Technology: How Does the IRmadillo Differ to Raman Spectrometers?
The IRmadillo is based on infrared absorption spectroscopy, while Raman spectroscopy uses an effect called “Raman scattering” – which is a type of emission spectroscopy. A brief explanation of the two is:
Infrared absorption spectroscopy works by shining a beam of infrared light made up of many different wavelengths through a sample, and some of these wavelengths are absorbed by the sample. The difference between the light that was emitted by the infrared source and the light detected by the detector is ratioed to create a spectrum.
Raman scattering is caused when molecules absorb laser light (of just one specific wavelength) to become excited in a “virtual state”. These molecules then relax down to their “ground state” and re-emit the light (the “scattering” effect), but a very small number of them absorb a small portion of light energy to relax into a different “vibrational state” to the one they started in – emitting light of a different wavelength to the original laser. This difference in energy can be detected and used to create a spectrum.
The key differences between the IRmadillo and Raman spectrometers are:
Light source: the IRmadillo uses a low-powered, broad-band infrared emitter, that generates a range of wavelengths at low energy. A Raman spectrometer will use a laser, sometimes with quite high power. The exact wavelength of laser will depend on the application, but in many cases will be either 785 or 1064 nm – known as “near infrared” wavelength. The user may need to take additional safety precautions with the Raman lasers – as they can be very dangerous if used incorrectly.
Scattering of light: the IRmadillo has an “ATR element” on the end of the probe that does the measuring of the sample. The way light is absorbed by the sample with an ATR means that bubbles or solids in suspension do not cause any problems with measurement. Because Raman spectroscopy is an emission process, the light is shone in all directions at once meaning scattering can be a problem from both particles and bubbles present in the process. One benefit of the Raman laser is that Raman instruments do not need to directly contact the sample, meaning they can be used through a window or sight glass (with adequate safety provisions for the laser).
Selection rule: the fundamental science behind the physics of infrared absorption spectroscopy compared to Raman spectroscopy means that different functional groups within molecules are stronger in either the infrared or Raman spectrum.
Infrared spectroscopy needs a change in the dipole moment of a molecule – meaning polar features such as C=O bonds, O-H bonds and C=N bonds have very strong infrared absorption.
Raman spectroscopy needs a change in the polarizability of a molecule – meaning strongly polar features can be invisible in the spectrum, for example water is almost impossible to see with Raman.
What is the problem with fluorescence?
One thing to note with Raman is that the need to use a laser to excite the molecules can cause strong fluorescence, making it very difficult to obtain a clean spectrum. The degree of fluorescence does change on the exact application, the wavelength of the laser and the type of probe used – so can sometimes be reduced to manageable levels.
What would we use a Raman instrument for?
Raman instruments have been available for many years, and have demonstrated successes in a multitude of different industries and applications. Some examples where Raman is a strong choice are:
Where it is not possible to insert a probe directly: by shining the laser through a sight glass or a window it can be possible to obtain a spectrum without wetting the probe or making contact with the sample. This could be very powerful in high temperature, high pressure or molten solids.
Where the molecules of interest have very low frequency features: Raman instruments can have extremely wide spectral range – even as low as 20 cm-1, which is much wider than almost all infrared spectrometers. Some molecules – such as those containing heavy metals – may have very low frequency spectral features and Raman is ideal for those.
For carbon fibres and carbon nanotubes: the selection rule for polarizability over dipole moment makes Raman an ideal choice for analysing giant covalent aromatic structures – such as those in carbon fibre and carbon nanotubes.
For topographic analysis (using a Raman microscope): many manufacturers offer Raman microscopes, where the laser is fed into the same objective as that used to take photographs. This makes using a Raman instrument ideal for mapping molecules on a substrate – such as certain molecules within a cell, or mapping processes on electrodes during electrochemistry.
What would we recommend choosing the IRmadillo over Raman for?
Although Raman instruments can be fantastic for the right application, there are many applications where they have been tried and one or more of their limitations makes them unsuitable. Here are some applications where we would suggest the IRmadillo is more suitable:
Biotechnology and fermentation: industrial biotechnology, fermentation for food, drink and fuel and other similar industries often have very turbid media with both biomass and bubbles. The combination of fluorescence and scattering means that analysis using Raman often needs very intense and complicated mathematical modelling. This makes the calibrations prone to instability at best, or even failure. The IRmadillo’s ATR tip does not have these problems, resulting in much easier calibration.
Any process with a polar molecule: the fundamental selection rule for infrared absorption makes it the ideal choice for any molecule that is polar. These include C=O, C-O, O-H, N-H and C=N groups. Moreover, if one of the chemicals of interest is water then it will be impossible to observe using Raman – you would need the IRmadillo.
Aqueous applications: a common myth is that the fact that Raman cannot observe water is beneficial for aqueous applications. Modern analysis techniques are based on multivariate analysis that actually uses the water spectrum as part of the calibration, meaning that much better detection limits can sometimes be achieved using an infrared instrument in place of a Raman.