4. CONCLUSION
Our results show that on-line removal of chloride with the clean-up column and split valve (Valve 2 in Fig. 1d) in the ion chromatograph is a viable method for simplifying the matrix of natural and experimental solutions for phosphite analysis. For an element such as phosphite, this tool can be combined with coupling of the IC to an ICP-MS to achieve detection limits below 0.003 µmol/L (< 0.1 ppb P), in line with previous studies[4, 6] but without the need for a large sample loop or pre-analytical sample treatment with OnGuard cartridges.
Without the ICP-MS, the removal of chloride also simplifies analyses with the conductivity detector of the IC alone, except for ions that elute close to chloride (as those may be difficult to separate from chloride) or those close to carbonate. The latter may be elevated by the introduction of external NaOH. The carbonate problem may be mitigated if a degasser is installed in-line with the NaOH supply, but we did not explore that in this study. However, even without carbonate-removal from the NaOH solution, we would expect that ions such as nitrate, sulfate or phosphate, which typically have much shorter or much longer retention times than carbonate with the AS17-C analytical column, would be easier to quantify at low concentrations after on-line removal of chloride with the setup described in this study. Our results show good linearity in the conductivity detector both with and without the Cl-removal setup. In addition, it may be possible to further modify the timing in the software such that additional ions can be cut out from the sample. Our study therefore presents a new approach for optimizing ion chromatography and taking full advantage of the low detection limits of ICP-MS.
We conclude that IC-ICPMS coupling with on-line chloride removal provides perhaps the best way forward for phosphite analyses at low concentrations, because detection limits are significantly better compared to NMR and UV-VIS spectrophotometry. NMR holds the major advantage that it can detect a more diverse range of phosphorus species, including large polyphosphate ions, while UV-VIS spectrophotometry is the most cost-effective method for phosphite analysis, but neither of the two methods is able to achieve similar detection limits. The IC-ICPMS approach may therefore be ideally suited for unlocking phosphorus redox chemistry in the environment.