Field-Portable Analyzers Based on Cavity-Enhanced Laser Absorption Spectrometry

Accurate measurements of greenhouse gases and pollutants are critical to emerging environmental regulations and industrial compliance monitoring. Until recently, many of these measurements involved taking discrete flask samples back to a laboratory for analysis by gas chromatography or mass spectrometry. This methodology typically results in a small number of measurements and does not provide temporal or spatial profiling. Alternatively, for some gases (e.g., carbon dioxide, nitrogen dioxide, and ozone), fixed sensors have been placed in environmental monitoring stations to provide continuous measurements at a specific location. Although this strategy provides high temporal resolution, it does not address the need for spatial profiling.

It is increasingly critical to obtain the spatial distribution of greenhouse gases and pollutants to help categorize source terms, quantify pollution migration, and determine the dependence of this migration on environmental factors. Such spatial mapping requires field-portable instrumentation that is capable of accurately measuring greenhouse gases and pollutants with very high precision. In order to facilitate deployments, these instruments must be compact and lightweight. They must require relatively low power to permit battery operation for several hours. The instruments have to be rugged enough to withstand field conditions and have minimal dependence on ambient temperature and pressure (e.g., for airborne deployments). In order to provide high spatial resolution aboard a moving platform, the sensor should have fast (>1 Hz) response and be readily integrated into a variety of vehicles, planes, ships, and human-portable carriers. The instrument would also benefit from a fast warm-up time and the ability to perform rapid field servicing if necessary.

Conventional laser absorption spectrometry (LAS)

Recently, laser absorption spectrometry has been used to develop field-portable instruments. In LAS (Figure 1), light from a tunable laser is passed through a gas sample and focused onto a detector. Either telecommunications-grade, near-infrared (1200–1900 nm) diode lasers and InGaAs detectors or quantum cascade lasers and mid-infrared detectors can be used. The laser wavelength is tuned over a small range (typically 0.5 nm) by varying its injection current. Specific molecules absorb at particular laser frequencies, resulting in a decrease in transmitted intensity at those frequencies. The measured transmission trace can then be converted to an absorption spectrum, and the integrated area under the absorption peak can be directly related to the concentration of the targeted species via the Beer–Lambert–Bouguer Law (Beer’s Law).

Figure 1 – Conventional laser absorption spectrometry (LAS) setup. A tunable laser is directed through a gas sample and focused onto a photodiode. The laser is frequency-tuned through a molecular absorption feature, and the change in transmission is directly related to the concentration of the targeted gas species via the Beer–Lambert–Bouguer Law.

This technology has several advantages over other conventional analytical methods. Foremost, due to the high spectral resolution of the laser (typically 1–3 MHz) and the discrete width of the absorption feature (typically 150–3000 MHz, depending on the measurement pressure), the technique is highly selective and exhibits minimal cross-interferences due to other background gases. By measuring all of the relevant parameters (e.g., integrated absorption area, gas temperature, gas pressure, and optical pathlength), Beer’s Law can be directly used to calculate the gas concentration, with little to no calibration or consumables (a critical feature for field deployments). Moreover, since the laser is rapidly and repeatedly tuned over the absorption feature (1 kHz, typical), LAS analyzers can provide fast data (1 Hz) for high spatial resolution aboard a moving platform. Finally, LAS analyzers normally have little to no moving parts, making them very robust and readily field-portable. These analyzers have been gaining acceptance in a wide variety of applications, including industrial process control. However, for trace gas monitoring (i.e., greenhouse gases and pollutants), conventional LAS cannot provide sufficient sensitivity, and more advanced techniques are required.

Cavity-enhanced laser absorption spectrometry (CELAS)

A simple method of improving the sensitivity of LAS involves increasing the optical pathlength over which the laser interacts with the gas sample. Traditionally, this has been implemented by directing the laser across a long open path; however, this technique cannot quantify specific points and is not suited for field deployment. Additionally, the technique’s relatively high measurement pressure (e.g., 1 atm) can make it difficult to specify trace gases accurately with high sensitivity.

