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Chemical Identification with Raman Spectroscopy: Part 1

Last week, I described the general idea behind Raman spectroscopy. Illuminate a molecule with a laser and look at the light that comes back. Most of light is Rayleigh-scattered at the laser wavelength. But a few photons—one in ten million— excite a vibrational or rotational mode, transferring some of their energy to the molecule.

These Raman photons scatter at a different wavelength from the excitation.  The absorbed energy is specific to the excited mode—the atoms directly involved in the bond (e.g., carbon-carbon, carbon-sulfur), the bond type (e.g., single, double, ring), their vibrational modes (e.g., breathing, rocking), their charge state, and other functional groups that might be attached to each atom in the bond. The sum of Raman-scattered photons provides a molecular fingerprint.

A Raman spectrum is typically presented with a vertical axis of intensity and a horizontal axis of wavenumber. Wavenumber, in units of inverse centimeters (cm-1), is a measure of the energy shift from the excitation wavelength. Since the energy shift is excitation wavelength independent, the Raman spectrum presented in this manner is excitation independent. Use an infrared or an ultraviolet laser, and you will get the same Raman spectrum.

Let’s consider toluene and xylene. Toluene and the xylenes are light aromatic solvents. Toluene is a benzene ring with a single methyl group. The xylenes, benzene with two methyl groups, consist of three isomers: ortho-, meta-, and para-xylene. Xylenes also typically contain up to 25% ethylbenzene (benzene with an ethyl group) that remains from the production process.

The figure shows the spectrum for toluene (green), xylene (black), and a mixture containing 1% toluene (blue). Note that the xylenes spectrum is actually four compounds (m-, o-, and p-xylene; and ethylbenzene). A few things to note. The Raman spectrum shows a characteristic peak at approximately 1000-cm-1 that is indicative of the aromatic ring in the m-xylene and ethylbenzene. There is a characteristic energy required to make this ring “breaths” while scattering a Raman photon.

There are many peaks that are not conserved between toluene and xylenes. These peaks can be used to identify each compound separately. Consider the xylene spectrum. At 767-cm-1,just to the left of the red circle, is a peak characteristic of ethylbenzene. Between 1200-cm-1 and 1300-cm-1, there are four peaks. The left peak is p-xylene. The next peak is o-xylene. And the last two peaks are m-xylene. This effect is remarkable. The only difference between these three xylene isomers is the location of the methyl groups on the ring, yet the Raman spectrum shows clear differences.

And this is where Raman becomes powerful. Suppose you want to identify a contaminant in your xylene supply, let’s say 1% toluene. We analyzed a 1% toluene in xylenes mixture. Toluene has a characteristic peak near 800-cm-1 (green) that is not present in the xylene spectrum (black). This toluene peak is clearly visible in the mixed spectrum, demonstrating the ability of Raman to identify contaminants in your chemical supplies. In one spectrum, we can identify all five compounds present.

As a QA/QC tool, a Raman spectrometer is a powerful addition to your lab. We present the spectra for a few solvents (methanol, hexane, xylenes, toluene, and isopropanol) in the document available here (login required).