A very large number of different analytical techniques are used for atmospheric pollutant analysis. Some of these whose uses are not confined to atmospheric analysis were discussed. A summary of some of the main instrumental techniques for air monitoring is presented in Table 30.2.
Analysis of Sulphur Dioxide:
The reference method for the analysis of sulphur dioxide is the spectrophotometric pararosaniline method first described by West and Gaeke, and subsequently optimised. It is applicable to the analysis of 0.005-5 ppm SO2 in ambient air. Figure 30.10 illustrates the various components involved in a sampling train employed to sample the atmosphere for sulphur dioxide to be analysed by the West-Gaeke method.
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The method makes use of a collecting solution of 0.04 M potassium tetrachloromercurate to collect sulphur dioxide according to the following reaction –
Typically, this involves scrubbing 30 litres of air through 10 ml of scrubbing solution with a collection efficiency of around 95 per cent. Sulphur dioxide in the scrubbing medium is reacted with formaldehyde –
The adduct formed is then reacted with uncoloured organic pararosaniline hydrochloride to produce a red-violet dye. Although NO2 at levels above about 2 ppm interferes, the interference may be eliminated by reducing the NO2 to N2 gas with sulphamic acid, H2NSO3H.
Performed manually, the West-Gaeke method for sulphur dioxide analysis is cumbersome and complicated. However, the method has been refined to the point that it can be done automatically with continuous monitoring equipment. A block diagram of such an analyser is shown in Fig. 30.11.
Generally, sulphur dioxide is collected in a hydrogen peroxide solution and increased conductance of the sulphuric acid solution is measured. Several types of sulphur dioxide monitors are based on amperometry, in which an electrical current is measured that is proportional to the SO2 in a collecting solution. Sulphur dioxide can be determined by ion chromatography, by bubbling SO2 through hydrogen peroxide solution to produce SO42–, followed by analysis of the sulphate by ion chromatography, a method that separates ions on a chromatography column and detects them very sensitively by conductivity measurement.
Flame photometry, sometimes in combination with gas chromatography, is also used for the detection of sulphur dioxide and other gaseous sulphur compounds. The gas is burned in a hydrogen flame and the sulphur emission line at 394 nm is measured. Several direct spectrophotometric methods are used for sulphur dioxide measurement, including non-dispersive infrared absorption, Fourier transform infrared analysis (FTIR), ultraviolet absorption, molecular resonance fluorescence and second-derivative spectrophotometry. The principles of these methods are the same for any gas measured.
Analysis of Nitrogen Oxides:
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Although as noted in Table 30.2, several methods have been used to determine nitrogen oxides, gas-phase chemiluminescence is the favoured method of NOx analysis. It results from the emission of light from electronically excited species formed by a chemical reaction. In the case of NO, ozone is reacted with NO to produce NO2, which loses energy and returns to the ground state through emission of light in the 600-3000 nm range. The emitted light is measured by a photomultiplier; its intensity is proportional to the concentration of NO. A schematic diagram of the device used is shown in Fig. 30.12.
Since the chemiluminescence detector system depends upon the reaction of O3 with NO, it is necessary to convert NO2 to NO in the sample prior to analysis. This is accomplished by passing the air sample over a thermal converter. Analysis of such a sample gives NOx, the sum of NO and NO2. Chemiluminescence analysis of a sample that has not been passed over the thermal converter gives NO. The difference between these two results is NO2.
Other nitrogen compounds besides NO and NO2 undergo chemiluminescence by reacting with O3, and these may interfere with the analysis if present in an excessive level. Particulate matter also causes interference which may be overcome by employing a membrane filter on the air inlet.
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This analysis technique is illustrative of chemiluminescence analysis in general. Chemiluminescence is an inherently desirable technique for the analysis of atmospheric pollutants because it avoids wet chemistry, is basically simple and lends itself well to continuous monitoring and instrumental methods.
