ammonia detection by use of near-infrared diode-laser-based

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Ammonia detection by use of near-infrareddiode-laser-based overtone spectroscopy

Ricardo Claps, Florian V. Englich, Darrin P. Leleux, Dirk Richter, Frank K. Tittel, andRobert F. Curl

We describe a portable diode-laser-based sensor for NH3 detection using vibrational overtone absorptionspectroscopy at 1.53 mm. Use of fiber-coupled optical elements makes such a trace gas sensor ruggedand easy to align. On-line data acquisition and processing requiring ,30 s can be performed with alaptop PC running LabVIEW software. The gas sensor was used primarily for NH3 concentrationmeasurements with a sensitivity of 0.7 parts per million ~signal-to-noise ratio of 3! over a two-week periodin a bioreactor being developed at the NASA Johnson Space Center for water treatment technologies tosupport long-duration space missions. The feasibility of simultaneous, real-time measurements of NH3

and CO2 concentrations is also reported. © 2001 Optical Society of AmericaOCIS codes: 300.6360, 140.3490, 280.3420.

26

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1. Introduction

Laser-based spectroscopic techniques to detect tracegases sensitively and selectively in real time in theinfrared ~IR! spectral region have been the focus ofonsiderable research and development activities inecent years. The principal optical gas sensor tech-ologies are based on absorption spectroscopy of fun-amental bands in the 3–25-mm spectral region andear-IR vibrational overtone and combination bandsrom 1–3 mm. Common radiation sources includeontinuous-wave ~cw! diode lasers ~GaAs-based,ntimonide-based, lead salt!, parametric frequencyonversion devices ~difference frequency generationnd optical parametric oscillators!, gas lasers ~COnd CO2!, and quantum-cascade lasers. Telecom-

munication distributed-feedback ~DFB! diode lasersare ideally suited for overtone spectroscopy of mole-cules with chemical bonds such as C-H, O-H, and N-Hin the near-IR region ~0.78–2.5 mm!. In this paperwe employ them to develop a gas sensor suitable forsimultaneous NH3 concentration measurements at

When this research was performed, all the authors were with theRice Quantum Institute, Rice University, Houston, Texas 77251-1892. R. Claps [email protected]! is now

ith Radiant Photonics, Incorporated, 1908 Kramer Lane, Build-ng B, Austin, Texas 78758. The e-mail address for F. K. Tittel [email protected].

Received 4 December 2000; revised manuscript received 17 April001.0003-6935y01y244387-08$15.00y0© 2001 Optical Society of America

the 1 3 10 level with a precision of 0.2 parts permillion ~ppm! and CO2 measurements between 1000and 20,000 ppm. The sensor was then used to mon-itor dynamic ammonia concentrations in bioreactorvent gases at the NASA Johnson Space Center ~JSC!.n addition, the performance characteristics of theioreactor in terms of the water chemistry could bevaluated. Other applications for compact, real-ime ammonia sensors include the monitoring of se-ective noncatalytic NOx reduction processes, process

control in semiconductor manufacturing, and envi-ronmental monitoring.

Ammonia has a rich spectrum in the near-IRregion.1–5 In the spectral range from 1450 to 1560nm ~;6400–6900 cm21!, Lundsberg-Nielsen et al.6have identified 1710 ammonia absorption lines andassigned 381 of them to rotational–vibrational tran-sitions in the combination band n1 1 n3 and the over-tone 2n3. Webber et al.7 examined these lines forinterferences and pointed out the best lines for mon-itoring purposes. From their report, one of the linesat 1531.7 nm ~6528.8 cm21! was chosen as the oper-ating wavelength for the NH3 sensor described here.At this wavelength, low-noise InGaAs detectors areavailable that operate with quantum efficienciesclose to unity and require no cooling or temperaturecontrol. Overtone spectroscopy exhibits absorptionline strengths that are typically approximately 1–2orders of magnitude weaker than those of the funda-mental vibrations in the mid-IR. To obtain the re-quired sensitivity in the near IR, longer absorptionpath lengths and optimal balancing of laser noise are

20 August 2001 y Vol. 40, No. 24 y APPLIED OPTICS 4387

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Table 1. NH and CO Absorption Lines

4

required. For this purpose, a compact multipass cellconfigured for a 36-m total optical path-length and anautobalance detection technique was used.8 Otherroups have used cavity-enhanced absorption spec-roscopy to monitor rovibrational overtone andombination band transitions.9,10 Also, use of cryo-

genically cooled tunable diode lasers, covering thefundamental absorption band of NH3 ~n2, at 10.07mm!, has been reported.11 The sensitivity obtainedin all cases is comparable to that described in thispaper, but those techniques have not yet been appliedto a portable system that has proven its applicabilityand performance in the field.

