Development of Calibration Systems for Measuring Total Reduced Sulfur and Sulfur Dioxide in Ambient Concentrations in the Parts per BiIIion Range T. L. C. de Souza” and S. P. Bhatia Pulp and Paper Research Institute of Canada, 570 St. John’s Boulevard, Pointe Claire, P.Q., Canada H9R 3J9
Mlnute amounts of pollutants like H2S, CH3SH, (CH3)2S, (CHd2S2,and SO2, present in the atmosphere due to emisslons of such operations as kraft pulping, can cause nulsance and sometimes damage to property and the health of humans, animals, and plants. A calibratlon system has been tested for total reduced sulfur (TRS) and SO2 In the 100- to 0.1-ppb concentration range, involving the use of permeation tubes coupled with an exponentlal dllutlon flask (EDF), a flame photometric detector (FPD), and a solid NaHC03 fllter to separate SO2 from reducedsulfur compounds. Separationand quantitative measurement of individual sulfur compounds was attempted successfully with the exception of SO2.
Minute amounts of sulfur compounds like HZS, COS, SOZ, CH3SH, (CH&S, and (CH&Sz, when discharged into the atmosphere, cause pollution and, if the exposure is of long duration, damage to property and the health of humans, animals, and plants. As governmental regulations restricting the levels of malodorous sulfur gases into the atmosphere at source become more and more stringent with time, the ability to accurately monitor the above pollutants in ambient (ppb) concentrations becomes very important. Also, if the sulfur compounds could be separated, identified, and individually quantified a t the low ppb levels (at or near their respective threshold values) by a suitable monitor, this could then replace the ultimate tool to sense offensive odors, viz., the human nose. In the past, human odor panels have been used to define the degree of pollution due to malodors caused by sulfur compounds. But, because of lack of specificity, uniformity, and accuracy, as well as the quick overloading of the human olfactory sensors, a human panel is not necessarily the best means to “monitor” the presence of odoriferous compounds. In addition, continuous monitoring in this fashion is totally impractical. Hence, the development of an instrument able t o analyze sulfurous odors at threshold levels would not only replace the human panel, but also help to prove or disprove the validity of complaints of residents around, say, a kraft mill, when the odor might be caused by compounds other than sulfur gases. Methods using a flame photometric detector (FPD) for the measurement of total reduced sulfur (TRS) and the WestGaeke method for SO2 are already available. However, the calibration of these systems in the ppb concentration range is much more difficult than the actual measurements. The history of development of calibration techniques for monitoring small concentrations of gaseous sulfur compounds is intricately connected with the development of the corresponding analytical and monitoring techniques; most of the calibration studies are reported in papers on related analytical research (1-28). This paper is an account of our efforts to design, construct, and test an improved calibration system, using permeation tubes manufactured by Ecology Board Inc. (29),for TRS and SO2 in the 100- to 0.1-ppb concentration range. It also covers 2234
an approach tested for achieving quantitative separation of gas mixtures containing six individual sulfur compounds in the low ppb concentration range.
EXPERIMENTAL Apparatus. The overall system devised for the calibration of low concentrations of TRS and SO2 can be seen in Figure 1. Standard sulfur gas diffusing from the permeation tube in the thermostated oven was swept to the FPD detector-equipped with a water-cooled photomultiplier tube-either directly or via an EDF flask. Sample air to be analyzed was pulled through a heated fiber glass particulate filter by means of a peristaltic pump. The sample was then passed through the oxidation chamber to convert TRS to SO2 before being fed to the detector. Thus, in actual effect, the detector sensed TRS as SOz. When the sample contained SO2 in the first place, it was passed through a solid NaHC03 filter for the removal of SO2 before the analysis by the FPD. All tubing in contact with sulfur gas was made of 3.2-mm 0.d. FEP Teflon to minimize loss due to adsorption. For the separation of individual sulfur compounds in the low ppb concentration range, use was made of an F&M 5750 dual column gas chromatograph equipped with an FPD detector, and an electrometer, both manufactured by MicroTek Instruments Inc., Austin, Texas. An “inert” 10-port valve, made of Carpenter 20 stainless steel, was used for sampling purposes. T o generate low concentrations of the six sulfur compounds of interest, a special oven was made in our laboratories to house six permeation tubes to give a standard mixture of these compounds. An improved type of permeation device (29)made of stainless steel tubing and filled with a liquified gas-which permeates through a small membrane imbedded in one end of the metal tube-was chosen for this study. These devices are more satisfactory because of their much reduced variation in permeation rates with temperature (
