Combining IR and Raman spectroscopy in 1 instrument

Infrared and Raman spectroscopies can observe different types of molecular vibrations. Using both methods on a compound provides a more complete molecular picture than either alone. But the two techniques are difficult to perform on a single instrument because of the difference in the wavelengths of light typically used as excitation sources—Raman spectroscopies use visible to near-infrared (NIR) light, and IR spectroscopy uses mid-IR (MIR) light.

A team led by Takuro Ideguchi of the University of Tokyo has now built a system that can perform simultaneous IR and Raman spectroscopy (Nat. Commun. 2019, DOI:10.1038/s41467-019-12442-9). By combining the two techniques into one instrument, researchers get more complete information and can save time and sample when analyzing a molecule’s vibrations.

The researchers use an NIR femtosecond Ti:sapphire laser as the light source. The light pulses are used as is for Fourier transform coherent anti-Stokes Raman spectroscopy. For IR spectroscopy, a nonlinear optical crystal converts some of the NIR pulses to MIR ones. After passing through a sample, the two types of light pulses are steered to different detectors.

The team demonstrated simultaneous spectroscopy with samples of toluene, chloroform, and a mixture of benzene and dimethylsulfoxide. This “a great accomplishment,” says Eric O. Potma, a spectroscopist at the University of California, Irvine. But he has reservations. “The authors demonstrate the technique on a series of pure organic liquids that are known to give exceptionally strong signals,” Potma says. “It would be interesting to see how well their approach fares when applied to more realistic and dilute samples.”