Detectors, FTIR

Such effects principally cannot be observed in multi band detectors such as a UV diode array detector or a Fourier transform infrared (FTIR) detector because all wavelengths are measured under the same geometry. For all other types of detectors, in principle, it is not possible to totally remove these effects of the laminar flow. Experiments and theoretical calculations show (8) that these disturbances can only be diminished by lowering the concentration gradient per volume unit in the effluent, which means that larger column diameters are essential for multiple detection or that narrow-bore columns are unsuitable for detector combinations. Disregarding these limitations can lead to serious misinterpretations of GPC results of multiple detector measurements. Such effects are a justification for thick columns of 8-10 mm diameter. 

Qualitative and quantitative analyses with HPLC are very similar to those with GC (Sections 12.7 and 12.8). In the absence of diode array, mass spectrometric, and FTIR detectors that give additional identification information, qualitative analysis depends solely on retention time data, tR and C (Remember that tR is the time from when the solvent front is evident to the peak) Under a given set of HPLC conditions, namely, the mobile and stationary phase compositions, mobile phase flow rate, column length, temperature (when the optional column oven is used), and instrument dead volume, the retention time is a particular value for each component. It changes only when one of the above parameters changes. Refer to Section 12.7 for further discussion of qualitative analysis. 

There are many GC detectors available although the flame ionisation detector remains the most widely used and the most widely applicable to quality control of pharmaceutical products. However, newer detectors such as the plasma emission detector for analysis of trace impurities or the GC-FTIR detector for the structural characterisation of components in mixtures are becoming increasingly important. 

The HPLC-FTIR technique has recently been used to identify six catechins and two methyl-xanthines present in green tea extracts." " A reversed-phase separation of the compounds was performed on a C-18 column equilibrated at 30°C using an isocratic mobile phase of acetonitrile-0.1% formic acid (15 85), prior to introduction to the deposition interface linked to the FTIR detector. The solvent was evaporated at 130°C and spectra were collected every 6 sec during the run. Two distinct designs for HPLC-FTIR interfaces have been developed flow cells and solvent elimination systems. Flow cell systems acquired spectra of the eluent in the solvent matrix through IR transparent, nonhydroscopic windows. The... 

The GPC/DRI/FTIR instrument is complementary to the UV detector for compositional distribution. It runs at 135°C in TCB and can be used for EP analysis. Typical applications include ethylene content as a function of molecular weight, maleic anhydride content in maleated EP, or PCL content in caprolactone-g-EP copolymers. The FTIR detector is off-line so that 5-10 fractions of the eluant are collected on KBr plates and analyzed. This procedure gives calibration of IR absorption bands. This method is much more labor intensive than the other techniques and should be used with discretion. 

The use of an on-line Fourier transform infrared (FTIR) detector with GC has allowed for the identification of unknowns and the distinction between structurally similar compounds. Many compounds with structural similarities cannot be identified by electron impact mass spectrometry because the fragmentation patterns are (or are nearly) identical. An example is the identification of positional isomers of substituted chlorobenzenes, whose mass spectra are identical. In these cases, chemical ionization can be used to highlight structural differences. The infrared detector (IRD) gives quite different spectra for positional isomers, and when compared to library spectra of authentic compounds, it gives unequivocal identification. 

The characteristics of some other detectors are summarized in Table 9. The popular MS and FTIR detectors (often used on-line) are not included because they are discussed in Chapter 11. A comparison of the working ranges of the most common detectors is shown in Figure 8.14. 

A rather broad applicability of FTIR as a detector in liquid chromatography can be achieved when the mobile phase is removed from the sample prior to detection. In this case the sample fractions are measured in a pure state without interference from solvents. Experimental interfaces to eliminate volatile mobile phases fromHPLC effluents have been tried with some success [133-135] but the breakthrough towards a powerful FTIR detector has only been achieved by Gagel and Biemann, who formed an aerosol from the effluent and sprayed it on a rotating aluminum mirror. The mirror was then deposited in a FTIR spectrometer and spectra were recorded at each position in the reflexion mode [136-138]. 

Combinations of highly efficient separation columns, with specific or selective detectors, such as electron capture detector (BCD), GC-mass spectrometer (MS), and GC-Fourier transform infrared (FTIR) detector, make GC a more favorable technique. Multidimensional GC systems, which contain at least two columns operated in series, have also proved to be a powerful tool in the analytical chemistry of complex mixtures. 

Bourne, S. An online direct-deposition FTIR detector for chromatographs. American Laboratory. 30 17F-17J, 1998. 

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. 

More recently a new filter-type IR detector has been developed [41] with a highly sensitive MCT thermoelectrically cooled sensing element. It has similar response in the C-H region to that of FTIR detectors but it does not require nitrogen cooling. The integration of this detector, in a thermostated compartment, into a GPC system has resulted in an improvement of sensitivity of around ten times... 

The RI, UV, LALLS and viscometer detectors have been successfully used in this work. The FTIR detector has been applied to study protein by Remsen... 

Looking again at Figure 2.7, it will be remembered that resins such as LLDPE made with heterogeneous Ziegler-Natta catalysts and tandem reactors have a rather complex relation between MWD and average comonomer composition. Because this relation has such an important impact on the mechanical properties of polyolefins, an FTIR detector is often added to the GPC (GPC/RI-FTIR) to measure comonomer fraction as a function of molecular weight. 

Fig. 1. Film stretching machine (1) FTIR detector, (2) pneumatically rotatable pt rizer unit, (3) clamp, (4) polymer film sample, stress transducer, (6) displaconent transducer, (7) driving motor, (8) heating accessory, (9) cartridge heater, (10) temperature control, (11) KBr window, (12) specimen preparation and transfer device...

A nondestructive and universal detector in SFC is the FTIR detector (see Section 12.2.6.7). Although its sensitivity is not too high, this technique is highly informative and often helps to identify... 

The Fourier transform infrared (FTIR) detector is a nondestructive and concentration-sensitive detector, which is universal. The detection limit is about 150 pg, depending on the compound, and the linearity is 10. ... 

Any technique for gas analysis can be applied to EGA. The most frequently used methods are mass spectroscopy (MS) and Fourier transform infrared spectroscopy (FTIR). Many instrument manufacturers provide the ability to interface their TGAs with MS or FTIR (see Section 3.7, on instrumentation). Temporal resolution between the TGA and the MS or FTIR detector is an important feature, for example, in distinguishing absorbed water from water as a reaction product and in assigning a decomposition product to a specific mass loss. Each method has its experimental requirements, limitations, and advantages. Mass spectroscopy is a very sensitive technique that identifies volatile species by their mass-to-charge ratio, referred to as m/z. The evolution of the sum of all mJz species can be plotted and compared with the derivative TGA plot to ensure temporal resolution between the TGA and the mass spectrometer. The evolution of a specific mJz, associated with species such as water or formaldehyde, can show the distinct evolution of these compounds. The most common ionization is by 70eV electron impact (El), which operates... 

Figure 10.78 shows a PAS-FTIR detector (a), with sample holder for powders (b) and for disk samples from plaques (c) [394]. 

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