Sensors andand Materials,
Materials, Vol.
Vol. 17,
17, No.
No. 77 (2005)(2005) 355– 364
Sensors
MYU Tokyo

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S & M 0613

“Electronic Nose”—A New Monitoring Device forEnvironmental Applications
Krishna C. Persaud*, Peter Wareham,
Anna Maria Pisanelli and Emmanuel Scorsone
School of Chemical Engineering and Analytical Science, The University of Manchester,
PO Box 88, Sackville Street, Manchester M60 1QD, UK(Received February 21, 2005; accepted May 9, 2005)
Key words:

odour sensing, sensor arrays, environmental monitoring

Environmental applications of odour sensing technology have been limited by problems of achieving stable measurements when confronted with highly variable environmental factors. We present three case studies for odour sensor technology applied to (a)detection of dry rot in buildings using a sampler and preconcentrator based on solid phasemicroextraction coupled to a metal oxide sensor array; (b) continuous monitoring of theodour of waste water; (c) discrimination of volatiles emitted by fires in the development ofa smart fire alarm system. Each of these applications has required understanding of thenature of the volatiles to be detected and discriminated, specific sampling methods anddedicated sensors, electronics and software.

1.

Introduction

The remarkable capabilities of biological chemosensory systems in detection, recognition and discrimination of complex mixtures of chemicals have stimulated the imaginationand interest of many researchers and commercial organisations for the development ofelectronic analogues. The responses of individual odour sensors combined into an array,
where each sensor possesses slightly different response selectivity and sensitivity towards
the sample odours, when combined using suitable mathematical methods, can provide
information to discriminate between many sample odours.(1) Arrays of gas and odour*Corresponding author, e-mail address: krishna.persaud@manchester.ac.uk

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sensors, made from different technologies, are finding their way into a variety of specialised
applications.(2) Some motives arise from the perceived limitations of traditional analyticalchemistry and instrumentation in classification of gas mixtures, or odours; applications
where the gestalt of a defined mixture of chemical species may be important in perception,
and where it is important to separate subjective and objective assessments; and also thecommercial drive to achieve devices capable of operating rapid, on-line process-measurement and control in areas of foods, beverages, chemical industries, and waste management.
Such devices have become known as “electronic noses” and consist of three elements: asensor array which is exposed to the volatiles, a converter of the sensor signals to a readable
format, and software analysis of the data to produce characteristic outputs related to theodour encountered. The output from the sensor array correspond to patterns of informationthat may be interpreted using a variety of methods such as principal component analysis,
discriminant function analysis, cluster analysis and pattern recognition algorithms such as
artificial neural networks to discriminate among samples.
Sampling methodology, sensors and material science, electronics and software techniques are important aspects of the development of the systems for any practical application. The selection of an effective set of descriptors of the patterns in question is highly
important. Where an odour is concerned, the problems to be addressed are whichcharacteristics of the molecules can be measured, and how can the resulting pattern be
encoded. The selection of a procedure for processing the parameters chosen to represent
the pattern is required to optimise the parameter values. This increases the resolution of thesystem so that certain representative pattern classes may be differentiated from others.
Solving these problems for diverse practical applications can be difficult.
In this paper, we describe three different environmental applications for such technology and illustrate how the problems were resolved. Common aspects of all three applications are the use of chemoresistive gas sensors using either metal oxide sensors or
conducting polymer sensors. A microprocessor-based system that measures gas sensor
resistance changes was developed. The types of sensor used were different for eachapplication. A micro-controller collected real-time data from resistance interrogation
circuitry and controlled a multiplexer, sequentially selecting different sensors from thearray. Data are either stored for later download or transferred to a host PC via an RS232
serial communication protocol.

2.

Detection of Dry Rot Infections in Buildings

We aimed to develop a portable sensing instrument that can be used by property
surveyors in the field to detect of volatiles at very low concentrations that are emitted fromfungal infections of Serpula lacrymans in buildings. Because of the low concentrations ofvolatiles, some mode of preconcentration of the volatiles during sampling was essential forthe success of the project. In this case we chose solid phase microextraction as an optimalmeans of preconcentration. Our approach was the analysis of dry rot chemical species andthe identification of markers. This involved growth of dry rot in culture medium,
innoculation of wood —pine and oak— followed by GC/MS analysis of volatile components, identification of key marker compounds, reconstitution of a dry rot surrogate
mixture, and confirmation of findings from field tests.(3)Sensors and Materials, Vol. 17, No. 7 (2005)

