Portable sensor detects trace elements of greenhouse gasses

The device, created by Rice engineer Frank Tittel and his group, uses a thumbnail-sized quantum cascade laser (QCL) as well as tuning forks to detect very small amounts of nitrous oxide and methane.

The QCL emits light from the mid- to far-infrared portion of the spectrum, allowing for better detection of gases than more common lasers that operate in the near-infrared.

The technique, quartz-enhanced photoacoustic absorption spectroscopy (QEPAS), was invented at Rice by Tittel, Prof Robert Curl and their collaborators in 2002. The Rice team’s new device is detailed in the Royal Society of Chemistry journal Analyst.

In tests, the device detected trace amounts of methane, 13 parts per billion by volume (ppbv), and nitrous oxide, six ppbv.

‘Methane and nitrous oxide are both significant greenhouse gases emitted from human activities,’ Tittel said in a statement. ‘The warming impact of methane and nitrous oxide is more than 20 and 300 times, respectively, greater compared to the most prevalent greenhouse gas, carbon dioxide over a 100-year period. For these reasons, methane and nitrous oxide detection is crucial to environmental considerations.’

The small QCL has only become available in recent years, Tittel said, and is far better able to detect trace amounts of gas than lasers used in the past. Previous versions of the QCL are just as effective, but far too bulky for mobile use.

What makes the technique possible is the small quartz tuning fork, which vibrates at a specific frequency when stimulated.

‘The ones we use are made for digital watches, and are very cheap,’ said Rice postdoctoral researcher and co-lead author Wei Ren. ‘The fundamental theory behind this is the photoacoustic effect.’

The laser beam is focused between the two prongs of the quartz tuning fork. When light at a specific wavelength is absorbed by the gas of interest, localised heating of the molecules leads to a temperature and pressure increase in the gas.

‘If the incident light intensity is modulated, then the temperature and pressure will be as well,’ Ren said. ‘This generates an acoustic wave with the same frequency as the light modulation, and that excites the quartz tuning fork.

‘The tuning fork is a piezoelectric element, so when the wave causes it to vibrate, it produces a voltage we can detect. That signal is proportional to the gas concentration.’

The unit can detect the presence of methane or nitrous oxide in as little as a second, he said.

To field test the device, the Rice team installed it on a mobile laboratory, which analysed pollution on the ground and from the air in September 2013. The lab analysed emissions from a Houston landfill, and the QEPAS sensor’s findings compared favourably to the lab’s much larger instrument, Tittel said.

‘This was a milestone for trace-gas sensing,’ Ren said. ‘Now we’re trying to minimise the size of the whole system.’

The research was supported by the US National Science Foundation and Robert Welch Foundation.