The selection of a suitable isolated absorption linefor the design of the gas sensor to be used for ammo-nia monitoring in the bioreactor depends on potentialabsorption interference by other gas species presentin the system. In the bioreactor environment, inaddition to NH3, the presence of H2O and CO2 needso be considered. The line selected for this study hashe merit of being more than 1 cm21 away from the

next H2O absorption line and approximately 0.13cm21 away from an adjacent NH3 and CO2 overtoneransition,12 which permits simultaneous concentra-

tion measurements of NH3 and CO2 at 6528.9 cm21.An ammonia ~or any other gas! concentration mea-

urement for narrow-linewidth radiation sources isbtained with the Beer–Lambert absorption law,hich relates the intensity of light entering into anbsorption medium I0 to the transmitted intensity Is follows:

II0

5 exp~2SnfnPxjl !, (1)

here Sn is the temperature-dependent absorptionine strength for a specific transition denoted by n

@cm21 ~atmycm!#, fn is the line-shape function ~cm!, xjis the fractional concentration of the species j, P ispressure ~atm!, and l is the optical path lengththrough the medium ~cm!.

Table 1 depicts the three absorption lines that areused in this study. Lines A and B are the two am-

onia lines used: A is an isolated line useful for theeasurement of ammonia concentrations whereas

he other ammonia line B overlaps with line C, whichs the ~00001!4 ~30011!, R~36! overtone transition of

CO2.The line-strength values reported in Table 1 corre-

spond to a temperature of 296 K. For measure-ments at the slightly higher temperatures used in the

3

Line

Line Strength

~cm21ymolecule cm22!@cm21y~atm cm

~at 296 K!

A 2.33 3 10221 0.057784B 1.24 3 10221 0.0307C 5.19 3 10225 1.29 3 1025

388 APPLIED OPTICS y Vol. 40, No. 24 y 20 August 2001

present study ~between 300 and 313 K!, an increasen line strength of less than 4% is to be expected.6

The line strength of C is approximately a factor of5000 less than A. However, in the bioreactor ex-haust, CO2 concentrations can be as high as 50% ormore, with the result that line A ends up buriedwithin line C. On the other hand, if CO2 levels arebelow 3%, overlapping between lines A and C is neg-igible and the ammonia concentration can easily bextracted with line A. Furthermore, under these

conditions, the three lines A, B, and C can be used toperform simultaneous NH3 and CO2 concentrationmeasurements.

2. Experimental

A. Ammonia Sensor

The diode-laser-based gas sensor developed for on-line NH3 monitoring in a bioreactor system is shownin Fig. 1. Its main components are a fiber-coupledDFB diode laser ~NLK1554STB from NTT Electron-ics Corporation, Japan!, a compact 36-m multipass,stigmatically compensated Herriott cell and a dual-eam, autobalanced InGaAs detector ~Nirvana 2017rom New Focus!, which can also operate as a singlehotodetector. A two-stage diaphragm pump wassed to flow the sample gas through the multipassell at a pressure of 100 Torr. The multipass cell

Fig. 1. Schematic of the DFB diode-laser-based ammonia sensor.

Frequency~cm21!

Wavelength~nm! Reference

6528.76 1531.68 76528.89 1531.65 76528.895 1532.65 12

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was heated to a temperature of 40 °C to minimizeammonia adsorption on its glass walls and to preventpotential water condensation on the cell mirrors.

