Towards a Quantitative Interpretation of Polymer Distributions from MALDI‐TOF Spectra

Abstract

The importance of proper conversion of MALDI‐MS signals into the number of polymers with a certain mass increment (by taking into account a nonconstant conversion factor) is demonstrated with calculated Schulz‐Zimm and Poisson distributions. Measurements of two narrow polystyrene standards were used to investigate the influence of measurement conditions (laser intensity, matrix) and data processing (signal conversion, baseline construction). We observed the following effects: (a) slightly broader distributions with DCTB as matrix at a given laser intensity in comparison to those obtained with all‐trans‐retinoic acid, (b) changes in the relative intensities of several peaks in a sample, and (c) a nonlinear baseline in the low molar mass region in most of the cases. Therefore, the implications of an assumed linear baseline instead of the correct one on the results were scrutinized experimentally and by simulations. In all, measurement of narrow distributions turned out to give reasonable results. Based on the assumption that the polymer standards can be described by Poisson distributions, overlap conditions between two distributions are rephrased. An alternative possibility of how mass discrimination could be determined, in principle, is presented, too.magnified image

A Perfect Couple: PLP/SEC/ESI‐MS for the Accurate Determination of Propagation Rate Coefficients in Free Radical Polymerization

“…[40,41] Some authors argue that in MALDI, sample preparation also has a major impact on mass discrimination. [42][43][44][45] In comparison to electrospray ionization, the mass bias in MALDI is somewhat reduced and ionization may be achieved for macromolecules with molecular weights of up to several hundreds of kg Á mol À1 depending on the monomer class. With the ionization process yielding mainly singly charged ions, an interpretation of the obtained mass spectrum in terms of a molecular weight distribution is possible after some data processing.…”

“…With the ionization process yielding mainly singly charged ions, an interpretation of the obtained mass spectrum in terms of a molecular weight distribution is possible after some data processing. [45,46] MALDI-MS has been investigated as a tool to measure MWD in conjunction with PLP-experiments using methyl methacrylate and styrene as model monomers as early as 1993. [31,[47][48][49][50] Although the general applicability was demonstrated, PLP/MALDI-MS has since not made the step towards an established technique for the determination of k p .…”

“…One likely reason for this has recently been summarized by Schnöll-Bitai et al, who showed that quantitative determination of MWD requires a number of data transformations to be made prior to analysis. [45] These include a proper baseline estimation, coordinate transformations from the time-of-flight to the mass axis and -most importantly -corrections for instrumental mass bias for broad (PLP-type) distributions. [45,46] Moreover, there is evidence that MALDI underestimates the polydispersity of narrow MWD with a potentially fatal impact in PLP, as this will inevitably bias the points of inflection in the MWD used for the determination of k p .…”

“…[45] These include a proper baseline estimation, coordinate transformations from the time-of-flight to the mass axis and -most importantly -corrections for instrumental mass bias for broad (PLP-type) distributions. [45,46] Moreover, there is evidence that MALDI underestimates the polydispersity of narrow MWD with a potentially fatal impact in PLP, as this will inevitably bias the points of inflection in the MWD used for the determination of k p . [45] However, there has been some promising research at converting MALDI-MS into an accurate tool for molecular weight analysis.…”

A Linearization Procedure for Determining Radical Reactivity Ratios in Restricted Penultimate Copolymerization Systems, 2

“…With respect to SEC, calibration (universal calibration, [39] multi-detector analyses [40,41] ) and the correction of band broadening phenomena [33,34,[42][43][44][45][46][47][48][49] are of utmost importance. With respect to mass spectroscopic techniques like matrixassisted laser desoprtion ionization (MALDI) [11,50,51] or electrospray ionization (ESI), [52,53] it is desirable to increase the quality of the data in particular for broad multimodal distributions so that a quantitative interpretation of spectra will yield more reliable data.…”