PhD (1995 - 1998)
Dr Alison Hollingsworth

I completed my Chemistry Degree at Heriot-Watt University in 1995 before coming to Edinburgh University to do my PhD. After completing my thesis I started a job with Nycomed Amersham in the Imaging Manufacturing Division.

The combination of lasers with time-of-flight mass spectrometry provides a powerful tool for many types of analytical and fundamental physical chemistry. Laser mass microscopy is a technique used to characterise samples of both industrial and fundamental significance. This technique offers information on the spatial distribution of such molecules within their host environments, as well as mass information, unique to that molecule.

Several types of microscopy exist for the elucidation of mixture composition and distribution. Laser mass microscopy has been used recently by Zare’s group for the analysis of meteorite samples. They were able to map across minute meteoric samples, detecting indigenous constituents such as polycyclic aromatic hydrocarbon compounds, and providing an overall picture of the spatial content of the sample.

Our instrument is a reflectron time-of-flight mass spectrometer with an automated sample stage, which can move in three dimensions in micron-sized steps. The apparatus comprises two differentially pumped chambers: the sample desorption / ionisation chamber, and the sample introduction chamber. The molecule to be studied is dissolved in a minimum amount of solvent and deposited onto a stainless steel probe, which is then introduced, via a fast load lock system, into the main chamber, which operates at a working pressure of 10^-7 to 10^-8 mbar.

Desorption of neutrals and ions into the gas phase can be carried out by a nanosecond pulsed TEA CO₂ laser. However, the diffraction limited spot size of light of this infrared wavelength is tens of microns, and for microanalysis we wish to study very small areas of a sample at a time, hence increasing our spatial knowledge of the sample. It is for this reason that ultraviolet wavelengths are used for desorption, as the diffraction limited spot size at a wavelength of 266 nm in the ultraviolet, is below one micron. Ionisation can be carried out by a variety of wavelengths, depending on the absorption spectroscopy of the sample in question. Positive product ions are then accelerated into a field-free drift region before entering the reflectron which turns the ions around, whilst compensating for variation in kinetic energy, resulting in high mass resolution of the signal at the microchannel plate detector.

We have performed some preliminary mapping experiments on our laser mass microscope. These illustrate the capability of our instrument to provide spatial information about a combination of samples desorbed onto a surface, and also some information on the relative abundance of different molecules. Future work for the microscope includes the analysis of more analytically appropriate samples from within their native environment.

There is an inherent trade-off between the high spatial resolution offered by small desorption spot sizes, and sample sensitivity. At particularly small spot sizes, the amount of material desorbed is too small for detection. We have made investigations into a solution to this problem, by carrying out some novel experiments using a state-of-the-art ultrafast laser system, which outputs femtosecond pulses.

In the case of sub-picosecond (10^-12 s) laser ionisation, the mechanism of photoionisation is not necessarily obvious. Several possibilities exist, such as multiphoton ionisation, tunnelling ionisation and barrier suppression. The latter two of these mechanisms have a high probability of occurring with high power femtosecond lasers, which can produce power densities in excess of 10¹⁴ W cm^-2. The strength of the electric field generated by such pulses can be sufficient to perturb the potential surface of the molecule, possibly rendering one molecule as equally likely to absorb photons as a second molecule with normally different photoabsorption properties.