable and is to be read on command after a peak is detected. Detecing and measuring the peak height is more involved because the maximum value of a fairly rapidly changing variable must be de- tected and measured with high accuracy.

Mass Digitizer Digital readout of the mass on a mass spectrometer

using an accelerating voltage scan can be performed wxth a modified self-balancing potentlometer circuit. To accom- plish this, the device must monitor the accelerating voltage and then deliver digital readings which are inversely pro- portional to the accelerating voltage. The circuit which does this is shown in Figure 1.

Operation of the circuit is as follows: The servo sys- tem adjusts R2 so that the voltage at point A equals the voltage at point B. Point B is a constant selected to give the desired mass calibration. The resistance, R2, required to maintain the specified voltage at A is inversely propor- tional to the current i, which is proportional to the accel- erating voltage. Thus the required inverse function is ob- tained and a F~scher and Porter Company shaft posthon

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dlgthzer coupled to R2 can be made to read &rect ly in mass units. R3 provides automatic gain control for the servo-amplifier.

This mass digitizer was designed for use wi th a Con- solidated Electrodynamws Corporahon 21-105 mass spec- trometer and covers the mass range from 10 to 110 mass units in 0.1 mass unit increments. A counter coupled to the mass digitizer servo system provides the operator with a continuous visual indication of the mass at which the spectrometer is set, or is scanning. To maintain mass cali- bration, the magnetic field is set to a specified value as indicated by a vibrat ing coil magnetometer (1) .

Peak Digitizer The design of the peak height recording system was

dictated to a large extent by our decision to use one of the lower priced commercially available digital voltmeters. These devices require a fraction of a second to give a four digxt reading and therefore are too slow to follow up a peak as i t is scanned. A separate, faster device xs needed which can follow the changing voltages accurately and store the maximum or peak value until the digital voltmeter mea- sures it. This peak storage unit is an integral part of the peak detector system also.

A peak or maximum in the ion current whether i t be a normal peak or a metastable peak is detected by noting when the electrometer output decreases a certain amount from the stored maximum. Therefore variations in the rate of scan, barring reversal of direction, do not affect the operation. The magnitude of the decrease necessary to initiate the measuring cycle is made large compared to the noise present. The noise problem vanes with peak size, since small peaks show a relatively large percentage of noise caused by the electrometer input resistor noise and large peaks show a relatively small percentage of noise which is nearly a constant percentage over a wide range of peak sizes; and i t is caused by pumping speed variations, power supply variations, and so forth. Therefore the peak detector is designed to be operated by a small, definite de- crease in peak height, about 1 my., when the peak is small. When the peak is large, a small percentage, about 2%, decrease in peak height i s required to operate the peak detector, and thus the system is reliable for all sizes of peaks.

The block diagram, Figure 2, shows the major com- ponents of the peak detecting and peak storage system, and will be used to describe the method of operation. Triangle E represents a chopper input amplifier which energizes RO if the output from the peak storage amplifier exceeds the chosen threshold value. Block F is an amplifier which takes its signal from the first stage of amplifier E, energizes R51 and shunts the output by a factor of ten ff the output exceeds 10 volts. Amplifier A, the diodes, capacitor C, and amplifier B form a chopper stabilized d.c. amplifier wi th overall feedback which stores the peak. Triangle D is an amphfier which detects when a severe unbalance occurs in the feedback amplifier and energizes R1 or R2, thereby indicating a peak or a valley has been passed. The magni- tude of unbalance required is determined by the peak height because of the automatic gain control voltage (AGC) supplied by amplifier G.

NUMBER 2, 1958 4]

INPUT

5 0 K

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4 7 K

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FIGURE 3. CHOPPER AMPLIFIER

AGC(G)

lOOK

.05

470K

P 2 2

PEAK DETECTER (D)

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AGC

FIG. 4. VARIABLE GAIN AMPLIFIER

± • 0 5 ~

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Sequence of Operations

When scanning a spectrum, two different situations can occur which require different treatment. There may be completely resolved peaks, or a group of partially resolved peaks which are to be recorded. With in limits, this system can record both types. A description of the sequence of operations in recording a peak in terms of the block dia- gram will ilustrate the method and show its limitations.

