EVALUATION OF 2-IMINOIMIDAZOLIDIN-4-ONE AND THYMINE AS RESPECTIVE INTERNAL STANDARDS FOR NORMAL-PHASE AND REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC DETERMINATION OF CREATININE IN HUMAN SERUM

We evaluated two internal standards for HPLC determination of creatini ne in human serum after ultrafiltration: 2-iminoimidazolidin-4-one for normal-phase HPLC on aluminium oxide, and thymine for C-18 reversed-p hase HPLC. Detection of 2-iminoimidazolidin-4-one was done at the same wavelength as that used for creatinine, i.e. 240 nm. For thymine, the wavelength was switched to 280 nm. The suitability of the selected co mpounds to serve as an internal standard in the described measurement procedures, including ultrafiltration of serum, was evaluated from the precision and accuracy obtained. The method based on normal-phase HPL C with 2-iminoimidazolidin-4-one showed an imprecision expressed as R. S.D. ranging from 0.8 to 3.4% (mean: 2.1%) and an inaccuracy, calculat ed from the deviations from target values determined by isotope-diluti on gas chromatography-mass spectrometry, ranging from -1.3 to +1.8% (m ean: +0.4%). For the reversed-phase HPLC procedure with thymine, the i mprecision ranged from 0.3 to 1.3% (mean: 1.0%) and the inaccuracy fro m +0.1 to +3.9% (mean: +1.7%). The occasional observation of interfere nces with 2-iminoimidazolidin-4-one limited the application of the nor mal-phase method to a certain extent.