On-Line Solid Phase Extraction-Capillary Electrophoresis for the Simultaneous Determination of Rare Earth Elements at Sub-Trace Levels

RAÚL GIL, LUIS DANTE MARTÍNEZ, NORA VIZIOLI, M. FERNANDA SILVA

Queretaro

Simposio; XII Simposio Latinoamericano en Aplicaciones de la Electroforesis Capilar y Tecnología del Microchip en Biotecnología, Biomedicina , Biofarmacia e Industria; 2006

UNIVERSIDADE DO SAO PAULO

The simultaneous determination of rare earth elements in the environment, as well as in body fluids of living species, at low concentrations is an urgent problem. A critical evaluation of possible risks for human health can only be given if reliable analytical data are available. During the past two decades, CE and related techniques have moved to the forefront of analytical chemistry and have excelled in the rapid analysis of ions replacing traditional separation methods.  Metal analysis by CE has the advantages of robustness and ruggedness, low-cost, rapidness and versatility. However, CE suffers from poor concentration sensitivity when using UV detection because of the small injection volumes and narrow optical pathlength. This presents a significant obstacle for routine analyses of metal ions at ppb levels in real samples. The need for enhancement of sensitivity has induced the development of highly sensitive detection and enrichment methods.  Consequently, preconcentration techniques in conjunction with CE represent a promising tool especially in the area of simultaneous determination of metals at sub-trace levels. Indeed, there is a continuing need for the development of alternative extraction/enrichment procedures which are safe, rapid, convenient, and accurate.  To overcome the detection sensitivity limitations of CE, nonspecific, on-line preconcentration has been employed in a variety of applications. This technique is based on inserting a small quantity of reversed-phase material (e.g., C18 particles or membrane) near the inlet of the CE capillary for sample enrichment by solid-phase extraction. A detachable cartridge containing the solid phase is relatively simple to construct and permits the injection of large sample volumes (1-100 ?ÊL) into the capillary. Elution of adsorbed analyte in less than 100 nL of solvent permits a 100-1000 fold improvement in the effective concentration limit of detection, depending on the hydrophobicity of the compound. The aim of the present paper is to evaluate the feasibility of coupling an on-line solid phase preconcentration step to Capillary Zone Electrophoresis with UV-Vis detection for the simultaneous determination of rare earth elements (REE) at ultra trace levels. The laboratory-made device of the analyte concentrator cartridge consisted of a fused-silica capillary piece (150_m i.d. ?~ 8mm in length) packed with silica-based C18 reversed-phase chromatographic material and coupled on-line near the inlet of the separation capillary (bare fused-silica capillary, 75_m i.d.?~28 cm from the concentrator to the detector). The REEs were retained as REE-2-(5-bromo-2-pyridylazo)-5-diethylaminophenol complexes in micelles of TX-100 at pH 9.00 (sodium tetraborate) and were eluted from the column with methanol. A BGE of 50 mM sodium tetraborate, pH =9.00 was found to be optimal for the  separation of metal chelates. Detection was performed at 585 nm. An enrichment factor of 1000 was obtained.  The method was successfully applied to the determination of spiked water samples.