If a completely resolved peak is scanned, the sequence is as follows. While following the baseline RO, R1, R2, RS, R4, R21A and R31 (see Figure 2) are all de-energized. Since R5 and R4 contacts are normally closed, the feed- back amplifier (A-B) can follow the small variations in the input voltage supplied by the electrometer amplifier of the mass spectrometer. Upon encountering the skirt of a peak, the input voltage exceeds the threshold value, about 1 my, and amplifier E energizes RO which in turn energizes RS. Then the output of the feedback amplifier can follow increasing values of input voltage but cannot decrease. Continuing the scan, the output voltage increases as we follow up the side of the peak until the maximum is reached. The output then is held at this maximum while the scan progresses down the other side of the peak. The difference between the stored value and the declining in- put signal drives amplifier A and activates the peak detector (D and R2) as soon as the difference exceeds 1 mv or 2% of the stored value, whichever is larger.

The appearance of this difference signal can be explained as follows. When following up the side of the peak, the feedback loop is complete and the feedback thru the 1 megohm feedback resistor balances out nearly all of the effect of the input voltage applied thru the 100,000 ohm input resistor. Then the net input to amplifier A is very small. However, after a maximum is passed, the diode shuts off and breaks the feedback. The relative values of the input and output are then no longer maintained and a substantial input appears at amplifier A. This results in a

large output voltage from A which Js apphed to amplifier D and causes the operation of R2. R2 initiates measure- ment of the stored value and pr int -out of the peak height and mass.

Af ter the pr int -out is complete, R5 is released and then R4 energized. The feedback amplifier will now follow down the side of the peak. In the case of a resolved peak, the input will continue to drop until the basehne is reached. RO will be released and the cycle is complete.

In the case of a part ial ly resolved peak, the valley or mlmmum is detected in a manner similar to that used to detect a maximum. Because R4 is energized during the time the amplifier follows down the side of the peak, the output can not increase. When the minimum is passed, a large unbalance voltage is again applied to amplifier A which drives D, and in this case energizes R1. R1 is acti- vated instead of R2 because the polarity of the unbalance is opposite to that which occured upon detecting a maxi- mum. Detection of the minimum or valley supplies the information needed to reset the program relays so that the ampbfier can follow up and record the next peak as de- scribed above.

Circuit Details

The Circuits of the separate units are shown m Figures 3 thru 8. Most of the c i rcui t ry is quite conventional and only the more important design features need to be men- tloned. The connections between units are specified in Table I.

Amplifier A, Figure 3, is a 400 cps chopper modulated and demodulated amplifier with a gain of 5000. Full wave input and output circuits are used. The output from amplifier A flows thru the diodes and charges capacitor, C. Amplifier B and capacitor C constitute a Miller integrator ctrcuif and in this case provide a storage unit for the peak or valley.

Details of the diode circuits and the low grid current d.c. amplifier B are shown in Figure 5. The diodes are operated with reduced filament voltage to lower the re- verse voltage needed to shut them off. The diodes are actually biased triodes used instead of simple diodes because their logarithmic cut off characteristic improved the over- all stability of the feedback amplifier. Since about 1 volt reverse voltage is required to shut off a diode and the gain of the amplifier ahead of it is 5000, the difference in the stored peak height from the true value is approximately 0.2 my in terms of input voltage.

The complete feedback amphfier (A and B) has an overall Rain of 10 and delivers a signal which is very accurately proportional to its input because of the large amount of feedback. I t becomes a peak reading or valley reading device depending on whether R4 or R5 is energized. Upon energizing R21A, the signal is stored with negligible decay by the capacitor unti l measured by the digital volt- meter. Af ter the stored signal has been measured, both R5 and R4 must be momentari ly released to permit the overall feedback to rebalance the circuit. Since most of the gain of the circuit is obtained in the chopper amplifier, no zero adjustments are needed between regular main- tenance checks at 3 month intervals.

