indicated which coefficients required adjustment. The treatments were modified as necessary to control the coefficientsfor accurate estimates of C. Data from the analyses were used to construct the following set of five simultaneous equations for mixtures containing about 6 % water:
1.00 CI
+ 1.00 cz + 1.00 c3 + 1.00 C4
0.95 Ci
+ 0.38 Cz + 0.72 C3 + 0.00 Cq
0.95 Ci
+ 0.93 Cz + 0.93 C3 + 0.00 C4
0.98 CI
+ 0.60 Cz + 0.27 C3 + 0.00 Cq
0.00 Cl
+ 0.00 cz + 0.00 c3 + 0.00 C4
+ 1.00 C5 = TI
(9)
+ 0.07 C5 = T2
(10)
+ 0.00 C5 = T3
(11)
+ 0.00 C5 = Td
(12)
+ 1.53 C5 = T5
(13)
where, C1--5 = concentrations of diacetyl peroxide, di-ptoluoyl peroxide, p-methylbenzyl hydroperoxide, peroxyacetic acid, and peroxy-p-toluic acid, respectively. The coefficients are unity in Equation 9 because total peroxides were determined without prior reduction by sulfide. The coefficients in Equations 10, 11, and 12 represent the fraction of each peroxide remaining after the sulfide reductions. Each coefficient times initial concentration of the individual peroxide represents the contribution of that peroxide to the titer found for this particular analysis. In the fifth analysis, T5is a direct measure of the peroxy-ptoluic acid which was extracted into the organic phase; therefore, the value of the coefficient in Equation 13 (1.53) is a function of the partition coefficient
and does not involve a rate constant. The coefficient is greater than unity because of a reduction in volume during the extraction. The first-order kinetics requirement for the sulfide reduction was demonstrated experimentally for each peroxide-sulfide combination whose coefficient was less than 0.93 but greater than 0.07. (For coefficients outside the range, either so little or so much of the peroxide has reacted that first-order kinetics is not necessary.) The method was tested with a mixture of known peroxides. The results are shown in Table 11. The accuracy of the method was good; the error ranged from 2 to 14% for the exact solution. A low concentration of peroxyacetic acid was calculated, although none was present in the mixture. This level of peroxide (0.0001 meqiml) was about the level of detection for the method as developed. When the concentration of peroxyacetic acid was set equal to zero and the four remaining peroxide concentrations were calculated by least squares method, the errors ranged from +2.7% to -12.2%. This least squares analysis decreased the relative error for four of the peroxides and increased it very slightly for one. CONCLUSIONS
The wide choice of sulfides and reaction conditions which can be used lends flexibility to the presented method of determining quantitatively the concentrations of individual peroxides in a peroxide mixture. The method should be adaptable to analyses of many different peroxide mixtures. T o apply the method to other mixtures, one must know which peroxides are present and determine the coefficients by tests on pure samples of each peroxide. RECEIVED for review September 3, 1971. Accepted January 13, 1972. Paper presented at the 159th National Meeting of the American Chemical Society, Toronto, Ontario, May 1970.
Determination of Dichloroacetylene in Complex Atmospheres Frederick W. Williams Chemistry Division, Code 6180, Naual Research Laboratory, Washington, D.C.20390
SEVERAL METHODS have been published on the determination of the extremely toxic compound, dichloroacetylene (DCA), but these methods are limited in effectiveness to concentrations far in excess of the toxicological limit (1) or are very time consuming (2-4). Previously, in enclosed environmental systems where problems have been experienced with DCA (5), the precursor compound 1,1,2-trichloroethene (TCE) was monitored (6). Dichloroacetylene is unstable under certain conditions and becomes spontaneously explosive when present in moderate concentrations in air, but at low concentrations it (1) American Conference of Governmental Industrial Hygienists, Threshold Limit Values of Airborne Contaminants, Cincinnati, Ohio, 1969. (2) J. Siege],R. A. Jones, and L. Kurlansik, J. Org. Chem., 35, 3199 (1970). (3) J. H. Wotiz, F. Huba, and R. Vendley, ibid., 26,1626 (1961). (4) M. E. Umstead and R. A. Saunders, Naval Research Laboratory, Washington, D.C., unpublished data, 1965. ( 5 ) R. J. Defalque, Clin. Pharmacol. Ther., 2,665 (1961). (6) L. S. Young, NASA TM X-62,004, October, 1970.
