Rapid, Parallel Imaging in Surface Chemistry and Biochemistry

Spatial imaging mass spectrometry is an analytical technique of growing importance, with a wide range of exciting applications ranging from forensics, tissue sampling, to parallel, high throughput chemical analysis, and is increasingly being applied to the characterization of biological samples. Spatial imaging MS is usually performed in one of two ways, microprobe or microscope mode. In the former, microprobe mode, an ionization source, such as a stream of ions or laser radiation, are focused to a small point on the sample and a mass spectrum is recorded at that precise location. If an ion beam is employed, the technique is usually referred to as secondary ion mass spectrometry, or SIMS. The sample or the ion source is then moved to a new position, and the process repeated until an entire spatially resolved mass spectrum is built up. By contrast, in microscope mode the entire surface is ionized, usually with an intense pulse of laser radiation, and the ions are typically recorded on a two-dimensional detector. Application of fast imaging sensors, with tens of nanosecond timing resolution, to microscope mode mass spectrometric imaging will allow the spatial imaging of all mass peaks in each experimental cycle. As all fragments can be detected simultaneously, far fewer laser shots and acquisition cycles are required for a full set of data to be acquired. A smaller amount of sample is required, samples suffer less degradation, and overall collection times are reduced.
Our proof-of-concept experiments in this area have yielded extremely promising results, and we are currently working to improve our spatial and mass resolution, and sample preparation techniques. SAI, a UK manufacturer of matrix-assisted laser desorption/ionization (MALDI) instruments, is very keen to collaborate on this aspect of the project, providing a specially modified commercial MALDI spectrometer, LaserToF LT2Plus, on which the fast imaging sensors can be tested. In the new instrument, the positions of release of the various molecular ions will be preserved and faithfully mapped onto a position sensitive detector. Their molecular weights can then be calculated from the time of flight of the various molecular ions. The spatial resolution of the instrument will be determined by a combination of the spherical aberration in the image forming electrostatic lens, the pore size of the ion detector, and the pixel size of the imaging sensor, and could in principle achieve 0.25 microns.
A number of established markets would benefit from this improvement in the measurement technology, in particular in the biochip testing industry. Biochips have been developed for high throughput analysis, and, in the case of reverse phase protein micro-arrays, some 500 samples are typically spotted in an area of less than five square millimetres. They are currently read sequentially, employing florescence or colorimetry techniques, which is costly and could be insufficiently sensitive. The proposed development will dramatically improve parallel measurements using biochip technology, yielding much faster analysis times and higher precision, whilst at the same time elimination the need for expensive photo-chemicals.