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Photoacoustic Spectroscopy in Trace Gas Monitoring Frans J.M. Harren, Gina Cotti, Jos Oomens, and Sacco te Lintel Hekkert in Encyclopedia of Analytical Chemistry R.A. Meyers (Ed.) pp. 2203 – 2226 John Wiley & Sons Ltd, Chichester, 2000

Photoacoustic Spectroscopy in Trace Gas Monitoring · 2 ENVIRONMENT: TRACE GAS MONITORING (a) B C D A (b) Figure 1 (a) The eye piece of a spectroscope is removed and substances are

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Page 1: Photoacoustic Spectroscopy in Trace Gas Monitoring · 2 ENVIRONMENT: TRACE GAS MONITORING (a) B C D A (b) Figure 1 (a) The eye piece of a spectroscope is removed and substances are

Photoacoustic Spectroscopy in Trace Gas Monitoring

Frans J.M. Harren, Gina Cotti, Jos Oomens, and Sacco te Lintel Hekkert

inEncyclopedia of Analytical Chemistry

R.A. Meyers (Ed.)pp. 2203–2226

John Wiley & Sons Ltd, Chichester, 2000

Page 2: Photoacoustic Spectroscopy in Trace Gas Monitoring · 2 ENVIRONMENT: TRACE GAS MONITORING (a) B C D A (b) Figure 1 (a) The eye piece of a spectroscope is removed and substances are
Page 3: Photoacoustic Spectroscopy in Trace Gas Monitoring · 2 ENVIRONMENT: TRACE GAS MONITORING (a) B C D A (b) Figure 1 (a) The eye piece of a spectroscope is removed and substances are

PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 1

Photoacoustic Spectroscopy inTrace Gas Monitoring

Frans J.M. Harren, Gina Cotti, Jos Oomens, andSacco te Lintel HekkertUniversity of Nijmegen, The Netherlands

1 Introduction 1

2 History 1

3 Devices and Equipment 23.1 Light Sources 23.2 Photoacoustic Cells 43.3 Limitations, Selectivity, Interference,

Detection Limits 6

4 Environmental Applications 114.1 Stack Gas Emission 114.2 Car Exhaust Emissions 114.3 Ambient Air Monitoring 11

5 Biological and Medical Applications 135.1 Postharvest Physiology 135.2 Plant Physiology 145.3 Microbiology 165.4 Human Health, Noninvasive Breath

Analysis 165.5 Entomology 17

6 Comparison with other SpectroscopicMethods in Trace Gas Monitoring 18

Acknowledgments 19

Abbreviations and Acronyms 19

Related Articles 19

References 20

Since ancient times people have searched for ways to under-stand processes occurring in the environment, atmosphereor living organisms. Study of the gaseous trace compoundspresent may shed new light on chemical reactions takingplace in the atmosphere or biochemical reactions insideorganisms such as plants, animals and human beings. Thisarticle presents photoacoustic spectroscopy as a sensitive,on-line and noninvasive tool for monitoring the concen-tration of trace gases. Following a short introduction anda historic overview, attention is focused on the descriptionof devices and equipment which determine the detectionlimits and selectivity. An overview is given of the currentdetection limits for photoacoustic detection. Applicationsare discussed with emphasis on environmental monitoring

(in ambient air, car exhaust and stack gas emission), onmedical applications and on biological applications (inpostharvest physiology, plant physiology, microbiologyand entomology).

1 INTRODUCTION

A gaseous molecule that absorbs electromagnetic radi-ation is excited to a higher electronic, vibrational orrotational quantum state. Generally, depopulation ofthis quantum state to lower lying states occurs eithervia fluorescence or collisions, the latter giving rise toa temperature increase of the gas due to energy trans-fer to translation. This nonradiative relaxation processoccurs when the relaxation time can compete with theradiative lifetime of the excited energy levels. Radia-tive decay has a characteristic lifetime of 10�7 s at visiblewavelengths as compared with 10�2 s at 10 µm. For nonra-diative decay these values depend on the pressure (decaytime t inversely proportional to the pressure) and canvary strongly at atmospheric pressures (10�3 –10�8 s).

By modulating the radiation source at an acousticfrequency the temperature changes periodically, givingrise to a periodical pressure change which can be observedas an acoustic signal; in the gas phase the effect can bedetected with a sensitive microphone.

Laser-based photoacoustic detectors are able to moni-tor trace gas concentrations under atmospheric conditionswith orders of magnitude better sensitivity than con-ventional scientific instrumentation; in addition they areable to monitor noninvasively and on-line under dynamicconditions.

2 HISTORY

The photoacoustic effect was first reported by AlexanderGraham Bell in 1880;.1/ he discovered that thin discsemitted sound when exposed to a rapidly interruptedbeam of sunlight. In a later experiment,.2/ he removedthe eye-piece of a commercial spectroscope and placedabsorbing substances at the focal point of the instrument.The substances were put in contact with the ear bymeans of a hearing tube (Figure 1a and b) and he found‘‘good’’ sounds in all parts of the visible and invisibleelectromagnetic spectrum of the sun.

Other publications on this phenomenon followed thisfirst work; we mention here the works of Rontgen,.3/

Tyndall.4/ and Preece..5/ However, due to the lack of aquantitative description and the lack of a sensitive micro-phone, interest in the photoacoustic effect soon declined.

In 1938 Viegerov refined the photoacoustic techniquefor the first spectroscopic gas analysis;.6/ hereafter Luft.7/

Encyclopedia of Analytical ChemistryR.A. Meyers (Ed.) Copyright John Wiley & Sons Ltd

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2 ENVIRONMENT: TRACE GAS MONITORING

(a)

B

C

DA

(b)

Figure 1 (a) The eye piece of a spectroscope is removed and substances are placed in the focal point of the instrument behinda slit. These substances are put in contact with the ear by means of a hearing tube..2/ (b) Sunlight is intercepted and aligned with asteering mirror (C, D), modulated with a chopper (B) and focused onto a glass bulb (A)..2/

measured trace gas absorption spectra with an infraredbroadband light source down to the parts per million level.

By the end of the 1960s, after the invention ofthe laser, scientific interest increased once again. In1968, Kerr and Atwood.8/ utilized laser photoacousticdetection to obtain the absorption spectrum of smallgaseous molecules. Due to the high spectral brightness oflasers and improved phase-sensitive lock-in techniquesthat amplified the acoustic signal, they were able todetermine low concentrations of air pollutants. Kreuzer.9/

demonstrated that it was possible to detect concentrationsof e.g. 10�8 (10 parts per billion) of methane in nitrogen,using an intensity modulated infrared (3 µm) He-Ne laser.Patel et al. demonstrated the potential of the technique bymeasuring the NO and H2O concentrations at an altitudeof 28 km with a balloon-borne spin-flip Raman laser..10/

After this the photoacoustic effect was introduced into thefield of trace gas detection with environmental, biologicaland medical applications.

3 DEVICES AND EQUIPMENT

3.1 Light Sources

With reference to the previous paragraph, lasers arenot essential to operate photoacoustic gas detection sys-tems. Although the spectral power density of broadband

infrared light sources is orders of magnitude lowercompared with lasers, their advantages are reliabilityand cost effectiveness. Infrared light sources in com-bination with various photoacoustic detection schemesare commercially available for trace gas detection atparts per million levels. Spectral selectivity is achieved byusing FTIR (Fourier transformed infrared) spectroscopyin combination with spectral band filters in front of thephotoacoustic cell; thus typically seven molecular gasescan be detected simultaneously at the 1–100 ppm level.

