## 3 Results
### Signal Linearity and Limit of Detection  
Based on the previous assessment, CBZ molecules are ionized through the combined action of two laser beams, so the relevant operating parameters of the two laser beams are directly related to the experimentally obtained MS signal intensity and resolution. There is a trade-off between achieving a desirable sensitivity and achieving a reasonable spectral resolution. We first studied the effects related to the instrument construction, specifically, the spatial distance, Z, between the sample surface and the VUV ionization laser, and the time delay control, T, between the two laser beams. According to previous experimental  
Figure 5: Mass spectra of carbendazim by LDPI-MS: (a) pure carbendazim sample, (b) carrot tubers with carbendazim, and (c) carrot tubers without carbendazim. findings, there is a critical value for the MS resolution, above which the intensity of MS signals may decrease. Therefore, we optimized both the resolution and signal intensity of the mass spectrometer used. To achieve this, we adjusted the time delay, T, between the two laser beams, and the distance, Z, between the entry alumina rod and the VUV ionization laser, and then measured the spatial resolution of the m/z 191 characteristic peak of the MS spectrum. Based on the experimental results shown in Figure 6a, the optimal resolution of the CBZ molecular MS signal is achieved when the time delay between the two laser beams, T, is 25 us and the distance between the entry alumina rod and the VUV ionization laser, Z, is 2.5 mm. Similarly, the optimal signal intensity of the CBZ molecular mass spectrometry signal is obtained when the time delay between the two laser beams, T, is 22.5 us and the distance between the entry alumina rod and the VUV ionization laser, Z, is 2.0 mm, as shown in Figure 6b. Considering both factors, we selected the following experimental conditions: a time delay of 22 us between the two laser beams and a distance of 2.0 mm between the alumina rod and the ionization laser; these achieved an optimal effect of more than 80%. In this study, the experimental intensity and resolution of the multi-residue signal were also influenced by sample preparation conditions. The concentration of multiple residues, the temperature of the water bath, the frequency of agitation during cultivation, and the thickness of the slices all influenced the detection results. To investigate the effects of sample preparation on CBZ residue detection in carrot tubers, it is necessary to discuss the influence of these factors on final residue distribution during sample cultivation. Table 1 lists the effects of cultivation temperature, agitation frequency during cultivation, and slice thickness during sample preparation on the multi-residue content of the samples. All other experimental conditions remained constant, and the multi-residue content was indicated by a signal intensity at m/z 191. Based on the results in Table 1, the water bath temperature (15-25 \({}^{\circ}\)C) exhibits a minor impact on the residual level of multiple residues in the carrot tubers under the selected experimental conditions, with a variation of approximately 5% when the water bath temperature increases. Therefore, a temperature of 20 \({}^{\circ}\)C, which closely approximates the temperature at which carrots grow, was chosen as the water bath temperature for the experiment. In terms of agitation frequency, little difference exists between agitation every 5 or every 10 min. ; only the presence of agitation has a significant impact, which may be related to the water solubility of multiple residues, since agitation can enhance the adsorption of multiple residues onto the surface of carrot tubers. On the other hand, theoretically, the thickness of slices should be positively correlated with the signal intensity.