## 3 Results  
### LDPI-MS Analysis of the Pure Sample of Carbendazim  
The experiment first needs to determine the carbendazim LDPI-MS signal, and because the experiment uses a mode of two-beam laser action, it also needs to determine whether this mode will interfere with the quality of the carbendazim spectrum signal.  
Figure 3 shows the LDPI-MS of the pure CBZ sample that covered the plate. After irradiation with the 532 nm infrared laser, the CBZ sample was desorbed, and gaseous clusters were formed. Figure 3b,c show no significant MS signals when solely the desorption laser or the ionization laser was used, since their separate application cannot generate a complete mass spectrum; the characteristic peaks of the target molecule can be generated only when both lasers work simultaneously. These gaseous clusters, containing several neutral CBZ molecules, automatically float toward the ionization base and undergo ionization with the 118 nm VUV laser after a delay of approximately 25 \(\upmu\)s. The ionized species are then focused, accelerated, repulsed, and deflected by the electric field force before they cross the mass spectrometer's free-flight tube, and they are ultimately detected by the MCP, producing experimental signals. Here, single-photon ionization using a 118 nm VUV laser was used as a soft ionization process. The experimental requirement for soft ionization is an ionization energy of the substance molecules of less than 10.48 eV. Theoretical calculations and previous experimental studies showed that the ionization energy of CBZ is lower than 10.48 eV, so the selected target pesticide molecules fully met the experimental requirements.  
Figure 2: Experimental flow chart of mass spectrometry imaging of pesticide residues.  
As shown in Figure 3a, two dominant peaks, with relative molecular masses of 191 and 160, appear in the mass spectrum. In order to verify that this mass spectrum is being obtained under the combined action of desorption light and ionization light, we use desorption light and ionization light to perform separate experiments on the sample. Figure 3b,c show the mass spectra under separate photoionization and electron ionization effects, respectively. Figure 3a presents the mass spectrum resulting from the combined action of both laser sources, further indicating that the CBZ mass spectrum cannot be obtained solely by desorption or ionization laser operations. The characteristic signal peaks of the target compound can only be successfully detected when both the desorption laser and ionization laser work together, emphasizing the importance of combined desorption and ionization processes. The peak at m/z 191 is assigned to the CBZ molecular ion with a relative molecular mass of 191 and is referred to as the parent ion peak. The peak at m/z 160 is plausibly attributed to the CBZ fragment ion [C8H6N3O]\({}^{+}\), because the desorption ionization stage of molecular clusters is a continuous process in which the internal energy of some molecules continuously increases, leading to fragmentation. The main bond-cleavage types for possible molecular fragmentation are depicted in Figure 4, with the predominant fragment having a relative molecular mass of 160. In subsequent experiments, the fragments at m/z 191 and m/z 160 were taken as the characteristic mass signals of CBZ [44].  
Figure 4: CBZ possible fragmentation pathways in LDPI-MS.  
Figure 3: LDPI-MS of carbendazim: (a) the LDPI mass spectrum of carbendazim with dual photon action; (b) the LDPI mass spectrum of carbendazim without postionization; and (c) the LDPI mass spectrum of carbendazim without laser desorption.