Amplifier D, Figure 4, is a variable gain amplifier tak- ing its a.c. input signal from the first stage of amplifier A. The two pentode stages use variable mu tubes so that the ~ain of the amplifier can be attenuated automaticallv when detecting large peaks. Since the output of a variable mu stage is approximately proportional to the logarithm of

NUMBER 2, 1958 43

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the AGC voltage which in turn is proportional to the peak height, the peak detector is operated by an approximately constant percentage of the peak height in the case of large peaks.

The amount of decrease in signal needed to operate R2 m the case of small peaks is chosen by adjusting the gain control in amphfier D. If the peak is large, the approxi-

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Fig. 6. THRESHOLD AMPLIFIER

mate percent decrease in the peak required to operate R2 is chosen by adjusting the at tenuator ahead of the AGC supply.

Amplifier E, the threshold amplifier, is shown in Figure 6. I t uses a 60 cps chopper and is a typical servo amplifier. In this case the two phase motor is used to drive a switch, RO. The output signal from the peak storage amplifier

44 API~LIED SDECTROSCO]~Y

is applied to the input of this amphfier. Thus an a.c. signal proportional to the stored peak is available from the first stage and this signal is used to drive the XIO shunt unit, F, and to furnish the AGC voltage for amplifier D by means of unit G.

The 400 cps choppers in amplifier A and the syn- chronous detector for amplifier D (see Figure 9) are driven by the oscillator shown in Figure 8.

Relay Program Table II lists the parts for the program relay unit,

Figure 9 shows the program relays which are operated by RO, R1, and R2 during the scan. The relay circmts are drawn in order of their operation reading from top to bottom. In describing the sequence of operatlons, an in- creasing output voltage will be said to result from an in- creasing input voltage because it is felt that this makes the description clearer. Actually this is only true in terms of absolute values since the output of a feedback amplifier of the type shown is 180 ° out of phase with the input.

The relay program is more complicated than one would expect for a dewce which is to alternately register peaks and valleys because of the transients which occur when the storage amphfier regains its equilibrium after being overdriven. The amplifier may be overdnven by sig- nals with an excessive rate of rise and is always overdriven when the input differs significantly from the stored value after the scan has passed a peak or a valley. By the use of time delays to give the transients time to die out and interlocks to prevent operations out of sequence, the tran- sients are prevented from causing false operations. In scanning a spectrum, the program relays operate as fol- lows. Upon encountering a peak, the input voltage increases and RO is closed. The 10 mf c a p a c i t o r be- comes charged and tends to hold the program relays if a fast transient releases RO momentarily. R5 and R3A are activated and lock themselves in; R7 also is energized. Be- cause R5 has operated, the output can now increase only. If the rate of rise of the signal exceeds the speed of the storage amplifier momentarily, R1 will operate R8 and block R2 so that the surges will not prematurely trigger the peak detector.

After passing the maximum, R2 is energized. This operates R5 and it locks in. R2 will stay closed during readout and reset of the feedback amplifier since the stored value exceeds the input. The diode holds R7 as in inter- lock. R5 operates R21 and R21A which initiates measure- ment of the stored votage by the digital voltmeter. R21A parallels RO so that switching transients cannot release the program system.

When readout is complete, R24 is activated by the ~rinter or punch and R3, 3A, R21 and R21A are released. Then after a delay which permits the feedback amplifier to regain equilibrium, R4 operates. The discharge of the 4 mf capacitor provides the delayed release of R7 and therefore delayed operation of R4. R6 operates with R4 also.

With R4 activated the feedback amplifier will follow the decreasing input voltage as we proceed down the side of the peak.

If the peak is completely resolved, the input will drop to zero and RO will open. This will release the relays R4 and RS.