is quite stable when stabilized by compounds such as ethers, or 1,1,2-trichloroethene (7). In the evaluation of atmospheres of enclosed environmental systems, the analysis for DCA is further complicated by the myriad of other compounds in the atmosphere. One other factor which makes such atmospheres even more complex is the potential interaction of air revitalization equipment with these contaminants (8). Several papers have appeared in the literature reporting the use of the gas chromatograph-microcoulometer combination for the determination of chlorinated hydrocarbons (9, IO). The microcoulometer is particularly attractive as a detector (7) D. W. F. Hardie, Kirk-Othmer Encycl. Chem. Techno/.,2nd ed., 5,203 (1964). (8) R. A. Saunders, Arch. Eiir;iron. Health, 14,380(1967). (9) D. M. Coulson and L. A. Cavanagh, ANAL.CHEM.,32, 1245 (1960). (IO) J. A. Stamm, “Lectures on Gas Chromatography-1964,’’ L. R. Mattick and H. A. Szymanski, Ed., Plenum Press, New York, N. Y., 1965, p 53. ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1317
were a 6-foot X l/&. stainless steel column packed with Porapak Q (Waters Associates, Inc.), 8OjlOO mesh; 6-foot x '/*-in. stainless steel column packed with diisodecylphthalate, 30% by weight on Chromosorb G, DMCS, and AW, 60/80 mesh; and polyethylene glycol 400, 15% by weight on Chromosorb P, 80/lOO mesh. The chlorinated hydrocarbons were purchased from Chem Service Incorporated. The chlorofluorocarbons were obtained from Du Pont. The dichloroacetylene was obtained from the U. S . Navy Toxicology Unit, Bethesda, Md. The method for preparing the standards is described elsewhere
HEATED INLET ( A LU\MIN U M 1
a.
H
MICROCOULOMETER
INTEGRATOR
1' I
a
(11).
RECORDER
SEPARATION COLUMN
--c
RESULTS AND DISCUSSION
PYROLYSIS
COULOMETRIC 4
FURNACE
CELL
I
I
MICROCOULOMETER
Figure 1. Block diagram of the experimental apparatus a . Gas chromatograph column instability studies b. Furnace temperature us. detectability curves
because its response (9) should be directly represented by Coulomb's law, and thus, if decomposition products from the DCA could be introduced into the coulometric cell in a titratable form, standards would not be necessary. A selective method has been developed for determining chlorinated hydrocarbons in complex atmospheric mixtures at the part per billion level (11). The method employs a gas chromatograph with a microcoulometric detector. An oncolumn concentrating step with a Porapak column is used to achieve the high sensitivity. Unfortunately this method cannot be used directly for the analysis of DCA, because this compound partially reacts with the Porapak and cannot be eluted quantitatively. The method described in this paper takes advantage of the fact that certain column materials can stabilize the DCA during the separation process. After elution, further advantage is taken of the instability of DCA which is then selectively pyrolyzed and the products are titrated coulometrically. EXPERIMENTAL
Equipment and Chemicals. For the studies on DCA instability on gas chromatographic columns, a MicroTek MT200 gas chromatograph equipped with a Dohrmann Microcoulometer operating in an oxidative mode with a silver cell was used. The furnace temperature-detectability curves were obtained using a Dohrmann Microcoulometer and a Carle gas sampling valve. When the coulometric peaks were amenable to automation, they were processed with a Hewlett-Packard 3370A electronic integrator. Block diagrams of the two experimental apparatus are given in Figure 1. A method has since been developed to automatically process all coulometric peaks and will be published at a later time. The gas chromatographic columns employed in the various determinations (11) F. W. Williams and M. E. Umstead, ANAL. CHEM., 40, 2232
(1968). 1318
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
The detection of a chlorinated hydrocarbon by microcoulometry depends first on passing the compound through a quartz tube filled with quartz chips in a pyrolysis furnace. The chlorine in the decomposition products is then titrated as a chloride ion with silver ion in a coulometric cell. The furnace according to the Dohrmann instruction manual (12) should be maintained at 850 "C. During a routine analysis of atmosphere samples, it was observed that with the pyrolysis furnace turned off,the coulometric detector responded to a gas sample containing DCA stabilized with TCE when separated on a diisodecylphthalate column. An attempt was made to determitle the exact nature of the thermal stability of the DCA sample by injecting gas samples containing 640 ppm of DCA stabilized with