3.1. Removal of chloride in ion chromatograph
The chromatograms of the IC in chloride-removal mode (Fig. 1c, d)
revealed complete removal of chloride while phosphite and other ions
contained in the solution were retained (Fig. 2b). The comparison to the
normal mode (Fig. 2a) shows that also nitrite was removed by this method
as it elutes close to chloride. However, nitrate, phosphite, sulfate and
phosphate were retained. Chloride removal was incomplete when the delay
on Valve 2 (i.e., the time before Valve 2 switches form ‘load’ to
‘inject’ mode) was less than 2 minutes. It may be possible to also cut
off anions that elute later than phosphite (such as sulfate or
phosphate) by switching Valve 2 back to ‘load’ as soon as phosphite has
been pushed from the clean-up column to the guard column. This was not
explored in this study.
The chromatograms with the Cl-removal method display a larger carbonate
peak than those generated in normal mode (Fig. 2) despite the use of
carbonate-free NaOH. We suspect that some carbonate was already
dissolved in the samples and standards, derived from atmospheric
CO2. Additional CO2 may have dissolved
in the NaOH solution during preparation. Care was taken to minimize
ingrowth of atmospheric CO2 (Section 2.1), but we could
not guarantee complete avoidance. The retention time of carbonate is
only slightly longer than that of phosphite, and therefore the carbonate
peak may reduce confidence in the detection and quantification of
phosphite with the conductivity detector on the IC. However, this
problem is resolved by IC-ICPMS coupling, as the ICP-MS measures
phosphite by mass and is therefore not impacted by the presence of
carbonate ions.