Correction of Time-of-Flight Shifted Polymeric Molecular Weight Distributions in Matrix-Assisted Laser Desorption/Ionization Fourier Transform Mass Spectrometry

摘要

Molecular weight distributions on an external injection matrix-assisted laser desorption/ionization Fourier transform mass spectrometer are subject to time-of, flight distortions as different ion velocities are probed with varied delay times between ionization and trapping (i.e., the trapping time). This phenomenon is used to advantage to reject low-mass matrix ions which would otherwise saturate the trapped ion cell; however, for accurate determination of molecular weight distributions of complex samples like polymeric systems, several mass spectra must be acquired at a series of different trapping times to compensate for this distortion. The spectra acquired should be superimposed (not summed) on the same m/z axis to yield the correct molecular weight distribution as summation of these spectra merely causes further distortions and can cause loss of signaloise. Distortions due to TOF effects are probed with a calibration compound, poly(ethylene glycol) of peak mass (M↓(p)) ~ 1000 Da, as well as the more difficult to ionize polystyrene, which was obtained as a chromatographic molecular weight standard (MW 950). This polystyrene reference material was determined to have a ~20% error in M↓(p).