Anal. Chem. 1084, 56, 1907-1912
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Detection and Differentiation of Nitrocellulose Traces of Forensic Science Interest with Reductive Mode Electrochemical Detection at a Pendent Mercury Drop Electrode Coupled with Size-Exclusion Chromatography J. B. F. Lloyd Home Office Forensic Science Laboratory, Priory House, Gooch Street North, Birmingham B5 SQQ, England
I n acetonltrlie, wlth the addition of 5% v/v water and 0.01 M tetramethylarnmonlum perchlorate, the nitrocellulose hydrodynamlc voltammogram at a pendent mercury drop electrode reaches a maxlmum at 0 V vs. Ag/AgCI and remains negligibly changed at lncreaslngiy cathodic potentlais. Because of the low residual current and noise level at electrode potentials in this region, nttrocelluloses may be detected In amounts as small as 100 pg In slze-excluslon chromatography effluents. The linear range extends to approxlmateiy 200 ng. Some examples are given of the detectlon and dmerentlatlon of nltroceiiuloses In trace amounts of propellants, explosives, palnts, lacquers, celluloids, adheslve, and a wood-filler composition.
A variety of the materials of significance to forensic science work contain nitrocelluloses. Of particular importance are firearms propellants, and explosives compositions, but there are many other common occurrences including paints, lacquers, adhesives, inks, various films and laminates, and artifacts in the bulk plastic, e.g., in celluloid and related materials (I). Often, the amount available for characterization is in the nanogram to microgram region at the most, which is well below the sensitivity limit of many of the usual techniques applied to polymeric materials. Neither are many of the techniques adequately selective. Frequently the detection of nitrocellulose in complex mixtures containing other polymers and monomeric nitrate esters is required. In explosives traces work, thin-layer chromatography with detection of the nitrite produced on the treatment of nitrocellulose with alkali (2) is used on occasions and distinguishes between nitrocelluloses (3,4). Sensitivity limits of 100 ng (3) and 300 ng (5)have been reported. The technique and others are comprehensively reviewed by Yinon and Zitrin (6). In other forensic applications infrared spectrometry is often used. It is, however, incapable of distinguishing between nitrocelluloses varying in molecular mass distribution, which is a major differentiating feature. Reductive mode electrochemical detection combined with high-performance liquid chromatography has been introduced for the identification of monomeric nitrate esters in explosives traces. Thin-layer electrodes (7) or a pendent mercury drop electrode (8)may be used. With the latter the detection limits for 18 explosives compounds are now in the range 2-20 pg, the nitrate esters lying in the higher part (9). The mercury drop electrode is renewable when necessary (between or during chromatograms) and affords, therefore, an important advantage over thin-layer electrodes, which are susceptible to a cumulative poisoning by adsorbed substances (8). Deleterious adsorption effects typically affect the electrochemistry of polymeric species, but with the use of a renewable electrode the consequences are insignificant within the limits of a single
chromatogram run under the appropriate conditions. Hence, the detection limits for nitrocelluloses may be improved now by several orders of magnitude, as the following describes. EXPERIMENTAL SECTION Nitrocelluloses. Industrial grade samples were obtained through the courtesy of the Imperial Chemical Industries Organics Division, U.K., and the Materids Quality Assurance and the Royal Armament Research and Development Establishments, Ministry of Defence, U.K. Eluent. To 1.74 g (0.01 mol) of tetramethylammonium perchlorate (Fluorochem, U.K.; Fluka purum grade) was added 50 mL of water and a similar volume taken from 1L of acetonitrile (Rathburn Chemicals, U.K.; HPLC S grade). The perchlorate was dissolved by sonication, and the solution added to the remainder of the acetonitrile. Chromatography Pumping System and Sample Introduction. The chromatographs were based on either a dual-piston reciprocating pump (Laboratory Data Control; Constametric pump model 111)fitted with the manufacturer’s pulsation dampener or a high-frequency single-piston reciprocating pump (Applied Chromatography Systems; Model 300/01). The detector noise levels due to eluent flow pulsation were least in the latter case. Usually the flow rate was set at 1.5 mL m i d , which minimized noise when the dual-piston pump was in use. To exclude oxygen, the eluent reservoir was maintained under