edited by H. BATT Kenyon College Gambier. OH 43022
RUSSELL
Mass Spectrometer Interface with an Apple 11 Computer John W. Stahl Oeneva College Beaver Falls, PA 15010 Mass spectroscopy is one of the basic techniques widely used in the structural study of chemical compounds. The need for experience with the mass spectrometer in the undergraduate laboratory is obvious. We have been able to upgrade a low-cost mass spectrometer into a very functional instrument using a commercial computer interface, some hardware modification, and custom-written software. The Varian EM-600 is a 60° sector, magnetically scanned mass spectrometer with a range of 1to 300 mass units (u) and an electron impact source. I t was introduced in the eady 1970'8 as a low-cost educational instrument. Original specifications indicated a scan linearity of 5%. hut over the years our instrument has degraded to the point where correlation of peak position with mass is very difficult and inaccurate. An Apple 11+ computer was available to us at no cost for dedicated use on this instrument. The Adalab interface hardware card is available a t moderate cost and provides one 12-hit AID input and one 12-bit DIA output channel.' The mass spectrometer is configured so that the computer controls the mass scan and simultaneously collects the intensity data. The overall configuration of the system is shown schematically in Figure 1.
if desired. With this modification, a digital output from the computer controls the masslcharge ratio selected by the mass spectrometer. By incrementing this digital output in a software loop, the mass is scanned. The reading of mass intensities from the mass spectrometer required no modifications,as a 0- to 1-V recorder output is available, which is connected directly to the AID converter. The Varian EM-600 has a selectable dc filter on the output that is useful for removal of noise in the signal.
Software A program entitled "MASS" was written in Applesoft Basic to control the mass spectrometer. Although Basic is a slow language in execution. the fact that the Varian EM600 is a maenetieallv scanned instrument. ~~. ~" ~, suhject to hysrererii,alrendy limitr themass .scanningrnre. Thc contni d t h c I) A and A, D interfaces is simplified grenrly hy thr use of one-word QUICK110 commands in the Basic program. QUICK110 is the operating sytmm that the manufacturrr provides t o control input to, output from, and timing oi t h r ADALAH rnterface. 'The . Drorram MASS is menu-based. To use the computer-interfaced mass spectrometer, a mass calibration experiment must be performed periodically. Per~
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fluorinated hydrocarbons are frequently usrd as mass standards. W e have also fvund an equal-volume mixture ofbrom
