Single-column ion chromatography of passive monitors for atmospheric pollution

Nitrite, nitrate and sulfate anions deposited on passive monitors were determined by using single-column ion chromatography. These monitors were exposed outdoors for various periods of time in an atmosphere with a very low degree of pollution, and they were used to evaluate the rate of deposition of some corrosive species on metallic materials. Two types of passive monitors were studied, namely sulfation and nitration plates. Comparison of the results obtained by using the classical turbidimetric method and single-column ion chromatography shows that turbidimetry is inappropriate for low sulfate concentrations (<2.5 mg/plate) deposited on sulfation plates. Thus, ion chromatography should be used to obtain a representative value of sulfate deposition because it allows the determination of sulfate in amounts as low as 200 μg/plate without further preconcentration. For nitration plates, ion chromatography permits simultaneous determination of nitrite, nitrate and sulfate from the same plate, whereas only nitrite can be determined by the classical colorimetric method. Both gluconate/borate and phthalate eluents were investigated and the results show that phthalate eluent is more appropriate for these plates. It was found that the pH of the sample has an important effect on the area of the sulfate peak. For both eluents, the area increases when the sample pH is lower than 6. This demonstrates the importance of an adequate sample treatment before injection in order to obtain reproducible and accurate values.