Sample depletion of the matrix-assisted laser desorption process monitored using radionuclide detection.

To investigate analyte consumption during the laser desorption process, matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) is combined with radionuclide detection. Radionuclide detection provides highly sensitive and quantitative information on the amount of radiolabeled analytes in a MALDI MS sample spot. 14C-Labeled cytochrome c is deposited with 2,5-dihydroxybenzoic acid in 10-nL volume spots. By comparing radioactivity levels of the labeled cytochrome c both before and after spectral acquisition, the reduction in labeled analyte molecules on the target allows monitoring of the moles of desorbed sample. Through a depletion study on this sample, the amount of analyte consumed for MALDI time-of-flight spectral acquisition and the average number of molecules desorbed per laser ablation are determined. When [14C]-cytochrome c is no longer detected by MALDI MS, approximately 70% of the original analyte remains in the sample spots. Redissolving the spots produced further desorption, indicating that the analyte before dissolution was in a physical environment that did not facilitate the desorption process. As a technique with a response that does not depend on the environment of the analyte, radionuclide detection allows characterization of mass-limited sampling methods to better understand the MALDI process.