The URAS gas analyzer of Hartmann & Braun uses aphotoacoustic detection scheme which is able to detect aspecific gas out of a multicomponent gas mixture avoid-ing cross interferences. In this instrument selectivity isachieved by comparing the direct absorption in a samplecell to that in a reference cell. After passing the samplingcells, each attenuated light beam enters a second detec-tion cell filled only with the gas of interest (Figure 2); thedetection cells are interconnected via a membrane con-nected to a capacitor. Since the dual beam is modulatedthe difference in acoustic energy reflects the difference inabsorption and thus the concentration difference betweensample cell and reference cell. The species under investi-gation enables the wavelength to be selected in such a waythat all wavelengths at which absorption occurs are simul-taneously active. When there is no spectral overlap fromother gases, additional absorptions in the sample cell willnot contribute to the acoustic signal; the light passes the

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PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 3

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M1

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Figure 2 Infrared gas analyzer of Hartmann & Braunwith photoacoustic detection scheme to detect a specificgas from a multicomponent gas mixture thereby avoidingcross-interference. Light from the infrared source is split intotwo paths. The chopper modulates the intensity for both paths.A filter volume in each path serves to filter out light of wave-lengths not needed for the detection process; they can be filledwith gases whose absorption spectra do not overlap with thoseof the species under scrutiny. M1 and M2 serve as measuring celland reference cell respectively. With the help of the equalizerboth light intensities become equal before entering the last cell.The last cell consists of two compartments with a membranebetween them. Both compartments are filled with the gas underinvestigation so that all wavelength characteristics for this gascontribute to the signal. If the attenuation differs in M1 fromthat in M2 the membrane starts to oscillate with the frequency ofthe chopper. This oscillation (typically a few hertz) is detectedcapacitively.

detection cell unattenuated. When a specific compound,e.g. H2O, causes spectral overlap, an extra cell can beplaced in the light path filled with the interfering gas. Thiscell completely attenuates the wavelengths where thisinterfering molecule absorbs, including the spectral over-lap regions. Thus, these wavelengths cannot contributeto the photoacoustic signal and a single component of acomplex multicomponent gas mixture can be detected.

High spectral brightness renders CW (continuouswave) laser sources ideally suitable for photoacoustictrace gas detection. In contrast to direct absorptiontechniques, the photoacoustic signal is proportional tothe laser power; from the Lambert–Beer law one findsfor small absorptions (Equation 1)

P D P0e�sNl ���! .P0 � P/ ³ P0sNl .1/

where P0 and P are the laser power before and afterthe photoacoustic cell, respectively, s is the absorptioncross-section per molecule (in cm2), N is the numberof absorbing molecules per cubic centimeters and l theabsorption path length (in cm). The absorbed energy(P0 � P) is converted into acoustic energy which isrecorded by the microphone.

Saturation, due to nonlinear absorption of the laserpower only occurs in focused high-power laser beams.The pumping rate to a higher rovibrational level isproportional to the laser light intensity; in the case ofsaturation it exceeds the collisional de-excitation rates.

Besides sensitivity, lasers achieve high selectivity. Thespectral selectivity is only limited by the pressure broad-ened absorption profiles of the gases under investigation.The first practical lasers which were used to detect tracegases were CO2 lasers..11/ These line tunable lasers coverthe infrared 9–11 µm wavelength region with a laserline spacing of 0.5–2 cm�1. Fingerprint absorption spec-tra can be achieved if we compare the laser line spacingto the pressure broadened absorption lines for the tracegas molecules (typically a few gigahertz at atmosphericpressures). Additionally, they are able to deliver highlaser powers (1–100 W) from a relatively small gas dis-charge tube.

A new development is the application of the CO laserin itsv D 1 (5.0–7.6 µm wavelength region) and v D 2(2.5–3.8 µm) version. Although less powerful (typically1 W) its performance can be improved by applying anintracavity set-up. CO lasers are line tunable with a linespacing between 0.5–1 cm�1.

Other CW lasers in the visible and infrared have beenapplied to photoacoustic trace gas detection such as a spinflip Raman laser,.10/ diode lasers.12,13/ and dye lasers..14/

Although dye lasers and titanium sapphire do not coverthe ideal wavelength region for trace gas detection theyare very well suited for photoacoustic spectroscopy ofweak absorption bands with their continuous tunabilityand a typical CW laser power of 1 W and they haveproved their potential in molecular spectroscopy of highlyvibrationally excited molecules..15,16/ However, for tracegas detection they are less applicable since the overtonemolecular absorption cross-sections are weak, therebyraising the detection limits. For the same reason infrareddiode lasers derived from telecommunication research(0.8 and 1.5 µm) are not very well suited.

Of periodically poled nonlinear crystals that havebeen developed, periodically poled lithium niobate(PPLN) is probably most well known..17/ Lithium niobate(LiNbO3) can be used in combination with pulsedlasers to generate mid-infrared radiation (2–5 µm) byparametric oscillation. CW operation was limited to afew microwatts of laser power due to phase matchingproblems. Quasi-phase matching with periodically poledmaterials overcomes this problem resulting in laserpowers up to a few watts in the infrared..18/

The advantage of using pulsed lasers for photoacousticdetection is their wider infrared tunability and conse-quently better spectral overlap with interesting moleculargases. Examples can be given of the experiments per-formed with a pulsed CO2 laser by Repond and Sigrist.19/

and experiments with a pulsed optical parametric oscil-lator (OPO)..20/ A disadvantage of pulsed lasers is theirhigh peak powers (megawatt) in relation to their relativelylow average energy (typically 1 W) which has to generatea relatively slow process as the gas phase photoacoustic

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4 ENVIRONMENT: TRACE GAS MONITORING

effect. Peak-to-peak fluctuation and nonlinear absorp-tion disturb the reproducibility of the generated signal,thereby raising detection limits. Typical detection limitsreported with these types of laser are in the parts permillion range.

3.2 Photoacoustic Cells

To perform trace gas detection the ideal photoacousticcell should amplify the generated sound originatingfrom the molecular gas absorption meanwhile rejectingacoustic (and electric) noise and in-phase infraredabsorption from other materials. Interfering gases shouldbe distinguished by spectroscopic or physical methods(see section 3.3)

For gas phase measurements, mainly resonant cellsare combined with modulated CW lasers and lock-inamplifiers; pulsed lasers are combined with piezoelectricdetectors and boxcars. These selective amplifiers arisefrom the necessity to lower acoustic and electric noiselevels thus improving the signal-to-noise ratio.

Other requirements for photoacoustic cells are a lowgas consumption or a fast response; for this the activevolume of the cell should be small so that no dilution cantake place when the trace gas and its carrier flow throughthe acoustic cell.

If we consider a nonresonant, cylindrical cell, itsperformance can be expressed as its efficiency to convertabsorbed photon energy into acoustic energy; this cellconstant F (Pa cm W�1).21,22/ is given by Equation (2)

Fnonresonant D G.g� 1/LwV

.2/

where L and V are the length and the volume of the cell,respectively, g is the specific heat constant, w D 2pn themodulation frequency and G is a geometrical factor in theorder of one. Within a nonresonant cell the gas absorptionsignal is independent of the cell length but inverselyproportional to its diameter. However, photoacousticsignals are also generated by infrared window absorption.To reduce these signals inside the cell its length should beas long as possible to spread this locally generated soundover the total cell volume.

For a resonant cell, the above cell constant F mustbe amplified with the quality factor Q of the generatedacoustic resonance, Equation (3)

Fresonant D QFnonresonant .3/

where Q is expressed by the ratio of the energy storedin the acoustical standing wave divided by the energylosses per cycle. This acoustical amplification process islimited by various dissipation processes which can bedivided into surface and volume effects. Main surfacelosses are viscous and thermal losses at the resonator

surface, microphone losses and acoustic wave scatteringlosses at obstacles in the cell. Volumetric losses are notas important as surface losses and are mainly due to freespace viscous and thermal losses and V –V, V –R, V –Trelaxation losses of polyatomic gases..23/

A cylindrical cavity can be a resonant cavity for soundwaves. The resonance frequencies of such a cavity aregiven by Equation (4)

fmnp D vs

((amn

2R

)2C( p

2L

)2)1/2

.4/

where vs is the sound velocity in the gas filling thecavity, R is the radius of the resonator, L is the length,p D 0, 1, 2, 3 . . . axial mode numbers and amn is a suitablesolution of Bessel equations with m D radial modenumber and n D azimuthal mode number (Figure 3)..24/

The cell constant F for all types of cylindrical resonantcells is proportional to L1/2/R..25/ Due to their largerdiameters resonances in the radial or azimuthal acousticmode have high Q values and high resonance frequenciesbut low F values. A longitudinally excited resonator willhave a low Q value but, due to its small diameter, a highF value.

In order to obtain an optimum signal-to-noise ratio,noise control and interfering signals have to be taken intoaccount. Theoretically, the ultimately lowest acousticalnoise results from random density fluctuations, i.e.Brownian motion, in the gas which is distributed overall frequencies in the sound spectrum. The total power ofthese density fluctuations is constant, but the frequencydistribution is dependent upon local resonances and theirQ values. Therefore, using a resonant cell the signal-to-noise ratio will not be improved by considering only theserandom thermal noise fluctuations.

