Abstract
The determination of total iodine and selenium in human breast milk (HBM) and infant formula (IF) by inductively coupled plasma mass spectrometry (ICP-MS) is presented. Instrumental memory effects associated with iodine (127I+) signals were reduced through the development of pneumatic nebulisation washout solutions (nitric acid sample digests: 5% H2SO4 / 0.5% Triton X-100; tetramethyl ammonium hydroxide (TMAH) sample digests; 1% TMAH). The use of nitric acid for sample dissolution did not result in the complete digestion of the fat and protein content. This resulted in poor levels of ICP-MS signal stability, precision and accuracy coupled with the blockage of ICP-MS nebuliser and interface components. For example, the calculated iodine concentration in NIST CRM 1549 non-fat milk powder was 1233 +/- 121 mug/kg (following digestion with HNO3-H2O2), compared to the certified value of 3380 mug/kg. However, sample dissolution with TMAH resulted in improved precision and accuracy levels of 3250 +/- 160 mug/kg. Similar problems were found for total selenium analysis (using 82Se+). TMAH dissolution did not result in the complete digestion of actual milk samples, with deposition of non-dissolved solids occuring on the ICP-MS injector. This also resulted in reduced precision levels and ultimately instrument shutdown. Enzymolysis of HBM and IF with enzymes: Candida rugosa, Mucor miehei and Pronase addressed this problem through improved sample clean-up, particularly when coupled with open vessel TMAH dissolution. This resulted in improved signal and instrument stability of ICP-MS measurements for total iodine and selenium. The prohibitive cost of enzymes led to the enzymolysis and analysis of a limited number of actual milk samples. Therefore, the single step method with TMAH dissolution was applied to the complete range of HBM and IF collected from around the World. Total iodine (95 - 140 mug/l) and selenium (16 - 50 mug/l) measured in human breast milk collected from the UK agreed with previously reported values. Iodine (265 - 953 mug/kg or 43 - 158 mug/l) and selenium (32 - 191 mug/kg or 3 - 32 mug/l) levels measured in UK infant formula samples were in agreement with reported literature values for reconstituted infant formula, but slightly higher for dry milk powder. In general, South American IF contained the higher iodine and New Zealand higher selenium levels. Speciation analysis of iodine by ion chromatography coupled to ICP-MS, confirmed that iodide present in HBM and IF (whey) was equivalent to 71 and 84% of the total iodine content. These findings were in general agreement with other published values.