RESUMO
The analytical methods for the determination of inorganic anions by water elution ion chromatography using a zwitterionic column (C14SB-coated column) have been explored, where pure water was used as the mobile phase. In the present ion chromatography, the complicated peaks derived from various "ion pairs" appeared on the chromatogram when the sample solution contained several kinds of cations and anions. The chromatograms with the complicated peaks were simplified by using a preconditioning cation-exchange column. In the preconditioning column, various kinds of countercations of the analyte anions were converted to a particular kind of common cation, and thus all the analyte anions were separated as the common cation form. The more effective separation was achieved by converting to a divalent cation (Mg(2+)) form than a monovalent cation (Na(+)) form, because the anions paired with the divalent cation provided longer elution times than those paired with the monovalent cation. All the analyte anions separated as the common cation form were simply determined from their calibration curves drawn from conductivity detection. In addition, the counterionic determination of the anions was also attempted by using ICP-AES. The analyte anions were determined by measuring their countercations with ICP-AES, where the calibration curve of the countercation measured by ICP-AES could be used.
RESUMO
An ODS column dynamically coated with zwitterionic bile acid derivative, 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS), was evaluated for direct injection determination of drugs in blood serum by HPLC. Polar functional groups such as sulfonate, ammonium and the three hydroxyl groups in CHAPS protruding towards an aqueous mobile phase formed a hydrophilic layer over the ODS reversed-phase surface, which resulted in high molecular mass compounds such as proteins being prevented from penetrating into the internal hydrophobic region. The bulk of the proteins were eluted as an unretained or nearly unretained band by using 0.2 mM sodium hydrogenphosphate solution (pH 7.4) as the mobile phase. In contrast, small molecules such as some inorganic anions and aromatic compounds were retained and thereby separated from one another. It was confirmed that the ODS column modified with CHAPS acts as a restricted access-type column with a hydrophobic interior and a hydrophilic exterior. Hence biological fluids could be directly injected into the CHAPS-coated ODS column. The present HPLC system using the CHAPS-coated ODS column was applied to the determination of theophylline and caffeine in human blood serum. The detection limits for the two drugs with UV absorption at 273 nm were 0.2 and 0.5 mg l-1 (injection volume 20 microliters) and the relative standard deviations of peak area measurements were < 1.4% and 2.2%, respectively, for 10 replicate measurements of serum spiked with 5 mg l-1 of each of the drugs.
