Permeative Amine Introduction for Very Weak Acid Detection in Ion Chromatography.

Abstract

A permeative amine introduction device (PAID) is placed after a conventional KOH eluent-suppressed conductometric anion chromatography (SCAC) system. The PAID converts the suppressed eluites from the acid form to the corresponding ammonium salt (NR2H + HX → NR2H2(+) + X(-)) and allows very weak acids HX (pKa ≥ 7.0) that cannot normally be detected by SCAC to be measured by a second conductivity detector following the PAID. Permeative reagent introduction is dilutionless, can be operated without pumps, and provides good mixing (baseline noise 0.8 nS/cm for 27 μM diethylamine) with low band dispersion (as small as 30 μL). Diethylamine (DEA) was chosen as the amine source due to its low pKb value (3.0), high vapor pressure, low toxicity, and low odor. The eluites are thus detected against a low diethylammonium hydroxide (DEAOH) background (5-31 μS/cm) as negative peaks because the equivalent conductance of OH(-) is greater than that of X(-). Reducing the background DEA concentration enhances the detectability of traces of weak acids. Lower background [DEA] will limit the maximum concentration of analyte acids that can be determined; a general concept of peak width measurement at a fixed height is proposed as a solution. Trace impurities formed during electrodialytic suppression play a role in background noise; for the first time, we look at the nature of such impurities. The appearance of silicate in a sample put in a glass container as a function of pH can be readily followed. The maximum silica level in high purity type 1 water is 50 nM (1.40 μg/L Si), which is a measurement challenge in particular. A large injection volume (1 mL) permits detection limits of 21 nM silicate, 3 nM taurine, 3 nM sulfide, and 13 nM cyanide.