Harald Pasch

HPLC of polymers

1 Introduction.- 2 Thermodynamics of Polymer Chromatography.- 3 Equipment and Materials.- 4 Size Exclusion Chromatography.- 5 Liquid Adsorption Chromatography.- 6 Liquid Chromatography at the Critical Point of Adsorption.- 7 Two-Dimensional Liquid Chromatography.

MALDI–TOF mass spectrometry of polyflavonoid tannins

Abstract Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (MALDI–TOF) appears to be a suitable method for examining polyflavonoid tannin oligomers. It appears capable to determine aspects of the structure and characteristics of polyflavonoid tannins, which are otherwise too difficult to determine by other techniques. It has been possible to determine by MALDI–TOF for the two major industrial polyflavonoid tannins which exist, namely mimosa and quebracho tannins, and some of their modified derivatives that: (i) mimosa tannin is predominantly composed of prorobinetinidins while quebracho is predominantly composed of profisetinidins, that (ii) mimosa tannin is heavily branched due to the presence of considerable proportions of ‘angular’ units in its structure while quebracho tannin is almost completely linear. These structural differences also contribute to the considerable differences in viscosity of water solutions of the two tannins. (iii) the interflavonoid link is more easily hydrolysable, and does appear to sometime hydrolyse in quebracho tannin and profisetinidins, partly due to the linear structure of this tannin, and confirming NMR findings that this tannin is subject to polymerisation/depolymerisation equilibria. This showed that the decrease of viscosity due to acid/base treatments to yield tannin adhesive intermediates does also depend in quebracho from a certain level of hydrolysis of the tannin itself and not only of the carbohydrates present in the extract. This tannin hydrolysis does not appear to occur in mimosa tannin in which the interflavonoid link is completely stable to hydrolysis. (iv) sulphitation has been shown to influence the detachment of catechol B-rings much more than pyrogallol-type B-rings. (vi) the distribution of tannin oligomers, and the tannins number average degree of polymerisation obtained by MALDI–TOF appear to compare well with the results obtained by other techniques.

Use of matrix-assisted laser desorption/ionization mass spectrometry for molar mass-sensitive detection in liquid chromatography of polymers

Abstract Matrix-assisted laser desorption/ionization mass spectrometric (MALDI-MS) detection is a powerful alternative to conventional refractive index or ultraviolet detection in the liquid chromatography of oligomers and polymers. As a molar mass-sensitive detector, MALDI-MS is capable of providing information on the molar mass and the chemical structure of fractions from liquid chromatographic separations. Although it is currently used only in the off-line mode, MALDI-MS yields fast and reliable results, consuming minimum amounts of a few nanograms. For a number of applications it is shown that MALDI-MS experiments can be carried out with fractions taken directly from the eluate after chromatographic separations on an analytical scale.

Two-dimensional chromatography of complex polymers Part 1. Analysis of a graft copolymer by two-dimensional chromatography with on-line FTIR detection

The grafting of butyl acrylate onto poly(styrene-block-butadiene) is investigated by two-dimensional (2D) liquid chromatography. Separating the graft backbone and the graft product by liquid chromatography at the critical point of adsorption in the first dimension and size exclusion chromatography in the second dimension, detailed information in the coordinates chemical composition and molar mass are obtained. It is shown that the grafting reaction results in the formation of a complex product due to the fact that in addition to grafting the graft backbone undergoes partial degradation. Combining 2D chromatography and FTIR spectroscopy in a quasi on-line setup, all components of the graft product can be identified. Via selective detection, the chemical composition drift of the different fractions can be determined.

Hyphenated Techniques in Liquid Chromatography of Polymers

Complex polymers are distributed in more than one direction of molecular heterogeneity. In addition to the molar mass distribution, they are frequently distributed with respect to chemical composition, functionality, and molecular architecture. For the characterization of the different types of molecular heterogeneity it is necessary to use a wide range of analytical techniques. Preferably, these techniques should be selective towards a specific type of heterogeneity. The combination of two selective analytical techniques is assumed to yield two-dimensional information on the molecular heterogeneity.

Chromatographic investigations of macromolecules in the “critical range” of liquid chromatography: II. Two-dimensional separations of poly(ethylene oxide-block-propylene oxide)

Abstract A polyethylene oxide-polypropylene oxide block polymer was characterized with respect to molar mass distribution and block length of the individual blocks using two-dimensional chromatographic techniques. In the first dimension the block polymer was separated according to the length of the polypropylene oxide block by liquid chromatography at the critical point of adsorption. The resulting polypropylene oxide uniform fractions were subjected to supercritical fluid chromatography or size-exclusion chromatography and the average length and the molar mass of the polyethylene oxide blocks were determined for every fraction.

