Polyatomics in zinc isotope ratio analysis of plasma samples by inductively coupled plasma-mass spectrometry and applicability of nonextracted samples for zinc kinetics

Inductively coupled plasma-mass spectrometry (ICP-MS) is a powerful tool for both quantitative multielement analyses of inorganic elements and measurement of isotope ratios (IRs). The main disadvantage of this technique is the existence of polyatomic isobaric interferences at some key masses. Zinc has been investigated for such potential interferences in serum or plasma. The Zn isotopes, ⁶⁶Zn and ⁶⁸Zn, have no apparent interferences, but ³²S¹⁶O₂ and ³²S₂ are isobaric with ⁶⁴Zn. The possible effects of S and other major components of blood plasma - Na, K, Cl, P, Ca - on Zn IRs were investigated using a series of mineral solutions which simulated human plasma with respect to these elements. The mixture of all mineral elements interfered only with ⁶⁴Zn (6.66 ng/mL) and ⁷⁰Zn (8.51 ng/mL). Interferences to ⁶⁶Zn, ⁶⁷Zn, and ⁶⁸Zn were minimal containing 0.90, 0.94, and 0.39 ng/mL, respectively. The copresence of Na or S shifted ³⁵Cl¹⁶O₂ (atomic mass 67 coming from Cl solution) to ³⁵Cl₂ which reduced the contribution to ⁶⁷Zn. The hypothesis that Zn IRs obtained from plasma at various intervals after the intravenous administration of enriched ⁶⁷Zn to humans would reflect those obtained after extraction of Zn was therefore tested. To compare the two pretreatment methods, 'extraction' versus 'nonextraction,' specimens were collected from 10 human subjects at intervals of 5 min to 24 h postinjection, and in 4 subjects from 5 min to 9 d postinjection. Two separate aliquots of plasma from each time-point were dried and digested with hydrogen peroxide, and the residue dissolved in nitric acid. One specimen was subjected to zinc extraction using ammonium diethyldithiocarbamate chelate followed by back extraction into nitric acid. The matching aliquot received no further pretreatment. The normalized IRs obtained from ⁶⁷Zn/⁶⁶Zn and ⁶⁷Zn/⁶⁸Zn in both the 'extracted' and 'nonextracted' samples agreed well (r² = 0.976 and r² = 0.985, respectively) compared to those from other ratios (r² = 0.838 for ⁶⁷Zn/⁶⁴Zn and r² = 0.747 for ⁶⁷Zn/⁷⁰Zn). Considering the minimum possibility of isobaric interferences in plasma samples, ⁶⁷Zn/⁶⁸Zn obtained from 'nonextracted' samples is sufficient for routine Zn kinetic analysis by ICP-MS.