title: Swab spray mass spectrometry: An electrospray-based ambient ionization technique for the analysis of surfaces
Creators: name: Muggli, Thomas Michael
Publisher: name: Universität Bern
Year: 2025
Subjects: subject: 540 Chemistry

description: In 2004, the first ambient ionization technique, termed desorption electrospray ionization (DESI), was presented. It was the beginning of an evolvement that yielded numerous ionization techniques, which allow minimization of sample preparation, permit ionization at ambient conditions, and moreover, offer the opportunity for insitu analysis. Direct Analysis in Real Time (DART), the first plasma-based ambient ionization technique, was introduced in 2005. Ionization of a broad range of analytes was achieved using a plasma produced through corona discharge. Electrospray continued to demonstrate its versatility for ambient ionization techniques with the introduction of probe electrospray in 2007 and paper spray in 2010. In 2015, swab spray was reported as a novel electrospray-based ambient ionization technique that merges sample collection and direct ion generation by application of solvent and high voltage directly to the swab head. Its potential has been reported in several forensic and medicinal applications, including the analysis of drugs and explosives, demonstrating that swab spray can deliver fast and reliable results while minimizing sample preparation. The use of swabs as electrospray emitters demonstrated new capabilities along with new challenges. On the one hand, swab materials must be carefully evaluated to minimize background ions and on the other hand, reproducible electrospray generation on swab tips requires diligent optimization of ion source parameters and electrospray solvent. In the first part of this thesis, a custom-made swab spray ion source was constructed and different swab materials were extensively investigated for their suitability to act as an electrospray emitter. Copan 160C swabs with a rayon head and aluminum applicator were found to be an optimal choice. They provide low background noise, high electrospray stability, and high tolerance for organic solvents. Moreover, swab spray ion source parameters, including the distance to the counter electrode, applied electric potential, solvent capillary position, and flow rate were evaluated. The influence of electrospray solvent properties, including surface tension, viscosity, and relative permittivity was investigated using photographic observation and signal yield, in order to predict solvent behavior in swab spray. It was observed that only spray solvents with a relative permittivity higher than 25 allow the generation of a short jet. The generation of a short jet is mandatory for the formation of a spray plume where the ionization takes place. Spray solvents with a lower relative permittivity exhibit a long jet, which is detrimental to ion formation. The influence of solvent relative permittivity in swab spray was demonstrated for the first time and results compared to paper spray and capillary-based electrospray ionization. In addition, different pulsation modes, such as the spindle mode and the dripping mode, the later occurring in spray solvents with a relative permittivity lower than 4, were examined. In the second part of the thesis, the swab spray setup was applied to rapid pesticide analysis on the surface of fruit. Pesticides were identified by their mass, isotopic pattern, and product ion spectra obtained by CAD experiments. Semi-quantitative analysis of the fungicide boscalid allowed to determine surface concentrations on a bell pepper surface. The occurrence of sodium adducts was linked to electrospray solvent composition and chemical structure of the analyte. The presence of ethyl acetate in the spray solvent was found to facilitate the formation of proton adducts. Further applications involved the analysis of skin for organic compound residues, identification of white powder, differentiation of ball point ink, and analysis of mycotoxins. The versatility of swab spray mass spectrometry makes it invaluable for applications requiring rapid analysis of organic compounds sampled from diverse surfaces, as it allows for easy sampling and requires no sample preparation. The analysis time can be significantly reduced compared to conventional laboratory-based methods, and furthermore, it enables in-situ mass spectrometry.