Automatic Mass Spectrometric Analysis B. F. DUDENBOSTEL, JR., and WILLIAM PRIESTLEY, JR. Esso Laboratories, Standard O i l Development Co., Linden,
A rapid method of obtaining mass spectrotneter analyses is of utmost importance in following pilot plant operation. Application of an automatic peak selector and an analog-to-digital converter makes possible direct tabulation of mass spectrometer ion currents for preselected anal?tical peaks. These data, furnished automatically by an electric typewriter or on punched cards or tape, may then be fed to a high speed computer for calculation and tabulation of the results. One such arrangement, using a Consolidated Spectro-Sadic, an IBM summary punch and card programmed calculator, is employed at the Esso Laboratories. With this system it is possible to report 30 gas analyses (20-component) per 8-hour shift. Only 1 1 / 1 men (11 man-hours) are required per shift to operate the mass spectrometer and computing equipment.
D
U R I S G 1953, a marked decrease in the time requirement for mass spectrometric analysis has been accomplished. In the past, the major portion of the time required to obtain a mass spectrometric analysis of a complex mixture has been that used to measure the analytical peaks on the recorder trace and to calculate the results. Actual sample-running time for a mass range of 2 to 100 requires only a few minutes as compared to 1 to 1.5 hours for the peak measurement and calculation steps. A typical example is the analysis of a hydrocarbon sample for Cg and lighter Components. The instrument running time is 5 to 7 minutes, a t the end of which a spectrum in the form of a photographic trace or a recorder chart is obtained. The first calculation step is the determination of the C;+ components by unique peak solution (using mechanical calculators) and correction of the remainder of the analytical peaks. Equations using the corrected peaks are then solved. In many cases a Consolidated analog computer is used. The remaining components (hydrogen, methane, air, etc.) are then obtained by mechanical calculation after the necessary corrections have been made. The total time of spectrum scanning, tabulation, and calculation by this procedure is from 1 to 1.5 hours per sample as contrasted to 5 to i minutes of instrument time. A more rapid method of obtaining mass spectrometer analyses is of utmost importance as the work load increases. In order to obtain the results from the analysis of 30 gas samples (20component) per 8-hour shift, two analog computers and six men would be required if the old method of measuring peaks from a record and calculation by employing analog computers were employed. Thus, it becomes an economic necessity to devise a means of rapid conversion of the analog information (ion collector currents) to digital information in the form of punched cards, magnetic tape, or punched tape. Also an automatic method of calculation, using high speed computers, is required. REDUCTION OF MASS SPECTROMETER ION CURRENTS TO DIGITAL INFORMATION
To date, two systems have appeared in the literature for reduction of mass spectrometer ion currents to digital form and calculation of the final results. One of these employs the Spectro-Sadie (a), manufactured by the Consolidated Engineering Corp. At the present time three of theFe itre in service. They are located a t the iinalytical
N. J. Service Laboratory of Consolidated Engineering Corp., Pasadena, Calif. ; Research and Development Department, Socony-Vacuum Laboratories, Paulsboro, N. J. ; and the Esso Laboratories, Standard Oil Development Co., Linden, N. J. The second system ( 6 ) , was developed by the Atlantic Refining Co., Philadelphia, Pa., in cooperation with G. B. Greene, Physics Research Corp., Pasadena, Calif. (now associated with Marchant Research, Inc., Oakland, Calif.) Both of these systems are easily adapted to present mass spectrometers. The conversion of ion currents to digital form consists of two parts. The first part is the automatic selection of the analytical peaks. The second is the actual conversion of the ion collector currents for the preselected analytical peaks to digital information. Both of the systems mentioned use presetting of the acceleration voltages for peak selection. The mass range covered is 12 to 122 or 40 to 150. Any 40 peaks within either one of these ranges can be programmed. It is not necessary to set up all 40 peaks but only the peaks desired for the type of analytical work being carried out.
-2
+%
i
TO
b
133-n
n
0
I
I
1;
l
ION SCLqCE
i.
