ICP-MS measurement of toxic and essential elements in human breast milk. A comparison of alkali dilution and acid digestion sample preparation methods

Background: For trace element measurement in human breast milk, an accurate high-throughput sample preparation method would be desirable. In this study, a quick alkali sample dilution method was systematically compared to the traditional acid digestion pretreatment for measuring 10 trace elements in 237 human breast milk samples by inductively coupled plasma mass spectrometry (ICP-MS). Methods: The alkali method involved dilution of the breast milk samples with a solution consisting of 2% butanol, 0.05% EDTA, 0.05% Triton X-100 and 1% NH4OH. The mixtures were sonicated, centrifuged and analyzed by ICP-MS. Acid digestion was performed in HNO3 at 250 degrees C using a Milestone Ultra CLAVE II microwave digestion system. Results: Overall, the alkali dilution method gave higher analytical precision compared to acid digestion while LODs and recoveries were similar between the two methods. Spearman's correlations between measured element concentrations by the two sample preparation methods were strong (r(s) > 0.70) for all elements, except for Ca and Cd (r(s) < 0.50). Passing-Bablok regression analyses showed best agreement between the two methods for Li, Mg, Fe, Zn, Se and Cd (slopes 0.8-1.2). Evaluation of Bland-Altman difference plots showed best method agreement (lowest bias) for Na, Mg, Ca, Zn and Cd. Highest difference in element concentration between the methods was observed for iodine (96 ng/g by alkali dilution and 33 ng/g by acid digestion), probably due to formation of volatile species such as HI and I-2 in the acid matrix. Conclusions: The results indicate that the alkali dilution high-throughput sample preparation method is comparable to the more commonly used acid digestion method for ICP-MS measurements of some toxic and essential elements in human breast milk.