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Wavelength-Scanned Cavity Ring Down Spectroscopy can combine the ppt sensitivity and species discrimination of separation science with the speed and simplicity of optical technology.

Introduction
Applications continue to grow and diversify for the detection and analysis of trace amounts of small gas-phase species. Examples include moisture and carbon dioxide in feedstock gases, ammonia and moisture in gases used in the semiconductor industry, and hydrogen sulfide in petrochemical gases. While there are several established technologies for trace gas analysis, each has significant inherent limitations in terms of speed or sensitivity; also, there are often practical issues, such as the need for a skilled operator, and frequent maintenance or re-calibration. However, a new technology now delivers species selectivity and parts-per-trillion sensitivity, combined with real-time speed, simplicity, no sample preparation requirements and no need for frequent re-calibration. This article explains this technique and shows how it delivers these key advantages in selected applications.

Limitations of Existing Technologies
Nearly every small molecule (e.g., H₂O, H₂S, NH₃) has a unique near-infrared absorption spectrum of sharp wellresolved lines. In principle, the concentration of any of these species can be measured just by recording the intensity of one or more of these absorption lines. However, traditional instruments based on light absorption have limited sensitivity. That’s because the amount of absorption due to a trace gas is often far less than the natural instrument noise fluctuations. This is true even in the new generation of gas sensors based on tunable laser diode technology, limiting their use to measurements in the ppmv (parts-per-million by volume) range or higher.

Moreover, even at the ppmv level, traditional optical instruments are subject to crosstalk between the target gas and other species, because of lack of spectral resolution: resolution: more than one gas type can be contributing to the absorption signal. This is a particular drawback for NDIR (non-dispersive infrared) gas sensors, where the performance can only be guaranteed in the absence of certain other trace gas species.

For this reason, trace gas applications have often turned to various non-optical solutions, such as mass spectrometry, chromatography, and even chemical sensors. But each of these has its own drawbacks (see Figure 1). Furthermore, these instruments are often large and operator intensive, limiting their use to laboratory analysis of captured samples, making sample handling another issue.

WS-CRDS Background
Fortunately, a clever optical technique called Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS) is fast gaining ground in many trace gas sensor applications, providing the ideal combination of sensitivity (down to parts-per-trillion), high speed (seconds), species selectivity (no crosstalk between different gases) and extended linear response range (above parts-per-million). Moreover, current WS-CRDS instruments are compact, rugged and can operate un-manned for months in even harsh industrial environments with no requirement of maintenance or re-calibration.

In a WS-CRDS instrument (see Figure 2), light from a wave length tunable laser diode enters the sampling cavity which contains three mirrors having exceptionally high reflectivity (>99.999%). The intensity of the light trapped within this cavity is sensed by a photo detector which measures the extremely small amount of light leaking through one of the mirrors. When the signal from this detector reaches a steady state condition, the laser is abruptly switched off . Because the mirrors do not have 100% reflectivity, the light intensity inside the cavity slowly leaks out and this ring-down (decay) is measured in real-time.

Now if the cavity contains a gas species that absorbs even weakly, the instrument introduces a second mechanism that drains the intracavity intensity. This results in a shortened decay time, from which the instrument calculates the sample absorbance and hence concentration. Even with a cavity of only 25 cm in length, the average distance or path length that any photon effectively travels within the cavity can be over 20 kilometers, resulting in sensitivity as high as parts-per-trillion. And because the measured decay rate is independent of the initial cavity intensity, WS-CRDS is immune to the effects of source noise, i.e., laser power fluctuations.

The key to species selectivity in WSCRDS is wavelength scanning with a narrow line laser. Specifically, the laser is systematically tuned over the target absorption line during each 1 Hz measurement cycle. This enables differentiation of various species even in the rare case that there are gas components with close overlapping lines. In addition, this wavelength scanning provides a very accurate measure of the absorption line intensity, providing significantly higher sensitivity than could be obtained with single point CRDS measurements.

Species selectivity also means that by using several compact lasers within the WS-CRDS instrument, multiple trace gas species can be simultaneously and independently monitored in a self-contained instrument about the size of a briefcase. Just as important, because these rugged instruments contain no moving parts, they can be remotely operated 24/7 for months on end without requiring re-calibration. Table 1 shows a partial list of trace gas species that can be measured with a WS-CRDS instrument together with typical values for sensitivity and lowest detection limit.

To better understand the advantages of WS-CRDS it is useful to briefly examine several real-world examples.

Figure 2. The basic principles of WSCRDS: light from a tunable laser is
trapped in a three-mirror cavity: when the laser is turned off the ring down
time for the light to decay is changed by the presence of any species that
absorbs at the laser wavelength.

Applications
HF in Semiconductor Fab and Aluminum Smelting Plants:
HF (hydrogen fluoride) is an example that nicely demonstrates two of the key advantages of WS-CRDS, namely extreme sensitivity and rugged, remote operating capability. HF is widely used as an etching agent in semiconductor fabrication, it is commonly used in making certain glass products, and it is generated as a by-product in the aluminum smelting industry. Because HF is extremely corrosive and toxic, all these applications need a real-time sensor with quantitative performance and the highest possible sensitivity. Moreover, the aluminum smelting industry needs an analyzer that can be located in a non pristine industrial environment (the pot room roof space) and operated continuously and remotely for long periods without recalibration. WSCRDS instruments are now meeting these needs.

Because it is a very polar molecule, HF gives rise to particularly intense infrared absorption. As a result the latest WS-CRDS instruments have very high HF sensitivity, far higher than any other type of HF sensor. Specifically, WS-CRDS can deliver a lower detectable limit of 10 parts per- trillion and a similar value for sensitivity (precision). Moreover, the concentration can be measured to four significant figures with a linear response up to parts-per-million.