v I
PEAK SE:EC70%
I
WA2YLT :LQ3>1-
"EFE7E\CE
VOLTACE
Figure 1. Peak Selection Circuit of the Spectro-Sadic
In Figure 1 is given a simplified circuit to achieve selection of the desired acceleration voltage in a Consolidated mass spectrometer. The condenser whose discharge varies the grid voltage of the lOOTH tube has been replaced by a servo-operated potentiometer. In addition, a bank of resistors has been connected across the acceleration voltage supply. By means of a selector switch any one of 40 tie-in points can be selected. The values of the resistors are such that the voltage output a t a tie-in point M ill focus a given mass. For instance, a t tap point one the voltage output of the circuit will be 3673 volts, which for a magnetic current of 0.587 ampere on the instrument used in this laboratory, will focus mass 12 on the collector. Similarly, a t tap point two mill be 3391 volts for focusing mass 13. One very important requirement in the use of a fixed acceleration voltage to focus a given mass-charge ratio, is that the magnetic field have a high degree of stability. This is not too important if a scanning technique of varying the ion acceleration voltage-for instance, discharging a condenser-is used. I n order to compensate for the drift in magnetic field strength, the acceleration voltage used is referred to a reference voltage which
1275
ANALYTICAL CHEMISTRY
1276
4. The signill is applied to the a n a l o a - t o - d i g i t a l converter which reschesa balance. 5. The data are read out to IBM nunoh and/or electrical tvnewriter. ' 6. Step'to next peak Gogram.
Figure 2.
Sp
As only the necessary analytical peaks (at a speed of 8 seconds per peak) are recorded, a d e h i t e decrease in the time required to run a sample is achieved. If desired, a photographic record of the selected analytical peaks can he obtained a t the same time. In addition, by throwing a single switch, it is possible to return to photographic recording of all the peaks. The Spectro-Sadic ( 1 ) is essentially a Thompson-Vdey self-balancing potentiometer. However, instead of operating on the nsud slide-wire basis, it incorporates a. unique 10-position, hidirectional stepping switch in each of the three stages to provide a 0- to 999digital output. The balancing process is accomplished by the stepping switches, which, being both reversible, and capable of simultaneous stepping in all three decades of the threedigit number, reach balance a t high speed. Linearity of the output voltage is ll_._l.__ .i ..E use of fixed precision resistors to establish the discrete voltage levels. The switch contacts providing the digital output parallel the potentiometer contacts to estahlish a positive relation of the output to the original analog input. When the system is in balance, the digital voltage value is tahulsted by any of a variety of devices. These may he card or tape punches, automatic typewriters, or any other read-out unit which may be aotuated by contact closures. A chart record of the spectrum (the analytical peaks only) can he obtained concurrently if desired. A photograph of the instrument is given in Figure 2. USE OF INVERSEMATRIXWITH PUNCHCARDCALCULATORS. Older calculation methods, employing analog computers, etc., did not use an inverse matrix for numerous rea6on8. When punch card calculators are employed, however, i t is advantageous to use an inverse matrix. Several methods for matrix inversion with punch card calcula-
.."..__.I_._._ ".-..
is derived from the magnetic current. As ea^.. voltage is selected, a oomparison is made to the reference voltage and a mrvo mechanism m a k e the necessary correction. This is also shown in Figure 1. The acceleration voltage goes to a servoamplifier, where it is compared to the reference voltage from the magnetic current. If an unbalance exists, the servo drives a potentiometer which changes the grid voltage of the lOOTH tube. This oorrectian is not striotly correct, since i t is a linear correction, while the effect of a drift in magnetic field is a square function. However, as only drifts of small magnitude are involved, this method works satisfactorily. The Spectm-Sadic System. In aotud operation this instrnment goes through several program steps for each mass peak.
1. The ion beam is defocused (electrically), a t which time the analog-to-digital converter is automatically zeroed. 2. The preprogrammed acceleration voltage is selected. 3. The ion beam is refocused.
Figure 3. IBM Card Programmed Calculator
V O L U M E 26, NO. 8, A U G U S T 1 9 5 4 tars have appeared in the literature (8, 3). The d a m i d iterative method of approximations (Gauss-Seidel) is employed a t the Esso Laboratories in conjunction with a card programmed calculator (CPC). This method requires Less than 6 hours for the inversion of the 20 equations containing the required mass spectrometer calibration data. (Matrix inversion is required only once for each instrument calibration.) Although faster methods are availa,hle, the approximation method is preferred because of its simplicity. A photograpb of a CPC setup is given in Figure 3. COMPUTING PROCEDURE. Once the inverse matrix is available, the computation of mass spectrometer data. proceeds in the following fashion. A the Ion the
sample is oharged to m a s s spectrometer. collector currents for preselected analytical peaks a r e c o n v e r t e d to digital information by the Spectra-Sadic a n d a u t o matically key-punched by an IBM reproducing or s u m m a r y punch. These cards ( o n e f o r e a c h a n a lytical p o i n t ) a r e f e d , along with the program deck, t o t h e card programmed calculator. The program deck contains the inverse matrix, molecular w e i g h t , compressibility, and other factors to permit the determination of all the desired information. The m e t h o d u s e d f o r calculation with the card moerammed calculator is the-same as that for the IBM 602A a8 described in a previous p u b l i c a t i o n from this laboratory (4). The time far this caleula t i o n o n t h e o a r d programmed calculator is 1
are shown in Table i. This is a mathematical synthetic blend containing 19 comFigure 4. Encoder (Atlanponents. It can be seen tic Refining Co.) that the analysis is reported in mole per cent (fixed ms free) and weight per cent. Specific gravities and the compressibility factor for the sample are also given.