However, these noise levels are far below other noisesources such as amplifier noise and acoustic disturbancesfrom outside the cell. The power of the amplifier noisevaries as 1/f , where f is the modulation frequency ofthe light beam. Therefore, in contrast to Brownian noise,it is more advantageous to operate at a cell resonanceto increase the generated acoustical signal above the 1/f

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Longitudinal Azimuthal Radial

L Z

r

k = 0,1,2.... m = 0,1,2.... n = 0,1,2....

2R

Figure 3 Resonant acoustic modes of a cylindrical closedchamber; the fundamental longitudinal, azimuthal and radialmodes.

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PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 5

noise. A prerequisite is that external acoustic disturbancesare shielded from the microphone by a proper cell wallconstruction, material choice and proper design of inletand outlet ports.

Noise fluctuations do not have a fixed phase relationwith the modulation of the light intensity. Other disturb-ing factors limiting the sensitivity do have a fixed phaserelation. Directly generated coherent acoustical back-ground signals, caused by the modulation process (e.g.chopper, current modulation) can be suppressed in thesame way as external acoustical disturbances.

Other, more serious interferences are coherent pho-toacoustic background signals which are caused byabsorption of the light beam in the window materialor light scattered or reflected off the resonator wall.They are generated at the same frequency and in-phasewith the modulated light beam. In resonant cells, windowabsorption signals can be suppressed by using large buffervolumes and l/4-tubes next to the windows..25/ These one-end-open tubes (Figure 4), placed near the window, actas a notch filter for the window signal at the resonancefrequency. The influence of the scattered light on thephotoacoustic background signal can be minimized byusing, for the resonator wall, a highly reflective polishedmaterial, with a thermally well conducting material assubstrate, e.g. in the case of the CO2 laser, a polished goldcoated copper tube..26/

In the past special designs have been developedfor longitudinally, azimuthally and radially resonantphotoacoustic cells,.27 – 30/ even without windows.31,32/ toimprove sensitivity.

In order to improve the selectivity the combinationof the Stark effect or Zeeman effect with photoacousticdetection represents an interesting solution for specificmolecules such as ammonia (NH3).33,34/ and nitricoxide (NO)..35/ The change in absorption at a specificlaser frequency depends on the shift and splitting ofthe molecular absorption lines of the species underinvestigation. Although the method does not sufferfrom interference problems within multicomponent gasmixtures, there are some limitations. The detectionis limited to molecules with a permanent electric ormagnetic dipole moment; in addition, the shift or splittingof the lines should be observable within the pressurebroadened Lorentzian profile at the overlap with thelaser frequency. For ammonia at the 10R8 transition ofthe 12CO2 laser these requirements are satisfied; Thonyand Sigrist.33/ showed that the absorption cross-sectionchanged by a factor of two at atmospheric pressure byapplying an electric field of 16.7 kV cm�1.

Another way to improve the sensitivity for a specificgas is by temperature change. By increasing the temper-ature the vapor pressure of, for example, an oil with lowvapor pressure is increased thereby improving the sensi-tivity. An elegant cell has been designed by Jalink andBicanic,.36/ who combined a heatpipe and a photoacous-tic cell thereby keeping the microphone and windowsat room temperature. In addition, a Helmholtz reso-nant photoacoustic cell (Figure 5) has been developedby Kastle and Sigrist for quantitative temperature-dependent absorption measurements of fatty acidvapors..37/ Helmholtz resonances (wH) can be constructed

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6 ENVIRONMENT: TRACE GAS MONITORING

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0 5 10 cm

Figure 5 Photoacoustic cell applied for temperature-depen-dent investigations on fatty acids. The temperature of the waterbath was varied between 278 and 350 K with a cold finger and twoimmersion heaters, while the temperature of the microphonewas kept constant with a cooling/heating device. (Reproducedwith permission from Kastle and Sigrist, Appl. Phys. B, 63,389–397 (1996), Copyright Springer Verlag.)

using a relatively large closed volume (V) connected tothe outer world via a relatively long narrow duct (lengthL, cross-section S at a sound velocity c) Equation (5):.24/

wH D√

c2SLV

.5/

Amplification of sound waves can also be achieved withthe thermoacoustic effect. The operating principle of suchan amplifier connected to a photoacoustic resonator canbe explained as follows. The gas inside the acousticalresonator is irradiated by an amplitude modulated laserbeam. Trace gas absorption inside the resonator tubewill produce a standing acoustical wave; this is theacoustic effect. The amplifier consists of a temperaturegradient over a stack of thin sheets separated by adistance equal to two times the thermal boundary layerof the gas (Figure 6). If the gas is moving up anddown over the temperature gradient inside the stack,heat energy is transferred to the gas and subsequentlyinto the acoustic standing wave. With such a cell, Qvalues have been improved by a factor of 20–30..38/

By adjusting the amplifier just below the onset of self-oscillation, the acoustical wave in the resonator willput the amplifier in sustained oscillation, leading to aconsiderable enhancement of the photoacoustic signal.Noise, acoustic and photoacoustic background signalswill also be enhanced. However, with careful design ofthe cell these levels can be suppressed thereby improvingthe signal-to-noise ratio.

3.3 Limitations, Selectivity, Interference,Detection Limits

Amplifying photoacoustic signals in resonant cells, reduc-ing noise, using high laser powers, and so on are all usedto gain an as high as possible sensitivity for moleculargas absorption. The sensitivity to trace a specific com-pound depends strongly on its spectroscopic properties.For example, closely spaced rotational absorption lineswithin a Q branch of a strong vibrational transition help toreach low detection limits of such a gas. One good exam-ple is the Q branch of the n7 band of C2H4 in the 10 µmregion. The 10P14 CO2 laser line is in exact resonancewith this Q branch. This results in an extremely low detec-tion limit for C2H4 of 6 parts per trillion (1 ppt D 1ð 1012)in nitrogen. Some gases can be detected even better (e.g.SF6) while others have higher limits due to less coinci-dence between available laser frequencies and molecularabsorption features. In the future this problem may beovercome by powerful tunable infrared lasers such asthe PPLN OPO.39/ and quantum cascade lasers..40/ Fordetection limits see Table 1 and references therein.

Reactive gases such as ozone are very difficult toquantify in absolute concentrations, although ozonehas a very strong absorption cross-section in the CO2

laser wavelength region, resulting in sub-parts perbillion detection limits. Ozone is highly reactive, thusinteractions with tubing walls, sampling cuvettes, andthe photoacoustic cell, cause rapidly decreasing ozoneconcentrations. This loss in concentration must bedetermined over the complete sampling system bycalibrating the system with a specific ozone analyzer andchecked for linearity over a large concentration range. Forexample, the breakdown of ozone over a 2-m samplingtube may be as high as 60%..67/ The degradation ofozone in a photoacoustic cell is determined to be 75% byinserting a second identical photoacoustic cell in the flowsystem. In spite of these limitations, successful studieshave been performed to quantify atmospheric ozoneconcentrations..68/

The first excited vibrational mode of N2 has a longvibrational lifetime (³1 ms at 1 atm (1 atm D 101.325 kPa)at n1 D 2200 cm�1). Using kilohertz modulation of thelaser beam a phase shift may occur in the generatedphotoacoustic signal. A well known example is thekinetic cooling effect of CO2 at CO2 laser wavelengths(9–11 µm)..54,69,70/ CO2 has a hot-band absorption in thiswavelength region and is excited to the n3 vibrational levelwhich is almost resonant with the first vibrational level ofN2. Therefore, under atmospheric conditions the energyabsorbed by CO2 will only slowly be converted into atemperature and pressure increase of the gas.

