Gas Detection [Aug 2009]
Analysis Methods For Particular Harmful Gases
GAS DETECTION
ANALYSIS METHODS FOR PARTICULAR HARMFUL GASES
Knowledge of the nature and amounts of pollutants in the atmosphere is of major importance in air quality. Consequently there is a need for precise and reliable instruments for analysis of particular gaseous pollutants. Such pollutants include sulphur dioxide (SO2), oxides of nitrogen (NOx) and carbon monoxide (CO). This article will review detection methods for these and, more briefly, for other pollutant gases.
Detection and measurement of sulpur dioxide
Sulphur dioxide is highly harmful to persons. Its level in the atmosphere is of the order of parts per hundred million (p.p.h.m.). Levels would have been higher than this in the days before regulation of gaseous emissions from industrial plant. It might be an interesting exercise in epidemiology to estimate how many persons in Britain died from the effects of sulphur dioxide over say the period 1850 to 1950. The present author's intuition is that whether it was millions or tens of millions would be as much as could from this distance in time be convincingly determined. Even nowadays a blip in the sulphur dioxide concentration of the atmosphere will be accompanied by excess deaths from such complaints as asthma, that is, deaths over the period of high sulphur dioxide concentration in excess of the expected number on the basis of statistics and records. Sulphur dioxide is also a strong factor in acid rain.
There are classical methods for determining sulphur dioxide, including infra-red spectroscopy and fluorescence. The former, the basis of which is absorption of infra-red radiation due to the vibrational functions of the molecule, might still find use in today's world where amounts of sulphur dioxide in effluent gases are being measured. Very roughly speaking, a gaseous pollutant will be diluted by a factor of 103 on release into the environment. An ambient level of parts per hundred million therefore converts to an emission level of parts per million or tens thereof, and modern infra-red analysers for sulphur dioxide can measure down to about 1 p.p.m. The basis of the fluorescence method is emission of radiation very distant in wavelength from infra-red. This can measure down to p.p.h.m. levels.
More modern sulphur dioxide measurement devices are of two types: semiconductor and electrochemical. A semiconductor is made by deliberate contamination ('doping') of a metallic compound, for example tin oxide, so as to give it either an excess or a deficit of electrons. The former is an n-type semiconductor and the latter a p-type semiconductor. The excess or deficit of electrons creates movement of electrons within the semiconductor, and the electrical current so generated is affected by contact with gas molecules. This is the basis of gas detection and measurement by semiconductors.
An important factor is choice of a dopant to give good selectivity in relation to the gas of interest. With a tin oxide semiconductor a nickel dopant gives good sulphur dioxide selectivity, and measurements to below 1 p.p.h.m. are possible.
The electrochemical approach is in principle applicable to anything oxidisable, as sulphur dioxide is:
SO2 + 2H2O ? SO42- + 4H+ + 2e
Such an instrument therefore operates as a galvanic cell the e.m.f. from which is the basis of the measurement. There are electrochemical measurement devices for sulphur dioxide which can measure down to ambient levels.
Detection and measurement of oxides of nitrogen
The term NOx means of course NO (nitric oxide) plus NO2 (nitrogen dioxide). NOx in the atmosphere is a factor in photochemical smog and in acid rain. The 'NOx meter' working on the principle of chemiluminescence has been in widespread use for decades. Rather inconveniently for field use, it requires a cylinder of oxygen. Oxygen entering the device is converted to ozone which in turn converts NOx to nitrogen dioxide in an electronically excited state. Emission of a photon by this:
NO2* ? NO2 + hv
is the basis of the measurement.
A modern NOx meter working along these lines can measure NOx in amounts of less than 1 p.p.m.
