TLC-FTIR

There is a need for increased chromatography-FTIR sensitivity to extend IR analysis to trace mixture components. GC-FTIR-MS was prospected as the method of choice for volatile complex mixture analysis [167]. HPLC-FT1R, SFC-FTIR and TLC-FTIR are not as sensitive as GC-FTIR, but are more appropriate for analyses involving nonvolatile mixture components. Although GC-FTIR is one of the most developed and practised techniques which combine chromatography (GC, SFC, HPLC, SEC, TLC) and FUR, it does not find wide use for polymer/additive analysis, in contrast to HPLC-FTIR. 

When considering libraries of spectra for identification purposes, the effect of sample preparation on spectral characteristics is also important. Two FUR sampling methods have been adopted for IR analysis of TLC eluates in the presence of a stationary phase, namely DRIFTS [741] and PAS [742], of comparable sensitivity. It is to be noted that in situ TLC-PA-FTIR and TLC-DRIFT spectra bear little resemblance to KBr disc or DR spectra [743,744]. This hinders spectral interpretation by fingerprinting. For unambiguous identification, the use of a reference library consisting of TLC-FTIR spectra of adsorbed species is necessary. 

To improve the performance of TLC-FTIR coupling, optimised sorbent layers are required which allow substantial reduction in identification limits. The full... 

TLC-FTIR is now in a rapid growth phase with commercial instrumentation. Somsen et al. [721] discussed applications of TLC in combination with FTIR, NIRS and PA-FTIR. TLC-FTIR has been reviewed [167]. 

Applications Identification of polymer additives by TLC-IR is labour intensive and comprises extraction, concentration of extracts, component separation by TLC on silica, drying, removal of spots, preparation of KBr pellets and IR analysis. The method was illustrated with natural rubber formulations, where N-cyclohexyl-2-benzothiazyl sulfenamide, IPPD and 6PPD antioxidants, and a naphthenic plasticiser were readily quantified [765]. An overview of polymer/additive type compounds analysed by transfer TLC-FTIR is given in Table 7.80. 

Applicability of in situ TLC-FTIR is shown in Table 7.81. Nonylphenol (NP) and alkylphenol ethoxy-lates (NP( 0) ), n = 3,9,14) were detected by means of TLC-FTIR [771],... 

Since SERS and SERRS are substance specific, they are ideal for characterisation and identification of chromatographically separated compounds. SE(R)R is not, unfortunately, as generally applicable as MS or FUR, because the method requires silver sol adsorption, which is strongly analyte-dependent. SE(R)R should, moreover, be considered as a qualitative rather than a quantitative technique, because the absolute activity of the silver sol is batch dependent and the signal intensity within a TLC spot is inhomogeneously distributed. TLC-FTIR and TLC-RS are considered to be more generally applicable methods, but much less sensitive than TLC-FT-SERS FT-Raman offers p,m resolution levels, as compared to about 10p,m for FTIR. TLC-Raman has been reviewed [721],... 

The preparation of fluorinated alcohols was carried out in multistep routes according to the reported procedures.1012 The synthesis of acrylic and methacrylic esters as shown in Table 11.1 was carried out in a fluorocarbon solvent such as Freon 113 by the reaction of the respective fluorinated alcohol with acryloyl chloride or methacryloyl chloride and an amine acid acceptor such as triethyla-mine with examples shown in Scheme 1. Other attempts to esterify the fluoroalcohols directly with acrylic acid or acrylic anhydride were not successful.11 Product purification by distillation was not feasible because of the temperature required, but purification by percolation of fluorocarbon solutions through neutral alumina resulted in products of good purity identified by TLC, FTIR, and H-, 13C-, and 19F- FTNMRs. 

Coupled spectroscopic methods such as TLC-UV (ultraviolet) and visible spectroscopy, TLC-mass spectrometry, and TLC-FTIR (Fourier transform infrared) have been developed to overcome this difficulty [7]. Their future application in the TLC analysis of natural pigments will markedly increase the information content of this simple and interesting separation technique. The automation of the various steps of TLC analysis (sample application, automated developing chambers, TLC scanners, etc.) greatly increased the reliability of the method, making it suitable for official control and legislative purposes [8]. 

