# Desorption electrospray ionization mass spectrometry imaging of latent fingerprints revealed by Oil Red O

Mariska Banidol, Sophia Kouider, Isaure Sergent, Hélène Pizzala, Laurence Charles

► To cite this version:

Mariska Banidol, Sophia Kouider, Isaure Sergent, Hélène Pizzala, Laurence Charles. Desorption electrospray ionization mass spectrometry imaging of latent fingerprints revealed by Oil Red O. Rapid Communications in Mass Spectrometry, 2024, 38 (8), pp.e9724. 10.1002/rcm.9724. hal-05007160

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RESEARCH ARTICLE

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WILEY

# Desorption electrospray ionization mass spectrometry imaging of latent fingerprints revealed by Oil Red O

Mariska Banidol<fnref n="1" />,<fnref n="2" /> | Laurence Charles<fnref n="1" />

Sophia Kouider<fnref n="1" />

Isaure Sergent<fnref n="1" />

Hélène Pizzala<fnref n="1" />

**Correspondence**

Laurence Charles, Aix Marseille Université, CNRS, UMR 7273, Institut de Chimie Radiculaire, 13397 Marseille Cedex 20, France.
Email: laurence.charles@univ-amu.fr

**Funding information**

Région Provence-Alpes-Côte d'Azur (Research Program 2016, project ENIGMass)

Mass spectrometry imaging (MSI) is increasingly used to produce chemical images of latent fingerprints. Yet, the actual benefits of MSI for real case studies have to be assessed for fingerprints previously processed by forensic techniques. Here, we have evaluated the compatibility of desorption electrospray ionization (DESI) with the fingerprint enhancement technique involving Oil Red O (ORO).

**Methods:** To optimize the ionization step independently from surface extraction, the ORO reagent and its mixture with model compounds (triolein and linoleic acid) were first studied in solution using high-resolution electrospray ionization tandem mass spectrometry (ESI-MS/MS). Then, DESI-MSI experiments were performed in both polarity modes for ORO-processed fingermarks deposited on pieces of paper used as porous substrates.

**Results:** ESI-MS of ORO reveals a complex mixture of azo dyes. Two main impurities detected beside the targeted species were characterized using MS/MS and then were usefully employed to produce DESI-MS images of fingermarks, decreasing the scanning rate to get sufficient ion abundance from natural fingerprints. ORO did not prevent chemical profiling, and one major added value of this pink dye was to produce MS images with contrast that cannot be obtained optically for some colored substrates.

**Conclusions:** DESI-MS has demonstrated imaging compatibility with the application of ORO used to enhance latent fingerprints on paper and could also enable chemical profiling in natural fingermarks. In addition, MS images of ORO impurities were of higher quality than optical ones for fingerprints revealed on colored paper.

# 1 | INTRODUCTION

The ability of mass spectrometry imaging (MSI) to achieve the profiling of chemicals together with their mapping at the surface of explored samples has attracted increasing attention in the forensic field, particularly for fingermark analysis.<fnref n="1" /> Marks left by individuals when touching a surface are complex chemical mixtures of

endogenous molecules excreted through the sweat pores and exogenous contaminants present on the skin, the former varying among individuals and the latter revealing contextual information.<fnref n="2" />⁻⁴ In MSI experiments, small areas (defined as pixels) of the sample surface are inspected sequentially in time, and spectral data recorded for each pixel are stored together with their spatial coordinates. MS detection of specific ions can then be used for chemical identification,

<footnote n="1">¹Aix Marseille Université, CNRS, UMR 7273, Institut de Chimie Radiculaire, Marseille, France</footnote>

<footnote n="2">²Institut de Recherche Criminelle de la Gendarmerie Nationale, Cergy-Pontoise, France</footnote>
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whereas their distribution can be visualized by the creation of chemical images, typically showing ridge patterns in the case of fingermarks.

