Electrodialytic Capillary Suppressor for Open Tubular Ion Chromatography

We describe an electrodialytic capillary suppressor for suppressed conductometric open tubular ion chromatography (SC-OTIC). For practical eluent concentrations, suppressor active lengths less than ∼1 mm are adequate. In its preferred embodiment, the suppressor consists of a solid polymer ion exchanger block. Two 0.45-mm-diameter parallel cylindrical passages are provided: these provide for passage of regenerant water and placement of platinum wire electrodes. The suppression channel is made by making a crack in the soft polymer block, using a needle 0.3 mm in diameter. The suppression channel runs parallel to and is flanked by the two aforementioned electrode channels through which water is pumped. The suppressor ends of the separation and the detection capillaries are tapered. The tapered ends of the capillaries are inserted into the suppression channel with the tips 0.4-1.1 mm apart. Using a suppression length of 1 mm, we were able to suppress 100 mM hydroxide @ 100 nL/min (10 neq/min). With such a suppressor coupled to an AS18 latex-coated surface-sulfonated cyclo-olefin polymer (COP) capillary column with an inner diameter (i.d.) of 28 μm and using an on-capillary admittance detector (AD), the feasibility of both isocratic and gradient SC-OTIC was demonstrated. At 170 nL/min (substantially above the Van Deemter optimum), the plate count for fluoride exceeded 70 000 plates/m under isocratic conditions. The dispersion induced by the suppressor could not be determined, because the peak half-widths after suppression were paradoxically less than those without suppression; this apparent negative dispersion is likely an artifact of the detector.