N2 is not unique. Another example is given inFigure 7 where CH4 absorbs the CO laser radiation in

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PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 7

Tunable piston

φ 23 mm

23 mm

Stainless steelsheets

Copper sheets

Stainless steelspacers

Resonatorφ15 × 150 mm

Microphone

Water cooling

Stack

Hightemperature

column

Heatingwire

12 mm

15 mm

23 mm

Figure 6 Expanded view of the thermoacoustic stack, dimensions are in millimeters. The tunable piston can slide in the upper partof the column which is cylindrically shaped (diameter 23 mm). The lower part (length 20 mm, square cross-section 23 ð 23 mm)forms the connection to the stack. This part is heated by Inconel wire. The exploded view of the stack shows the stainless steelplates with the temperature gradient and copper plates to maintain the gradient; stainless steel spacers separate the plates. At theconnection between thermoacoustic stack and the photoacoustic resonator, a water cooler removes the excess heat and keeps thelower end of the stack at constant temperature. (Reproduced with permission from Bijnen et al.).38/

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8 ENVIRONMENT: TRACE GAS MONITORING

Table 1 A number of photoacoustic trace gas studies with laser type and molecules investigateda

(a) CO lasers

CO laser, 5–6.4 µm.41/a ppb CO laser, 5–8 µm.42/ ppb CO laser.11/ ppb

Nitric oxide 15.4 Carbon disulfide 0.01 Ammonia 0.4Nitrogen dioxide 0.88 Acetaldehyde 0.1 1,3-Butadiene 1Nitrous oxide 50 Water vapor 0.1 1-Butadiene 2Carbon dioxide 5ð 104 Carbon dioxide 1ð 103 Nitric oxide 0.4Water vapor 65 Nitrogen dioxide 0.1 Nitrous oxide 0.1Ethylene 42 Nitric oxide 0.3 Propylene 3Vinyl chloride 10.6 Nitrous oxide 1 Water vapor 14Propylene 47 Acetylene 11,3-Butadiene 22 Ethylene 1Formaldehyde 7.7 Ethane 1Acetaldehyde 4.5 Methane 1Acrolein 2.1 Pentane 3Acetone 4.2 Trimethylamine 1Benzene 70 Carbonyl sulfide 1Toluene 40 Dimethyl sulfide 3m-Xylene 19 Sulfur dioxide 0.1o-Xylene 70 Methanethiol 3p-Xylene 97 Hydrogen sulfide 100

(b) Various lasers

Laser Species cmin (ppb) Ref.

Dye laser 290–310 nm Sulfur dioxide 0.12 14Dye laser 570–620 nm Nitrogen dioxide 10 43Dye laser 600 nm Iodine 260 44Dye laser 600 nm Bromine 7.9ð 104 44HeNe laser 3.39 µm Methane 10 9HeNe laser 3.39 µm CnH2nC2 104 –105 45CO Spin flip Raman laser Nitric oxide 0.1 46CO laser, Zeeman effect Nitric oxide 0.01 35CO laser 4.75 µm Carbon monoxide 150 47CO laser 5.40 µm Phosgene Few 48CO laser 5.42 µm Nitric oxide 0.1–1 45CO laser 6.13 µm Nitrogen dioxide 0.01–0.1 45DF laser 3.87 µm Methane 1.6ð 103 49PbS1�xSex4.8 µm Carbon monoxide 5ð 104 12InP/InGaAsP 1.13 µm Water vapor 1ð 104 13Diode 10.5 µm Ethene 200 50

(c) CO2-lasers

ppb ppb

Ref. 51 Ref. 58Ethylene 46 Hydrazine 0.2Benzene 2.05ð 103 Monomethylhydrazine 0.3Ammonia 35 Unsymmetrical-Trichloroethylene 0.55ð 103 dimethylhydrazine 0.3Freon-12 0.13ð 103 Dimethylamine 1.2Ethanol 0.36ð 103 Trimethylamine 0.3Toluene 0.8ð 103 Methanol 0.05

Ref. 52 Ref. 61b

Ethylene 0.02 Acrolein 36Ozone 0.06 Styrene 36Hydrogen sulfide 170 Ethyl acrylate 141,2-Dibromoethane 0.4 Trichloroethylene 7Ethyl acetate 0.06 Vinyl bromide 25

Vinylidene chloride 8

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PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 9

Table 1 (continued)

(c) CO2-lasers (continued)

ppb Ref. atm�1cm�1

Ref. 62 Ref. 65

Nitroglycerine 0.28 Acetonenitrile 0.75Ethylene glycol dinitrate 1.5ð 103 Ammonia 69.5Dinitrotoluene 16ð 103 Arsine 5.3

Benzene 2.7Ref. 63 ppb W�1 Butadiene 4.4

Ethrane 21.7Ethylene glycol dinitrate 8.26 Ethylenechlorohydrin 9.3Nitroglycerine 0.23 Ethylene oxide 0.642,4-Dinitrotoluene 0.5 3.4b

Cyclohexanone 20.2 Formic acid 17.1Methylamine 25.3 Freon-12 99.8Ethylene 0.3 53 Germane 1.9Ammonia 1 54c Hydrogen selenide 0.5Nitrous oxide 2ð 104 55 Perchloroethylene 25.9Sulfur hexafluoride 0.01 56 Phosgene 1.1Ethylene, Ammonia, Freons, etc. �4 57 Phosphine 4.0Carbon dioxide 7.4ð 103 31 Styrene 56Ammonia 0.43 31 6.0b

Ethylene 1.3 31 Sulfur dioxide 0.24Ethylene 1–10 45 Sulfuryl fluoride 0.19Ammonia 1–10 45 53.3b

Tricloroethane 111 24.0Ref. 59c Ref. 60c Trichloroethylene 15.510�20 cm2 ppb Vinyl chloride 6.6

Ref. 66Ethylene 130 0.7Water vapor 0.005 2ð 104 Freon-13 1.24b

Propylene 8.2 27 Freon-22 0.74b

Vinyl chloride 29 2.6 Freon-12 79.37b

Benzene 7.2 7.2 Freon-11 4.23b

Toluene 4.5 20 Ammonia 40.87b

Methanol 72 1.4Ethanol 17 6 Ref. 29 ppbCarbon dioxide 0.0151,3-Butadiene 28 Acetonitrile 670o-Xylene 4.3 Benzene 48m-Xylene 2.8 Cyclohexane 250p-Xylene 2.7 1,2-Dichloroethane 450Ethylbenzene 2.3 Ethyl acetate 8.3Dichloromethane 0.17 Freon-12 5.5Chlorobenzene 13 Freon-113 5.0o-Dichlorobenzene 21 Furan 25m-Dichlorobenzene 11 Isopropanol 29p-Dichlorobenzene 5.5 Methylchloroform 11

Methyl ethyl ketone 83t-Butanol 26

atm�1cm�1 Vinyl chloride 12Iodopropane 360

Ref. 64 Acetylene 3Ammonia 0.8

Sulfur hexafluoride 646 Ozone 9Tungsten hexafluoride 0.091 Perchloroethylene 3.2Uranium hexafluoride 8.5ð 10�4 Trichloroethylene 13

(continued overleaf )

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10 ENVIRONMENT: TRACE GAS MONITORING

Table 1 (continued)

(c) CO2-lasers (continued)

atm�1cm�1 atm�1cm�1

Sulfur hexafluoride 0.2 Butane 2001,1-difluoroethylene 5.3 Ethylene glycol dinitrate 38Freon-11 18 Nitroglycerine 0.7Freon-114 3.6 Methylamine 120Ethylene 2.6

a Unless indicated otherwise, detection limits are reported for single-component mixtures with N2 at 1 atmosphere as buffergas. Data are given in ppb (1 part per billion volume D 1ð 109). In a number of references only absorption strengths aregiven: s (in cm2) or a (in atm�1cm�1), with a D Ntot Ð s (Ntot D 2.5ð 1019 molecules cm�3 at 1013 hPa and 20 °C). When thedetection system has a sensitivity of typically amin D 1ð 10�8 the minimum detectable gas concentration can be calculatedfrom cmin D amin/.s ÐNtot/.

b At 13CO2 laser lines.c Multicomponent study.

170 Collisions

1533

1306

0CH4 O2

E(c

m–1

)

t V – T =10 ms

Thermal equilibrium1554

Figure 7 Vibrational energy transfer due to near-resonantenergy levels and a slow V–T relexation process, in comparisonwith the modulation frequency (1010 Hz), may lead to anamplitude decrease and phase shift of the photoacoustic signal.This effect can, for instance, be observed when comparingmixtures of methane in nitrogen and in air, due to rapidvibrational energy transfer between methane and oxygen (seetext for details).

the n4 vibrational mode centered around 1306.2 cm�1.This is the lowest vibrational mode of the molecule;the collisional relaxation lifetime is therefore largecompared with the lifetime of other molecules witha smaller energy gap between their lowest vibrationalmodes and the ground state. The adjacent n2 mode(1533.3 cm�1) becomes thermally populated too (rateconstant 13 µs�1 atm�1)..71/ Oxygen has a near resonantvibrational level (1554 cm�1) and only 170 collisions areneeded to transfer energy to the vibrational mode of O2

(rate constant ³28 µs�1 atm�1)..72/ Since this is a ratherfast process most energy is deposited into O2 and thenrelaxes slowly to the ground vibrational state; the averagenumber of collisions required for relaxation to the groundstate is 8.3ð 107 in pure O2 (at standard temperatureand pressure 5ð 109 collisions per second take place).The much larger number density of O2 compared withCH4 creates a buffer of vibrational energy leading to aneffective transient cooling of the translational degreesof freedom. Due to our high modulation frequency

(1010 Hz), we observed this transient cooling by O2 asan amplitude decrease and a phase change relative to thesignal caused by the same amount of CH4 in N2.

