Comparison of two CE-ICP-MS interfaces based on microflow nebulizers:: application to cadmium speciation in metallothioneins using quadrupole and double focusing mass analyzers

Two different interfaces, based on microflow nebulizers, were investigated in detail and compared for metal speciation studies by CZE-ICP-(Q)MS and CZE-ICP-(DF) MS. The model species selected were rabbit liver metallothioneins (MTs). Small differences among MT sub-isoforms determine the need for an adequate interface, which is critical if the resolution achieved (e. g., by UV) is to be maintained with ICP-MS detection. HEN and MicroMist nebulizers were compared to interface the flows emerging from the CE capillary with the ICP-MS instrument. The observed analytical characteristics of the hybrid systems were compared in terms of calibration graphs, resolution, repeatability and detection limits (DLs). The separation profiles matched perfectly those obtained with UV detection for both interfaces under scrutiny. Similar performances were observed by CE-ICP-(Q)MS in terms of resolution and DLs achieved. The MicroMist interface proved to offer better performance than the HEN-based one using the ICP-(DF) MS instrument, because the introduction of an additional Ar "make-up'' gas into the spray chamber is not required (very important if the HEN-based interface is used). In general terms, the MicroMist interface was easier to operate, providing the best DLs for Cd (about 2 pg in each isoform by CE-ICP-( DF) MS, a value 6 times lower than that obtained in a quadrupole system).