reflux, and all pipe work was of stainless steel rather than of poly(tetrafluoroethy1ene). Samples were deoxygenated in a modified 1-mL glass syringe mounted on an injection valve (Rheodyne; Model 7125) fitted with a nitrogen purge through one of the waste lines (10)and with a 10-pLsample loop. In turn from the pump, the eluent line also included a silica presaturation column, a dump valve to enable the upstream part of the system to be rapidly purged, an injector bypass (II), and a precolumn filter (Scientific Systems; part no. A-3100-2). Size-Exclusion Columns. Lichrospher Si 300 5 pm (Merk) was slurried in methano1:chloroform (l:l, v/v) (12) and driven into 250 X 4.5 mm columns with methanol at a constant flow rate of 4.5 mL min-’. The pressure rose to approximately 330 bar. Hence, compared to the usual constant pressure technique, particle breakdown was minimized. One packed column (250 X 5 mm in this case) was silanized (13). Before use new columns were purged, e.g., overnight, until their chromatographic performance was stabilized. Detection. The electrochemical detector was an EG&G Brookdeal Princeton Applied Research Model 310 polarographic detector and a Model 174A control unit operated in the direct current mode, usually with a time constant setting of 0.3 s. The Ag/AgCl reference electrode compartment contained 5 M aqueous lithium chloride. The flow cell was modified as previously described (14) so that the distance between the tip of the mercury capillary and the jet, from which the eluent projects at the drop, could be varied. Also, the jet orifice was expanded to 0.8 mm. All of the results were obtained with a tip-to-jet separation of 0.25 mm and, except for the flow rate experiments, with a drop size of 3 mg given by the “M” setting on the dispensing control. Under these conditions the “pendent” drop is partly supported by the eluent flow, and provided that all of the pipe work is of stainless steel the deoxygenation of the contents of the eluent receiver in
0003-2700/84/0356-1907$01.50/0Published 1984 by the Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
which the flow cell is immersed is unnecessary (14).The mercury drop was renewed before the injection of each sample. The signal from the detector was presented to a variable-range strip chart recorder, by which the displayed readout could be expanded to 400 pA full scale. The ultraviolet absorbance of the eluent was monitored with a detector (Pye Unicam; model LC-UV) inserted in-line prior to the electrochemical detector. Chromatograph Standby and Setting-Up Conditions. The previously refluxed contents of the filled eluent reservoir were left on standby at room temperature with a nitrogen purge discharging into the lower part of the reflux condenser. An eluent flow of 100 pL min-' was maintained through the whole system. When the system was required for use, the purge was introduced into the body of the reservoir, and the eluent brought to reflux. The pump was turned off meanwhile. After 5 min the dump valve was opened, the eluent flow was set at 8 mL min-', the pump and the presaturation column were purged for 3 min, and then the flow was reduced to 1.5 mL min-' throughout, with the dump valve closed. Given day-to-day operation,at the wual working electrode potential of -200 mV w. Ag/AgCl, the system could be made ready for use within 30 min from cold (standing current less than 500 PA). With more negative electrode potentials the setting-up time increased because of the increased sensitivity to oxygen. If the system became entirely aerated approximately 90 min was necessary. Sample Manipulation. On contact with glass surfaces nitrocellulose in dilute solution was found to decompose. When such contact could not be avoided, e.g., in the samples' deoxygenation, the surface was silanized. Even so, the contact was kept to a minimum because a slow decomposition could still occur. Otherwise, sample solutions were handled in polyethylene/propylene centrifuge tubes and in centrifugalmicrofilters fitted with poly(tetrafluoroethy1ene) 0.2-pm membranes (Bioanalytical Systems). Solid samples were dispersed in acetonitrile, filtered, and diluted with the eluent. When a sample could not be dispersed by vigorous agitation and heating, as in the case of the majority of plastics other than nitrocellulose, brief sonication was applied. No significant effect attributable to shear-degradation occurred in any sample. RESULTS AND DISCUSSION Chromatography Conditions. Usually tetrahydrofuran
