Ammonia sensors and their applications—a review.pdf

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Sensors and Actuators B 107 (2005) 666–677
Ammonia sensors and their applications—a review
Bj¨ rn Timmer
∗
, Wouter Olthuis, Albert van den Berg
o
MESA
+
Research Institute, University of Twente, Enschede, P.O. Box 217, 7500AE Enschede, The Netherlands
Received 14 May 2004; received in revised form 12 November 2004; accepted 15 November 2004
Available online 16 March 2005
Abstract
Many scientific papers have been written concerning gas sensors for different sensor applications using several sensing principles. This
review focuses on sensors and sensor systems for gaseous ammonia. Apart from its natural origin, there are many sources of ammonia,
like the chemical industry or intensive life-stock. The survey that we present here treats different application areas for ammonia sensors
or measurement systems and different techniques available for making selective ammonia sensing devices. When very low concentra-
tions are to be measured, e.g. less than 2 ppb for environmental monitoring and 50 ppb for diagnostic breath analysis, solid-state ammonia
sensors are not sensitive enough. In addition, they lack the required selectivity to other gasses that are often available in much higher
concentrations. Optical methods that make use of lasers are often expensive and large. Indirect measurement principles have been de-
scribed in literature that seems very suited as ammonia sensing devices. Such systems are suited for miniaturization and integration to make
them suitable for measuring in the small gas volumes that are normally available in medical applications like diagnostic breath analysis
equipment.
© 2005 Elsevier B.V. All rights reserved.
Keywords:
Gas sensors; Ammonia; Miniaturization
1. Introduction
Thousands of articles have been published that deal with
some sort of gas sensor. This makes it virtually impossible to
write a review article, completely covering this area. When
looking in the scientific literature, summarizing articles can
be found that deal with specific application areas or specific
types of gas sensors. Examples of review articles about ap-
plications for gas sensors are: high volume control of com-
bustibles in the chemical industry
[1],
exhaust gas sensors for
emission control in automotive applications
[2,3]
or monitor-
ing of dairy products for the food industry
[4].
Articles that
emphasize a specific type of gas sensor are written about, for
example, solid state gas sensors
[5],
conducting polymer gas
sensors using e.g. polyaniline
[6],
mixed oxide gas sensors
[7],
amperometric gas sensors
[8],
catalytic field-effect de-
Corresponding author. Tel.: +31 53 489 2755; fax: +31 53 489 2287.
E-mail address:
b.h.timmer@el.utwente.nl (B. Timmer).
URL:
http://www.bios.el.utwente.nl.
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2004.11.054
∗
vices
[9]
or gas sensor arrays used in electronic noses
[4,10].
The review presented here will focus on one specific gas,
ammonia.
After a brief introduction of the origin of ammonia in
the earth’s atmosphere, we consider various artificial sources
of ammonia in the air, such as intensive life-stock with
the decomposition process of manure, or the chemical in-
dustry for the production of fertilizers and for refrigera-
tion systems. Subsequently, different application areas for
gaseous ammonia analyzers are investigated with a sum-
mary of the ammonia concentration levels of interest to
these different areas. Applications in the agricultural and
industrial chemistry areas are discussed, as well as envi-
ronmental, automotive and medical applications for ammo-
nia sensing devices. The overview of application areas pro-
vides us with an indication of the required specifications,
like detection limits and response time, which will be used
as a guideline for the consideration of different measur-
ing principles and techniques, as discussed in the next sec-
tion.
B. Timmer et al. / Sensors and Actuators B 107 (2005) 666–677
667
2. Sources of ammonia
Ammonia is a natural gas that is present throughout the
atmosphere. The relatively low concentrations, of low-ppb
to sub-ppb levels
[11],
have been significantly higher in the
past. Earth history goes back over 4.5 billion years, when it
was formed from the same cloud of gas and interstellar dust
that created our sun, the rest of the solar system and even the
entire galaxy. The larger outer planets had enough gravita-
tional pull to remain covered in clouds of gas. The smaller
inner planets, like earth, formed as molten rocky planets with
only a small gaseous atmosphere. It is thought that the early
earth formed a chemically reducing atmosphere by 3.8 to
4.1 billion years ago, made up of hydrogen and helium with
large concentrations of methane and ammonia. Most of this
early atmosphere was lost into space during the history of
the planet and the remaining was diluted by a newly form-
ing atmosphere. This new atmosphere was formed mostly
from the outgassing of volatile compounds: nitrogen, water
vapour, carbon dioxide, carbon monoxide, methane, ammo-
nia, hydrochloric acid and sulphur produced by the constant
volcanic eruptions that besieged the earth.
