ANALYSIS OF METAL SPECIES BY USING ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY AND CAPILLARY-ELECTROPHORESIS ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

Metal speciation was carried out by on-line hyphenation of capillary e lectrophoresis (CE) with mass spectrometry (MS) via an electrospray io nization (ESI) interface. The commercially available interface was har dly able to produce stable electrospray conditions over an extended pe riod of time, mainly caused by an insufficient positioning of the CE c apillary inside the ESI stainless steel tip. A device was developed, w hich allowed an infinitely variable adjustment of the capillary. The o ptimum position for stable electrospray conditions was set to 0.4-0.7 mm outside the ESI tip. Off-line ESI-MS investigations of free metal i ons [Cu(II)], metal ion-containing complexes [CuEDTA, (CH3)(3)SbCl2] a nd covalent organometallic compounds (selenocystamine, selenomethionin e) were carried out in order to assess the suitability of the techniqu e for metal speciation. The usefulness of ESI-MS as a detection method largely depends on the stability of the analysed species. Inorganic s pecies (i.e. metal ions) alter their composition when being electrospr ayed. Parts of the weakly complexing ligands will. be exchanged by sol vent molecules, mainly originating from the sheath liquid. The destruc tion of ion-solvent clusters by heating, collision-induced decompositi on or use of a sheath gas may lead to charge reduction of transition m etal ions. Organometallic complexes with strongly complexing ligands r emain intact, while those with weakly complexing ligands suffer from t he same disadvantages as inorganic species. ESI-MS is best suited for the speciation of covalent organometallic compounds. The ionisation pr ocess does not alter their structure and they will mostly be detected as singly charged molecular ions. The application of CE-ESI-MS for sel enium speciation to an existing method using an alkaline buffer system (Na2CO3-NaOH) gave unsatisfactory results. The non-volatile electroly te affects the ESI process dramatically. The final CE method used an a cidic background electrolyte (2% acetic acid) for the separation of th ree organometallic selenium species [selenomethionine (SeM), selenocys tamine (SeCM) and selenocystine (SeC)]. The Se species were sufficient ly separated from each other and appeared at 6.49 min (SeCM), 19.47 mi n (SeM) and 20.60 min (SeC). Detection limits were calculated as 1-6 m g/l for the organic Se species. (C) 1998 Elsevier Science B.V. All rig hts reserved.