I f the peak is not completely resolved, after passing the minimum the signal will increase again. The signal

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FIG. 7. Xl0 SHUNT

T A B L E I . C A B L E S

Umt F*gure Connector Connected to

Chopper Amphf ie r 3 P10-1 P5-1, P60-1 -2 -2

P l l - 1 System's Inpu t , P30-1 -2 Ground , P30-2

P12 P22 P13-1 P40-1, P55-1

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-2 -2

P22 P12 Storage A m p h f l e r 5 P I -1 P14-1

-2 -2 P2 P51 P3 P52 P4 P53 P5-1 P10-1, P60-1

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T h r e s h o l d A m p h f i e r 6 P30-1 P I0 -1 , PS-1, P60-1 -2 -2, -2, -3

P31 P20, P62 P33-5 P54-1

-6 -2

X10 Shunt 7 P60-1 P10-1, P5-1 -2 O u t p u t to digital vol tmeter -3 P10-2, P5-2 Gnd.

P61-1 Con tac t closure -2 Ind ica t ing X10 shun t

P62 P31, P20

400 cps Oscd la to r 8 P40-1 PI3 -1 , P55-1 -2 -2 -2

P rog ram Relay 9 PS0-1 Con tac t closure to -2 act ivate digital vol tmeter

P51 P2 P52 P3 P 53 P4 P54-1 P33-5

-2 -6 P55-1 P40-1, P13-1

-2 -2 -2 P56-1 P21-1

-2 -2

Note Each u m t has its connectors numbered m one decade. All cables are shielded except P13, P33, P40, P50, P54, P55, and P61. Cables to P2, P3 and P4 should be sMelded polyethylene msulated wire or cable of eqmvalent insulation resistance.

will then exceed the stored mimmum and R1 will be energized. This will release R4 and R5. After a delay, caused by R6, introduced so that the feedback amplifier can regain its equihbnum, R3 and 3A again operate. Wzth

NUMBER 2, 1958 4 5

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$

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FIG. 9. PROGRAM UNIT

TABLE II. PARTS LIST FOR PROGRAM RELAY UNIT

Notation Desc rtphon

R2 SPDT relay 2000 ohm colt, Sigma Type 4 R3, R3A SPDT relay 2000 ohm cod, Sxgma Type 4

RI DPDT mlcrosw~tch relay 1000 ohm cod, Clare Type G R4 DPDT mlcrosw~tch relay 3300 ohm cod, Clare Type G R5 DPDT rmcroswxtch relay 3300 ohm cod, Clare Type G R6 SPDT relay 2000 ohm coil, Sxgma Type 4 R7 SPDT relay 2000 ohm coil, Sigma Type 4 R8 SPDT relay 2000 ohm coil, Sxgma Type 4

R21, R21A DPDT nucroswltch relay 1000 ohm cod, Clare Type G R24 DPDT mxcroswltch relay 3300 ohm coil, Clare Type G

15 ma Selemum rectifiers, Federal No. 1159 350 ma Selemum rectifiers, Federal No. 1023

T1 Stancor No. A4711 T2 Stancor No 3850

R3 and 3A operated, the sys tem is again set to fo l low up the side of a peak and the sequence ]s as described above.

P e r f o r m a n c e

T h e peak s torage and de tec to r u n i t dr ive a Delaware Products Company electromc dzg#al voIfmeter w h i c h wil l give a fou r digi t r eadmg in 0.2 seconds w i t h a fu l l scale reading of 9.999 volts . T h e m a n u f a c t u r e r claims an accur - acy of ± l m v or 0 . 1 % . Repea t readings t aken w i t h the comple te peak reading sys tem on a k n o w n vol tage of 0.8 v have shown a s t anda rd devia t ion of 0 . 0 4 % . W h e n repeat readings are t aken on a large mass spec t romete r peak, the reproduc ib i l i ty is 3-5 t imes poorer and this appears to be the f a u l t of the spec t rometer , n o t the record ing system.

T h e desired five digi t r ecord ing range is ob ta ined by use of an XIO shunt . O n the X1 range, i n p u t signals of zero to 0.9 v give readings f r o m zero to 9000 because the s tor- age amphf ie r has a gain of 10. T h e n the least c o u n t is 0.1 m y on the inpu t . T h e storage amplif ier o u t p u t is shun t ed b y a f a c t o r of ten for signals exceeding 0.9 v and ano the r

46 A P P L I E D SPECTROSCOPY

decade of range is available. Since the digital voltmeter can provide a four digit reading, the rated accuracy can be realized after shunting, i.e. after 9000 changes to 900.

