SPE of 5‐lipoxygenase metabolites and the effect of head‐column field‐amplified sample stacking in MEKC

The SPE of leukotrienes and eicosatetraenoic acids using anion exchange materials was compared to the classical extraction with C₁₈ columns. A silica‐based strong anion exchanger, a polymer‐based weak anion exchanger, and a polymer‐based mixed‐mode strong anion exchanger were studied. All anion exchange materials displayed a higher recovery of the analytes with values between 70 and 90% when extracting standard solutions and analyzing by HPLC. The effect was less pronounced for the analysis of the compounds in incubations of polymorphonuclear leukocytes. Using MEKC with head‐column field‐amplified sample stacking for analyte quantification, much lower values of the peak areas were observed compared to the determination of the recovery of the analytes by HPLC. Using MEKC analysis, the highest values were found for the polymer‐based weak anion exchange material, while values below 10% were found for the polymer‐based mixed mode strong anion exchanger. This could be attributed to the presence of electrolytes in the eluates that compromised the stacking efficiency. The extent of residual electrolytes depended on the SPE protocol, resulting in large differences of the amount of analyte determined by MEKC when applying head‐column field‐amplified sample stacking for online analyte concentration.