An analytical methodology for the determination of cadmium bound to whey-proteins by laser-induced breakdown spectroscopy at low pressures.

In this study, a dried-droplet LIBS methodology for determining cadmium in cow milk has been developed. The performance of the methodology was shown by standard and real protein samples. A standard protein, bovine serum albumin (BSA), and whey protein extracted from skim cow milk were incubated in standard Cd solutions, and the complex solution was filtered through cut-off filters by centrifugation. The unreacted cadmium in the filtrate and Cd-bound protein in the filtered fraction were loaded separately onto a Si-wafer substrate and analyzed via dried-droplet LIBS methodology. Measurements were performed at reduced pressures by taking advantage of the signal enhancement effect. The optimum pressure for most Cd emission lines was found to be 100 mbar. It has been shown that the dried-droplet LIBS methodology at reduced pressures can be used for the identification and determination of free and protein-bound Cd in the whey matrix. The concentration-based detection limit of Cd bound to whey proteins was determined to be 20.2 ng mL^-1, which corresponds to as low as 10 pg in absolute amount with a sample volume of 500 nL. The LOQ value is estimated as 67.3 ng mL^-1 and 33.3 pg, in terms of concentration unit and absolute amount, respectively. The use of small sample volumes is important in the analysis of limited amounts of samples, such as body fluids. Preconcentration studies with multiple loadings of the sample on the same spot resulted in improvements in concentration-based detection. 8 ng mL^-1 Cd in the whey matrix that could not be determined by a single droplet loading due to being below the detection limit; could be determined after 10 consecutive loadings. The methodology may also be applied to the determination of other toxic metals bound to proteins for food quality control.