TCE into the chromatograph equipped with a diisodecylphthalate column at 115 "C at various pyrolysis furnace temperatures. At a pyrolysis furnace temperature of 7 5 "C, 43 % of the DCA was decomposed. At approximately 500 "C, the DCA was 100% decomposed and titratable by the coulometric cell. Finally, at 850 "C, both the DCA and the TCE were quantitatively decomposed and titrated with the microcoulometer. The initial 43 % decomposition of the DCA at 7 5 "C was due to an interaction of the compound with the diisodecylphthalate column when operated above 70 "C. This premature decomposition of the DCA on the analytical column tends to broaden the peak. An additional component of the experimental set-up which could influence the stability of the DCA was the heated aluminum transfer line from the gas chromatograph to the microcoulometer (See Figure 1). The stability of the DCA with respect to the temperature of the transfer line was determined. The data show that DCA which has been separated in the G C column is stable at temperatures up to approximately 170 "C over aluminum. The exact stability of the DCA as compared to other chlorocarbons in the pyrolysis furnace was determined. This was done to explore the possibility of selectively decomposing the DCA. Figure 2 is a plot of furnace temperature cs. per cent of chlorinated hydrocarbon decomposed, that is, the percentage titratable by the coulometer. These data were obtained by direct injection of standard gas mixtures into the pyrolysis furnace, without the use of a gas chromatographic column. As can be seen from Figure 2, of the compounds studied, CFC13, CC14, CH3CC13,CHCI3, CF2C12,and CHK12 would interfere with the determination of DCA for selective decomposition if a furnace temperature of 500 " C was used. It should also be noted that CClF3 cannot be determined quantitatively unless the furnace temperature is maintained at (12) "Sub-Micro Elemental Analysis by Microcoulometry," Dohrmann Instrument Co., J. A. McNulty and L. W. Hoppe, Ed,
Mountain View, Calif., 1969.
a ln W
g z 8 w a
0 CHsCCI3 0 CIC
i CCI ( S T A B I L I Z E D / C H C I * CC12)
+ CFC13 x
cc14
Figure 2. Thermal stability of various chlorinated hydrocarbons over quartz
100
200
300
400
500
600
700
800
900
1000
F U R N A C E T E M P E R A T U R E , 'C
li
+
Figure 3. Line chromatogram of chlorinated hydrocarbons that have been identified in enclosed environments with a PEG 400 column
-
N
0 N
I 0
I
I
I
I
I
I
I
T I M E , MIN
1100 "C, which is 250 "C above the recommended temperature for this apparatus. The finer detail of the decomposition curves is beyond the scope of this paper, but it should also be noted that the CH3CC13curve is clearly a two-step decomposition with the first chlorine appearing at about 200 "C. The reaction probably yields vinylidene chloride, as shown in the equation, since the second portion of the decomposition curve corresponds exactly to the vinylidene chloride decomposition curve. CH3CC13 + HC1
+ CHFCC12
Neither of the above two methods tried for the determination of DCA in complex chlorinated hydrocarbon mixturesfinding a suitable gas chromatographic column which would resolve the mixture and not react with the compound of interest, or selective decomposition of the DCA with subsequent analysis by microcoulometry-proved completely satisfactory. Consequently, a combination of the two techniques was studied in an attempt to solve the problem. A separation column was needed which would stabilize the isolated DCA and prevent premature decomposition. One
particular gas chromatographic column packing which stabilizes DCA when it is separated from an atmospheric stabilizer is polyethylene glycol 400 (PEG 400) (13). The stabilization is probably due to the ether linkages in the PEG, since it is known that ethers are stabilizers for DCA (3). The exact mechanism of this stabilization is not known. Figure 3 is a line chromatogram of chlorinated hydrocarbons of a typical closed environmental atmosphere sample as separated on a 6-foot X 1/4-in,PEG 400 column at 38 "C. Because of the slow response of the microcoulometric detector and the close proximity of several chlorocarbon retention times, if DCA is present at low concentration in the presence of CC12F2,CHF2Cl, CC13F, CC12FCC1F2,CBrFzCBrF2, and/or CH2=CC12, it might be masked. Using a PEG 400 column at room temperature just prior to the pyrolysis furnace, Figure l b , a series of DCA samples in nitrogen stabilized with diethylether vapor were determined at
(13) M. E. Umstead, Naval Research Laboratory, Washington, D.C., unpublished data, 1965. ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1319
