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1、International Standard ( 7996 0 Q 8 4lisiB INTERNATIONAL ORGANIZATION FOR STANDARDIZATIONMEYHAPOHAR OPrAHM3AUHR IlO CTAHAAPTH3AWlMORGANlSATlON INTERNATIONALE DE NORMALISATION Ambient air - Determination of the mass concentration of nitrogen oxides - Chemiluminescence method Air ambiant - De terminat
2、ion de la concentration en masse des oxydes d azote - Mthode par chimiluminescence First edition - 1985-12-15 UDC 614.71 : 543.272.32 Descriptors : air, quality, chemical analysis, determination of content, nitrogen oxides. Ref. No. IS0 7996-1985 (E) Price based on 9 pages Copyright International Or
3、ganization for Standardization Provided by IHS under license with ISO Licensee=Defense Supply Ctr/9960866100 Not for Resale, 04/24/2007 10:52:02 MDTNo reproduction or networking permitted without license from IHS -,-,- Foreword IS0 (the International Organization for Standardization) is a worldwide
4、federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carried out through IS0 technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that c
5、ommittee. International organizations, govern- mental and non-governmental, in liaison with ISO, also take part in the work. Draft International Standards adopted by the technical committees are circulated to the member bodies for approval before their acceptance as International Standards by the IS
6、0 Council. They are approved in accordance with IS0 procedures requiring at least 75 % approval by the member bodies voting. International Standard IS0 7996 was prepared by Technical Committee ISO/TC 146, Air quality. Users should note that all International Standards undergo revision from time to t
7、ime and that any reference made herein to any other International Standard implies its latest edition, unless otherwise stated. 0 International Organization for Standardization, 1985 0 Printed in Switzerland Copyright International Organization for Standardization Provided by IHS under license with
8、ISO Licensee=Defense Supply Ctr/9960866100 Not for Resale, 04/24/2007 10:52:02 MDTNo reproduction or networking permitted without license from IHS -,-,- INTERNATIONAL STANDARD IS0 7996-1965 (El Ambient air - Determination of the mass concentration of nitrogen oxides - Chemiluminescence method 1 Scop
9、e and field of application This International Standard specifies a chemiluminescence method for the determination of the mass concentration of nitrogen oxides present in ambient air. The method is applicable to the determination of the mass con- centration of nitrogen monoxide present in ambient air
10、 up to approximately 12,5 mg/m3 and of nitrogen dioxide up to approximately 19 mg/m3 respectively ) at 25 “C and 101,3 kPa. For interferents, see 6.6. 2 References IS0 6142, Gas analysis - Preparation of calibration gas mix- tures - Weighing methods. IS0 6144, Gas analysis - Preparation of calibrati
11、on gas mix- tures - Static volumetric methods. IS0 6349, Gas analysis - Preparation of calibration gas mix- tures - Permeation method. IS0 6711, Gas analysis - Checking of calibration gas mixtures by a comparison method. 3 Principle The electric signal obtained is proportional to the total amount of
12、 nitrogen oxides, and the nitrogen dioxide contribution is calculated from the difference between this value and that ob- tained for nitrogen monoxide only - when the air sample does not pass through the converter. Analysers may be of the dual or cyclic type. In the dual type, the air flow is divide
13、d into two streams, one passing directly to the reaction chamber and the other via the converter. There are two reaction chambers and one or two detectors respectively monitoring the nitrogen monoxide and nitrogen oxides content of the air sample. In the cyclic type analyser, there is only one react
14、ion chamber and detector and the output alternates between nitrogen monoxide and total nitrogen oxides measure- ments, i.e. the air sample alternately bypasses or passes through the converter. 4 Reactions Luminescence is a characteristic of many substances from which light is emitted when they are e
15、xcited. This phenomenon is called chemiluminescence when produced as a result of a chemical reaction. This chemiluminescence method is based on the reaction NO + O3 -+ NO; + O2 Excited nitrogen dioxide emits radiation centred around the near infra-red (1 206 nm). 5 Apparatus Passage of the air sampl
16、e through a filter (to prevent mal- operation of the chemiluminescence analyser) at a constant flow rate into the reaction chamber of the analyser, where it is mixed with an excess of ozone. Filtering of the emitted radiation, proportional to the amount of nitrogen monoxide present in the air sample
17、, by means of a selective optical filter and conversion of the filtered radiation into an electric signal by a photomultiplier tube. Measurement of nitrogen dioxide in the air sample by reducing Schematic diagrams of typical analysers are given in figures la) it to nitrogen monoxide by passage throu
