My research focuses on improving mass spectrometry instrumentation for the analysis and quantification of large biomolecules, including whole proteins. Using mass spectrometry, it is easier to analyze small molecules than large molecules. One reason for this bias toward smaller molecules is sample introduction. In order for samples to be analyzed by mass spectrometry, the sample needs to be in the gas phase and ionized. It is much easier to introduce smaller molecules into the gas phase using traditional methods, because large molecules tend to decay rather than evaporate. Methods such as electrospray ionization (ESI) and matrix assisted laser desorption/ ionization (MALDI) are capable of introducing large molecules into the gas phase, however, these methods have their own disadvantages. With electrospray ionization, for example, each species can have many different charge states leading to complex spectra from which quantitative data is difficult to extract.
Another reason for mass spectrometer bias toward small molecules involves current detection systems, which show better response and sensitivity with small molecules. This small molecule bias results in the need to digest large proteins into small peptides in order for the sample to be analyzed. Digesting proteins into small peptides increases the complexity of the resulting spectra.
In order to improve mass spectrometers for the analysis of biomolecules, it is necessary to improve ionization methods and detectors. New ionization techniques need to be developed that provide uniform ionization efficiencies for all peptides and proteins, regardless of sequence or functional groups. Furthermore, it is desirable for ionization techniques to yield molecules in a single charge state, eliminating the complexity and difficulty of extracting quantification data from spectra of molecules in multiple charge states. We are developing an ionization technique that creates neutral gas phase molecules and then allows them to undergo controlled gas phase proton transfer reactions. These reactions would result in molecules having a single positive charge state. By using this method under the proper conditions, ionization efficiency will only be dependent upon molecular weight, which can be accounted for by using physical collision models or calibration curves with known masses. In addition, by improving detector response to high molecular weight molecules, it will be possible to analyze whole proteins without the need for digestion. The new detectors developed should also have high sensitivity with low noise levels, a large dynamic range, fast response times, fast recovery times, and a large active area.
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