Semiconductors have also been applied to NOx measurement. Reference [1] reports good performance of a semiconductor composed of Fe2O3 doped with niobium in the measurement of oxides of nitrogen. The sensitivity of the semiconductor detector depends how much of the NOx is present as the oxidised form NO2, that is, on the ratio:
NO2/( NO + NO2)
and this sensitivity also depends upon the amount of dopant. With the dopant amount used in the majority of the experiments the response of the semiconductor was remarkably sensitive to the above ratio. With a value of 1 for the ratio, that is, NO2 only to the exclusion of NO, amounts of less than five p.p.m. are easily measurable. There is progressively poorer response with values of 0.5 and 0.25 for the above ratio. With a value of zero, that is NO only, the limit of measurement is about 100 p.p.m. The reader should be aware of two points. First, it is a simple matter to convert the NOx to NO2 entirely, as indeed is done in a chemiluminescence meter. Secondly, with semiconductors there is much scope for variation of conditions including temperature and dopant amounts and to give response most suitable for a particular application.
Amounts of NOx can also be determined by superionic conductors. NASICON (sodium superionic conductor, formula Na3Zr2Si2PO12) is an important example. Such a substance resembles an ionic solution in that movement of ions produces current. When NASICON is used in a NOx sensing device, the NOx first encounters a noble metal surface contacting the NASICON. This influences current flow through the NASICON, and this can correlated with the NOx concentration. There are alternatives to noble metals for the fabrication of the sensing electrode, notably certain oxides containing three metallic elements. These are sometimes loosely called 'pyrochlores'; one such having found application to NOx sensing is an oxide of lead, ruthenium and vanadium.
For sulphur dioxide, infra-red methods of measurement are seen as being dated if not obsolescent. For NOx newly developed infra-red devices for measurement are available (e.g., [2]) and these are arguably starting to replace the time-honoured chemiluminescence method.
Carbon monoxide
The background level of carbon monoxide in the atmosphere is a few p.p.m. It will seldom be of interest to measure such a concentration distant from a major source such as an industrial complex or a heavily used road. It is however important to be able to measure amounts of carbon monoxide in vehicle exhaust emissions and this is very widely done on a routine basis. In the UK MoT test, the level of carbon monoxide in the exhaust of a vehicle is measured by infra-red, absorption being at a wavelength of 47?m. Levels in vehicle exhaust gas will be typically four orders of magnitude higher than ambient levels. There are also (e.g. [3]) infra-red devices for measuring carbon monoxide in flue gases from combustion plant, for example at power stations.
Carbon monoxide poisoning during a fire or an incipient fire causes many deaths every year. A carbon monoxide detector for prevention might be a semiconductor, or it might work along electrochemical principles. Perhaps most interestingly of all, the detector might be biomimetric in its operation. Each of these will be explained in turn.
It is clear from what has already featured in this article that semiconductors have very wide scope for development in gas detection. For carbon monoxide a tin oxide semiconductor with a cerium oxide dopant is very suitable. Electrochemical detection of carbon monoxide is also widely practised. The basis of this is as follows. In a galvanic cell with sulphuric acid solution as the electrolyte the following processes occur at the respective electrodes:
0.5 O2 + 2H+ + 2e -> H2O
CO + H2O -> CO2 + 2H+ + 2e
CO + 0.5 O2 -> CO2
Clearly a rise in the carbon monoxide concentration will be manifest as a change in the e.m.f. of the cell.
Perhaps the most interesting of the devices for carbon monoxide detection is the biomimetic sensor. Such a sensor uses a substance which changes colour when carbon monoxide reacts with it, similarly to the action of carbon monoxide on hemoglobin in the blood. The sensor can therefore be said to have mimicked biochemistry.
Detection of ozone and volatile organic compounds (VOC)
The 'ozone layer' about 20 km above the earth's surface needs to be protected as it filters out parts of solar radiation which are harmful to persons. However, ozone much closer to the earth's surface is itself harmful, being an agent in the formation of photochemical smog. Ozone absorbs in the ultraviolet, so air to the extent that it contains ozone loses its transparency in this wavelength range and this provides on means of measuring amounts of ozone in air.
