Plasma characterisation and optimisation of methods

In this project, the Miniaturisation group works out the basics of their novel ionisation and sample supply techniques that are afterwards applied to health research problems in cooperation with the other ISAS groups.

One important aspect of this work is plasma characterisation, as the different plasma modes probably determine the methods for which a plasma can best be deployed: filamentary plasmas appear to be better suited to element spectroscopy, whilst homogenous plasmas could well be deployed for the soft ionisation of molecules, for instance in mass spectrometry. The Miniaturisation group therefore focuses on the question of how the different modes can be optimised and used in analytical chemistry. Currently, this work primarily concentrates on measuring electrical currents and emissions with temporal resolution to depict the plasma creation process as well as mode changes. Moreover, this method offers the opportunity of intervening at specific points in the process and encouraging the plasma to move towards one mode or the other.

The group is also developing two new methods for the soft ionisation of samples, primarily to be used in mass spectrometry: one is a plasma jet created by dielectric barrier discharge and the second is a dielectric barrier electrospray. The plasma jet is best suited to ionising non-polar molecules. Compared to the common methods of ionising non-polar molecules, such as APCI (atmospheric pressure chemical ionisation), the advantage is that the plasma jet is created outside the capillary, so that the structure cannot be polluted by vaporised material. It also seems to ionise some polar molecules, which would extend the range of its application still further. Polar, large molecules can be softly ionised by DB-ESI and subsequently identified by mass spectrometry. The electrodes do not corrode because they are separated from the sample by an isolating dielectric barrier, so that DB-ESI is much better suited to long-term measurements than conventional nano electrosprays.

Coupling both techniques should enable the parallel identification of completely different components in one sample. Therefore, the group is investigating ways to operate a plasma jet and DB-ESI simultaneously at the inlet to a mass spectrometer, and is furthermore endeavouring to couple both ionisation methods with separation techniques, such as gas chromatography or liquid chromatography.

Another aspect of the work is to find new methods of supplying samples for MS analyses, which allow the analytes to be separated directly from the surface of liquid or dried samples, for instance by means of laser desorption. This would render lengthy sample preparation workflows unnecessary, making analyses faster and more reliable. Desorbed molecules could then be simultaneously excited by the synchronised ionisation techniques DB-ESI and plasma jet and measured by ion mobility spectrometry or mass spectrometry. The group has already been able to demonstrate the feasibility of laser desorption by detecting pesticides on the skins of oranges. However, the technique may also be suitable for numerous biomedical applications because it requires no sample preparation and the analyses can be performed at ambient pressure conditions. In theory, the technique might even be usable for analyses directly from cell cultures. In order to further develop laser desorption into a reliable technique, the group is currently testing various lasers and laser powers and is working on the preparation of appropriate surfaces for sampling.