(15,16)or, on one occasion, ethyl acetate (17)has been used as eluent in the size-exclusion chromatography of nitrocellulose, but because of their poor ion-solvating characteristics they are of restricted value in electrochemical work. In this respect, acetonitrile is superior. The nitrocelluloses of interest were freely soluble in it, and the solutions (e.g., 1 mg mL-') were tolerant to the presence of up to 20% v/v of water. Hence, water could be added to the eluent to provide the proton source generally necessary for the extensive reduction of organic functional groups and to supress adsorption and tailing effects during the chromatography. On bare silica surfaces typical of the packing material employed here adsorption effects often occur (18). They are evident in the present system, but unimportant within the constraint that the objective was the development of a rapid and sensitive fingerprinting technique rather than the acquisition of accurate molecular mass data. The packing (Lichrospher Si 300) is one of the few suitable 5 pm materials available commercially in loose form. Columns can be made from it readily at,a fraction of the cost of bought-in columns. Size-exclusion chromatograms from samples of industrial grade nitrocelluloses are shown in Figure 1. The system's total exclusion and permeation volumes were 1.29 mL and 3.14 mL, respectively, corresponding, e.g., to the large and the small peaks in chromatogram A. The mass-average relative molecular mass (MARMM) values quoted by the suppliers of the samples varied between 279 000 and 19 500, as indicated in the figure caption. Clearly, this range considerably exceeds the permeation limits of the column packing (for polystyrene in chloroform the total exclusion limit given by the manu-
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Figure 1. Size-exclusion chromatograms of samples of a blasting grade nitrocellulose (A) and some nonexplosive8 grades (6-E). The MARMM values quoted by the suppliers are 270 000 (A), 140 000 (B), 75 000 (C), 40 000 (D), 19 500 (E). The amounts represented vary between 10 ng and 70 ng, with arbitrarily varied instrumentalsensitivity. The chromatograms were run on an unmodified SI 300 column, with detection at -200 mV vs. Ag/AgCI. Other conditions are given in the Experimental Section.
facturer is (1.5-3) X lo6)as does the distribution within several of the samples. However, the chromatograms rapidly make obvious the differences between the samples, both in their overall molecular mass distributions and in the composite nature of the distribution in some cases. At the flow rate used (1.5 mL min-') the total permeation volume is reached in about 2 min; and the flow rate may be doubled if necessary with slight effect on chromatographic performance (see below). Apart from the addition of water or other highly polar compounds, silanization of the silica supresses the adsorption (13,18). Some publications have dealt with the chromatography of nitrocelldoses on the commercially available columns of such materials, with ultraviolet absorbance detection (17, 19). An in situ silanized column was used in the present work, with unchanged eluent, when adsorption effects might have influenced the results significantly, as in the subsequently described examination of flow rate effects. For most purposes the two types of column gave equivalent results, and unmodified columns were used. The tetramethylammonium perchlorate used as the supporting electrolyte gave a lower residual current and more stable solutions than a variety of other ammonium, tetraalkylammonium, and lithium salts. If the use of perchlorates is objectionable, e.g., under circumstances where the detection of traces of perchlorate-based explosives or propellants may be of interest, tetrabutylammonium tetrafluoroborate may be substituted. This gives equivalent results except that a longer time is taken for the base line to stabilize after a new electrode drop has been formed. Electrochemical Detection. The hydrodynamic voltammograms plotted in Figure 2 show the variations with the working electrode potential of the cathodic current peak response from chromatograms of a nitrocellulose (MARMM ca. 50000) and of nitroglycerin and the corresponding variations of the residual current and its noise level. In general, the response from the nitrate esters is due to their reduction to nitrous acid, which may be reduced in turn under some conditions (20). Typical voltammetric behavior is illustrated by the plot for nitroglycerin in Figure 2, where
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the chromatographic noise level (O), the reslduai current (0),and the peak response (A)to nitrocellulose(NC) and nitroglycerin(NG). The respecthre quantltles were 200 ng and 3.5 ng. The conditions are given in the Experimental Section (unmodified column: L.D.C. pump).
