Dual detector analyses

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. 

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. 

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. 

Figure 13 represents an instrument having dual-channel optics. This means that there are facilities for two line sources and one continuum source, each line source having its own monochromator and detector. Many workers have advocated the usefulness of this type of instrument. Certainly the recently introduced microcomputerised dual channel models are easier to use than their earlier counterparts, so it is worthwhile to summarise the possible attractions of simultaneous dual element analysis. One obvious possibility is the analysis of two elements in a sample at the same time, thus halving analysis time. A less obvious attraction is the possibility of analysing via an internal standard. This is where a second element, either already present or added to the sample, is measured and ratioed to the analytical element. The... 

Fig. 7.15. Coupled SFE-SFC system with dual detector for the total and individual analysis of a coating fibre extract. (Reproduced with permission of Springer-Verlag.)...

Actinomycin D dissociation kinetics were measured on a Cary 219 spectrophotometer equipped with a magnetic stirrer and thermostated cell holders. Sodium dodecyl sulfate (SDS) was used to sequester dissociating actinomycin D, and the resulting Increase In absorbance was monitored at 452 nm as a function of time. Stop-flow studies (daunorubicin and daunorubicin/ actinomycin D) were conducted with a Durrum-Glbson Model 110 stopped-flow spectrophotometer equipped with a dual detector accessory and a Tektronix storage oscilloscope Interfaced with a PDF 11/34 computer. Experiments were done In a 0.01M Na phosphate buffer, 0.1M NaCl, 0.001M NaEDTA, pH=7. Dissociation time constants were computed with a multlexponentlal analysis computer program. 

SP-2401" and 3% SP-2250. ° Detectors used by EPA standards procedures, include photoionization (PID)," electron capture (ECD)," Eourier transform infrared spectrometry (PTIR), " and mass spectrometry detectors (MSD)." ° Method 8061 employs an ECD, so identification of the phthalate esters should be supported by al least one additional qualitative technique. This method also describes the use of an additional column (14% cyanopropyl phenyl polysiloxane) and dual ECD analysis, which fulfills the above mentioned requirement. Among MSDs, most of the procedures employ electron impact (El) ionization, but chemical ionization (CI) ° is also employed. In all MSD methods, except 1625, quantitative analysis is performed using internal standard techniques with a single characteristic m/z- Method 1625 is an isotope dilution procedure. The use of a FTIR detector (method 8410) allows the identification of specific isomers that are not differentiated using GC-MSD. 

As implied above, the use of two columns of disparate phases may provide useful supplementary results for identification. This is especially useful when the two phases have complementary resolution performance for compounds in a specific analysis. The expectation is that compounds not resolved on one phase may be resolved on the second phase, provided sufficiently different retention mechanisms are available on these phases for the compounds of interest. Figure I3B shows the possible instrument arrangement for a dual column-dual detector single injection analysis. Often the injectors will be the same if it is only the phase difference and therefore resolution differences that are of interest. It is necessary to have a well-characterized set of component overlap problems, so that routine data interpretation is possible. An alternative approach to obtaining two sets of retention data simultaneously is shown in Figure I3A, where outlet splitting of the effluent from the column... 

Figure 13 (A) Dual detection analysis where the effluent from the single column is split into two different detectors. (B) Dual column analysis provides for different columns to be used to attempt to maximize resolution of components in complex sample analysis.

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. ... 

Figure 3.22 Analysis of the molecular weights ot the polymers in a polymer blend of polystyrene and poly(methyl methacrylate). This method requires a dual detector GPC system.

Qualitative analysis is the process of the determination of the presence (or absence) of a particular element or group of elements in a sample. The ability to perform a comprehensive qualitative analysis is directly related to the sensitivity of the analysis method and hence the detection capability. Ideally, it is desirable to determine major, minor, trace, and ultratrace concentration level elements simultaneously on the same sample aliquot, which requires an instrument and technique that exhibits a wide dynamic range of measurement of the ion currents for the various element isotopes. However, in practice, it is often difficult to determine the high intensity of major elements on the same sample dilution as that required for ultratrace concentration element determination (usually measured on undiluted sample). This problem is often accommodated by the use of very low abundance isotopes of the major concentration analyte, reducing the analysis sensitivity. Where no low abundance isotope is available, instrumentation with dual detectors (electron multiplier for low concentration analytes and Faraday analog detectors for high concentration elements) can be used effectively. 

Two examples of dual-channel manifolds for use In flow Injection analysis where R1 and R2 are reagent reservoirs P Is the pump S Is the sample I Is the Injector B Is a bypass loop W Is waste C Is the mixing and reaction coll and D Is the detector. 

Recently, multidimensional GC has been employed in enantioselective analysis by placing a chiral stationary phase such as a cyclodextrin in the second column. Typically, switching valves are used to heart-cut the appropriate portion of the separation from a non-chiral column into a chiral column. Heil et al. used a dual column system consisting of a non-chiral pre-column (30 m X 0.25 mm X 0.38 p.m, PS-268) and a chiral (30 m X 0.32 mm X 0.64 p.m, heptakis(2,3-di-(9-methyl-6-(9-tert-butyldimethylsilyl)-(3-cyclodextrin) (TBDM-CD) analytical column to separate derivatized urinary organic acids that are indicative of metabolic diseases such as short bowel syndrome, phenylketonuria, tyrosinaemia, and others. They used a FID following the pre-column and an ion trap mass-selective detector following the... 

The concrete block walls of the cell housing the generator tube and associated components are 1.7 meters thick. The facility also includes a Kaman Nuclear dual-axis rotator assembly for simultaneous transfer and irradiation of reference and unknown sample, and a dual Na iodide (Nal) scintillation detector system designed for simultaneous counting of activated samples. Automatic transfer of samples between load station to the rotator assembly in front of the target, and back to the count station, is accomplished pneumatically by means of two 1.2cm (i.d.) polyethylene tubes which loop down at both ends of the system and pass underneath the concrete shielding thru a pipe duct. Total one-way traverse distance for the samples is approx 9 meters. In performing quantitative analysis for a particular element by neutron activation, the usual approach is to compare the count rates of an unknown sample with that of a reference standard of known compn irradiated under identical conditions... 

Accordingly GPC analysis of dried latex samples were carried out. The carrier solvent was tetrahydrofuran and peaks were monitored by a Waters dual absorbance detector at wavelengths of 2 h and 3U0 nm. The latter detection was the closest to 350 nm available. 

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