A Forensic Science Laboratory Experiment Molecular Weights of Drugs and Diluents by Osmometry Robert Rothchild City University of New York, John Jay College of Criminal Justice, 445 West 59th Street, New York, NY 10019 Forensic science mav be defined as the application of the natural sciences (chemistry, biology, phys&s) to studies of physical evidence relevant to matters of law. The forensic scientist must he a eeneralist. broadlv trained in manv fields. because of the vasiarray of different kinds of pbys;cal evi: dence that mav move informative in a case. The ahilitv to apply diverse ai&tical techniques to characterize uniquely, or individualize. a sample. . . lies a t the heart of forensic science. In order to offer trulv. probative testimonv. . . . whether for the defense ur the pruserution, as much curroboration as possible s h ~ ~ uhe l d availahle to, fur example, identify a snmplc. Forensir science students should be able toapply as many techniques as art. available that wuuld help to arhieve this end. Analysis of illicit drug samples is a major prohlem encountered by forensic laboratories. Street drugs are rarely pure, having often been adulterated or cut with a variety of substances. These substances may serve merely as a filler or diluent. nossessine little or no nhvsioloeical activitv in themseives. Thus, Tactose and mannitol arecommonly &ed to extend various "white ~owders."Alternativelv. cheaper and more readily availahle sibstances are often used to i i m i c or substitute for a more exoensive. more desirable material. Amphetamine or methamphetamine ("speed") may be used as a diluent for cocaine, since all three drugs share central nervous system stimulant effects. Sometimes a local anesthetic mav be added to amphetamines to simulate cocaine's anesthetic effect. Forensic identification of various substances incorporated into a confiscated drug sample can suggest common origins and suggest patterns of drug traffic and use. Different batches can be individualized based on a "chemical fingerprints" of the specific substances and quantities present. We have used an Osmette A model 5002 automatic osmometer (Precision Systems, Inc., Sudbury, MA 01776) t o characterize different drugs. diluents. and adulterants commonly encountered in samples of forensic interest. This unit operates on the principle of a freezing point depression of a solvent produced by sblutes. A threeydigit LED display directly indicates the milliOsmolality of the aqueous sample. Osmolality refers to the total concentration of all (non-solvent) particles present in the solution, whether ions, molecules. etc. Thus. a 1m solution of NaCI. assumine near-complete'dissociatibn, would yield a 2 0sxn soluti&. I t is the concentration of imnuritv-derived oartides which is measured by Osmolality andkhiLh direct1;correlates with such colligative properties as freezing-point depressions. The instrument usedoperates by first &percooling the sample aolutim and then induring nucleation bv hiah-amplitude \ibration of a metal "stirring rod." ~ a m ~ l e & m ~ e r a tthen & e rises to the actual solution melting point. The experiment serves t o ilhsrrate not unly the bas'k concepts of freeing point depress i m as a rolligative property but also the sometimes confusing aspects of supercooling and latent heats of fusiun associated with these systems. The instrument permits very rapid ( 1 5 min) determinations with good reproducibility and permits use by large numhers of students with little training. Since forensic labs are often called upon to work with exceedingly small samples, this technique has a virtue in being capable of producing useful rcsults with milligrams of mate-
672
Journal of Chemical Education
