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

Abstract

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 signal/noise. 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 .Matrix-assisted laser desorption and ionization (MALDI) has rapidly become a routine tool for analysis of polymeric systems, 1-3 is currently used extensively for determination of molecular weight distributions, copolymerization structure, and homogeneity, 4 and is beginning to be used for branching studies. 5 The rapid, lowcost analysis provided by MALDI time-of-flight (TOF) instruments has allowed mass accuracies of 0.1% with oligomeric mass resolution, and the advent of delayed extraction has extended this to isotopic resolving power for polymers up to ∼4000 Da with mass accuracies of ∼150 ppm 6-8 and, recently, 10-15 ppm mass accuracy for peptides from 1 to 4 kDa. 9

A Novel 9.4 Tesla FTICR Mass Spectrometer with Improved Sensitivity, Mass Resolution, and Mass Range

“…The most direct method to reduce TOF effects is to reduce the ion flight length between the ion accumulation trap and the ICR cell. Other reported methods include use of multiple transfer events with different ion flight periods [14,43,44], segmentation of the transfer multipole to provide axial containment throughout the ion transfer event [45], conversion of axial into radial energy as the ions enter the ICR cell (sidekick) [46], or m/z-dependent ion velocity adjustment during the transfer event [47,48].…”

Quantitative mass spectrometry of technical polymers: a comparison of several ionization methods

“…In addition, it is possible that the lack of ions in the higher m/z range indicates a difficulty in transport to the ICR trap. FTMS analysis of synthetic polymers; 24 it is likely that this time-of-flight discrimination also occurs, to some extent, in external source ESI-FTMS, hence the narrower oligomer distribution and the observed lower polydispersity. There are also some fragment ions (not shown) in the low m/z region; detailed investigation in the determination of their structures will appear as a separate publication.…”

External electron ionization 7T Fourier transform ion cyclotron resonance mass spectrometer for resolution and identification of volatile organic mixtures

“…44 However, for the 15 n-alkylbenzenes mixture, the m / z distribution obtained by superposition was unexpectedly similar ͑relative peak heights agreed to within ±20%͒ to the m / z distribution obtained from a single mass spectrum ͑at an intermediate transfer interval of 0.7 ms͒. For singly charged ions at the same rf field, lower-mass ions are confined by a higher-magnitude electric potential than highermass ions.…”