Measurement of perchlorate in water by use of an 18O-enriched isotopic standard and ion chromatography with mass spectrometric detection

Currently, the most promising analytical methodology for low-level determination and confirmation of perchlorate (ClO4−) in drinking water is ion chromatography followed by electrospray ionization mass spectrometric detection (IC–ESI-MS). However, there are still potentially limiting situations that must be considered when analyzing real world samples by IC–ESI-MS. They are: (1) co-elution of contaminants with fragment ions having the same m/z as ClO4−, (2) high background counts at the m/z of interest leading to a subsequent decrease in signal-to-noise, (3) gradual loss of sensitivity occurring over time as the sampling cone and/or ion optics of the mass spectrometer are fouled, and (4) suppression of gas phase ion formation (ionization suppression) that can occur if high concentrations of contaminants co-elute with ClO4−. An internal standard whose response, on the column and in the electrospray, is similar to that of ClO4− would give the highest degree of accuracy possible in situations 3 and 4 listed above. Commercially available NaClO4 that was enriched with oxygen-18 was evaluated for its potential as an internal standard. The level of oxygen-18 (18O) enrichment was verified, the stability of the enriched ClO4− in aqueous solutions was determined, and method performance parameters such as method detection limits, accuracy, precision and ruggedness using the enriched ClO4− as an internal standard were determined. Internal and external calibration yielded similar results with respect to calibration, detection limits, and short-term precision. However, the enriched isotopic internal standard showed superior results with respect to ruggedness and percent recovery of spikes in ground water and synthetic water samples.