The first ionization technique with surface sampling capabilities reported for MSI of fingermarks was desorption electrospray ionization (DESI),⁵ which makes use of an electrosprayed solvent to examine sample surfaces at atmospheric pressure. In these seminal papers,⁶,⁷ DESI was employed to produce high-quality images of chemicals used as models for exogenous species (e.g., drugs of abuse and explosives) or endogenous components in latent fingermarks deposited on various substrates or tape-lifted from surfaces. DESI was also used to identify and map small additives and synthetic oligomers typically found in marks left by fingers after handling condoms.⁸ Quantitation capabilities of DESI were also demonstrated for cocaine in simulated fingerprints deposited on paper,⁹ whereas its high throughput was underlined when profiling cocaine metabolites in latent fingermarks.¹⁰ DESI-MSI data recorded for lipids in latent fingerprints left on glass were also processed with machine learning models, enabling the distinction of individuals in overlaid fingermarks.¹¹ However, the number of DESI-MS studies of fingerprints remains quite low compared to those employing matrix-assisted laser/desorption ionization (MALDI),¹² which has undoubtedly become the most popular ionization technique in the field.¹³ Yet, DESI offers substantial advantages over MALDI for fingermarks analysis. As an ambient method, DESI analysis is performed on samples in their native state, with no need for any preparation step such as matrix adjunction as required in MALDI. The low m/z range where many relevant analytes are expected is made crowded by intense matrix signals in MALDI mass spectra, whereas the risk for such spectral interferences is minimized in DESI, which uses a stream of charged droplets comprised only of solvent molecules and eventually a salt to promote specific ionization mode. This liquid medium may, however, induce a "washing effect"¹⁴ toward those analytes weakly adsorbed on surfaces, negatively impacting the spatial resolution of MS images. This effect can be minimized through proper optimization of sample scanning or using synchronized DESI,¹⁵ as reported by Perry and coworkers with high-resolution images (≤75 µm) of fatty acids in fingerprints.¹⁶ Although the actual influence of the substrate on ionization efficiency is still not fully understood,¹⁷ DESI can also be successfully performed on a wide variety of substrates, with no need for the support to be conductive. Last but not least, DESI is nondestructive toward sample substrates, allowing confident and safe analysis of questioned objects or documents.

One major issue still to be addressed before capitalizing all benefits of DESI for MSI of fingermarks is its compatibility with fingerprint enhancement techniques (FETs), which, in contrast to MALDI,¹³,¹⁸⁻²¹ has barely been investigated. FETs are routinely employed to locate latent fingermarks (that otherwise remain invisible) on investigated surfaces. They also enable sufficient contrast between secretion residues and the substrate to best evidence ridge patterns historically used for identification purposes. So-processed fingermarks have thus their chemical composition modified by the presence of the reagent and/or products resulting from the FET

reaction. As reported for MALDI, this can happen either beneficial or deleterious to MSI. For example, it was found that titanium oxide, a development powder commonly used for dark nonporous surfaces, usefully acts as a MALDI matrix to produce high-quality MS images of both endogenous and exogenous components in fingerprints.²² Similarly, gold deposited on latent fingerprints by vacuum metal deposition was sometimes shown to act as a strong enhancer of MS signals, depending on the substrate.¹⁸ In contrast, no MALDI-MS image could be obtained due to major suppression effects caused by 1,8-diazafluoren-9-one, used to reveal latent fingermarks on porous surfaces once reacted with amino acids.¹⁸ Between these two extreme cases, sequential use of FET and MSI may also require either the FET or the MSI method to be reoptimized, as shown with the need to minimize the thickness of the polymer formed upon cyanoacrylate fuming to best perform MALDI-MSI.¹⁹

So far, the compatibility of DESI has been assessed with only one FET, namely, cyanoacrylate fuming. Clear DESI-MS images of fingermarks contaminated with condom lubricants were produced based on major poly(ethylene oxide) and poly(dimethylsiloxane) oligomers, whose ionization was not compromised by the deposited cyanoacrylate film.²³ Here, we have explored the compatibility of DESI-MSI with the FET using Oil Red O (ORO), a lipophilic stain well known in histopathology and proposed in 2004 by Beaudoin to reveal, with white light, latent fingermarks on dry and wet porous surfaces such as paper.²⁴

## 2 | EXPERIMENTAL

### 2.1 | Chemicals

ORO (1-[2,5-dimethyl-4-(2,5-dimethylphenylazo)phenylazo]-2-naphthol) and triolein and linoleic acid (LA) used as model compounds were purchased from Sigma Aldrich (Saint Louis, MO, USA). HPLC-grade methanol and water were obtained from SDS (Peypin, France). Ammonium acetate (used as an additive in electrospray solutions) and sodium hydroxide (used to prepare the ORO stain solution) were purchased from Sigma Aldrich.

