IR interfacing

Figure 7.11 Gas chromatography-IR interfaces. After White [166]. Reprinted from R.L. White, in Encyclopedia of Spectroscopy and Spectrometry, Academic Press, J.C. Lindon (ed.), pp. 288-293, Copyright (2000), with permission from Elsevier...

Furnace, Curie-point or heated filament pyrolysers linked to packed column or capillary column gas chromatograph. GC-MS or GC-FT-IR interfaces. 

Commercial GC/IR/MS instruments are available from Mattson Instruments (using a matrix isolation GC/IR interface) and from Hewlett Packard (using a highly optimized light pipe GC/IR design). Each uses a Hewlett Packard Mass Selective Detector to obtain electron impact MS data. The instrument in our laboratory is a prototype version of the Mattson instrument, built in collaboration with Mattson Instruments. 

IR Spectra of Benzyl Benzoate and Testosterone Cypionate Obtained from the LC/IR Interface... 

Instruments IR-85 Fourier Transform infrared spectrometer, through an IBM GC-IR interface. The interface consisted of a gold-coated Pyrex light-pipe with potassium bromide windows. A scan rate of 6 scans/sec and a spectral resolution of 8 cm- - were used for data acquisition. Samples were introduced into the system via splitless injections. A fused silica capillary column, 30 m x 0.32 mm i d DB-WAX (dj 1.0 pm), was employed with the outlet end connected directly to the GC-IR light-pipe entrance. Helium was used as the carrier gas at an average linear velocity of 41.4 cm/sec (35°C). No make-up gas was employed in the system. The column temperature was programmed from 35°C to 180°C at 2°C/min. The GC-IR light-pipe assembly was maintained at 170°C. 

System 2. This system consists of a Model 501 supercritical fluid chromatograph (Lee Scientific, Salt Lake City, UT) and a Nicolet 20 SXC FT-IR spectrometer. The SFC/FT-IR interface is a prototype design containing a 600 urn I.D. by 5mm pathlength SFC/FT-IR flow-through cell, narrow band MCT detector, and optics designed to match the collimated beam from the main bench to the flow cell and detector. 

Comparison of FID and FT-IR Detection. Figure 2a shows the flame ionization detector (FID) chromatogram from the capillary SFC separation of the pesticide mixture. This trace was obtained without the SFC/FT-IR flow cell by connecting the capillary separation coluurm directly to the end-of-coluurm restrictor mounted in the FID. This serves as a reference to show the chromatographic separation obtained before connection to the SFC/FT-IR interface. 

The authors thank D. Wymer of the Procter Gamble Company for assistance with the citrus oils SFC/FT-IR analyses, J. Freal of Adams Veterinary Research Laboratories for the pyrethrin extract, G. Adams and K. Kempfert of Nicolet Instrument Corporation for assistance with the SFC/FT-IR interface and pyrethrin extract analysis respectively. 

Kuehl, D., 1981, Hardware Considerations for a Capillary GC/FT-IR Interface, Paper No. 242, Pittsburgh Conf. Anal. Chem. Appl. Spectrosc., Atlantic City, NJ. 

The major problem with this approach to GC-IR (which can to a certain extent be shared with vapor-phase measurements) is the lack of extensive libraries of appropriate reference spectra. This disadvantage has largely been overcome by the final type of GC-IR interface to be described below. 

The two IR interfaces in common use are the light pipe [8] and so-called matrix isolation [9]. In the former method, the column effluent is passed through a heated IR gas cell (light pipe), and in the latter, it is condensed and frozen into a matrix suitable for analysis by IR [10]. 

Heaps, D.A. Griffiths, P.R. Reduction of detection limits of the direct deposition GC/FT-IR interface by surface-enhanced infrared absorption. Anal. Chem. 2005, 77, 5965. 

The system applied in the study mentioned above consisted of a CDS model 122 Pyroprobe with a ribbon filament as the heating surface (see Chapter 3 and Appendix 1). This pyrolyser heats by varying the resistance of the platinum element. Temperature rise times for flash pyrolysis are typically of the order of milliseconds. IR spectra were obtained with an FT-IR bench system equipped with a CDS pyrolysis/FT-IR interface. The data were collected at 8 cm" with a deuterated triglycine sulfate (DTGS) detector. The interface is cylindrical in shape with two potassium bromide windows for the IR beam to pass through. 

Dwyer [62] used a combination of chromatography and IR spectroscopy to provide a versatile tool for the characterisation of polymers. The HPLC-FT-IR interface systems described, deposit the output of a chromatograph onto an IR optical medium, which is then scanned to provide data as a time-ordered set of spectra of the chromatogram. Polymer analysis applications described include the identification of polymer additives, the determination of composition/molecular weight distributions in copolymers, the mapping of components of polymer alloys and blends, the molecular configuration changes in polymers, and component identification in complex systems. 

Perkin Elmer supply a system 2000 TGA-IR interface. This is a single supplier fully optimised system with a powerful range of advanced graphics and post-run processing software including the ability to produce stack plots showing small changes and trends... 

These techniques have been discussed by Castle and McClure [51] who applied the Perkin Elmer system 2000 TGA-FT-IR interface system to thermal decomposition studies on PVC-polyvinyl acetate copolymers. They identified acetic acid and hydrogen chloride as two of the decomposition products. 

Supercritical fluid chromatography (SFC)/ FT-IR spectroscopy, generally with carbon dioxide as the mobile phase, bridges the gap between GC/FT-IR and LC/FT-IR, and is particularly useful for separating nonvolatile or thermally labile materials not amenable to gas chromatographic separation [109J. Flow cells, mobile phase elimination and matrix-isolation techniques are used as SFC/FT-IR interfaces. 

