Elemental mapping of cocoa beans with laser-induced breakdown spectroscopy

Abstract

Cocoa and chocolate are important trading goods and serve as a source of minerals and essential trace elements, important in the human diet. The standard industry practice for quantifying elements in cocoa samples involves the use of inductively coupled plasma mass spectrometry and atomic absorption spectrometry. Within an experimental lab environment, X-ray fluorescence with micrometer resolution (µXRF) can reveal the spatial distribution of elements across cocoa beans. Over the past years, laser-induced breakdown spectroscopy (LIBS) has become an emerging analytical technique for a wide range of applications. Imaging cocoa beans with LIBS is challenging because it exhibits areas with low and high density (shell vs inner structure) and the inner structure has a very low melting point. To minimize melting and ensure an optimal lateral resolution for LIBS imaging, a thorough assessment of the experimental parameters and data preprocessing is necessary. This study reveals the capabilities and limitations of LIBS for mapping mineral and trace element distribution in cocoa beans within only 90 min. A carbon-based normalization method was adopted to rectify the experimental variations resulting from sample ablation initiated by the laser beam, thus allowing for reliable sample comparisons. A side-by-side comparison of µXRF and LIBS data from the same cocoa beans shows that both techniques yield similar elemental images, although LIBS displays somewhat lower resolution. Furthermore, detailed images of five individual cocoa beans are examined to illustrate how LIBS effectively detects variations in elemental distributions. The advantages of multi-element analysis, fast response, little to no sample preparation and ease of use position LIBS alongside µXRF as a promising technique for the cocoa sector.