The laser diode was driven by a compact currentsupply ~MPL-250, Wavelength Electronics! with rip-

le noise ,1 mA so that the frequency fluctuations ofhe laser line that are due to current noise are neg-igible ~,1 MHz!. The current supply is scanned at

20 Hz about an average current of 67 mA with asawtooth ramp function. The laser temperaturewas controlled to within 0.003 °C, near 32 °C, by athermoelectric current driver ~HTC-1500, Wave-length Electronics!. The scan range of the laser un-der these conditions was 0.3 cm21, which allowed allthree spectral lines, A, B, and C, to be accessed onevery scan.

The fiber-pigtailed DFB laser diode delivers 15mW at 1531.7 nm with a specified linewidth of ,10MHz. The fiber was fusion spliced to a 70y30~%!directional coupler with an insertion loss of lessthan 2%. The 70% power arm of the directionalcoupler was sent to the multipass cell by use of alens ~ f 5 7 mm, 0.5 N.A.! mounted in a precisionholder with 5 deg of freedom ~Optics for Research!o set the beam waist at the midpoint of the multi-ass cell. The 30% power arm was used as theeference beam for the balancing detector. Thisntegrated laser and optical fiber configuration de-ivers 10 mW of laser light at the input of the mul-ipass cell. The output power of the beam obtainedrom the cell after 182 passes was 17 mW, for ahroughput efficiency of 0.17%. This correspondso a 96.5% reflectivity for the cell mirrors, indicat-ng degradation from the original reflectivity of 98%fter three years of use. The output beam from theell is focused on the signal and reference photo-iodes of the dual-beam detector by a gold-coatedarabolic mirror of 1-in. ~2.54-cm! diameter ~ f 5 2!nd a lens ~ f 5 7 mm, 0.5 N.A.! mounted on arecision holder, respectively. Because the refer-nce beam power ~Pref! was much greater than

the power of the signal beam coming from the cell~Psignal!, it was attenuated with a variable fiber at-tenuator. For optimum performance of the auto-balanced detector, the reference power was set asPref 5 2.2 3 Psignal at the center frequency of theaser scan. The attenuation level was set below 25B, because above this level interference fringesrom fiber-to-fiber etalons produced in the attenua-or module started to affect the observed signal.

A laptop PC running LabVIEW 5.0 software wassed for data acquisition and processing. In theutobalance mode, 500 scans were averaged forach single concentration measurement. For theoncentration measurements made with the detec-or in the linear mode, 1000 scans were used for aingle measurement because of the lower signal-to-oise ratio of this mode. The total data collectionime, averaging, and processing to obtain a singleoncentration measurement is less than 30 s. Foronvenience, the sensor was configured to take mea-urements every minute to correlate the ammonia

measurements to external daily events and specificchemical actions performed on the bioreactor. Wefound it convenient to monitor the sensor perfor-mance and concentration data remotely using PCAnywhere software.

B. Bioreactor and Measurement Setup

The bioreactor referred to above is part of a waterrecovery system being developed at the JSC in Hous-ton, Texas. The water recovery system is composed ofseveral subsystems, among them the biological waterprocessor ~BWP!, which is illustrated in Fig. 2. Thissystem produces potable water from a wastewaterstream using anaerobic microbial processes. Aque-ous ammonia is produced in the BWP and must beremoved to preserve the bacterial population. This isdone at the nitrifier stage of the system, where ammo-nia is oxidized into NO3

2. The gases resulting fromthe process are separated from the fluid at the gas–liquid separator stage and vented into the atmosphere,whereas the liquid is returned to the BWP for a newcycle. The concern is that effluent from the gas–liquid separator might transfer potentially harmfulamounts of NH3 to an enclosed environment. This isthe principal factor motivating the monitoring of NH3concentration levels in the water recovery system gasexhaust.

The packed-bed reactor contains anaerobic bacte-ria that decompose organic molecules from wastewa-ter collected from the donation center moving fromthe waste tank at a rate of 16 mlymin. As a residualof this initial process, known as total organic carbon~TOC! reduction, an equilibrium amount ofammonia–ammonium ions dissolved in water is pro-duced, along with other compounds, in particularCO2. The nitrifier consists of a 4000-ft- ~1219-m-!long coil of 0.25-in. ~0.64 cm! polypropylene tubing,

Fig. 2. Diagram depicting the main subsystems of the NASA JSCBWP system and attached NH3 diode-laser-based gas sensor.

he actual size of the BWP is 2 m 3 2 m 3 3 m.


