## 3 Results
### Signal Linearity and Limit of Detection
However, considering the actual laser efficiency, excessively thick slices may result in incomplete sample vaporization, e.g., a thickness of 50 um leads to difficulties in sample preparation, sample breakage, and issues related to secondary ionization. Considering the above data, the optimal experimental conditions for this experiment were chosen as  
Figure 6: Effect of delay T between two laser beams and distance Z between the sample surface and ionization laser on the LDPI: (**a**) mass spectrometry resolution and (**b**) peak relative intensity. follows: a thickness of all slices of 100 \(\upmu\)m, and all the samples were placed in a water bath at 20 \({}^{\circ}\)C, and shaken every 5 min. To establish a relationship between the MS signal intensity and the actual concentration of CBZ, systematic experiments were performed. Since the signal measurement of CBZ molecules involves absorption from a 532-nm laser, only the content of CBZ molecules at a single point needs to be calculated in the experiment. The sample has an approximately rectangular shape, with dimensions of 20 mm in length and 10 mm in width. The surface concentration of CBZ was calculated by evenly spreading 1 \(\upmu\)L of 15.7 pmol/L standard sample solution over an area of approximately 200 mm\({}^{2}\). The 532 nm laser was used; it had a diameter of \(\sim\)200 \(\upmu\)m and an area of \(\sim\)0.314 mm\({}^{2}\). As shown in Figure 7a,b, the CBZ concentration of the sample ranges from 0.25 to 50 mmol/mm\({}^{2}\) (0.0785-15.7 pmol/L). The concentration is estimated based on the average value of at least five measurements. The linear regression equation is represented as \(Y=-0.11811+0.92533X\), with R\({}^{2}\) = 0.99488. Y denotes the electrical signal intensity detected for the m/z 191 target, expressed in millivolts (mV), and X represents the CBZ concentration on the sample slice. The lowest detected CBZ concentration in the present study is 0.157 pmol/mm\({}^{2}\), indicating that the LOD (S/N = 3) for residual CBZ in carrot tubers, using this experimental method, reaches 3.14 pmol/sampling point. Herein, the LOD is estimated to be 0.019 ppm. Specific calculations are given in the Supporting Information. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Condition**} & \multicolumn{5}{c}{**CBZ (m/z 191) Signal Intensity (a.u.)**} \\ \hline \multicolumn{3}{c}{**Variate**} & \multicolumn{3}{c}{**Duplicate Experimental Data**} & \multicolumn{3}{c}{**Average**} \\ \hline \multirow{3}{*}{Temperature (\({}^{\circ}\)C)} & 15 & 46.3 & 45.8 & 46.6 & 47.1 & 45.5 & 46.3 \\  & 20 & 46.8 & 49.2 & 48.5 & 46.1 & 50.2 & 48.2 \\  & 25 & 47.3 & 46.7 & 48.0 & 51.1 & 54.2 & 49.4 \\ \hline \multirow{3}{*}{Shaking frequency (/min)} & 0 & 38.2 & 41.0 & 38.5 & 37.6 & 38.1 & 38.7 \\  & 5 & 46.8 & 49.2 & 48.5 & 46.1 & 50.2 & 48.2 \\  & 10 & 45.8 & 47.3 & 50.0 & 48.9 & 46.3 & 47.7 \\ \hline \multirow{3}{*}{Thickness (\(\upmu\)m)} & 50 & 25.3 & 24.8 & 23.9 & 24.1 & 23.9 & 24.4 \\  & 100 & 46.8 & 49.2 & 48.5 & 46.1 & 50.2 & 48.2 \\  & 150 & 55.2 & 51.6 & 54.5 & 55.3 & 54.0 & 54.1 \\ \hline \hline \end{tabular} The standard experimental conditions were as follows: a thickness of all slices of 100 \(\upmu\)m, and all of the samples were placed in a water bath at 20 \({}^{\circ}\)C and shaken every 5 min. The conditions are consistent with the standard conditions, with the exception of the control variable conditions.  
\end{table} 
Table 1: The impacts of different experimental conditions on residual carbendazim in carrot tubers. Figure 7: _Cont._