Preparation and characterisation of certified reference materials for furazolidone and nitrofurazone metabolites in prawn

Abstract

Nitrofuran residues in food, particularly intensively farmed prawns, were identified as an important trade and health issue. No certified reference materials were available to allow laboratories to demonstrate satisfactory performance of their analytical methods. Freeze-dried prawn reference materials MX012A and MX012B were prepared at the National Measurement Institute, Australia, from blank and contaminated prawn tissue. A high-accuracy, exact-matching isotope dilution mass spectrometry method was developed for the analysis of two nitrofuran metabolites in the prawn materials. Residues were extracted by acid hydrolysis and derivatisation with 2-nitrobenzaldehyde, extracts were purified by liquid–liquid extraction followed by solid-phase extraction, and instrumental analysis was by ultra-high-pressure liquid chromatography with electrospray tandem mass spectrometry (UHPLC-ESI-MSMS). The method was used to certify the reference materials for mass fraction of 3-amino-2-oxazolidinone (AOZ) and semicarbazide (SEM), including the preparation of detailed measurement uncertainty budgets. MX012A contains AOZ at (137.5 ± 8.5) ng/g of freeze-dried material, and MX012B contains AOZ at (30.2 ± 1.8) ng/g and SEM (fortified) at (70.3 ± 3.1) ng/g. When reconstituted as directed on the certificate, the materials contain AOZ at (20.6 ± 1.3) ng/g and (4.53 ± 0.27) ng/g (MX012A and MX012B, respectively) and SEM at (10.5 ± 0.5) ng/g (MX012B). The individual components contributing to the measurement uncertainty estimates were the mass fractions assigned to calibration standards, gravimetric mass measurements, precision of the analytical method, reproducibility, long-term storage stability of the material at −20 °C, stability of the material during transportation, and potential bias due to extraction efficiency of the incurred analyte and matrix interference.