no response is seen at potentials less negative than -600 mV vs. AgJAgCl, but beyond which point a continuous increase occurs. The behavior of nitrocellulose is in striking contrast. A substantial response is seen even at positive potentials, which reaches a maximum a t 0 V and varies only slightly beyond this point. Even in the region of +300 mV a cathodic contribution due to nitrocellulose can still be seen against the high anodic residual current from the dissolution of the electrode. At this (offscale, Figure 2) point the residual current was -32 nA. Although the ease of reduction of monomeric nitrate esters increases with the degree of nitration (21),the present result for nitrocellulose is assumed to be promoted by the adsorption onto the electrode of a reduction product, whose free energy of adsorption offsets the free energy change in the formation of a solvated species (22). The signal-to-noise ratio for nitrocellulose (Figure 2) is at a maximum in the region of -200 mV, where the residual current and the noise level are 200 pA (cathodic) and less than 3 PA, respectively. At more positive potentials the sensitivity to oxygen, which is largely responsible for the residual current, rapidly falls, and the deoxygenation of the eluent might be omitted. However, the rise in the noise level from the increasingly anodic current at increasingly positive potentials would degrade detection limits considerably. The dual-piston pump was used for these particular results. Overall, the high-frequency pump employed in most of the work gives lower noise levels. Flow Rate Effects. The variation in the relative integrated response of the size-exclusion peaks with eluent flow rate (silanized column) is shown in Figure 3 for two partly excluded nitrocelluloses (examples E and C, Figure 1) detected at -200 mV, and for the totally permeating nitroglycerin detected at -1 V. To preserve the stability of the electrode drop at high flow rates its size was reduced to 1.5 mg in all of these experiments. A wider diameter colum than usual (5 mm as opposed to 4.5 mm) was used throughout to provide higher flow rates at the detector for a given back-pressure. The linear flow rate in the column correspondingly decreased by a factor of 0.81, and the retention volumes correspondingly increased by 1.23. The results are expressed relative to the number of
Figure 3. Variation with eluent flow rate of the relative integrated response of size-excluslon peaks from nitroglycerin (O),and samples of nitrocellulose with quoted MARMM values of 19 500 (0) and 75 000 (0). The sample mass was 26.7 ng in each case. The detectlon was at -1 V (nitroglycerin) and -200 mV (nitrocellulose) vs. Ag/AgCi. A silanized column was used.
nitrate groups present, calculated for the nitrocelluloses from the 11% nitrogen content specified for both of them by the supplier. The specified MARMM values are 19 500 and 75 OOO. As Figure 1 indicates, the molecular mass distributions are characteristically broad and complex. At the lowest flow rate the results (Figure 3) given by the nitrocelluloses are much less than the nitroglycerin results. As the flow is increased the response rises virtually linearly, and the nitroglycerin plot is overtaken by the lower molecular mass nitrocellulose plot. The response from each of the materials at the highest relative to the lowest flow rate is 2.7, 5.9, and 6.5, in order of increasing molecular mass. The result for nitroglycerin agrees with the value 2.65 predicted from the dependence of the limiting current on (flow rate)1/2for flow cells of similar configuration (23). However, in logarithmic coordinates all of the plots are nonlinear and cannot be represented by the usual equations for hydrodynamic electrodes. This is due at least partly to the variation of the effective area of the electrode with the flow rate (14).The difference between the nitrocelluloses is expected from the inverse dependence of polymer diffusion coefficients on molecular masses, which presumably could be applied independently in the acquisition of molecular mass data. For nitroglycerin, evidently the effect of a relatively high diffusion coefficient must be countered by a reduced rate of electron transfer at the electrode surface. Because the relative response of nitrocelluloses differing in molecular mass varies with the flow rate, the apparent peak retention volumes of these