Signal Enhancement of Cadmium in Lettuce Using Laser-Induced Breakdown Spectroscopy Combined with Pyrolysis Process

Abstract

Fast detection of heavy metals in lettuce is significant for food market regulation and the control of heavy metal pollution. Advanced methods like laser-induced breakdown spectroscopy (LIBS) technology have been tried to determine the cadmium (Cd) content. To retard the negative effect of complex matrix composition from samples and improve quantitative performance of LIBS technology, the pyrolysis process combined with LIBS was adopted to determine the cadmium (Cd) content of lettuce. Adaptive iteratively reweighted penalized least squares (airPLS) was used to preprocess the LIBS spectra and solve the baseline drift. For multivariate linear regression based on the three selected Cd emission lines correlation coefficient in the prediction set Rp2 increased from 0.9154 to 0.9969, and the limit of detection (LOD) decreased from 9.1 mg/kg to 0.9 mg/kg after the pyrolysis process. The partial least squares (PLS) regression and support vector regression (SVR) were applied to construct calibration models based on full spectra. In addition, the least absolute shrinkage and selection operator (LASSO) was implemented to choose limited lines to predict the Cd content. The PLS model with the pyrolysis process obtained the best results with Rp2 = 0.9973 and LOD = 0.8 mg/kg. The results indicated that the pyrolysis method could enhance the spectral signal of cadmium and thus significantly improve the analysis results for all the models. It is shown in this experiment that proper sample preprocessing could effectively amplify the Cd signal in LIBS and make LIBS measurement an efficient method to assess Cd contamination in the vegetable industry.

A tutorial review on methods of agricultural product sample pretreatment and target analysis by laser-induced breakdown spectroscopy

“…The pyrolysis of agricultural samples is a possible method to improve the physicochemical properties of samples, reduce matrix effects and enhance the LIBS spectral signal. Chen et al 59 adopted the method to pyrolyze dried and milled lettuce powder, which enhanced the signal intensity of Cd in lettuce leaves and the LOD of Cd was reduced from 9.1 mg kg −1 to 0.9 mg kg −1 . Lázaro et al’ s 60 dry heat treatment of botanical (citrus leaves), agronomic (soil) and industrial samples (carbon steel alloy) revealed an enhanced LIBS signal.…”

“…The results showed that DP-LIBS and MA-LIBS could effectively improve the detection sensitivity of Cd, but they did not reach the standard of rice quality safety. Chen et al 59 adopted the pyrolysis process combined with LIBS to determine the cadmium (Cd) content in lettuce. Adaptive iteratively reweighted penalized least squares (airPLS) was employed to preprocess the LIBS spectra and solve the baseline drift.…”

Crater–Spectrum Feature Fusion Method for Panax notoginseng Cadmium Detection Using Laser-Induced Breakdown Spectroscopy

“…The emission lines of LIBS that characterize the substance's features can be applied for qualitative and quantitative analysis in coal production [13], agriculture [14,15], soil [16], and so on. Cd detection using LIBS has been researched for use in lots of plants, such as cabbage [17], herbs [18,19], lettuce [20], and rice [21]. The bottleneck that causes a relatively lower measurement precision and accuracy of LIBS toxic metal quantification is signal uncertainty, which hinders further development [22].…”