The earth’s surface began to cool and stabilize, creating
the solid crust with its rocky terrain. Clouds of water began to
form as the earth began to cool, producing enormous volumes
of rain water that formed the early oceans. The combination
of a chemically reducing atmosphere and large amounts of
liquid water may even have created the conditions that led to
the origin of life on earth. Ammonia was probably a compo-
nent of significant importance in this process
[12–17].
Today, most of the ammonia in our atmosphere is emitted
direct or indirect by human activity. The worldwide emission
of ammonia per year was estimated in 1980 by the European
community commission for environment and quality of life
to be 20–30 Tg
[18].
Other investigations, summarized by
Warneck
[11],
found values between 22 and 83 Tg.
Fig. 1
shows an estimate of the annual ammonium deposition rate
world wide, showing a maximum deposition in central- and
Western Europe
[11].
In literature, three major classes of current ammonia
sources are described
[11].
Although the earth’s atmosphere
comprises almost 80% nitrogen, most nitrogen is unavail-
able to plants and consumers of plants. There are two natural
pathways for atmospheric nitrogen to enter the ecosystem,
a process called nitrification. The first pathway, atmospheric
deposition, is the direct deposition of ammonium and nitrate
salts by addition of these particulates to the soil in the form of
dissolved dust or particulates in rain water. This is enhanced
in the agricultural sector by the addition of large amounts of
ammonium to cultivated farmland in the form of fertilizer.
However, when too much ammonium is added to the soil,
this leads to acidification, eutrophication, change in vegeta-
tion
[19]
and an increase in atmospheric ammonia concentra-
tion
[20].
The second way of nitrification is bacterial nitrogen
fixation. Some species of bacteria can bind nitrogen. They re-
lease an excess of ammonia into the environment. Most of this
ammonia is converted to ammonium ions because most soils
are slightly acidic
[6].
The contribution of nitrogen fixation
to the total worldwide ammonia emission is approximated to
be 1.0 Tg/year
[18].
A larger source in the overall nitrogen cycle is ammoni-
fication, a series of metabolic activities that decompose or-
ganic nitrogen like manure from agriculture and wildlife or
leaves
[12].
This is performed by bacteria and fungi. The
released ammonium ions and gaseous ammonia is again con-
verted to nitrite and nitrate by bacteria
[12,21].
The nitrogen
cycle is illustrated in
Fig. 2.
The worldwide ammonia emis-
sion resulting from domestic animals is approximated to be
20–35 Tg/year
[11].
A third source of ammonia is combustion, both from
chemical plants and motor vehicles. Ammonia is produced
by the chemical industry for the production of fertilizers and
for the use in refrigeration systems. The total emission of
ammonia from combustion is about 2.1–8.1 Tg/year
[11].
Fig. 1. Annual ammonium deposition (100 mg/m
2
)
[11].
668
B. Timmer et al. / Sensors and Actuators B 107 (2005) 666–677
Fig. 2. Nitrogen cycle (Copyright University of Missouri, MU Extension WQ252).
There are numerous smaller sources of ammonia, e.g. sur-
face water. Normally seas and oceans act as a sink for ammo-
nia but occasionally they act as an ammonia source
[22,23].
Ammonia is produced because of the existence of ammonium
ions that are transformed to gaseous ammonia by alkaline
rainwater
[23].
3. Application areas of ammonia sensors
There are many ways to detect ammonia. High concentra-
tions are easy to detect because the gas has a very penetrat-
ing odour. With respect to other odorous gasses, the human
nose is very sensitive to ammonia. To quantify the ammonia
concentration or determine lower concentrations of ammo-
nia, the human nose fails. However, in many occasions, the
ammonia concentration has to be known, even at ultra low
concentrations of less than parts per billion in air (ppb)
[24].
This section focuses on four major areas that are of inter-
est for measuring ammonia concentrations; environmental,
automotive, chemical industry and medical diagnostics, and
describes why there is a need to know the ammonia con-
centration in these fields. Where possible the concentration
levels of interest are given for the different application areas.
3.1. Environmental gas analysis
The smell of ammonia near intensive farming areas or
when manure is distributed over farmland is very unpleasant.
Furthermore, exposure to high ammonia concentrations is
a serious health threat. Concentration levels near intensive
farming can be higher than the allowed exposure limit. This
results in unhealthy situations for farmers and animals inside
the stables, where the concentrations are highest.
Another interesting point is the formation of ammonium
salt aerosols. Sulphuric acid and nitric acid react in the at-
mosphere with ammonia to form ammonium sulphate and
ammonium nitrate
[25].
These salts are condensation nuclei,
forming several nanometre sized airborne particles. There-
fore, ammonia reduces the quantity of acids in the atmo-
sphere. These ammonia aerosols have a sun-blocking func-
tion, as can often be seen above large cities or industrial areas,
as shown in
Fig. 3.
These clouds of smog have a temperature
reducing effect. This effect however, is presently hardly no-
ticeable due to the more intense global warming caused by
the greenhouse effect.