highly disperse samples should vary also. However, any such effect has been too small to be detected, although minor differences have occurred at the extremities of some distributions. The coefficients of variation of the retention volumes of the three samples (Figure 3), in the previous order, were 1.3% (nine results), 2.7% (seven results), and 0.9% (ten results). The peak width at half height varied to some extent. For nitroglycerin it was at a minimum at 1 mL min-l and increased by 70% at 3.5 mL min-l. For the lower molecular mass nitrocellulose the width increased by 14% between 0.5 mL min-I and 3.5 mL min-l. For the other nitrocellulose no significant trend was detected; the coefficient of variation was 1.4%. Detection Limit, Range, and Reproducibility. The variation in chromatographic peak height of 100 pg to 10 ng amounts of an explosives grade nitrocellulose is shown in Figure 4. Clearly, an amount as low as 100 pg can be detected
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Table I. Plastics Materials Teated for Possible Interferences in the Detection of Nitrocelluloses
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ABS (acetonitrile-butadiene-styrene copolymer) cellulose acetate epoxy resin melamine-formaldehyde neoprene nylon phenol-formaldehyde polyacetal polyacrylonitrile polycarbonate polyester polyethylene polyisocyanurate poly(methy1 methacrylate) polypropylene polystyrene poly(tetrafluoroethylene) poly(viny1 chloride) poly(viny1idene chloride) polyurethane urea-formaldehyde
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Figure 4. Size-exclusion chromatograms (unmodified column) from the indicated amounts of an explosives grade nitrocellulose, with electrochernlcal detection at -200 mV vs. Ag/AgCI.
readily. The limit is governed largely by the degree of overlap of the low molecular mass region with the totally permeating contaminants. Other nitrocelldoses have given similar results. Twofold-increasing amounts from 100 pg to 204 ng of the nitrocellulose gave a linearly increasing peak response. In log-log coordinates the coefficient of linear regression of the peak height on the mass injected was 0.997, which is not significantly different from the expected value of 1. The correlation coefficient was 0.995. There was no detectable dependence of the peak width at half height (beyond the 3.2 ng point, where the contribution from the totally permeating peaks became negligible) or of the retention volume on the mass injected. The respective coefficients of variation were 3.8% and 1.7%. In another experiment, a sequence of 10 replicate injections of 10-ng amounts gave the respective coefficients of 2.0% and 0.72%, and for the peak height, 3.3%. In the range 204 ng to the upper limit examined, 1.6 pg, the peaks became increasingly distorted. Their height relative to the mass injected decreased, their low molecular mass wings were steepened, and their apparent retention volume decreased. In this region and in the higher part of the linear range, the peaks could be detected by their ultraviolet absorbance (220 nm). No significant variation in peak retention volume and profiie from those detected by the electrochemical technique in its linear range occurred. Presumably, the linearity of the electrode response is limited by a deactivation due to the adsorption of the reduction product. A related effect was seen if the electrode drop was left unchanged between chromatograms, when the response decreased after a number of injections had been made. Were it necessary to detect quantities beyond the linear range of the present technique, the deactivation could be avoided by polarographic detection. However, for trace analysis, involving the detection of peak currents in the pA region, this technique is precluded by the high noise level characteristic of the dropping electrode. Specificity. Samples of 21 common plastics materials, listed in Table I, were processed under the same conditions as for nitrocellulose (working electrode, -200 mV vs. Ag/AgCl), the volume of injected extract corresponding to 10 pg per sample. Most samples were poorly soluble in acetonitrile and generally lack readily reducible functional groups. Some halo
nSamples2, 5, 9, and 19 were from a laboratory collection; the others were from a collection available from the Building Research Establishment, Department of the Environment, U.K.
