# Chemical Images on Fingerprints Revealed with Mass Spectrometry
## 5 Conclusions  
The analysis of small molecules with inorganic NPs, has been described as a powerful alternative to conventional matrices for latent fingerprints [18; 41]. All our experimental evidence indicated that commercially available P25 nanoparticles assist the ionization of small molecules in MSI experiments. The strong thermochemical stability of TiO\({}_{2}\) under laser irradiation resulted in negligible background noise in the low mass region (\(m/z<500\)). Based on these results, proof of concept of an innovative method using MALDI-MSI was developed. This new MSI method is suitable for the deconvolution of overlapping latent finger marks and sensitive enough to reveal exogenous trace compounds embedded in human skin. We also showed that, with the high sensitivity of this approach, one can image not only endogenous compounds but also residues of exogenous compounds on LFM. These chemical images can prove the contact between the finger and tablet, and the chemical images reflected the shape of the contaminated object (paper clip). Thus, MALDI-MSI may be a practical analytical approach useful to support the forensic application of mass spectrometry. This approach, to the best of our knowledge, is the first demonstration of visualization by mass spectrometry of chemical images of exogenous compounds on human skin, with high sensitivity and direct analytical abilities. Future developments will be devoted to increasing the ionization efficiency of P25, using UV photo-adsorber compounds and plasmon resonance nanostructures. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app11125624/s1](https://www.mdpi.com/article/10.3390/app11125624/s1)]([https://www.mdpi.com/article/10.3390/app11125624/s1](https://www.mdpi.com/article/10.3390/app11125624/s1)). Figure S1: NP MALDI/TOF analysis of caffeine ([M+Na]+ m/z 217, [M+K]+ m/z 233) and stearic acid ([M-H]- m/z 283). MS spectra of caffeine: A (160 pmol/mm2), B (16.0 pmol/mm2), and C (1.60 pmol/mm2); MS spectra of stearic acid in box-D (111 pmol/mm2), box-E (11.1 pmol/mm2), and box-F (1.11 pmol/mm2). Figure S2. NP MALDI/TOF measurements of caffeine 10 pmol/mm2. (A) caffeine ionized by P25 H-doped ([M+H]+ m/z 195), (B) caffeine ionized by P25 Li-doped ([M+Li]+ m/z 201), C caffeine ionized by P25 Na-doped ([M+Na]+ m/z 217), and D caffeine ionized by P25 K-doped ([M+K]+ m/z 233). Figure S3. Diffuse reflectance spectrum of P25 sample. Figure S4. \(\zeta\) potential vs. pH of P25 aqueous slurry. Figure S5. MALDI/TOF full mass spectrum of a fingermark showing different fatty acids (probable structures, see Table S1 for identification) and different images created on six analytes. Table S1. Probable structures of endogenous substances (fatty acids) revealed in latent fingermarks by NP MALDI/TOF and DESI/Orbitrap experiments. Table S2. Exogenous substances (caffeine, acetaminophen and acetyll salicylic acid) revealed in latent fingermarks by MALDI/TOF and DESI/Orbitrap. **Author Contributions:** Conceptualization, A.C., E.D., L.F. and G.C. ; methodology, A.C. and L.M. ; formal analysis, A.C. and L.M. ; writing--original draft preparation, A.C. and E.D. ; writing--review and editing, all authors; supervision, E.D. ; funding acquisition, E.D. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was supported by Cariplo Foundation for the project \"Nanostructured-initiators for matrix-free, surface-based mass spectrometry imaging of antitumour drugs in tissues\" (Project 2013-0692) to E.D. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Data is contained within the article or supplementary material. **Acknowledgments:** This work was supported by Cariplo Foundation for the project \"Nanostructured-initiators for matrix-free, surface-based mass spectrometry imaging of antitumour drugs in tissues\" (Project 2013-0692). The authors wish to thank [PERSON] for editorial assistance.