rial. If a microbalance is availahle to allow accurate nreoara. . tion of solutions from milligram-sized samples, the forensic utilitv is considerahlv enhanced. However. a standard analytical balance reading to 0.1 mg is quite satisfactory for our purposes. Even 1mg accuracy can suffice if larger sample size is available. In this case a larger quantity of (or more concentrated) solution can he prepared and then aliquots can be taken to get a useful sample concentration. We would recommend that unknowns be assigned to students. Such unknowns might include solids such as sucrose, lactose, mannitol, glycine, as well as liquids such as ethanol and acetone. If controlled drugs are availablefor these experiments, salts of amphetamine, methamphetamine, codeine, and various barbiturates are ~articularlv as un. a~nronriate .. . knowns which should generate considerat~lestudent interest. Hased on the experimental milliOsmolalitv of their solutions. students can calEulate effective molecular weights. Where salts are examined, students must appreciate the total number of anionic and cationic particles produced by the sample. The molecular weight of a sample is commonly obtainable by mass spectrometry, but such expensive instrumentation may not he a part of often-underfunded forensic labs. Indeed, even when such instrumentation is availahle, many samples fail to produce observable molecular ions, particularly under electron impact ionization conditions. The t w e s of watersoluhle compounds of greatest interest here inciude salts and carbohvdrates which are freouentlv"mite . reluctant to oroduce molecular ions. In a sense, the freezing point depression method can be regarded as a relativelv low cost comolement to mass spectrometry. Furthermore, varinus deviations from idealitv in sulutions mav t ~ eassociated to different deerees with different samples. T h e errors in molecular weight determination based on freezing point methodology may thus provide a unique capability for sample individualization. While our runs have utilized the Osmette's 2 mL sample holders, 0.2-mL sample holders are available. If samples are prepared directly in these smaller holders, weights of the unknowns being analyzed may be just a few milligrams and the accuracy of a microbalance would be justified. For a typical undereraduate lahoratorv if sufficient sa&Dle is .exneriment. . providkd to allow adequate concentrations when lo.& g of water is added. the 2-mL samole holders can be used conveniently, and a" ordinary anaiytical balance will suffice for weighings. In practicing forensic laboratories. of course. the sensitivzy provided h; use of 0.2-mL sample tubes and a microhalance are essential. A number of materials availahie in our labs were analyzed by this method. We routinely weighed the sample to 0.1 mg and diluted it with 10.00 g distilled water. Concentrations on the order of 1% w/w were used unless solubility problems limited this. Samnles were analvzed a t least in trinlicate. E:xperimentally determined milli0smolality \.alum could he redicated usuallv within *I l e s t dienificant dieit. Calibration wss carried out ;mmediately betbre the runs (and rechrcked immrdintely after) using 100 and 500 mOsm standard solu-
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Johnson, Jr., R. El. and Hoch. H.. in "Standard Methods of Clinical Chemisby," Vol. 5, S. Meites, (Editor-in-ChiefJ,Academic Press, New York, 1965, p. 764.
tions of NaCl containing 3.094 and 15.93 g NaCl per Kg H20, respectively.' Agreement with theory was fairly good for nonionic substances. Larger deviationsobserved for ionizable
materials are consistent with incomplete dissociation, ion pairing, etc. The accompanyingtable summarizes some typical results.
Typical Student Results
Sample Weight, m9'
Average experimental miiliosmoiaiityb
Theoretical Numbr of Nan-Solvent Particles per "Molecule"
31.5 28:3 80.7 178.7 35.
I 1 1 1 1
360.3 360.3 182.2 75.1 151.2
363.2 383.7 178.8 75.8 157.5
201.7
114.4 100.9 108.5 135.5 137.8 (in 25.00g H20) 105.8 109.5 140.3
64.7 67. 130
3 3 2
142.8 142.8 100.9
163.2 163.4 107.9
272.8
108.2
71.8
2
136.4
151.1
248.3
111.4
78.8
2
124.1
141.4
184.2
92.4 (in 20.009 H20)
339.8
98.8
55
2
169.9
179.8
Calculated Fwmula We2ht. M
Sample lactose monohydrate maltose monohydrate mannitol glycine acetaminophen CHsCONHCeHIOH (Tylenolm) ephedrine sulfate U.S.P. (CsHsCHOHCHCHsNHCHs).H2SOI ephedrine hydrochloride N.F. (CeHsCHOHCHCHaNHCH&HCI Rocaine hydrochloride NH2CsHP02C2H4N(C2H5)2.HCI (Novacainm) Sodium pentobarbital H
360.3 360.3 182.2 75.1 151.2
428.5
Theoretical m9/ miiliOsmol
Experimental mgf miiliO~moi
CH-CH / 0 GH, (Amand) Sodium barbital
(Amend) Ccdeine sulfate N F
1
0
,
CHX) OH .H2SO4. 5H20 (Merck) Meperidine hydrochlorideU.S.P. C& WEQC e~~cH&ICi (Demerol") (S.B. Penick) Cocaine hydmchloride U.S.P.
& .HCI
0,rn.H. (S.B. Penick)
Sample was dissolved in 10.00 g H , 0 except as noted. 'Experimentaiiy determined milliOJmolaliNis the average of three to six runs.
Volume 59
Number 8
August 1982
673