### 2.2 | ORO FET on fingerprint samples

All experiments were conducted using a single donor, following ethical approval 0062023-1 granted by the Data Protection Officer of General Data Protection Regulation of Aix Marseille University. Enriched fingermarks were created by instructing the volunteer to wash his or her hands before immersing a finger into solutions of model compounds (triolein, 1mM; LA, 3mM). The donor deposited the groomed fingerprint onto an office white paper (80 g m⁻²) used as a porous substrate. For experiments performed on natural fingermarks, the donor was instructed to engage in regular activities for a minimum of 2 h (neither hand washing nor control was applied). Fingermark depositions were performed on pieces of office white paper as well as
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pink and black papers produced with printer ink. Depositions were allowed to air-dry for 15 min before being subjected to the ORO enhancement technique.²⁴ The stain solution was prepared by mixing a methanolic solution of ORO (1.54 g in 770 mL) with an aqueous solution of NaOH (9.2 g in 230 mL). Paper samples were immersed in the ORO stain solution until a contrast between ridges and the substrate was achieved (typically after 30–40 min). Owing to the impossibility of visualizing contrast on dark surfaces, black papers were processed using the same timing as other clear substrates. Then, paper samples were immersed in water for 1–2 min and left to dry. Digital images were obtained using a Nikon D3000, 10 Mpx camera, with an AF-S 60 mm lens.

## 2.3 | Electrospray ionization mass spectrometry

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) experiments were performed using a Synapt G2 HDMS instrument from Waters (Manchester, UK) operated either in the positive ion mode (capillary ESI voltage, +2.8 kV; sampling cone voltage, +20 V; extraction cone voltage, +6 V) or in the negative ion mode (capillary ESI voltage, -2.27 kV; sampling cone voltage, -20 V; extraction cone voltage, -6 V), with the desolvation gas (N₂) flow set to 100 L h⁻¹ at 35°C. MS/MS experiments were carried out with collision-induced dissociation (CID) using argon as the collision gas. In the MS mode, accurate mass measurements were achieved after external calibration of the orthogonal acceleration time-of-flight mass analyzer (with a resolving power of about 25 000 at m/z 500), whereas the precursor ion was used as an internal standard in MS/MS. A syringe pump was used to introduce sample solutions in the ESI source at a 10 µL min⁻¹ flow rate. Instrument control, data acquisition, and data processing were achieved with the MassLynx 4.1 programs provided by Waters.

## 2.4 | DESI mass spectrometry

DESI analyses were performed on the same Synapt G2 instrument after switching the ESI source to a 2D DESI ion source (Prosolia, Indianapolis, IN, USA). The DESI source was operated at 35°C with a nebulizing gas (N₂) pressure of 10 bar and the following geometrical factors: a 3 mm distance between the capillary tip and the sample surface, a 4 mm distance between the solvent capillary and the transfer line, 0.5 mm between the transfer line and the sample surface, an incident angle to the surface of 55°, and a sampling angle of 5°. The source was operated in the positive ion mode (capillary voltage, +2.8 kV; sampling cone voltage, +20 V; extraction cone voltage, +6 V) and in the negative ion mode (capillary voltage, -2.27 kV; sampling cone voltage, -20 V; extraction cone voltage, -6 V). The sprayed solution was delivered at a 3 µL min⁻¹ flow rate. All DESI experiments were performed on freshly deposited and revealed fingermarks, using double-sided tape to fix paper samples on the glass slide. DESI-MS imaging was performed by scanning the 10 mm width × 3 mm height surface at a velocity of 100 or

50 µm s⁻¹ on 30 rows to define pixels of about 100 × 100 µm. Instrument control, data acquisition, and data processing were achieved with the MassLynx 4.1 programs provided by Waters and Omnispray provided by Prosolia. After raw data were converted into imzML using Firefly (v. 2.1.06), chemical images were generated with Biomap (v. 3.8.04).

