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Primer on Raman Spectroscopy

Our specialty at OndaVia is the application of portable Raman spectroscopy to quantitative chemical analysis. Although first discovered nearly one-hundred years ago, Raman has primarily been a specialty laboratory technique. Over the past decade or so, technological improvements have allowed Raman to move from a laboratory-only technique to portable, in-field analysis tools for a variety of fields. Raman spectrometers are used by law-enforcement for illicit drug identification, first-response teams to identify hazardous materials, and recyclers to determine the composition of plastics.

The Raman effect is named after the Indian scientist Sir C. V. Raman, who observed the effect in 1928 and subsequently won the 1930 Nobel Prize in Physics. When a source of monochromatic light—light of a single color from, for example, a laser—illuminates a material, the molecules absorb and scatter the incoming photons. Some photons are absorbed, an effect that is used in infrared spectroscopy, known as Rayleigh scattering. Some of the photons are scattered elastically. Their energy is unchanged from the interaction. And a few photons are scattered inelastically. The molecule absorbs some of the energy, which then is converted into vibrational modes.

These three possibilities can be described using the illustration in Figure 1. Consider a molecule with electrons in the lowest energy state. For absorption spectroscopy, photons with an energy equal to the difference to a higher state are absorbed, moving the electron into the higher state. The electron returns to the ground state, releasing the energy via non-photonic pathways. For infrared spectroscopy, we illuminate the sample and look at which colors are absorbed by the material. 

When performing scattering spectroscopy, we illuminate the sample with a single color. The impinging photons excite electrons into a higher virtual energy state. Most electrons return to the ground state, releasing a photon of the same color. This scattering is elastic as no energy is lost in the process. This process is known as Rayleigh scattering.

However, a few electrons do not return to the ground state. They return to an excited lower state. A photon of a different color is released, with the difference in energy being the distance between the starting and ending energy states. This energy difference provides information about the vibrational states of the molecule—in other words, it provide a measure of bonds within the molecule. The total spectrum acts as a “fingerprint”, providing a method to identify a material based solely on the Raman spectrum. The energy difference is independent of the excitation wavelength. Higher energy (shorter wavelength) excitation will yield more Raman photons, but the energy difference does not change. The Raman spectrum is independent of the color used to illuminate the sample.

This ability to identify chemicals through the Raman spectrum is powerful. Raman is non-destructive and optical, meaning that we need neither to touch nor to damage the sample to perform a measurement. The drawback is that very few photons excite a Raman state, maybe one in ten million. When building a Raman spectrometer, we must have a high-power laser to provide enough excitation photons. We must also filter out the Rayleigh scattering before measuring the weak Raman signal.

A Raman spectrometer contains a light source, typically a laser; a set of lenses to focus and collected the light from the sample; a filter to eliminate the Rayleigh scattering; a dispersive element (e.g., a grating) to separate the light into its colors; and a detector. But the Raman photons are special—they are literally one in ten million. So, through the late 1990’s, the challenge of observing these photons limited Raman spectroscopy to laboratory-scale instruments. These instruments were large, expensive, and complex.

Modern filter designs, diode lasers, and sensitive CCD detector arrays provide excellent performance in portable systems. An OndaVia spectrometer is about the size of a shoe box, weighs less than 8 kg, and operates on a 12-VDC supply. And we are experimenting with spectrometers that fit in a pocket. The technology will continue to improve and Raman will find more and more applications for in-line process control and in-field chemical analysis.