Robert Rothchild Science Department John Jay College of Criminal Justice City University of New York 445 West 59th Street New York, NY 10019
An Organic Chemistry Experiment for Forensic Science Majors
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Teaching organic chemistry within a program for Forensic Science majors presents a special challenge; the course is a n essential part of a satisfactory background for future required ~olicacourses in biochemistrv a n d forensic science. M a n -v a .. tions are frequently encountered by t h e practicing forensic science orofessional who must have a n understanding of ors variuu.icharacteri7ation ganic renrtim m w h a n ~ s m tosupport tests :and analyticd results, particularly if these resulti are heing prest.nrtd n i part of s o m . courtroom proceedings. To some extent, the organir chemistry course r a n he "slanted" toward the firld of specialirntiun with no sacrifice of course rontent and, i~idet,d,with enhancement oistudent motivation. Eiferrive mpthods for doinr" this would inrlude freouent use of compounds of special interest t o forensic scientists, such as drugs, controlled substances, toxic materials a n d reagents used for specific purposes i n modern criminalistics laboratories. Of particular concern i n presenting courtroom evidence is t h e degree of certainty t o he ascribed t o a n experimental conclusion. Has t h e Identity of a sample been confirmed "hevond a reasonable doubt"? A wav of introducine this concept in a numerical sense (hut n o t a philosophic one!) has develooed a s a simole and reasonable offshoot of one of t h e organiE chemistry Lboratory experiments. T h e basic experiment is one which is oarticularlv. a .. ~ ~ r .o o r i a for t e future criminologists. I t is t h e familiar use of thin-layer chromatography' (tlc) for qualitative analysis, specifically theiden. tification of d r u ~ sWhile . student analysis of suhstances such as mornhine.. heroin. cocaine.. etc... might - he e s ~ e c i a l.l vaDpropriate for eliciting student interest, there are real problems associated with access t o and inventow control of such strictly student response has regulated materials. I n practice, .been encountered during analyses of various commercial over-the-counter analgesic formulations, including assorted headache, pain, a n d cold tablets. ~~~
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Specllics Ire w~uldurge some mdification* in the performance of this rxprrimmt ,>r it 1% wmetimwdelrrit,cd in lnhorarury manuals. Common solvents such as benzene, chloroform, and carbon tetrachloride a r e classified as cancer suspect agents and should not be used for extraction of drugs from sample tablets, for solution of known reference standards or for development of the tlc chromatograms. We have managed almost totally to phase out the use of these solvents in our organic chemistry laboratory by use of substitutes such as hexane, toluene, or methylene c h l ~ r i d eAnother .~ modiGcation that we suggest is the use of commercial Silica Gel tle plates impregnated with fluorescent indicator. This allows multiple visualization techniques to be applied for more effective sample characterization. Visualization of spots can be performed with both shortwave and longwave radiation and students can note plate appearance, spot fluorescence and color while separately observing with 254 nm and 360 nm light sources. Students are often quite surprised to note the behavior of different sample spots and readily appreciate the added information for sample identification that can be provided in this way. Students should be cautioned against direct viewing of the ultraviolet light sources? Visualization with iodine (in a hood) should be applied after use of the ultraviolet lamp. To expand the experiment with development of concepts of polarity, solvent strength, and eluotropic series, students should run plates with low and high polarity solvents, such as methylene chloride and acetone, and then prepare solvent mixtures of intermediate polarity. Of special interest would be the use of the newly available reversed-phase tlc plates4 in which octyl- or oetadecylsilyl groups are chemically bonded to silica gel. With these plates, solvent strength
(eluting power) will generally increase with decreasing solvent polarity; methanol will be a more powerful solvent than water. Use of reversed-phase tlc plates is especially relevant because of the preponderance of analyses and separations being carried out on reversed-phase columns with high performance liquid chromatography (HPLC). Applications of this technique in'forensic (and other) labs is growing very rapidly. While HPLC has not yet become common in undergraduate organic chemistry courses because of cost, etc., the concepts of reversed-phase chromatography which is so useful in HPLC may be cheaply and easily introduced by using the corresponding tlc plates. Finally, students can perform a very elementary statistical treatment of their tlc results if they carry out replicate runs of their known reference materials and the unknowns. This is useful to evaluate reproducibility of student technique. Comparisons of methods involving side-by-side spotting across asingle tlc plate with spatting on different plates and with superimposed spotting of two different materials should be made and discussed from the viewpoint of achieving maximum confidence in results. For forensic science laboratory results to lead to effectiveprobative evidence. it could be useful to have a auantitative sense of haw sienificant or believable the data mav h.without reauirine an extensite hnckpmnd d~srurrmnoi~tntirti&,studens can drtrrmmr standard drr i;nionr for the ti, valut-s determined tn,m replienre sputrlnoi on a single and on different tlc plates. If different brands of commercial ~
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from a single individual with valuesbbtained from Rrvalues determined by different students. Reproducibility using a single solvent for development should he compared with results from a solvent mixture; difficulties of using mixed solvent systems should be discussed, including inaccuracies in mixture preparation by different individuals and selective solvent evaporation from the mixture. Students can determine whether Rrvalues are susceptibleto matrix effects.Thus, a student might plot average Rrtslues determined from solutions of pure known reference compounds against values obtained after extraction from a commercial tablet containine several compounds HS wrll as an excipient (tinder, zurh a\
