FTIR detection

As to the main limitation of MS vs. FTIR detection, namely the inability to distinguish closely related isomers, this rarely plays a role in additive analysis. Notable examples of isomeric additives are the bifunctional stabilisers C22H30O2S as 4,4 -thio-bis-(6-t-butyl-m-cresol), 2,2 -thio-bis-(4-methyl-6-f-butylphenol) and 4,4 -thio-bis-(2-methyl-6-f-butylphenol) (Section 6.3.6), the bisphenolic antioxidants C23H32O2 (Plastanox 2246 and Ethanox 720) and the phenolic antioxidants C15H24O (nonylphenol and di-f-butyl-p-cresol). 

Microbore HPLC-FTIR detection limits are about 10 times lower than analytical-scale HPLC-FTIR detection limits. The lowest reported LC-FTIR detection limits are approximately 100-1000 times higher than the best GC-FTIR detection limits. The main characteristics of flow-cell HPLC-FTIR are summarised in Table 7.44. Because of mobile-phase interferences, flow-cell HPLC-FTIR is considered as a powerful tool only for the specific detection of major components but is otherwise a method of limited potential, and SFE-SFC-FTTR has been proposed as an alternative [391]. 

For microbore HPLC, with a flow of less than lOOpLmin-1, off-line LC-FT1R has been developed using matrix isolation techniques. The solutes are deposited on a moving IR salt window [504] or on a rotating plated disc [486], and are measured afterwards with the aid of a FITR microscope or a reflectance accessory. FTIR detection was first applied to the analysis of microbore HPLC eluent by Teramae and Tanaka [505]. In microbore HPLC-FTIR the amount of mobile phase required for separation is much less than for conventional scale HPLC. This simplifies both flow-cell and mobile-phase elimination interfaces. Flow-cell... 

The two-step degradation of commercial ethylene-vinyl acetate (EVA) copolymers has been investigated using TGA coupled with FTIR detection of the pyrolytic products evolved [48]. Acetic acid was evolved from the first... 

Trimethylsilyl derivatives of ten hydroxy- and methoxyhydroxyflavonoids have been studied by the GC-FTIR technique." " The correlation found between retention and gas-phase IR data was used in structural identification of compounds having very similar chromatographic behavior. The shift of the carbonyl frequency gave information on the presence of substitution. Some hydroxy- and methoxy-substituted flavones have been studied following carbon dioxide supercritical fluid chromatography on polymethylsiloxane capillary columns using flame ionization and FTIR detection." " " ... 

Gas FTIR detection wavenumber (cm-1) FTIR detection limit1 (ppb at L = 1 km) TDLS detection wavenumber (cm-1) TDLS detection limit1 (ppb at L = 150 m) Matrix isolation detection limit (ppb)... 

To gain further information concerning the hexane fractions, on-line supercritical fluid extraction/SFC was coupled with on-line FTIR detection. FT1R should have an advantage in that it is more sensitive to aliphatic hydrocarbons than the UV detector. 

The on-line extraction technique does not perform as well as traditional methods for the toluene fraction in contrast to the hexane fraction. The later eluting peaks are much more intense for the traditional sampling in all cases. It seems that CO favors the lower molecular weight molecules, even at high densities. On line FTIR detection did not yield any new information for the aromatic fractions. Highly aromatic material does not absorb infrared radiation greatly. In contrast, UV detectors are quite sensitive and yield significant information for aromatic compounds. 

Noguchi T. FTIR detection of water reactions in the oxygen-evolving centre of photosystem II. Phil Trans R Soc B. 2008 363(1494) 1189-95. 

Initially packed columns were used, but capillary columns are most frequently used today. A variety of polar and non-polar column phases have been used to determine VOCs depending on the particular compound(s) analyzed. Numerous examples were summarized by Wille and Lambert.5 Similarly, a range of detection devices have been utilized. Flame ionization detection (FID) and mass spectrometry (MS) in the selected-ion monitoring mode (SIM) are most frequently used, but other techniques include electron-capture detection (ECD) and Fourier transform infrared (FTIR) detection. 

Other examples of instruments that can be used for real-time monitoring of VOCs are gas analyzers based on non-dispersive infra red (NDIR) detection and Fourier transform infrared (FTIR) detection. The use of FTIR technique may under certain circumstances enable identification and quantification of individual VOCs at low concentrations (Hicks et al., 1992 and Bunding Lee et al., 1993). 

The molecular information that FTIR detection offers that MS, the other common hyphenated GC technique, cannot, includes elucidation of aromatic ring substitution (for example, with di-substitution, ortho, meta, or para), cis or trans or geminal substitution on a carbon-carbon double bond, the arrangement of rings in PAHs (benz[a]anthracene versus chrysene or triphenylene or benzo(c]phenan-threne or tetracene for the four-ring ortho-fused PAHs), aUcyl-chain branching isomers, and alcohols, which often shows in the MS as the easily dehydrated product aUcene [84]. 

There are two main types of FTIR detection for GCs, in the gas-phase using an in-stream optical system and through vapor deposition with detection being away from the GG flow stream. In the first, a light pipe that can transmit IR radiation is positioned on either side of a detection cell. Transparent windows pass the IR radiation into the flow ceU. The whole assembly is maintained at temperatures of 250 °C to 350 °C to prevent deposition of sample molecules. Most interfaces for this type of GC-FTIR also have heated transfer lines to and from the flow cell to ensure that no deposition occurs before introduction into the spectrometer. 

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