Dual detector

In particular, for copolymers this required an orthogonal coupling of one GPC to another to achieve the desired cro fractionation before application of dual detectors. This method is really a new polymer analysis member of a family of approaches developed in the literature which we are now terming "Orthogonal Chromatogr hy . It not only provides both a cro fractionation approach for copolymers and a new way of determining the GPC s "imperfect resolution" it also enables separation mechanisms previously reserved for the liquid chromatography of small molecules to be used for polymer analysis. 

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. 

The same can be said for the sections concerning the instrumental techniques of GC, IR, NMR, and HPLC. The chromatographic techniques of GC and HPLC are presented as they relate to thin-layer and column chromatography. The spectroscopic techniques depend less on laboratory manipulation and so are presented in terms of similarities to the electronic instrumentation of GC and HPLC techniques (dual detectors, UV detection in HPLC, etc.). For all techniques, the emphasis is on correct sample preparation and correct instrument operation. 

The molecular-weight distributions were measured using a Waters GPC in the dual-detector mode (DRI and UV). The UV detector was operated at 254 nm. The samples were prepared by dissolving 2 mg of polymer in 10 ml of THE The injection volume was 200 pi. Separations were effected using two Polymer Labs 10-gm PL mixed-B columns. THF was used as the mobile phase. The molecular-weight distributions were calculated relative to narrow polystyrene standards ranging from 102 to 4 x 106 M ,... 

Perkin— Elmer 8400 Yes and 8500 Yes Dual detector instrument (detectors chosen from Can be fitted with any Yes combination of above injection systems Yes Yes Yes Yes (CP 100 printer plotter Yes/Yes Yes, down to -80°C... 

Seawater, rain Solvent extraction Dual capillary GC/dual detector (ECD, ELCD) IDL 8.4 pg/n (ECD) 11.5 pg/ L (ELCD) No data Durel1 and Sauer 1990... 

Durrell GS, Sauer TC. 1990. Simultaneous dual-column, dual-detector gas chromatographic determination of chlorinated pesticides and polychlorinated biphenyls in environmental samples. Anal Chem 62(17) 1867-1871. 

Figure 3. Gel permeation chromatogram of nonylphenol-formaldehyde adducts with dual detectors.

Figure 6. Gel permeation chromatograms with dual detectors. Key A, di-2-ethylhexyl phthalate and B, chlorinated paraffin.

Quinones and Hydroqulnones. In the analysis of quin-ones and hydroqulnones, the use of two different dual detector systems was required. The retention data for hydroqulnones shows the normal behavior of hydroxyl groups associating with the solvent, THF. Thus octyl quinone and hydroquinone elute almost together. Similarly dioctylquinone and octyl hydroquinone elute together (Figure 7). The UV/RI response ratio for benzoquinone is 3.75. Hydroquinone and dioctylquinone show similar disparities in the UV/RI responses. This information provides a very good method for detecting impurities in dioctyl hydroquinone. 

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.)...

Figure 6.28. Simplified schematics of practical dual detector and/or column configurations.

Thermal conductivity detectors usually are built as dual detectors because originally a reference filament formed one leg of the Wheatstone bridge. Today most TCD units manufactured still require a reference gas. In dual column compensation mode the compensating column becomes the reference side. 

Fig. 34.11. Electropherograms for pAP using a microchip with a gold-based dual detector applying different Ed, (A) (in-channel Au film) +0.8 V (B) (end-channel Au wire) —0.1 V. Conditions Vsep = +250 V injection 5s at +2000V running buffer 50 mM Tris-Gly pH 9.0. Reprinted from Ref. [77], Copyright (2006), with permission from Elsevier.

The measurements by ultraviolet analyzers can utilize (1) single-beam (2) split-beam (3) dual-beam, single-detector (4) dual-beam, dual-detector (5) flicker photometer (6) photodiode and (7) retroreflector designs. The standard errors of these measurements are 2% FS, whereas it is 1% FS for the fiber-optic diode-array designs. These analyzers can handle process pressures up to 50 barg (750 psig) and temperatures up to 450°C (800°F). 

These ratios are sensitive to detector calibration and should always be determined on a given dual detector system by monitoring the chromatography of standards. 

Dual Detectors. Most dual detectors are run in parallel, the column effluent being split and run through both of them simultaneously. In GC the technique is known as dual channel GC usually, one of the detectors chosen is universal and the other is highly selective. Figure 6.6 shows the analysis of a commerical gasoline sample with dual detection by flame ionization (FID) and electron capture (ECD). The FID detects all the hydrocarbons, but the ECD is selective for the alkyl lead additives in gasoline and permits their detection without interference from the hydrocarbons. 

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