Analysis of Oxidants:
Atmospheric oxidants that are commonly analysed include ozone, hydrogen peroxide, organic peroxides and chlorine. The classic manual method for the analysis of oxidants is based upon their oxidation of I– ion followed by spectrophotometric measurement of the product. The sample is collected in 1 per cent KI buffered at pH 6.8. Oxidants react with iodide ion as shown by the following reaction of ozone –
The absorbance of the coloured I3– product is measured spectrophotometrically at 352 nm. Generally, the level of oxidant is expressed in terms of ozone, although it should be noted that not all oxidants— PAN, for example, react with the same efficiency as O3. Oxidation of I– may be employed to determine oxidants in a concentration range of several hundredths of a part per million to approximately 10 ppm. Nitrogen dioxide gives a limited response to the method and reducing substances interfere seriously.
Now the favoured method for oxidant analysis uses chemiluminescence. The chemiluminescent reaction is that between ozone and ethylene. Chemiluminescence from this reaction occurs over a range of 300-6000 nm, with a maximum at 435 nm. The intensity of emitted light is directly proportional to the level of ozone. Ozone concentrations ranging from 0.003 to 30 ppm may be measured. Ozone for calibrating the instrument is generated photochemically from the absorption of ultraviolet radiation by oxygen.
Analysis of Carbon Monoxide:
Carbon monoxide is analysed in the atmosphere by non-dispersive infrared spectrometry. This technique depends upon the fact that carbon monoxide absorbs infrared radiation strongly at certain wavelengths. Therefore, when such radiation is passed through a long (typically 100 cm) cell containing a trace of carbon monoxide levels, more infrared radiant energy is absorbed.
A non-dispersive infrared spectrometer differs from standard infrared spectrometers in that the infrared radiation from the source is not dispersed according to wavelength by a prism or grating. The non- dispersive infrared spectrometer is made very specific for a given compound, or type of compound, by using the sought for material as part of the detector, or by placing it in a filter cell in the optical path. A diagram of a non-dispersive infrared spectrometer selective for CO is shown in Fig. 30.13.
Radiation from an infrared source is ‘chopped’ by a rotating device, so that it alternately passes through a sample cell and a reference cell. In this particular instrument, both beams of light fall on a detector which is filled with CO gas and separated into two compartments by a flexible diaphragm. The relative amounts of infrared radiation absorbed by the CO in the two sections depend upon level in the sample.
The difference in the amount of infrared radiation absorbed in the two compartments causes slight differences in heating, so that the diaphragm bulges slightly toward one side. Very slight movement of the diaphragm can be detected and recorded. By means of this device, carbon monoxide can be measured from 0 to 150 ppm, with a relative accuracy of ±5 per cent in the optimum concentration range.
Flame-ionisation gas chromatography detection can also be used for the analysis of carbon monoxide. It is selective for hydrocarbons and conversion of CO to methane in the sample is required. This is accomplished by reaction with hydrogen over a nickel catalyst at 360°C –
A major advantage of this approach is that the same basic instrumentation may be used to measure hydrocarbons.
Carbon monoxide may also be analysed by measuring the heat produced by its catalytic oxidation to CO2 over a catalyst consisting of a mixture of MnO2 and CuO. Differences in temperature between a cell in which the oxidation is occurring and a reference cell through which part of the sample is flowing are measured by thermistors. A vanadium oxide catalyst can be used for the oxidation of hydrocarbons, enabling their simultaneous analysis.
Analysis of Hydrocarbons:
Monitoring of hydrocarbons in atmospheric samples takes advantage of the very high sensitivity of the hydrogen flame ionisation detector to measure this class of compounds. Known quantities of air are run through the flame ionisation detector 4 to 12 times per hour to provide a measure of total hydrocarbon content. A separate portion of each sample goes into a stripper column to remove water, carbon dioxide, and non-methane hydrocarbons.
Methane and carbon monoxide, which are not retained by the stripper column, are separated by a chromatographic column, passed through a catalytic reduction tube then to a flame ionisation detector. Eluting first, methane is not changed by the reduction tube, and is detected as such by the detector. The carbon monoxide is reduced to methane, then detected as the methane product by the flame ionisation detector. Concentrations of non-methane hydrocarbons are given by subtracting the methane concentrations from the total hydrocarbons.
Using the method described above, total hydrocarbons can be determined in a range of 0-13 mg/m3, corresponding to 0-10 ppm. Methane can be measured over a range of 0-6.5 mg/m3 (0.10 ppm).