IR Instrumentation

Until the early 1980s, most mid-IR spectrometer systems were double-beam dispersive grating spectrometers, similar in operation to the double-beam system for ultraviolet/visible (UV/VIS) spectroscopy described in Chapter 2. These instruments have been replaced almost entirely by FTIR spectrometers because of the advantages in speed, signal-to-noise ratio, and precision in determining spectral frequency that can be obtained from a modem multiplex instrument. There are NIR instruments that are part of double-beam dispersive UWVIS/NIR systems, but many NIR instruments are stand-alone grating instruments. 

Note A single arrow - ) denotes a single-bond stretching vibration, a double arrow ( ) denotes a double-bond stretching vibration, and so on. A vertical arrow (J,) denotes a bending vibration. 

Material Transmission Range (pm) Solubility (g/100 g Water) Refractive Index (Rl) Comments 

Sodium chloride (NaCI) 0.25-16 36 1.49 Most widely used  

Potassium chloride (KCI) 0.30-20 35 1.46 reasonable range and low cost. Wider range than NaCI  

Let us begin with the instrumentation. Dispersive IR instruments, similar to the double-beam instruments described for UV-VIS spectrophotometry, have been used in the past but have become all but obsolete. While some laboratories may still use these instruments, we will not discuss them here. 

The methodology that involves instruments that utilize the interferometer and Fourier transformation mentioned in Section 8.6 has come to be known as Fourier transform infrared spectrometry (FTIR). 

FIGURE 8.15 An illustration of an FTIR instrument showing the light source, the interferometer, the sample compartment, and detector. 

The advantages of FTIR over the dispersive technique are 1) it is faster, making it possible to be incorporated into chromatography schemes, as will be seen in Chapters 12 and 13, and 2) the energy reaching the detector is much greater, thus increasing the sensitivity. 

---

Like NMR spectrometers some IR spectrometers oper ate in a continuous sweep mode whereas others em ploy pulse Fourier transform (FT IR) technology All the IR spectra in this text were obtained on an FT IR instrument... 

In principle, emission spectroscopy can be applied to both atoms and molecules. Molecular infrared emission, or blackbody radiation played an important role in the early development of quantum mechanics and has been used for the analysis of hot gases generated by flames and rocket exhausts. Although the availability of FT-IR instrumentation extended the application of IR emission spectroscopy to a wider array of samples, its applications remain limited. For this reason IR emission is not considered further in this text. Molecular UV/Vis emission spectroscopy is of little importance since the thermal energies needed for excitation generally result in the sample s decomposition. 

Whilst nothing can improve upon the disadvantage of low molar absorption coefficients, instrumental designs and improvements with ratio recording and FT-IR instruments have virtually overcome the accuracy and instrumental limitations referred to in (b) and (c) above. As a result, quantitative infrared procedures are now much more widely used and are frequently applied in quality control and materials investigations. Applications fall into several distinct groups ... 

A number of newly introduced abbreviations describes the special techniques used which depend also on the IR instrumentation (conventional dispersive optics or Fourier transform devices). 

Hi) F. The flow cell would have to be made from a water insoluble material that is transparent to infrared radiation, eg KRS5 (TIBr/Tll). Glass cannot be used for optical components in ir instruments, as it absorbs ir radiation. 

The major progress that has been made in in situ IR spectroscopy to date has been achieved with commercial IR instruments. However, an important... 

Laboratory measurements of the vibrational spectra of C3, C5 and C7 show transitions that lie in the spectral region 2300-1700 cm-1 or 4.35-5.88 p,m, but again the usual caveat about the resolution of the IR instrument and the precise identification of molecules still applies. Infrared astronomy is still best at identifying families of molecules containing C-H or C-C stretch, whether aromatic or aliphatic. Laboratory measurements are, however, possible for these species both in the IR and in the visible, and the positive identification of C2 emission in the Red Rectangle is without question, as in the identification of long chains up to HCm . 