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treated on the inner surface with a biofilm of aerobic,nitrifying bacteria, which in the presence of air pro-duce the reaction

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NO22O¡

O2NO3

2.

he NH3yNH41 concentration level indicates the ef-

ficiency of the nitrifying process in the BWP, once thenitrifier loop is set in operation. If the pH is known,the total NH3yNH4

1 concentration can be calculatedfrom measurements of the NH3 concentration in thegas stream. The CO2 concentration measurementalso gives a quantitative indication of the conversionof TOC in the system, which is an important param-eter to determine the quality of the water that hasbeen recycled. At the end of the loop, a fraction~1y20th! of the recycled water is extracted from the

WP for postprocessing, whereas the rest is sentack into the packed-bed reactor. To keep ammoniaevels in the effluent gas low, nitric acid is added asecessary to the BWP waste tank to maintain a pH of.5. The gaseous residual is vented through the ex-aust line into the atmosphere. Water vapor is re-oved from the gas exhaust by a water condenser.The diode-laser-based sensor was connected to the

xhaust line after the water condenser, as shown inig. 2, and placed ;3 m away from the BWP unit.

We used approximately 6 m of Teflon tubing @0.25 in.0.64 cm!# to satisfy NASA JSC safety protocols. Alass flask ~;150 ml! was inserted along the Teflonine to collect any remaining water droplets in theas. The flow rate to the multipass cell was adjustedo be less than the exhaust line rate to ensure sam-ling of the BWP and not outside air. For this pur-ose, a flow meter and controller were installedmmediately upstream from the sensor to measure aonstant flow rate of 10 SCCM ~SCCM denotes cubicentimeters per minute at STP!, well below the bio-eactor exhaust rates of several hundreds of SCCM.

3. Results and Discussion

A. Calibration and Fitting Routines

A convenient method for determining the accuracy ofconcentration measurements of the sensor systemwas to measure the absorption of a 100-Torr sampleof CO2, which resulted in a concentration of 97.6%when we used HITRAN96 line-strength andpressure-broadening coefficients and assumed an op-tical path length of 36 m. When we measured thesame sample using a second multipass cell and a2.0-mm diode laser, accessing a CO2 line whose line-strength and pressure-broadening coefficient werecharacterized independently,13,14 the CO2 concentra-tion was found to be 100 6 5%. Thus the accuracy ofthe laser-based ammonia sensor reported in thisstudy is better than 5%.

A second calibration procedure was performed witha calibrated ammonia sample ~100 6 2 ppm in N2!hat we diluted with N2 to a nominal 5-ppm mixture


using a gas mixer capable of controlled temperature,pressure, and flow-rate operating conditions. NH3concentration measurements were performed simul-taneously with the overtone-based sensor operatingat 1.53 mm and with a quantum-cascade laser-basedsensor that uses the fundamental NH3 absorptionband n2 at 10.07 mm.15 The results for the two pro-cedures are shown in Fig. 3 and demonstrate anagreement of better than 5% between the two inde-pendent measurements. These are within the stan-dard deviation of the overtone measurements of 60.2

pm, or 7%. We obtained the fundamental absorp-ion measurement using a line strength availablerom the HITRAN96 database, whereas the overtoneeasurement uses a value reported in Ref. 7. Botheasurements yield values below the expected NH3

concentration, most likely because ammonia sticks tothe stainless-steel surface of the gas mixer, and in-sufficient time elapsed to reach a steady state in ad-sorption and desorption.