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Flgure 5. Size-exclusion chromatograms (Unmodified column) of neoprene (A), ABS (B), and nitrostarch (C); the amounts represented are 10 pg, 10 pg, and 15 ng, respectively: (full Ilne) electrochemical detection (-200 mV vs. Ag/AgCI) at the Indicated sensitivities;(dotted
line) ultraviolet absorbance detection at 220 nm; the amplitude of the most Intense peak is 0.0044 absorbance unlts.
compounds are reducible, however, and one of the three chloropolymers, neoprene, gave a weak and partly excluded peak (Figure 5A). On a mass basis the peak height was a fador of approximately 0.002 that of nitrocellulose. Presumably, because the sample did not disperse significantly, the peak is due to a minor component. It was not detected at a potential of 0 V and, therefore, is unambiguously differentiated from nitrocellulose. One other sample, ABS (Table I; Figure 5B), gave a partly excluded peak, which was clearly distinguished from nitrocellulose by a first-derivative-like profile, probably due to an adsorption process, and by a coincidence with an intense, undistorted ultraviolet absorbance (220 nm) peak. No other of the samples gave any partly excluded, electrochemicallydetected peak. Many samples gave cathodic peaks a t the total permeation volume, which are assumed to be due to additives and impurities. The only substance likely to be confused with nitrocellulose is nitrostarch, which occurs as a rare explosives component. A chromatogram of a sample, from wheat starch, is shown in Figure 5C. However, the sample's voltammetric curve was clearly distinguished from that of nitrocellulose by a cathodic
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
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Flgure 6. Size-exclusion chromatograms (unmodified column), with detection at -200 mV vs. Ag/AgCI, from samples of a firearms propellant (A), a semigelatinous explosive (B), an ammongelignite (C), and
two nitrocellulose-based spray palnts (D). The amount represented of each sample and the maximum peak amplitude are, respectlvely: 100 ng, 6 nA (A); 25 ng, 0.4 nA (B); 1 pg, 2.5 nA (C); 50 ng, 2.5 nA (D).
displacement of approximately 200 mV. Presumably the reduction product is less strongly adsorbed on the electrode. Examples. In Figure 6 are shown chromatograms from a propellant (A), two explosives (B, C), and two nitrocellulose-based spray paints (D). The amounts represented were varied between 25 ng and 1pg according to the sample composition. These quantities are well above the detection limits. The only preparation required for each sample was its dissolution and filtration. The propellant nitrocellulose (A, Figure 6) appears at the exclusion limit as a spike that extends in a broad peak to the total permeation limit. The chromatogram from the semigelatinous explosive (B) shows the presence of two partly resolved distributions. The pattern is highly reproducible, it is independent of the amount injected within the linear range, it is independent of the potential of the working electrode, and is the same on a silanized column. Hence, the separation is a size-exclusion process, which is not significantly influenced by adsorption processes that might result, e.g., in the separation of fractions having different nitration levels. The chromatogram from an ammon-gelignite (C) indicates the presence of a nitrocellulose with a molecular mass distribution largely beyond the column’s exclusion limit. The occurrence of a substantial proportion of the sample in the relatively high molecular mass range is a usual feature of propellants and explosives grades of nitrocellulose (1,15). The spray paints (D, Figure 6) were automobile touch-up paints indistinguishable in color. Their peaks are at the same point, but are distinguished by differing dispersions. The samples were also differentiated by pyrolysis gas chromatography. The two techniques reflect independent features and, therefore, are complementary to one another. However, the size-exclusion technique is substantially the more sensitive. The results shown represent 50-ng amounts, which could be reduced considerably if necessary. In Figure 7 are shown the results given by three samples of celluloid-type materials (A-C) and by samples of an adhesive (D), a fingernail lacquer (E), and a wood filler (F). All of these vary widely between one another as the chromatograms show. Two other samples of nail lacquer were not distinguished from the sample shown, In the examination of firearms propellants it is often of interest to determine whether a composition is double or single based, i.e., if nitroglycerin is present or not. This is readily effected if the electrode potential is reduced to or below -700 mV. Examples are shown in Figure 8, where chromatograms
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Flgure 7. Size-exclusion chromatograms (unmodified column), with detection at -200 mV vs. Ag/AgCI, of three different celluloid-type materials (A-C, 80 ng each), an adhesive (D, 20 ng), a fingernail lacquer (E, 100 ng), and a wood filler (F, 180 ng). The respective peak amplitudes are 1.1, 2.6, 2.6, 1.3, 8.4, and 1.4 (2nd peak) nA.
