LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Analysis of a Suspected Drug Sample Eric J. Schurter,* Lois Anne Zook-Gerdau, and Paul Szalay Department of Chemistry, Muskingum University, New Concord, Ohio 43762, United States
bS Supporting Information ABSTRACT: This general chemistry laboratory uses differences in solubility to separate a mixture of caffeine and aspirin while introducing the instrumental analysis methods of GCMS and FTIR. The drug mixture is separated by partitioning aspirin and caffeine between dichloromethane and aqueous base. TLC and reference standards are used to identify aspirin precipitated by acidifying the aqueous layer and the caffeine is recovered by evaporating the dichloromethane. FTIR analysis of the isolates is intended to provide an introduction to both the basic operation of a FTIR spectrometer and experience in matching library reference spectra to FTIR spectra of unknowns. GCMS analysis parallels the wet chemistry separation and FTIR identification of the components in the drug mixture. Used as a re-introduction to GCMS, emphasis is placed on how GCMS combines sample separation and component analysis into one operation. This laboratory is intended to be part of a suite of vertically integrated laboratory exercises linked by a forensic theme. Proceeding experiments in the suite are centered on the theory and application of TLC for forensic analysis. Subsequent experiments are focused on the use and interpretation of FTIR and GCMS for analysis. KEYWORDS: First-Year Undergraduate/General, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Chromatography, Drugs/Pharmaceuticals, Forensic Chemistry, IR Spectroscopy, Mass Spectrometry, Thin Layer Chromatography
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novel pedagogical approach applying vertical integration of topics through the four-year chemistry curriculum is highlighted by an experiment that showcases the parallel use of complementary techniques. Related analytical methods for separating and identifying components in a mixture, specifically thin-layer chromatography (TLC), gas chromatography mass spectroscopy (GCMS), and solvent extraction followed by Fourier transform infrared spectroscopy (FTIR), are used in the experiment described here. The multiple laboratories in the vertically integrated sequence were developed with the initial laboratories introducing the analytical methods and emphasizing the principles of separation. Each subsequent laboratory in the sequence builds on the last laboratory and compares the different methods. As a consequence, each laboratory expands the students theoretical understanding of the method while increasing their responsibility for setting experimental parameters and operation of the instrument. In the final laboratories in the vertically integrated sequence, performed during the upper-level courses, the students operate the instruments independently to solve individual research problems. Key to this approach is that any given analytical method is revisited multiple times using different applications with increasing levels of sophistication as the student progresses through the curriculum. This vertically integrated experiment, appropriate for general chemistry, is the third in the sequence and can be used as a stand-alone experience. The components of a drug mixture (caffeine and aspirin)1 are isolated by wet chemistry methods. The isolation and identification of the components by TLC2 5 is designed to parallel the isolation of components by solvent extraction and identification with FTIR. FTIR is used to confirm the identification of the isolates and provides students with their first exposure to this analytical technique.6,7 The time needed to prepare and acquire spectra is greatly reduced by using an ATR accessory for sample handling. As a result, there is sufficient time to train students in sample preparation and FTIR operation, with Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
interpretation of the experimental FTIR spectra limited to matching unknown spectra to library standards. Additionally, the instrumental analysis method of GCMS for determining the composition of mixtures is introduced. The GCMS is used as a parallel method of analysis that combines sample separation and component analysis in one operation.8,9 Using GCMS with this laboratory helps to link GC to TLC and solvent extraction as related separation methods and link MS to FTIR as related compound identification methods. As with FTIR spectral analysis, interpretation of mass spectra is limited to identification of separated components based on molecular mass and library matching to reference mass spectra. This laboratory is intended to be part of a suite of vertically integrated laboratory exercises.10 14 As part of a vertically integrated curriculum, this laboratory’s role in the progression is to reinforce, then build upon previous work involving TLC,15 solubility,16 and GCMS17 instrumental analysis. This is in addition to introducing the instrumental method of FTIR. Subsequent chemistry laboratories expand on GCMS instrumental analysis and the introduction to FTIR by further developing theory, operation, and data interpretation. Proceeding experiments in the suite of laboratory exercises center on the theory and application of TLC for forensic analysis and the establishment of the crime scene (see the Supporting Information). A forensic theme is used to increase student interest and involvement in the experiments while serving to unite previous laboratory work.18,19 This has the added benefit of aiding recall and makes it easier for students to build upon concepts that have been introduced weeks earlier.20 25 A problem-based approach, such as solving a forensic mystery, has been shown to produce Published: August 19, 2011 1416
dx.doi.org/10.1021/ed101001u | J. Chem. Educ. 2011, 88, 1416–1418
Journal of Chemical Education
Figure 1. Structures of aspirin 1 (o-acetoxybenzoic acid or 2-acetylsalicylic acid) and caffeine, 2 (1,3,7-trimethylxanthine).
more motivated, independent students with better developed critical thinking skills.26
’ MATERIALS All GCMS data are collected in EI mode on a PerkinElmer Clarus 600 gas chromatograph and PerkinElmer Clarus 600C mass spectrometer with a PerkinElmer Elite-5MS, 30 m 0.25 mm i.d. capillary column. The FTIR spectra are collected on a PerkinElmer FTIR Spectrometer, model Spectrum RX1 fitted with a Pike Miracle ATR accessory. ’ EXPERIMENTAL PROCEDURE The experiment takes two, 3-h laboratory periods. The drugs, aspirin and caffeine, are separated by partitioning between methylene chloride and aqueous base (sodium hydroxide). The caffeine is recovered from the methylene chloride through evaporation of the solvent. The aspirin is recovered by acidifying the aqueous layer with hydrochloric acid and filtering out the precipitate. The separation demonstrates principles of solubility and how changes in pH can affect the solubility of acids (aspirin) with an emphasis on intermolecular forces.25 27 Small quantities of materials (