With the dispersive IR instrument, it is important to use computer control and data acquisition. For conventional experiments also, the attractiveness of such a system has been pointed out by Peri (24). 

The essential instrumentation is divided into three parts (a) the pyrolyser, (b) the gas chromatograph and (c) the MS or FT-IR instruments. In this chapter interest focuses on pyrolysers as the other instruments are discussed elsewhere. 

There are many IR instruments, and since they are so different, you need your instructor s help here more than ever. But there are a few things you have to know. 

Fourier transform spectroscopy technology is widely used in infrared spectroscopy. A spectrum that formerly required 15 min to obtain on a continuous wave instrument can be obtained in a few seconds on an FT-IR. This greatly increases research and analytical productivity. In addition to increased productivity, the FT-IR instrument can use a concept called Fleggetts Advantage where the entire spectrum is determined in the same time it takes a continuous wave (CW) device to measure a small fraction of the spectrum. Therefore many spectra can be obtained in the same time as one CW spectrum. If these spectra are summed, the signal-to-noise ratio, S/N can be greatly increased. Finally, because of the inherent computer-based nature of the FT-IR system, databases of infrared spectra are easily searched for matching or similar compounds. 

It will be obvious from my account that although I consider myself primarily a physical chemist, my concern was always with finding the most efficient solution to a problem. If that meant doing phase-rule studies with a vacuum line (hardly ever attempted before), or getting infra-red spectra of polymers to identify end-groups when the only IR instrument in the City of Manchester at that time was in the ICI laboratories at Blackley, or synthesising new model compounds or super-pure A1C13, we just overcame the practical obstacles and got on with it. 

Photomultiplier tubes or photodiodes (light sensors) are used as detectors in UV-VIS spectrophotometers, while thermcouples (heat sensors) are used as detectors for infrared (IR) spectrometry. This is the reason UV-VIS instruments are called spectrophotometers while IR instrument are called spectrometers. 

Both the GC-MS and GC-IR instruments obviously require that the column effluent be fed into the spectrometer detection path. For the IR instrument, this means that the IR cell, often referred to as a light pipe, be situated just outside the interferometer (Chapter 8) in the path of the light, of course, but it must also have a connection to the GC column and an exit tube where the sample may possibly be collected. The infrared detector is nondestructive. With the mass spectrometer detector, we have the problem of the low pressure of the mass spectrometry unit coupled with the ambient pressure of the GC column outlet. A special method is used to eliminate carrier gas while retaining sufficient amounts of the mixture components so that they are measurable with the mass spectrometer. 

IR instruments are available in filter-based, grating-based, and FT-based models. The usual approach is to use a full-spectrum model to ascertain the working wavelengths for a particular reaction, then to apply simpler filter instruments to the process. This works where one, two, or three discrete wavelengths may be used for the analysis. If complex, chemometric models are used, and full-spectrum instruments are needed. 

As noted, IR instruments fall into several categories that range from simple photometers (single or multiple filter devices) to relatively complex full-spectrum devices, such as FTIR instruments. Today s process engineers often prefer optical methods of measurement rather than the traditional chromatographs because of perceived lower maintenance and ease of implementation. However, the final selection is often based on the overall economics and the practicality of the application. 

The other forms of blackbody sources are adaptations of those used widely in conventional laboratory-style IR instruments, which feature exposed electrically heated elements. Various designs have been used, with metal filaments, made from Kanthral and Nichrome, being simple solutions in lower cost laboratory... 

In a Fourier transform IR instrument the principles are the same except that the monochromator is replaced by an interferometer. An interferometer uses a moving mirror to displace part of the radiation produced by a source (Fig. 5.4) thus producing an interferogram which can be transformed using an equation called the Fourier transform in order to extract the spectrum from a series of overlapping frequencies. The advantage of this technique is that a full spectral scan can be acquired in about 1 s compared to the 2-3 min required for a dispersive instrument... 

---