On-line chloride removal from ion chromatography for trace-level analyses of phosphite and other anions by coupled ion chromatography-inductively coupled plasma mass spectrometry

RationaleIon chromatography (IC) combined with inductively coupled plasma mass spectrometry (ICPMS) is an ideal tool for measuring low concentrations of anionic species such as phosphite; however, the high concentration of chloride and other anions in natural solutions may negatively impact chromatographic separation and data quality.MethodWe developed an on-line mechanism of removing chloride from a sample within an ion chromatograph, using an additional valve and a separation column that transfers chloride to waste while phosphite and most other anions are retained. We installed this system in a coupled IC/ICPMS system (ICS6000 and Element 2 in medium-resolution mode) and determined linearity and detection limits. In addition, we measured phosphorus species by NMR for comparison as an alternative method for phosphite determination.ResultsChloride was fully removed from the samples while phosphite was retained and could be analysed by IC/ICPMS. Concentrations could be measured down to 0.003 mu mol/L and possibly less with good linearity over the explored range (up to 1.615 mu mol/L; r2 = 0.999). In contrast, the detection limit by NMR was 6.46 mu mol/L.ConclusionsThe on-line removal mechanism works well for simplifying sample matrices. It removes the need for costly pre-analytical sample treatment with OnGuard columns. We confirm that IC/ICPMS is the most powerful technique for quantifying phosphite in natural solutions. The new chloride-removal method may also be applicable to analyses of other anions.