Molecular Fingerprinter for Trace-Gas Detection Could Improve Diagnostic Devices

Scientists at JILA (Boulder, CO), a joint institution of the National Institute of Standards and Technology (NIST; Gaithersburg, MD) and the University of Colorado (Boulder), have demonstrated that an improved laser-based "molecular fingerprinting" technique can pick out traces of key hydrogen-containing and other molecules from a billion other particles in a gas in 30 seconds or less. This performance could render the technique suitable for breathalyzers used to diagnose diseases.

Described in Optics Express, the research extends the range of an existing NIST/JILA invention to cover the mid-infrared region of the electromagnetic spectrum, a critical range because it includes the frequencies associated with strong molecular vibrations, including various hydrogen bonds. The technology can thus identify a much wider variety of molecules than the previous-generation instrument, including virtually any containing hydrogen, and can measure lower concentration levels.

The heart of the JILA system is an optical frequency comb, a tool generated by ultrafast lasers that precisely identifies a wide range of different colors of light. Researchers identify specific molecules based on which colors of light, or comb "teeth," are absorbed by a gas, and in what amounts. The comb light usually passes through a gas mixture many times, significantly improving detection sensitivity. Concentrations are measured with the help of molecular "signatures" assembled from databases. The technique works quickly and reliably even when molecules have overlapping, continuous, or otherwise confusing absorption signatures. Because of its rapid data collection capability, the technology is suitable for surpassing or replacing conventional Fourier transform infrared spectrometers for many applications.

In the demonstration, scientists measured a dozen important molecules at parts-per-billion precision, including the greenhouse gases methane, carbon dioxide, and nitrous oxide and the pollutants isoprene and formaldehyde. In addition, the system detected molecules useful in human breath analysis: ethane (a sign of asthma) and methanol (a sign of kidney failure).

Collaborators from IMRA America Inc. (Ann Arbor, MI) developed the fiber laser used to make the frequency comb. The comb itself is based on a nonlinear optical process that shifts the light from the near-infrared to the mid-infrared. The JILA researchers now plan to extend the system further into longer wavelengths to cover a second important molecular fingerprinting region, to identify a more diverse set of complex molecules containing carbon, and to modify the instrument to make it portable. Planning is also under way for clinical trials of the breathalyzer application.