Characterization and uncertainty evaluation of aqueous reference materials for cholesterol

Abstract

Objective: Characterization and evaluation of the uncertainty of the reference materials of cholesterol in aqueous solution. Methods: The certified reference material GBW09203b was weighed accurately and dissolved into methyl cyclodextrin aqueous solution to prepare six kinds of candidate reference materials of cholesterol according to the concentration. The materials were tested for homogeneity and stability using routine methods and were evaluated the uncertainty derived from the inhomogeneity and instability. The uncertainty due to measurement was derived from the purity of the raw material, the weighing and the density measurement. The certified values and expanded uncertainties were calculated according to the ISO Guide. The expanded uncertainty of the certified value was the combination of standard uncertainty of measurement, inhomogeneity and instability. The coverage factor, k, was determined from the Student's t-distribution corresponding to the calculated effective degrees of freedom and 95% level of confidence. Results: It showed that the six candidate reference materials were homogeneous and were tested to be stable for at least 1 year at -70 ℃ and -20 ℃ using the isochronous stability study. The relative uncertainties due to the inhomogeneity were 0.14%, 0.11%, 0.13%, 0.47%, 0.39% and 0.11%. The relative uncertainty due to the instability was 0.25% at -70 ℃. The uncertainties of measurement were 0.06%, 0.05%, 0.05%, 0.05%, 0.05% and 0.05%.The certified values (reference value ± expanded uncertainty, mmol/L) were as follows: (0.651±0.004), (1.310 ±0.007), (2.574±0.015), (5.210±0.055), (7.710±0.072), (10.242±0.056) mmol/L. Conclusions: Six kinds of certified reference materials for cholesterol in aqueous solution have been prepared, which were homogeneity, stable and reliable. It showed that the uncertainty of measurement was very small compared with the uncertainty due to inhomogeneity and instability.