For specific molecules (such as NH3, NO) spectro-scopic interference can be avoided by combining thephotoacoustic effect with the Stark or Zeeman effect;the selectivity is enhanced by intermodulating chop-per and electric or magnetic fields. A more generallyapplicable method is to separate gases, either by gaschromatographic methods, selective trapping inside a coldtrap,.26/ or by a specific chemical reaction (e.g. CO2 byKOH ���! K2CO3 and water). In most cases, a smallamount of the interfering compound remains present.To take this into account, a multicomponent analysis isnecessary selecting a number of laser lines on which inter-fering species and the species of interest have both strongand weak absorptions.

For a multicomponent analysis of a gas mixture,the laser is subsequently tuned to L laser lines. For theanalysis it is preferable to obtain an overdeterminedsystem, G < L with G being the number of gases; inthis way, more information can be obtained from thegas mixture resulting in more accurate results..60,73/ Forpractical convenience the background signal generatedby window absorption and/or resonator wall absorptionis added as a fictive gas component. In the ideal case themeasured spectrum is equal to the calculated spectrum.If the absorption coefficient

∑lg of a gas g at laser line l is

known, the total absorption at a laser line, al is given byEquation (6)

al DG∑

gD1

lg Ð cg .6/

with cg the concentration of component g. In matrixnotation one writes Equation (7)

Ea D EE Ð Ec .7/

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PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 11

If no measurement errors are specified in the absorp-tion coefficient matrix EE and the measured spectrum( Qal), the gas concentration Ec can be calculated by.26/

Equation (8)

Ec D . EEtÐ EE/�1 Ð EE

tÐ E1 .8/

where EEtis the transposed matrix ofij,ij D

∑ij /ai and

1i D 1

4 ENVIRONMENTAL APPLICATIONS

4.1 Stack Gas Emission

Several attempts have been made to measure stackgas emissions from power plants. Due to the highamount of nitrogen oxide (NO and NO2) containingcompounds in the emissions, they contribute significantlyto photochemical smog formation and acidification ofthe soil. To reduce the total amount of NOx in thestack gas, NH3 is added in the exhaust gas towards thechimney. Using a voluminous catalyst, NOx reductionwithin the stack gas takes place. In order to checkthe performance and to avoid an excess of ammoniainjection, the ammonia concentration in the chimney isdetermined. Olafsson et al. have successfully applied aCO2 laser-based photoacoustic detection system for insitu monitoring of ammonia concentrations..74/ Due tothe difficult hostile environment halfway up the chimney(vibrations, temperature fluctuations, etc.), concessionshave to be made as to the sensitivity of the apparatus. Witha nonresonant photoacoustic cell at 125 °C, a detectionlimit of 1 ppm NH3 was achieved in a multicomponentgas mixture containing 10–15% CO2.

Gas detection using pulsed lasers in combination withphotoacoustic spectroscopy in the UV (ultraviolet) andvisible has been developed by Stenberg et al. to analyzegas concentrations in fluidized beds and other combustionenvironments..75/ The probe has been calibrated fortypical combustion gases such as N2O, NO, NO2, NH3,SO2 and H2S at temperatures between 20 and 910 °C.

Another example of the potential of the photoacousticsystems to control industrial processes is given bySigrist.22/ who was able to follow the ethanol andmethanol emissions from a pharmaceutical productionplant with a CO2 laser-based set-up with a time resolutionof 5 min. With this time resolution, strong concentrationfluctuations were observed in the exhaust emission,whereas the gas chromatographic system which was usedfor comparison proved inadequate.

4.2 Car Exhaust Emissions

CO and CO2 lasers were used to analyze the air pollutingemissions from car exhausts. A thorough study was

performed by Bernegger and Sigrist to unravel car exhaustmixtures quantifying CO2, NO and 12 hydrocarbons (seeTable 2)..76/ To this end, absorption cross-sections ofthe individual species at each laser line were measured;certified trace gas mixtures in a buffer (mostly N2)or other certified methods were used to quantify theabsolute values. Once the absorption cross-sections aredetermined, care has to taken to avoid saturation effects(i.e. nonlinearity of the photoacoustic effect with thelaser power.52/) and phase shift effects (see previoussection). From the absorption cross-sections detectionlimits can be obtained. In the literature, extremely lowdetection limits are mentioned based on a signal-to-noise-ratio of one (S/N D 1, see Table 1), extrapolated fromlarger quantities of trace gases in a buffer gas. In realitythe detection limits are higher due to multicomponentgas mixtures which induce cross-sensitivities in theabsorption coefficients. This mathematical analysis of thephotoacoustic spectra is based on the weighted leastsquares fit of the measured spectra with iteration steps.By choosing the best set of laser lines the error in thecalculated concentrations can be reduced..70,77/

Car exhaust gas mixtures have been investigated bycollecting exhaust samples from different motorcycles,diesel and gasoline engines equipped with catalytic con-verter. The photoacoustic measurements were comparedwith conventional gas analytical methods. The engineexhaust stream was mixed with air and a fraction of themixture was collected in a Tedlar sampling bag. Dueto the strong water vapor absorption at the CO laserwavelengths and the high water vapor concentrations of1–2% in the sampling bag, the gas was flowed over a dry-ice cold trap before analysis thereby reducing the watervapor concentration to 0.1%..76/

4.3 Ambient Air Monitoring

Examples of laboratory photoacoustic studies on multi-component studies can be found elsewhere..22/ Here wemention two examples of mobile systems which weredeveloped and used in field campaigns. A completelaser photoacoustic system which fitted into two mediumsized boxes (0.3 m3 each) was developed by Rooth et al.to detect ammonia gradients above the heath in theNetherlands..54/ The potential danger of nitrification byNH3 of such nutrition poor areas motivated the measur-ing campaign to follow the deposition/emission ratio ofammonia over the area. The system operated for severalmonths, taking data for water vapor, CO2 (every 40 min)and ammonia (every 6 min).

Another system was developed by Sigrist et al.Installing the whole set-up in a small trailer the stresson the equipment is less severe. Thanks to this approachthe system has been operational for years and has been

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12E

NV

IRO

NM

EN

T:T

RA

CE

GA

SM

ON

ITO

RIN

G

Table 2 CO laser photoacoustic analyses of vehicle exhausts collected at idling operation of the enginea (after Bernegger and Sigrist).76/

Vehicle Dried CO2(%) NO Ethylene Propylene Benzene Toluene m-Xylene o-Xylene p-Xylene Formal- Acetal- Acroleindehyde dehyde

Volvo Yes 10.3š 2.5 105š 9 260š 20 191š 9 37š 5 26š 14 16š 10 42š 18 4š 19 5.5š 1.9 2.7š 0.8 2.5š 0.4(leaded)Jeep Yes 9.1š 1.1 23š 2 191š 11 109š 3 48š 3 268š 9 67š 7 114š 10 7š 8 1.2š 0.6 2.6š 0.2 1.2š 0.1(unleaded) No 11.1š 1.3 40š 4 206š 13 107š 5 78š 4 196š 13 188š 12 103š 18 67š 13 0.1š 1.5 4.1š 0.3 2.4š 0.2VW Bus Yes 10.8š 1.1 11.8š 1.5 172š 10 109š 4 34š 3 149š 7 31š 7 59š 10 2.8š 8 0.1š 0.8 3.6š 0.3 1.7š 0.2(cold catalytic No 11.1š 1.5 27š 3 178š 14 98š 13 45š 5 87š 15 39š 14 33š 19 11š 11 1.2š 2.6 3.4š 0.9 1.9š 0.4convertor)Renault Master Yes 2.8š 0.3 10.4š 0.9 32š 3 18.2š 1.2 2.7š 0.7 18.5š 1.9 4.0š 1.7 7.7š 2.6 0.5š 2.5 8.1š 0.8 7.6š 0.4 3.2š 0.2(diesel) No 2.7š 0.6 24.5š 2.2 33š 6 14š 8 0.3š 2.8 16š 7 6.4š 6 5.6š 10 1.2š 6 7.1š 2.5 8.7š 1.0 4.4š 0.5Unimog Yes 2.6š 0.4 15.7š 2.2 5š 3 5š 2 2.0š 0.8 6š 2 2.5š 2 1š 3 5š 3 2.6š 0.7 2.5š 0.3 1.3š 0.1(diesel) No 2.2š 1.0 43š 5 7š 10 – – – 3.5š 4 – 8š 8 3š 7 0.9š 2 1.5š 1.2FBW Truck Yes 1.9š 0.4 37š 3 16š 3 17.5š 1.3 – 12š 2 1.1š 1.5 0.3š 3 – 4.2š 0.6 7.4š 0.2 3.4š 0.1(diesel) No 2.7š 0.9 78š 6 23š 10 23š 9 2š 5 20š 10 6š 7 11š 12 3š 10 3š 2 8.4š 0.5 6.9š 0.3Puch motorcycle Yes 7.9š 3 0.3š 5 220š 10 329š 7 203š 7 700š 20 47š 18 135š 15 15š 10 15.9š 1.8 4.1š 0.7 6.2š 0.3(2-stroke) No 8.5š 3 0.1š 2 234š 10 284š 7 217š 6 405š 16 95š 13 148š 15 20š 9 12.6š 2.2 5.2š 0.8 6.5š 0.3