# 3 | RESULTS AND DISCUSSION

DESI can be regarded as a process combining ionization in line with the instantaneous extraction of analytes from substrate surfaces. The desorption step makes use of high-velocity charged droplets directed to the surface where they first form a thin film enabling analyte dissolution before further impact of primary charged droplets at the sample surface creates secondary droplets carrying analytes. These molecules are then emitted as intact gas phase ions according to an ESI-like ionization step and sampled in the mass spectrometer via a transfer line.²⁵ Because solvents allowing the best extraction of analytes may not be optimal for ESI (and vice versa), the ionization step can be decoupled from the analyte pickup step by studying analyte solutions using ESI to investigate any effects specific to the production of gas phase ions.

## 3.1 | ESI-MS of reagent and model compounds

ORO and model compounds, namely, LA and triolein, were first mass analyzed using ESI to evaluate their ionization behavior and any suppression effects when in a mixture. These experiments were performed in methanol supplemented with ammonium acetate (3 mM). This volatile organic salt was specifically selected as it promotes the ionization of triglycerides as ammonium adducts (also in DESI²⁶) as well as protonation of ORO in the positive ion mode while favoring the deprotonation of fatty acids in the negative ion mode. ESI(+)-MS of ORO yields the expected m/z 409.2 ion (Figure 1A) consistent with the C₂₆H₂₅N₄O⁺ elemental composition of this molecule when protonated (Figure 1B, with R₁ = R'₁ = CH₃). Yet, this mass spectrum also exhibits two other major signals at m/z 395.2 and m/z 381.2, that is, with either 14 or 2 × 14 Da mass defect compared to ORO, respectively. Based on accurate mass measurements (Table S1), each 14 Da mass defect corresponds to CH₃/H substitution in the ORO structure. The three ions detected in ESI(+)-MS of the ORO solution were then submitted to CID, and fragmentation of the two unknown species was studied with regard to the MS/MS pattern of [ORO + H]⁺.

As proposed in Scheme S1 and supported by accurate mass data in Table S2, two main dissociation pathways would proceed by cleavage at each N=N group, the first one yielding m/z 290.1 (in red) and the second one producing m/z 252.1 (in blue) from [ORO + H]⁺ in Figure 1C1. From [II + H]⁺ lacking two methyl group (Figure 1C3), the red pathway produces the m/z 276.1 fragment, which indicates R'₁ = H, whereas the m/z 224.1 ion obtained via the blue pathway
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<figure><img src="image_6.png" /><figcaption>**FIGURE 1** (A) Positive ion mode electrospray ionization mass spectrometry (ESI[+]-MS) of ORO in methanol with 3 mM ammonium acetate. (B) The fragmentation scheme proposed to account for main product ions observed upon collision-induced dissociation (CID) of (C1) [ORO + H]⁺ at m/z 409.2 (R₁ = R'₁ = CH₃), (C2) [I + H]⁺ at m/z 395.2 (R₁ ≠ R'₁, H/CH₃), and (C3) [II + H]⁺ at m/z 381.2 (R₁ = R'₁ = H). [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>
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indicates R₁ = H. In contrast, two fragments are generated according to the red pathway in the case of [I + H]⁺ (Figure 1C2): this suggests that m/z 395.2 precursor ions consist of two isomers, Iₐ with R₁ = CH₃ and R'₁ = H to account for m/z 276.1 and Iₐ with R₁ = H and R'₁ = CH₃ to account for m/z 290.1, both yielding the same m/z 238.1 via the blue pathway. These results are consistent with fragments expected from the third main dissociation reaction (in purple), which yields m/z 105.1 for species with R₁ = CH₃ (i.e., ORO and Iₐ) and m/z 91.1 for species with R₁ = H (i.e., Iₐ and II). These findings are also supported by signals recorded using ¹H NMR (Figures S1-S4), which also reveal positional isomers regarding the location of the R₁ methyl group. Although not excluding the presence of other isomers, ¹H NMR provides clear evidence of species with CH₃ in para, expected to exhibit the same MS/MS pattern as the structure with this methyl group in meta. Although commercially available as a 408.2 Da molecule, ORO is instead a mixture of compounds whose composition can be rationalized based on the patent that describes the preparation of oil-soluble red dyes in four steps:²⁷ (a) diazotization of a mixture of toluidines and xylidines, the result of which is (b) coupled with a similar toluidines/xylidines mixture to yield an aminoazo intermediate further (c) subjected to diazotization before (d) being coupled with beta-naphthol. Our set of MS(/MS) and NMR data would therefore indicate that the analyzed ORO reagent was produced using, in the two first steps, 2-amino-1-methylbenzene as the main toluidine and different xylidines, including 1-amino-2,4-dimethylbenzene (Scheme 1).

