In our modern laboratory, we use Raman spectroscopy for residual dirt analysis for the assessment of fibres, plastics or salts, for the verification and the identification of filmic contamination as well as particulate contamination and in chemical analytics, in particular in plastics analytics. There the method is used, e.g. for the identification of deposits, residues, inclusions, media, substances, additives and materials (plastics).

Constant-scan plane grating monochromators have been designed50 but have not been widely adopted, due to the complexity of the required mechanisms for the precise movement of the slits. Hunter described a constant-scan monochromator for the vacuum ultraviolet in which the entrance and exit slits moved along the Rowland circle. The imaging properties of the constant-scan monochromator with fixed entrance and exit arms have not been fully explored, but since each wavelength remains on blaze, there may be applications where this design proves advantageous.

Raman spectroscopyPDF

Since the incident light is not collimated, the grating introduces wavelength-dependent aberrations into the diffracted wavefronts. Consequently, the spectrum cannot remain in focus at a fixed exit slit when the grating is rotated (unless this rotation is about an axis displaced from the central groove of the grating). For low-resolution applications, the Monk-Gillieson mount enjoys a certain amount of popularity, since it represents the simplest and least expensive spectrometric system imaginable.

Two monochromator mounts used in series form a double monochromator. The exit slit of the first monochromator usually serves as the entrance slit for the second monochromator (see Figure 6-5), though some systems have been designed without an intermediate slit. Stray light in a double monochromator with an intermediate slit is much lower than in a single monochromator: it is approximately the product of ratios of stray light intensity to parent line intensity for each single monochromator.

The Raman spectrum of each substance has certain areas with higher and lower areas of Raman intensity (so-called bands); these areas produce a characteristic image. This image can be compared to known patterns in a spectral library and the type of sample and its characteristics determined beyond doubt.

Raman spectroscopy is suitable for the analysis of a large number of substances. It is possible to analyse liquids, gases and solids.

A plane grating is one whose surface is flat. Plane gratings are normally used in collimated incident light, which is dispersed by wavelength but is not focused. Plane grating mounts generally require auxiliary optics, such as lenses or mirrors, to collect and focus the energy. Some simplified plane grating mounts illuminate the grating with converging light, though the focal properties of the system will then depend on wavelength. For simplicity, only plane reflection grating mounts are discussed below, though each mount may have a transmission grating analogue.

For years we have used Raman spectroscopy reliably and routinely to analyse samples for our customers. For this reason, we are also able to obtain exact measurement results in challenging conditions. The spectrometers and databases we use are from renowned brand-name manufacturers and as such guarantee not only precise results, but also maximum protection for the material.

Raman spectroscopyapplication

The Raman spectrum is characterised by the bands mentioned. These areas of higher Raman intensity are characteristic for every substance. In this way the spectrogram of an unknown substance can be compared to samples from a spectral database. If the bands are in the same places, the classification is unambiguous, as in our example for the comparison of a sample to the spectrum for polypropylene.

Raman spectroscopysample preparation

At Quality Analysis we offer a broad spectrum of measuring and analytical services. These services also include the use of Raman spectroscopy to identify inorganic and organic samples as well as their composition and crystal orientation. Having spectroscopic analysis undertaken by us offers you a whole string of technical and commercial advantages.

Depending on the specific characteristics of the material to be analysed (e.g. the area of the excitation wavelength), Raman spectroscopy also has disadvantages. In particular, these include:

For footnotes and additional insights into diffraction grating topics like this one, download our free MKS Diffraction Gratings Handbook (8th Edition)

Raman spectroscopyinstrumentation PDF

Using Raman spectroscopy, it is possible to draw conclusions about the following material characteristics, among others:

A grating used in the Littrow or autocollimating configuration diffracts light of wavelength λ back along the incident light direction (Figure 6-4). In a Littrow monochromator, the spectrum is scanned by rotating the grating; this reorients the grating normal, so the angles of incidence α and diffraction β change (even though α = β for all λ). The same auxiliary optics can be used as both collimator and camera, since the diffracted rays retrace the incident rays. Usually the entrance slit and exit slit (or image plane) will be offset slightly along the direction parallel to the grooves so that they do not coincide; this will generally introduce out-of-plane aberrations. True Littrow monochromators are quite popular in laser tuning applications.

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Since the invention of more powerful lasers that are at the same time less aggressive on the material, Raman spectroscopy has become established in almost all areas of chemical analytics. Thanks to the high information density, chemicals can not only be reliably identified, pure material concentrations can also be assessed in complex mixtures. Raman spectroscopy offers many other possible applications.

Raman scattering describes the interaction of the photons (particles of light) with the molecules of the medium. The photons create molecular vibration in the sample. During this process the photons lose energy. Because the wavelength of the light is dependent on its energy, the wavelength is reduced by the loss of energy, in other words: the frequency changes compared to that of the incident light. The frequencies produced by Raman scattering are dependent on the material on which the light is incident. The frequency differences are dependent on various energies in the material such as the rotation, spin-flip and vibration processes. Part of this energy is transferred from the material to the light and changes the frequency of the light. This is the so-called Raman effect.

For the analysis of the reflected, scattered light, first all the light at the excitation wavelength (that is the Rayleigh scattering) must be removed using an optical filter. The remaining scattered light (the Raman scattering) is guided to an optical grid and split into its individual wavelengths. A CCD sensor produces a spectrum from this light.

Raman spectroscopyinstrumentation

Aberrations caused by the auxiliary mirrors include astigmatism and spherical aberration (each of which is contributed additively by the mirrors); as with all concave mirror geometries, astigmatism increases as the angle of reflection increases. Coma, though generally present, can be eliminated at one wavelength through proper choice of the angles of reflection at the mirrors; due to the anamorphic (wavelength-dependent) tangential magnification of the grating, the images of the other wavelengths experience higher-order coma (which becomes troublesome only in special systems).

