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Impact of cryogenic grinding on iodine homogeneity in NIST SRM 1548a Typical Diet
 Kameník, Jan ; Kučera, Jan
The reference material NIST SRM 1548a Typical Diet has a certified iodine mass fraction of 0.759±0.103 mg kg-1. The expanded uncertainty is rather large (relative value 13.6 %), thus the material does not provide strict requirements for quality control results. Milling of NIST SRM 1548a at the temperature of liquid nitrogen yielded free-flowing material. Iodine mass fraction in the original and cryo-ground materials was determined by epithermal neutron activation analysis (ENAA) and radiochemical neutron activation analysis (RNAA). The average and standard deviation from both methods was 0.73±0.03 mg kg-1 (20 values). The determined mass fraction was in a close agreement with the certified value of NIST SRM 1548a, thus iodine total content was not influenced by the milling procedure. In comparison to iodine mass fraction determined in the original NIST SRM 1548a 0.72±0.07 mg kg-1 (average ± standard deviation, 14 values), the standard deviation for cryo-ground material was considerably lower. The prepared quality control material sets up more stringent requirements on the validation and quality control procedures of the measurement of iodine mass fraction in such a composite food matrices. We believe that the iodine random inhomogeneity in the original NIST SRM 1548a is due to insufficient mixing of some iodine-rich component in the complex food matrices. Milling at the temperature of liquid nitrogen offers an effective option for production of future reference materials of this kind.
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