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Anal. Chem. 1982, 5 4 , 2121-2122
AIDS FOR ANALYTICAL CHEMISTS Determination of Chloroacetaldehyde in Air by Differential Pulse Polarography Roger G. Williams The Upjohn Company, Kalamazoo, Michigan 4900 1
Chloroacetaldehydle has been identified as a mutagen and has been assigned a ceiling exposure limit of 1ppm (3 mg/m3) by the American Conference of Governmental Industrial Hygienists. A sensitive assay method is required to monitor the workplace air at this concentration. The standard methodology involving charcoal tube atdsorption, solvent desorption, and gas chromatographic analysis fails in this case because of low recovery from the charcoal and because of the low response of the flame ionization detector toward chloroacetaldehyde (I). The NIOSH procedure for analysis of chloroacetaldehyde (2) overcomes these problems by use of a silica gel tube for sampling, 50% methanol for desorption, and an electron capture detector, The procedure gives good recoveries with very high sensitivity, lbut attendant upon it are the problems associated with use of the electron-capture detector. These include nonlinearity (3) and variations in response with changes in detector temperature (4) or carrier gas purity (5). In addition, if the detector is used intermittently, the time required for stabilizaticin after start-up lowers the efficiency of the assay. The operation manual for the detector in our laboratory recommends a stabilization time of several hours, and in practice an overnight wait has been necessary. Therefore, especially in situations in which assays are to be run at irregtdar intervals, the polarographic method of analysis may offer a simpler alternative. The equipment is relatively inexpensive, the setup time is short, and the analysis time is competitive with that for gas chromatography. Accordingly, we have investigated the use of polarography in the determination of chloroacetaldehyde. The polarography of chloroacetaldehyde (as well as its diand trichloro analogues) has been studied extensively (6-8). Reduction waves are seen, but the current is lower than expected because of hydration of the carbonyl group. In the case of some other carbonyll compounds, sennicarbazone formation has been shown to enhance polarogralphic detectability (9). A similar enhancement for chloroace taldehyde would give sensitivities in the rztnge required for air analyses.
EXPEltIMENTAL SECTION
All chemicals were reagent grade and were used without further purification. The nominal 50% chloroacehtaldehyde solution waq purchased from Pfaltz ,and Bauer. The solution was standardized by the titration procediure of Yasnitmikii and Satanovskaya (IO). A PARC Model 303 static mercury drop electrode was used, with a Ag/AgCl reference electrode and platinum wire counterelectrode. A PARC Mcdel174A polarographic analyzer was used, operating in the differential pulse mode with a drop time of 1 s, scan rate of 2 mV/a, modulation amplitude of 50 mV, and sensitivity of 5 WAfs. The differential pulse polarograms were recorded with a Hewlett-Packard Model 7015A XY recorder. The polarographic ,solution of citrate buffer, EDTA, semicarbazide, and lithium chloride was made up according to the formula of Afghan and co-workers (9). While the solution could be stored for a period of weeks, a gradual increase was seen in a blank peak in the region of interest. For maximum sensitivity, it was found desirable t o use the solution within 1 day after
addition of the semicarbazide hydrochloride. The samples were collected on NIOSH-type large silica gel adsorption tubes obtained from SKC Inc. The tubes contained a front section of 520 mg of silica gel and a back section of 260 mg. The two sections were added to separate 10-mL portions of the polarographic buffer. The solutions were heated to 70-80 OC (with occasional shaking) for 10 min, cooled to room temperature, and decanted from the silica gel into a 10-mLpolarographic cell. Nitrogen was bubbled through the solutions for 4 min prior to analysis. The potential was scanned from -0.7 V to -1.5 V (vs. AglAgCU.
