Article pubs.acs.org/ac
Highly-Precise Measurements of Ambient Oxygen Using NearInfrared Cavity-Enhanced Laser Absorption Spectrometry Manish Gupta* Los Gatos Research, 67 East Evelyn Avenue, Suite 3, Mountain View, California 94041-1529, United States ABSTRACT: Highly precise measurements of ambient oxygen have been used to constrain the carbon budget, study photosynthesis, estimate marine productivity, and prescribe individual pollution events to their point of origin. These studies require analyzers that can measure ambient oxygen with ppm-level precision. In this work, we utilize near-infrared off-axis integrated cavity output spectroscopy (off-axis ICOS) to quantify ambient oxygen with a precision (1σ, 100s) of ±7 ppm. By periodically calibrating the instrument, the analyzer is capable of making oxygen measurements to better than ±1 ppm (1σ). The sensor is highly linear (R2 > 0.9999) over a wide dynamic range (0−100% oxygen). The sensor was combined with a commercial CO2/CH4/H2O Analyzer, and used to make measurements of respiration and fossil fuel pollution events with oxidative ratios ranging from 1.15−1.60. Future improvements will increase the analyzer precision (1σ, 100s) to better than ±1.4 ppm, and decrease the periodic referencing interval to >1 h. By including an additional diode laser, the instrument can be extended to make simultaneous measurements of O2, CO2, and H2O to enable improved understanding of carbon dioxide production and loss.
S
ince the pioneering work of Keeling,1 highly precise oxygen measurements have been used to constrain the carbon budget,2 study terrestrial photosynthesis and respiration,3 and estimate marine productivity.4 Measurements of the O2/N2 ratio have become more routine and have been recently used to detect strong increases in phytoplankton abundance (spring bloom),5 track seasonal changes in atmospheric potential oxygen,6−8 and prescribe individual pollution events to their point of origin.9 In most of these studies, O2/N2 ratios were measured with a 1σ precision of ±1.4 to ±14 per meg (∼ 0.3−3 ppm) in a measurement time of 10−20 min, and such high precision was required to discern typical oxygen concentrations changes of 20 ppm that occurred over several hours. A variety of methods have been developed to precisely quantify ambient oxygen to the parts per million-level. Initial work relied on using interferometry1 and mass spectrometry10 to make laboratory-based measurements with very high precision. Gas chromatography coupled with a thermal conductivity detector (GC/TCD) has also been successfully used for oxygen quantification11 in both laboratory flask samples12 and field measurement stations;5 however, the apparatus requires considerable sample conditioning and exhibits some cross-interference from argon. More compact, field-portable instrumentation was developed using a paramagnetic oxygen sensor13 and vacuum ultraviolet absorption spectrometry,14 both of which required extensive customization. Recently, a differential fuel-cell analyzer has been adapted15 to provide very precise measurements of ambient oxygen in a compact, cost-effective, autonomous system that requires minimal user intervention or sample conditioning. Note that none of these technologies can be extended to © 2012 American Chemical Society
carbon dioxide, prohibiting the development of a multiplexed O2/CO2 sensor. An alternate measurement scheme involves using tunable diode laser absorption spectrometry (TDLAS) to quantify oxygen. This scheme is attractive because the resulting instrument can be highly selective, robust, and field-portable, while providing simultaneous measurements of other key gases (e.g., CO2 and H2O). However, the strongest, readily accessible optical transition in oxygen absorbs light near 760 nm (B ← X transition) and provides an optical absorption of ∼2% in a 1 m path length containing an ambient concentration of oxygen. Thus, a typical TDLAS system (e.g., ΔI/I0 = 10−5−10−4, 1 m optical path length) can only provide an oxygen measurement precision of ±100−1000 ppm. In this work, we utilize nearinfrared, cavity-enhanced optical absorption spectrometry to increase the effective optical path length of TDLAS and quantify ambient oxygen with high precision.
■
METHODS We have utilized a cavity-enhanced optical absorption spectroscopy termed Off-Axis Integrated Cavity Output Spectroscopy (Off-Axis ICOS) to quantify O2 using near-infrared optical transitions (Figure 1). The technique has been described previously16 and only a brief overview will be provided below. A temperature-controlled, tunable, distributivefeedback (DFB) diode laser operating near 1.272 μm was coupled into a 28 cm long high-finesse optical cavity consisting Received: June 27, 2012 Accepted: August 27, 2012 Published: August 27, 2012 7987
dx.doi.org/10.1021/ac301790p | Anal. Chem. 2012, 84, 7987−7991
Analytical Chemistry
Article
fitted Voigt was converted into a molar mixing ratio (e.g., μmol oxygen per mol air) using measured gas pressure and temperature. The gas inlet was connected to a solenoid valve array to permit rapid switching between gas standards and samples for periodic calibration. As noted above, the analyzer measures the molar mixing ratio of oxygen in air (μmol/mol); however, for the ambient air studies presented below, small changes in atmospheric oxygen are usually expressed as O2/N2 ratios relative to a standard.17 Thus, the measured value molar mixing ratio must first be converted to an O2/N2 ratio
Figure 1. Schematic diagram of the off-axis integrated cavity output spectroscopy (off-axis ICOS).
