Charge Detector for the Measurement of Ionic Solutes

摘要

We describe a flow-through ionic charge detector in thenform of a three-compartmented system. A central waternchannel is separated from two outer channels bearingnwater (or a dilute electrolyte) by a cation-exchange membranen(CEM) and an anion-exchange membrane (AEM).nIndependent fluid input/output ports address all channels.nOne platinum electrode is put in each outer channel.nWhen the AEM-side electrode is positive with respect tonthe CEM-side electrode and voltage (∼1-10 V) is applied,nthe observed background current is from the transportnof H+/OH- through the CEM/AEM to the negativepositive electrodes, respectively. The H+ and OH- arengenerated by the ionization of water, in part aided bynthe electric field. If an electrolyte (X+Y-) is injected innto the central channel, X+ and Y- migrate through thenCEM and AEM to the negative and positive electrodes,nrespectively, and generate a current pulse. The integratednarea of the current signal (coulombs) elicitednby this electrolyte injection is dependent on a numbernof variables, the most important being the centralnchannel residence time and the applied voltage (Vapp);nthese govern the transport of the injected electrolytento/through the membranes. Other parameters includenelectrode placement, fluid composition, and outernchannel flow rates. For strong electrolytes, dependingnon the operating conditions, the current peak arean(hereinafter called the measured charge signal, Qm)ncan both be less or more than the charge representednby the electrolyte injected (Qi). Qm is less than Qi ifntransport to/through the membranes is subquantitative.nQm can be greater than Qi at higher Vapp. Atnconstant Vapp more voltage is dropped across thenmembranes as the central channel becomes morenconductive and water dissociation at the membranensurface is enhanced. Effectively, the membranes experiencena greater applied voltage as the central channelnbecomes more conductive. The resulting additionalncurrent accompanying analyte introduction to thendetector can substantially augment Qm. Thus, thendevice is not an absolute coulometer although Vapp cannbe deliberately chosen to have Qm ) Qi over at least an10-fold concentration range. Importantly, equivalentnamounts of diverse strong electrolytes (with substantiallyndifferent conductivities) injected into the centralnchannel produce the same charge signals. In ion chromatography,nthis results in identical calibration curvesnfor all strong acid anions, obviating individual calibrations.nWhereas with a conductivity detector (CD) onlynthe ionized portion of a weak electrolyte responds, innthe present charge detector (ChD), ions are actuallynremoved, leading to further ionization and the detectionnof a proportionately greater analyte amount.