David M. Hercules
Research in the Hercules group deals with the development of new instrumental analytical techniques. The major focus has been on application of novel method of mass spectrometry to the study of synthetic polymers and to biomedical, environmental and analytical chemistry.
The Hercules group is active in the characterization of polymers using mass spectrometry. The focus is both on surface analysis and bulk analysis. Surface analysis of polymers involves studying the surface segregation of one component of a block copolymer. For example, in a styrene-siloxane diblock copolymer, the surface layer will be entirely the siloxane component even though the bulk composition is 10% or less siloxane. The extent of surface segregation is measured by secondary-ion mass spectrometry (SIMS).
Matrix Assisted Laser Desorption/Ionization (MALDI)
MALDI mass spectrometry is being used for bulk analysis of copolymers. Of particular interest is determining the molecular weight distribution of a component of a block copolymer as it exists in the polymer chain. Polyurethanes are specific examples studied recently. Polyurethane properties are determined by the molecular weight of the soft block in the polymer chain. Combination of selective chemical reactions with MALDI permit measurement of the soft block (or hard block) molecular weight distributions in polyurethanes.
Single Molecular Weight (SMW) Oligomers
Synthesis of single molecular weight (SMW) oligomers of reasonable size (molecular weight >5000) is being pursued for a variety of reasons. SMW oligomers are important as calibration standards for polymer chromatography, as models to study fragmentation of polymers in mass spectrometry, and as model compounds for the behavior of intractable polymers in mass spectrometry. They are also valuable for comparing ionization processes in the different types of mass spectrometry.
Electrospray ionization (ESI)
ESI mass spectrometry is a particularly versatile method because one can inject solutions directly into the mass spectrometer. This allows one to determine the species of a given element in solution and to observe changes in speciation as solution conditions are changed. This is particularly attractive for environmental chemistry because the chemical behavior (toxicity) of an element varies significantly with its specific molecular form. Because ESI is a soft ionization method, non-covalent complexes can be observed and these can be used to measure the chirality of molecules, something almost impossible with other forms of mass spectrometry.
MALDI is very useful as a rapid screening, quantitative method which makes it ideal for use with combinatorial studies. Rapid screening in clinical and forensic chemistry is very important, and MALDI, coupled with solid-phase extraction, can be used to screen individual or multiple analytes. It is possible to screen for members of a given drug type (benzodiazepines) or a given clinical component (bile acids). Current research is focused on improved ways to use MALDI to accomplish rapid screening, coupled with chemically-selective solid-phase extraction methods for separation of trace analytes from complex matrices.