An alternate method involves using a high-finesse optical cavity to provide an extraordinarily long (5–10 km typical) effective optical pathlength (Figure 2). In this scheme, the windows of the gas cell are replaced by highly reflective mirrors (R > 99.9%). The laser light passes through the mirror and reflects back and forth in the cavity over 1000 times, providing a large effective optical pathlength, and enhancing the molecular absorption. In off-axis integrated cavity output spectroscopy (off-axis ICOS),1 a particular variant of cavity-enhanced LAS, the laser is aligned off-axis with respect to the cavity to prevent optical interference within the cavity and optical feedback to the laser from the mirrors. A typical laser scan takes 100 msec (100 Hz) and involves turning on the laser, scanning over the absorption feature, and turning the laser off. The last step simultaneously provides a measurement of both detector offset and effective optical pathlength via the established cavity ringdown technique.2 Although for most applications, near-infrared diode lasers are used for lowest cost, the off-axis ICOS technique may be applied using lasers at essentially any wavelength from the near-UV to the mid-infrared.

Figure 2 – Top: Schematic of the off-axis ICOS technique. The highly reflective mirrors provide an effective optical pathlength of many kilometers, sufficient for greenhouse gas, isotope, and pollutant quantification. Bottom, left to right: Off-axis ICOS instruments in various field-portable configurations (airborne, deep-sea, and ultraportable) to address a wide array of field deployments.

This technology retains all of the benefits of traditional LAS (i.e., selectivity, speed, minimal calibration/consumables, robustness, and cost-effectiveness), but improves the analyzer sensitivity by a factor of 1000–10,000, allowing for the quantification of trace greenhouse gases, isotopes, pollutants, and other weak optical absorbers. Moreover, the technology is highly robust. The exact trajectory of the laser into the cavity is not critical, helping make the system immune to small changes in optical alignment due to mechanical and thermal perturbations (e.g., vibrations, shock, relative motion, etc.).

The mirrors utilize dielectric coatings that do not degrade with time or chemical contact. The sample gas is filtered (0.5–2 μm, typical) to prevent fouling of the mirrors during routine operation. Since the optical alignment is not precise, the mirrors can be removed, cleaned, and replaced in the field. This serviceability aspect further differentiates the off-axis ICOS technique from other long-path LAS technologies, including cavity ringdown spectroscopy and multipass (Herriott, White) cell measurements. Finally, as noted below, off-axis ICOS sensors have been packaged in a variety of styles for industrial, mobile, and laboratory deployment. The technology has gained wide acceptance for environmental (air and water quality) applications, including isotope measurements.

Field-portable applications

Due to the advantages discussed above, off-axis ICOS instruments have been used in a variety of field deployments, including mobile measurements of greenhouse gases, field isotope monitoring, and spatial and airborne profiling of pollutants.

Mobile greenhouse gas monitoring

There is growing awareness of the need to accurately monitor greenhouse gases (e.g., carbon dioxide, methane, and nitrous oxide) for impending environmental regulations. These measurements require new instrumentation that can provide highly precise, accurate greenhouse gas measurements in a field-portable package. Mobile monitoring of greenhouse gases can help better quantify key sources, including vehicles, landfills, dairies, and pipelink leaks. For example, methane measurements made in Palo Alto, CA using an LGR UltraPortable Greenhouse Gas Analyzer (Mountain View, CA) mounted inside a moving vehicle (Figure 3) clearly show elevated methane levels near a capped, repurposed landfill. Similarly, mobile methane measurements near Fremont, CA (Figure 3) show elevated methane levels that correlate with the location of a known “high-risk” gas pipeline, suggesting fugitive emissions. This work has been extended to include quantification of carbon dioxide and nitrous oxide emissions from vehicles passing through the Caldecott Tunnel.

Figure 3 – Left: Mobile greenhouse gas measurements taken in Palo Alto, CA show elevated methane levels near a repurposed landfill. Right: Similar measurements taken near Fremont, CA suggest fugitive gas pipeline emissions.

A similar analyzer was mounted in a small aircraft and used for airborne measurements of gas pipeline leaks. The analyzer provided 10 Hz data and, due to its high measurement precision, could quantify small methane leaks while flying 700 ft over a gas pipeline. Such measurements can be used to reduce methane emissions and enhance consumer safety.

Isotope monitoring in the field

Measurements of the stable isotope ratios of greenhouse gases (e.g., 13C/12C ratio in carbon dioxide or methane), water (e.g., 18O/16O ratio), and pollutants can help identify sources and monitor migration of these important species. Traditionally, these analyses have required that discrete flask samples be obtained in the field and taken to an isotope ratio mass spectrometer (IRMS) for analysis. This procedure is expensive, time-consuming, and requires a dedicated operator for the IRMS, making spatial profiling prohibitive. With the advent of CELAS, real-time stable isotope measurements in the field can now be readily performed, enabling several new possibilities. For example, by injecting deuterium-enriched water into an established groundwater system, samples taken from multiple downstream wells can be measured on-site using a field-portable Liquid Water Isotope Analyzer. The resulting measurements (Figure 4) can be used to discern the groundwater flow and help track pollutant migration.

