Sample Preparation in Mass Spectrometry - Matrix-assisted Laser Desorption/ionization

Matrix-assisted Laser Desorption/ionization

For matrix-assisted laser desorption/ionization (MALDI) mass spectrometry the sample is usually mixed with a matrix solution and spotted on the target plate. The matrix crystallises together with the sample and the analyte molecules are transferred to the gas phase by the pulsed laser irradiation.

The concentration of salt in the sample is thereby not so critical as it is for electrospray ionization. However, interfering signals are observed due to side reactions of the matrix with alkali metal ions which can impair the analysis of the spectra. Nevertheless, a separate desalting step is usually not necessary as in previous steps of sample preparation e.g. in-gel digestion of proteins, the selection of appropriate buffer salts prevents the occurrence of this problem. Furthermore the crystallised matrix/analyte mixture allows removing of salts by washing on the target and the addition of ammonium phosphate before crystallisation can improve the signal quality.

MALDI matrices used for identifying peptides include CHCA and DHB. These solutions are typically saturated solutions in a 50% Acetonitrile/0.1% TFA; TFA being the protonating agent. Typically the sample solution and matrix solution is mixed in a 1:1 ratio and then a half a microliter to a microliter is added to the MALDI plate. Proteins use SA (sinapinic acid) as their matrix; which is prepared in the same way as the peptide matricies.