Robert S. BrownRobert S. Brown

Associate Professor
Analytical Chemistry
B.S., 1978, University of Massachusetts, Lowell
Ph.D., 1983, Virginia Tech
Postdoctoral, 1983-87, University of California, Riverside
435.797.0545 brownusu@cc.usu.edu
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Our research is primarily concerned with understanding and improving the analysis of large, nonvolatile molecules by mass spectrometry. Our principle analysis technique involves the use of high powered pulsed lasers (laser desorption) to effect ionization of a wide range of molecule classes.These include biologically relevant molecules such as proteins, peptides and glycoproteins and synthetic commercial polymers. Mass analysis is carried out via either time-of-flight (TOF) or Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. Both of these mass spectrometers are well suited for pulsed laser ionization sources. FTICR mass spectrometers provide impressive mass resolution and multistage (MS/MS) analysis capabilities while TOF mass spectrometers combine very high sensitivity with an extended mass range for analyte analysis. Intact biomolecules, such as antibodies with molecular weights in excess of 150,000 are easily analyzed via TOF mass spectrometry with sensitivities typically in the femtomole range.

In order to both volatilize and ionize such large molecules, a small quantity (»1 picomoles) of analyte (for example a protein) is mixed with a large excess of a small molecule "matrix" (10,000:1) and the two materials coprecipitate as a solid. The matrix is chosen so that it can provide a source for proton transfer to the analyte for ionization as well as being strongly absorbing at the laser wavelength. For our work, cinnamic acid derivatives are one class of compounds utilized. The mechanism for this matrix assisted laser desorption/ionization (MALDI) process is still not completely understood, but appears to involve the formation of a plume of matrix molecules which are desorbed from the sample surface upon laser irradiation. The nonabsorbing analyte molecules are carried along into the gas phase with the matrix molecules as "free" ions which can then be analyzed by mass spectrometry. Recent work in our laboratory has involved the development of delayed extraction techniques for MALDI time-offlight mass spectrometry. This allows significant improvements in the mass resolution obtainable with time-of-flight mass spectrometers. The delayed ion extraction methodology which our group was the first to demonstrate for MALDI, also allows the study of MALDI metastable ions. In the case of peptides and proteins, metastable ions produced by MALDI fragment along the backbone of the amino acid chain and provide a wealth of sequence specific information.

In addition to improving the instrumental aspects of this technique and its applicability to various chemical and biochemical problems, our research is actively engaged in more fundamental studies of the underlying mechanisms involved. In particular, we are interested in determining the exact role which the matrix plays in the ionization processes. This should facilitate development of improved matrices for current applications and expand the applicability of the technique to additional compound classes. Additional fundamental questions which our research is addressing include: the role of excited state species in the ionization process; the nature and source of the ion activation processes that induce MALDI metastable ion decay; the effect of the laser wavelength (IR vs. UV) and pulse width employed on the MALDI process; and the proton transfer processes involved in analyte ionization. This combination of both fundamental and applied studies provides a better understanding of a very complex process. This combined approach has already produced tangible experimental improvements to one of the most important new analysis method for large biomolecules.

Selected Publications

R.S. Brown and J.J. Lennon, "Mass Resolution Improvement by Incorporation of Pulsed Ion Extraction in a Linear Time-of-Flight Mass Spectrometer," Anal. Chem., 67, 13, 1998, 1995.

R.S. Brown and J.J. Lennon, "Sequence Specific Fragmentation of Matrix-Assisted Laser Desorbed Protein/Peptide Ions,"Anal. Chem., 67, 21, 3990, 1995.

R.S. Brown, B.L. Carr and J.J. Lennon, "Factors Influencing the Observed Fast Fragmentation of Peptides in Matrix-Assisted Laser Desorption," JASMS, 7, 225, 1996.

R.S. Brown, J. Feng and D.C. Reiber, "Further Studies of In-Source Fragmentation of Peptides in Matrix-Assisted Laser Desorption/Ionization," Int. J. Mass Spectrom. Ion Processes, 169-170, 169, 1997.

D.C. Reiber, T.A. Grover and R.S. Brown, "Identifying Proteins Using Matrix Assisted Laser Desorption/Ionization In- Source Fragmentation Data Combined with Database Searching," Anal. Chem., 70, 4, 673, 1998.

D.C. Reiber, S. Weinberger, J. Kenney, J. Bailey and R.S. Brown, "Unknown Peptide Sequencing Using Matrix-Assisted Laser Desorption/Ionization and In-Source Decay," Anal. Chem., 70, 6, 1214, 1998.