Detectors in size-exclusion chromatography

Conductivity detectors, commonly employed in ion chromatography, can be used to determine ionic materials at levels of parts per million (ppm) or parts per bUHon (ppb) in aqueous mobile phases. The infrared (ir) detector is one that may be used in either nonselective or selective detection. Its most common use has been as a detector in size-exclusion chromatography, although it is not limited to sec. The detector is limited to use in systems in which the mobile phase is transparent to the ir wavelength being monitored. It is possible to obtain complete spectra, much as in some gc-ir experiments, if the flow is not very high or can be stopped momentarily. 

Multiple Detectors in Size Exclusion Chromatography Signal Analysis... 

The application of UV spectrophotometers to the analysis of styrene containing copolymers has been extensively reported in the literature. However, hypochromic effects and band shifts which result in deviations from Beer s Law have limited the use of UV detectors as mass or composition detectors in size exclusion chromatography applications. Deviations from Beer s Law for low conversion styrene-acrylonitrile copolymers in tetrahydrofuran have been experimentally investigated and compared with results previously reported in the literature. The behaviour of the extinction coefficient as a function of the copolymer composition is discussed in view of the information obtained from infrared and nuclear magnetic resonance measurements on the same polymers. As a result of this investigation, a quantitative correlation of the extinction coefficients of styrene-acrylonitrile copolymers with the length of the styrene sequences has been obtained which, in turn, allows for the use of UV spectrophotometers as sequence length detectors. 

Figure 1.38 shows the dependence. The data were obtained by using an on-line lightscattering detector in size exclusion chromatography. In the study, R and M were measured simultaneously and instantaneously as the polymer was separated in... 

Garcia-Rubio, L.H., MacGregor, J.F., Hamielec, A.E., "Copolymer Analysis Using GPC with Multiple Detectors , presented at the Symposium on Recent Developments in Size Exclusion Chromatography , 178th ACS National Meeting, Washington, D.C., September 9-14, 1979. 

Advances in size-exclusion chromatography, coupled with refractive index, absorption, viscosity, and lightscattering detectors, and MALDI-ToFMS, have made it possible to accurately determine molecular weight distribution (oligomer profiling), even at the relatively low values of polymeric additives (up to about 5000 Da). Advances in column design, e.g. high-resolution PS/DVB columns (> 105 plates m-1) mean that SEC can provide a valuable alternative to conventional HPLC techniques for the separation of small molecules. 

In the first section, the mechanisms involved in size exclusion chromatography are discussed this is an area where additional understanding and clarification still are needed. Data treatment with respect to statistical reliability of the data along with corrections for instrumental broadening is still a valid concern. Instrumental advances in the automation of multiple detectors and the developm.ent of a pressure-programmed, controlled-flow supercritical fluid chromatograph are presented. 

C. Jackson, H.G. Barth, Molecular weight sensitive detectors for size exclusion chromatography, in Handbook of Size Exclusion Chromatography and Related Techniques, vol. 91 (Chromatographic Sciences Series), C.-s. Wu, ed., Marcel Dekker, Inc., New York, 2004, p. 99. 

A more sensitive viscometer than the drop-time glass capillary method is also needed in size exclusion chromatography (SEC) such as the gel permeation chromatographic (GPC) analysis of polymer molecular weight distribution (MWD). In an SEC system, a concentration detector is commonly used for providing the weight concentration profile of the polymer elution curve. 

A New Detector for Determining Polymer Size and Shape in Size Exclusion Chromatography... 

A low-angle laser light scattering (LALLS) detector was developed for use in size exclusion chromatography [133] and has been used coupled with FFF [134]. The advantage of this detector is that it can, in combination with a concentration detector (refractometer, UV/VIS, or IR photometric detector), provide direct data on molar masses of the eluted sample. 

Jackson, C. Barth, H. G. In Molecular Weight Sensitive Detectors for Size Exclusion Chromatography Wu, C.-S., Ed. Dekker New York, 1995 Chapter 4, pp 103-145. 

The use of multiple detectors with size-exclusion chromatography (SEC) can greatly increase the information content available from a typical SEC analysis. This multidetector approach permits more accurate measurement of polymer properties than conventional SEC. The additional information, however, is obtained at the expense of an increase in the complexity of the instrumentation and data handling. In particular, a number of concerns arise in data acquisition and processing that are not present in conventional SEC. Some of these difficulties are outlined, and possible solutions are discussed. 

J. W. Mays and N. Hadjichristidis, Polymer characterization nsrng dilnte solntion viscometry, in Modern Methods of Polymer Characterization (H. G. Barth and J. W. Mays, eds.), John Wiley Sons, New York, 1991, pp. 227-269. C.-Y. Kno, T. Provder, M. E. Koehler, and A. F. Kah, Use of a viscometric detector for size exclnsion chromatography, in Detection and Data Analysis in Size Exclusion Chromatography (T. Provder, ed.), American Chemical Society, Washington, DC, 1987, pp. 130-154. 

Refractive index detectors can also be used. Their main advantage is that they can used for compounds with small to no UV absorption coefficients. They are, however, much less sensitive than UV detectors for the majority of compounds encountered as natural products. These detectors are generally more difficult to set up to obtain a stable baseline. They are best used with isocratic mobile phases such as those used in size-exclusion chromatography. 

Since the initial introduction of laser diffraction instrumentation in the 1970s, many different applications to panicle si/e aniilysis have been reported. Ihese have included measurements of si/e distributions of radioactive tracer particles, ink particles used in photocopy machines, zirconia fibers, alumina particles, droplets from electronic fuel injectors, crystal growth particles, coal powders, cosmetics, soils, resins, pharmaceuticals, metal catalysts, electronic materials, phoiugraphic emulsions, organic pigments, and ceramics. About a dozen instrument companies now produce LALLS instruments. Some I.AI.LS instruments have become popular as detectors for size-exclusion chromatography. 

The latter methods were almost fully substituted with size exclusion chromatography, though especially light scattering and viscometry are often used for detection in SEC (see section 11.7.3.2, Mobile Phases and Detectors for Size Exclusion Chromatography). 

Thus calibration of the apparatus with, for example, polystyrene standards with known values of K and a in the particular solvent at a particular temperature permits the absolute molecular mass to be calculated via the universal calibration method [9]. A further advantage of the detector is its ability to be used at temperatures up to 150°C, thus permitting the analysis of polyolefins. The study of branching in polymers is also possible with this system. This new detector probably represents the most significant advance in detectors for size exclusion chromatography in recent years. 

Kuo, C.-Y. Provder, T. Koehler, M.E. Kah, A.E. Use of a viscometric detector for size exclusion chromatography. In Detection and Data Analysis in Size Exclusion Chromatography, Provder, T., Ed. American Chemical Society Washington, D.C., 1987 130-154. 

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