Ammonia levels in the natural atmosphere can be very
low, down to sub-ppb concentration levels above the oceans.
The average ambient ammonia concentration in the Nether-
lands is about 1.9 ppb. Very accurate ammonia detectors with
a detection limit of 1 ppb or lower are required for measuring
such concentrations. Near intensive farming areas, ammo-
nia concentrations are much higher, up to more than 10 ppm
[26].
It depends on the actual application what concentration
levels are of interest. This also determines the time resolu-
tion of the required analysis equipment. Monitoring ambient
ammonia levels for environmental analysis does not demand
B. Timmer et al. / Sensors and Actuators B 107 (2005) 666–677
669
of NO
x
with NH
3
, according to Eq.
(1) [32].
Therefore, am-
monia is injected into the exhaust system.
4NO
+
4NH
3
+
O
2
→
4N
2
+
6H
2
O
(1)
Fig. 3. Smog, or clouds of aerosols, has a sun-blocking effect.
for extremely fast detectors. When an analyzer is used in a
controlled venting system in stables, a shorter response time
is required in the order of a minute.
3.2. Automotive industry
The automotive industry is interested in measuring atmo-
spheric pollution for three reasons
[27].
First, exhaust gasses
are monitored because they form the major part of gaseous
pollution in urban sites. For instance, ammonia exhaust is
associated with secondary airborne particulate matter, like
ammonium nitrate and ammonium sulphate aerosols, as dis-
cussed in the previous section. Ammonium aerosols are mea-
sured to be up to 17% of the particulate matter concentration
smaller than 2.5 m
[27].
Ammonia emissions have been
measured up to 20 mg/s or up to 8 ppm ammonia in exhaust
gas
[28,29].
A second reason for the automotive industry to be inter-
ested in detectors for atmospheric pollution like ammonia, is
air quality control in the passenger compartment
[27].
Mod-
ern cars are frequently equipped with an air conditioning sys-
tem. This system controls the temperature and the humidity
of the air inside the car. Fresh air can be taken from the outside
of the car or it can be created by conditioning and circulat-
ing air inside the car. When there is low quality air outside
the car, like air with smoke near a fire or a factory, the sys-
tem should not take up new air from outside. A major source
of unpleasant smell is the smell of manure near farms and
meadows. This smell is caused by the increased ammonia
concentration in these areas. For indoor air quality monitors,
the detection limit should lie around the smell detection limit
of about 50 ppm. Moreover, for such an application it is im-
portant that the sensor responds very fast. The air inlet valve
should be closed before low-quality gas is allowed into the
car. A response time in the order of seconds is required.
A third application for ammonia sensors in the automotive
area is NO
x
reduction in diesel engines. Modern diesel en-
gines operate at high air-to-fuel ratios that result in an excess
of oxygen in the exhaust gas, resulting in large concentrations
of NO and NO
2
(NO
x
)
[30,31].
Toxic NO
x
concentrations are
lowered significantly by selective catalytic reduction (SCR)
It is unfavourable to inject too much ammonia for this is emit-
ted into the atmosphere where it adds to the total pollution,
known as ammonia-slip. The injected amount can be opti-
mised by measuring the excess ammonia concentration in
the exhaust system. The concentration level that is of interest
for this application depends on the controllability of the setup.
When the controllability of the ammonia injection is very ac-
curate, the used sensor should be able to measure very low
ammonia concentrations in a few seconds. The sensors that
are currently used have detection limits in the order of a few
ppm
[30]
and a response time of about 1 min. Because mea-
surements are performed in exhaust pipes, the sensor should
be able to withstand elevated temperatures.
3.3. Chemical industry
The major method for chemically producing ammonia is
the Haber process. The German scientist Fritz Haber started
working on a way to produce ammonia in 1904
[33].
In
1918 he won the Nobel Prize in Chemistry for his inven-
tion. Ammonia is synthesized from nitrogen and hydrogen
at an elevated temperature of about 500
◦
C and a pressure
of about 300 kPa using a porous metal catalyst. The process
was scaled up to industrial proportions by Carl Bosch. The
process is therefore often referred to as the Haber–Bosch
process.
Ammonia production was initiated by the demand for an
inexpensive supply of nitrogen for the production of nitric
acid, a key component of explosives. Today, the majority
of all man made ammonia is used for fertilizers or chemical
production. These fertilizers contain ammonium salts and are
used in the agricultural sector.
Another substantial part is used for refrigeration. Ammo-
nia was among the first refrigerants used in mechanical sys-
tems. Almost all refrigeration facilities used for food pro-
cessing make use of ammonia because it has the ability to
cool below 0
◦
C
[34,35].
The first practical refrigerating ma-
chine was developed in 1834 and commercialised in 1860.