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Flgure 8. Size-exclusion chromatograms (unmodified column) from 50-ng amounts of single- and double-based propellants, with detection at -200 mV vs. AglAgCl (full Ilne) and at -770 mV vs. Ag/AgCI (dotted
line) and at the same sensitivity in each case. detected at -200 mV and at -700 mV are compared. The difference between the chromatograms at the two potentials in the single-based case is negligible, only nitrocellulose is apparent; in the double-based case also a strong peak due to nitroglycerin is apparent at the total permeation volume. If the potential is further reduced the peak is greatly intensified (-700 mV was used here to retain the peak on-scale). The dependence of the peak response on the electrode potential could be used to distinguish some other electroreducible compounds occasionally present in propellants, e.g., aromatic nitro compounds and degraded stabilizers. It is unlikely that such compounds could be confused with nitroglycerin, from which the response is particularly intense at optimal potentials. The principal limitation here is the presence of the negative peak, presumably an electrode adsorption effect, that occurs at strongly cathodic potentials and perturbs totally permeating peaks when small samples, e.g., less than 30 ng, are taken. This peak is weakly apparent in Figure 8. Marked
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differences are present between the molecular mass distributions of the propellant nitrocelluloses in Figure 8 and between these and the example in Figure 6. The three propellants could be differentiated solely on this basis. CONCLUSION
It is emphasized that the technique described here is designed purely as a fingerprinting technique. Obviously, for the acquisition of accurate molecular mass data a chromatography system having wider exclusion limits and calibrated with well-characterized standard nitrocelluloses would be required. However, in its intended usage the present technique now enables both the sensitivity and the selectivity of detection of nitrocellulose-containing traces to be greatly increased and will make amenable to characterization very much smaller amounts of all of the materials mentioned above than is possible with any of the other usual techniques. Among the more important applications is likely to be the detection of firearms propellants and their residues in handswabs and on clothing. The technique should be applicable directly in the analysis of the residues from clothing found by Douse et al. (24) with their micro vacuum-sampling technique. This area will be examined specifically in future work. Registry No. Nitrocellulose, 9004-70-0. LITERATURE CITED (1) Bogan, R. T.; Kuo, C. M.; Brewer, R. J. I n "Kirk-Othmer Encyclopedia of Chemical Technology", 3rd ed.; Grayson, M., Ed.; Wiley: New York, 1979; Volume 5; pp 129-143.
(2) Lloyd, J. B. F. J. Forensic Scl. Soc. 1967, 7 , 198. (3) Elle-Calmet, J.; Forestler, H. Int. Crim. Police Rev. 1979, 325, 38-47. (4) Peak, S.A. J. Forensic Sci. 1980, 2 5 , 679-681. (5) Douse, J. M. F. J. Chromatogr. 1982, 234, 415-425. (6) Ylnon, J.; Zltrln, S. "The Analysis of Explosives"; Pergamon: Oxford, 1961. (7) Bratin, K.; Kissinger, P. T.; Briner, R. C.; Bruntlett C. S. Anal. Chlm. Acta 1981, 130, 295-311. (8) Lloyd, J. B. F. J . Chromatogr. 1983, 257, 227-236. (9) Lloyd, J. B. F. Proceedings of the International Symposium on Analysis and Detection of Explosives; FBI Academy, Quantico, 1983; in press. (10) Lloyd, J. B. F. J . Chromatogr. 1983, 156, 323-325. (11) DICesare, J. L.; Dong, M. W.; Gant, J. R. Chromatographla 1982, 15, 595-598. (12) Kirkland, J. J. J. Chromatogr. 1976, 125, 231-250. (13) Kirkland, J. J.; Antie, P. E. J. Chromatogr. Sci. 1977, 15, 137-147. (14) Lloyd, J. B. F. Anal. Chim. Acta 1983, 154, 121-131. (15) Cunnlngham, A. F.; Heathcote, C.; Hlllman, D. E.; Paul, J. I.Chromatogr. Scl. 1980, 13 (Liq. Chromatogr. Polym. Relat. Mater. 