16.3: Infrared Instruments
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Instrumentation for infrared spectroscopy use one of three common optical benches: non-dispersive instruments, dispersive instruments, and Fourier transform instruments. As we have already examined non-dispersive and dispersive instruments in Chapter 13, and because they are no longer as common as they once were, we give them only a brief consideration here. Fourier transform instruments, which dominate the current marketplace, receive a more detailed treatment.
Non-Dispersive Instruments
The simplest instrument for IR absorption spectroscopy is a filter photometer similar to that shown earlier in Figure 13.4.1 for UV/Vis absorption. These instruments have the advantage of portability, which makes the useful in the field, and typically are used as dedicated analyzers for gases such as HCN and CO.
Dispersive Instruments
Infrared instruments using a monochromator for wavelength selection use double-beam optics similar to that shown earlier in Figure 13.4.3. Double-beam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than those for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and H2O vapor when using double-beam optics. Resolutions of 1–3 cm–1 are typical for most instruments.
Fourier Transform Instruments
We covered the basic concepts of the Fourier transform in Chapter 7, which you may wish to review. In this section we take a more detailed look at the application of Fourier transforms to infrared instrumentation.
Components of a FT-IR
In a Fourier transform infrared spectrometer, or FT–IR, the monochromator is replaced with an interferometer (Figure \(\PageIndex{1}\)). There are four key components that make up the interfometer: the drive mechanism that moves the moving mirror, the beam splitter, the light source, and the detector.
Drive Mechanism
As we learned in Chapter 7, the Fourier transform encodes information about the wavelength or frequency of source radiation absorbed by the sample by observing how the signal reaching the detector varies with time. As the moving mirror is displaced in space, some frequencies of light experience complete constructive interference, some frequencies of light experience complete destructive interference, and other frequencies fall somewhere in between giving rise to a time domain spectrum. As the signal is monitored as function of time and the moving mirror is traversing a variable distance, the drive mechanism must allow for a precise and accurate relationship between the two. The mechanism of the moving mirror must be capable of moving the mirror through a distance of up to 20 cm at a scan rate as fast as 10 cm/s; it must also accomplish this while maintaining the mirror's orientation relative to the axis of its movement. To maintain accuracy, a HeNe laser, which emits visible light with a wavelength of 632.8 nm, is aligned with the light source so that they follow the same optical path.
Beam Splitter
The beam splitter is designed to reflect 50% of the source radiation to the fixed mirror and to pass the remaining 50% of the source radiation to the moving mirror. The materials used to construct the beam splitter depends on the range of wavelengths being used. The most common range of wavelengths, which is called mid-IR, runs from approximately 670 cm–1 to 4000 cm–1. Instruments for mid-IR use a beam splitter that consists of silicon or germanium coated onto a substrate of KBr or NaCl.
Sources and Transducers
The most common sources for FT-IR are those discussed in the previous section, such as a Nernst glower. The most common transducer for FT-IR is pyroelectric triglycine sulfate.