day 142: raman, not ramen

From HORIBA Scientific:

RAMAN DATA AND ANALYSIS

Raman Spectroscopy for Analysis and Monitoring
The Raman scattering technique is a vibrational molecular spectroscopy which derives from an inelastic light scattering process. With Raman spectroscopy, a laser photon is scattered by a sample molecule and loses ( or gains) energy during the process. The amount of energy lost is seen as a change in energy (wavelength) of the irradiating photon. This energy loss is characteristic for a particular bond in the molecule. Raman can best be thought of as producing a precise spectral fingerprint, unique to a molecule or indeed and individual molecular structure. In this respect it is similar to the more commonly found FT-IR spectroscopy. However, unlike FT-IR, there are a distinct number of advantages when using Raman.
•    Raman can be used to analyse aqueous solutions since it does not suffer from the large water absorption effects found with FT techniques.
•    The intensity of spectral features in solution is directly proportional to the concentration of the particular species
•    Raman spectra are generally robust to temperature changes
•    Raman requires little or no sample preparation. It does not need the use of Nujol, or KBr
matrices and is largely unaffected y sample cell materials such as glass.
•    The use of a Raman microscope such as the LabRAM provides very high level of spatial
resolution and depth discrimination, not found with the FT methods of analysis
These advantages and its highly specific nature, mean that Raman has become a very powerful tool for analysis and chemical monitoring. Depending upon instrumentation, it is a technique which can be used for the analysis of solids, liquids and solutions and can even provide information on physical characteristics such as crystalline phase and orientation, polymorphic forms, and intrinsic stress.

ENHANCEMENTS

Raman scattering is a relatively weak process. The number of photons Raman scattered is quite small.

However, there are several process which can be used to enhance the sensitivity of a Raman measurement.

I. Techniques

Resonance Raman – can be used when the laser wavelength utilized is close to the absorption wavelength of the molecule. By irradiating the sample with a wavelength close to this wavelength an order of magnitude greater detectivity may be achieved. Not all samples will show resonance enhancement with common Raman lasers, but generally species such as porphyrins and those with a heavy central atom can show such an enhancement.

SERS Raman – SERS or SERRS Raman (surface enhanced (resonance) Raman spectroscopy) is arguably a less well understood enhancement technique. It requires a further moiety to be present (eg. A SERS prepared surface or colloid). The presence of such an agent can provide quite dramatic enhancements and has been used successfully in the study of biological samples such as DNA, peptides and proteins.

Active Substrates – here a specialized coating can be used to enhance the sampling sensitivity of a liquid or solution sample. The sample does not ‘wet out’ the surface remaining in a concentrated micro-droplet. Enhancements for such applications as assay screening have been shown to be a likely candidate for such technology.

II. Raman Instrumentation

Best suited Laser wavelength – The correct selection of the laser wavelength can be an important consideration for Raman spectroscopy. With modern equipment, often several laser wavelengths may be employed so as to achieve the best detection of the Raman signal:

For instance, many samples, especially those of an ‘organic’ or ‘biological’ nature will be quite fluorescent species. Exciting these samples with a laser in the green (532 nm) may promote this fluorescence, and may swamp any underlying Raman spectrum to such an extent that it is no longer detectable.

In this instance, the use of a laser in the red (633 nm) or NIR (785 nm) may provide a solution. With the lower photon energy, a red or NIR laser may not promote the electronic transition (and hence the fluorescence) and so the Raman scatter may be far easier to detect.

Conversely, as one increases the wavelength, from green to red to NIR, the scattering efficiency will decrease, so longer integration times or higher power lasers may be required.

Thus, it is often most practical to have a number of laser wavelengths available to match the various sample properties one may encounter, be it resonance enhancements, penetration depth of fluorescence.

Green, Red and NIR 785 nm laser excitation of a fluorescent sample.

The strong background seen with the green and red lasers swamps the Raman signal, whereas the 785 nm excitation is outside of the fluorescence range, enabling the Raman to be detected.

Raman Microscope

Raman microscope – The Raman microscope is by far one of the best instrumentation enhancements one can make. The new generation of Raman microscope can offer a powerful non-destructive and non-contact method of sample analysis.

First introduced by HORIBA Scientific in the mid 1970s, the micro Raman system can open up a whole new dimension of spectroscopic analysis. They are now far easier to operate, and laser adjustment and alignment are virtually eradicated. It becomes a simple operation to use the micro Raman instrument with even computer control of laser switching and grating selection now possible.

One of the greatest benefits is the use of a true CONFOCAL Raman microscope design. This enables a very small sample area or volume to be analysed – down to the micron scale. Combine this micro Raman analysis with automated focusing, XYZ movement, and it becomes possible to produce ‘chemical’ images of a sample.

Localisation, distribution, phase and other such properties can be imaged as never before. It has become a most powerful technique.

Raman Imaging

Raman imaging is a powerful technique for generating detailed chemical images based on a sample’s Raman spectrum.  A complete spectrum is acquired at each and every pixel of the image, and then interrogated to generate false colour images based on material composition and structure:

  • Raman peak intensity yields images of material concentration and distribution
  • Raman peak position yields images of molecular structure and phase, and material stress/strain
  • Raman peak width yields images of crystallinity and phase

Thus with a single data set a wide variety of Raman images can be created which take the researcher well beyond what the eye can see.

Standard point-by-point mapping affords the ultimate sensitivity for materials with extremely low Raman scattering properties, and additionally allows high resolution, large spectral range capability.  Typical acquisition times for such maps can be in the order of 1s-10s per point (or longer), and thus total measurement times can be significant.  The LineScan and SWIFT™ Ultra-fast Raman Imaging modules offer drastically reduced measurement times with acquisition times down to <5ms/point, allowing large area survey scans and detailed Raman images to be completed in seconds or minutes!

The latest developments in Raman imaging offer adaptive laser spot sizes for true macro-scale Raman imaging using the unique DuoScan™ optics.  In addition, combination of Raman imaging with piezo stages (including the DuoScan™ module) and AFM systems is moving Raman imaging far into the sub-micron regime, making Nanoscale Raman imaging a reality at last.

Raman image of a meteorite section, showing distribution of six major minerals
Raman image of 3µm polystyrene beads on a Silicon substrate
Raman image of sodium sulfite crystals on glass

Visit our Raman Image Gallery for more astonishing Raman images

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