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.