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2.1 GC-MS Headspace analysis
In the first instance it was important to determine which key volatile components could
be used as markers for the volatiles emitted from the fungal infections. We found three
SPME fibre types more applicable for this work. These consist of 100 μm
polydimethylsiloxane (red hub), 65 μm polydimethylsiloxane/divinylbenzene (blue hub)and 75 μm carboxen/polydimethylsiloxane (black hub), all available from Supelco (UK).
For this study, these three SPME fibres were used to concentrate the head space volatiles
from the samples. The analysis was carried out with an HP-GC 5890/MS on a ZB-1701(14% cyanopropylphenyl, 86% dimethylsiloxane phase) column.

2.2 Procedure
Samples were placed in a 20 ml headspace vial. In the case of cultured samples,
samples were placed in a larger wide-mouthed container, with the headspace extracted via
a pre-drilled septum. The Teflon-lined septum covering the vial was pierced and the fibre
was exposed to the sample headspace for 30 min. The fibre was then retracted into theneedle and immediately transferred to the gas chromatograph and desorbed for 5 min in the
GC injection port. Compounds were resolved on a ZB-1701 column (30 m × 0.25 mm ×0.25 μm) under the following conditions: injection port temperature 250°C; helium flow
2.5 ml/min; oven temperature program 40°C (4 min hold) then 4°C/min to 220°C.

2.3 Results
Results from GC-MS analysis indicated that the presence of 1-octen-3-ol (mushroom
odour) and 3-octanone (resinous odour) appear to be common to all samples of dry rot,
whether cultured, recovered or distilled, and to a slightly lesser extent, 3-octanol was also
identified. Where dry rot fungus is present on pine wood substrate, additional compounds
also appear in the headspace including α-pinene (pine odour), hexanal (cut grass odour)and carene (lemon odour), likely originating from the wood itself.
For a practical device that could be used in the field, we developed an automated system
for sampling and preconcentration of these volatiles that was incorporated into a probe that
could be inserted into crevices or cracks and be used to probe wall cavities or spaces under
floors. This was based on the solid phase microextraction fibres that were encapsulated
into a probe (Fig. 1(a)) that allowed rapid sampling of odour at very low concentrations
from previously inaccessible locations.
For this application —because of the requirements that volatiles be thermally desorbed
from the SPME fibres— we chose commercially available metal oxide sensors such as
Taguchi gas sensors (TGS) (Figaro Inc., Japan) or Capteur sensors (now City Technology,Ltd.) as the core sensing element for an array of odour detectors. These devices consist ofan electrically heated ceramic pellet onto which a thin porous film of SnO2 doped withvarious precious metals has been deposited. The doped SnO2 behaves as an n-typesemiconductor and the chemisorption of oxygen at the surface results in the removal ofelectrons from the conduction band. Gases interact with the surface adsorbed oxygen andthereby affect the conductivity of the SnO2 film. The devices are run at elevatedtemperatures (typically 300–400°C) to achieve rapid response/recovery times and to avoid
interference from water. This results in relatively high power consumption. The response

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Sensors and Materials, Vol. 17, No. 7 (2005)(a)

Response(b)Time (s)(c)
Fig. 1. (a) Portable field sampler based on a solid phase microextraction. (b) Design of themeasurement system (inset -SPME fibre). The sensors are mounted in a symmetrical 3-dimensional
array within a stainless-steel header block. The array encloses a small headspace volume that is
rapidly heated to elevated temperatures by the sensors themselves. The header block allows theintroduction of the SPME fibre. The sample volatiles are thus desorbed directly into the enclosed
region between the sensors as shown. (c) The desorbed volatiles are sensed and the resulting
response profile depends on the selectivity and sensitivity of the sensors to the mixture of volatiles.

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characteristics can be tailored by varying the operating temperature and the doping agent.
The final measurement system was based on an array of metal oxide sensors (Capteur,UK), incorporated into a suitable header (Fig. 1(b)) so that when a SPME fibre wasinserted, thermal desorption of trapped volatiles occurred and dynamic responses of thesensors could be recorded (Fig. 1(c)). A neural network based on radial basis function
architecture was utilised to give real-time classification of the incoming volatiles andsignal the presence of dry rot.(5)Two hand-held protototype devices were built and tested in the field and gave similar
results. Greater than 70% positive detection of dry rot was achieved in the field since Feb.
2003. The automated sampler device developed was tested in the field with excellent
results.