What about corrosion? In WSCRDS, the only optical components that come into contact with the gas are the critical high refl ectivity mirrors. But these use dielectric coatings made from fluoride materials, so they are naturally immune to reaction with HF, even in the presence of moisture. This is a key factor in enabling WS-CRDS to accurately monitor HF in ambient air for months on end without any drift in baseline or sensitivity.

Monitoring H₂S with no crosstalk
H₂S (hydrogen sulfide) is an example that highlights the species selectivity of WSCRDS. This is a highly poisonous gas that must be detected at trace levels in several different industries and applications, from semiconductor to petrochemicals and in greener automotive engine development. For instance, it must be removed from natural gas in order to meet safety regulations. But the removal process is costly, making it beneficial to keep the levels just below regulatory values, thus optimizing operational costs. Yet traditional H₂S detection is often surprisingly crude. The gas industry often uses lead acetate tape that changes color in response to H₂S. Conversely, high-tech solutions, such as ultraviolet spectrophotometers, are often susceptible to crosstalk from other chemicals. Fortunately, WS-CRDS instruments now supports these applications with quantitative analysis at a 50 ppbv lowest detection limit and no crosstalk with other gas species.

To demonstrate the selectivity of WSCRDS for H₂S, we conducted a study in which zero-air was introduced to the analyzer and the H2O concentration was varied from 2% to 5% while the CO₂ was varied from 0 to 7%. As shown in Figure 3 there was no detectable change in the H₂S signal during this test. Similar tests were performed with other gases including propane, SO₂, CO and NO. Again, any interference from these gases was shown to be significantly below the 50 ppbv lowest detection limit of the analyzer.

Environmental Monitor for CO₂/CH₄/H₂O
Given its geopolitical and economic ramifications, environmental monitoring for greenhouse gases is arguably the most important applications space for WS-CRDS. It demonstrates two other advantages of WS-CRDS: the ability to simultaneously monitor multiple gases and the ability to distinguish isotopes. Multi-species environmental sensors based on WS-CRDS are now replacing multiple instruments and even multiple technologies formerly required to measure several species simultaneously. The most common combination preferred in this application is configured for the two most important greenhouse gases (CO₂ and CH₄), which can both be measured down to parts per-billion precision without the need for water vapor removal. (In fact some users opt for three species systems that also quantify water vapor.)

In a related application, WS-CRDS is now being used to determine the source of CO₂. Every different type of carbon dioxide source (e.g., burning fossil fuels, plant respiration) has a well-known and unique value of the ratio of the carbon 13 and 12 isotopes. Portable, remotely operated WS-CRDS instruments can now make measurements in the field, replacing bulky, high-cost, lab-bound instruments such as the IR-MS (isotope-ratio mass spectrometer). Here, the heights of two closely-positioned absorption lines belonging to ¹³CO₂, and12 isotopes. Portable, remotely operated WS-CRDS instruments can now make measurements in the field, replacing bulky, high-cost, lab-bound instruments such as the IR-MS (isotope-ratio mass spectrometer). Here, the heights of two closely-positioned absorption lines belonging to ¹³CO₂, and ¹²CO₂ are measured (see Figure 4). In most samples, these lines have very different intensities because ¹³C has a much greater natural abundance than ¹³C. But the high sensitivity and dynamic range of WS-CRDS allows both to be measured to several significant figures.

Conclusion
WS-CRDS is a powerful new tool for trace gas analysis that delivers a unique combination of speed, sensitivity, selectivity, portability and no need for frequent re-calibration. While no one is claiming WS-CRDS is a panacea for all gas monitoring situations, its advantages are leading to increasing acceptance in an ever growing list of applications.

AARON VAN PELT I S A MARKETING ENGINEER AT PICARRO, 480 OAKMEAD PARKWAY, SUNNYVALE, CA 94085. HE FOCUSES ON MARKET AND APPLICATIONS DEVELOPMENT LEVERAGING PICARRO’S WS-CRDS-BASED ANALYZERS IN KEY MARKETS INCLUDING ENVIRONMENTAL, PROCESS, AND EMISSIONS MONITORING. AARON WAS FORMERLY A SENIOR RESEARCH SCIENTIST AT PHYSICAL SCIENCES, INC. AND AN APPLICATIONS ENGINEER AND PRODUCT MANAGER FOR NEW FOCUS, INC. HE HAS A BS IN PHYSICS FROM THE UNIVERSITY OF WYOMING AND A MS IN LASER SPECTROSCOPY FROM WASHINGTON STATE UNIVERSITY. HE CAN BE CONTACTED AT 408-962-3200 OR AVANPELT@PICARRO.COM. IAIN GREEN IS DIRECTOR OF MARKETING AT PICARRO.

IAIN GREEN HAS NEARLY 20 YEARS OF COMMERCIAL EXPERIENCE IN THE ANALYTICAL EQUIPMENT INDUSTRY. PRIOR TO JOINING PICARRO IN 2008, IAIN HELD KEY MARKETING POSITIONS FOR VARIAN INC.’S GLOBAL MAGNETIC RESONANCE BUSINESS AND THERMO’S LC-MS/MS PRODUCT GROUP. IAIN RECEIVED HIS B.SC. IN CHEMISTRY FROM THE UNIVERSITY OF ST. ANDREWS, SCOTLAND IN 1984, AND HIS PH.D. IN CHEMISTRY FROM THE UNIVERSITY OF LONDON, ENGLAND IN 1988. HE CAN BE CONTACTED AT 408-962-3942 OR IGREEN@PICARRO.COM

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