Ochratoxin A Levels in Flour Measured Using Isotope Dilution Mass Spectrometry

Scientists from the National Research Council Canada and the Canadian Grain Commission in Ottawa, Canada and Winnipeg, Canada recently measured how effective different solutions are in measuring ochratoxin A levels in flour samples.

Fresh flour and ears of wheat isolated on white | Image Credit: © New Africa - stock.adobe.com

Modern liquid chromatography–mass spectrometry (LC–MS) instruments can easily detect low-level contaminants, which can help in monitoring organic contaminants; sadly, accurate quantitation can be challenging in complex matrices. Ionization suppression or enhancement of trace analytes can be done by co-eluting high-abundance matrix components, though this can bias results. As such, matrix effects that lead to ionization suppression must be measured to ensure that LC–MS methods are accurate.

Different strategies can be used to mitigate these effects, with internal standard-based methods, such as isotope dilution mass spectrometry (IDMS), being the most optimal. In this experiment, the scientists tested several internal calibration strategies for quantifying ochratoxin A (OTA) in wheat samples using LC–MS. Specifically, single (ID1MS), double (ID2MS), and quintuple (ID5MS) isotope dilution mass spectrometry, as well as external standard calibration were used and compared to each other. MYCO-1, a certified reference material (CRM) of flour, was used to evaluate each method’s accuracy.

External calibration led to results 18–38% lower than the certified value for MYCO-1, with this likely spawning from matrix suppression effects. Additionally, lower OTA mass fractions were obtained for the wheat samples via external calibration as opposed to the other methods. That said, all isotope dilution methods led to results within 3.17–4.93 µg/kg, which is the average range for MYCO-1. However, ID1MS-obtained results yielded an average 6% decrease in OTA mass fractions. Under scrutiny, these differences were attributed to an isotopic enrichment bias in the isotopically labelled internal standard [13C6]-OTA that was used for ID1MS. However, for many analytes, an appropriately labelled reference material may be unavailable; in these cases, ID2MS would be the next best approach. ID5MS, which was used to quantify OTA via multi-point calibration with internal standard, may require more sample preparation, but it proved most ideal for samples where OTA levels are unknown and differ between samples.

Reference

(1) Bates, J.; Bahadoor, A.; Tittlemier, S. A.; Melanson, J. E. Comparison of Calibration Strategies for Accurate Quantitation by Isotope Dilution Mass Spectrometry: A Case Study of Ochratoxin A in Flour. Anal. Bioanal. Chem. 2024, 416, 487–496. DOI: 10.1007/s00216-023-05053-3