Aerosol dilution and formic acid for the direct analysis of bovine liquid milk by microwave-induced plasma optical emission spectrometry (MIP OES) as a faster and environmentally friendly analytical method

For the first time, the analysis of milk solutions without previous acid decomposition is presented using microwave-induced plasma optical emission spectrometry (MIP OES) with aerosol dilution (AD) as a sample introduction strategy. The milk samples were previously diluted using formic acid (concentration 2 % v v^-1) and directly introduced in the MIP OES instrument equipped with an AD system of laminar flux. The system introduced a controlled nitrogen gas stream (dilution gas, DG) between the spray chamber and the plasma torch. The milk and formic acid proportions, the samples' dilution, and the DG and the nebulizer gas (NG) flow rates were evaluated, considering plasma robustness (N₂⁺/OH ratio) and the intensity of the emission lines. A 50-fold dilution of milk and flow rates of 0.6 L min^-1 for DG and 0.4 L min^-1 for NG were chosen as a compromise condition. This simple strategy allowed the determination of Ca, K, Na, Mg, and P by MIP OES, with recoveries of 96-109 % obtained in addition and recovery assays and RSD lower than 6 %. The developed methodology used minimal reactants, and the aerosol dilution was performed online with the plasma gas, generating less waste. Besides, the sample treatment was fast, and the multi-elemental determination was performed using the cost-effective multi-elemental spectrometric technique of MIP OES.