ESI-MS data were then recorded for an equimolar (10 µM) mixture of ORO and model compounds, showing no evidence for any reaction products between the FET reagent and either triolein or LA, as also confirmed by ¹H NMR (Figure S5). In the positive ion mode, besides the three major signals previously observed from the ORO reagent, triolein is detected as an ammonium adduct at m/z 902.8 (Figure 2A). In the negative ion mode, deprotonated molecules are detected for LA at m/z 279.2 and, to a lower extent, for ORO-related species (Figure 2B). As compared to data recorded for individual

solutions, a slight decrease in ion abundance is observed for all species, the extent of which ranges from about 10% for protonated ORO to about 18% for ammoniated triolein (Figure S6). Interestingly, for ORO components, suppression effects decrease as the number of CH₃ groups increases, consistent with the increased hydrophobicity expected for methyl-rich species that would favor their release as gas phase ions from the charged droplets.²⁸,²⁹ Nevertheless, the extent of overall suppression effects remains quite low, which should not jeopardize the detection of targeted species in DESI.

## 3.2 | DESI-MSI of ORO-processed fingermarks

DESI-MS was first performed for imaging portions (10 × 3 mm) of fingermarks enriched with triolein and LA and revealed on white paper by ORO. Using the optical image as a reference (Figure 3A), the spatial fingerprint pattern is clearly detected in chemical images recorded in positive ion mode for protonated species of the ORO reagent at m/z 409.2, m/z 395.2, and m/z 381.2 (Figure 3B). A similar quality level is obtained for the image reconstructed with the triolein ammonium adduct, [Tr + NH₄]⁺ at m/z 902.8 (Figure 3B): as compared to data recorded before treatment, only a 10% decrease was measured for the abundance of this ion after ORO application (Figure S7). In contrast, substantial delocalization of LA occurred upon ORO application: as compared to the image reconstructed for the m/z 279.2 anion before fingerprint enhancement where the ridges and minutiae can clearly be distinguished, no pattern can be observed after the application of ORO (Figure 3C). In other words, although still detected, deprotonated LA can no longer be used to create chemical images once fingerprints are treated with ORO. The same effect was previously reported by Chaurand and coworkers while using silver-assisted laser/desorption ionization³⁰ to image ORO-processed latent fingermarks in MS.²¹

The same instrumental settings were then used to perform positive mode DESI-MSI of natural fingermarks revealed by ORO, but

<figure><img src="image_8.png" /><figcaption>**SCHEME 1** Synthesis of oil-soluble red dyes involving a mixture of toluidine (R₁ = R'₁ = H) and xylidine (R₁ = R'₁ = CH₃) to account for the three main species detected in the Oil Red O (ORO) sample.</figcaption></figure>
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<figure><img src="image_10.png" /><figcaption>**FIGURE 2** (A) Positive and (B) negative mode electrospray ionization mass spectrometry (ESI-MS) of a 10 µM solution of Oil Red O (ORO, in red), triolein (in blue), and LA (in green) in methanol containing 3 mM ammonium acetate. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**FIGURE 3** (A) Optical and (B) positive ion mode desorption electrospray ionization (DESI[+]-MS) images of enriched fingermark revealed by Oil Red O (ORO). (C) Negative ion mode desorption electrospray ionization (DESI[−]-MS) images of deprotonated linoleic acid (LA) at *m/z* 279.2 before and after ORO application. Chemical images (10 × 3 mm) were recorded by scanning the surface at a 100 µm *s*⁻¹ rate (acquisition time ≈1.5 h). [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

only poor-quality images were obtained due to a very low abundance of targeted cations, including those of the ORO reagent (Figure S8A). This shows that the actual amount of ORO dyes left in the revealed fingermarks after the washing step with water highly depends on the

amount of lipids in which they can dissolve. One influential factor found to improve the DESI extraction yield was to decrease the scanning velocity, of course, at the expense of the time requested to record images. Scanning sample surface at 50 µm *s*⁻¹ permitted to
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**FIGURE 5** Optical and positive ion mode desorption electrospray ionization (DESI[+]-MS) images of natural fingermarks revealed by Oil Red O (ORO) on (A) black and (B) pink paper substrates. Chemical images (10 × 3 mm) were recorded by scanning the surface at a 50 µm s⁻¹ rate (acquisition time ≈3 h). [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

reconstruct chemical images but only for ORO components (Figure S8B). On average, decreasing the scanning rate by a factor of 2 allows a fivefold increase in ion abundance (Figure S8C).

