Fast assay of angiotensin 1 from whole blood by cation-exchange restricted-access solid-phase microextraction

A novel restricted-access material with cation-exchange properties was evaluated for the extraction of the peptides angiotensin 1 (Ang1) and angiotensin 2 (Ang2) from whole blood. The ion exchange diol silica (XDS) material revealed enhanced efficiency for extraction of peptides when compared with the conventional alkyl diol silica (ADS) with reversed phase extraction centers. The outer surface of XDS particles contains diol groups while the internal surface has sulphonic acid moieties with strong cation-exchange properties toward compounds with low molecular weight. This new coating for solid-phase microextraction (SPME) offers the possibility to combine all initial sample preparation steps into a single one, even for complex biological samples such as whole blood or plasma. The XDS material was immobilized on a stainless steel wire with a binding agent; the reproducibility of the procedure was determined to be 5.56%. Compounds extracted from whole blood were separated by narrow-bore liquid chromatography (LC) and Ang1 was quantified by electrospray mass spectrometry, with good linearity over the range of 25-500 pM. Total analysis time was less than 90 min. The detection limit for Ang1 from whole blood was determined to be 8.5 pM. A single fiber could be used more than 150 times before a noticeable decrease in extraction capacity was observed. Although the limit of quantification is not as low as that of radioimmunoassay techniques, this method is much faster, less expensive and still provides enough sensitivity at physiological levels of Ang1. (c) 2004 Elsevier B.V. All rights reserved.