The real-time fitting routine implemented in Lab-VIEW has been reported elsewhere,16 and only minorsoftware modifications were required for this study toprocess the logarithmic signal from the detector oper-ating in the autobalance mode. Figure 4 illustrateshow a direct transmission spectrum from the logarith-mic output of the Nirvana detector, operating in auto-balance mode, is obtained. A third-order polynomialis fitted to the baseline of the absorption scan, asshown in Fig. 4~a!. If a voltage V*~n! is assigned tothe baseline values, and the logarithmic signal valuesare assigned a voltage V~n!, then the correspondingoptical transmission for a specified frequency valueT~n!, in percent, is given by17

T~n! 5 100expF2 A

TV*~n!G 1 1

expF2 AT

V~n!G 1 1, (2)

where A is a constant determined by the gain of thedetector amplifier (A 5 273 KyV) and T is the temper-ature of the photodetector ~operating at 300 K!. Theresult of this operation is depicted in Fig. 4~b!. Thisprocedure provides a balanced transmission spectrumand simultaneously corrects for the baseline of theabsorption signal. To this spectrum ~after we divideby 100 and take the natural logarithm!, an absorptionline-shape function is fitted to obtain the gas concen-tration as described below. The convenience of thisprocedure is that only one detection channel is neededin this configuration to account for both the signal andthe reference signals from the photodetector.

The fit employs a nonlinear, least-squaresLevenberg-Marquardt algorithm and uses a given ab-sorption profile to fit the data. Because the pressureregime of operation ~100 Torr! lies between the pre-dominantly Doppler and pressure-broadened regimes,a Voigt profile with fixed pressure broadening andDoppler width was used. A Lorentz function overes-timates the area under the curve by ;6%. Figure 5

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shows a typical absorption spectrum obtained, wherewe removed the baseline using a third-order polyno-mial and to which we fitted a Voigt profile,18,19 with ainewidth of 0.027 cm21 and a pressure-broadening

contribution of 0.020 cm21. Because of pressure fluc-tuations in the bioreactor exhaust of 62 Torr, not ac-ounted for by the fitting routine, and theuctuations observed in the CO2 concentration from

the exhaust gases of 62%, the linewidth of the ammo-nia absorption peaks is expected to fluctuate by ap-proximately 60.002 cm21.5,6,20 This introduces anerror for the ammonia concentration measurement inthe LabVIEW calculation routine of 4%. The residualof the Voigt fit yields an uncertainty level for a singlemeasurement ~with 500 averages! of 60.01% absorp-

Fig. 3. Intercomparison of concentration measurements of a4-ppm NH3 in a N2 mixture with a 1.53-mm DFB diode laser and

single-frequency quantum-cascade laser-based sensor operatingt 10.07 mm.

Fig. 4. Algorithm developed to obtain the transmission spectrum~%! from the logarithmic output of the Nirvana detector in theautobalanced mode. See text for details.

ion @note that 100 3 ln~IyI0! is equivalent to percentabsorption for small absorptions#. The rms uncer-tainty level is then 0.014% absorption. From Fig. 5,where a peak absorption of 0.085% corresponds to 1.4ppm of NH3, it can be seen that detection with a signal-to-noise ratio of 3 corresponds to a concentration of 0.7ppm ~i.e., 0.7 6 0.12 ppm!. This is the minimum con-centration that can be measured reliably with the re-ported gas sensor design. Optical fringes frominterference effects introduced by the multipass celllimit the sensitivity and are the primary source ofuncertainty in the ammonia concentration values re-ported. Other groups implementing a dual-beam au-tobalancing technique like the one reported in thisstudy have obtained similar sensitivity values.21,22

To our knowledge, the sensitivity reported here is thebest obtained so far in a field application for NH3 mea-urements by use of overtone absorption spectroscopy,aking advantage of recent advances in single-requency DFB diode-laser and optical fiber technol-gy.We obtained simultaneous measurements of NH3

and CO2 concentrations by fitting the three lines A,B, and C in real time as mentioned in Section 1.This technique uses the fact that the three peaks canbe obtained in a single diode-laser scan of ;0.3 cm21.The algorithm begins with a Voigt fit on peak A.With the NH3 concentration thus obtained, peak B ismodeled and subtracted from the absorption feature.The remaining absorption signal at the location ofpeaks B and C is fitted again to a Voigt profile, whichyields the CO2 concentration. This algorithm re-moves the problem of having to fit the two peaks thatoverlap B and C simultaneously. It also allows forthe subtraction of the baseline in two separate sec-tions, one for line A in the first step and the other forines B and C in the second step. This techniqueliminates unwanted effects of spurious baseline fea-

Fig. 5. Typical NH3 absorption line at 6528.76 cm21 observed fromthe NASA JSC bioreactor, with the detector operating in autobal-ance mode. The residual obtained from the Voigt line-shape fit isshown below the NH3 spectrum and indicates a limiting detectionensitivity of 1024 optical density @optical density is given by 2ln~Iy

I0!#. The NH3 concentration from the Voigt fit is 1.4 ppm.


