Size-exclusion chromatography -MALDI, polymer

Nielen, M. W. R, Polymer Analysis by Micro-Scale Size-Exclusion Chromatography/MALDI Time-of-Flight Mass Spectrometry with a Robotic Interface, Anal. Chem., 70, 1563, 1998. 

In addition, the determination of a polymer s endgroup(s) [125,126] and the analysis of random and block-copolymers [127,128] can be achieved by MALDI. However, care has to be taken when judging the MALDI spectra because of the mass-dependent desorption and detection characteristics of the experiment. In case of higher polydispersity (PD >1.1) high-mass ions are underestimated from MALDI spectra. [93,124] The current practice to deal with such samples is to fractionate them by gel permeation chromatography (GPC) [123] or size-exclusion chromatography (SEC) prior to MALDI analysis. [124,129]... 

MALDI (matrix assisted laser desorption and ionization) combined with size-exclusion chromatography is increasingly being used to study the microstructure of polymers [542-544], MALDI allows the desorption and ionization of macromolecules with molecular mass up to hundreds of kilodaltons with little or no fragmentation. The MALDI mass spectra provides structural information for identification of the polymer repeat units or copolymer sequence, end group mass and mechanism of polymerization for polymers with a narrow polydispersity (< 1.25). For samples of high polydispersity MALDI fails to provide reliable information. The separation of polydisperse samples by size-exclusion chromatography provides fractions suitable for characterization by MALDI. MALDI can also be used for mass calibration of size-exclusion columns. 

Jackson, C., Larsen, B., and McEwen, C., Comparison of Most Probable Peak Values As Measured for Polymer Distributions by MALDI Mass Spectrometry and by Size Exclusion Chromatography, Anal. Chem., 68, 1303, 1996. 

The analysis of a broad-polydispersity polymer (PD > 1.2) can be carried out by combining GPC or size-exclusion chromatography (SEC) with MALDI-MS [161-172]. In this approach, a wide-polydispersity polymer is first separated by GPC and fractions at a defined time interval are collected. The time interval is properly chosen so that the individual fraction would contain only a narrow-polydispersity (PD < 1.2) polymer, which can then be analyzed by MALDI-MS for accurate molecular mass determination. The molecular mass information generated from the MALDl analysis of aU individual fractions can be used to convert the time domain in the GPC chromatogram into a mass domain. The polymer distribution can be determined from this chromatogram. 

Esser, E., Keil, C., Braun, D., Montag, P, Pasch, H. (2000) Matrix-assisted laser desorption/ionization mass spectrometry of synthetic polymers. 4. Coupling of size exclusion chromatography and MALDI-TQF using a spray-deposition interface. Polymer, 47,4039-4046. 

Many of the problems of MALDI of polydisperse polymers can be overcome by using a combination of an initial fractionation by size exclusion chromatography (sec) followed by MALDI, taking advantage of the enormous sensitivity of MALDI to analyze the minute amoimts of polymer eluted from the sec. This is probably now the best available method for getting MWD information but is complex. 

Jackson et al" compared the most probable peak values as measured for polymer distributions by matrix assisted laser desorption/ionization (MALDI) mass spectrometry and by size exclusion chromatography (SEC). MALDI provided the most probable peak (Mp) values for polymethyl methacrylate polymers that are low reletive to Mp values supplied by the polymer manufacturers which were obtained by SEC. The reasons for this discrepancy are discussed. 

The cyclic undecamer (molecular mass 1244,8), present as the most abundant species in MALDI-TOF, has also been deteaed by Size Exclusion Chromatography (SEC) (Figure 24) as the maximum of a distinrt peak in the low-molar-mass region, well separated from the high polymer trace. HPLC analysis of the cyclic oligomer fraction up to the undecamer for another anionic PA6 sample not previously... 

Montaudo and co-workers [8] reported on a SEC/MALDI study of molar mass distribution in two random copolyesters polybutylene adipate - polybutylene sebacate (PBA - PBSe) and polybutylene sebacate - polybutylene sccinate (PBSe - PBSu). MALDI allows desorption and when polydisperse polymer samples are fractionated by size exclusion chromatography, they yield fractions with very narrow distributions which, when analysed by MALDI give mass spectra with molar mass values in excellent agreement with those obtained by conventional techniques. These workers derived the following equation for the molar mass of the copolymer molecule ... 

Large polymers of the type found in plant cell walls are difficult to analyze directly. Therefore, most studies use prefractionation by size-exclusion chromatography (SEC) or reduction to smaller units by partial acidic hydrolysis or enzymolysis. For mixtures of large polymers such as dextrans ionized by MALDI (Figure 19.5), there is a fall in signal intensity with increasing mass that has been attributed to signal suppression by the smaller. 

Size exclusion chromatography and MALDI-TOF mass spectrometry was coupled via a robotic interface. For polydisperse synthetic polymers a continuous track of matter was deposited onto the matrix surface. Spectra were taken from different positions of the polymer track. Data are presented for PS, PMM A and butyl methacrylate-methyl methacrylate copolymers. 20 refs. 

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