1. INTRODUCTION
Ion chromatrography is a common tool for quantifying the concentration of different species of the same element in natural and experimental solutions. Examples include, but are not limited to, nitrate and nitrite, bromide and bromate, chloride and perchlorate, as well as an array of organic compounds. In recent years, with the discovery of reduced phosphorus species in natural settings[1-3], phosphate, phosphite and hypophosphite have moved into the focus of anion chromatography[2, 4-6]. Phosphite has even been detected in ancient sedimentary rocks and holds potential as an important substrate for prebiotic chemistry and early life[7, 8]. Reconstructing its biogeochemical history over geologic time and its distribution in modern environments therefore promises to yield new insights into the evolution of Earth’s biosphere. Phosphite is thermodynamically unstable in water (i.e., it forms in the stability field of H2), but it is kinetically stable, because oxidation to phosphate is slow and primarily catalysed by micro-organisms today[7-9]. Phosphite can be produced biologically[10] or abiotically under dry, hot conditions[8, 11], by lightning[12-14], or from the dissolution of meteoritic FeP-minerals[15]. However, as these pathways are relatively rare, phosphite concentrations can be low in natural fluids (e.g., 0.15-3 µmol/L in waters from Florida[1] and up to 0.45 µmol/L in a eutrophic lake in China[3], but less than 0.06 µmol/L in some geothermal waters[2] and often undetectable). Concentrations at the lower end of this range pose analytical challenges, because the phosphite anion peak is typically dwarfed by those of other anions such as chloride, sulfate, and phosphate in typical chromatographic setups. Saturation of the column and/or detector by those other more abundant anions may impact element separation and detection of phosphite[6].
To overcome this issue, Han et al.[6] as well as Ivey & Foster[4] implemented the use of OnGuard cartridges that simplify the sample matrix, particularly by the removal of chloride – the most common anionic species in most natural systems. This allowed injection of larger sample volumes (500-800 µL loop size) to overcome the detection limit. By coupling the ion chromatograph (IC) with an inductively-coupled plasma mass spectrometer (ICP-MS), Ivey & Foster[4] were able to achieve detection limits around 0.002 µmol/L for phosphite. However, the authors also noted that OnGuard cartridges can impact the phosphate and phosphite content of the samples, introducing additional uncertainty into the analytical yield. Furthermore, OnGuard cartridges add extra costs and labor to the sample preparation protocol. Alternatively, phosphite can be analysed by UV-VIS spectrophotometry[16]; however, this may suffer from interferences with other ions, and the reported detection limit of 0.36 µmol/L[16] is not as good as with the coupled IC-ICPMS system. Lastly, phosphite measurements can be made by nuclear magnetic resonance (NMR)[1], but this technique is generally not optimized for trace quantities. A detection limit has to our knowledge not yet been published, possibly because it is dependent on numerous instrument parameters (discussed below).
Here we present a new approach of removing chloride from the sample matrix on-line within the ion chromatograph by splitting the sample stream after passage of the chloride fraction through an additional clean-up column. By coupling this method with an Element 2 ICP-MS in medium-resolution mode, this allows us to achieve detection limits better than 0.003 µmol/kg with a small sample volume (37.5 µL loop size on Valve 1, Fig. 1a). We further present data collected by NMR with a detection limit of ca. 6.46 µmol/L, which highlights the value of the IC-ICPMS setup for natural samples.