Analytical Sciences

Electrospray Ionization Mass Spectrometry for the Quantification of Inorganic Cations and Anions

Hiroki HOTTA* and Kin-ichi TSUNODA**

*Department of Chemistry, Nara University of Education, Takabatake, Nara 630-8528, Japan
**Department of Chemistry and Chemical Biology, Gunma University, Kiryu 376-8515, Japan

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_ncydtaⁿ⁻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_nntaⁿ⁻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, H_nparⁿ⁻²), i.e., [⁶³Cu^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.

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