Compared to other spectroscopic methods, for example FTIR spectroscopy, Raman spectroscopy offers a few advantages that result above all from the usage of different lasers in the visible to near IR range for a very wide range of materials. Specifically, these include:

A monochromator is a spectrometer that images a single wavelength or wavelength band at a time onto an exit slit; the spectrum is scanned by the relative motion of the entrance and/or exit optics (usually slits) with respect to the grating.

it is the scan angle Φ rather than the half-deviation angle K that remains fixed. In this mount, the bisector of the entrance and exit arms must remain at a constant angle to the grating normal as the wavelengths are scanned; the angle 2K(λ) = α(λ) - β(λ) between the two arms must expand and contract to change wavelength.

This design involves a classical plane grating illuminated by collimated light. The incident light is usually diverging from a source or slit, and collimated by a concave mirror (the collimator), and the diffracted light is focused by a second concave mirror (the camera); see Figure 6-1. Ideally, since the grating is planar and classical, and used in collimated incident light, no aberrations should be introduced into the diffracted wavefronts. In practice, since spherical mirrors are often used, aberrations are contributed by their use off-axis.

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This is the Raman spectrum for a particle of polypropylene (red) compared to a reference from a spectral database (blue). The identification is unambiguous.

Fastie improved upon the Ebert design by replacing the straight entrance and exit slits with curved slits, which yields higher spectral resolution.

The vast majority of monochromator mounts are of the constant deviation variety: the grating is rotated to bring different wavelengths into focus at the (stationary) exit slit. This mount has the practical advantage of requiring a single rotation stage and no other moving parts, but it has the disadvantage of being “on blaze” at only one wavelength – at other wavelengths, the incidence and diffraction angles do not satisfy the blaze condition

If a material is treated thermally or mechanically, its internal stress can change. If you now compare the Raman spectra of a sample of the treated and untreated material, the changes in the stress can be detected in the form of frequency shifts. Higher frequencies are indicative of a compressive stress, while lower frequencies indicate an increase in the tensile stress.

During the analysis of the structure of chemical substances, the process determines the structure of the molecules in a chemical substance. This process, important in chemistry and pharmaceuticals, determines the polarisation of the Raman scattered light. If this light is completely polarised, the molecules are isotropically polarised; on the other hand if the polarisation of the scattered light is incomplete, the molecules are anisotropically polarised. The exact degree of depolarisation is determined by placing various polarisation filters in the beam path.

Raman spectroscopydiagram

Raman spectroscopy is a method for the analysis of the inelastic scattering of light at molecules or solids and is used for the analysis of material characteristics, among other aspects.

Like all monochromator mounts, the wavelengths are imaged individually. The spectrum is scanned by rotating the grating; this moves the grating normal relative to the incident and diffracted beams, which changes the wavelength diffracted toward the second mirror. Since the light incident on and diffracted by the grating is collimated, the spectrum remains at focus at the exit slit for each wave-length, since only the grating can introduce wavelength-dependent focusing properties.

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An alternative design that may be considered is the constant-scan monochromator, so called because in the grating equation

A triple monochromator mount consists of three monochromators in series. These mounts are used when the demands to reduce instrumental stray light are extraordinarily severe.

Raman spectroscopyprinciple and instrumentation PDF

In this mount (see Figure 6-3), a plane grating is illuminated by converging light. Usually light diverging from an entrance slit (or fiber) is rendered converging by off-axis reflection from a concave mirror (which introduces aberrations, so the light incident on the grating is not com-posed of perfectly spherical converging wavefronts). The grating diffracts the light, which converges toward the exit slit; the spectrum is scanned by rotating the grating to bring different wavelengths into focus at or near the exit slit. Often the angles of reflection (from the primary mirror), in-cidence and diffraction are small (measured from the appropriate surface normals), which keeps aberrations (especially off-axis astigmatism) to a minimum.

The basic prerequisite for Raman spectroscopy is a monochromatic light source. Because the light scattered during Raman scattering is of relatively low intensity, the light source must also have a very high radiation intensity. Lasers have both characteristics, lasers are available on the market with different fixed frequencies or as tunable devices.

Light incident on a non-transparent medium is predominantly scattered without changing its wavelength. This effect is termed Rayleigh scattering. A small part of this visible light is, however, scattered in a different wavelength. This phenomenon is called Raman scattering or the Raman effect, after the Indian physicist and Nobel laureate C. V. Raman. But what exactly is Raman scattering?

The applications of Raman spectroscopy in medicine are very varied. In this way, for example, the chemical composition of kidney stones can be analysed immediately after their removal. Then the patient can be given tailored recommendations for the prevention of new stones without complex analyses in specialist laboratories. It is also possible to analyse a living biological sample using its Raman spectrum. Neither of these methods has become established as standard yet.

This design is a special case of a Czerny-Turner mount in which a single relatively large concave mirror serves as both the collimator and the camera (Figure 6-2). Its use is limited, since stray light and aber-rations are difficult to control – the latter effect being a consequence of the relatively few degrees of freedom in design (compared with a Czerny-Turner monochromator). This can be seen by recognizing that the Ebert monochromator is a special case of the Czerny-Turner monochromator in which both concave mirror radii are the same, and for which their centers of curvature coincide. However, an advantage that the Ebert mount provides is the avoidance of relative misalignment of the two mirrors.

However, these disadvantages are minimised to a large extent by using modern lasers such that they are increasingly irrelevant for the practical use of Raman spectroscopy.

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