Matrix-Assisted Laser Desorption Ionization (MALDI)

MALDI MS was first published by Michael Karas and Franz Hillenkamp1 in 1985 when they used tryptophan as a matrix for other amino acids in laser desorption MS experiments. Koichi Tanaka’s group accidentally discovered that mixing sample with metal particles allows the analysis of large molecules such as intact protein. In 1988, both groups published the application of MALDI MS on intact protein using fine metal granules2 and organic acids3 as matrices, respectively.

Preparation of the Samples

The analyte of interest is dissolved (usually in aqueous solution) and mixed with a large excess of matrix (1000:1 to 10000:1). The matrix usually an organic acid, such as derivatives of benzoic acid or cinnamic acid, but other compounds also work. A small aliquot of this mixture (0.5 - 1.0 microliter) is deposited on the stainless steel sample stage and allowed to air-dry. For calibration, an internal standard may be spiked into the matrix solution or another spot may be placed nearby for external calibration.

Analysis of the Samples

After the sample deposits have dried, the plate is placed in the instrument via a vacuum interlock chamber. Each sample is irradiated with a pulsed nitrogen laser (lambda = 337 nm) to create ions in the gas phase (desorption/ionization). The mass-to-charge ratios (m/z) are measured using an axial TOF mass spectrometer. The ions can be detected in linear mode (higher sensitivity, but lower resolution) or in reflected mode (lower sensitivity, but higher resolution).

Karas, M., Bachmann, D., and Hillenkamp, F. (1985). Influence of the Wavelength in High-Irradiance Ultraviolet Laser Desorption Mass Spectrometry of Organic Molecules. Anal. Chem. 57, 2935-2939
Tanaka, K., Waki, H., Ido, Y., Akita, S., Yoshida, Y., & Yoshida T. (1988). Protein and polymer analysis up to m/z 100,000 by laser desorption time-of-flight mass spectrometry. Rapid Commun. Mass Spectrom. 2, 151-153.
Karas, M., Hillenkamp, F. (1988). Laser desorption ionization of proteins with molecular masses exceeding 10,000 daltons. Anal Chem. 60, 2299-2301.