Certification and Long-Term Stability Assessments of 5-Methyltetrahydrofolate in Spinach and Kimchi Cabbage Certified Reference Materials Using Tri-enzyme Method and Isotope Dilution Ultra-performance Liquid Chromatography/Tandem Mass Spectrometry

This study presents the certification of spinach and kimchi cabbage certified reference materials (CRMs), (KRISS CRM 108-05-010 and 108-05-011), for 5-Me-THF. The fresh spinach and kimchi cabbage process involved washing, freeze-drying, pulverizing, sieving, V-mixing, and bottling in 10-g portions. The 5-Me-THF in the CRMs was then extracted and analyzed using a tri-enzyme method and isotope dilution ultra-performance liquid chromatography/tandem mass spectrometry. The method validation demonstrated that the method was both repeatable and reproducible, with the recovery ranging from 96.5 to 107.2%. The stability monitoring was performed for 7 days at room temperature, 1 month at − 20 °C, and 13 months at − 70 °C, confirming the stability of the CRMs under most conditions. The certified values were (16.93 ± 0.73) mg/kg and (11.66 ± 0.25) mg/kg for spinach and kimchi cabbage CRMs, respectively. The homogeneity of the CRMs was 1.85% and 0.91% for spinach and cabbage CRMs, respectively. Additional long-term stability assessments were performed, and this suggested that the CRMs will remain valid for at least another 48 months at − 70 °C.