In desorption electrospray ionisation (DESI) the interaction between the electrospray and the surface is key to two important analytical parameters, the spatial resolution and the sensitivity. We evaluate the effect of the electrospray solvent type, organic solvent fraction with water, analyte solubility and substrate wettability on DESI erosion diameter and material transferral into useful ion signal. To do this five amino acids, glycine, alanine, valine, leucine and phenylalanine are prepared as thin films on three substrates, UV/ozone treated glass, glass and polytetrafluoroethylene (PTFE). Four different solvents, acetonitrile (ACN), methanol (MeOH), ethanol (EtOH) and propan-2-ol (IPA), are used with organic solvent fractions with water varying from 0.1 to 1. These model systems allow the solubility or wettability to be kept constant as other parameters are varied. Additionally, comparison with electrospray ionisation (ESI) allows effects of ionisation efficiency to be determined. It is shown that the DESI efficiency is linearly dependent on the solubility (for these materials at least) and for analytes with solubilities below 1.5 g kg(-1), additional strategies may be required for DESI to be effective. We show that the DESI erosion diameter improves linearly with organic solvent fraction, with an organic solvent fraction of 0.9 instead of 0.5 leading to a 2 fold improvement. Furthermore, this leads to a 35 fold increase in DESI efficiency, defined as the molecular ion yield per unit area. It is shown that these improvements correlate with smaller droplet sizes rather than surface wetting or ionisation.

Latent fingerprints provide a potential route to the secure, high throughput and non-invasive detection of drugs of abuse. In this study we show for the first time that the excreted metabolites of drugs of abuse can be detected in fingerprints using ambient mass spectrometry. Fingerprints and oral fluid were taken from patients attending a drug and alcohol treatment service. Gas chromatography mass spectrometry (GC-MS) was used to test the oral fluid of patients for the presence of cocaine and benzoylecgonine. The corresponding fingerprints were analysed using Desorption Electrospray Ionization (DESI) which operates under ambient conditions and Ion Mobility Tandem Mass Spectrometry Matrix Assisted Laser Desorption Ionization (MALDI-IMS-MS/MS) and Secondary Ion Mass Spectrometry (SIMS). The detection of cocaine, benzoylecgonine (BZE) and methylecgonine (EME) in latent fingerprints using both DESI and MALDI showed good correlation with oral fluid testing. The sensitivity of SIMS was found to be insufficient for this application. These results provide exciting opportunities for the use of fingerprints as a new sampling medium for secure, non-invasive drug detection. The mass spectrometry techniques used here offer a high level of selectivity and consume only a small area of a single fingerprint, allowing repeat and high throughput analyses of a single sample.

Ambient mass spectrometry (ambient MS) is a powerful and rapidly growing new field that provides high sensitivity MS directly from surfaces at ambient pressure. There is now a rich evidence base in the published literature of the success of these methods for forensic analysis including: detection of explosives at nanogram levels; chemical composition of counterfeit pharmaceutical tablets, detection of drugs of abuse from biological liquids such as urine and plasma; breath analysis of metabolites; and imaging analysis for document verification and fingerprint identification. Recent developments in miniaturised (shoe-box sized) mass spectrometers have enabled these developments to be translated to portable on-scene detection and first responder usage. In this review, we illustrate and compare the effectiveness of the most popular and promising techniques of desorption electrospray ionisation (DESI), direct analysis in real time (DART), plasma assisted desorption ionisation (PADI) and extractive electrospray ionisation (EESI). Forensic analysis by its very definition must stand up to scrutiny in a court of law. It is therefore essential that measurements are repeatable, valid, traceable and fit for purpose. The establishment of a measurement infrastructure is therefore essential to ensure that the methods used may be accredited and conform to relevant quality systems and procedures.

Results are presented on the optimization and characterization of a plasma-assisted desorption ionization (PADI) source for ambient mass spectrometry. It is found that by optimizing the geometry we can increase ion intensities for valine and by tuning the plasma power we can also select a more fragmented or less fragmented spectrum. The temperature of the surface rises linearly with plasma power: at 19 W it is 71 °C and at 28 W it is 126 °C. To understand if the changes in signal intensity are related to thermal desorption, experiments using a temperature-controlled sample stage and low plasma power settings were conducted. These show markedly different signal intensities to experiments of equivalent surface temperature but higher plasma power, proving that the mechanisms of ionization and desorption are more complicated than just thermal processes. Four different polymers, poly(methyl methacrylate) (PMMA), poly(ethylene terephthalate) (PET), poly(lactic acid) (PLA), and poly(tetrafluoroethylene) (PTFE), are analyzed using PADI. Mass spectra are obtained from all the polymers in the negative ion mode and from PMMA and PLA in the positive ion mode. For each polymer, characteristic ions are identified showing the ability to identify materials. The ions are formed from bond cleavage with O and CH(2) as common adducts. Ions were detected up to m/z 1200 for PTFE.