18、gh a converter before and lb) (dual type) and figure Ic) (cyclic type). An apparatus it enters the reaction chamber. consists of the following principal components (5.1 to 5.10). 1) At the temperature and pressure given, the following conversion factors apply. NO: 1 mglm3 P 0,81 ppm (V/V) 1 ppm (VI
19、VI P 1,23 mglm3 N02: 1 mglm3 P 0,53 ppm (VIV) 1 ppm (VI VI P 1,88 mglm3 1 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=Defense Supply Ctr/9960866100 Not for Resale, 04/24/2007 10:52:02 MDTNo reproduction or networking permitted without lice
20、nse from IHS -,-,- IS0 7996-1985 (E) 5.1 Sample line. 5.6 Reaction chamber. The sample intake shall consist of a small inverted funnel con- nected to the sample line - the latter being as short as prac- tically possible. Both shall be made of material which is inert to nitrogen monoxide and nitrogen
21、 dioxide. It is very important not to use any materials made from copper based alloys. From experience, the best materials to use are polytetrafluoro- ethylene (PTFE) or perfluoro(ethylene/propylene) (FEP). In order to avoid disturbance of the nitrogen monoxide-nitrogen dioxide-ozone equilibrium cau
22、sed by reduced daylight intensity in the sample line, the residence time in the sample line should be as short as possible. The sample line may be moderately heated to avoid any condensation. 5.2 Particle filter. The particle filter shall retain all particles likely to alter the per- formance of the
23、 analyser. At the same time, it and its support shall be made from material which will not retain nitrogen monoxide or nitrogen dioxide, for example PTFE or stainless steel. The particle filter shall be replaced or cleaned periodically as required by the dust loading at the sampling site. Overloadin
24、g of this filter may cause loss of nitrogen dioxide by sorption on the particulate matter. 5.3 Control and regulation of sample flow rate. The sample flow rate shall be maintained to +2 % of that specified by the manufacturer of the analyser. Flow rate control is usually maintained by a controlled p
25、ressure drop across restrictors. 5.4 Converter. The converter shall consist of a heated furnace maintained at a constant temperature and is made of stainless steel, copper, molybdenum, tungsten or spectroscopically pure carbon. It shall be capable of converting at least 95 % of the nitrogen dioxide
26、to nitrogen monoxide at a temperature not exceeding 409 OC. The role of the converter is very important since it controls the determination of nitrogen dioxide. 5.5 Ozone generator. Ozone is generated from oxygen by either ultra-violet radiation or by a high voltage silent electric discharge. If oxy
27、gen in am- bient air is used for ozone generation by a high voltage silent electric discharge, it is essential that the air be thoroughly dried and filtered before entering the generator. If the ozone is generated using oxygen of recognized analytical grade from a compressed gas cylinder, this oxyge
28、n can be fed directly into the generator. The concentration of ozone produced shall be greater than the maximum concentration of nitrogen oxides being measured. The flow rate of air or oxygen to the ozone generator shall be kept constant. This shall be constructed of an inert material. Its dimension
29、s determine the characteristics of the chemiluminescence reac- tion (residence time, speed of reaction). The reaction chamber may be heated slightly to avoid condensation. The reaction is generally carried out at reduced pressure to minimize quench- ing effects and increase sensitivity. 5.7 Optical
30、filter. This filter shall remove all radiation at wavelengths below 600 nm, thus avoiding any interference produced by the chemi- luminescence reaction with unsaturated hydrocarbons which radiate at these wavelengths. 5.8 Photomultiplier tube. The output of the analyser is closely affected by the ch
31、arac- teristics of the photomultiplier tube. In order to reduce background noise and the effect of temperature changes, the tube is usually housed in a thermostatically controlled refrigerated container. 5.9 Ozone filter. The ozone shall be removed from the gas on leaving the reac- tion chamber by p
32、assage through activated charcoal. This prevents pollution of the immediate ambient air and protects the sampling pump. 5.10 Sampling pump. Situated at the end of the circuit, the sampling pump draws air through the apparatus. It shall be capable of operating under the pressure conditions required f
33、or the reaction chamber. 5.11 Apparatus for checking the analyser converter efficiency (see 6. I). 5.11.1 Source of nitrogen monoxide, compressed gas cylinder containing nitrogen monoxide in nitrogen at a concen- tration of the order of 1 ppm (V/V). The actual concentration need not be known provide
