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.