Table 11. Analytical Results Obtained with CAD (At r = 20 seconds) Sulfate, ,ug/ml Standard deviation Taken Found n ,ugh1 4.00 3.07 12 0.04 1 .OO 6.00 6.01 6 0.04 0.67 8.00 7.96 12 0.06 0.75 10.00 10.01 6 0.06 0.60 12.00 11.98 12 0.08 0.67 14.00 14.01 6 0.08 0.57 16.00 16.04 12 0.12 0.75 18.00 18.00 6 0.06 0.33 20,oo 20.01 12 0.14 0.70
z
run to run. Results of the analysis of variance indicated that the variation between duplicate samples in the same run was approximately the same as that obtained on duplicates in different runs. These studies also indicated that the number of runs previously made had no effect on the performance of the BaClz procedure. Analytical results of long term (one month) precision studies with the BaClz procedure (using freshly prepared reagents for each run) were nearly identical to those reported in Table I. These results are considerably better than those reported for the standard procedure ( I ) . CAD PROCEDURE. Table I1 presents analytical results obtained via the CAD procedure. Inspection of these data shows that the analytical performance of the CAD procedure is superior to the BaClz procedure. Again, as was the case with the BaClz procedure, there is an optimum delay time between mixing and observation-20 seconds in this case. Moreover, the slope and intercept values for the standard curves with the CAD procedure varied by as much as 20% from run to run. Variations of this magnitude require internal calibration for each run, as is the case with the BaClz procedure. An important advantage of the CAD procedure
is that the cells are easily cleaned between runs. No deposits on the cells were observed after expulsion of the samples and rinsing. Analysis of variance for the CAD procedure indicated that the measurement error was approximately constant within and between runs. This indicates that the observed deviations are largely due to errors in delivery of the small sample and reagent volumes. Long term (1 month) precision results with the CAD procedure were consistent with those reported in Table 11. During the course of obtaining the long term precision data, the CAD reagent was found to be stable for at least one month if stored in stoppered brown glass containers. Results obtained with aged reagent at the end of one month were not significantly different from those obtained with freshly prepared reagent. CONCLUSIONS
The parallel analysis technique has been found to be especially attractive for turbidimetric analysis. This technique ensures that all samples and standards are treated alike, which is the major limitation in conventional turbidimetric procedures. Parallel analysis obviates the requirement of duplicating particle growth from run to run since standardization and analysis are performed simultaneously. Indeed, with the parallel-analysis approach, the observation time can be optimized to gain sensitivity and reliability, and to circumvent interferences. The results of this study suggest strongly that turbidimetric procedures warrant re-evaluation using the parallel photometric analysis approach. RECEIVED for review April 15, 1971. Resubmitted January 24, 1972. Accepted January 28, 1972. One of the authors (RLC) wishes to express his sincere appreciation to Oak Ridge Associated Universities for an Oak Ridge Graduate Fellowship which provided support for portions of this work. This research was sponsored by the U S . Atomic Energy Commission under contract with the Union Carbide Corporation.
I NOTES
1
A Rapid, Sensitive Method for Calibration of Permeation Devices Larry J. Purdue and Richard J. Thompson Encironmental Protection Agency, Research Triangle Park, N.C. 27711
SINCEPUBLICATION of directions for the preparation and use of permeation tubes by O’Keeffe and Ortman in 1966 ( I ) , permeation devices have been used extensively for the preparation of known concentrations of a variety of gaseous atmospheric pollutants. Illustrated directions for the construction of picogram dispensers (microbottles) have been presented ( 2 ) and detailed, and explicit directions for the (1) A. E. OKeeffe and G. C. Ortman, ANAL.CHEM.,38, 760 (1966); 41, 1598 (1969). (2) Zbid.,39, 1047 (1967). 1034
ANALYTICAL CHEMISTRY, VOL. 44,
NO. 6, MAY 1972
use of permeation devices (3) make these devices amenable to the generation of almost any condensable substance with a vapor pressure above a few tenths of a Torr under ambient conditions. The procedures ( I , 3) describe gravimetric calibration of permeation devices at permeation rates between 2.8 X low6 and 6 X 1O-Il g/cm/min. The device is placed in a watertight gas washing bottle, immersed in a constant-temperature (3) F. P. Scaringelli, A. E. O’Keeffe, E. Rosenberg, and J. B. Bell, ANAL.CHEM., 42, 871 (1970).
