SELECTIVE PEAK ENHANCEMENT AND SUPPRESSION IN CONDUCTOMETRIC ION CHROMATOGRAPHY AND ITS APPLICATION TO THE DETERMINATION OF TRACE LEVELS OF ORTHOPHOSPHATE IN ENVIRONMENTAL-SAMPLES

In non-suppressed conductometric ion chromatography, when a carboxylic acid eluent of low pH is used, the signal intensity (DELTA-H(c))is proportional to the sum of the conductivity change caused by changes in the sample concentration (DELTA-C(s)) as well as conductivity changes caused by changes in the eluent concentration (DELTA-C(e)). This can be expressed as DELTA-H(C) = B(DELTA-C(s)) + B(DELTA-C(e)), where B is a constant. In this equation, DELTA-C(s) is constant for any sample species. However, DELTA-C(e) varies with changes in the sample capacity factors. This value is positive when the sample elutes before the system peak (k(s)' < k(e)'), and is negative when it elutes after the system peak (k(s)' > k(e)'). Therefore, the sample peak intensity (DELTA-H(C)) is enhanced prior to the system peak and is suppressed after it. These effects increase the closer the sample and the system peaks. The effect of this theory was proved by a determination of orthophosphate using a TSK gel IC-Anion-PW column and a 0.75 mM phthalic acid eluent. The detection limit of this method for orthophosphate (1 x 10(-11) mol, with an S/N of three) was smaller (by a factor of 10-30) than those by previous conventional ion chromatographic methods. Also, the interference of chloride or nitrate, often observed in the previous methods, was removed by suppressing their peak intensities. When this method was applied for the determination of orthophosphate in environmental samples, other interfering compounds were effectively removed by a column-switching system attached to the ion chromatograph, and a trace level of the orthophosphate was determined.