adding gas chromatography–mass spectrometry data to a melting point and thin-layer chromatography...

Adding Gas Chromatography−Mass Spectrometry Data to a MeltingPoint and Thin-Layer Chromatography LaboratoryAdam M. Azman, Julie A. Barrett, Megan Darragh, John J. Esteb, LuAnne M. McNulty, Paul M. Morgan,Stacy A. O’Reilly, and Anne M. Wilson*

Department of Chemistry, Butler University, Indianapolis, Indiana 46208, United States

*S Supporting Information

ABSTRACT: The addition of gas chromatography−mass spectrometry (GC−MS) data interpretation to a thin-layer chromatography (TLC) and meltingpoint (mp) laboratory for an introductory organic course is described. Studentsare given a sample containing one unknown and a list of 24 unknowncompounds, organized in groups by similar mp ranges. Students identify theunknown through mp and TLC. Students are then given GC−MS datacontaining three unknowns and are asked to determine GC peak correspondingto their unknown using the MS spectra. Upon completion of the laboratory,students have gained an understanding of how many components are presentgiven GC−MS data and how to identify each component from the molecular ion.They also become familiar with the physical characteristics of melting point,retention factor, and retention time. These skills are utilized in other experimentsthroughout the academic year

KEYWORDS: Second-Year Undergraduate, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives,Problem Solving/Decision Making, Gas Chromatography, Mass Spectrometry, Physical Properties, Thin Layer Chromatography

Melting point (mp) and thin-layer chromatography(TLC) are often utilized in organic experiments and

are covered early in a laboratory curriculum to introducestudents to the concept of using these properties for compoundidentification. There are several experiments in the literaturedesigned as an introduction to mp,1,2 TLC,3−5 or both of thesetechniques.6 Gas or liquid chromatography can be covered atthe same time as melting point as both of these techniques relyon physical properties of compounds. Gas chromatography(GC) relies on similar physical characteristics (polarity andboiling point).Spectroscopic identification of organic compounds is usually

deferred until late first semester of organic chemistry or morefrequently to second semester. Mass spectrometry (MS),although not a spectroscopic technique, is often covered withspectroscopic methods for compound identification both inlecture and the laboratory.7 In the literature, there are examplesof structure identification using combined techniques in anupper-level laboratory.8−11 However, many undergraduatecurricula include this in the second semester of organicchemistry.Organic chemists rely on MS to confirm the molecular

weight of a compound, determine the presence of certain atomsin a molecule (Br, Cl, N, etc.), and occasionally, to determinethe most stable fragment ion (the base peak). Literatureexperiments that utilize MS often focus on full spectroscopicanalysis,7,9,12 identification of compounds by fragments,13 orlibrary searches for the best match.14 These concepts require a

familiarity with organic structures typically not achieved untillater in the curriculum.Early introduction of GC−MS analysis as an analytical tool

allowed the use of this method throughout the academic year.The goals of the addition of GC−MS to this experiment wereto have students determine the number of components by GC,identify each component by MS, and utilize GC−MS as ameans of verifying product mixtures in future experiments.

■ HAZARDS

Gloves and appropriate eyewear are required. In general, thechemicals used may be harmful if inhaled, cause respiratorytract irritation, be harmful if absorbed through the skin, causeskin irritation, cause eye irritation, and be harmful if swallowed.See Supporting Information for specific hazards.

■ THE EXPERIMENT

Students were given a sample containing one unknown and alist of 24 unknown compounds, organized in groups by similarmp ranges (see Table 1 in the Supporting Information).Students determined the mp of their unknown to narrow thelist to compounds with similar melting points. TLC analysiswas performed with the unknown along with standards of thethree similar compounds and the identity of their unknown wasdetermined similar to the procedure outlined by Levine.6

Published: November 2, 2012

Communication

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The GC−MS data for the mixtures of the eight groups ofthree melting point-related compounds was posted onlineaccording to group number. Students identified the appropriategroup by the mp of their unknown and retrieved the GC−MSdata during the laboratory period. Each student identified theirunknown compound in the mixture by locating the molecularion peak (molecular weight) (see the Supporting Information).The retention time for all three compounds was recorded andcompared to the retention factor, Rf, determined by TLC.At the end of the first semester (before a detailed discussion

of mass spectrometry), a laboratory quiz was given showing theGC−MS of an incomplete reaction. The quiz results indicatedthat 65−70% of students have gained some understanding ofGC and determined that the reaction was incomplete. Many(85%) students identified the starting material and product bytheir molecular ions in the mass spectra, determined the relativeamounts of each compound (65%), and correctly utilized all thedata presented in the GC−MS spectra (40%).

■ CONCLUSIONS

The introduction of GC−MS analysis to a physical character-istic experiment allowed the use of this instrumental methodthroughout two semesters of an organic chemistry laboratorycourse. GC−MS analysis was used in four additional experi-ments in the first semester and three experiments in the secondsemester. This has been successfully implemented in twoacademic years to over 400 undergraduate students. By the endof the first semester of organic chemistry, many students weresimply able to match the correct molecular ion to their desiredmolecular weight, but this provided a strong foundation for thedetailed discussion of MS the following semester.The analysis of mixtures by GC−MS is a common modern

method for determining purity and identification of constitu-ents. In addition to the traditional methods of mp and TLC,GC−MS analysis allowed students to have a more completeunderstanding of the composition of a mixture and theconfidence to support their answer with data analysis. Byincorporating GC−MS analysis of reaction mixtures through-out the semester, students gained an understanding of how thistechnique is utilized by synthetic chemists.

■ ASSOCIATED CONTENT

*S Supporting Information

Student experiment; instructor information; MS data for theunknowns. This material is available via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author

*E-mail: amwilson@butler.edu.

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors acknowledge the 2010 and 2011 organic chemistrystudents for experiment testing, funding from the NationalScience Foundation, Ben Clark (Rf values), and Jeff Godbey atDow AgroSciences (CI MS data).

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Journal of Chemical Education Communication

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