Simplified pretreatment method for plasma samples applicable to zinc kinetic studies

Common biochemical indicators of zinc (plasma Zn) are insensitive. Prasad et al (JLCM 1963:61 -5.17) utilized ⁶⁵Zn to show the rapid plasma Zn disappearance in Zn deficiency. Radiation exposure limits its application. Stable Zn isotopes are alternatives. Sample purification is usually required to obtain accurate results for mass spectrometric analysis, but also increases the chance of contamination. We compared two pretreatment methods (extraction vs nonextraction) for Zn isotope ratio (IR) measurement by inductively coupled plasma-mass spectrometry. Plasma samples collected from ten human subjects 5 min to 24 h after iv dose of ⁶⁷Zn (2 mg) were used for comparison. The plasma (1.5 ml) was digested by hydrogen peroxide (Alcock. BTER 1987;13:363) and dissolved in nitric acid. "Extraction": Zn in the digestate was extracted into CCl₄ as diethylammonium diethyldithiocarbamate chelate followed by back extraction of Zn in nitric acid. The solution was then heated overnight at 80 °C to remove traces of CCl₄, and made up to 10 ml with high putrity water after adding yttrium (Y) internal standard. "Nonextraction": Y and high purity water was directly added to the digestate. After subtraction of the baseline the plasma Zn IR was divided by the natural Zn IR to give the normalized IR (NIR). NIR obtained by both the methods agreed well (⁶⁷Zn/⁶⁴Zn, r²=0.997; ⁶⁷Zn/⁶⁶Zn, r²= 0.999; ⁶⁷Zn/⁶⁸Zn, r²=0.999). Considering the minimum possibility of isobaric interferences in plasma samples. ⁶⁷Zn/⁶⁸Zn obtained from nonextracted samples is sufficient for routine Zn kinetic analysis.