In MALDI-TOF mass spectrometry, protein or peptide samples are mixed with a matrix compound (usually an aromatic organic acid that donates a proton to the analyte) and dried onto a metal sample plate. After the plate is placed in a high vacuum source chamber in the mass spectrometer, a small portion of the sample is vaporized (desorption) by blasts from a nitrogen laser. The ions produced ‘fly’ up a tube to the mass analyzer and their masses (actually their mass-to-charge ratio) are determined by their ‘time-of-flight’. The range of measurements using MALDI is in excess of 100 kDa although accuracy falls off at this size. The lower practical limits are several hundred daltons as there is considerable interference in this range of the spectra from matrix and other low mass contaminants.