Performance Characterization and Optimization of the AgNO₃-Filter/FMA Fluorimetric Method for Atmospheric H₂S Measurements

Abstract

This sensitive, albeit precarious, method for measuring ppb-ppt (V/V) concentrations of H 2 S was examined for various sources of potential error within the procedure. Filter preparation, filter storage, filter extraction, fluorimetric reagent stabilities, matrix differences between standards and samples, and possible interferences from other sulfur-containing compounds were separately studied for their effects on the analytical performance of the method. The overall method showed no Interference from SO 2 , CS 2 , COS, CH 3 SH, CH3SCH3, and SO 4~2 . To minimize bias and obtain a reliable estimate of precision, the method should be calibrated with H 2 S standards rather than liquid bisulfide standards. The measurement precision is a function of the quantity of H 2 S collected as Ag 2 S and/or AgSH on the impregnated filters. Because of the method's linear dynamic range, sufficient air should be sampled to achieve filter loadings of 15 to 35 ng S/filter. A quality control method based on fluoresceln mercuric acetate (FMA) is presented that ensures data quality while reducing the otherwise frequent need for fluorimetric calibration.A sensitive method for measuring low and sub-parts-perbillion (ppb V/V) concentrations of atmospheric hydrogen sulfide has been described by Natusch et al. 1 and modified by Jaeschke et al. 2 for automated field sampling. The method employs the collection of H2S on a AgNCVimpregnated filter. The filter is then leached with a basic cyanide solution to liberate the sulfide collected as Ag2S and/or AgSH on the filter. Quantification is achieved by the selective quenching effects of sulfide [S"~2] (or more correctly bisulfide [HS~] since pK2 = 19 ± 2 for H2S( aq ) 3 ) on the fluorescence of fluorescein mercuric acetate (FMA). This fluorimetric procedure was initially described by Karush et al.,4 Grunert et al., 5 and Axelrod et al. 6The work described in this paper was performed as a portion of a more comprehensive evaluation of potential analytical techniques to measure natural emissions of volatile sulfur compounds. 7 The major objectives for the specific evaluation of the AgNCVfilter/FMA fluorimetric method were to: a) compare the responses of liquid sodium sulfide standards, which are normally used to calibrate the method, with gaseous H2S standards, which better simulate actual analyte and sample matrix components; b) clarify previous conflicting reports 1 ' 2 ' 8 concerning potential positive interferences from CH3SCH3 and CH3SH; c) assess the sources and magnitudes for both indeterminate and systematic errors in the sampling and fluorimetric procedures; and d) suggest modifications to the basic method and/or operational procedures that improve both analytical ruggedness and data quality. Experimental Methods ApparatusAn Aminco-Bowman spectrophotofluorometer with an analog readout was used for the fluorimetric determinations. The excitation and emission wavelengths were 499 nm and 520 nm, respectively. Conventional fused silica cuvettes were used for the fluorimetric measurements. A...

A New All-Season Passive Sampling System for Monitoring H2S in Air

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