Characterization of polydisperse synthetic polymers by size-exclusion chromatography/matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Abstract

Synthetic polymers having a polydispersity greater than 1.1 cannot be reliably characterized by matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry alone. This complication has been overcome by off-line coupling of size-exclusion chromatography (SEC) with MALDI. Data are presented for a wide variety of synthetic polymers having polydispersities from 1.7 up to 3.0. Polystyrene (15 kDa and 48 kDa), polybutylacrylate (62 kDa), polycarbonate (28 kDa), aromatic polyester resin (8 kDa) and a methyl methacrylate-methacrylic acid copolymer (34 kDa) were injected on a conventional SEC system with refractive index (RI) or UV absorbance detection and 40 fractions collected over the polymer distribution. In a recent comparison between most probable peak values (M p ) as measured for polymer distributions by mass spectrometry and size-exclusion chromatography (SEC) it has been concluded, using theoretical considerations, that, in the case of polydisperse polymers, i.e. in most routine cases, the molecular weight distribution (MWD) data provided by mass spectrometry does not resemble the distribution as obtained by SEC because of the fundamental difference between recording the number fraction versus m/z (as in mass spectrometry) and the weight fraction versus the logarithm of the molecular mass (as in SEC with refractive index (RI) detection).1 As a practical consequence, the high-mass tail of the polydisperse distribution disappears much earlier into the baseline noise of the mass spectrometer system than into the noise of the SEC-RI system, thus yielding incorrect calculation of the so-called molecular averages: This polydispersity complication has limited the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry 3 to the characterization of synthetic polymers; it has been concluded that MALDI-MS is only applicable to narrowlydistributed synthetic polymers (polydispersity less than 1.1). 4 Recently, Montaudo et al. suggested a procedure to obtain improved MALDI-MS data from polydisperse polymers.5 In our experience, however, data thus obtained are extremely sensitive to the baseline subtraction procedure; even minor changes in baseline level cause huge deviations in the final MWD calculation which becomes inaccurate and irreproducible. Moreover, it is hard to believe that detector sensitivity does not alter over a range of say m/z 500 up to 300 000.Several groups have investigated experimental parameters in MALDI time-of-flight (TOF) analysis in order to get a better understanding and/or to improve the results for polydisperse synthetic polymers. Different accelerating voltages and detection efficiencies were found to cause severe distortion of the molecular weight distribution in case of polydisperse synthetic polymers. 6 The MWD of polymethylmethacrylate

Small molecule analysis by MALDI mass spectrometry

“…Offline GPC coupling has been accomplished by fraction collection, primarily for high molecular weight polymers. [51] Monomers' structure and end groups were determined using the delayed extraction reflectron mode. Offline fraction collection is both laborious and time consuming but MS settings can be optimized for each individual fraction.…”

Data Processing Method for the Determination of Accurate Molecular Weight Distribution of Polymers by SEC/MALDI-MS

“…(8) is 1, the constant a can be calculated by (9) Solving Eqs. (6) and (9) for kf gives (10) Using k′f obtained from Eq. (5) and observed If,i, one can calculate the response factor kf for each f-th fraction.…”

“…The average molecular weight of the original polymer samples can be calculated by using SEC chromatograms thus calibrated. Various applications using SEC/MALDI-MS have been reported for the characterization not only of homopolymers such as polystyrene (PS), 10 poly(methylmethacrylate) (PMMA), 11,12 polycarbonates, 10,13,14 and polyesters, 10,15 but also of some copolymers 10,11,[15][16][17] and polymer blends. 18 However, it should be noted that correct molecular weight distribution might not be necessarily obtained even by SEC/MALDI-MS, mainly due to the band broadening on the SEC chromatogram and inadequate calibration, as was theoretically pointed out by Lou et al 19 The former band broadening is frequently encountered in SEC measurements.…”

Characterization of Tars from the Thermal Processing of Baganuur and Tavan Tolgoi Coals from Mongolia, Using SEC, UV-F, IR and Mass Spectrometry

“…Briefly, Garozzo et al [11] showed that agreement between molecular mass distributions of polysaccharides determined by MALDI-MS and SEC was improved if polydispersities were low. Similarly Montaudo et al [12] found that molecular mass distributions of polydimethylsiloxanes determined by MALDI and SEC agreed if polydispersities were low, while Nielen and Malucha [13] showed that synthetic polymers with a polydispersity greater than 1.1 could not be reliably characterized by MALDI-MS. The high mass components of blends of two polymers of narrow polydispersity suffered from mass discrimination [14], giving lower intensities than expected for the high mass ions.…”