FTIR spectrometers

For radiofrequency and microwave radiation there are detectors which can respond sufficiently quickly to the low frequencies (<100 GHz) involved and record the time domain specttum directly. For infrared, visible and ultraviolet radiation the frequencies involved are so high (>600 GHz) that this is no longer possible. Instead, an interferometer is used and the specttum is recorded in the length domain rather than the frequency domain. Because the technique has been used mostly in the far-, mid- and near-infrared regions of the spectmm the instmment used is usually called a Fourier transform infrared (FTIR) spectrometer although it can be modified to operate in the visible and ultraviolet regions. 

The most important component of an FTIR spectrometer is an interferometer based on the original design by Michelson in 1891, as shown in Figure 3.11. 

One of the main design problems in an FTIR spectrometer is to obtain accurate, uniform translation of Mj over distances 6 which may be as large as 1 m in a high-resolution interferometer. 

As in all Fourier transform methods in spectroscopy, the FTIR spectrometer benefits greatly from the multiplex, or Fellgett, advantage of detecting a broad band of radiation (a wide wavenumber range) all the time. By comparison, a spectrometer that disperses the radiation with a prism or diffraction grating detects, at any instant, only that narrow band of radiation that the orientation of the prism or grating allows to fall on the detector, as in the type of infrared spectrometer described in Section 3.6. 

An FT-Raman spectrometer is often simply an FTIR spectrometer adapted to accommodate the laser source, filters to remove the laser radiation and a variety of infrared detectors. 

Atr—ftir can be readily performed on most commercial ftir spectrometers through the use of an attachment for atr spectroscopy. These devices provide ir-transparent internal reflection elements that are typically made of Ge, KRS-5, ZnSe, or ZnS. These internal reflection elements are made of materials that are of extremely high purity to avoid losses from absorption by impurities in these devices. Coupling of a thin film or surface sample to one of these reflection elements is accompHshed by pressing the sample against the element while acquiring the spectmm. 

Diffuse reflection iavolves reflecting the iafrared beam off of a soHd sample, as ia specular reflectioa, but it is the aoaspecular portioa of the reflected radiatioa that is coUected. Whea an ftir spectrometer is used, diffuse reflection is caUed DRIFTS (diffuse reflectance iafrared Fourier-transform... 

When the spectral characteristics of the source itself are of primary interest, dispersive or ftir spectrometers are readily adapted to emission spectroscopy. Commercial instmments usually have a port that can accept an input beam without disturbing the usual source optics. Infrared emission spectroscopy at ambient or only moderately elevated temperatures has the advantage that no sample preparation is necessary. It is particularly appHcable to opaque and highly scattering samples, anodized and painted surfaces, polymer films, and atmospheric species (135). The interferometric... 

Infrared (in) spectrometers are gaining popularity as detectors for gas chromatographic systems, particularly because the Fourier transform iafrared (ftir) spectrometer allows spectra of the eluting stream to be gathered quickly. Gc/k data are valuable alone and as an adjunct to gc/ms experiments. Gc/k is a definitive tool for identification of isomers (see Infrared and raman spectroscopy). 

In an FTIR spectrometer, a source (usually a resistively heated ceramic rod) emits infrared radiation that is focused onto an interferometer whose main components consist of a beamsplitter, fixed mirror, movable mirror, and detector. The beamsplitter divides the beam into two beams. One beam is reflected off the beamsplitter toward the fixed mirror and is then reflected back through the beamsplitter to the detector. The other beam is transmitted through the beamsplitter toward the movable mirror and is then reflected off of the beamsplitter and to the detector [1],... 

In an industrial-design FTIR spectrometer, a modified form of the G enzel interferometer is utilized.A geometric displacement of the moving mirrors by one unit produces four units of optical path difference (compared with two units of optical difference for a Michelson type interferometer). The modified Genzel design reduces the time required to scan a spectrum and further reduces the noise effects asstxiated with the longer mirror translation of most interferometers. 

Kauppinen, Jyrki K. FTIR. Spectrometers for Industrial Applications. Lecture notes, Universitv of Turku, Department of Applied Physics (1993). 