Table I. Chlorinated Hydrocarbon Profiles for Some Typical Enclosed Environmental Samples Concentration (mrnP .~ Samples Compound A B C D E F G CFiCli 52 18 11 29 36 3 12 CFiClCFZCl 1 0.4 0.7 0.9 1 0.02 0.7 11 3 0.4 3 8 0.2 CFCla 0.5 CH?=CCl? . . .b ... ,.. 0.4 0.2 4 0.03 CF*ClCFCl? 0.1 0.09 0.2 0.02 0.06 0.2 0.03 CF2BrCF2Br 0.2 ... ... 0.1 0.1 ... ... CHC13 0.1 ... 0.01 0.03 0.06 0.1 0.03 CHaCC13 0.2 0.2 0.2 0.1 0.6 4 0.03 CHCl=CClz 0.04 ... 0.01 ... ... ... 0.03 ccl,=Ccl~ 0.03 ... ... 0.03 0.02 0.05 ... 12.0 11 .o 0 0 0 0 ClC=CCl (Added) 0 ClCECCl (Found) 12.0 10.6 ... ... ... ... thep precision ofthemethodis ~ k 5 above 2 lopprn, i.10zabove 1 pprn, and = 2 0 z above IOOppb, and *50% above 10 ppb. * Less than 0.01 pprn if present. ~
100 -
a
0 W
I
0
I
cIc:ccI
I-
$
80-
PRESEPARATION O N PEG 400
I-
W
0
a
60-
V
0
12.0
w
1
d j 0
-
20
I
I
$--
N 0 PR E SE PA R A T ION
I
1
1
I
I
FURNACE TEMPERATURE,
OC
Figure 5. Selective decomposition of an enclosed environmental sample doped with DCA
various furnace temperatures. The resulting curve of chloride found is given in Figure 4. The curve shows that at a furnace temperature below 250 "C, only one of the two chlorine atoms of the DCA molecule is detected in the coulometric cell. Thus the molecule is being 50% "decomposed." The lower curve labeled "No Preseparation" is for the same set of experiments except the PEG column was replaced with an empty tube. The lower curve is essentially a repeat of the DCA curve in Figure 2; that is, the DCA is stable in the quartz pyrolysis furnace t o about 225 "C in the presence of diethylether. The results depicted in Figures 2 and 4 show that if the coulometric furnace is maintained at below 200 "C with preseparation of the sample on a room temperature PEG 400 column, low level concentrations of DCA can be determined without interferences. Since only half the available chlorine in the molecule is titrated in the coulometer under these conditions, the final concentration must be multiplied by two. By this means, DCA can be determined quantitatively in the presence of high concentrations of the interferences listed previously. In addition to the compounds shown in Figure 2, other chlorinated hydrocarbons which were typically found in the more complex enclosed atmospheres were tetrachloroethene, cis- and trans-dichloroethene, p-chlorotoluene, trifluorobromomethane, 1,l ,Z,Z-tetrafluoro- 1,2-dichloroethane, chloroethene, dichlorofluoromethane, trifluorochloromethane, trichloromethane, I,l,l-trichloroethane, and 1320
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1,I ,2-trichloroethane. It was determined experimentally that none of the above compounds interfered with the method. Table I contains the chlorocarbon profiles of a number of atmosphere samples taken from enclosed environmental systems. DCA was deliberately added to two of these atmospheric samples (A and B). Two chlorinated compounds in these samples, CF2C12and CFC18,can cause significant interference with DCA during a normal GC analysis of these samples using a PEG 400 column and pyrolysis furnace temperature of 1000 "C. The DCA and the CFC13are masked by the high concentration of CF?C12and thus cannot be determined. Using the method described in this paper, consisting of preseparation on a PEG 400 column and selective decomposition at a pyrolysis furnace temperature of 200 "C, the DCA can be determined quantitatively. The method of selective decomposition depends on the DCA being 50% decomposed at temperatures below 200 "C. The results of two doped atmospheric samples are given in Table I. Sample A was further evaluted in a more definitive way by studying the thermal stability of the chlorocarbons in the mixture. Figure 5 shows that at a furnace temperature above 275 "C, the CFC13 becomes unstable and would add t o the chloride ion concentration and above 425 "C the CF2CI2interferes. Samples C through G listed in Table I, which had been taken from an enclosed environmental system, were analyzed for DCA and for possible interferences by the method of selec-
tive decomposition. As can be seen from the table, neither DCA nor interferences in the method were detected. Although most unlikely, the possibility of an interference that would have the same retention time on PEG and stabilize the DCA still exists. This would make the analysis less than quantitative. A simple test for this would be to add a known partial pressure of DCA to the sample and to repeat the
analysis. If the newly-determined concentration agreed with the calculated value, it could be concluded that the original analysis was quantitative.