It used vapour compression as the working principle. The
basic principle: a closed cycle of evaporation, compression,
condensation and expansion, is still in use today
[36].
Because the chemical industry, fertilizer factories and re-
frigeration systems make use of almost pure ammonia, a leak
in the system can result in life-threatening situations. All fa-
cilities using ammonia should have an alarm system detect-
ing and warning for dangerous ammonia concentrations. The
maximum allowed workspace ammonia level is tabulated to
be 20 ppm. This is a long-term maximum and no fast detec-
tors are required, a response time in the order of minutes is
sufficient. Especially in ammonia production plants, where
ammonia is produced, detectors should be able to withstand
670
B. Timmer et al. / Sensors and Actuators B 107 (2005) 666–677
Fig. 4. Electron micrograph of
H. pylori.
the high temperature, up to 500
◦
C, applied in the production
process.
3.4. Medical applications for ammonia sensors
High concentrations of ammonia form a threat to the hu-
man health. The lower limit of human ammonia perception
by smell is tabulated to be around 50 ppm, corresponding to
about 40 g/m
3
[37].
However, even below this limit, am-
monia is irritating to the respiratory system, skin and eyes
[38,39].
The long term allowed concentration that people
may work in is therefore set to be 20 ppm. Immediate and
severe irritation of the nose and throat occurs at 500 ppm. Ex-
posure to high ammonia concentrations, 1000 ppm or more,
can cause pulmonary oedema; accumulation of fluid in the
lungs. It can take up to 24 h before the symptoms develop:
difficulty with breathing and tightness in the chest. Short-
term exposure to such high ammonia concentrations can lead
to fatal or severe long term respiratory system and lung disor-
ders
[40].
Extremely high concentrations, 5000–10,000 ppm,
are suggested lethal within 5–10 min. However, accident re-
constructions have proven that the lethal dose is higher
[41].
Longer periods of exposure to low ammonia concentration
are not believed to cause long-term health problems. There
is no accumulation in the body since it is a natural body
product, resulting from protein and nucleic acid metabolism.
Ammonia is excreted from the body in the form of urea and
ammonium salts in urine. Some ammonia is removed from
the body through sweat glands.
As being a natural body product, ammonia is also pro-
duced by the human body
[12].
The amount of produced am-
monia is influenced by several parameters. For instance, the
medical community is considerably interested in ammonia
analyzers that can be applied for measuring ammonia lev-
els in exhaled air for the diagnosis of certain diseases
[42].
Measuring breath ammonia levels can be a fast diagnostic
method for patients with disturbed urea balance, e.g. due to
kidney disorder
[43]
or ulcers caused by
Helicobacter pylori
bacterial stomach infection, of which an image is shown in
Fig. 4 [44–46].
For such applications, often only a few ml of
exhaled air is available and, at present, no suitable ammonia
breath analyzer exists
[47].
Fig. 5. Immune system cells infiltrate the area of the ulcer to attack the
bacteria, leading to inflammation and damage.
After infection, the bacterium penetrates the stomach wall
through the mucous barrier used by the stomach to protect
itself against the digestive acid gastric juice
[45].
The bac-
terium’s most distinct characteristic is the abundant produc-
tion of the enzyme urease
[48].
It converts urea to ammonia
and bicarbonate to establish a locally neutralizing surround-
ing against penetrating acid. This is one of the features that
make it possible for the bacterium to survive in the human
stomach.
The immune system responds to the infection by sending
antibodies
[45].
H. pylori
is protected against these infec-
tion fighting agents because it is hidden in the stomach wall
protection layer. The destructive compound that is released
by the antibodies when they attack the stomach lining cells
eventually cause the peptic ulcer, as illustrated in
Fig. 5 [45].
The conversion of urea to ammonia and bicarbonate led to
H. pylori
infection diagnosis tests. A first method is based on
a gastric CO
2
measurement, directly related to the bicarbon-
ate concentration. It makes use of an endoscopic procedure
[48].
Non-invasive test methods are shown based on measur-
ing exhaled CO
2
or NH
3
levels
[46,48].
Because the normal
exhaled CO
2
levels are relatively high, isotopically labelled
urea is used. Subsequently, labelled CO
2
concentrations are
measured. The results are excellent but the test is expensive
and it requires a radionuclide, limiting the applicability. Us-
ing a breath ammonia analyzer would be a more appropriate
solution. Suitable ammonia analyzers should be able to mea-
sure down to 50 ppb ammonia in exhaled air, containing CO
2
concentrations up to 3%
[42].
When measuring in exhaled
air, the used analysis equipment should have a reasonable re-
sponse time of at most a few minutes and often only small
volumes of analyte gas will be available.
Ammonia levels in blood are also of interest in the sports
medicine. During activity the human body produces ammo-
nia. Ammonia can diffuse out of the blood into the lungs when
the ammonia levels become higher than the ammonia levels

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