Analysis of milk samples using calibration by proxy directly in the digestion vessel

Calibration by Proxy (CbPx) is a new, matrix-matched calibration method that uses just two solutions to provide a complete calibration curve simultaneously with the multi-element analysis of a sample. CbPx uses stand-in elements as proxies for the analytes of interest to build the calibration curve. Solution 1 contains the sample and known, varying concentrations of proxy elements. Solution 2 contains the same amount of sample and the same proxy elements at higher concentrations, plus an aliquot of a stock solution with analytes at known concentrations (standards). The calibration curve plots the signal ratio of each element in Solution 1 to the additional signal from the aliquot in Solution 2 (S₁/S_add) on the y-axis, against the concentration ratio (C₁/C_add) for each proxy element on the x-axis. This study applies CbPx to the analysis of solid samples by Inductively Coupled Plasma Optical Emission Spectrometry. Calibration is performed directly in the microwave-assisted digestion vessels, bypassing the need for additional sample preparation. Validation was performed using a powdered milk reference material, yielding recoveries for analytes ranging from 87 % to 107 %, with relative standard deviations (RSDs) within a few percent. The method was applied to the analysis of 10 commercial infant formula and milk samples (8 powders and 2 liquids). Limits of detection ranged from 0.01 (Mn) to 10 (K) mg/kg of analyte in the powder samples. Calibration by Proxy is a matrix-matched, cost-effective approach using only two solutions, enabling simultaneous measurement of multiple analytes for efficient, routine analysis.