Quantitation of perchlorate ion by electrospray ionization mass spectrometry (ESI-MS) using stable association complexes with organic cations and bases to enhance selectivity

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Abstract
Quantitation of trace levels of perchlorate ion in water has become a key issue since this species was discovered in water supplies around the United States. Although ion chromatographic methods presently offer the lowest limit of detection, ≈40 nM (4 ng ml–1), chromatographic retention times are not considered to be unique identifiers and often cannot be used in legal proceedings without confirmatory testing. Mass spectrometry can provide such confirmation; however, detection capabilities can impose a practical limitation on its use. Moreover, quadrupole mass spectrometers cannot provide sufficient accuracy and precision in m/z to identify conclusively an ion as perchlorate when samples are run directly without prior chromatographic or electrophoretic separation. We report on the abilities of (1) tetralkylammonium cations and (2) minimally nucleophilic, sterically hindered organic bases to increase selectivity in the electrospray ionization mass spectrometric (ESI-MS) determination of perchlorate ion without concomitant loss of sensitivity. Selectivity arises from the formation of a stable association complex between a base molecule and a perchlorate anion. The best results were obtained using 10 µM chlorhexidine in methanolic solution; the lower limit of detection (LLOD) for S/N ≥ 2 was less than or equal to 0.10 µM (10 ng ml–1). This compares favorably with the LLOD determined for perchlorate in the absence of any complexing agents (≈0.05 µM = 5 ng ml–1). For the other bases, which were diazabicyclo compounds (DBN, DBU, DBO), sensitivity was lower by 90% or more. The chlorhexidine–perchlorate complex (m/z = 605) can be observed even in the presence of equiformal nitrate, nitrite, hydrogensulfate, chloride, bromide, bromate, and chlorate (all together) down to approximately 1 µM; thus, the method is rugged enough to find application to systems containing multiple inorganic anions.