Programming a Mass Spectrum Digitizer

James F. Light, W. R. Grace & Co., Clarksville, Md.

NE of the greatest problems in using 0 the Consolidated Electrodynamics Corp. Type 34-201 Mascot is the large amount of data obtained which makes data reduction a lengthy and time-con- suming operation. Either computer or hand selection to remove the unneces- sary peaks must be done before the data can be processed by the usual computer program. Normally, the data, whether from tape punch or card punch readout, must have as much of the unnecessary material removed as possible before the computer can be used. In most labora- tories a computer with enough storage capacity or, more likely, enough time available for sorting and eliminating the unnecessary portion, is generally not available. In this case, eliminating the unneeded portion a t the source of the data is preferable. This is the reasoning behind the design presented here.

The Mascot is essentially a dual ana- log-to-digital converter to be used with a CEC 21-103C Series mass spec- trometer. When attached to a normally scanning mass spectrometer, it converts

the ion current amplifier output and in- stantaneous ion-accelerating voltage into decimal digital form and actuates read- out devices which tabulate these values.

While the peak amplitude output of the mass spectrometer is being measured the mass number digitizer circuit is continuously measuring the ion-acceler- ating voltage. This digitizer is an electromechanical servo system that de- tects the decreasing ion-accelerating voltage during the mass spectrometer scan. As the servo shaft rotates, the angular position is inversely propor- tioned to the instantaneous value of the accelerating voltage. A shaft-position digitizer is attached to the servo shaft and is geared to indicate the mass num- bers. In the servo system is a double ended potentiometer (R-121). One end of the shaft of this potentiometer is unused. If the servo system had enough reserve power, this shaft could be used to drive a programmer to select the desired portions of the data output.

The main problem in programming the Mascot is selectivity. Because

space is a t a premium in the top drawer of the Mascot, no elaborate system could be installed. To obtain maximum selec- tivity, a rotating disk system was de- vised. Another problem was circuit switching. With the IBM card sum- mary punch, two circuits would have to be switched. For simplicity and to prevent any problems with the IBM card summary punch, switching the readout pulses from the Mascot was much easier than trying to control the printer and punch independently.

This method of switching has one disadvantage, however, in that only the signal being read out is printed so that if a peak is missed because of shifts in magnet current, it cannot be picked up without rescan. To prevent loss of peaks, the technician must be careful of instrument adjustments so that if shifts do take place they can be corrected before peaks are lost.

With these objectives in mind, the present programmer was designed (Fig- ure 1). (Detailed mechanical drawings are available from the author on re-

118 PLEXIGLAS x 7"DIA. 0

$ BEAR IN

TOP V I E W

MICRO SWITCH CHAIN TENSION ADJUSTING SLOTS

SIDE VIEW 6

Figure 1 . Programmer for CEC Mascot mass spectrum digitizer

VOL. 37, NO. 12, NOVEMBER 1965 1627

T O SCAN CONTROL

CAM OPERATED MICRO SWITCH

quest.) This programmer uses the digitizer servo system to drive a cam- operated micro switch which handles only biasing voltage. With a pair of miter gears and a miniature chain drive (Pic Design Corp., Long Island, N. Y.), the cam could be located in the large space to the rear of the digitizer. This allows ample room for the 7-inch diam- eter, inch thick Plexiglas cam. The rim of the cam was marked correspond- ing to the desired peaks, and notched to a depth of inch and a width of "18

inch for each peak marked. A micro switch (Micro Switch Corp., Freeport, Ill.) with a short operating stroke was modified by silver soldering a l/4-

inch length of 1/8-inch 0.d. brass rod a t right angles to the switch lever. This allowed the switch lever to ride on the surface of the cam and, because of the short stroke of the micro switch, re- quired little force to activate the switch.

To conserve space and reduce mainte-

Figure 2. Switching circuits

Components above broken line are existing parts in Mascot

nance, the two switching circuits were designed around silicon-controlled recti- fiers (Figure 2). This eliminated the problem of arcing relay contacts which could have caused interference in the Mascot. The easiest way to get to the circuits to be switched is through the rear of the cabinet where space is avail- able on the back of one of the drawers for mounting the switches. By using solid state switches, the load is removed from the programmer switch and only a low biasing voltage is switched. This bias voltage can be supplied by dry batteries or from a small d.c. power sup- PlY *

A separate switch is used to bypass the programmer when all peaks are to be read out. If, for any reason, the programmer should need to be removed, a Cinch-Jones plug is used so that by plugging in a jumper plug the instru- ment is returned to its original state.

The programmer, as described, is

being used successfully in the deter- mination of the carbon number distribu- tion of aromatic compounds by mass spectrometry (ASTM 01658). In this computer program only 67 peaks are needed, while two or three times that many may be evolved from a sample containing the smallest number of com-

The time required for a technician to eliminate the unnecessary data and re- duce the number of IBM cards to a number acceptable to the computer program has been eliminated. Using the programmer, it is now possible to go directly to the computer without the problem of eliminating unnecessary data.

' ponents.

ACKNOWLEDGMENT

The author thanks J. P. Conrey and E. V. Brown for their valuable assistance in the design and construction of the sys- tem described.

1628 ANALYTICAL CHEMISTRY