The TGA/FT-IR data were collected by using a Bio-Rad FTS 40 spectrometer coupled to a PL Thermal Sciences TGA 1000. A Bio-Rad TGA/IR interface bench was located between the spectrometer and TGA. The interface bench was equipped with a high temperature gas cell (where the evolved gases from the TGA experiment are analyzed by IR), an infrared detector (MCT), and two Watlows to heat and control the temperature of the gas cell and transfer line. The 1 mm ID transfer line was made of stainless steel widi a thin silica lining. 

In this chapter we show how the design of the GC/FT-IR interface has led to improvements in sensitivity. We will then examine how various other instrumental techniques that produce an output stream where the composition is changing with time have been interfaced to FT-IR spectrometers. These include high-performance liquid chromatography (HPLC), supercritical fluid chromatography (SFC), and thermogravimetric analysis (TGA). 

LIGHT-PIPE-BASED GC/FT-IR INTERFACES 23.2.1. Instrumental Considerations... 

Figure 23.1. Two types of light-pipe used in the GC/FT-IR interface (a) with effluent from the GC column passing through a fitting (b) with GC effluent passing through a laser-drilled hole in the boro-silicate tube from which the light-pipe is constructed.

Several variations on the basic technique described above have been reported. For example, the absorbance values between 4000 cm and the detector cutoff can be integrated. Since all compounds give rise to a signal when this approach is used, the GC/FT-IR system switches from being a selective detector to being a universal detector. In an alternative approach, the chromatogram is simply constructed from the value of the largest absorbance measured in each spectmm. GC/FT-IR interfaces used in this manner would also be considered universal detectors. 

Figure 23.5. Cryolect matrix-isolation GC/IR interface. (Reproduced from [17], by permission of the American Chemical Society copyright 1985.)...

In practice, however, the matrix-isolation GC/FT-IR interface has three main drawbacks ... 

Even without the application of SEIRA, the sensitivity of the DD-GC/FT-IR interface has now reached the point that it rivals, and sometimes exceeds, that of quadrupole GC/MS instruments. Indeed, one user of a DD-GC/FT-IR interface had to purchase an ion-trap GC/MS interface because her quadrupole GC/MS system had lower sensitivity than the Bio-Rad Tracer [24] However, the popularity of GC/FT-IR has, in anything, waned since 1990. Although this is partly a matter of expense it is also a matter of education. Regrettably, many GC/MS users are unaware that the first hit obtained by a spectral search routine is often not the correct answer and hence do not understand the need for a complementary technique. Finally, the DD-GC/FT-IR interface is mechanically complex and more prone to down-time than GC/MS. The DD-GC/FT-IR interface must be simpler and more robust than the versions that are commercially available at the time of this writing if this technique is to gain more widespread popularity. For more details of GC/FT-IR interfaces of all types, the review article by Visser is strongly recommended [25],... 

In general, the interface between chromatographs and FT-IR spectrometers can be divided into two types those based on flow cells (such as the light-pipe GC/FT-IR interface) and those based on elimination of the mobile phase (such as the DD-GC/ FT-IR interface). Although not without its limitations, as outlined earlier in this chapter, the flow-ceU (light-pipe) approach for GC/FT-IR has proved to be a useful tool for the solution of a number of important problems of chemical analysis. Unfortunately, the same cannot be said for HPLC/FT-IR measurements, in which the column effluent is simply passed continuously through a flow cell. Let us examine the reason for this situation. 

Since most of the problems associated with flow-cell HPLC/FT-IR interfaces are caused by the presence of the mobile phase, the obvious solution is to eliminate the solvent prior to measurement of the infrared spectrum of the solute. A number of techniques for mobile-phase elimination have been reported in the past, with varying degrees of success. For conventional HPLC separations performed with 4.6-mm-i.d. columns, the biggest difficulty is complete evaporation of the solvent, for which the flow rate is typically 1 mL min while the solute is deposited simultaneously in a small area on a suitable substrate. 

More recent efforts became geared toward the development of a device that is analogous to the DD-GC/FT-IR interface (i.e., one in which the mobile phase is eliminated while depositing the analytes in as small an area as possible), so that ideally at least, the spectrum of each eluate can be measured in real time. Many different techniques for solvent elimination have been applied to the DD-HPLC/ FT-IR interface, including thermospray [33], concentric flow nebulizer [34], particle beam (sometimes called a monodisperse aerosol generator) [35], ultrasonic nebulizer [36], and pneumatic nebulizer [37,38]. A comparison of many of these techniques has been made by Somsen et al. [39], but at the time of this writing, the book is still out as to the identity of the optimum approach. An excellent summary of HPLC/FT-IR interfaces is to be found in a review article by Kalasinsky and Kalasinsky [40]. 

The first commercially successful off-line DD-HPLC/FT-IR interface was the LC Transform, made by Lab Connections [41]. With this device, nebulization is initiated ultrasonicaUy and the solvent is evaporated with either a thermospray or a concentric flow nebulizer. The solutes are first deposited on a rotating germanium disk on the underside of which a thick layer of aluminum has been deposited. After the deposition step, the disk is then moved to a specular reflection accessory that is mounted in the sample compartment of a standard FT-IR spectrometer. The developers of the LC Transform recognized that it is more convenient to measure the spectra of the components that had been deposited on the disk by reflection spectrometry than by transmission. However, the deposition of a very thin film of each eluate on a metal substrate would not allow its reflection-absorption spectrum to be measured with adequate efficiency without resorting to grazing incidence measurements, for which the disadvantages were discussed in Section 23.3.3. 

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