Adaptation of a Mass Spectrometer for Gaseous Samples Of d l the insrruments m thr modern lahoratury. the interpr~rat~on of mass spectra is rhe easiest fur the student LU undemrand. The mass spectra bf ..lmple moleculer pravlde a lurid manner fur preseormg the cuncepta of atoms and molecules; thus, it is advantageous to introduce students to the mass spectrometer while these concepts are being taught. To accomplish this, the spectrometer must he very easy t o operate, samples should have no preparation, the compounds must be very simple, and the entire procedure from the sample gathering to completed spectrum should take only a few minutes. Gas samples are excellent for this purpose because they are simple molecules, available anywhere (air, breath, natural gas, auto exhaust, etc.) and easily collected using disposable plastic syringes. The Varian EM-600 mass spectrometer, an instrument commonly found in laboratories devoted to undergraduate education, is suitable for this use. To convert the EM-600 to accept gaseous samples, which can be done in half an afternoon using material found in every stockroom and no special tools, the capillary sample injector is removed and replaced with a glass tube of the same diameter. The glass tube is attached to a I-ft length of soft copper tubing using Picien wax. At least 6 in. of glass tubing should be exposed so that heat from the injection port does not melt the wax. The copper tubing is terminated with a Swedgeloc coupling, and a septum is placed in the other side of the coupling. The copper tubing is fastened to a board which is held in d a c e hv the weieht of the soectrometer. so that samoles mav be iniected " without disturbing the glass tubing. Approximately 4 ml of gaseous sample a t atmospheric pressure produces an adequate signal. The figure shows several spectra obtained on the altered EM-MK) using samples collected by freshman chemistry students. The spectra demonstrate the instrument is sensitive enough to show the presence of argon in air. The different chemical compositions of air, breath, and auto exhaust are dramatically illustrated, and the lines due to fragments are easily identified and explained to students because of the simple nature of the parent compounds. Other samples which give excellent spectra are taken from the air space inside bottles of soda pop, methanol, ethanol, acetone, ammonia, ether, chloroform, i.e., any substance having an appreciable vapor pressure. The air in the samples serves as convenient reference peaks. Our students have shown enthusiasm and interest when allowed to gather, inject, and record a spectrum themselves; and they attain quickly the ability to interpret and explain the results. Washburn University Topeka, Kansas
D. P. Miller
Volume 57, Number 8, August 7974
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