Matrix-Assisted Laser Desorption Ionization

MALDI is an ionization technique for large and/or labile molecules such as peptides, proteins, polymers. One to two microliters of the sample in solution is mixed with or deposited on a UV-absorbing matrix on a stainless steel target and allowed to dry. The sample must be soluble in a solvent that will air-dry. The target holds 26 sample spots. Liquid matrices cannot be used. The laser light (337 nm Nitrogen) strikes the target and ions and neutrals are desorbed. The matrix greatly increases the desorption efficiency. MALDI produces both positive and negative ions of the form (M+H)+ and (M-H)-. The technique also produces multiply charged ions, usually up to +3, as well as dimers, trimers, etc. The upper mass limit for MALDI is about 350,000 amu. MALDI is useful for mixtures such as tryptic digests since the technique produces predominantly (M+H)+ species for lower molecular weight compounds.

Buffers Salts

Low levels of some salts, buffers, and detergents can be tolerated as well as less than 2% of glycerol. Water, acetonitrile, methanol, THF, and other organic solvents can be used.
DMSO cannot be used, since even traces of DMSO will cause problems.

Sample Submission Requirements

Sample size depends on molecular weight, the higher the molecular weight the less the sample that is needed. Samples are dissolved in a suitable solvent. Proteins and peptides are usually dissolved in H2O with 0.1% TFA. 10-20 pmol/microliter is needed for MW 1000, 5-10 pmol/microliter for MW 20,000, and 1 pmol/microliter is needed for MW 60-100,000. In some cases as little as 250 fmoles of sample is needed on the target.

It is best not to add acid or solvent to the sample before submitting for ESI or MALDI. I will dilute the sample to the proper concentration just before running the sample. Samples should be submitted in Eppendorf-type tubes 0.5ml or 1.5ml. You can obtain some from me if you need them.
I generally require a minimum of 1 mg of sample for analysis. Note I will return samples back to you when completed.

Last Update Friday, 02 October 2009
By Frank Antolasic
e-mail frank.antolasic@rmit.edu.au