34、d that it remains constant through- out the test. 5.11.2 Source of oxygen, compressed gas cylinder contain- ing oxygen or air. 5.11.3 Ultra-violet lamp, capable of varying the size of the lamp window for producing varying amounts of ozone from oxygen. 5.11.4 Chemiluminescence analyser in which the t
35、em- perature of the converter furnace is controllable. 2 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=Defense Supply Ctr/9960866100 Not for Resale, 04/24/2007 10:52:02 MDTNo reproduction or networking permitted without license from IHS -,-,
36、- IS0 7996-1985 (El 6 Procedure The operation of the apparatus will vary from one to another. Follow the manufacturer s instructions regarding setting up operations, and flow rates of air sample and ozone. The fre- quency of calibration will vary from one apparatus to another, but this should be car
37、ried out as frequently as is practically possible. where R, and P, are the total nitrogen oxides and nitrogen monoxide concentration responses respectively for each position of the window of the ultra-violet lamp. If this efficiency falls below 95 %, replace or regenerate the converter. 6.2 Calibrat
38、ion of the chemiluminescence analyser 6.1 Converter efficiency One of the most important components of the apparatus (5.2 to 5.10) is the converter, which reduces nitrogen dioxide to nitrogen monoxide, and permits, by difference, the measure- ment of the nitrogen dioxide concentration. Before calibr
39、ating the analyser, it is essential to ensure that the converter efficiency lies as close to unity as possible. The deter- mination of the efficiency is based on the principle that the response of the apparatus to the total amount of nitrogen oxides (NO,) does not change if analyses are made of vary
40、ing mixtures of nitrogen monoxide and nitrogen dioxide, but for which the total concentration of nitrogen oxides is constant. 6.2.1 Principle Generation of calibration gas mixtures containing accurately known concentrations of nitrogen dioxide by means of a per- meation system (see IS0 6349). Diluti
41、on of the nifrogen dioxide emitted with zero air of a relative humidity of about 50 % to ob- tain nitrogen dioxide concentrations suitable for the calibration of the nitrogen dioxide and total nitrogen oxides channels. Calibration of the nitrogen monoxide channel by using the same system in conjunct
42、ion with a converter, the efficiency of which has been determined as in 6.1. 6.2.2 Apparatus A schematic diagram of a suitable arrangement is given in figure 2 (see also 5.11). A typical system for generating the required concentrations of nitrogen monoxide and nitrogen dioxide is shown in figure 3.
43、 Ensure that the total flow rate of nitrogen monoxide and oxygen (or air) is greater than the flow rate of gas through the analyser - the excess being led away as shown in figure 2. In each of the following steps, note both the nitrogen monoxide and total nitrogen oxides concentration responses of t
44、he analyser. This then checks the level of nitrogen dioxide being produced which should be in the range IO to 90 % of the total nitrogen oxides level. a) With the ultra-violet lamp extinguished, note the total nitrogen oxides concentration response, RI, and the nitrogen monoxide concentration respon
45、se, PI. b) Switch on the ultra-violet lamp. Ozone is formed which reacts with the nitrogen monoxide to produce nitrogen dioxide before the gases enter the analyser. Note the total nitrogen oxides concentration response, R, and the nitrogen monoxide concentration response, P2. c) Vary the output of t
46、he ultra-violet lamp, and note the total nitrogen oxides concentration responses (R3, R, R, etc.) and the nitrogen monoxide concentration responses (Ps, Pa, P5, etc.). The ratios R2/R1, R3/R1, R41R, etc. should be as close to unity as possible (within the range of repeatability of the analyser), i.e
47、. the total nitrogen oxides concentration response should be constant in each instance and independent of the ratio of the concentrations of nitrogen dioxide to nitrogen monoxide. d) Determine the actual efficiency, expressed as a percen- tage, of the converter from the formula (R, - P,J - (R, - P,)
48、 x loo PI - P” 6.2.2.1 Air flow rate controllers and flow meters, capable of maintaining and measuring flow rates to + 1 %. 6.2.2.2 Drier, to remove moisture from the air passing over the permeation cell. Satisfactory driers are permeation driers or suitable large tubes of desiccants containing sili
49、ca gel or similar. 6.2.2.3 Constant temperature chamber, capable of hous- ing the nitrogen dioxide permeation cell and maintaining its temperature to within fl OC. It shall incorporate a tem- perature measuring device having an accuracy of +0,05 OC. 6.2.2.4 Valves. Two valves - one to direct the nitrogen monoxide (or nitrogen dioxide) flow into the diluent zero air flow and one to allow the nitrogen dioxide to flow through or bypas