Matrix-assisted laser desorption/ionization mass spectrometry of synthetic polymers: 2. Analysis of poly(methyl methacrylate)

Abstract Matrix-assisted laser desorption/ionization mass spectrometry (m.a.l.d.i.-m.s.) is shown to be a unique method for the determination of the functional heterogeneity of poly(methyl methacrylate)s. In group-transfer polymerization of methyl methacrylate, oligomers with cyclic end-groups are formed in addition to the expected linear oligomers. The cyclics are identified via preparative separation of the cyclic trimer by high-performance size exclusion chromatography. Using m.a.l.d.i.-m.s. the different homologous series are identified and their molar-mass distributions are calculated. In addition, high-performance size exclusion chromatography with dual refractive-index and ultra-violet detection provides information on the molar-mass distribution of the cyclic fraction. For technical products, it is shown that oligomers with cyclic end-groups may be formed in significant amounts and, therefore, have to be accounted for in structural characterization.

Recent advances and trends in the liquid-chromatography-mass spectrometry analysis of flavonoids.

Flavonoids have elicited significant attention as a result of their importance in plants, their influence on the properties of natural-product derived commodities and especially as a consequence of their purported health benefits. Research in all of these fields relies heavily on accurate analytical data, and in this LC-MS has come to play an influential role by allowing relatively fast tentative identification and accurate quantification of low levels of flavonoids in a variety of matrices. The field has undergone rapid expansion in the last decade due to important developments in both HPLC and MS instrumentation, which nowadays allow much faster and more accurate analysis of flavonoids. This contribution aims to provide an overview of these developments and their application in flavonoid analysis since 2009. The discussion is focussed first on methodologies which provide improved LC separation of flavonoids in terms of speed and/or resolution, including ultra high pressure LC (UHPLC), monolithic and superficially porous phases, high temperature LC (HTLC) and comprehensive two-dimensional LC (LC×LC). The fundamental background relevant to each of these will be briefly outlined, as well as the implications and promise of their hyphenation to MS. Secondly, the possibilities and limitations of a range of the latest MS instruments available in combination with advanced LC analysis will be discussed, including ion trap, triple quadrupole, time-of-flight, Orbitrap, ion mobility and various hybrid instruments. Examples from the latest literature will be used to illustrate the performance gains achievable in flavonoid analysis by the hyphenation of advanced LC separation and high-end MS instrumentation.

Dispersion Polymerization of Methyl Acrylate in Nonpolar Solvent Stabilized by Block Copolymers Formed In situ via the RAFT Process

The free-radical dispersion polymerization of methyl acrylate (MA) in isododecane was carried out in the presence of a poly(2-ethylhexyl acrylate) macromolecular RAFT (reversible addition-fragmentation chain transfer) agent bearing a trithiocarbonate reactive group in the middle of the chain (P2EHA-TTC). The presence of the trithiocarbonate function was crucial for the synthesis of monodisperse colloidal poly(methyl acrylate) (PMA) particles stabilized by the P2EHA segments. The hydrodynamic diameters ranged from 100 to 300 nm, using particularly low amounts of the macro(RAFT agent) (1-6 wt % vs. MA) in dispersion polymerizations carried out at 20 wt % solids content. As shown by 2D liquid chromatography, P2EHA-b-PMA or P2EHA-b-PMA-b-P2EHA block copolymers formed in situ at the early stage of the dispersion polymerization due to the reversible transfer process and played the role of particle stabilizer. The glass-transition temperature of the derived polymer films was not affected by the low amount of the chosen macromolecular stabilizer and the mechanical properties were mainly those of PMA, which makes the technique very attractive for coating applications.

Two-dimensional chromatography of complex polymers ☆: 6. Method development for (meth)acrylate-based copolymers

The free-radical copolymerisation of various acrylates and methacrylates resulting in complex copolymers for cosmetic applications were investigated using different chromatographic techniques including HPLC and on-line coupled two-dimensional (2D) liquid chromatography. The complete separation of all polymerisation products was achieved by gradient HPLC. A computated optimisation procedure, using the Polymer Chromatographic Model allowed us to design a step mobile phase gradient to improve resolution of homopolymer chromatographic separation. By combining gradient HPLC and SEC (Size Exclusion Chromatography) in a fully automated two-dimensional chromatography setup, the complex distributions of chemical composition and molar mass could be simultaneously described and fingerprinted.