Figure 4 – Measurements of deuterium enrichment in groundwater during a tracer experiment that shows water flow through a system over several days (left to right). All of the data were taken in the field using a Liquid Water Isotope Analyzer from LGR.

Similar off-axis ICOS systems have been used to detect simulated carbon dioxide leakage via its isotopic signature in a carbon sequestration field site.3 In this experiment, the 13C/12C isotope ratio was used to distinguish leaked carbon dioxide from natural, biogenic sources. These analyzers are now being extended to characterize carbon isotopes for methane attribution, nitrogen isotopes for soil bacterial studies, and oxygen and hydrogen isotopes for bodily fluids.4

Mobile pollutant monitoring

CELAS has also been extensively used to measure a variety of pollutants (e.g., NO2, NH3, and CO) and industrial emissions (e.g., H2S, HCl, and HF). These monitors are field-deployable and portable, enabling mobile monitoring of pollution sources. For example, Figure 5 shows mobile measurements of nitrogen dioxide (NO2) in Oakland, CA. These measurements clearly show the large degree of inhomogeneity in the pollution distribution. Moreover, current, stationary air-quality monitoring stations in Oakland miss this variability close to NO2 sources and may underestimate the NO2 levels experienced in impacted neighborhoods near highways and shipyards.

Figure 5 – Mobile measurements of NO2 made on a street-by-street basis in Oakland, CA show large spatial variability in pollution.

Similar measurements of pollution inhomogeneity have been made using an off-axis ICOS ammonia (NH3) analyzer aboard a Gulfstream-1 aircraft. The plane was flown at low altitudes (i.e., 1000 ft) over Sunnyside, WA, and large ammonia plumes were correlated with cattle feedlots (Figure 6). This deployment clearly demonstrates the utility of field-deployable sensors in characterizing source emissions, and the need for fast, sensitive instrumentation for field measurements.

Figure 6 – Mobile measurements of ammonia made 1000 ft above the Yakima Valley, WA. Large ammonia plumes correlate with cattle feedlots and dairies.

Future outlook

The advent of field-portable CELAS analyzers enables a new era of mobile greenhouse gas, isotope, and pollutant monitoring. These data can be used to better characterize the spatial inhomogeneity of these species and develop spatial maps to better place stationary monitors for long-term monitoring.5 Similarly, mobile isotope monitoring enables the emerging field of isoscapes.6 The emergence of new light sources such as external cavity quantum cascade lasers and difference-frequency generation sources will allow this technology to be extended to other species, including hydrocarbons and volatile organic carbon compounds. Other applications of the CELAS technology include industrial process control, medical diagnostics, and homeland security.

References

  1. Baer, D.S.; Paul, J.B. et al. Sensitive absorption measurements in the near-infrared region using off-axis integrated cavity output spectroscopy. Appl. Physics B: Lasers and Optics  2002, 75, 261.
  2. O’Keefe, A.; Deacon, D.A.G. Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources. Rev. Sci. Instr.  1988, 59, 2544.
  3. McAlexander, I.; Rau, G.H. et al. Deployment of a carbon isotope ratiometer for the monitoring of CO2 sequestration leakage. Anal. Chem.  2011, 83, 6223.
  4. Berman, E.S.F.; Fortson, S.L. et al. Direct analysis of δ2H and δ18O in natural and enriched human urine using laser-based, off-axis integrated cavity output spectroscopy. Anal. Chem.  2012, 84, 9768.
  5. Norris, G.; Larson, T. Spatial and temporal measurements of NO2 in an urban area using continuous mobile monitoring and passive samplers. J. Exposure Anal. Environ. Epid.  1999, 9, 586.
  6. Bowen, G.J. Isoscapes: spatial pattern in isotopic biogeochemistry. Ann. Rev. Earth Planet Sci.  2010, 38, 161.

The authors are with Los Gatos Research, 67 East Evelyn Ave., Ste. 3, Mountain View, CA 94041, U.S.A.; tel.: 650-965-7772; e-mail: m.gupta@lgrinc.com.

Comments