2), 173-196, and references therein. (16) Yau, W. W.; Kirkland, J. J.; Bly, D. D. "Modern Size-Exclusion Liquid Chromatography"; Why: New York, 1979, and references therein. (17) Sjobom, R. A.; Oreston, H. G. Propellants Expbs. W80, 5 , 105-110. (16) Unger, K.; Ringe, P. J. Chromatogr. Scl. 1971, 9 , 463-466. (19) Marx-Flglnl, M. Papier 1982, 3 6 , 577-582. (20) Kaufman, F.; Cook, H. J.; Davis, S.M. J . Am. Chem. SOC.1952, 74, 4997-5001. (21) Whitnak, G. C.; Nielsen, J. M.; Gantz, E. St. C. J. Am. Chem. Soc. 1954, 76,4711-4714. (22) Heyrovskq, J.; Kuta, J. "Principles of Polarography"; Academlc Press: New York, 1966; Chapter 16. (23) Stulik, K.; PaEkovB, V. J. Electroanal. Chem. 1981, 129, 1-24. O'Callaghan, K. A. Proceedings of the Inter(24) Douse, J. M. F.; Jane, I.; national Symposlum on Analysis and Detection of Explosives; FBI Academy, Quantico, 1983; in press.
RECEIVED for review January 10,1984. Accepted April 9,1984.
Polarographic Adsorptive Complex Wave of Light Rare Earths with o -Cresolphthalexon Xiaoxia Gao* and Manping Zhang' Department of Chemistry, Peking University, Beijing, People's Republic of China
The adsorptlve complex wave of rare earths wlth o-cresolphthalexon can be used In the polarographlc determlnatlon of light rare earh elements, such as lanthanum and praseodymlum. I n the slngle-sweep polarography, the reductlon POtentlal of the complex wave Is more negative than that of the free dye, and the amount of peak potential dlfferences decrease wlth the Increase of atomlc number In lanthanide serles. The peak height of the complex Is proportlonal to the concentratlon of light rare earth Ions In the range of lo-' to M. This wave has been proven to be an adsorptlve complex wave by experlments of cycllc voltammetry and double-step chronocoulometry. The surface excesses of the adsorbed specles were calculated. I t was further found that the praseodymlum-o -cresolphthalexon complex does not dlssoclate but reduces directly on the mercury electrode surface In the complex form.
China's rare earth resources are richly endowed by nature. In general, analyses of small amounts of metals by means of polarographic methods are rather quick and easy to perform Present address: Department of Chemistry, Shandong College of Oceanology, Quingdao, People's Republic of China.
and polarographic instruments are less expensive. But unfortunately, most rare earth ions, except Eu, Sm, and Yb, are not reduced at the mercury electrode in aqueous solutions, and so there are few papers which deal with the polarographic analysis of rare earths. Willard and Dean (2) reported that the polarographic wave of Eriochrome Violet B splits if aluminum ion is added to the solution. Florence (2) has shown that the polarographic wave of Eriochrome Violet B splits in the presence of rare earth ion in DC/AC and single sweep polarography. Thakur (3) proposed a method for determination of gadolinium with cathode ray polarography by using this dihydroxyazo dye. We have been working on the polarographic study of rare earths. We have found that several dyes and organic reagents, which are reducible at the mercury electrode, can be used as complexing ligands in forming adsorptive complex waves of rare earths (4-7), among which some systems were used for the determination of trace rare earth contents in ores, phosphors, and plants (8, 9). In this paper we shall discuss another triphenylmethane dye, o-cresolphthalexon (OCP) and its adsorptive complex wave with rare earths in some detail. EXPERIMENTAL SECTION Apparatus. An electrochemistry measuring instrument, Model DHZ-1, a single-sweep oscillopolarograph, Model JP-lA, a voltammetric analyzer, Model 79-1, a hanging mercury electrode,
0003-2700/84/0356-1912$01.50/00 1984 American Chemical Soclety