a All data are given in parts per million (ppm) with their uncertainty, except for CO2 (which is in %). The effect of drying of the gas could be observed.

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PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 13

applied in several field campaigns in urban and ruralenvironments..78/ For example, multicomponent gas mix-tures were analyzed during sunny days in the summer,using nine laser lines, for the concentration of H2O, CO2,NH3, O3 and C2H4 with a 5 min time resolution.

5 BIOLOGICAL AND MEDICALAPPLICATIONS

5.1 Postharvest Physiology

To delay the effects of aging and ripening, many harvestedcrops are stored under reduced oxygen concentrationsthereby lowering the respiration rate and the rate atwhich metabolic processes take place. However, whenthe oxygen concentration falls below a certain limit,crops switch to fermentation, an alternative methodto generate the energy needed to sustain their vitalfunctions. This anaerobic metabolism, converts glucose,via several intermediate steps, into acetaldehyde whichis then quickly reduced to ethanol. As accumulation ofthese products in the tissue may affect the quality of thecrop, it is necessary to control the rate of fermentationduring storage. The concentrations of the volatile in the

headspace is an accurate quantitative indicator to assessthe rate of fermentation.

Another gas of interest during crop storage is theplant hormone ethylene, which is mainly emitted dur-ing ripening of climacteric fruits, i.e. fruits whichshow a sudden rise in respiratory rate just prior tofull ripening. Furthermore, carbon dioxide may beanalyzed to assess both respiration and fermentationrates and ethane may be analyzed as the end prod-uct of lipid peroxidation causing breakdown of cellmembranes.

The noninvasive nature of the photoacoustic tracedetection technique renders it extremely suitable forapplications in the field of postharvest physiology.Moreover, the high sensitivity combined with fast analysisallows study of an individual piece of fruit so thatmetabolic activity may be followed in real time. Here wegive an overview of some recently performed experimentson single pieces of harvested crops. In general, the crop isplaced in a glass container which is connected to the flow-through system leading the released metabolites to thephotoacoustic cells. Storage conditions are simulated bysupplying the sample with a premixed flow of oxygen andnitrogen (Figure 8); the storage temperature is controlledusing a refrigerator.

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O2

O2

N2

N2

10

1012

13

1

1 2 3

4 5

7

6

2

3

11

9

8

cell 1 cell 2 cell 3

Figure 8 Laser photoacoustic detection set-up. 1, 2 and 3, trace gas flows; 4, triple photoacoustic cell; 5, liquid nitrogen cooled COlaser; 6, grating to select the appropriate transition; 7, chopper; 8, catalyst to remove hydrocarbons from the flow toward the fruit;9, KOH scrubber to remove CO2; 10, switching valve for (an)aerobic conditions; 11, cuvettes, one containing a tomato, the otherempty as reference; 12, switching valve to select cuvette; 13, cold trap to freeze out interfering gases selectively.

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14 ENVIRONMENT: TRACE GAS MONITORING

The ethylene emission of climacteric fruit shows adistinct rise during ripening, reaching a maximum whenthe sample is at its best; subsequent degradation of theproduct accompanies a decline in the ethylene release.To localize the ethylene release of different crops, DeVries et al. applied the extreme sensitivity of CO2-laser based photothermal and photoacoustic detectors..79/

Huge differences in emission pathways were foundranging from more than 99% through the skin for banana,apple and citrus fruits, to around 85% through the pedicel‘‘stem’’ for tomato and bell pepper. In addition, theamount of ethylene released varied from a few tenths toa few thousands of nanoliters per hour per fruit.

When stored crops are returned from controlled atmo-sphere (CA, low oxygen, high CO2, low temperature)conditions to normal atmospheric conditions, a rapidpeak in the acetaldehyde release occurs due to oxidationof the ethanol accumulated in the tissue. This process wasfollowed in real time for red bell peppers by Zuckermannet al. using a CO laser-based photoacoustic detector..80/

Returning a pepper from anaerobic conditions to normaloxygen conditions (21%) caused a peak in the acetalde-hyde release with a maximum of about four times theanaerobic level occurring within 25 min after the switch.

The high velocity with which the process takes placecaused the authors to conclude that the effect is mainlydue to the peroxidative action of the enzyme catalase..81/

Further investigations.82/ have shown that the acetalde-hyde peak occurs even at much lower postanaerobic O2

concentrations with roughly equal intensity, though notas fast (Figure 9).

During CA storage of apples it is common practiceto apply low concentrations of carbon dioxide externallyas it is known to inhibit respiration. As both respirationand fermentation produce carbon dioxide, it is difficult topredict a priori how the fine balance that exists betweenrespiration and fermentation at low O2 concentrations,will be affected. For pears, at oxygen levels around1% a substantial rise in the acetaldehyde release couldbe observed upon addition of a few percent of CO2.The crop is no longer able to retrieve its energy fromrespiration and is forced to generate additional energythrough fermentation.

5.2 Plant Physiology

One of the major environmental factors influencing cropproductivity is flooding. Tolerant species often possess

0

0

100

200

3001

10

100

400

500

600

5 10 15 20 25

0.4% O2added

Ace

tald

ehyd

e (p

pb)

Pepperinserted

2 4 6 8

Time (h)

Figure 9 Emission of the precursor of ethanol in the fermentation process, acetaldehyde, is followed on-line while anaerobicconditions are imposed on a red bell pepper. Adding only 0.4% of O2 results in a strong postanaerobic effect due to the oxidationof ethanol by transient free oxygen radicals in the tissue.

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PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 15

‘‘escape’’ mechanisms such as fast underwater growththat minimizes the duration of total submergence..83/

However, at early vegetative stages, even in ‘‘deep-water’’ types of rice, such escape is precluded by a lackof sufficiently vigorous shoot elongation for resurfac-ing. Survival of submergence under these circumstancesdepends on intrinsic physiological and biochemical toler-ance. Screening of cultivars is needed by plant breeders,agrotechnology and biotechnology to achieve sustain-able improvements in productivity of rice farming inthe rainfed lowland of southeast Asia..84/ Submergenceis not an easily quantified stress compared with, forexample, cold, heat, or pollution, since it cannot beassessed by one simple analysis. This is because itinvolves exclusion and entrapment of several key gasesand interference with illumination. Normal rates of res-piration and photosynthesis are affected; differences insubmergence tolerance are also reflected in a differentreaction to low oxygen concentrations. Precise measure-ments of fermentation rates can result in a powerfultool to shed some light on submergence stress. As men-tioned in the previous chapter anaerobic fermentationin plant tissue can be studied by use of photoacoustictechniques, through detection of ethanol, acetaldehydeand CO2 by means of a liquid nitrogen cooled COlaser. Experiments in the laboratory show that seed andseedlings of rice species with different degrees of toler-ance toward submergence possess different fermentationrates under anaerobic conditions. Figure 10 shows thatafter a period of 5 h in which both species have simi-lar ethanol production rates, the most tolerant species(FR13A) starts to slow down fermentation. After about20 h the production rate for the less tolerant species(CT6241) is doubled with respect to the other; it probably

0 4 8 12 160

50

100

150

200

250

Time (h)

Eth

anol

(nL

h–1

see

d–1 )

Figure 10 Production rate of ethanol plotted against time forrice seeds during germination under anaerobic conditions. Theflooding resistant species (FR13A)(�) does not ferment as muchas the intolerant species (CT6241)( ) indicating that it preservesits energy for the postanaerobic period.

runs out of storage material much faster than the resis-tant one.