XO2 O2 ≈ N2 1 − XO2 − XCO2 − X H2O − 0.01
of 2 highly reflective mirrors (R ≈ 99.93% as determined from the measured absorption of pure oxygen) and provided an effective optical path length of 400 m. The A ← X band near 1270 nm was selected over the B ← X band near 760 nm due to the availability of very robust, telecommunications-grade DFB diode lasers near 1270 nm and minimal absorption effects due to ambient oxygen outside the measurement cell. Light transmitting through the cavity was focused onto an amplified InGaAs detector. A diaphragm pump continuously flowed gas through the cavity at ∼0.5 SLPM and a proportional solenoid valve retained the pressure inside the cavity at 139 Torr, assuring good absorption peak contrast and baseline quantification. The laser frequency was repeatedly tuned over 20 GHz to span over a single oxygen molecular absorption feature (Figure 2). Each scan required 0.83 ms (e.g., 1200 Hz
where XO2, XCO2, and XH2O are the measured molar mixing ratios of O2, CO2, and H2O, respectively, and the factor of 0.01 accounts for the remaining air constituents (e.g., argon, neon, methane, ...). This value is then referenced to a standard and expressed as δ(O2/N2) in per meg units: ⎛ (O2 /N2)sample ⎞ δ(O2 /N2) = ⎜⎜ − 1⎟⎟ × 106 ⎝ (O2 /N2)ref ⎠
Note that, by using a reference tank that has been calibrated to the ppm-level, the analyzer would provide accurate quantification of O2 in addition to precise O2 measurements. However, currently, such oxygen standards are difficult to obtain and most previous work has involved making precise measurements of O2/N2 against a fixed reference gas cylinder.
■
RESULTS AND DISCUSSION Analyzer Performance. The precision, linearity, and time response of the analyzer were gauged by repeated, long-term measurements of dry, compressed ambient air. The time response of the instrument is limited to 8 s (1/e) by the time it takes for a gas sample to move through the measurement cell. The stability and precision of the sensor were determined by measuring a constant, flowing (0.5 SLPM) air sample for 48 h, and the resulting Allan deviation plot is shown in Figure 3. The data suggests that a 1σ measurement precision of ±7 ppm in 100 s of data averaging, that increases at longer times due to measurement drift. Similar to other high-precision gas sensors, this drift can be mitigated by periodically referencing. The instrument inlet was
Figure 2. Measured (red points) cavity-enhanced off-axis ICOS absorption spectrum of ambient oxygen (∼20.1%) fit to a single Voigt profile (black line) to yield the fit residual shown atop the figure. The cavity provides an effective optical path length of 400 m and the measurement signal-to-noise ratio suggests a 1 Hz precision of approximately ±54 ppm (1σ).
laser tuning) and 1200 transmission spectra (1 s) were averaged prior to fitting the O2 absorption feature. The laser tuning rate was characterized prior to the experiment by passing the laser through a glass etalon (FSR = 2.00 GHz) and measuring its change in frequency with injection current for the given scan rate. The measured transmission spectrum was fit to a baseline, second-order polynomial function coupled with a single Voigt profile. Correcting for the baseline transmission yielded the cavity-enhanced absorption spectrum shown in Figure 2. This spectrum has a signal-to-noise ratio (peak signal to rms noise) ratio of 958:1. Taking into account that there are ∼16 points on the peak (full-width half-maximum), this noise level suggests a 1 Hz measurement precision of approximately ±54 ppm (1σ), consistent with the results presented below. The area of the
Figure 3. Allan deviation plot showing the oxygen measurement precision (1σ) as a function of data averaging time with (black squares) and without (red circles) the periodic calibration described in the text. 7988
dx.doi.org/10.1021/ac301790p | Anal. Chem. 2012, 84, 7987−7991
Analytical Chemistry
Article
Note that, similar to other off-axis ICOS analyzers, the sensor can operate over a wide ambient temperature range (0−50 °C) and is relatively immune to mechanical vibrations. Thus, it can be field-deployed in a wide variety of locations, including field stations,18 air monitoring towers,19 aircraft,20 and ships.21 Ambient Air Measurements. To demonstrate the utility of the oxygen analyzer for ambient air studies, the sensor was combined with a commercial Greenhouse Gas Analyzer (Los Gatos Research model 911−0011) to provide simultaneous, highly precise measurements of O2, CO2, CH4, and H2O as shown in Figure 6. The Greenhouse Gas Analyzer was
switched between two constant air samples every 194 s (Figure 4) for over 24 h. The first 80 s of data taken after switching
Figure 4. Periodic calibration scheme involves switching between two sources (e.g., tank 1 and tank 2) every 194 s (red points). The first 80 s of data is disregarded and the next 90 s of data is averaged (thick blue and green lines) to provide an oxygen reading. Thus, the difference in the oxygen molar mixing ratio, ΔO2 (ppm), is obtained every 388 s.