3.

Monitoring Waste Water Volatiles

The abatement of odour nuisance and increased efficiency within a plant is now seen asa major issue for water companies. The development of green field sites for sewage
treatment or the major refurbishment of existing treatment plants requires that the company
considers the problem at the planning stage and shows that the “best available technology”
is being used.
Spot measurements of a single compound, hydrogen sulphide, previously used, wererecognised as not effectively describing the multi-component sludge and sewage odours.
In recent years chemical analysis has been supplemented with dynamic dilution olfactometric measurements of odour concentration of the emissions.(6) This method is time
consuming and costly and provides only a snap-shot of the emission. Continuous monitoring by any of these techniques is not representative or feasible.
To meet the needs of water companies, monitoring equipment is required that is bothreliable and robust and that can objectively and continuously monitor odours. If thetechnology could be applied using distributed systems, this would enable individual
processes to be monitored, optimised and controlled. The system may have wider
application where volatile compounds are indicators of process variations and, being noninvasive, should have a high reliability.
We have been developing instrumentation that is capable of sensing and recording inreal time changes in “odour” correlated to concentrations of volatile organic chemicals
emitted at a sewage site. This technology is of interest because it does not only usehydrogen sulphide monitoring as a surrogate for odour emission, but it is able to sense a
broad spectrum of organic chemicals that are emitted.
The equipment has been tested for monitoring the inlet stream of a sewage plant. It iscapable of transmission of data in real time to a computer screen in a control room. The
software produces a strip chart display of odour changes with time, and set points andalarms can be programmed remotely (Fig. 2). This can be used to trigger collection of asample via an autosampler, to control a valve for chemical release, or to control diversion
of a stream to a holding tank.
Odours from sewage treatment plants arise principally from the biological degradationof the constituents of domestic sewage and are particularly associated with anaerobic

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Sensor Response ΔR/R (%)

Sensors and Materials, Vol. 17, No. 7 (2005)

Time t (h)Fig. 2. Continuous monitoring of odour at the inlet of a sewage plant. The trace shows a 24 hrecording using an array of four sensors sensitive to sulphurous odours. Odour events are recorded inreal time; peaks are observed between between 0800–1100 and 2100–2200 h in this trace.

microbial activity. Additional odours may arise directly or indirectly from industrial
wastewater or other discharges to sewers. Odorous components such as solvents, petroleum derivatives and other volatile organic compounds contribute directly to an odour
problem, whereas indirect odour formation can be caused by highly degradable or warm
effluents, or effluents with high sulphur content. In this case we had to choose a sensor
system capable of measuring volatile organic chemicals, but which would not be degradedby the high sulphur content of the environment.
The requirements were for a device to monitor the odour of waste water at the inlet ofa sewage plant, where continuous measurement of the organic load can be measured. The
requirements are stringent —the device has to operate continuously in real-time transmitting odour concentration data so that abnormal organic or industrial loads are detected intime. Because this device needed to be operated in a very harsh outdoor environment, thepackaging requirements were also stringent.
For this application —because of the constraints implied in using systems in an outdoor
environment where temperature and humidity would vary greatly— we have chosencommercially available metal oxide sensors such as Taguchi gas sensors (TGS) (Figaro
Inc., Japan) or Capteur sensors (now City Technology Ltd) as the core sensing element for
an array of odour detectors.
We constructed microprocessor based electronics to measure the percentage change inresistance of the sensor array in response to volatile organic chemicals in the environment
and incorporated automated sampling and filter systems to optimise the performance of the
instrument in a harsh environment. These have proven to be robust in monitoring changes
in odour level over time (see Fig. 2).

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4.

