size exclusion chromatography Dotaz Zobrazit nápovědu
Distribuce molárních hmotností je jednou z velmi důležitých vlastností polymerů stanovovaných již od doby, kdy Staudinger prosadil koncept makromolekul. První metody používané pro stanovení této distribuce byly však zdlouhavé a náročné na obsluhu. To se změnilo v roce 1964, kdy J. C. Moore, pracovník firmy Dow Chemical Company, zveřejnil metodu jím nazvanou gelová permeační chromatografie (GPC) a dnes známou spíše jako vylučovací chromatografie (size exclusion chromatography, SEC). Její princip spočíval v dělení makromolekul podle velikosti na základě jejich permeace do pevné stacionární fáze, která obsahovala síť pórů různých velikostí. Dělené molekuly difundovaly pouze do pórů, které byly větší, než byla jejich velikost. Takže menší molekuly permeovaly do většího počtu pórů než molekuly velké, a tudíž se zdržely v koloně déle. Jednotlivé polymerní molekuly pak opouštěly kolonu v pořadí od největších po nejmenší. Aby se tato technika mohla rozšířit, bylo potřeba nalézt výrobce odpovídajícího přístroje, kterým se stal podnik vlastněný J. L. Watersem. Byl to důležitý počin pro jeho firmu, z níž se díky SEC stal dnes jeden z největších světových producentů zařízení pro kapalinovou chromatografii, kam SEC také patří.
Molar mass distribution is one of the most important properties of polymers that has been determined since Staudinger introduced the concept of macromolecules. However, the first methods used to determine this distribution were tedious and difficult to use. This changed in 1964 when J. C. Moore of the Dow Chemical Company published a method he called gel permeation chromatography, now better known as size exclusion chromatography (SEC). The principle was to separate macromolecules by size based on their permeation through a solid stationary phase containing a network of pores of different sizes. The separated molecules diffused only into pores larger than their size. As a result, the smaller molecules permeated into a greater number of pores than the larger molecules and therefore remained in the column longer. The individual polymer molecules then left the column in order from largest to smallest. To scale up this technique, it was necessary to find a manufacturer of a suitable instrument, which was a company owned by J. L. Waters. This was a major achievement for his company, which, thanks to SEC, is now one of the world's largest manufacturers of equipment for liquid chromatography, of which SEC is a part.
In analyses of polystyrene standards by size-exclusion chromatography for polymers of molecular weight about thirty thousand and higher, the separation improved with decreasing flow-rate is in accord with Giddings observation of transversal diffusion as a factor decreasing the broadening of the band broadening function. The variance of the elution curves is larger than it corresponds to dispersity M¯w/M¯n and the shape of the elution curves depends on the experimental conditions which suggests that it reflects properties of the band broadening function. The skew and the excess kurtosis of the elution curves increase near the exclusion limit in accord with theoretical prediction for the band broadening function.
The main factors affecting the mesopore porosity of methacrylate-ester based monolithic columns were investigated. We prepared 40 monolithic capillary columns with porosity controlled by varying the proportions of butyl methacrylate (BMA) and ethylene dimethacrylate (EDMA) monomers and of 1,4-butanediol (BUT) and 1-propanol (PROP) as the porogen solvent in the polymerisation mixtures by thermally initiated in situ polymerisation in fused-silica capillaries. Using mixture design software, we systematically varied the composition of the polymerisation mixtures to find significant factors affecting mesopore formation. Multivariate analysis of the experimental data obtained for the fabricated columns yielded a model for prediction of the mesopore porosity in monolithic beds as a function of the composition of the polymerisation mixture used to prepare polymethacrylate monolithic capillary columns. The mean absolute deviation of predicted porosities is 0.029 for most of the columns, with only eight columns showing deviations exceeding 0.050. The main factor affecting the mesopore porosity proved to be the combination of the concentration of hydrophobic monomer (BMA) and the concentration of the less-polar solvent, 1-propanol, in the porogen mixture. The proportion of mesopores in the monolithic capillary columns increases with increasing concentration of 1-propanol and with decreasing concentration ratios of the cross-linker (EDMA) to monomer (BMA) and of BUT to PROP porogenic solvents.
A new method for the decomposition of non-baseline-resolved multimodal elution curves of SEC with the concentration, light scattering and viscosity detection is presented. The method makes possible the characterization of the polymer-sample components, represented by the peaks forming multimodal elution curves, individually and reduces also the error in the calculation of molecular-weight averages. The procedure is demonstrated on narrow molecular-weight distribution polystyrene standards and their mixture as well as on a grafted polymer sample.
The SEC separation of a randomly branched polymer, in particular local dispersity due to branching, are theoretically examined. A model of the SEC separation of randomly branched polymer with tetrafunctional branch points enabling the estimation of local dispersity was developed. Measurable quantities (branching indices) were compared with real data. Local dispersity due to branching is demonstrated to depend on elution volume and degree of branching and in the area of the beginning of the elution curve it can reach non-negligible values.
