Analytical measurement uncertainty of food carotenoid determination

Resumo publicado em: EURACHEM/CITAC 2011 workshop: Recent Developments in Measurement Uncertainty, 6‐7 June 2011: book of abstrats, p. P - 25 [Disponível em: http://eurachem2011.fc.ul.pt/pdf/Eurachem_CITAC2011-BookofAbstracts.pdf]INTRODUCTION. Carotenoid determination in food is a complex analytical process involving several mass transfer steps (extraction, evaporation, saponification, etc.). For consistent interpretation of an analytical method result it is necessary to evaluate the confidence that can be placed in it; this can be provided by the quantification of its accuracy (trueness and precision) in the form of a measurement uncertainty estimate. The Guide to the expression of Uncertainty in Measurement issued by the International Organization for Standardization1 establishes rules for evaluating and expressing uncertainty. Although it is a very powerful tool2, it is even more complex when analytical methods include mass transfer steps that lack descriptive models for the behaviour of the analyte in the analytical system. The guide was interpreted for analytical chemistry by EURACHEM, whose second edition3 already includes the possibility of using interlaboratory information and also the use of information obtained from analytical methods inhouse validation. MATERIAL AND METHODS. Analytical measurement uncertainty was estimated based on intralaboratory data, studies of precision and analytical process performance, and on contribution of standard stock solution concentration not covered by precision data (bottom-up approach). After estimation of all sources of uncertainty the combined standard uncertainty (uc) was calculated according to their laws of combination3. The expanded uncertainty (Uc) was calculated as Uc = k uc, where k is the coverage factor3. This methodology was applied to different food matrices of fruits and vegetables. RESULTS. Relative measurement expanded uncertainty was mainly between 0.10-0.31, higher values were found for measurements near instrumental quantification limits (ex. 0.75 for 8-cryptoxanthin, and 0.99 for lutein, in pear) or when sample chromatograms presented interferences with the analyte peak (ex. 0.44 for ;- carotene in orange). Lower relative measurement expanded uncertainty values (0.028-0.13) were obtained for food matrices not requiring the saponification step. Trueness studies showed good laboratory performance. CONCLUSIONS. Analytical measurement uncertainty of food carotenoid determination was 10-30% of the final result in the great majority of cases. The saponification step should be avoided if food carotenoids are not present in the ester form. Carotenoid values included in Food Composition Data Bases should be expressed taking into account the measurement uncertainty, to give correct information on data accuracy. 1ISO. Guide to the expression of uncertainty in measurement (GUM). International Organization for Standardization (1995), Geneva, Switzerland. 2Analytical Methods Committee. Uncertainty of measurement: implications of its use in analytical science. The Analyst, 120 (1995), 2303-2308. 3EURACHEM/CITAC Guide CG4: Quantifying Uncertainty in Analytical Measurement (2000), 2nd edition ; www.eurachem.ul.pt