Cavity Leak-Out Spectroscopy Aids Ethane Detection
Lynn M. Savage
Ethane is an important chemical that is used in several industrial processes, such as the manufacturing of ethylene, but it also is an important biomarker for human health. Trace amounts of the odorless gas are created as a by-product of oxidative stress in the body, and they escape through exhalation of breath. The ability to confidently track ethane in one’s breath possibly could help identify people who have cardiopulmonary disease, atherosclerosis, cancer or Alzheimer’s disease.
Several methods exist to test ethane content in a mixture of gases, such as breath. The best known of these is gas chromatography, which can identify ethane at concentrations as low as 0.2 parts per billion (ppb). The technique, however, has several disadvantages: It requires precise calibration; cross-sensitivity with other molecules, such as water, must be avoided by careful operation of the equipment; and, most importantly, preconcentration of test samples is required, which slows results.
A schematic illustrates the optical setup of a cavity leak-out spectrometer used to detect ethane in breath. Emissions from a 1064-nm Nd:YAG laser operating at 0.9 W and from an 800-nm external cavity diode laser (ECDL) operating at 580 mW combine in a periodically poled LiNbO3 crystal (PPLN) to generate 3.34-μm radiation that is used for spectroscopic analysis. EOM = electro-optic modulator; PZT = piezoelectric transducer. Reprinted with permission from the American Chemical Society.
Recently, researchers at the University of Düsseldorf and at University Hospital of Rostock, both in Germany, compared the ability of gas chromatography to detect ethane in breath with that of another technique, cavity leak-out spectroscopy. A variant of cavity ringdown spectroscopy, the latter technique uses mid-IR continuous-wave lasers that are frequency-locked to a high-finesse cavity.
Using a custom spectrometer developed by the university’s institute for laser medicine, the investigators tested several samples of ethane mixed with other gases. The samples were collected and bagged in such a way that they could be shared between the two institutions; spectroscopy was performed in Düsseldorf, chromatography in Rostock, about 400 miles away. The spectrometer comprised a continuously tunable laser that generated 3.34-μm radiation via difference frequency generation. The effective absorption path length of the device was 3.4 km, which provided high sensitivity.
The group in Rostock used an Agilent/Hewlett-Packard gas chromatography-flame ionization detection system for its part of the comparison.
The researchers found that, after repeated tests during the course of a single day or over the span of up to a year, the results from each type of measurement showed satisfying agreement. However, the spectroscopy setup could detect ethane in less than 1 min, whereas chromatography required 30 to 60 min, making the former setup more suitable for continuous or online monitoring of patients. In addition, the spectrometer does not require recalibration and can distinguish ethane from interfering molecules, such as methane and chloromethane, that often are present in breath.
According to the researchers, however, chromatography remains preferable if other sets of substances are to be monitored or if unknown ones must be identified. Furthermore, neither technique may be useful when large amounts of interfering substances may be present.
Analytical Chemistry, ASAP Edition, March 15, 2008, doi: 10.1021/ac702282q.
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