Besides fermentation, the availability of oxygen in theroots is of major concern for the plant. In case of partialor complete submergence, rice plants must rely on a well-developed system for gas transport through the plant. Thereal path of the gas (especially in the shoots) and how thischanges with long periods of submergence is unknown.Photoacoustic trace gas detection can also be very usefulin this case: a known amount of the tracer gas SF6

(biologically inactive; atmospheric lifetime 3200 years)is injected into the nutrient solution around the roots anddetected coming out of the tip of the leaves, while the restof the plant is completely submerged in water. We useSF6 as the tracer gas because the CO2 laser photoacousticsystem is particularly sensitive to this gas (down to partsper trillion level). Figure 11 represents a typical outputfrom the tip of a leaf (enclosed in a cap) after the injectionof 4.5 mL of H2O saturated with SF6. The plant is keptunder water during the experiment.

A question we can readily answer with our systemis what kind of gas transport is predominant in theshoots of a young rice seedling. It could be eitherthrough aerenchyma (i.e. intercellular spaces present inroots) or through the air layer that is wrapped aroundthe hydrophobic surface of the leaves. This air layeris destroyed by dissolving a few drops of detergentin the water surrounding the plant; because the SF6

signal dropped to zero immediately afterwards, we canexclude the presence of aerenchyma in the shoots of theseedling.

00

1

2

SF

6 (p

pb)

3

4

1 2

Clean waterInjection 4.5 mL

SF6/H2O

2-mL detergentSDS 10%

2-mL detergentSDS 10% 3-weeks old

rice seedling(FR13A)

3

Time (h)4

Figure 11 Depletion of the air layer around the submergedrice leaves by addition of detergent. The effect was monitoredby SF6 diffusion which was added into the root compartment(physically separated from the shoot compartment). SF6 wascollected from a small cap filled with air around the tip of a leaf.The air layer is completely restored by replacing the detergentwith clean water. During the whole experiment the plant waskept completely submerged.

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16 ENVIRONMENT: TRACE GAS MONITORING

Another example of trace gas detection within plantphysiology is the intriguing question of whether ethyleneis a reaction to the growth of the pollen tube insidethe pistil (i.e. a form of wounding) or if the pollen tubegrowth is a consequence of the hormone production bythe flower (i.e. a real signal for the plant). It is known thata pollinated flower emits ethylene at the parts per billionlevel, with a characteristic double peak shaped curve..85/

To answer this question, incongruous pollination has beenperformed on tobacco flower (Nicotiana tabacum), i.e. bypollinating the flower with pollen from other species thatshow a certain degree of compatibility with the tobaccoplant. For this we used pollen with a very low compatibilityto the tobacco flower Nicotiana repanda; it enters thestigma and stops immediately thereafter. The productionof ethylene, detected by photoacoustic spectroscopy, inthese cases is very different, as shown in the Figure 12.

The first peak is always reproduced in both heightand time and, comparing these results with microscopywork,.86/ it corresponds to the entrance of the pollentube in the stigma. The second peak has a morecomplicated shape which is never perfectly reproducedwith incongruous pollination and which does not simplyfollow the growth of the pollen tube in the pistil. Thismight indicate that the second peak is a specific signalfrom the plant and not a simple wounding effect.

5.3 Microbiology

Biological systems cannot always use molecular nitrogenfor their metabolic processes. To this end, dinitrogen

0

4

8

12

16

20

24

Eth

ylen

e (n

L h–

1 )

0 20 40 60 80 100 120

Time (h)

Figure 12 The in vivo production of ethylene during polli-nation of the Nicotiana tabacum flower with two types ofpollen: Nicotiana tabacum and the incongruous pollen Nicotianarepanda. The latter enters the stigma and stops immediatelythereafter, whereas the ‘‘right’’ pollen grows down the pis-til into the ovary for fertilization. �, Nicotiana trigonofilla; ,N. repanda.

(N2) must be reduced, i.e. the triple bond must bebroken to convert the molecule into more appropriatecompounds such as ammonium (NH4

C), nitrate (NO3�)

or amino acids. Only few organisms are capable ofreducing dinitrogen; among them are the diazotrophiccyanobacteria..87/ The nitrogen fixation process is inducedby the enzyme nitrogenase, which is very sensitive at lowoxygen concentrations. However, like other organisms,cyanobacteria need energy which they derive fromphotosynthetically produced oxygen. In order to activateboth photosynthesis and nitrogen fixation, these processesare separated either in time or in place..88,89/ Nodulariaspumigena belong to the last category. They consist oflarge strings of cells formed mainly by vegetative cells inwhich photosynthesis takes place alternated every 10 to20 cells by one heterocyst in which the nitrogen fixationtakes place..90/

The acetylene (C2H2) reduction method is commonlyused to follow the process of nitrogen fixation..91/ Asfor N2, the triple bond of acetylene is broken andreduced to ethylene (C2H4).92/ which can be detected verysensitively with laser photoacoustics..52/ In the past, gaschromatography utilizing flame ionization detection andphoton ionization detection were applied to investigatethe nitrogen fixation process..91,93/ Long incubationtimes, up to several hours, are necessary to reach gasconcentrations observable for the equipment makingdynamic measurements unfeasible.

Laser photoacoustic experiments have been performedto study the dynamic behavior of nitrogen fixation..94/

To avoid the buffering volume of the water, the algaeNodularia spumigena were put on filter paper and agas mixture of O2 and N2 with different concentrationsof acetylene was flushed over the sample. In this waythe effect of parameters such as light intensity andtemperature could be studied with a 20 s time resolution.

Utilizing the CO laser as radiation source instead ofthe CO2 laser in the photoacoustic spectrometer, it ispossible to detect other interesting gases. Methane anddimethyl sulfide emission in rice field paddies forms animportant threat to the environment. Methane is, afterCO2, the second most efficient greenhouse gas whosebiological production from the man-made rice cultures,needs further clarification..95,96/ Dimethyl sulfide is aclimate-active natural marine emitter and responsible forsulfur transport into the atmosphere. The fact that bothgases can be detected very efficiently by photoacoustictechniques, at the water surface as well as at various waterdepths, forms an important challenge for future activitiesat field sites.

5.4 Human Health, Noninvasive Breath Analysis

Since ancient times it has been known that the smell ofexhaled air can be used as an indicator of several processes

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PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 17

taking place in the human body;.97,98/ uncontrolleddiabetes produces a sweet fruity odor, advanced liverdiseases entail a musty fishy reek, failing kidneys bringabout a urine-like smell and a lung abscess can bebrought to light by its putrid stench. In the past,several attempts to use trace gas detection of exhaledair have been performed with variable success becauseof the nonsufficient detection limits of the detectorsavailable. Gases produced in the body are transportedto the lungs and are diluted (at rest, healthy personsexhale approximately 1000 L h�1) before being exhaled.To measure them, it is necessary to enrich the air samplesby adsorbing the gases on an agent and releasing theconcentrated gas at a certain moment rendering themeasurements less accurate and difficult to reproduce..99/

Measurements on the effects of UV radiation on thehuman skin are presented to illustrate the possibilities ofphotoacoustic trace detection in this field. In these experi-ments lipid peroxidation of the skin is monitored..100/ UVradiation causes reactive oxygen species to be formedin the skin..101/ These radicals can damage the lipids inthe cell membranes producing small hydrocarbons such asethane, pentane and ethylene..102/ With the CO2 laser set-up it is advantageous to study these latter molecules..21,52/

A small amount of air is sampled from the exhaled airand cleaned from CO2 (typically 5%), water vapor andother spectroscopically interfering gases like acetone andethanol. The test persons are measured while restingunder a solar bench. Under UV radiation a steadyincrease of the exhaled ethylene is observed with a2 min time delay (Figure 13). The increase continues untilthe solarium is switched off after 15 min (the maximumexposure time advised). During the subsequent decreaseof ethylene emission two decays can be distinguished;a fast and a slow decay. The first decay is caused bythe washout of ethylene from the blood; the seconddecay results from ethylene stored in the body tissue.The dynamics of the system, i.e. transport of gasesthrough the body, yields information for pharmakineticresearch. In order to determine the locally induceddamage, a specially designed cell was placed on theskin. Immediately after the start of the UV exposurea steady and constant production of ethylene in theskin was observed. Combining the two results, it isconcluded that the slow increase in ethylene signal inthe exhaled air is caused by the buffering effect of thehuman body.

