3.3. Comparison of IC-ICPMS to alternative methods
Previous workers also used NMR to quantify phosphite concentrations in
solutions[1]. Our own measurements reveal a
detection limit around 6.46 µmol/L (200 ppb), which is substantially
higher than what is achievable by IC or IC-ICPMS. To first order, this
method therefore appears less suitable for phosphite-lean samples than
IC, IC-ICPMS and even UV-VIS spectrophotometry (Table 1). However, we
note that in NMR, the detection limit depends on the strength of the
magnetic field (i.e., resonance frequency), the nature of the probe, and
the number of scans (which in turn determine the length of the run per
sample). In our case, the strength of the magnetic field was 500 MHz.
The detection limit could be comparatively lower (< 200 ppb)
in an instrument with a stronger magnetic field (e.g., 700 mHz) and
higher (> 200 ppb), if the magnetic field is weaker (e.g.,
400 mHz). The NMR used in this study is equipped with a
liquid-nitrogen-cooled broadband cryoprobe. If an uncooled probe were
used, it is estimated that the detection limit could be roughly 2.5
times higher (nearly 500 ppb) if all other parameters remain the same.
Finally, we performed 7000 scans for phosphite analysis, which equated
to 4 hours of run time per sample, but it is possible to detect even
lower concentrations if the samples are analyzed with a higher number of
scans and accordingly longer runs.
A major advantage of NMR is its ability to detect a much wider range of
phosphorus species, including various
polyphosphates[17]. These are relatively large
molecules that would likely be difficult to elute from the separator
column of an IC. And to our knowledge, no UV-VIS method has so far been
developed to measure polyphosphates other than pyrophosphate in
solution. Hence NMR, despite its limitations in the detection limit,
analytical time and installation costs (Table 1), is perhaps the best
method for measuring polyphosphate species. It is also unaffected by the
presence of chloride or other interferences.
UV-VIS spectrophotometry is probably the most cost-effective method for
phosphite[16] as well as
phosphate[18] measurements, and it too can be
conducted in the presence of high chloride
concentrations[16]. However, other interferences
may persist, and its detection limit is significantly higher than with
the IC and ICP-MS, making it unsuitable for many environmental samples.