Adding Gas Chromatography-Mass Spectrometry Data to a Melting Point and Thin-Layer Chromatography Laboratory Adam 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 melting point (mp) laboratory for an introductory organic course is described. Students are given a sample containing one unknown and a list of 24 unknown compounds, organized in groups by similar mp ranges. Students identify the unknown through mp and TLC. Students are then given GC-MS data containing three unknowns and are asked to determine GC peak corresponding to their unknown using the MS spectra. Upon completion of the laboratory, students have gained an understanding of how many components are present given 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 experiments throughout 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 M elting point (mp) and thin-layer chromatography (TLC) are often utilized in organic experiments and are covered early in a laboratory curriculum to introduce students to the concept of using these properties for compound identification. There are several experiments in the literature designed as an introduction to mp, 1,2 TLC, 3-5 or both of these techniques. 6 Gas or liquid chromatography can be covered at the same time as melting point as both of these techniques rely on physical properties of compounds. Gas chromatography (GC) relies on similar physical characteristics (polarity and boiling point). Spectroscopic identification of organic compounds is usually deferred until late first semester of organic chemistry or more frequently to second semester. Mass spectrometry (MS), although not a spectroscopic technique, is often covered with spectroscopic methods for compound identification both in lecture and the laboratory. 7 In the literature, there are examples of structure identification using combined techniques in an upper-level laboratory. 8-11 However, many undergraduate curricula include this in the second semester of organic chemistry. Organic chemists rely on MS to confirm the molecular weight of a compound, determine the presence of certain atoms in a molecule (Br, Cl, N, etc.), and occasionally, to determine the most stable fragment ion (the base peak). Literature experiments that utilize MS often focus on full spectroscopic analysis, 7,9,12 identification of compounds by fragments, 13 or library searches for the best match. 14 These concepts require a familiarity with organic structures typically not achieved until later 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 were to have students determine the number of components by GC, identify each component by MS, and utilize GC-MS as a means of verifying product mixtures in future experiments. ■ HAZARDS Gloves and appropriate eyewear are required. In general, the chemicals used may be harmful if inhaled, cause respiratory tract irritation, be harmful if absorbed through the skin, cause skin 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 a list of 24 unknown compounds, organized in groups by similar mp ranges (see Table 1 in the Supporting Information). Students determined the mp of their unknown to narrow the list to compounds with similar melting points. TLC analysis was performed with the unknown along with standards of the three similar compounds and the identity of their unknown was determined similar to the procedure outlined by Levine. 6 Published: November 2, 2012 Communication pubs.acs.org/jchemeduc © 2012 American Chemical Society and Division of Chemical Education, Inc. 140 dx.doi.org/10.1021/ed300490x | J. Chem. Educ. 2013, 90, 140-141