Semiconductors have been developed for ozone detection and these include indium oxide (In2O3) doped with iron oxide (Fe2O3) [4]. A semiconductor will always respond to gas impingement, and the conventional way of expressing the effectiveness of a semiconductor as a sensor for a particular gas is the ratio of the response from that gas to that from air, other things being equal. An indium oxide semiconductor so evaluated [4] gave responses up to 100 times that for air. Factors in the response include the temperature of the semiconductor. The temperature range 300 to 700°C was examined in the work in [4] and the most promising results were obtained at the lower end of the range. Also relevant is the temperature at which the doped semiconductor was prepared. In [4] this range was 900 to 1300°C. Other semiconductors having been adapted into ozone detectors include tungsten oxide [5].
The pollutants considered so far in this article have been single organic compounds. By contrast 'Volatile organic compounds' (VOC) are considered collectively in air pollution control and their coverage in the same section as that of ozone is appropriate as VOC are involved in ozone formation in the atmosphere. The VOC level inside a building will exceed that of the air surrounding the building by a factor of between 2 and 5. This is because of VOC release by such substances as paints, polishes, office equipment and cosmetics. The primary sources of VOC in the atmosphere are however vehicles and refineries.
A common method of detection and measurement of VOC is photoionisation. A u.v. light source within the detection instrument ionises any VOC in gas drawn in for analysis. The electrical current so created is the basis of the signal. Such instruments are now available which can measure down to 1 p.p.b. of VOC. As one would expect, semiconductors have been developed for VOC detection and here again tin oxide has proved its worth [6]. VOC can be measured with a semiconductor comprising SnO2 doped with platinum and palladium. Tungsten oxide which, as noted above can be the basis of a semiconductor detector for ozone, can also be used for VOC if suitably treated with one or more 'minority elements'.
Concluding remarks
In 1934, an article was published 'Journal of Scientific Instruments' entitled 'Note on gas analysis with modified Orsat apparatus'. An Orsat apparatus consists of three glass containers of aqueous reagent into which samples of flue gas are passed. Analysis is for carbon dioxide, carbon monoxide and oxygen. The Orsat apparatus has been used much more recently than 1934. The present author supervised its operation as an undergraduate experiment at the University of New South Wales over the period 1987 to 1995. He saw it as being messy and uninteresting, a view that was almost certainly shared by the majority of the students. When more advanced instruments for gas analysis first came into use they required mains electricity. The Orsat apparatus and others of its genre, that is 'wet chemistry' devices having no power requirement, retained a role. It is clear from this article that gas analysis has advanced a long way since the Orsat apparatus, although it is unlikely that that has disappeared totally. Methods discussed in this article have included spectroscopic and electrochemical ones. Semiconductors have featured centrally and it is clear that scope for the application of semiconductors to gas analysis is very wide indeed. <
References
[1] Cantalini C., Sun H.T., Faccio M., Ferri G., Pelino M. 'Niobium-doped ?-Fe2O3 semiconductor ceramic sensors for the measurement of nitric oxide gases' Sensors and Actuators B 24-25 673-677 (1995)
[2] http://www.yokogawa.com/iab/appnotes/ iab-app-noxremovale-en.htm
[3] http://www.landinst.com/combustion/ products/combustion/Series9000_Carbon_ Monoxide.htm
[4] Kim S-R., Hong H-K., Kwon C.H., Yun D.H., Lee K., Sung Y.K. 'Ozone sensing properties of In2O3-based semiconductor thick films' Sensors and Actuators B 66 59-62 (2000)
[5] http://iopscience.iop.org/0957-0233/12/ 6/305/?ejredirect=.iopscience
[6] http://ci.nii.ac.jp/naid/10021135098/en
Author
J.C. Jones
School of Engineering, University of Aberdeen.
j.c.jones@eng.abdn.ac.uk
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