samples was disregarded to allow for the instrument to fully respond to the new sample. The next 90 s of data was averaged to provide an oxygen reading that was precise to better than ±7 ppm (1σ). The difference in the oxygen molar mixing ratio, ΔO2 (ppm), was obtained every 388 s. On the basis of the precision of each measurement and the instrument drift between measurements, the difference between the two oxygen measurements (ΔO2) has a precision of ±21 ppm (1σ, 388 s). This precision improves with subsequent measurements of the oxygen molar mixing ratio difference and the 1σ measurement precision of ΔO2 is also included in the Allan deviation plot (Figure 3). With periodic referencing, the sensor is capable of quantifying differences in oxygen concentration (ΔO2) with a precision of better than ±1 ppm (1σ) in less than 7 h. The linearity of the analyzer was determined by using mass flow controllers to dilute a pure oxygen sample with dry nitrogen to achieve oxygen concentrations ranging from 0 to 100%. The measured results are shown in Figure 5 versus those
Figure 6. Experimental configuration for simultaneous, high-precision measurements of O2, CO2, CH4, and H2O with periodic referencing. This configuration was used to measure a constant gas source (compressed air cylinder), laboratory room air, and outdoor air in an urban environment (Mountain View, California).
calibrated on NOAA-certified cylinders and quantified CO2, CH4, and H2O with a precision (1σ, 10s) of ±0.12 ppm, ± 0.95 ppb, and ±70 ppm, respectively. A timed 3-way inlet valve (Omron #H3CR) was periodically switched every 388 s to select between a constant reference gas and other gas sample. The selected input gas was pulled at ∼1 SLPM through a Nafion drying unit (Permapure #PD-50T-12MPS) with a ∼3 SLPM counterflow of ambient air at reduced pressure that was dried to 1000 s of data averaging and a periodic calibration frequency of >1 h to achieve a long-term oxygen precision of better than ±1 ppm (1σ). The instrument’s accuracy can be further improved by altering the sample handling strategy15 to minimize fractionation effects at gas inlets and tubing junctions, maintain constant pressure throughout the system, eliminate dead volumes, and further dry the sample to obviate any effects due to water vapor. Finally, by using dual-coated mirrors that provide high reflectivity at both 1272 and 1600 nm and geometrically multiplexing two lasers into an Off-Axis ICOS cavity,24 the technique can be extended to produce a single instrument that simultaneously measures O2, CO2, and H2O with very high precision, thus obviating the need for a second Greenhouse Gas Analyzer and data logger.
events with oxidative ratios ranging from 1.15−1.60. Future improvements should enable this sensor to achieve a higher measurement precision (±1.4 ppm, 1σ, 100s) with a less frequent calibration interval (>1 h). Moreover, by including an additional DFB diode laser operating near 1600 nm, the instrument can be extended to make simultaneous measurements of O2, CO2, and H2O for an improved understanding of carbon dioxide production and loss.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare the following competing financial interest(s): Dr. Gupta is an employee and partial owner of Los Gatos Research..