Hyphenated TLC techniques. TLC has been coupled with other instrumental techniques to aid in the detection, qualitative identification and, occasionally, quantitation of separated samples, and these include the coupling of TLC with high-pressure liquid chromatography (HPLC/TLC), with Fourier transform infra-red (TLC/FTIR), with mass spectrometry (TLC/ MS), with nuclear magnetic resonance (TLC/NMR) and with Raman spectroscopy (TLC/RS). These techniques have been extensively reviewed by Busch (1996) and by Somsen, Morden and Wilson (1995). The chemistry of oils and fats and their TLC separation has been so well established that they seldom necessitate the use of these coupling techniques for their identification, although these techniques have been used for phospholipid detection. Kushi and Handa (1985) have used TLC in combination with secondary ion mass spectrometry for the analysis of lipids. Fast atom bombardment (FAB) has been used to detect the molecular species of phosphatidylcholine on silica based on the molecular ion obtained by mass spectrometry (Busch et al, 1990). 

TLC remains one of the most widely used techniques for a simple and rapid qualitative separation. The combination of TLC with spectroscopic detection techniques, such as FTIR or nuclear magnetic resonance (NMR), is a very attractive approach to analyze polymer additives. Infrared microscopy is a powerful technique that combines the imaging capabUities of optical microscopy with the chemical analysis abilities of infrared spectroscopy. FTIR microscopy allows obtaining of infrared spectra from microsized samples. Offline TLC-FTIR microscopy was used to analyze a variety of commercial antioxidants and light stabilizers. Transferring operation and identification procedure by FTIR takes about 20 min. However, the main drawbacks of TLC-FTIR are that TLC is a time-consuming technique and usually needs solvent mixtures, which makes TLC environmentally unsound, analytes must be transferred for FTIR analysis, and TLC-FTIR cannot be used for quantifying purposes. 

On a TLC plate, the entire sample is available for separation and visual detection. There are no problems with unrecognized loss of peaks or unexpected appearance of peaks from previous samples as in HPLC. Zone identification is facilitated in TLC by the visual nature of the detection using colors and shades and many different reagents and temperatures, and inspection in daylight and under short- and long-wavelength lamps. Separated substances can be subjected to subsequent analytical procedures (e.g., coupled TLC-UV-Vis, TLC-MS, TLC-FTIR) at a later time. 

Figure 9.1 shows an AMD (Chapter 7) separation of a complex mixture of pesticides, with multiple wavelength scanning to provide confirmation of identity (Butz and Stan, 1994). On-line TLC-UV and TLC-FTIR were used to identify drugs and metabolites (Chambuso et al., 1994 Kovar and Pistemik, 1996). Computer software for identification of separated zones by spectrodensitometry was described (Aginsky, 1995). 

An overview of TLC-FTIR evaluation was given by Frey et al. (1993), and qualitative and quantitative uses of near-IR reflectance densitometry were reviewed by Ciurczak et al. (1991), Mustillo and Ciurczak (1992), and Fong and Hieftje (1994). TLC on microchannels of zirconia was shown to provide 500 times better sensitivity compared to IR detection on microscope slides (Bouffard et al., 1994). Recent applications of TLC coupled with FTIR include identification of adenosine in biological samples (Pfeifer et al., 1996), surfactants (Buschmann and Kruse, 1993), EDTA in water (Wolff and Kovar, 1994b), and drugs (Mink et al., 1995 Pfeifer and Kovar, 1995). 

The combination of TLC with FTIR in situ evaluation is a useful method particularly for the identification of complex mixtures and their constituents. Although determination limits are higher than those for UV spectroscopy, the method can also be used for the quantification of substances with no suitable UV response. TLC/FTIR is described in detail in Chapter 8 of this volume. 

The large quantity of data generated by TLC-FTIR coupling can be printed out as the 3-D plot of a spectral series, with the wave numbers on the a -axis, the distances on the z-axis, and the absorptions on the y-axis (Fig. 19). However, since the whole picture can then become very complex, the (2-D) contour plot (Fig. 20) is better for recognition of band overlap and small quantities of impurities. 

Figure 18 TLC-FTIR window chromatogram (A and B, dashed line) and Gram-Schmidt chromatogram (B, dotted line) of an analgesic containing paracetamol, phenazone, and caffeine.

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