Mass Spectrometry for Large Undergraduate Laboratory Sections

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edited bv SUSAN H. HIXSON

Highlights

National Science Foundation Arlington, VA22230 CURTIS T. SEARS, JR. Georgia State University Atlanta, GA 30303

Projects supported by the NSF Division of Undergmduate Education Mass Spectrometry for Large Undergraduate Laboratory Sections A. Illies, P. B. ~hevlin,' G. Childers, M. Peschke and J. Tsai Auburn University, AL 36849-5312 Mass spectrometry is routinely covered in undergraduate organic chemistry courses and a number of valuable laboratory experiments featuring its use have been discussed (1-7). Although such experiments work well a t institutions with limited laboratory enrollments, we typically teach laboratories with enrollments of 160 or more in which it is difficult to allow each student to carrv out a meaningful "hands on" mass spectrometry experiment. Since we feel that some uraetical exoerience with this technique is important, we have designed a simple gas chromatography-mass spectrometry (gc/ms) exercise that allows each student to analyze the products of a simple synthesis that they have performed. The exercise starts with the microscale S N synthesis ~ of l-bromobutane from l-butanol a s described by Williamson (8). The students complete the synthesis and place one drop of the distilled product in a screw capped vial. The vials are then sealed, labeled with the students name and taken to the mass spectrometry laboratory by a teaching assistant. Students are instructed to sign up for a 20-min block of time over the next few days in order to analyze their sample. When the student arrives a t the laboratory, he or she adds 1 ml CHzClz to the sample and injects 0.3 p of the solution into the gas chromatograph. The samples typically contain the l-butanol starting material and the 1bromobutane product along with traces of dibutyl ether. The figure shows a mass chromatogram along with the mass spectra of the starting material and product from an actual student run. For this analysis to be applicable to large numbers of students. the .. ec seoaration must be a s . rapid 81s pr~.ssihlr.\Ve have Iwtrn ;il,lt: tu :ln.~lvzet:;~rlisamvlc: in 6 minutes 011 a :lO nl DB-5cav~llar\'column w ~ t hthe ?allowing temperature program: 70°C foi 1min, 70-80 "C a t 10 " C h i n , 86-140 "C a t 67.5 "Clmin, 140-210 "C a t 70 " C h i n , and 210 "C for 1min. Amass range of 20-200 amu is scanned with a solvent delav of 2 min. Under these conditions each analysis takes the student nhout 10 rnin and two itudrnti art. svhrdulrd o w 20 rnin i h c k . Since the instrument is Lnder computer control, students operate the computer during the run. As the peaks appear on the mass chromatogram, their mass spectra are obtained and the student decides which corresponds to product and evaluates product purity and the structure of impurities. There is ample time to display all spectra, conduct library searches, and print data. This relatively simple laboratory exercise has the advantage of allowing each student to carry out a n analysis on his or her own product. The fact that a brominated product is obtained introduces a discussion of isotopic patterns in mass spectrometry. The experiment is scheduled to coin'Author to whom correspondence should be addressed.

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. 0 0 0 Retention time (minutes)

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l-butauol

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l-bromobutane

a) Mass chromatogram of a typical student sample. b) Mass spectrum of 1 butanol. c) Mass spectrum of l-brornobutane. cide with the lecture discussion of spectroscopic structure determination and after S N reactions ~ have been covered. One of our mass spectrometer satellite data stations is interfaced with a "3-gun" projector in a large lecture hall allowing the display of actual mass spectra from our instrument to lecture sections. Acknowlegement T h i s work w a s p a r t i a l l y s u p p o r t e d by a g r a n t , DUE9350846, from the National Science Foundation Division of Undergraduate Education Instrumentation and Laboratory Improvement Program. Literature Cited 1. Bru3h.R. C.: Rice, G. W J Chem. Edue. 1994.71.A293-A294

6. Mabb0tt.G. A J. Cliem. Educ. 1990.67.4415. 7. Hill. D. W.; McSharry.B.T.;li.rupek.J Chern. Educ. 1988. 65,907-10, 8. Williamson. K. L. M n c m m i e nnd Microscnl~Organic Experiments. 2nd ed.: D. C. Heath: Lexington. MA. 1994:pp 24731.

Volume 72 Number 8 August 1995

717