Dr Andrew Bowfield,
Post-doctoral researcher at the University of Liverpool and the National Physical Laboratory

The innovation of desorption electrospray ionisation (DESI) in 2004 heralded the increasingly important field of ambient mass spectrometry (AMS). Within this growing domain, plasma based desorption and ionisation techniques have developed as one of the leading variants, with particular focus at NPL upon plasma assisted desorption/ionisation (PADI).

Recent research conducted by NPL in collaboration with The University of Liverpool and The University of Nottingham is pioneering novel PADI systems in order to facilitate further applications to AMS, namely 2D chemical imaging. Design and construction of microplasma devices so as to limit the size of the plasma plume to < 20µm and hence the desorption/ionisation footprint on a subject surface to a similar area, has already resulted in a publication of ambient mass spectra produced by a plasma confined to the tip of a 200µm diameter tungsten wire in ambient air (Rev. Sci. Instrum. 83, 063503 (2012)). This design is free from the need of a discharge gas, enabling a truly mobile ‘gasless’ plasma device that does not require compressed gas storage.

Efforts to further reduce the volume of the plasma plume for high-resolution imaging are now focused upon two separate designs; a micro hollow cathode discharge (MHCD) device and a microfluidic chip.

(1) MHCD
These devices are microfabricated silicon wafers each with a hole, or ‘hollow’, of varying diameter (100 µm, 50 µm, 20 µm and 10 µm) in the centre of the wafer. A gas, such as He, flows through the hollow and a DC voltage is applied to breakdown the discharge gas. The photograph below shows the plasma produced in a 100µm diameter hollow. The metal cylinder is electrically grounded and one can see the constrained plasma plume. Recent ambient MS studies have shown a profusion of water clusters in the background ions produced by this device, a phenomenon observed using other plasma devices.

(2) Microfluidic Chip
Here, lab-on-a-chip technology is used to fabricate four channels which are 30µm in depth are etched out of a quartz chip and filled with a liquid metal. The distance of closest approach of these ‘electrodes’ is designed to be close to the exit of a gas channel which runs through the centre of the chip and is also 30µm in depth. This should ensure that the plasma plume is able to exit the device over a short distance and remotely activate any surface placed in front of it. Shown below are a diagram of the design of the chip and also a photograph of the chip in operation.

The advantage of developing these devices can be found in the cheap and simple instrumentation required to strike and sustain a stable plasma. Also, the lack of preparation procedures for subject samples, since the plasma plume can interact with a surface remotely, could also allow for high-throughput analysis. This would be of great benefit both academically and commercially. Further, the potential number of different areas this technique could potentially be applied to is vast; from analysing drugs and explosives to breaking down volatile chemicals and hazardous waste products.