18.2: Instrumentation
- Page ID
- 386420
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The basic instrumentation for Raman spectroscopy is similar to that for other spectroscopic techniques: a source or radiation, an optical bench for bringing the source to the sample, and a suitable detector.
Sources
One of the notable features of the Raman spectrum for CCl4 (see Figure 18.1.1) is the low intensity of the Stokes lines and the anti-Stokes lines relative to the line for Rayleigh scattering. The low intensity of these lines requires that we use a high intensity source so that there are a sufficient number of scattered photons to collect. For this reason, a laser is the most common source for Raman spectroscopy, providing a high intensity, monochromatic source. Table \(\PageIndex{1}\) summarizes some of the more common lasers.
| type of laser | wavelengths (nm) |
|---|---|
| Ar ion | 488.0 or 514.5 |
| Kr ion | 530.9 or 647.1 |
| He/Ne | 632.8 |
| Near Infrared (NIR) Diode Laser | 785 or 830 |
| Nd/YAG | 532 or 1064 |
The intensity of Raman scattering is proportional to \(\frac{1}{\lambda^4}\), where \(\lambda\) is the wavelength of the source radiation; thus, the smaller the wavelength, the more intense the intensity of scattered light. For example, the intensity of scattering using an Ar ion laser at 488.0 nm is almost \(23 \times\) greater than the intensity of scattering using a Nd/YAG laser at 1064 nm
\[\frac{(1/480)^4}{(1/1064)^4} = 22.6 \nonumber \]
The increased scattering when using a smaller wavelength laser comes at a cost, however, of an interference from fluorescence from species that are promoted into excited electronic states by the source. The NIR diode laser and Nd/YAG laser, when operated at 1064 cm–1, discriminate against fluorescence and are useful, therefore, for samples where fluorescence is a problem.
Samples
Raman spectroscopy has several advantages over infrared spectroscopy. Because water does not exhibit much Raman scattering it is possible to analyze aqueous samples; this is a serious limitation for IR spectroscopy where water absorbs strongly. The ability to focus a laser onto a small area makes it possible to analyze very small samples. A liquid sample, for example, can be held in the tip of a 1-mm inner diameter capillary tube, such as that used for measuring melting points. Solid samples and gaseous samples can be sampled using the same types of cells used in IR and FT-IR (see Chapter 17). Fiber optic probes make it possible to collect samples remotely. Figure \(\PageIndex{1}\) shows the basic set-up. A small bundle of fibers (shown in blue) brings light from the source to the sample where a second bundle of fibers (shown in green) brings the scattered light to the slit that passes light onto the detector.
Optical Bench
Raman spectrometers use optical benches similar to those for UV/Vis or IR spectroscopy, which were covered in Chapter 7. Dispersive instruments place the laser source and the detector at 90° to each other so that any unscattered high intensity emission from the laser source is not collected by the detector. A filter is used to remove the Rayleigh scattering. To record a spectrum one either uses a scanning monochromator or a multichannel detector. Fourier transform instruments are similar to those used in FT-IR and include a filter to isolate the Stokes lines.