bath, and purged with 20-50 ml of dry air per minute. After an hour or more equilibration time, the tube is removed from the bath and weighed to the nearest 0.1 mg on an analytical balance. The tube is then replaced in the gas bottle and the process repeated; weighings are made at daily, weekly, or monthly intervals, depending on the rate of permeation of gas from the tube. Five to ten weighings are usually needed to determine whether the gas is diffusing from the tube at a constant rate. The major disadvantage of this method of calibration is that establishing a mean permeation rate often requires several weeks. Devices having very low rates, such as multiwalled tubes, drilled rods, and microbottles, may require several months for accurate calibration. This method of calibration is also inconvenient when permeation tubes are needed for nonroutine calibration of instruments in the field. Another drawback is encountered in the use of the tubes for diffusing NO2, for which the permeation rate may not be constant with time. For laboratory use, tubes can be kept in a constant-temperature bath for the life of the tube, and calibration can be continued during the period of use by reweighing the tube at convenient intervals. For field use, this method is unsatisfactory because tubes cannot be held at constant temperature during transportation to and from the field operation. Several days and possibly weeks of calibration time are required to gravimetrically recheck the permeation rate of a tube suspected of changing rate during field use. This paper describes the rapid calibration of permeation devices by use of a recording microbalance for dynamic determination of weight loss. This technique provides accurate rapid calibration and also makes possible use of devices charged with NOz and other systems for which the permeation rate may not be linear. EXPERIMENTAL
Figure 1. Apparatus for gravimetric calibration of permeation devices
08001 0
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Apparatus. A Cahn Model 2500 RH electrobalance in a No. 2505 Cahn glass vacuum bottle, equipped with a Cahn RH No. 2508 weighing unit replete with an O'Haus No. 2574 tare weight set and a Cahn No. 2503 REI control equipped with a custom hang-down tube (Figure 1) was used to measure the weight loss, which was recorded at a chart speed of 1-inch per hour. The temperature in a Model F. S. Haake constanttemperature circulator was maintained to AO.1 "C with the aid of a Polyscience Corporation refrigeration unit and the temperature monitored with a Model 46 Yellow Spring Instrument telethermometer. Reagents. Gases, generally stated to be of 95z purity or better, were obtained from established suppliers of laboratory gases. Liquid reagents were of analytical reagent grade. Procedure. The permeation tube to be calibrated is suspended from the balance in the water-jacketed hang-down tube, which is held at a fixed temperature by circulating water from the constant-temperature bath through the water jacket. This tube (Figure 1) has a gas exit port at the top and a gas entry port and a temperature sensor port at the bottom. The tube as shown will accommodate permeation devices up to 12-inches long. The weight loss is recorded continuously on a (1 mV) recorder at a chart speed of 1-inch per hour. The permeation rate of the tube is determined by calculating the slope of the resulting curve for weight cs. time. A recorder range of 0 to 1 mg full-scale is adequate for calibrating most permeation devices in less than 24 hours. A tube with a permeation rate of 0.1 pg per minute loses 144 pg in 24 hours, which is represented by 14.4 divisions on 100-division full-scale chart paper. A tube with a rate of 10 pg per minute loses 1 mg in 100 minutes, which causes a full-scale deflection on the recorder. Devices with very low
1
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Figure 2. Typical calibration chart for SO, permeation tube
rates require more sensitive ranges of 0-200 or 0-400 pg full-scale, and more time for calibration. A microbottle with a permeation rate of 0.01 pg per minute loses 28.8 pg in 48 hours, which is represented by 14.4 divisions on the strip chart when the 0-200 pg full-scale recorder range is used. A typical calibration chart for an SOz permeation tube is shown in Figure 2. RESULTS
An electromicrobalance offers many advantages over use of a standard analytical balance for the calibration of permeation devices. With the electromicrobalance apparatus, using a continuous recording of weight US. time on a strip chart recorder, the permeation rate of most devices can be determined in hours or days instead of weeks or months. Devices containing substances that permeate at very low rates are not ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
1035
normally calibrated gravimetrically; with the procedure described in this paper they can usually be calibrated in less than 7 days. For example, a mercury permeation tube with a permeation rzte of 10 nanograms per minute was calibrated in 5 days. The apparatus is ideally suited for calibration of permeation tubes for use in the field, because tubes returned to the laboratory after field use can be recalibrated in only a few hours. Also, the operator time necessary for calibration on a continuing basis is reduced; only 1.5 man-hours per day is required for calibration of two tubes per day. DISCUSSION
With the exception of the permeation tube housing, the components of this system are readily available from customary laboratory supply sources. The equipment described has been employed for a year without problems except for
background drift due to fluctuations in ambient temperatures. It is of interest that such variations would not be noted with the calibration techniques used earlier. The rapid response of this system makes possible dynamic blending of gases into a multicomponent mixture of known composition. By proper temperature programming, known synthetic atmospheres approximating those of different cities can be produced for laboratory work with this technique instead of more cumbersome dilution systems ( 4 ) . RECEIVED for review September 24, 1971. Accepted November 30, 1971. Mention of a commercial product does not constitute endorsement by the Environmental Protection Agency. (4) H. D. Axelrod et al., Amos. Emiron., 4,209 (1970).
