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.