Apart from the effect of UV radiation on the skin,other effects induced by lipid peroxidation can be studied:patients suffering from chronic obstructive pulmonarydisease have increased cell membrane damage in thelungs; patients with pancreas problems cannot digestlong-chain fatty acids, resulting in a lack of ethylenein the exhaled air after consumption of these fatty

Time (h–1)0.0 0.2 0.4 0.6

C2H

4 (p

pb)

3

2

1

UV exposure

UV shielding

(a)

Time (h–1)1.0 1.2 1.4 1.6

C2H

4 (p

pb)

6

5

3

2

4

UV on

UV off

(b)

Figure 13 Ethylene emission from the human lungs inducedby UV radiation from a solar bench. As a product of lipidperoxidation ethylene is found in low amounts in the exhaledair (a) and directly on the skin (b) utilizing a 5 cm diametersample cell. The time during which the skin is irradiated (intotal 15 min) is indicated by the black bar. Shielding the bodyfrom UV radiation using Lexan prevents generation of ethylene.

acids; patients suffering from cardiac insufficiency haveincreased ethylene levels in the exhaled air.

5.5 Entomology

Methane has been recognized as one of the principalgreenhouse gases, second only to CO2. Its estimatedcontribution to the enhanced greenhouse effect variesbetween 15–20% (Intergovernmental Panel on ClimateChange, 1990) depending on the time window forwhich the calculation is made. Methane is not onlya potent greenhouse gas, it is also chemically activein the atmosphere thus influencing concentrations ofseveral important species, e.g. OH, O3 and CO. Biogenicsources are responsible for the major part of theglobal methane flux (>70%)..103/ Some arthropods (i.e.millipedes, cockroaches, termites and scarab beetles)

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18 ENVIRONMENT: TRACE GAS MONITORING

0.0

0.5

1.0 CH4(µmol h–1)

0

50

25

75 CO2(µmol h–1)

5.2 5.6 6.0 6.4

Time (h–1)

Figure 14 Methane release together with the CO2 emission, during a complete respiration sequence of breathing (starting at 5.15 hand 6.08 h) and constriction (starting at 5.48 h and 6.32 h) for the cockroach species Gromphadorhina portentosa.

are believed to contribute up to 25% of the totalbudget..104/

Gas chromatography allows measurement of themethane release of single animals only after incubationfor several hours. Consequently these measurementscannot provide information about the dynamic characterof the gas emission. Infrared absorption measurementsfeaturing fast response time permitted observation of thedynamics of CO2 emissions.

Besides CH4 and CO2 emissions from insects, watervapor release has been the subject of many studies.Information about water loss dynamics in insects isbased upon theoretical modeling, sensitive weighing andrelatively slow direct water vapor measurements..105 – 108/

With laser photoacoustic detection Bijnen et al. showedthat cockroaches at rest show a regular breathingpattern that optimizes oxygen uptake and reduces waterloss..26/ Respiration patterns of cockroaches and beetleshave been recorded where microbiologically producedmethane was found to be co-emitted with CO2 andwater vapor (Figure 14). This on-line observation is

to be utilized in a study to establish conditions forhyperventilation of insects so that the minimum amountof insecticide will suffice to kill noxious insects which, forexample, are known to transmit viral diseases.

6 COMPARISON WITH OTHERSPECTROSCOPIC METHODSIN TRACE GAS MONITORING

Fast and sensitive trace gas detection is not exclusivelyreserved for photoacoustics. In general, all spectroscopictechniques are able to monitor gas absorptions on asubsecond timescale. However, depending on the type ofapplication not all techniques possess a high sensitivity.

Air pollution can be very well studied with long-path light absorption techniques rendering low detectionlimits; the sensitivity of the method relies strongly onthe path length. A broadband light source (often UV) isdirected towards a receiver over an open path in theatmosphere..22/ For stratospheric studies the sun has

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PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 19

also been used as a light source applying a 0.01 cm�1

resolution Fourier transform spectrometer on board thespace shuttle as part of the Spacelab 3 mission..109/ Laser-based long-path absorption techniques are used eitherwith multipass arrangements, LIDAR (light detectionand ranging) or the related DIAL (differential absorptionlidar) methods..22/ In contrast to CW operation, usingpulsed lasers has the advantage that the backscatteredlight gives spatial information about the atmosphericabsorption pattern..110/

All methods described above derive their sensitivityfrom long-path absorption thereby integrating overhundreds of meters. Laser-based photoacoustic detectionhas the advantage of tracing gases locally and cantherefore also be used in laboratory studies. Otherlaser-based techniques which can also achieve a highsensitivity within a small volume of gas are intracavitylaser absorption spectroscopy, which relies on very longeffective absorption length within a high optical Qfactor laser cavity;.111/ cavity ring down spectroscopycombines very accurate decay time measurements oflight pulses and a high Q optical cavity resulting inlong effective absorption lengths..112/ In comparison withthese techniques laser photoacoustics has the advantagethat it is background free: it does not rely on a decreaseof the transmitted light but on an increase from thezero baseline, i.e. on a collisional release of energy afterabsorption.

Other background-free and therefore strongly com-petitive techniques are laser-induced fluorescence (LIF)and multiphoton ionization (MPI). These techniques areboth extremely sensitive due to the high collection effi-ciency of photomultipliers and ion counters; both can beapplied locally within a small gas volume. LIF and laserphotoacoustics are complementary in energy release. Theadvantage of LIF is that it can also detect free radicals(e.g. OH) with relatively low laser powers. MPI can bevery selective and sensitive for molecules with a highermass. An elegant example is the detection of OH with LIFin the upper atmospheric layers with an aircraft, avoidingspectroscopic cross interferences from other molecules,laser-induced generation of OH, depletion of OH duringsampling, etc..113/ A restriction of LIF is the lifetime of theexcited levels. As stated in the introduction, the compe-tition between collisional relaxation and radiative decayrestricts LIF mainly to the visible and the UV part of theelectromagnetic spectrum.

ACKNOWLEDGMENTS

The authors wish to thank D.H. Parker and J. Reussfor their helpful discussions and continuous sup-port. We are also grateful to H. Zuckermann (Open

University of Israel), H.S.M. de Vries, J. Oosterhavenand R.H. Veltman (ATO-DLO) for their collabora-tions in postharvest research; M.B. Jackson (Univer-sity of Bristol, UK) for his indispensable contributionin the rice plant research; C. Mariani (University ofNijmegen), D. de Martinis (ENEA, Italy) for their pol-lination research; M. Staal, L. Stal (NIOO) for theircontributions in the field of microbiology; R. Berkelmans,K. Kuiper, P. Hollander (Free University, Amsterdam),P. Scheepers, R. Dekhuijzen (University of Nijmegen) fortheir help in breath analysis; and P. Kestler (Universityof Osnabruck, Germany) and J. Hackstein (Univer-sity of Nijmegen) for their collaborations on entomo-logical applications. Furthermore we thank F. Bijnen,S. Persijn, T. Groot, E. Santosa for their experimentalsupport and C. Sikkens, C. Timmer, H. Schoutissen andL. Gerritsen for technical assistance. Finally we wouldlike to thank the Dutch Technology Foundation (con-tracts no: NNS44.3404, NNS.4596 and NNS55.3921), theDutch Royal Academy of Science (contract 95BTM04)and the European Union (contracts no: BRFSCI-CT91-0739, ERB3514-PL95-0708, PL95-0468 and ERB4062-PL970089) for their financial support.

ABBREVIATIONS AND ACRONYMS

CA Controlled AtmosphereCW Continuous WaveDIAL Differential Absorption LidarFTIR Fourier Transformed InfraredLIDAR Light Detection And RangingLIF Laser-induced FluorescenceMPI Multiphoton IonizationOPO Optical Parametric OscillatorPPLN Periodically Poled Lithium

NiobateUV Ultraviolet

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Environment: Trace Gas Monitoring (Volume 3)Environmental Trace Species Monitoring: Introduction žUltraviolet/Visible Light Detection and Ranging Appli-cations in Air Monitoring

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20 ENVIRONMENT: TRACE GAS MONITORING

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Food (Volume 5)Infrared Spectroscopy, Gas Chromatography/Infrared inFood Analysis

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