Chemical Senses 2005 30(Supplement 1):i252-i253; doi:10.1093/chemse/bjh210
Chemical Senses Vol. 30 No. suppl 1 © Oxford University
Press 2005; all rights reserved
‘Electronic Nose’—New Condition Monitoring Devices for Environmental Applications
Krishna C. Persaud,
Peter Wareham,
Anna Maria Pisanelli and
Emmanuel Scorsone
Department of Instrumentation and Analytical Science, UMIST, Manchester, UK
Correspondence to be sent to: Krishna C. Persaud, e-mail:
kcpersaud{at}umist.ac.uk
Key words: environmental monitoring, odour sensing, sensor
arrays
 |
Introduction
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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 by
suitable mathematical methods, can provide information to
discriminate
between many sample odours (Pearce
et al., 2002
). Arrays of
gas and odour sensors, made using different
technologies, are
finding their way into a variety of specialized applications
(Persaud, 2001
). Software techniques
and material science
are important aspects of the development of the systems. These
devices have become known as ‘electronic noses’
and consist of three
elements: a sensor array which is exposed
to the volatiles; conversion of the sensor
signals to a readable
format; and software analysis of the data to produce characteristic
outputs related to the odour encountered. The output from the
sensor array may be
interpreted via a variety of methods—such
as pattern recognition algorithms,
principal component analysis,
discriminant function analysis, cluster analysis and
artificial
neural networks—to discriminate between samples.
We describe three different environmental applications for such technology and
illustrate how the problems were resolved.
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Detection of dry rot infections in buildings
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We aimed to develop a portable sensing instrument that could
be used by property
surveyors in the field for detection of
volatiles at very low concentrations that are
emitted from
fungal infections of
Serpula lacrymans in buildings. The volatiles
are emitted at very low concentrations and a GC/MS analysis
indicated that key marker
compounds included 3-octanone, 3-octanol
and 1,3-octen-3-ol. 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 under-floor spaces. This
was based on solid phase microextraction fibres (SPME)
(Figure
1a) and allowed rapid sampling of
odour at very low concentrations
from previously inaccessible locations.
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Figure 1 (a) Solid phase microextraction fibre: thermal desorption of
adsorbed volatiles onto a heated sensor array. (b) Design of the measurement
system. The sensors are mounted in a symmetrical three-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 the
introduction 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 is dependent on the selectivity and sensitivity
of the sensors to the mixture of volatiles.
|
|
The measurement system was based on a array of metal oxide
sensors, incorporated into
a suitable header (Figure
1b)so
that when a SPME fibre was
inserted, thermal desorption of
trapped volatiles occurred and dynamic responses of the
sensors
could be recorded (Figure
1c).
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Monitoring of waste water volatiles
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The requirements were for a device for monitoring the odour
of waste water at the
inlet of a 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 in time. Because this
device needed to be operated in a very harsh
outdoor environment,
the packaging requirements were also stringent. We utilized
in this
case an array of metal oxide sensors sensitive to sulphurous
compounds that are
characteristic of organic decay and these
have proven to be robust in monitoring changes
in odour level
over time (see Figure
2).
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|
Figure 2 Continuous monitoring of odour at the inlet of a sewage
plant. The trace shows a 24 h recording using an array of four sensors sensitive to
sulphurous odours. Odour events are recorded in real time and can be seen between
0800–1100 and 2100–2200 h.
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|
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Smart fire detection systems
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A big problem in fire detection is that of false alarms and
many optical detectors do
not discriminate between different
types of smokes. We have been developing an array of
conducting
polymer sensors that are capable of carrying out the rapid discrimination
of
volatiles from different types of fires. GC/MS analysis
would indicate that the odorous
volatile profiles emitted from
different type of standard fires can be characterized. We
have
developed a conducting polymer sensor array that allows discrimination
of these
profiles (Figure
3).
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Conclusion
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We show that odour sensor array based technology has improved.
When coupled to
suitable sampling systems and pattern recognition
software odour sensor array technology
can produce instruments
that fulfil a variety of sensing needs, where a change of
condition
needs to be monitored.
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Acknowledgements
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This work was supported through a DTI Link project and EPSRC
UK, in collaboration with
ECOLAB (Terminix), Stockport, UK
and IST-2001-38404 project IMOS, through the European
Commission,
Brussels.
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References
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Pearce, T.C., Schiffman, S.S. Nagle, H.T. and
Gardner, J.W. (eds) (2002) Handbook of Machine Olfaction: Electronic Nose Technology. Wiley-VCH, Weinheim.
Persaud, K.C. (2001) Olfactory system cybernetics artificial noses. In Doty, R.L. (ed.), Handbook of Olfaction and Gustation, 2nd edn. Marcel Dekker, New York, pp. 295–308.
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Introduction
Detection of dry rot...