On-line back-extraction field-amplified sample injection method for directly analyzing cocaine and thebaine in the extractants by solvent microextraction

This paper describes a novel method that applies on-line back-extraction field-amplified sample injection (OLBE-FASI) to the extractants by solvent microextraction (SME) in capillary zone electrophoresis (CZE). To our knowledge, it provides the first report that the water-immiscible solvent samples were directly analyzed by CZE. The water-immiscible solvent sample, sealed with a water plug in the sample vial, was used for direct electroinjection. The water plug with a moderate content of organic solvent, low-conductivity, and the presence of a small amount of H+ provided the highest sensitivity for analyzing positively chargeable compounds, such as cocaine and thebaine. The linear range was at least 2 orders of magnitude, with the square of the correlation coefficient (r(2)) > 0.9999, and a separate calibration over the range 0.016-10 mu g/mL showed the linear range to be approaching 3 orders of magnitude. Detection limits were in the range of 2 - 10 ng/mL. Because the need to perform solvent exchange (from organic to aqueous solution) was eliminated, the OLBE-FASI method could be conveniently coupled with SME. In the present work, SME-OLBE-FASI-CE was validated for quantitative purposes, and applications to human urine were demonstrated.