Oligomers, analysis MALDI-ToFMS

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

The short pulse of ions produced by laser desorption is ideally suited to mass analysis by time-of-flight (ToF), which makes it possible to record a complete mass spectrum for a single laser shot. Actually, MALDI has greatly contributed to the revival of ToF-MS. The detection efficiency decreases as the ion mass increases. Key instrumental parameters of MALDI-MS are mass resolution and mass accuracy. Mass resolution of MALDI-ToFMS has increased considerably by pulsed extraction techniques. Resolutions of 30,000 or better and mass accuracy at low ppm level are achieved, which allow separation of single oligomers. The sensitivity of ToF is in the sub-fmol range. Recently also the... 

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