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Introduction to MALDI-TOF Mass Spectrometry

What is MALDI-TOF?

Matrix Assisted Laser Desorption - Ionization

Time Of Flight mass spectrometry

University of Wisconsin-Madison's Bruker Reflex II MALDI spectrometer
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In the MALDI-TOF experiment, a laser is pulsed at a target surface, which contains the sample of interest (the analyte) mixed with a chemical matrix. The matrix is chosen so that it strongly absorbs the laser light energy. When this absorption of the energy pulse occurs, a supersonic explosion results, and matrix molecules are ejected from the surface, taking some analyte molecules with them into the evacuated chamber.
As is shown in the diagram, ionization of the matrix and analyte and the matrix often occurs. Ionized particles are then accelerated down the flight tube by an applied voltage. The experimenter can choose to accelerate and detect either cations or anions. As in all "time-of-flight" experiments, the mass/charge (m/z) ratio is determined by carefully measuring the time it takes for the charged particles to reach the detector after ejection from the source. Particles with larger m/z move more slowly down the tube, and thus have a longer time-of-flight.
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The data presented here were taken in "reflectron" mode, which means that the path from the source to the detector is not a single straight line. Instead, the particles' direction is reversed by a second voltage and bent back towards the detector. Reflectron mode is more versatile than linear mode, which uses a straight path.

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The more traditional electron ionization method (EI) is very harsh, and it is often difficult to observe parent ions. Liquid secondary ion mass spectrometry (LSIMS) is a milder method, but the often highly reactive molecules studied by organometallic chemists can interact with the liquid matrix before ionization. Electrospray is another method that has been applied to organometallics, but it too is a "wet" method, and is prone to some of the same interactions. The matrix and sample in MALDI-TOF are in the solid phase, so pre-ionization reactions are minimized. In addition, this poster will demonstrate that the light absorption properties of some organometallics make it possible to eliminate the matrix entirely.
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Useful references:

Electrospray: L. A. P. Kane-Maguire, R. Kanitz, M. M. Sheil, "Electrospray mass spectrometry of neutral p-hydrocarbon organometallic complexes." Inorg. Chim. Acta 245, 209-214 (1996).
C. E. C. A. Hop, J. T. Brady, R. Bakhtiar, "Transformation of neutral rhenium compounds during electrospray ionization mass spectrometry." J. Am. Soc. Mass Spectrom. 8, 191-194 (1997).
LSIMS: A. L. Bandini, G. Banditelli, S. Catinella, P. Traldi, "Fast-atom bombardment in the characterization and reactivity study of some iridium (I) complexes." Rapid Commun. Mass Spectrom 11, 936-938 (1997).
MALDI-TOF: M. J. Dale, P. J. Dyson, B. F. G. Johnson, P. R. R. Langridge-Smith, H. T. Yates, "Laser-desorption mass spectrometry of [Ru6C(CO)17] and its derivatives: cluster aggregation in the gas phase." J. Chem. Soc., Dalton Trans. 771-774 (1996).
C. K. L. Wong, T. D. Chan, "Cationization processes in matrix-assisted laser desorption/ionization mass spectrometry: attachment of divalent and trivalent metal ions," Rapid Commun. Mass Spectrom. 11, 513-519 (1997).
F. Hillenkamp, M. Karas, R. C. Beavis, B. T. Chait, "Matrix-assisted laser desorption/ionization mass spectrometry of biopolymers," Anal. Chem. 63 1193A-1203A (1991).

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Send comments and questions to John R. Nash.