The four methods for the determination of formic, acetic, propionic, isobutyric and butyric acids in rain water by ion exclusion chromatography were investigated. In method (A) and (C), 0.05 mM H2SO4 was the eluent, and in method (B) and (D), 0.1 mM 1-octanesulfonic acid and 0.1 mM HC1 were the respective eluents.Method (A) used Na2SO4 as the scavenger while the other methods used tetrabutylammoniurn hydroxide as the scavenger. Carboxylic acids were detected by conductivity detector (CD) and ultra violet detector (UV) at 210 nm. The analytical sensitivity of method (A) was the highest of the four methods in the case of detection by CD. However, the analytical sensitivities of the four methods by UV differed little. It is considered that the decrease in the analytical sensitivity of methods (B), (C) and (D) using CD was caused by ion association between tetrabutylammoniurn ion and carboxylic acid ion. The detection limits by CD were 0.001~0.002 µg/ml for formic acid, 0.001 µg/ml for acetic acid, 0.003~0.004 µg/ml for propionic acid, 0.006~0.008 µg/ml for isobutyric acid and 0.006~0.007 µg/ml for butyric acid. The detection limits by UV were inferior to those by CD.Repeatability of the analysis data for carboxylic acids in standard solutions was good. The concentrations of these carboxylic acids in 14 rain samples obtained at Hiyoshi, Yokohama, were measured from June to November, 1988 by using method (A). The concentration range was 0.07~1.98 µg/ml for formic acid. 0.03 ~ 0.85 µg/ml for acetic acid, 0.02~0.15 µg/ml for propionic acid, 0.06 ~ 0.44 µg/ml for isobutyric acid and 0.06 ~ 0.16µg/ml for 1-butyric acid.
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