Comparison of calibration strategies for accurate quantitation by isotope dilution mass spectrometry: a case study of ochratoxin A in flour

• Published in: Analytical and bioanalytical chemistry 2024-01, Vol.416 (2), p.487-496
• Main Authors: Bates, Jennifer, Bahadoor, Adilah, Tittlemier, Sheryl A., Melanson, Jeremy E.
• Format: Article
• Language: English
• Subjects: Analytical Chemistry; Biochemistry; Calibration; Characterization and Evaluation of Materials; Chemistry; Chemistry and Materials Science; Contaminants; Dilution; Flour; Food safety; Food Science; Isotopes; Isotopic enrichment; Laboratory Medicine; Liquid chromatography; Mass spectrometry; Mass Spectrometry - methods; Mass spectroscopy; Monitoring/Environmental Analysis; Ochratoxin A; Organic contaminants; Quantitation; Research Paper; Scientific imaging; Wheat
• ISSN: 1618-2642; 1618-2650; 1618-2650
• DOI: 10.1007/s00216-023-05053-3

Analysis of low-level organic contaminants in complex matrices is essential for monitoring global food safety. However, balancing sample throughput with complex experimental designs and/or sample clean-up to best reduce matrix effects is a constant challenge. Multiple strategies exist to mitigate these effects, with internal standard-based methods such as isotope dilution mass spectrometry (IDMS) being the most advantageous. Here, multiple internal calibration strategies were investigated for the quantification of ochratoxin A (OTA) in wheat samples by liquid chromatography-mass spectrometry (LC-MS). Internal standard-based quantitation methods such as single (ID 1 MS), double (ID 2 MS), and quintuple (ID 5 MS) isotope dilution mass spectrometry, as well as external standard calibration, were explored and compared. A certified reference material (CRM) of OTA in flour, MYCO-1, was used to evaluate the accuracy of each method. External calibration generated results 18–38% lower than the certified value for MYCO-1, largely due to matrix suppression effects. Concurrently, consistently lower OTA mass fractions were obtained for the wheat samples upon quantitation by external calibration as opposed to ID 1 MS, ID 2 MS, and ID 5 MS. All isotope dilution methods produced results that fell within the expected range for MYCO-1 (3.17–4.93 µg/kg), validating their accuracy. However, an average 6% decrease in the OTA mass fraction was observed from results obtained by ID 1 MS compared to those by ID 2 MS and ID 5 MS. Upon scrutiny, these differences were attributed to an isotopic enrichment bias in the isotopically labelled internal standard [ 13 C 6 ]-OTA that was used for ID 1 MS, the OTAL-1 CRM. The advantages and limitations of each isotopic method are illustrated.