Despite being in low abundance, many other signals could be observed in mass spectra extracted from these images, notably in the m/z range expected for the detection of triglycerides as ammonium adducts (Figure 4 and Table S3). Detailed analysis of these species is out of the scope of the present study, but these data show that chemical profiling is still possible in ORO-processed natural fingerprints, although structural characterization using MS/MS would require the DESI experiments to be performed in the static mode to further increase ion abundances.

Yet, chemical imaging of the FET reagent remains a valuable option to provide a contrast that cannot be obtained optically in the case of colored substrates. This is illustrated in Figure 5A for ORO-processed natural fingermarks deposited on a piece of black paper, where chemical images of the three ORO components reveal clear details of the fingermark that remain optically unnoticed. Although spectacular, this proof-of-concept example remains an unrealistic scenario because FET-processed fingermarks first need to be visually localized to define the substrate area to be imaged using MS (therefore precluding the use of ORO as a FET for black porous

substrates). One more realistic case is depicted in Figure 5B, where DESI-MS images of the ORO cations offer the visual contrast lacking in optical images of ORO-processed natural fingermarks on pink paper. In particular, compound I found to be the most abundant component in ORO reagent allows the best chemical images upon extraction of the m/z 395.2 ion.

4 | CONCLUSIONS

In this work, DESI-MSI has been successfully applied to latent fingermarks deposited on paper and following their optical revelation by ORO. Application of this FET reagent did not prevent the extraction and subsequent ionization of the fingermark molecular material using DESI. For enriched fingermarks, ridges and minutiae were clearly distinguished in MS images generated from the ammonium adduct of triolein and protonated ORO species. This was no longer the case when examining the spatial distribution of LA found to be strongly delocalized upon ORO application. In natural fingermarks, the scanning velocity of the DESI probe had to be lowered to increase the amount of extracted analytes in the secondary droplets. In these conditions, MSI was possible only for
<figure><img src="image_14.png" /></figure>

ORO species, but DESI-MS showed potential for the chemical profiling of endogenous molecules such as triglycerides. ORO impurities described here for the first time were particularly useful in producing MS images of higher quality than optical ones for natural fingermarks revealed on colored porous substrates.

## AUTHOR CONTRIBUTIONS

**Mariska Banidol:** investigation (MS and DESI-MS imaging experiments). **Sophia Kouider:** investigation (NMR experiments). **Isaure Sergent:** investigation (theoretical calculation). **Hélène Pizzala:** supervision of NMR experiments. **Laurence Charles:** conceptualization, supervision of MS and MSI expertiments, writing, review and editing.

## ACKNOWLEDGMENTS

This project was supported by the Forensic Science Department of the French National Gendarmerie. L.C. thanks the Région Provence-Alpes-Côte d'Azur (Research Program 2016, project ENIGMass) for financial support for the acquisition of the Prosolia DESI source and acknowledges support from Spectropole, the Analytical Facility of Aix-Marseille University, by allowing a special access to the instruments purchased with European Funding (FEDER OBJ2142-3341). This work was also supported by the computing facilities of the Centre Régional de Compétences en Modélisation Moléculaire (CRCMM) of Aix Marseille University.

## CONFLICT OF INTEREST STATEMENT

The authors declare no competing financial interest.

## DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## ORCID

Laurence Charles | https://orcid.org/0000-0003-3807-8375

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## SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

**How to cite this article:** Banidol M, Kouider S, Sergent I, Pizzala H, Charles L. Desorption electrospray ionization mass spectrometry imaging of latent fingerprints revealed by Oil Red O. *Rapid Commun Mass Spectrom.* 2024;38(8):e9724. doi:10.1002/rcm.9724