The quick and dirty sample preparation technique is used by numerous MALDI practitioners to get fast results without consideration of signal optimization [References, 20].
It is simple, fast and broadly applicable. It completely decouples the matrix handling from the sample handling. In most cases very little or no sample purification is needed prior to the analysis.
- Deposit a 1 uL drop of analyte solution (0.1-10 mM) on the sample plate
- Immediately add a 1 uL drop of matrix solution (1-10 uM) on top
- Mix the two solutions thoroughly with the pipette tip before the mixture dries
- Dry the droplet in a stream of air or nitrogen
- Introduce the sample into the mass spectrometer
Crystal Washing: When signal supression is suspected, or if no signal is observed, the dried sample spot can be rinsed, one or two times, with a 2-5 uL drop of 4? C water. If the supression effect is suspected to be caused by salt impurities in the protein sample itself, the rinses can be done with cold matrix solution. Sample purification can also be performed using a pipette tip fitted with C18 reversed phase resin bed (i.e. ZipTip C18 from Millipore) followed by direct elution onto the sample plate. Zip Tip C18 pipette tips are an excellent choice for desalting, concentrating and removing detergents from small amounts of biological samples.
The advantages of the Q&D method are multiple:
- It is fast. It decouples the sample and matrix preparation steps.
- It can be used for the analysis of in-plate protein digestions. In this case a drop of protein solution in the required buffer is mixed with a drop of the proteolytic enzyme directly on the MALDI sample pate. The protein/enzyme drop is stored in a high humidity chamber for the amount of time necessary to ensure complete digestion, and a drop of matrix solution is then added to halt the enzymatic process. The final drop is dried as usual and the peptide fragments analyzed by MALDI.
- It makes it very easy to add a calibration standard to the sample. A drop of mass calibrant solution (typically 0.5 uL) can be easily mixed into the sample drop before the matrix is added.
The main disadvantge of the method is that it provides the least controlled conditions of all sample preparation procedures. Results are not as reproducible or consistent as those obtained with the similar dried-droplet method.