Feasibility of rare earth element determination in edible insects by USN-ICP-MS after microwave-assisted UV digestion using diluted acids

Abstract

In the present study, a microwave-assisted ultraviolet digestion (MAWD-UV) method with diluted acids was evaluated as a sample preparation method for edible crickets for the subsequent determination of rare earth elements (REEs; Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) by inductively coupled plasma mass spectrometry (ICP-MS) with ultrasonic nebulization (USN). The use of diluted HNO₃, as well as the use of H₂O₂ and HCl, was evaluated in the digestion solution. When 15 mL of 3 mol L⁻¹ HNO₃ + 0.8 mol L⁻¹ HCl were used, quantitative results were obtained for all REEs (agreement >90% with the reference method – MAWD using concentrated HNO₃). The accuracy of the proposed method was evaluated by using a certified reference material (BCR-668, mussel tissue), and no significant difference was observed between the results and the certified values. Using the optimized conditions, REE concentration in blank solutions was always negligible. The digests obtained by MAWD presented high carbon (3604 mg L⁻¹) and acidity (2.9 mol L⁻¹) content, increasing the possibility of occurrence of non-spectral interferences during REE determination by USN-ICP-MS. The interferences presented are mainly due to differences in the properties of digests (viscosity, density, and surface tension) and the calibration solutions, requiring a high dilution factor before the determination step. The proposed method allowed the digestion of up to 500 mg of sample using 15 mL of 3 mol L⁻¹ HNO₃ + 0.8 mol L⁻¹ HCl, resulting in digests with low carbon (1370 mg L⁻¹) and acidity (1.0 mol L⁻¹) content. The proposed MAWD-UV method provided LOQs between 0.15 (Eu) and 37 ng g⁻¹ (Ce), allowing the determination of REEs at very low concentrations.