Lower limits and nonlinearity of calibration in suppressed ion chromatography with strong acid eluent

Theoretical and experimental studies on nonlinearity of calibration in suppressed ion chromatography have been conducted. In suppressed cation chromatography with a strong acid as an eluent, nonlinear response in conductivity detection caused by water dissociation equilibrium shift has not been of concern in general. However, this issue can be of great consequence for analysis at the μg/L level. Theoretical calculation shows that the calibration curves at μg/L level are highly curvatured and a negative response range exists, making it impossible to make a determination. The extent of this phenomenon highly depends on the types and concentrations of ionic impurities present in the eluent. The presence of nonlinearity of calibration at the μg/L level is demonstrated experimentally. In order that linear calibrations are assured and negative response is eliminated, an addition of small amount of cation impurities into the eluent is recommended, as excessive amount tends to increase background conductance and their detection limits.