Molar mass-sensitive detectors

To overcome the problems associated with classical SEC of complex polymers, molar mass-sensitive detectors are coupled to the SEC instrument. Since the response of such detectors depends on both concentration and molar mass, they have to be combined with a concentration-sensitive detector. The following types of molar-mass-sensitive detectors are used frequently [25-28] ... 

Coupling with Molar-Mass-Sensitive Detectors... 

As has been pointed out, for SEC of complex polymers no simple correspondence exists between elution volume and molar mass. It is, therefore, useful to determine the molar mass not via a calibration curve but directly from the SEC effluent. This can be done by using molar-mass-sensitive detectors based on Rayleigh light scattering or intrinsic viscosity measurements [45]. 

As has been discussed, the combination of SEC and molar-mass-sensitive detectors is a powerful tool for the analysis of complex polymers. However, it is important to distinguish between claimed versus actual capabilities and between potential expectations and demonstrated performances. Tables 1 and 2, taken from a critical review of different techniques, summarize the informational content and additional details of SEC-light scattering and SEC-viscometry coupling... 

Table 1. SEC analysis using molar-mass-sensitive detectors ...

Gores F, Kilz P (1993) Copolymer characterization using conventional SEC and molar mass-sensitive detectors. In Provder T (ed) Chromatography of polymers, chap 10. ACS Symp Ser 521, American Chemical Society, Washington, DC... 

The introduction of molar mass-sensitive detectors overcame problems in SEC analysis of various polymer topologies, if the chromatographic technique is able to separate them properly (S). 

CHROM software. It is also possible to use molar mass sensitive detectors in this setup to get independent of calibrations or assumptions for the calculation of molar masses for copolymers with unknown structures. Multidetection analysis for independent copolymer composition calculations (S) and 2D segment distributions are already fully implemented in the PSS 2D-CHROM software package. 

Gel permeation chromatography (GPC) is the established method for the determination of molar mass averages and the molar mass distributions of polymers. GPC retention is based on the separation of macromolecules in solution by molecular sizes and, therefore, requires a molar mass calibration to transform elution time or elution volume into molar mass information. This kind of calibration is typically performed with narrow molecular mass distribution polymer standards, universal, or broad calibration methods or molar-mass-sensitive detectors like light-scattering or viscosity detectors. 

FIA Uses existing equipment Saves eluent No separation Limited time gain Not applicable for copolymers/blends Requires molar mass sensitive detectors Only primary information (cone., Mw, TV) Needs method change Needs special software Samples difficult to separate Utilize existing instruments... 

This method uses the high-performance liquid chromatography (HPLC) equipment for sample handhng and requires molar mass sensitive detectors (such as light scattering and/or viscometry) to obtain a mean property values from each detector (Mw and/or IV, respectively). The FIA result from a concentration detector yields polymer content in a sample, which can also be determined with other well-established methods. The FIA approach requires expensive and well-maintained equipment, and will not save much time or solvent furthermore, no distribution information is available. 

Application of a molar mass sensitive detector eliminates the column calibration. In principle, there are two kinds of the molar mass sensitive detectors. They are the light scattering photometers and viscometers. Actually the viscometers are not truly molar mass detectors since the measured quantity is the intrinsic viscosity and not the molar mass. The molar mass is determined from the experimental intrinsic viscosity and the so-called universal calibration, i.e., the relation log(M[j ]) versus elution volume that is independent of polymer composition and structure. Likewise, in conventional SEC the obtained results are affected by the flow rate and temperature... 

Consider the BB problem when molar mass sensitive detectors are employed. First, let us analyze the ideal case of a chromatograph fit with perfect detectors and not exhibiting BB, secondary fractionations, or interdetector volumes. In this case, the instantaneous MMD is strictly uniform, and any molar mass detector type would provide the same result ... 

CORRECTION METHODS FOR MOLAR MASS SENSITIVE DETECTORS... 

Now, we wish to determine an unbiased MMD from measurements of mass and molar mass sensitive detectors. Rewrite Eq. 3 as follows ... 

Table 2 Simulated example for molar mass sensitive detectors true and recuperated average molar masses from several MMDs.

The commercially available molar mass sensitive detectors do include a correction for BB in their software. Unfortunately, the applied correction procedures are not fully disclosed, but they seem to involve an interdetector volume readjustment. For example, the Viscotek Model 200 detector combines a DR in parallel with a specific viscometer. First, the (mass and molar mass) chromatograms of several narrow standards must be measured to determine the interdetector volume and a (uniform EMG) BB function. Then, the MMD is corrected for BB in an unspecified manner. Similarly, Wyatt Corp. has recently introduced a patented BB correction procedure for their triple-detector system (multiangle LS, DR, and specific viscosity sensors). 

The techniques that avoid numerical inversions, correct the MMDs in a single step, and are based on either 1) rotating the linear calibration (when only a mass chromatogram and a linear calibration are available) or 2) modifying the interdetector volume shift (when molar mass sensitive detectors are employed). Their main advantage is that they produce smooth and unique solutions. Their limitation, however, is that they produce only approximate solutions. 

The following types of molar mass-sensitive detectors are in common use ... 

Momabal et al. [76] proposed this new approach which combines separation power of CRYSTAF and TREF and provides very fast analysis in comparison to long analysis times of other crystallization based techniques. CEF can be performed in a typical column based TREF instrument and can easily adapt viscometry, light scattering, composition or other molar mass sensitive detectors. In another paper, Momabal et al. [77] demonstrated that dynamic crystallization analysis can be optimized by knowing the crystallization range of the sample and adapt the experimental conditions accordingly. CRYSTAF and dynamic crystallization... 

High-temperature SEC is the premier technique for information regarding molar masses. A number of different concentration detectors as well as molar mass sensitive detectors can be used. The coupling of SEC with spectroscopic techniques like FTIR and H-NMR reveals the chemical composition across the MMD of the sample. 

Tab. 20.7. Specific (molar mass sensitive) detectors for size exclusion chromatography.

Molecular-mass detectors. Size-exclusion molar-mass calibration is a complicated matter. Calibration curves differ for different polymer types, and for most commercial polymers, direct molar-mass calibration is not possible because of the lack of suitable known molecular-weight standards of sufficiently narrow mass distribution, and of the same chemical structure. The alternative is to determine the polymers molar mass in the SEC eluent, in situ by the use of an on-line molar-mass-sensitive detector. Two such detectors are now commercially available, the light-scattering detector and the recently introduced viscosity detector[5-8]. 

Molar mass-sensitive detector (light scattering, viscometry, MS) + concentration detector (Rl, UV/DAD)... 

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