DETERMINATION OF NANOMOLE AMOUNTS OF SULFUR-DIOXIDE IN AIR BY FLOW-INJECTION CONDUCTIMETRY WITH ONLINE PRECONCENTRATION

A simple and sensitive flow-injection technique for measuring nanomole amounts of sulphur dioxide in gaseous atmospheres is presented. The samples are passed through 0.2 M sulphamic acid solution to eliminate constituents interfering with the final electrolytic conductivity measurement. Sulphur dioxide is preconcentrated from the carrier gas stream by absorption in a capillary denuder tube coated with 0.02 M hydrogen peroxide solution. The sulphuric acid formed in the liquid film is then eluted with a stream of the absorbent circulated through an electrolytic conductivity microdetector. The change in specific conductance due to the presence of the sulphuric acid in the effluent stream is linearly related to the amount of sulphur dioxide in the sample (up to 55 nmol). The method can also be applied to the analysis of aqueous sulphite solutions. In this case the samples are injected into the sulphamic acid, and the acidic solution is then stripped with a stream of nitrogen. The limit of detection for aqueous samples is 1 mug l-1 and for air samples 4 mug m-3. A simple determination, including sample preparation, takes 7 min. The precision is better than 5% (P = 95%) for 2.5-50 nmol of sulphur dioxide. The selectivity of the system with respect to nitrogen oxides, ammonia, hydrogen chloride and hydrogen sulphide was also examined.