RESULTS AND DISCUSSION A freshly prepared solution of chloroacetaldehyde in the semicarbazide-containing buffer gave a differential pulse polarograpic peak at -1.10 V (Figure lb). When the solution was allowed to stand at room temperature for approximately 1h, the initial peak decreased to about half its original height, and a second peak grew in at -0.96 V (Figure IC).This process was accelerated by heating of the solution. A polarogram carried out in the same buffer except in the absence of semicarbazide showed no detectable peak under the same conditions. In order to determine the cause of the time dependence of the polarographic behavior, we studied the reaction of chloroacetaldehyde with semicarbazide on a larger scale. The lH NMR spectrum of chloroacetaldehyde in D 2 0solution showed a doublet at 3.5 ppm and a triplet at 5.1 ppm relative to Me4Si (J= 6 Hz). Addition of semicarbazide hydrochloride led to the immediate formation of a white precipitate. The reaction was accompanied by a shift in the NMR peaks to 4.1 ppm for the doublet and 7.3 ppm for the triplet. Elemental analysis of the isolated product showed that the chlorine atom was still present. The data are consistent with formulation of the initial product as the straightforward semicarbazone I (Figure 2). Polarography of the product showed it to be the source of the peak a t -1.10 V (Figure Ib). Several sets of conditions were tried for conversion of I to the species giving the polarographic peak at -0.96 V. Heating of an aqueous solution led to degradation to undefined products. Treatment with base, however, gave a product with the expected polarographic behavior. The reaction was carried out most conveniently by treatment of an aqueous solution of I with the hydroxide form of Dowex 1-X8. The IH NMR spectrum of the product was unchanged except for broadening of the triplet at 7.3 ppm so that it was not resolved. Elemental analysis showed that the chlorine atom had been lost. When the reaction of I with Dowex 1-X8 was carried out in methanol solution, a polarographic peak was again generated a t -0.96 V, and the NMR spectrum showed that a methoxy group had been incorporated. It is therefore clear that the reaction being observed in the polarographic cell is solvolysis of the chlorine atom in I to 111,probably through the intermediacy of I1 (11). As confirmation, the polarographic assay was carried out on authentic methoxyacetaldehyde, and the -0.96 V peak was again seen.
0Q03-:!700/82/0354-2121$01.25/0 0 1982 American Chemical Society
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Anal. Chem. 1982, 5 4 , 2122-2123
Table I. Recovery of Chloroacetaldehyde from Silica Gel Tubes p g of ClCH,CHO added
% recoverya
4.8 a
A
I a
.o
-0 6
-1.0 E,V
8
.1 2
-1 4
Flgure 1. Differential pulse polarograms of 10 pg of chloroacetaldehyde in 10 mL of semicarbaride-citrate buffer: (a) buffer blank, (b) freshly mixed sample solution, (c) sample solution after heating to
80
OC.
+
C1-CHi-CHO
HI 0
YzN-NH-C-NHz
11 0
C1-CHi-C-d
I1 N 1
YH c=o 1
I
NHI
1
1
IO-CHI-C-H I1
KOH
r-
H2C.C-H
OHc ROH
1
c__
u I
VH 1
99
19.3
102
Average of three determinations.
(Table I) showed satisfactory recovery a t all levels. The relative standard deviation for six replicates at the 9.6 pg/tube level was found to be 2.8%. The lower limit of detection of the assay is approximately 1pg/tube, equivalent to about 0.1 x TLV for a typical sample and therefore adequate for assay at the levels required. The sensitivity of the polarographic method itself is inherently higher, but it is limited by the variable blanks associated with “clean” silica gel tubes, which average about twice the buffer blank. Among common interferences which may be encountered in the assay, formaldehyde is the most serious, giving a strong peak at the -0.96 V potential. Chloral gives a peak in the same region, but with about one-tenth the response of chloroacetaldehyde. The peak for dichloroacetaldehyde comes at -0.68 V and does not interfere. Acetone gives two peaks at -1.25 V and -1.34 V, causing no interference unless the level is extremely high.
ACKNOWLEDGMENT
C=O 1 NHi
