Dual detector applications

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. 

Cotton fibers are single cells composed primarily ( 96%) of the polymer cellulose. In our laboratory (5), cotton fibers were dissolved directly in the solvent DMAC-LiCl. This procedure solubilizes fiber cell wall components directly without prior extraction or derivatization, processes that could lead to degradation of high MW components. MW determinations have been carried out by a size-exclusion chromatography (SEC) system using commercial columns and instrumentation with DMAC-LiCl as the mobile phase. Incorporation of viscometry and refractive index (RI) detectors (6) allowed application of the universal calibration concept (7) to obtain MW distributions (MWDs) based on well-characterized narrow-distribution polystyrene standards (5). The universal calibration concept used by incorporation of dual detectors bypasses the need for cellulose standards. There are no cellulose standards available. Polystyrene standards for a wide range of MWs dissolved readily in DMAC-0.5% LiCl with no activation necessary. 

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 flame ionization detector (FID) is the detector most often used in steroid analyses. For very low concentrations of steroids, the application of ECD is needed. Thermal conductivity detectors (TCDs) cannot be used in the analysis of steroids because of their very low sensitivity. For steroidal alkaloids, a nitrogen-specific detector (NPD) has also been used. By the use of dual detector systems (e.g., FID and NPD), closely related nitrogen-containing and non-nitrogen-containing steroids can be easily differentiated. The application of MS as detector was already discussed in a previous entry in this encyclopedia." By using a GC/MS system, the identity of the peak(s) can be determined in an undisputed manner. ... 

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

TWO DETECTORS IN PARALLEL. This mode of operation has sometimes been referred to as dual channel operation. It has practical applications where additional qualitative information about a peak or sample is desired. There are several ways this additional information can be obtained. The most common ones are ... 

Samples are injected onto the turbulent-flow column similar to single-column methods. The analytes of interest are retained in the turbulent-flow column while the large macromolecules are eluted to waste. Once the analytes are separated from the matrix, the samples are then eluted into the analytical column. The characteristics of the analytical column determine the peak shape and separation seen at the MS detector. Flow rates which are compatible with the mass spectrometer can then be used and the chromatograms are based on conventional HPLC parameters. The key to dual-column methods is that the retentive properties of the analytical column must be sufficiently stronger than that of the turbulent-flow column the dual-column approach is performed in such a manner so that the mobile-phase composition needed to elute the analyte from the turbulent-flow column does not elute the analyte from the analytical column. The sample is then focused at the head of the analytical column until the mobile-phase conditions are changed to elute the analyte. The choice of columns is critical to the success of dual-column methods. Table 10.2 lists some of the applications of dual-column methods found in the literature. 

In the event that key temperature sensing instruments become inoperative, there are several alternate methods that may be used. Some applications utilize installed spare temperature detectors or dual-element RTDs. The dual-element RTD has two sensing elements of which only one is normally connected. If the operating element becomes faulty, the second element may be used to provide temperature indication. If an installed spare is not utilized, a contact pyrometer (portable thermocouple) may be used to obtain temperature readings on those pieces of equipment or systems that are accessible. 

For all the above reasons, it is to be preferred to measure two solute properties in one detector, especially if both measurements can be performed simultaneously. An example of this is the application of dual-wavelength absorption detection in LC. The application of this technique for the purpose of selectivity optimization has been investigated by Drouen et al. [584]. For the purpose of peak assignment or recognition, ratio recording may be used. The principle of this technique is based on Beer s law and may be explained from the following equation for the absorption ratio Ra ... 

Further information on quantitative aspects of SEC with dual detection can be obtained from the work of Trathnigg et al. [38]. Different applications of dual detection SEC in the analysis of segmented copolymers [39], block copolymers [40,41], star polymers [42], and polymer blends [43,44] are also available. The limitation of SEC with dual detection is that only binary combinations of monomers can be investigated successfully. In the case of ternary combinations, more than two detectors must be used or one of the detectors must be able to detect two components simultaneously. 

The application of refractive index and differential viscometer detection in SEC has been discussed by a number of authors [66-68]. Lew et al. presented the quantitative analysis of polyolefins by high-temperature SEC and dual refractive index-viscosity detection [69]. They applied a systematic approach for multidetector operation, assessed the effect of branching on the SEC calibration curve, and used a signal averaging procedure to better define intrinsic viscosity as a function of retention volume. The combination of SEC with refractive index, UV, and viscosity detectors was used to determine molar mass and functionality of polytetrahydrofuran simultaneously [70]. Long chain branching in EPDM copolymers by SEC-viscometry was analyzed by Chiantore et al. [71]. 

Ultraviolet spectrophotometers cont.), single-beam, 225 standardisation, 226 Ultraviolet spectrophotometry, 221-232 absorption cells, 226 colorimetry, 228 derivative, 230 difference method, 229 dual-wavelength, 229 identification by, 231 influence of pH, 224 influence of solvent, 224 laws of absorption, 222 quantitative applications, 227 stray-light effects, 224 Ultraviolet-visible detector, 202 multiwavelength, 211 Unicontin, 1011 Unidiarea, 474 Unidone, 356 Uniflu, 557, 893 Unilobin, 709 Unimycin, 846 Uniphyllin, 1011 Uniprofen, 677 Unisom, 576... 

The application of dual detection [UV and refractive index (RI)] to the SEC analysis of polystyrene-poly(methyl methacrylate) (PS-PMMA) has already been studied in this laboratory (2). Both MWD and CCD were determined using a methodology outlined by Runyon et al. (3). This approach relies on SEC column calibration with narrow polydis-persity standards for each of the homopolymers as well as a measure of the detector response factors for each homopolymer to produce a copolymer MWD. In the case of PS and PMMA this is feasible, but in other block copolymer systems the availability of suitable molecular weight standards may be more limited. In addition, this procedure does rely on true SEC and is not valid for block copolymers for which the universal calibration does not hold true for both blocks in a given solvent system. 

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