The storage amplifier shows a flat response up to 10 cps. However, the program relays and digital voltmeter opera- tion limit the maximum scan rate to about one peak per second. This scan rate may be further limited by the type of readout. The maximum scan rate can be used with a Clary Corporahon printers At present, an IBM Model 523 card punch is being used which slightly lengthens the operating cycle. One IBM card is punched for each peak and it contains a five digit code number, a four digit peak height with the XIO shunt factor shown as an overpunch, and a four digit mass number. The scan is stopped during peak measurement and readout by interupting the decay of the capacitor in the grid circuit of the type 100TH ion accelerating voltage control tube. The base scan rate is the same as ,s customarily used, RC ~ 100 sec.

Two of these recording systems have been constructed. The first is the one described in this article and is used with the usual oscillographic recorder on a Consolidated Electro- dynamics 21-103 mass spectrometer. The second was de- signed for use with a 90 ° magnetic scanning mass spec- trometer. It differs in the type of mass digitizer used and

in the type of base line recorder used. A Leeds and Northup recorder is used to draw the baseline and small peaks.

A digital recorder can provide substantial sawngs in man power in prowding direct data for manual compu- tlon and also improved accuracy by avoiding the reading errors of an operator. The time required per peak to as- semble peak height data can be reduced by a factor of four or more. The advantage is particularly great when a great many peaks are to be measured because then the operator becomes fatigued and inefficient. Of course, even greater savings result if the punched cards are utilized directly by an electronic computer.

The digital recorder with a pen and ink recorder could be used with mass spectral scanning rates which previously required a magnetic oscillograph. A pen and ink recorder can draw the baseline and small peaks with acceptable ac- curacy even though not fast enough to draw the large peaks accurately. This combination then allows one to avoid the work of developing oscillograph records without ac- cepting a slower scan rate and/or less accuracy.

Literature Cited 1. V. J. Caldecourt and S. E Adler, Rev. Sci. Instr. 25,

953-5 (1954). Submitted November 7, 1957, accepted April 12, 1958.

A Three Stage Mass Spectrometer F. A. White, F. M. Rourke, and J. C. Sheffield

General Electric Co., Knolls Atomic Power Laboratory, Schenectady, N. Y.

Abstract

A three stage mass ~pcctrooaeter, utilizing two consecutive nlag- netic focusing lenses of 20" radii followed by a 20" radms of curva- ture electrostatic analyzer, is described An abundance sensitivity of 10 a° was measured for this i n s t r u m e n t in the low mass region. Appli- cation O~ this instrument to analytical and research problems in the field of mass spectrometry and results obtained are discussed.

Introduction

The design and construction of the instrument herein described was undertaken after a fairly serious appraisal of current and antic~pated future problems in isotopic ratio measurements. Lim~tatmns in isotopic ratio determinations may be said to fall into two broad categories, (1) those which are indigeneous to a specific experiment, and (2) basic or more fundamental restrictions which ~mpose physi- cal limits of observation and detection. In the first cate- gory might be included difficulties of producing a suffi- ciently copious number of ions from a high boIhng point metal, for example. The second group would comprise boundary conditions such as the lower limit of detection of a singly charged particle of a given kinetic energy, or the resolution of various ions having very small differences in momenta. All signal-to-noise limitations could be m- eluded in this group.

The isotopic problems of interest to this laboratory and the objectives of this instrument are definitely in the latter class. While significant improvement in abundance sensiti- vity was noted for a two stage analyzer (7), a need for still higher abundance sensitivity exists in some analytical problems. Therefore the destgn and construction of a three stage analyzer which would have application in the latter class of problems was undertaken.

General Design

The arrangement as sketched in Figure 1 is justified on the basis of flexlblhty as a research instrument. Two 90 °

MAGNETIC ANALYZER

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FIGURE 1. SCHEMATIC DIAGRAM or THREE-STAGE Mass SPECTROMETER