Comparison of three different ICP-MS instruments in the study of cadmium speciation in rabbit liver metallothionein-1 using reversed-phase HPLC and post-column isotope dilution analysis†
The coupling of reversed-phase high-performance liquid chromatography (RP-HPLC) with inductively coupled plasma mass spectrometry (ICP-MS) using post-column isotope dilution analysis (ID) was applied to the study of cadmium binding to standard rabbit liver metallothionein isoform 1 (MT-1). The analytical performance of three different ICP mass spectrometers, time of flight (TOF), double focussing (DF) and quadrupole (Q), was compared according to their performance for cadmium detection during chromatographic separation. Chromatographic conditions were optimised on a Vydac C8 VHP 259 RP column using a water–methanol gradient. Seven different sub-isoforms were distinguished by using this column coupled to a UV–Vis spectrophotometer, which greatly improved the resolution power exhibited by conventional anion-exchange chromatography. The ICP-DF-MS instrument was seriously affected by the methanol gradient and, thereafter, showed the worst performance in all cases. In contrast, the ICP-TOF-MS was able to resist up to 50% methanol with no serious changes in sensitivity or plasma stability. This instrument exhibited the lowest detection limits. The ICP-Q-MS provided results in between the other two instruments. In all cases, the use of post-column isotope dilution analysis was able to correct for matrix effects and signal drift, allowing for accurate determination of total MT concentration and the actual distribution of Cd-binding sub-isoforms.