2.2. Chloride removal set-up
In the standard IC set-up, the sample is transferred from the autosampler to a loop that is attached to a Valco-style (hereafter Valve 1). Initially, Valve 1 remains in load position until sample transfer is complete. Then it switches to inject position, and KOH that comes from the eluent generator pushes the sample from the sample loop to the guard and separator column (Fig. 1a). For on-line chloride removal, the IC was modified with the installation of an additional valve (hereafter Valve 2, supplied by Sunquest Scientific). This was placed between the sample loop on Valve 1 and the guard column (Fig. 1c, d). The software (Chromeleon) was modified such that Valve 2 remained in load position for an additional 2.5 minutes after Valve 1 had switched from ‘load’ to ‘inject’ (Fig. 1). During this time, a 1 mmol/L NaOH solution at a flow rate of 1 ml/min was used to push the sample from the loop on Valve 1 to the clean-up column installed on Valve 2. This additional NaOH solution was supplied by an external pump (Dionex GP50 Gradient Pump). Alternatively, an N2-pressurized reservoir may be used. The clean-up column on Valve 2 was a AG11-HC 4x50mm column, which separates anions contained in the solution similar to the guard and analytical columns. Chloride eluted first and was allowed to pass into the waste (Fig. 1c, d). Valve 2 was switched from ‘load’ to ‘inject’ after chloride had passed but prior to the elution of other anions from the clean-up column. The timing was calibrated manually at the start of the installation. After 2.5 minutes, Valve 2 switched automatically to ‘inject’, and the KOH eluent pushed the sample out of the clean-up column onto the guard column.
When the instrument was not in chloride-removal mode, Valve 2 was set to stand-by (Fig. 1b). The clean-up column was flushed with NaOH and kept moist with DI-water supplied from the external gradient pump. This solution was sent to waste and did therefore not interfere with analyses carried out in standard mode.