Application of capillary electrophoresis-electrospray ionisation mass spectrometry to arsenic speciation

The on-line coupling of capillary electrophoresis (CE) and electrospray ionisation mass spectrometry (ESI-MS) is a very useful tool for speciation analysis. This hyphenated technique provides elemental (isotopic pattern, if the element is not monoisotopic) as well as structural (molecular mass and/or fragmentation) information on an unknown species. Owing to several properties (high separation efficiency, low 'flow rates'), CE is best suited as the separation device for this coupling. The geometrical dimensions of both systems require the use of rather long CE capillaries (up to 100 cm), which leads to long total analysis times. The application of pressure along the capillary during or after the CE separation shortens the total analysis time dramatically. The effects of this 'pressure mobilisation' on detection limits, peak shape and resolution are discussed in detail. The technique was applied to the speciation of arsenic. A CE method was developed, providing the separation of six arsenic species of interest in a single run {arsenite [As(III)], arsenate [As(V)], methylarsonic acid (MMA), dimethylarsinic acid (DMA) arsenobetaine (AsB) and arsenocholine (AsC)}. The method used an acidic electrolyte system (ammonium acetate-acetic acid) for pH stacking. With the exception of As(III) and MMA, the arsenic species were baseline-separated from each other. Detection limits were calculated as 60-480 mu g L-1 for the arsenic species. The only exception was arsenite, As(III), with a detection limit of 50 mg L-1. The method was applied to standard mixtures and urine samples.