MALDI FTMS analysis of polymers: improved performance using an open ended cylindrical analyzer cell

Improvements in the matrix assisted laser desorption/ionization analysis of polymers are obtained with Fourier transform mass spectrometry by using an open-ended cylindrical analyzer cell. These improvements include a reduced sensitivity to experimental parameters in determining the oligomer molecular weight distribution, and better mass accuracy. Ion trajectory calculations and experimental data show that the open-ended cylindrical cell is less sensitive to changes in gated trapping time for capturing a range of masses. For poly(ethylene)glycol 1000, a shift of only 20 Da is observed for a 200 μs change in gated trapping time and shift of 100 Da for the weight average is observed for a 540 μs change. The mass accuracy for determinations of end-group and repeat unit masses are presented for internal and external calibration of polymer samples. Using a linear regression method, repeat group masses were determined to within 0.1 mmu and 0.7 mmu for internal and external calibration at 2000 Da. The mass accuracy at high molecular weight with external calibration was tested with poly(methyl) methacrylate 6000, which yielded a mean squared error of 8.1 mmu for the repeat mass and 23.6 mmu for the end-group mass. Space charge is shown to affect mass accuracy. By accounting appropriately for this effect, a root mean error of 1.4 ppm is obtained for nine peaks in an externally calibrated poly(methyl) methacrylate 6000 mass spectrum.