Near-IR Diodes Help Make Gas-Trace Measurement Faster

Jörg Schwartz

In our everyday lives, we all are exposed to "BTX" -- air pollutants that are found at gas stations, at workplaces or on roads. BTX stands for benzene, toluene and xylene, three organic substances that are suspected carcinogens. Unfortunately, concentrations as low as tens of micrograms per liter of air, which are difficult to measure in the field, can be harmful. Researchers at the Hydrochemistry Institute of the Technical University of Munich have developed a new method for detecting these concentrations in the field.

The difference in this technique, which is described in the September 1999 issue of Applied Spectroscopy, lies in the use of laser diodes. The principle of photoacoustic spectroscopy is not new, but the technique has been confined to lab use -- unable to perform fast in-line monitoring -- because of the bulky CO₂ lasers typically used.

Reinhard Niessner, head of the research institute, said the team replaced the cumbersome CO>₂ lasers with fiber optically coupled near-IR laser diodes to achieve not only the desired compactness, but also easier modulation.

Wanted: More suitable lasers

The fiber optic approach allows the use of two lasers with different wavelengths to measure more than one analyte at a time. The researchers have shown that it is possible to distinguish two similar substances and to compensate for their spectral interference using a dual-wavelength measurement.

In addition, the procedure enables the researchers to eliminate the background effects that are caused by water vapor, which is present under most conditions.

Measurements of numerous substances have been reported, applying laser diodes emitting at 1.67 and 1.31 µm. But the availability of proper diodes has put constraints on the detection limit so far.

According to Niessner, both higher laser power and a mid-IR wavelength would have a dramatic impact on the quantum efficiency. With cheap and rugged design features -- e.g., avoiding use of monochromators -- the photoacoustic spectroscopy technique is a good candidate for commercialization.

But first a hundredfold decrease in the detection limit is necessary. Niessner said this is within reach with better laser diodes.