361

Smart Fire Detection Systems

Fire detectors such as discrete heat, ionisation or optical fire detectors are widely used
in industrial premises. These detectors use a single sensor and generally are not able todiscriminate between real fires and nuisance alarms. Fire brigades deplore a large numberof false alarms that can be costly in both financial and human terms. Sensor combinations
including gas sensors and specific signal processing algorithms have been used to address
this problem. For instance, carbon monoxide sensors have been added to standard fire
detectors since CO is generated in most fires.(7) However, all other sources of CO such as
cigarette smoke or diesel engines potentially cause false alarms. Only the combination
with another sensor type may avoid a false alarm. Another approach is to measure volatileorganic compounds (VOCs) in smoke as they are produced in large numbers during
combustion and can constitute “fingerprints” of fires.(8) This could be achieved at low costusing an e-nose with a dedicated sensor array for smoke detection. In this paper, we report
the development of an e-nose for fire detection based on an array of eight sensors. One
requirement of such a device is low power consumption, as it should run continuously andremotely from power mains or with a battery backup. Sensors made of conducting
polymers (CP) based on aromatic or heteroaromatic compounds are therefore potentiallysuitable, as they can operate at room temperature. A big problem in fire detection is that offalse alarms, and many optical detectors do not discriminate between different types ofsmoke. GC/MS analysis indicates that the odorous volatile profiles emitted from different
type of standard fires can be characterised. We have developed a conducting polymer
sensor array that allows discrimination of these profiles (Fig. 3).

4.1 Analysis of smoke and identification of possible markers
Four common fires (wood, paper, cotton, polyurethane) and a common source of
nuisance (cigarette smoke) were generated in a purpose-built fire cabinet. Smoke wasanalysed using a portable Fourier transform ispectrometer GASMET DX-4000 (Ansyco,
Germany) and a model Saturn 2200 gas chromatograph—mass spectrometer (Varian)equipped with a splitless injector, 30 m ZB1701 capillary column (Phenomenex) and aflame ionisation detector. Samples for GC-MS analysis were collected using solid phasemicro-extraction (SPME) fibres. SPME fibres were obtained from Supelco (UK) and havea stationary phase 75 μm thick made of carboxenTM/polydimethylsiloxane.

4.1.1 Instrumentation
The prototype system incorporated a microprocessor-based system that measures
sensor resistance changes, as described earlier. A dedicated sampling system has been
built that includes a small pump and filters that protect the pump and the sensors fromparticulate materials in smoke.

4.2 Results
A number of markers have been identified for the five fire or nuisance events investigated and they are listed in Table 1. The sensors were interrogated with 10 ppm of a marker
from each test fire. Fig. 3(b) shows the sensor array response of the system to hexanoic

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Sensors and Materials, Vol. 17, No. 7 (2005)% Change in resistance(a)Time (s)(b)
Fig. 3. (a) Conducting polymer sensor array developed for fire detection. (b) Response of the array
to a pulse of 10 ppm hexanoic acid, a marker for smouldering wood.
Table 1Markers identified in smoke from four test fires and cigarette smoke using GC-MS/FTIR analysis.
Test fires
Smouldering cotton
Smouldering wood
Smouldering paper
Cigarette smoke
Flaming polyurethane

Markers
furfural, 2-(3H) furanone
hexanoic acid, guaiacol and heptanoic acid
acetohydroxamic acid, hydrochloric acidpyridine, limonene, nicotine, isoprene,ammonia
benzonitrile, NO, NO2, N2O, HCN

acid, a marker for smouldering wood. Different response patterns were obtained for eachmarker tested. The responses of an in-line FTIR spectrometer and optical particulate
detector sensors (Fig. 4 (top) are compared to the conducting polymer array (Fig. 4(bottom)) for volatiles emitted from smouldering wood. A time course from the start of
combustion is shown in the figure. The data indicate that the conducting polymer array
may be just as sensitive as the conventional detectors of particulates but may also provide
further discriminatory information.

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Fig. 4. Example of gas sensing using a conducting polymer array (bottom), VOC measurements
using an FTIR (top), and optical particulate detector (Wagner GmBH) (top—bold trace), obtained forsmouldering wood (a: fire starts, b: detection limit of optical detector, c: “fire alarm” according toparticulate detector).

5.

Conclusions

We show that odour sensor array-based technology can be applied to a variety ofenvironmental applications providing the problems to be solved are well understood.
When coupled to suitable sampling systems and pattern recognition software, odour sensor
array technology can be used to produce instruments that fulfil a variety of sensing needs
where a change of condition needs to be monitored.

Acknowledgements
This work was supported through a DTI Link project and EPSRC UK, in collaboration
with: ECOLAB (Terminix), Stockport, UK, Yorkshire Water plc, and IST-2001-38404
project IMOS, through the European Commission, Brussels. This work was presented as
an oral communication at ISOT 2004, Kyoto, Japan, July 2004.

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