Two-dimensional detection in ion chromatography: Sequential conductometry after suppression and passive hydroxide introduction

An improved method that uses sequential suppressed and nonsuppressed IC for the sensitive detection of both common anions and very weak acid anions is described. After suppressed conductometric detection of an electrolyticall y generated hydroxide eluent and an electrolytic suppressor, the eluent is passed into a membrane device where KOH is passively introduced into the el uent stream using Donnan forbidden leakage. A second conductivity detector then measures the conductivity of the stream. The background conductance of the second detector is typically maintained at a relatively low level of 2 0-30 muS/ cm. The weak acids are converted to potassium salts that are full y ionized and are detected against a low KOH background as negative peaks. The applicability of different commercially available cation exchange membr anes was studied. Device configurations investigated include planar, tubula r, and a filament-filled annular helical (FFAH) device. The FFAH device pro vided more effective mixing of the penetrated hydroxide with the eluent str eam, resulting in a noise level of less than or equal to7 nS/crn and a band dispersion value of less than 82 muL. Optimal design and performance data are presented.