An improved approach for analysis of chocolate by laser induced breakdown spectroscopy (LIBS) using L-menthol as matrix modifier.

Abstract

Chocolate, derived from cocoa beans and butter, is widely consumed globally. However, environmental pollution, fertilization practices, and industrial processing can lead to the accumulation of toxic metals (e.g., Cu, Pb, Mn, and Cd) in raw materials and finished products, while packaging materials can be a source of contamination. On the other hand, evaluating nutrients such as Ca, Mg, and Fe is also relevant. However, traditional digestion methods for chocolate are challenging due to its high-fat content, 27-35 % (w/w), typically requiring concentrated acids (14 mol L-1 HNO3) and high temperatures (220 °C) for microwave-assisted digestion (MW-AD). Direct Laser-Induced Breakdown Spectroscopy (LIBS) analysis faces additional issues, including sample fragility and high background signals. This study presents an improved approach for chocolate analysis using LIBS, in which L-menthol acts as a matrix modifier, yielding a stable solid, uniform craters, and more reproducible ablated masses. Sample preparation involved heating chocolate with L-menthol (1:1 w/w) at 65 °C for 30 min, followed by solidification in silicone molds at 4 °C. Approaches for qualitative, semi-quantitative (single-point calibration), and quantitative analysis (matrix-matched external standard calibration using a L-menthol plus stearic acid deep eutectic solvent) are discussed. Under the optimized conditions, coefficients of variation ranged from 0.7 to 7.8 % (macroelements) and 4.2 to 4.9 % (toxic metals), with limits of detection within 2.2 and 9.4 mg kg-1. Background signals were reduced from 2 (Cu) to 14-fold (Ca) compared to direct chocolate analysis. This approach reduces hazardous reagent use, simplifies sample preparation, and provides an environmentally friendly alternative with AGREEprep score of 0.70 (LIBS) vs. 0.28 (MW-AD) for rapid analysis of samples with high-fat matrices like chocolate.