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4

tures that may occur between the two absorptionlines. Figure 6 shows an absorption spectrum of thetwo NH3 and the CO2 lines listed in Table 1, obtainedin a single diode-laser scan. The baseline subtrac-tion obtained with this algorithm for the peaks A, B,and C yields a fit residual of less than 60.02% ab-sorption within the peaks. Such a residual wouldnot have been realized if a single baseline had beenfitted across the entire scan range.

B. Bioreactor Measurements

NH3 concentration measurements were performed atNASA JSC for a period of 13 days, from 25 August to7 September 2000. Figure 7 shows the ammoniaconcentration as a function of time, measured duringthe first three days of data collection ~25–28 August2000!. The NH3 concentration fluctuates in periodsof ;10 h, ranging from levels below the sensitivity ofthe instrument and up to 5.6 ppm. Figure 8 showsthe overall time history of the ammonia measure-ments performed on the bioreactor system, except fora three-day period from 30 August to 1 September2000, when the bioreactor was monitored by a 2.0-mm

iode laser from Stanford University.14 Critical bio-chemical procedures taken on the different parts ofthe BWP by NASA personnel are indicated. Thestandard deviation of the autobalanced output data iscalculated to be 0.1 ppm, whereas for the linear case

Fig. 6. Measured NH3 and CO2 absorption lines listed in Table 1,data obtained from a typical laser scan over ;0.3 cm21. The two aResult of the two-step fitting routine applied to line A, in which aabsorption feature that includes both lines B and C, showing a C


it increases to 0.2 ppm. Such a difference is ex-pected because the autobalanced scheme minimizesmost common sources of noise in both the signal andthe reference arms of the laser beam. In particular,laser amplitude fluctuations produced by electronicnoise from the power supplies and temperature driftsare removed. As mentioned above, the linear outputmeasurements were averaged for 1000 scans, as com-

red at 6528.8 cm21, observed from the BWP vent gases. ~a! Rawtion features at 6528.76 and 6528.89 cm21 are clearly visible. ~b!

3 concentration of 2.1 ppm is obtained. ~c! Fit performed on thencentration of 0.9%.

Fig. 7. Time history of NH3 concentration from NASA JSC bio-reactor vent gases for a three-day period.

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Table 2. Aqueous- and Gas-Phase NH Concentrations Calculated

pared with only 500 scans for the autobalanced mea-surements. The main sources of uncertainty in thisexperiment are the optical fringes produced in themultipass cell. The autobalance technique does notremove the fringes because they do not affect thesignal and reference beams simultaneously. Weperformed a background ammonia concentrationmeasurement by disconnecting the system from thebioreactor and sampling room air. The NH3 back-ground was measured to be ;0.5 ppm ~see Fig. 8! andis due to the residual ammonia that has accumulatedon the walls of the gas line from the BWP to theammonia sensor. During the entire measurementperiod the ammonia levels never exceeded 5.6 ppmafter the packed-bed reactor and the nitrifier coilswere integrated. The NH3 concentration fluctuatedduring the period ~25–30 August! before the BWP

as operated in the recycling mode. In this mode,o more wastewater samples were added and themmonia levels decreased ~shown in Fig. 8 and alsoerified in Fig. 13, Ref. 14!.NH4

1 aqueous-phase concentration measurementswere performed once a day by NASA personnel usingstandard wet-chemical techniques. From this andthe pH value of the aqueous solution, the concentra-tion of NH3 in solution can be calculated. An esti-

ate of the NH3 concentration in the gas phase wascalculated with Henry’s law.23 The resulting valuesare shown in Table 2, which is in agreement with theresults obtained with the laser-based ammonia sen-sor. It is observed that, in general, the calculatedvalues from the wet-chemical measurements aresomewhat larger than the measured concentrationswith the laser-based sensor. However, it should benoted that the wet-chemical measurements provideonly an order-of-magnitude estimate of the overallammonia concentration expected in the gas phase.

