Simultaneous determination of metal ions, amino acids, and other small biogenic molecules in human serum by capillary zone electrophoresis with transient isotachophoretic preconcentration

CE with indirect UV detection was used for the simultaneous determination of lithium, magnesium, calcium, creatinine, carnitine, and a number of amino acids in human serum. The target analytes, positively charged under acidic electrolyte conditions, were separated with positive separation voltage polarity using 10 mM 4-methylbenzylamine, 4.5 mM citric acid, 25% (v/v) methanol at pH 4.05 as background electrolyte providing optimal separation. When analyzing real samples, however, some peaks were broadened due to essentially destacking conditions. In order to maintain the separation efficiency and also enhance the detection sensitivity, transient isotachophoresis (tITP) sample stacking was applied and yielded theoretical plate numbers in the range from 160,000 (arginine) to 350,000 (creatinine). The limit of detection values with tITP preconcentration were 0.11-0.26 mg L-1 for metal cations, 1.0 mg L-1 for creatinine, and 1.3-3.9 mg L-1 for histidine, lysine, arginine, and ornithine. The method precision for peak areas was from 0.4 to 5.0% relative standard deviation using the matrix sodium as internal standard. The accuracy of the developed tITP-CZE system was verified by consistent results for Li+, Mg2+, Ca2+, and creatinine obtained on analyzing two serum certified reference materials. The only sample preparation required was ultrafiltration and acidification (to release protein-bound alkaline earths), and working ranges for individual analytes corresponded well to clinical concentration ranges.