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.