Evaluation of Ca/P Ratios in Infant Milk Samples by CH4-Ar Mixed Plasma-DRC-MS

Abstract

Methane mixed plasma dynamic reaction cell mass spectrometry was successfully used for accurate determination of Ca and P and Ca/P ratios in infant milk samples. The interfering ⁴⁰Ar⁺ at m/z 40 was reduced in intensity by up to five orders of magnitude using 1.1 mL min^-1 CH₄ as reaction gas in a dynamic reaction cell. Polyatomic ion interferences (¹⁴N¹⁶O¹H⁺, ¹⁵N¹⁶O⁺ and ¹⁴N¹⁷O⁺ etc.) at m/z 31 were eliminated by detecting P⁺ as the product oxide ions ³¹P¹⁶O⁺ using O₂ as the cell gas. The poor sensitivity of ³¹P¹⁶O⁺ was improved to 3-fold by addition of 3 mL^-1 methane to the plasma. An extending dynamic range technique based on adjusting the dynamic reaction cell bandpass parameters (RPa) was successfully employed to extend the linearity of ⁴⁰Ca⁺ from 1 mg L^-1 up to 100 mg L^-1. LOD of Ca and P was 0.3 and 4 ng mL^-1, respectively. Results of three milk powder standard reference materials were in good agreement with the certified values. The proposed method was applied to determination of 160 milk samples collected from twenty different markets of China and the results of Ca/P ratio ranged from 1.3 to 1.9, which accorded with the official standards (1-2) of FAO/WHO.