Low-angle laser light-scattering detector

The response of a LALLS detector is basically determined by the molecular mass of the polymer, its concentration, and the differential refractive index (dn/dc) for the polymer-solvent combination. Clearly, dn/dc must be measured separately this is a simple matter, but can require a comparatively large amount of sample and can be quite time-consuming. This combined technique can in theory be used to determine the absolute MMD of a single sample, or it can be used to produce a calibration which can subsequently be used by the SEC system on its own. 

The technique is not particularly well suited to materials of low molecular mass. In these cases, the amount of material required to obtain a good LALLS response may be such that it interferes with the chromatography. Also, while dn/dc is constant for reasonably high molecular masses, at lower molecular masses (less than 50 000 for polystyrene in toluene) dn/dc varies, and can be difficult to incorporate into the calculation. 

Problems are also introduced into this combined technique by the concentration detector. As the LALLS detector gives considerably more response for the higher molecular masses, there is often difficulty regarding the calculation of molecular mass at the high end of the distribution when there may be a strong LALLS response, but the concentration detector does not respond to the small amount of material present (an underestimate of the amount of material present produces a large overestimate of molecular mass in the limit, a LALLS response with no measurable concentration detector response corresponds to an infinite molecular mass ). Usually the concentration detector will be a differential refractometer. These are difficult to calibrate reliably for response, and this will lead to errors in the calculated molecular masses. Alternatively, a known mass of sample is injected into the SEC system and it is assumed that this corresponds to the total area of the detector response. This alternative approach works as long as no sample is lost 

With SEC-LALLS, noise on the LALLS trace, due to a variety of causes, is often observed, and can result in a number of problems [18,19]. This noise presents difficulties for any automatic data handling system and must be filtered out. However, in the experience of the author, in low-temperature applications of SEC/LALLS the SEC columns act as very efficient dust filters, and no problems have been encountered with noise. 

In the opinion of the author, the biggest difficulty with the use of SEC-LALLS calibration is conceptual. It is not clear whether it is because the weight average is measured at each point or because the LALLS response is emphasized at the higher molecular masses, but the calculated MMDs for samples always appear narrower for SEC-LALLS than for SEC on its own. This would be acceptable if users of the techniques took due account of these differences, but the general preference for polymers with narrow MMDs appears to cloud some users interpretation of the results. 

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Optical Diagram of a Low Angle Laser Light Scattering Detector Courtesy of LDC Abalytical, Thermo Instruments Corporation. 

In addition to refractive index detectors, specialized detectors such as on-line SEC detectors and low angle laser light scattering detectors are used for determining the distribution of molecular weights by SEC. 

Polymer Laboratories, Inc., Amherst Fields Research Park, 160 Old Farm Road, Amherst, MA 01002 low-angle laser light-scattering detector. 

Tosoh Corporation, 707, 1-Chome, Akasaka, Minato-ku, Tokyo, Japan low-angle laser light-scattering detector. 

Mrkvickova investigated PMMA grafted with poly(dimethylsiloxane) (PDMS) by SEC coupled with refractive index and low-angle laser light scattering detectors [144, 145]. Using toluene and THF as isorefiractive index eluents for PMMA and PDMS, respectively, a variation in chemical composition and molecular weight of individual copolymer blocks as a function of hydrodynamic volume were measured. Kramer et al. characterized poly(S-co-EA) by SEC coupled with NMR [ 146]. 

LOW-ANGLE LASER LIGHT SCATTERING DETECTOR (LALLS)... 

LALLS low-angle laser light scattering (detector)... 

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