Preparation and evaluation of the homogeneity of milk as a candidate reference material for carbon-14

A pilot project was initiated to study the feasibility of preparing milk as a candidate reference material for ¹⁴C near environmental levels. Two materials, MK-B at natural level of ¹⁴C and MK-C4 at an elevated level, have been prepared from pasteurized 2% dairy milk. MK-C4 was spiked with an appropriate amount of ¹⁴C-methylated casein tracer to achieve the elevated level. Several samples from MK-B and MK-C4 have been analyzed to test the homogeneity of these materials for the distribution of ¹⁴C. The samples were combusted in oxygen under 20 atmospheres pressure using a Parr bomb. The ¹⁴C concentrations were determined by liquid scintillation counting using Carbo-Sorb/Permafluor E⁺ cocktail. The results indicate that these materials are homogeneous with respect to ¹⁴C concentration even in sub-sample sizes of 0.25 g of the freeze-dried material. The precision of our ¹⁴C measurements, as expressed by the % relative standard deviation, is within 5%. The accuracy has been tested by analyzing replicate samples of the IAEA ¹⁴C quality assurance materials, C-3 (cellulose) and C-6 (ANU sucrose) and found to be within 3%. The lower limits of detection are 0.08, 0.05 and 0.02 Bq.g^–1 of carbon for 20 ml of liquid scintillation mixture (Carbo-Sorb/Permafluor E⁺ = 0.67) loaded with up to 0.4 g of carbon from the sample and counted for 3 cycles of 60, 180 and 1000 min each, respectively. Our measurements of ¹⁴C specific activities of MK-B and MK-C4 are 0.26±0.01 and 15.3±0.4 Bq.g^–1 of carbon, respectively.