Dual concentration detector

Dumelow (123) used SEC-LALLS with dual concentration detectors to study the variation in compositional heterogeneity with molecular weight in polystyrene-polydimethylsiloxane block copolymers. The results showed that some of the copolymers were in fact blends. The largest errors in the analysis... 

Trathnigg and co-workers [97] have quantitated copolymers and blends using a dual concentration detector. Xu and co-workers [98] used UV absorbance and differential refractive index detectors in conjunction with SEC to study the copolymer composition of chlorinated butyl rubber/PS comb graft copolymers as a function elution volume. 

Figure 11 illustrates an SEC separation of a sample of 3-conponent polystyrene mixture with the dual concentration and viscosity detectors of Figure 10. The top trace shows the concentration elution profile of the SEC separation as detected by a uv-photometer. The bottom trace records the same SEC separation, except with the viscometer signal from the log-amplifier output.

Multiple detection applied to the SEC characterization of copolymers is attractive because it yields both CCD and MWD information. A dual detection system based on two concentration detectors, for example, RI and UV, is useful where narrow standards of the homopolymers are available and where both homopolymers obey universal calibration. However, in other copolymer systems the addition of a third detector, LALLS, can offer the advantage of on-line determination of molecular weight for each eluting species. The triple detection approach gave similar values to the dual detector approach for a model copolymer system (PS-PMMA) studied. It was also applicable to a more difficult copolymer system (PS-PEO), although it appeared that where one homopolymer was present in very small quantities, an average p value gave more consistent results than correction for pi across the distribution. 

The above methods are specific to SEC systems employing a single concentration detector, as has been noted in the previous section and some molecular weights can be obtained by the use of a dual detector system. 

Determination of Chemical Composition Distribution by Dual Detection Based on detection with independent concentration detectors, this approach provides information on the chemical composition of copolymers or polymer blends and the variation with the molar mass [26]. An independent detector signal is required for each comonomer in the macromolecule. Many results have been published for chemically heterogeneous samples using two detectors. Obviously, there is a practical limit (four signals have been reported so far), as there are not many detectors to choose from. In addition, the approach is limited by the contribution of each detector to band broadening. 

Recently, the state-selective detection of reaction products tluough infrared absorption on vibrational transitions has been achieved and applied to the study of HF products from the F + H2 reaction by Nesbitt and co-workers (Chapman et al [7]). The relatively low sensitivity for direct absorption has been circumvented by the use of a multi-pass absorption arrangement with a narrow-band tunable infrared laser and dual beam differential detection of the incident and transmission beams on matched detectors. A particular advantage of probing the products tluough absorption is that the absolute concentration of the product molecules in a given vibration-rotation state can be detenuined. 

Interpretation of copolymer chromatograms in the literature does not include axial dispersion correction (3, 6) and little is known regarding it (5). The usual approach( is to utilize dual detectors and to assume that both detectors respond to at mos both composition and concentration. The two chromatograms then provide two equations in these two unknowns at each retention time. 

In addition to the emission due to the test element, radiation is also emitted by the flame itself. This background emission, together with turbulence in the flame, results in fluctuations of the signal and prevents the use of very sensitive detectors. The problem may be appreciably reduced by the introduction into the sample of a constant amount of a reference element and the use of a dual-channel flame photometer, which is capable of recording both the test and reference readings simultaneously. The ratio of the intensity of emission of the test element to that of the reference element should be unaffected by flame fluctuations and a calibration line using this ratio for different concentrations of the test element is the basis of the quantitative method. Lithium salts are frequently used as the reference element in the analysis of biological samples. 

Figure 4 Dual detector SECs Solution of the two equations for the two unknown concentrations leads to plots of the concentration ratio versus retention time. (Reproduced frcni Ref. 6. Copyright 1983, American Chemical Society.)...

Sane et al. reported the determination of diloxanide furoate in pharmaceuticals by gas chromatography [36]. A sample of powdered tablets equivalent to 250 mg of drug was dissolved in chloroform and diluted to a concentration of 5 mg/mL. A mixture of 2 mL of the sample solution and 1 mL of bromhexine hydrochloride solution (the internal standard) was diluted to 5 mL with chloroform, was used for the analysis. The injection volume was 400 nL, which was analyzed at 265°C on a stainless steel column (3 m x 2 mm) containing 3% OV-13 on Chromosorb W-HP (80-100 mesh). Nitrogen was used as the carrier gas at a flow rate of 50 mL/min, and analyte detection was effected using a dual flame ionization detector. 

TABLE 10.12 Concentrations of 10 PAHs in National Institute of Standards and Technology (NIST) Standard Reference Material SRM 1649 Air Particles Determined by Dual Programmable Fluorescence Detector Method... 

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