Matrix-Assisted Laser Desorption Ionization
MALDI is an ionization technique for large and/or labile molecules such as peptides, proteins, polymers, dendrimers, and fullerenes. This technique involves embedding analytes in a matrix which absorbs energy at the wavelength of the laser. The detailed mechanism of the MALDI process, a combination of desorption and ionization, is still being investigated. MALDI produces both positive and negative ions, usually in the forms of (M+H)+, (M+Na)+ and (M-H)-. The technique also produces multiply charged ions, usually up to +3, as well as dimers, trimers, etc.
Low levels of some salts, buffers, and detergents can be tolerated as well as less than 2% of glycerol. However, data quality and sensitivity may be compromised. Water, acetonitrile, methanol, THF, and other volatile organic solvents can be used. DMSO and DMF cannot be used. Samples should be submitted in small Eppendorf-type tubes.
Sample size depends on molecular weight, the higher the molecular weight the more the sample that is needed. Samples are dissolved in a suitable solvent. Proteins and peptides are usually dissolved in H2O with 0.1% TFA. 1-10 pmol/µl is needed for MW 1000, 10-50 pmol/µl for MW 20,000, and 100-500 pmol/µl is needed for MW 60-100,000. In some cases as little as 250 fmoles of sample is needed on the target. See the table below for the amounts needed for different types of samples. The upper mass limit for MALDI is about 350,000 Da. MALDI is useful for mixtures such as tryptic digests since the technique produces predominantly (M+H)+ species for lower molecular weight compounds.
Output for MALDI is in the form of smoothed averaged data.
Sample amounts needed for MALDI
|Dry Sample||In Solution (No DMSO unless necessary)|
|Proteins/Peptides||6 nmol||0.1 µM|
|DNA/RNA||50-100 pmol in 5-10µL|