Simple and robust two-layer matrix/sample preparation method for MALDI MS/MS analysis of peptides.

Recent advances in MALDI MS/MS instrumentation allow a high degree of automation in the efficient detection of peptide fragment ions that can be used for protein identification. However, the performance of the technique is dependent on the MALDI sample preparation. We present a simple and robust two-layer sample preparation method tailored for sensitive and reproducible generation of MALDI MS/MS data. This method produces a strong and uniform crystal layer which allows acquisition of high quality MS/MS spectra over the entire sample surface area. Furthermore, due to its crystal strength, the matrix/sample layer can be washed extensively on target, enabling direct analysis of samples containing impurities, such as salts and surfactants. This method is demonstrated to be very useful in routine analysis of in-gel tryptic digests of silver-stained protein gel spots, without the need of desalting steps or hunting for "hot" spots. As an example, seven threonine-phosphorylated proteins involved in signal transduction in response to growth factor stimulation within the lipid raft fractions of the IMR5 neuroblastoma cells have been identified using differential gel display, in-gel digestion and MALDI MS/MS with the new two-layer sample preparation method. Some of these proteins have the functions of maintaining raft structure or cell signaling.