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18 Apr 2008
Wavelength-scanned cavity ring down spectroscopy avoids optical feedback and reflection effects, and can detect 10 parts per trillion of hydrogen fluoride.
Picarro's G1205 hydrogen fluoride analyser is claimed to be capable of measuring concentrations as low as 10 parts per trillion in ambient air, using a proprietary development of cavity ring down spectroscopy (CRDS).
"Our system uses wavelength-scanned CRDS, or WS-CRDS, a unique implementation that uses a wavelength monitor to scan and fit the entire absorption line profile," Aaron Van Pelt of Picarro told optics.org. "By contrast, conventional single-point CRDS techniques are locked to a particular spectral peak."
In conventional CRDS an infrared laser pulse traverses a cavity multiple times, bouncing between mirrors until it eventually dissipates through loss at each reflection and leakage to the outside.
If a light-absorbing substance is introduced into the cavity, the light will undergo fewer detections at the mirrors before it is all gone. CRDS measures the time taken for the light to dissipate by a certain percentage, which can then be converted into a concentration of the absorbing substance.
WS-CRDS develops this approach by integrating the entire lineshape, and also allows the measurement, fitting and subtraction of any nearby spectral pedestals or fine structure on the line of interest. These would show up as spurious concentration changes under the conventional method.
"Our laser is a standard near-infrared distributed-feedback system that is continuously interrogated by the wavelength monitor," said Van Pelt. "It can be driven to any position along the absorption profile of the molecule, and indeed to any position within the laser's tuning range, allowing more than one analyte gas to be interrogated concurrently with one laser. Measurements can be taken off-resonance, so that any device-dependent losses can be subtracted out for each concentration measurement."
The laser's high spectral resolution combined with an effective path length of 20 km from a physical cell length of 20 cm allows the technique to distinguish closely spaced spectral lines, making it insensitive to any interference or contamination crosstalk between the species of interest or any other gas species in the sample.
"WS-CRDS is certainly not limited to HF," noted Van Pelt. "Anything that is infrared-active and has appropriately well-defined spectral lines can potentially be measured. This includes substances having absorption lines in the regions of the spectrum where narrow-linewidth, tuneable lasers are available."
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