■ ■
ACKNOWLEDGMENTS LGR gratefully acknowledges funding for this work through the NASA STTR program (Grant NNX11CI23P). REFERENCES
(1) Keeling, R. F. J. Atmos. Chem. 1988, 7, 153. (2) Keeling, R. F.; Shertz, S. R. Nature 1992, 358, 723. (3) Severinghaus, J. P.; Bender, M. L.; Keeling, R. F.; Broecker, W. S. Geochim. Cosmochim. Acta 1996, 60, 1005. (4) Balkanski, Y.; Monfray, P.; Battle, M.; Heimann, M. Global Biogeochem. Cycles 1999, 13, 257. (5) Yamagishi, H.; Tohjima, Y.; Mukai, H.; Sasaoka, K. Atmos. Chem. Phys. 2008, 8, 3325. (6) Kozlova, E. A.; Manning, A. C.; Kisilyakhov, Y.; Seifert, T.; Heimann, M. Global Biogeochem. Cycles 2008, 22, No. GB4020. (7) Sirignano, C.; Neuber, R. E. M.; Rödenbeck, C.; Meijer, H. A. J. Atmos. Chem. Phys. 2010, 10, 1599. (8) van der Laan-Luijkx, I. T.; Karstens, U.; Steinbach, J.; Gerbig, C.; Sirignano, C.; Neubert, R. E. M.; van der Laan, S.; Meijer, H. A. J. Atmos. Chem. Phys. 2010, 10, 10691. (9) Minejima, C.; Kubo, M.; Tohjima, Y.; Yamagishi, H.; Koyama, Y.; Maksyutov, S.; Kita, K.; Mukai, H. Atmos. Chem. Phys. 2012, 12, 2713. (10) Bender, M. L.; Tans, P. P.; Ellis, J. T.; Orchardo, J.; Habfast, K. Geochim. Cosmochim. Acta 1994, 58, 4751. (11) Tohjima, Y. J. Geophys. Res., [Atmos.] 2000, 105, 14575. (12) Tohjima, Y.; Mukai, H.; Machida, T.; Nojiri, Y. Geophys. Res. Lett. 2003, 30, 1653. (13) Manning, A. C.; Keeling, R. F.; Severinghaus, J. P. Global Biogeochem. Cycles 1999, 13, 1107. (14) Stephens, B. B.; Keeling, R. F.; Paplawsky, W. J. Tellus 2003, 55B, 857. (15) Stephens, B. B.; Bakwin, P. S.; Tans, P. P.; Teclaw, R. M.; Baumann, D. B. J. Atmos. Oceanic Technol. 2007, 24, 82. (16) Baer, D. S.; Paul, J. B.; Gupta, M.; O’Keefe, A. Appl. Phys. B: Laser Opt. 2002, 75, 261. (17) Keeling, R. F. The atmospheric oxygen cycle: The oxygen isotopes of atmospheric CO2 and O2 and the O2/N2 ratio. Reviews of Geophysics, Supplement, U.S. National Report to the IUGG 1991− 1994; 1995; p 1253. (18) Berman, E. S. F.; Gupta, M.; Gabriell, C.; Garland, T.; McDonnell, J. J. Water Resour. Res. 2009, 45, W10201. (19) Tuzson, B.; Hiller, R. V.; Zeyer, K.; Eugster, W.; Neftel, A.; Ammann, C.; Emmenegger, L. Atmos. Meas. Tech. 2010, 3, 1519. (20) Berman, E. S.F.; Fladeland, M.; Liem, J.; Kolyer, R.; Gupta, M. Sens. Actuators, B 2012, 169, 128. (21) Gülzow, W.; Rehder, G.; Schneider, B.; Schneider, J.; Deimling, V.; Sadkowiak, B. Limnol. Oceanogr. Methods 2011, 9, 176. (22) Steinbach, J.; Gerbig, C.; Rödenbeck, C.; Karstens, U.; Minejima, C.; Mukai, H. Atmos. Chem. Phys. 2011, 11, 6855.
■
CONCLUSION We have fabricated an Off-Axis ICOS oxygen analyzer that provides highly precise measurements of ambient oxygen (±7 ppm, 1σ in 100 s) that improve to better than ±1 ppm (1σ) with periodic calibration. The sensor is highly linear (R2 > 0.9999) over a wide dynamic range (0−100% oxygen). By combining this system with a commercial Greenhouse Gas (CH4, CO2, H2O) Analyzer (model 911−0011), we were able to make measurements of respiration and fossil fuel pollution 7990
dx.doi.org/10.1021/ac301790p | Anal. Chem. 2012, 84, 7987−7991
Analytical Chemistry
Article
(23) Aemisegger, F.; Sturm, P.; Graf, P.; Sodemann, H.; Pfahl, S.; Knohl, A.; Wernli, H. Atmos. Meas. Tech. Discuss. 2012, 5, 1597. (24) Berman, E. S. F.; Fladeland, M.; Liem, J.; Kolyer, R.; Gupta, M. Sens. Actuators, B 2012, 169, 128.
7991
dx.doi.org/10.1021/ac301790p | Anal. Chem. 2012, 84, 7987−7991