The samples were submitted to the sulfidation procedure described above, followed by 2 h of heating at 673 K, under vacuum (about 2x10 3 Pa). After cooling under vacuum, pyridine was adsorbed at room temperature for 30 minutes. The samples were then outgassed in three steps of 1 h the first one at room temperature and the others at 423 K and 523 K. Spectra were taken before pyridine adsorption and after each outgassing step, with a FTIR spectrometer Bruker IFS-88 (spectral resolution set at 1 cm ). Each spectrum represented the average of at least 50 scans. 

The reaction on the catalyst surface was followed by in situ i.r. spectroscopy using a Bruker IFS88 FTIR spectrometer for the characterisation of sorbed species and mass spectroscopy for the analysis of gas phase. The state of Pt was further investigated by in situ X-ray absorption spectroscopy (Daresbury, UK, beamline 9.1, transmission mode, Si(220) monochromator, Pt-Lj, edge). Details of catalyst characterisation techniques are reported elsewhere [13,14]. 

The samples were characterized by chemical analysis induced coupled plasma and atomic absorption techniques apparatus), nitrogen adsorption isotherms (at 77 K), XRD patterns ( Siemens diffractometer and (3uKa radiation), SEM observations (Hitachi S800 apparatus of the University C. Bernard, Lyon I) and TGA-DTA (Setaram 92-12 apparatus). The IR spectra were recorded with a Bruker IPS 48 FTIR spectrometer. 

Since modern FTIR spectrometers can operate in a rapid scan mode with approximately 50 ms time resolution, TRIR experiments in the millisecond time regime are readily available. Recent advances in ultra-rapid scanning FTIR spectroscopy have improved the obtainable time resolution to 5 ms. Alternatively, experiments can be performed at time resolutions on the order of 1-10 ms with the planar array IR technique, which utilizes a spectrograph for wavelength dispersion and an IR focal plane detector for simultaneous detection of multiple wavelengths. ... 

One major problem is that of sensitivity (i.e. the signal is very weak owing to the small number of adsorbing molecules). Typically the sampled area is 1cm2 with less than 1015 adsorbed molecules (i.e. about 1 nmol). With modern FTIR spectrometers, however, such small signals (0.01-2% absorption) can still be recorded at relatively high resolution ( 1 cm-1). 

The use of IR pulse technique was reported for the first time around the year 2000 in order to study a catalytic reaction by transient mode [126-131], A little amount of reactant can be quickly added on the continuous flow using an injection loop and then introduce a transient perturbation to the system. Figure 4.10 illustrates the experimental system used for transient pulse reaction. It generally consists in (1) the gas flow system with mass flow controllers, (2) the six-ports valve with the injection loop, (3) the in situ IR reactor cell with self-supporting catalyst wafer, (4) the analysis section with a FTIR spectrometer for recording spectra of adsorbed species and (5) a quadruple MS for the gas analysis of reactants and products. 

Diffuse reflectance FTIR (DRIFT) spectra were recorded on a Bio-Rad FTIR spectrometer (EXCALIBUR FTS3000). A high-temperature cell was attached to a flow system that allows in-situ sample treatment, adsorption and desorption of probe molecules at different temperatures. 

Metal-modified silicas were exposed to excess BuOOH vapor in order to generate the supported feri-butylperoxide complexes, followed by evacuation to remove PrOH and unreacted BuOOH. Reaction kinetics were monitored as the uptake of cyclohexene from the gas phase, using a ThermoNicolet Nexus FTIR spectrometer to measure the intensity of the o(C=C) mode. In situ spectra were recorded in custom-made glass reactors under vacuum. Formation of cyclohexene oxide was confirmed by GC/MS on an HP 6890 equipped with a DBI capillary column (J W Scientific). 

The more sophisticated treatment of Ingle and Crouch [7] comes very close but also misses the mark for an unexplained reason they insert the condition ... it is assumed there is no uncertainty in measuring Ert and Eot... . Now in fact this could happen (or at least there could be no variation in AEr) for example, if one reference spectrum was used in conjunction with multiple sample spectra using an FTIR spectrometer. However, that would not be a true indication of the total error of the measurement, since the effect of the noise in the reference reading would have been removed from the calculated SD, whereas the true total error of the reading would in... 

---