Feasibility of Laser-Induced Breakdown Spectroscopy and Hyperspectral Imaging for Rapid Detection of Thiophanate-Methyl Residue on Mulberry Fruit

Abstract

An effective and rapid way to detect thiophanate-methyl residue on mulberry fruit is important for providing consumers with quality and safe of mulberry fruit. Chemical methods are complex, time-consuming, and costly, and can result in sample contamination. Rapid detection of thiophanate-methyl residue on mulberry fruit was studied using laser-induced breakdown spectroscopy (LIBS) and hyperspectral imaging (HSI) techniques. Principal component analysis (PCA) and partial least square regression (PLSR) were used to qualitatively and quantitatively analyze the data obtained by using LIBS and HSI on mulberry fruit samples with different thiophanate-methyl residues. The competitive adaptive reweighted sampling algorithm was used to select optimal variables. The results of model calibration were compared. The best result was given by the PLSR model that used the optimal preprocessed LIBS-HSI variables, with a correlation coefficient of 0.921 for the prediction set. The results of this research confirmed the feasibility of using LIBS and HSI for the rapid detection of thiophanate-methyl residue on mulberry fruit.

Keywords: chemometrics; hyperspectral imaging; laser-induced breakdown spectroscopy; mulberry; pesticide residue; variable selection.

Figure 1

Typical LIBS profiles of the control group and pesticide residue group of mulberry fruit with two concentrations of the pesticide solution (0 g mL⁻¹ and 0.0001 g mL⁻¹).

Figure 2

Typical Vis-NIR (visible and near infrared reflectance) hyperspectral profiles measured based on (a) system I and (b) system II for uncontaminated mulberry samples and samples contaminated with fungicide residue (concentrations of the fungicide solutions used in the samples from groups 1 to 6 were 0 g mL⁻¹, 0.0050 g mL⁻¹, 0.0025 g mL⁻¹, 0.0017 g mL⁻¹, 0.0013 g mL⁻¹, and 0.0001 g mL⁻¹).

Figure 3

PCA plots using the LIBS data (a) before preprocessing and (b) after preprocessing, and the LIBS data combined with the HSI data (c) before preprocessing and (d) after preprocessing (concentrations of the pesticide solutions used in the samples from groups 1 to 6 were 0 g mL⁻¹, 0.0050 g mL⁻¹, 0.0025 g mL⁻¹, 0.0017 g mL⁻¹, 0.0013 g mL⁻¹, and 0.0001 g mL⁻¹).

Figure 4

A typical LIBS analysis system setup.