Refractometer detector applications

Use of the differential refractometer detector is applicable to all polymers having refractive indices different from that of the solvent. However, a correction must be made if the polymer refractive index depends on molecular size, such as at very low molecular weights. 

Liquid chromatographs are equipped with a means to continuously monitor the column effluent and recognize the presence of solute. Only small sample sizes are used with most HPLC columns, so a detector must have high sensitivity. The type of detector that has the most universal application is the differential refiractometer. This device continuously monitors the refractive index difference between the mobile phase (pure solvent) and the mobile phase containing sample (column effluent). The sensitivity of this detector is on the order of 0.1 ju,g, which, compared to other detectors, is only moderately sensitive. The major advantage of the refractometer detector is its versatility its main limitation is that there must be at least 10 7 refractive index units between the mobile phase and sample. 

Note that in liquid phase chromatography there are no detectors that are both sensitive and universal, that is, which respond linearly to solute concentration regardless of its chemical nature. In fact, the refractometer detects all solutes but it is not very sensitive its response depends evidently on the difference in refractive indices between solvent and solute whereas absorption and UV fluorescence methods respond only to aromatics, an advantage in numerous applications. Unfortunately, their coefficient of response (in ultraviolet, absorptivity is the term used) is highly variable among individual components. 

Fluorescence detectors can also be used and while their sensitivity may be greater, they are less widely applicable owing to the smaller number of fluorescent compounds. Differential refractometers will detect changes in the refractive index of the solvent due to the presence of solutes and, while they are less sensitive than the other detectors and often cannot be used effectively with gradient elution techniques, they are capable of detecting the presence of any solute. 

GPC is a promising method for examination of template polymerization, especially copolymerization. Copolymerization of methacrylic acid with methyl methacrylate in the presence of polyCdimethylaminoethyl methacrylate) can be selected as an example of GPC application for examination of template processes. The process was carried out in tetrahydrofurane as solvent at 65°C. After proper time of polymerization, the samples were cooled, diluted by THF, filtered, and injected to GPC columns. Two detectors on line UV and differential refractometer, DRI, were applied. UV detector was used to measure concentration of two monomers, while the template was recorded by DRI detector (Figure 11.3) The decrease in concentration ofboth monomers can be measured separately. It was found that a big difference in the rate of polymerization between template process and blank polymerization exists. The rate measured separately for methacrylic acid (decrease of concentration of methacrylic acid in monomers mixture) was much higher in the template process. Furthermore, the ratio ofboth monomers changes in a different manner. Reactivity ratios for both monomers can be computed. Decrease in concentration during the process is shown in Figure 11.4. 

Earlier work in the HPLC analysis of TGs used a differential refractometer as the detector a number of papers have detailed isocratic systems combined with refractive index (RI) detectors, often with acetonitrile/acetone mobile phases. Although aqueous mobile phases were generally used with alkyl-bonded phase columns, due to the lipophilicity of TGs, water could not be used in the mobile phase for this particular application therefore the mobile phases generally employed consisted of mixtures of acetone and acetonitrile and occasionally tetrahydrofuran, methylene chloride, or hexane (the conspicuous absence of water in the mobile phase prompted the term nonaqueous reverse phase, or NARP, to describe these systems). 

Refractive index and specific refractive index increments - (k = dn/dc) of polymers in solution have been studied extensively in connection with light scattering measurements and size exclusion chromatography applications to polymer characterization for which refractometers are used as standard concentration detectors. Contrary to the observations made in the infrared region (12), refractive index increments have been shown to be a function of the molecular weight of the polymers (2) and, in some cases, of the copolymer composition (17). Therefore, the assumptions of linearity and additivity (Eq. 1 to 4) have to be verified for each particular polymer system. In the case of styrene/acrylonitrile copolymers, there is an additional uncertainty due to the... 

The detection problems. Due to preferential solvation of mactomolecules in mixed solvent (see section 11.2.4), the composition of the bulk solvent among polymer chains differs from the composition of the original sample solvent, which is mobile phase. In LC CC, macromolecules elute together or in the vicinity of their bulk solverrt. This prevents application of the non-specific detectors such as differential refractometers (see sections 11.6.1.4 and 11.7.3.2), which sensitively respond to changes in... 

In 1980, Fukutomi et al. reported on the application of LALLS detection for proteins, enzymes, and a variety of synthetic water-soluble polymers, including PVA (21). A differential refractometer was used as the concentration detector in tandem with the LALLS photometer. TSK-PW columns were used for... 

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