Systematic Studies on the Breakdown of p,p’-DDT in Tobacco Smokes Investigations into the Presence of Methyl Chloride, Dichloromethane, and Chloroform in Tobacco Smokes N. M. Chopral and Larry R. Sherman Department of Chemistry, North Carolina Agricultural and Technical State University, Greensboro, N.C. 27411
IN OUR EARLIER papers ( I , 2) of the series we had reported the ,I ,I-trichloropresence of p,p’-DDT (2,2-di-(p-chlorophenyl)-l ethane), p,p’-DDE(2,2-di-(p-chlorophenyl-l,1 -dichloroethylene), p,p’-TDE (2,2-di-(p-chlorophenyl)-l,l-dichloroethane), p,p’-DDM (2,2-di-(p-chlorophenyl)-l-chloroethylene), trans-4,4’-dichlorostilbene (DCS), bis-(p-chloropheny1)methane (BCPM), and 4,4’-dichlorobenzophenone(DCBP) in p,p’DDT treated tobacco smokes. Of these compounds, the first five have the same number of carbon atoms as p,p’-DDT, and represent the products of dehydrochlorination, hydrogenation, and in the case of DCS, rearrangement reactions. The last two of these compounds, i.e., BCPM and DCBP, represent the compounds obtained from the p,p’-dichlorophenylmethyl moiety of p,p’-DDT. The other part of the p,p’-DDT molecule, i.e., the trichloromethyl moiety, on pyrolysis, could yield dichlorocarbene and trichloromethyl free radicals. In the reducing atmosphere present in the tobacco burning zone [ c j : Chopra (31 dichlorocarbene could give dichloromethane and methyl chloride, while trichloromethyl radical could give chloroform and methyl chloride. Of these three compounds, only methyl chloride has been reported to be present in tobacco smokes so far ( 4 , 5 ) . Author to whom correspondence should be addressed. ( 1 ) N. M. Chopra. J. J. Domanski, and N. B. Osborne, Beitr. Tubakforscli., 5, 167 (1970). (2) N. M. Chopra and N. B. Osborne, ANAL.CHEM.. 43,849 (1971). (3) N. M. Chopra, Proc. Second Ititerti. Cotigr. Pesticide Cliem., Tel Aviv, Israel, 1971, in press. ( 4 ) R. J. Philippe and M. E. Hobbs, ANAL.CHEM.,28, 2002 (1956). (5) J. R . Newsome, V. Norman, and C. H. Keith, Tobacco Sei., 9, 102 (1965).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
Methyl chloride is a unique chlorohydrocarbon in that it can be formed not only by the reduction of dichlorocarbene and trichloromethyl radical but also by the action of methyl free radicals in tobacco smokes on the natural organic and inorganic chlorine present in tobacco. Of the three compounds we found only methyl chloride and chloroform in tobacco smokes, and in this paper we are reporting on our investigations into the presence of methyl chloride, dichloromethane, and chloroform in tobacco smokes, and the significance of the amounts in which they are present. EXPERIMENTAL
Materials. All solvents used were of “pure” grade and were distilled before use (“Pure” grade refers to the quality of the reagent as mentioned on the reagent bottles), and p,p’-DDT used was 99.9 pure. REFERENCE COMPOUNDS.Methyl chloride (“High Purity” grade), dichloromethane (Nanograde), and chloroform (“Spectrophotometric” grade) were purchased from Matheson Co., Mallinckrodt Chemical Works, and Merck and Co., respectively. The three compounds gave only one peak each when chromatographed on three different GLC columns. Methods and Results. SMOKING OF ~ , ~ ‘ - D D T - T R E A T E D TOBACCO SAMPLES.Pesticide-free flue cured tobacco samples containing different amounts of p,p’-DDT were smoked as reported by Chopra and Domanski (6). A continuous flow of air was maintained through the apparatus throughout the smoking, and the tobacco smoke was collected in six traps containing heptane at - 80 “C. (6) N. hl. Chopra and J. J. Domanski, Beifr. Tubukforscll., 1971, in press.
