A method for the determination of formic acid and acetic acid in rain water by ion chromatography was investigated. Three different methods were tested for formic acid and acetic acid determination. One is ion exchange chromatography (IC) by using Na2B407 solution as an eluent, and the others are ion exclusion chromatographys (ICE) with Ag+-type suppressor column and cation exchange fiber suppressor. The ion exchange chromatography by using 2 mM Na2B407 eluent was the most suitable method for formate and acetate ions determination from the viewpoint of the separation of these ions and the analytical sensitivity. This IC method could obtain a good separation of formate and acetate ions even in the case of sample volume as large as 1000 μ1, while the ICE methods Cbuld not obtain a good separation. The detection limits of the IC method were 0.003 ppm for formate ion and 0.005 ppm for acetate ion at 1000 jil injection volume. Formate and acetate ions were eluted within 6 minutes by 2 mM Na2B4O7 eluent at 1.9 ml/min flow rate. Most of nitrate and sulfate ions in rain water were retained on the anion exchange resin of the separation column and could not be eluted by using 2 mM Na2B407 eluent due to its wesk elution ability. It is necessary to elute out these ions from the separation column in order to maintain a good reproducibility of the analysis. So, 4 mM Na2CO3 mM NaHGO3 solution was injected to the separation column with the injector after formate and acetate ionswere detected, so that nitrate and sulfate ions were eluted out before the measurement of the next sample. It takes about 20 min to complete the analysis of a rain water sample by using the above method, and the relative standard deviation of the analysis was 3.7% for formate ion and 7.1% for acetate ion, respectively. The measurements of formic acid and acetic acid in rain water at Hiyoshi, Yokohama were done from April to October 1987 by this method. The concentration range was 0.05~0.91 ppm for formic acid and 0.04~0.56 ppm for acetic acid, respectively.
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