Fig. 8. Ammonia concentration measurements for an 11-day pe-riod. Filled circles correspond to data points obtained in the au-tobalance mode, and triangles correspond to data obtained in thelinear mode of the detector. Measured ammonia levels never ex-ceeded 5.6 ppm during this period. A three-day gap occurredwhen a 2-mm DFB diode laser14 was used in the sensor instead ofhe 1.53-mm diode laser.

An accurate measurement would require a preciseaccount of all the ions in the solution, the pH of thesolution, and the variation of these quantities in thevarious subsystems of the bioreactor.

Figure 9 shows the simultaneous NH3 and CO2concentration measurements for a portion of theoverall measurement time, obtained when we appliedthe triple Voigt fit algorithm described in Section 3.A.The two measurements are clearly anticorrelated,which is to be expected from the fact that ammoniaand carbon dioxide gas-phase concentrations shouldhave opposite dependence on the pH in the BWPwater solution. The CO2 concentration shown inFig. 9 provides information that we can use to im-prove the BWP operation by monitoring the efficiencyof the BWP in performing TOC reduction. Through-out the time periods when CO2 concentrations weremeasured, largely fluctuating values were obtained.CO2 levels from less than 1000 to 20,000 ppm werebserved, with values decreasing after the BWP waset into its recycling mode on 30 August 2000.

4. Conclusions

We described the performance of a trace gas sensorbased on overtone absorption spectroscopy at 1.53

Fig. 9. Simultaneous CO2 and NH3 concentration measurements.The CO2 levels do not exceed 20,000 ppm. The time period can beelated to the ammonia measurements reported in Fig. 7. Theontinuous curves represent moving averages of ten data points forhe two gases being measured.

3

from Measurements of @NH41#, pH, and with Henry’s Law

DateNH3 Aqueous

~mgyl!NH3 Gas

~ppm!

25 Aug. 32.5 13.026 Aug. 12.5 5.027 Aug. 8.7 3.528 Aug. 8.6 3.429 Aug. 20.7 8.330 Aug. 19.9 8.031 Aug. 10.0 4.01 Sept. 14.5 5.82 Sept. 6.1 2.43 Sept. 11.0 4.44 Sept. 5.2 2.15 Sept. 3.7 1.56 Sept. 4.4 1.8


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itoring near 1.5 mm with diode laser absorption sensors,” Appl.

1

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4

mm, applied to ammonia and carbon dioxide detectionin a bioreactor exhaust line, with a time resolution of30 s and a sensitivity of 0.7 ppm at a signal-to-noiseratio value of 3. The advantages of this sensor com-pared with other methods are compactness, portabil-ity, simplicity of operation, and the ability to performnear-real-time measurements. The NH3 concentra-tion measurements reveal a dynamic behavior of thebioreactor that could not be detected and measuredotherwise. These measurements also indicate that,during the period of this study, ammonia emissionsinto the environment remained below harmful lev-els.24 This evaluation may have to be revised whenlonger periods of operation in a closed environmentare considered. The ability to perform simultaneousNH3 and CO2 measurements in real-time has been

emonstrated. A clear anticorrelation between theoncentrations of these two gases is observed, whichs in agreement with the expected operating condi-ions of the NASA JSC bioreactor.

The diode-laser-based ammonia sensor can be usedn other applications, especially those with concentra-ion levels of 1 ppm and higher, that need to be mon-tored with a fast time response. Furthermore, otherases such as CO2, HF, HCl, NO, CH4, and H2O can be

monitored by the appropriate wavelength selection ofthe DFB diode laser or a tunable multiwavelengthDFB diode-laser source, which provides additionalwavelength channels for multispecies gas detection.

Funding for this project was provided by NASA, theTexas Advanced Technology Program, and the WelchFoundation. R. Claps thanks M. E. Webber of Stan-ford University for fruitful discussions on Voigt fittingroutines. F. V. Englich and R. Claps also expresstheir gratitude to the NASA JSC Crew and ThermalSystems Division personnel for providing the neces-sary logistic support to perform the bioreactor mea-surements. D. Leleux acknowledges the support bythe NASA JSC Graduate Student Research Program.

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