RECEIVE[)
for review September 27, 1971. Accepted January
10,1972.
Emission Spectrometric Determ ination of Trace Amounts of Mercury F. E. Lichte and R. K. Skogerboe Department of Chemistry, Colorado State University, Fort Collins. Colo. 80521
RECENTRESEARCH on the widespread contamination of the environment with mercury has generated a demand for highly sensitive, selective, and reliable means for determining this element in samples of diverse materials. Of the various analysis methods available, the cold cell atomic absorption method developed by Hatch and Ott ( I ) has received extensive attention because of its simplicity and sensitivity. Kalb ( 2 ) , for example, has adapted the technique for determining parts per billion concentrations in water and sediments while Uthe, Armstrong, and Stainton (3) have utilized it to analyze fish tissues. One can conclude that the method generally satisfies the analysis requirements for a majority of problems. There are cases, however, where the lack of adequate sample or the unusually low concentration of mercury precludes the use of this technique and greater sensitivity is consequently required. An interesting report by April and Hume ( 4 ) describes the use of a capacitively-coupled radiofrequency plasma torch in conjunction with a mercury reduction-vaporization cell for emission spectrometric determinations. Data presented suggests a limit of detection of 2 nanograms of mercury and a useful working range of 10 nanograms to 10 micrograms is cited ( 4 ) . Accepting the 10-ml sample size cited by the authors ( 4 ) , mercury concentrations as low as 1 ppb can be quantitatively determined in water with the system used. Thus, the method appears to be more sensitive than the atomic absorption technique, on an absolute basis, by approximately one order of magnitude (1-3). Previous reports from this laboratory have dealt with the utilization of a low power, microwave induced plasma as an excitation medium for spectrochemical analyses (5-7). In addition to the relative simplicity of this system, the high absolute sensitivity that can be obtained can be cited as a primary advantage. While this plasma resembles that used by Hume and associates ( 4 , 8 ) , the methods used to couple (1) W. R. Hatch and W. L. Ott, ANAL.CHEM., 40,2085 (1968). (2) G. W. Kalb, A t . Absorprioti Newsleft.,9, 84 (1970). (3) J. F. Uthe, F. A. J. Armstrong, and M. P. Stainton, Fisheries Research Board of Canada, Freshwater Institute, Winnipeg,
Canada, 1970. (4) R. W. April and D. N. Hume, Science, 170,849 (1970). ( 5 ) J. H. Runnels and J. H. Gibson. ANAL.CHEM.. 39. 1398 11967). (6) H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, ibid., 42, 876 (1970). (7) Ibid., p 1589. (8) C. D. West and D. N. Hume, ibid., 36,412 (1964).
ANHYDRONE
+ARGON IN
-PLASMA ClNG SOLUTION U
Figure 1. Schematic diagram of mercury reduction chamber the power into the discharges are quite different as are the characteristics of the plasmas produced. The principal problem associated with the microwave plasma originates from the fact that the rate of sample introduction must be limited if a stable plasma is to be maintained (5-7). Consequently, the technique involving the reduction of mercury compounds to the metal followed by volatilization into the plasma (1-4) offers a nearly ideal means for eliminating the sample introduction problem. Experiments carried out using the reduction-vaporization approach indicate that microwave plasma excitation can be used to determine ultratrace concentrations of mercury in a variety of sample types. The accuracy and precision of the method, as inferred from measurements made at ppb to ppm concentration levels, is estimated to be generally better than 10%. EXPERIMENTAL
Apparatus. The instrumentation utilized is listed in Table I. Specific features of this system are discussed below where appropriate. A schematic of the closed reduction chamber is presented in Figure 1. Reagents. The reagents specified by Hatch and Ott ( I ) were utilized for reduction of the mercury. Acids used for preparation of the reducing solution, for dissolution of samples, and for preserving water samples, were redistilled from A.R. grades or were commercial products of a purity designated as higher than that of A.R. grade. Standards were prepared from triply distilled mercury. Procedure. Approximately 5 ml of freshly prepared reducing solution were placed in the reduction-vaporization chamber and argon was bubbled through for several minutes ANALYTICAL CHEMISTRY, VOL. 44, NO. 7 , JUNE 1972
1321
