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SummaryCapillary zone electrophoresis (CZE) has been coupled with mass spectrometry to enable the identification of mineral and organometallic compounds of arsenic in speciation studies. The electrophoretic effluent was introduced through a concentric interface into the mass spectrometer. Make-up liquid was added to enable electric contact at the outlet of the separation capillary and to assist the electronebulization process. After ionization, the ions were analyzed and quantified with an ion-trap detector. Optimization of the coupling conditions (geometry of the concentric interface, composition and flow rate of the sheath liquid, electronebulization and detection conditions) is described. The results show that the geometry of the concentric interface and the positioning of the outlet of the separation capillary have a critical effect on stability and sensitivity. Programming the electronebulization and detection conditions throughout the analysis enabled identification and quantification of the seven arsenic compounds of interest (neutral, and positively or negatively charged species) in less than 20 min at the ppm level. Limits of detection ranged from 0.5 to 3.3 mg L 1, corresponding to amounts injected ranging from 15 to 60 pg. The linear dependence of mass spectrometric response on arsenic concentration was verified for concentrations ranging from 5 to 200 mg L 1. For the two positively charged species, arsenobetaine and arsenocholine, an on-line preconcentration technique (field-amplified sample injection) enabled reduction of the detection limits by aP?arOXimately one order of magnitude to 110 and 160 #g L-, respectively.