Continuous monitoring of sulfur dioxide with a gas permeation denuder-based system

This study describes a continuous flow procedure for monitoring atmospheric sulfur dioxide using a planar gas permeation denuder (GPD) and a fiber optic spectrometer. When gaseous samples are directed through a GPD which consists of a gas-permeable membrane of poly(vinylidene) difluoride and two perspex blocks with engraved channels of mirror image, the fraction of sulfur dioxide passing the membrane is absorbed in a flowing stream of 5.0 x 10(-4) mol L-1 5,5'-dithiobis(2,2'-dinitrobenzoic acid) (DTNB) in 0.025 mol L-1 phosphate buffer of pH 7.0, and reacts with DTNB to yield a yellow product. This product is monitored spectrophotometrically at 4 10 nm with a miniature charge-coupled device (CCD) fiber optic spectrometer. The analytical range of sulfur dioxide is easily adjusted via the flow rate of DTNB solution, normally from 200 ppb to 94 ppm (v/v). The procedure is hardly interfered with by coexisting metal and non-metal species, except for hydrogen sulfide in the atmosphere. The error caused by hydrogen sulfide at a sulfur dioxide level (in ppm) of 2% is less than +/-10%. With the proposed procedure, the result for a mimetic air sample is very close to that of the ultraviolet fluorescence method.