Box 22-4 Matrix-Assisted Laser

Major methods for introducing proteins and other macromolecules into mass spectrometers are electrospray and matrix-assisted laser desorption/ionization (MALDI).18-27 Most often, MALDI is used with a time-of-flight mass spectrometer, which can measure mlz up to 106. Typically, 1 p,L of a 10 jxM solution of analyte is mixed with 1 p,L of a 1-100 mM solution of an ultraviolet-absorbing compound such as 2,5-dihydroxybenzoic acid (the matrix) directly on a probe that fits into the source of the spectrometer. Evaporation of the liquid leaves an intimate mixture of fine crystals of matrix plus analyte. 

To introduce ions into the gas phase for mass spectrometry, a brief infrared or ultraviolet pulse (600 ps) from a laser is directed 

Sequence of events in matrix-assisted laser desorption/ionization. (a) Dried mixture of analyte and matrix on sample probe inserted into backplate of ion source. (foZ) Enlarged view of laser pulse striking sample. (t 2) Matrix is ionized and vaporized by laser and transfers some charge to analyte. (b3) Vapor expands in a supersonic plume. 

Partial mass spectrum of cow s milk (containing 2% milk fat) observed by MALDI/time-of-flight mass spectrometry. [From R. M. Whlttal and L Li, Time-Lag Focusing MALDI-TOF Mass Spectrometry, Am. Lab. December 1997, p. 30.] 

SOURCE Data from T. Wachs and J. Henion, Electrospray Device for Coupling Microscale Separations and Other Miniaturized Devices with Electrospray Mass Spectrometry, Anal. Chem. 2001, 73, 632. See also M. Mann, C. K. Meng, and J. B. Fenn, Interpreting Mass Spectra of Multiply Charged Ions Anal. Chem. 1989, 61, 1702. 

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