The Atlantic System. In the Atlantic setup the same method of peak selection is used as in the Spectro-Sildic. In fact, the development of this method was a joint effort of the Consolidated Engineering Corp. and the Atlantic Refining Co. From this point on, however, the Atlantic system is quite different, in that an entirely different method of recording the peak heights is used and the computer is an integral part of the mass spectrometer setup. The Atlantic system consiEts of an encoder (the Marchant Codemaster), a digital computer (Minisc), and a typewriter. The encoder converts galvanometer deflections to derived binary numbers. At this point the data can be stored on a magnetic drum in the computer, can be oonverted to the deoimal system and recorded by the typewriter, or e m be punched on tape in a binary form. The encoder is based on a newly developed cathode ray technique, involving a monoscope tube with an internal screen containing a 1024line binary pattern. It can resolve 1024 units of voltage to the nearest half unit and is free from ambiguity. The encoder is equipped with a logadhmie amplifier which
1217 Table I.
Mass Spectrometer Analysis Mole % Inert Free
11.76
Be"Se"e Toluene Total Nitrogen Carbon dioxide Air Total Compressibility Gravity inert free
5.88 5.88 5.88 5.88 5.87 5.89 5.89 5.88 5.87 5.89 5.88 5.87 5.88 5.88 5.88 9996 5.00 5.00 5.00 15.00
Wt. % 0.45 1.80 3.37 3.15 4.95 4.71 6.53 6.53 6.29 8.08 8.10
7.87 9.65 9.44 8.76 10.33
__ 100.01
Mole % 10.00 5.00 5.00 5.00 5.00 4.99 5.01 5.01 5.00 4.99 5.01 5.00 4.99 5.00 5.00 5.00 5.00 5.00 5.00 100.00 0.9736 1.8115
___(
provides single scale operation, an important adjunct to automatic operation, with constant precision regardless of the magnitude of the analog voltage. In this manner, one of the 1024 horizontal lines of the monoscope tube is chosen through vertical deflection to correspond to the instantaneous magnitude of the analog voltage. Aotual reading is accomplished by sweep ing the beam horizontally across the pattern along the line so chosen. The encoder ordinarily opcrates only upon interrogation by the Miniac computer. The encoder is shown in Figure 4. Because the Miniao computer is 8. general-type digital computer, it can be used for many types of calculations other than mass spectrometry. Therefore, provision has been made for either calculating the results as the samples am run or for storing the peak values on punched tape for calculations a t a later time. If i t is desired to calculate the results as samples are run, the calibration data and instructions to the computer are fed into the computer on punohed tape prior to running the sample. Under this system the final reesults BTB wailable some 3 to 4 minutes after scanning a sample. An inverse matrix solution method is used for routine work. A 20 x 20 matrix may be solved in less than 2 minutes. In some
Figure 5.
Digital Computer (Atlantic Refining Co.)
1278
ANALYTICAL CHEMISTRY
cases it is desirable to calculate particular solutions for each problem and this direct solution can he accomplished by reiteration in about 6 minutes. Two photographic views of the Miniac computer are presented in Figures 5 and 6. Time Requirement Comparison. Three methods of obtaining mass spectrometric results have been employed over the past few years a t this Iahoratory: photographic record plus analog
Table 11. Time Requirements for Various Methods of Measuring and calculation of Mass Spectrometer Data (Basis. 30 20-component gas samples) PhotogrsDliio Time Record Photographic Requirement. Analog Record Man-Hours CornLlutcr 602A (IBM) 10 Instrument' 10. Chart measurement 10 10 Csleulation 30 7.5 T n. t~.~ al 60 27.5 . Man-hr./sample 1.65 0.92 II This includes sample oharging and poml,-oiit.
Sadie CPC (IBM) 8 0 3
calculator reduced this still further, to 0.37 man-hour. These figures include time for instrument pump-out, sample charging, checking, eto. It is now possible with 11/2 men, one mass spectrometer, digital conversion, and card programmed calculator computation t o report 30 gas samples (20 components) per &hour shift. ACKNOW LEDCMENT
The authors wish to thank the Atlantic Refining Co., Phil* delphia, Pa., for the photographs and description of the encoder and digital computer used in their system. LITERATURE
