Ion-pair chromatography of methotrexate in a column-switching system using an alkyl-diol silica precolumn for direct injection of plasma.
The retention behaviour of methotrexate as an ion-pair with tetrabutylammonium in a column-switching system, based on an alkyl-diol silica C8 precolumn, combined with an analytical column, LiChrospher RP 18, was studied. Methotrexate is mainly present as a divalent anion at pH 7.4, however, the retention data was consistent with the formation of a 1 + 1 ion pair with the counter ion. The concentration of the tetrabutylammonium and the acetonitrile in the mobile phase could be used to regulate the retention in the system. Relevant chromatographic parameters to estimate the enrichment effect in the column-switching system are also identified and discussed. The column-switching system was applied to direct injection of plasma (100 microliters) giving a limit of detection of 10 ng/ml for methotrexate using UV detection at 307 nm.
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Analytical Pharmaceutical Chemistry, Uppsala University Biomedical Centre, Sweden.
Westerlund
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D000097:Acetonitriles / Q000737:chemistry; D000838:Anions / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid / Q000295:instrumentation / Q000379:methods; D006863:Hydrogen-Ion Concentration; D016014:Linear Models; D008727:Methotrexate / Q000032:analysis* / Q000737:chemistry* / Q000378:metabolism; D008956:Models, Chemical; D010949:Plasma / Q000737:chemistry; D000644:Quaternary Ammonium Compounds / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
D000097:Acetonitriles; D000838:Anions; D000644:Quaternary Ammonium Compounds; D012822:Silicon Dioxide; C009405:tetrabutylammonium; D008727:Methotrexate; C032159:acetonitrile