Charge Detector for the Measurement of Ionic Solutes

We describe a flow-through ionic charge detector in the form of a three-compartmented system. A central water channel is separated from two outer channels bearing water (or a dilute electrolyte) by a cation-exchange membrane (CEM) and an anion-exchange membrane (AEM). Independent fluid input/output ports address all channels. One platinum electrode is put in each outer channel. When the AEM-side electrode is positive with respect to the CEM-side electrode and voltage (∼1−10 V) is applied, the observed background current is from the transport of H⁺/OH⁻ through the CEM/AEM to the negative/positive electrodes, respectively. The H⁺ and OH⁻ are generated by the ionization of water, in part aided by the electric field. If an electrolyte (X⁺Y⁻) is injected in to the central channel, X⁺ and Y⁻ migrate through the CEM and AEM to the negative and positive electrodes, respectively, and generate a current pulse. The integrated area of the current signal (coulombs) elicited by this electrolyte injection is dependent on a number of variables, the most important being the central channel residence time and the applied voltage (V_app); these govern the transport of the injected electrolyte to/through the membranes. Other parameters include electrode placement, fluid composition, and outer channel flow rates. For strong electrolytes, depending on the operating conditions, the current peak area (hereinafter called the measured charge signal, Q_m) can both be less or more than the charge represented by the electrolyte injected (Q_i). Q_m is less than Q_i if transport to/through the membranes is subquantitative. Q_m can be greater than Q_i at higher V_app. At constant V_app more voltage is dropped across the membranes as the central channel becomes more conductive and water dissociation at the membrane surface is enhanced. Effectively, the membranes experience a greater applied voltage as the central channel becomes more conductive. The resulting additional current accompanying analyte introduction to the detector can substantially augment Q_m. Thus, the device is not an absolute coulometer although V_app can be deliberately chosen to have Q_m = Q_i over at least a 10-fold concentration range. Importantly, equivalent amounts of diverse strong electrolytes (with substantially different conductivities) injected into the central channel produce the same charge signals. In ion chromatography, this results in identical calibration curves for all strong acid anions, obviating individual calibrations. Whereas with a conductivity detector (CD) only the ionized portion of a weak electrolyte responds, in the present charge detector (ChD), ions are actually removed, leading to further ionization and the detection of a proportionately greater analyte amount.