Why does my mass spectrum have a peak at approximately half of the calculated molecular weight?
Midland oligos are analyzed by delayed-extraction (DE) matrix assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry. Ideally the laser creates a single ionic charge per molecule, but in reality it can generate a molecule with two negative charges. Since the mass spectrometer actually measures the mass-to-charge ratio of a molecule, the molecules having 2 charges will migrate up the tube to the detector in half the time it takes the singularly ionized molecule of the same mass. This phenomenon creates a peak with a mass that is almost exactly half the expected mass for the molecule.
