Electrospray ionization mass spectrometry for the quantification of inorganic cations and anions

Recent studies from our laboratory on electrospray ionization mass spectrometry (ESI-MS) for the quantification of inorganic cations and anions are reviewed. Metal ions were determined by ESI-MS in negative-ion mode as monovalent negative ions of their aminopolycarboxylate (APC) complexes, where excess amounts of the APC agents were added to sample solutions. Among the APCs studied, we found trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (CyDTA, the chemical forms in a complex were expressed as H(n)cydta(n-4)) as the best chelating agent. A size exclusion column was used for on-line separation of the metal-APC complexes from matrix salts in samples. Total amounts of Al, Ni, Cu, Zn, and Pb in the biological certified reference materials (CRM), Olive Leaves (BCR-062) and Plankton (BCR-414), and in a soil CRM (JSAC-0401) were successfully determined by the proposed method. Halide ions (X(-) = F(-), Cl(-), Br(-) and I(-)) and cyanide (CN(-)) were determined by ESI-MS based on the formation of ternary complexes of metals, chelating agents and the analyte anions. Negative ions of the ternary complexes of group 13 elements, nitrilotriacetic acid (NTA, H(n)nta(n-3)), and halides, i.e., [AlF(nta)](-) for F(-), and [InX(nta)](-) for other halides, were measured; the limits of detection (LODs) were 10 nmol dm(-3) for F(-), 0.31 μmol dm(-3) for Cl(-), 3.8 nmol dm(-3) for Br(-), and 1.6 nmol dm(-3) for I(-), respectively. In the case of CN(-), an LOD of 20 nmol dm(-3) was obtained based on measurements of the ternary complex of Cu(II), CN(-) and 4-(2-pyridylazo)resorcinol (PAR, Hnpar(n-2)), i.e., [(63)Cu(II)(CN)(par)](-) (m/z 302). Moreover, quantitative methods for Cr(VI) and Cr(III) by ESI-MS were developed, where HCrO4(-) (m/z 117) for Cr(VI) and [Cr(III)(cydta)](-) for Cr(III) were used for measurements.