1、Designation: E 1865 97 (Reapproved 2002)Standard Guide forOpen-Path Fourier Transform Infrared (OP/FT-IR) Monitoringof Gases and Vapors in Air1This standard is issued under the fixed designation E 1865; the number immediately following the designation indicates the year oforiginal adoption or, in th
2、e case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide describes active open-path Fourier transforminfrared (OP/FT-IR) monitors a
3、nd provides guidelines forusing active OP/FT-IR monitors to obtain concentrations ofgases and vapors in air.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety
4、and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 131 Terminology Relating to Molecular Spectroscopy2E 168 Practice for General Techniques of Infrared Quanti-tative Analysis2E 1421 Practice for Describing and Meas
5、uring Performanceof Fourier Transform Mid-Infrared (FT-MIR) Spectrom-eters Level Zero and Level One Tests2E 1655 Practices for Infrared, Multivariate, QuantitativeAnalysis23. Terminology3.1 For definitions of terms relating to general molecularspectroscopy used in this guide refer to Terminology E 1
6、31. Acomplete glossary of terms relating to optical remote sensing isgiven in Ref (1).33.2 Definitions:3.2.1 background spectrum, na single-beam spectrum thatdoes not contain the spectral features of the analyte(s) ofinterest.3.2.2 bistatic system, na system in which the IR source issome distance fr
7、om the detector. For OP/FT-IR monitoring,this implies that the IR source and the detector are at oppositeends of the monitoring path.3.2.3 monitoring path, nthe location in space over whichconcentrations of gases and vapors are measured and averaged.3.2.4 monitoring pathlength, nthe distance the opt
8、icalbeam traverses through the monitoring path.3.2.5 monostatic or unistatic system, na system with theIR source and the detector at the same end of the monitoringpath. For OP/FT-IR systems, the beam is generally returned bya retroreflector.3.2.6 open-path monitoring, nmonitoring over a path thatis
9、completely open to the atmosphere.3.2.7 parts per million meters, nthe units associated withthe quantity path-integrated concentration and a possible unitof choice for reporting data from OP/FT-IR monitors becauseit is independent of the monitoring pathlength.3.2.8 path-averaged concentration, nthe
10、result of divid-ing the path-integrated concentration by the pathlength.3.2.8.1 DiscussionPath-averaged concentration gives theaverage value of the concentration along the path, and typicallyis expressed in units of parts per million (ppm), parts perbillion (ppb), or micrograms per cubic meter (gm3)
11、.3.2.9 path-integrated concentration, nthe quantity mea-sured by an OP/FT-IR monitor over the monitoring path. It hasunits of concentration times length, for example, ppmm.3.2.10 plume, nthe gaseous and aerosol effluents emittedfrom a stack or other pollutant source and the volume of spacethey occup
12、y.3.2.11 retroreflector, nan optical device that returns ra-diation in directions close to the direction from which it came.3.2.11.1 DiscussionRetroreflectors come in a variety offorms. The retroreflector commonly used in OP/FT-IR moni-toring uses reflection from three mutually perpendicular sur-fac
13、es. This kind of retroreflector is usually called a cube-cornerretroreflector.3.2.12 single-beam spectrum, nthe radiant power mea-sured by the instrument detector as a function of frequency.3.2.12.1 DiscussionIn FT-IR absorption spectrometry thesingle-beam spectrum is obtained after a fast Fourier t
14、ransformof the interferogram.3.2.13 synthetic background spectrum, na backgroundspectrum made by choosing points along the envelope of asingle-beam spectrum and fitting a series of short, straight linesor a polynomial function to the chosen data points to simulatethe instrument response in the absen
15、ce of absorbing gases orvapors.1This guide is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and is the direct responsibility of Subcommittee E13.03 on InfraredSpectroscopy.Current edition approved March 10, 1997. Published July 1997.2Annual Book of ASTM Standards, Vol 03.06.3
16、The boldface numbers in parentheses refer to a list of references at the end ofthis guide.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4. Significance and Use4.1 This guide is intended for users of OP/FT-IR monitors.Applications o
17、f OP/FT-IR systems include monitoring forhazardous air pollutants in ambient air, along the perimeter ofan industrial facility, at hazardous waste sites and landfills, inresponse to accidental chemical spills or releases, and inworkplace environments.5. Principles of OP/FT-IR Monitoring5.1 Long-path
18、 IR spectrometry has been used since themid-1950s to characterize hazardous air pollutants (2). For themost part, this earlier work involved the use of multiple-pass,long-path IR cells to collect and analyze air samples. In the late1970s a mobile FT-IR system capable of detecting pollutantsalong an
19、open path was developed (3). The 1990 amendmentsto the Clean Air Act, which may require that as many as 189compounds be monitored in the atmosphere, have led to arenewed interest in OP/FT-IR monitoring (4). The OP/FT-IRmonitor is a spectrometric instrument that uses the mid-IRspectral region to iden
20、tify and quantify atmospheric gases.These instruments can be either transportable or permanentlyinstalled. An open-path monitor contains many of the samecomponents as those in a laboratory FT-IR system, for examplethe same types of interferometers and detectors are used,except that the sample volume
21、 consists of the open atmosphere.In contrast to more conventional point monitors, the OP/FT-IRmonitor provides path-integrated concentration data. Unlikemany other air monitoring methods, such as those that usecanisters or sorbent cartridges, the OP/FT-IR monitor measurespollutants in situ. Therefor
22、e, no samples need be collected,extracted, or returned to the laboratory for analysis. Detectionlimits in OP/FT-IR depend on several factors, such as themonitoring pathlength, the absorptivity of the analyte, and thepresence of interfering species. For most analytes of interest,detection limits typi
23、cally range between path-integrated con-centrations of 1.5 and 50 ppmm.NOTE 1The OP/FT-IR monitor can be configured to operate in twomodes: active or passive. In the active mode, a collimated beam ofradiation from an IR source that is a component of the system istransmitted along the open-air path.
24、In the passive mode, radiation emittedfrom objects in the field of view of the instrument is used as the source ofIR energy. Passive FT-IR monitors have been used for environmentalapplications, such as characterizing the plumes of smoke stacks. Morerecently these systems have been developed to detec
25、t chemical warfareagents in military applications. However, to date, the active mode has beenused for most environmental applications of OP/FT-IR monitoring. Inaddition to open-air measurements, extractive measurements can be madeby interfacing a closed cell to an FT-IR system. This type of system c
26、anbe used as a point monitor or to measure the effluent in stacks or pipelines.6. Description of OP/FT-IR Systems6.1 There are two primary geometrical configurations avail-able for transmitting the IR beam along the path in activeOP/FT-IR systems. One configuration is referred to as bistatic,while t
27、he other is referred to as monostatic, or unistatic.6.1.1 Bistatic ConfigurationIn this configuration, the de-tector and the IR source are at opposite ends of the monitoringpath. In this case, the optical pathlength is equal to themonitoring pathlength. Two configurations can be used forbistatic sys
28、tems. One configuration places the IR source,interferometer, and transmitting optics at one end of the pathand the receiving optics and detector at the other end (Fig.1(A). Typically a Cassegrain or Newtonian telescope is usedto transmit and collect the IR beam. The advantage of theconfiguration dep
29、icted in Fig. 1(A) is that the IR beam ismodulated along the path, which enables the unmodulatedambient radiation to be rejected by the systems electronics.The maximum distance that the interferometer and the detectorcan be separated in this configuration is limited becausecommunication between thes
30、e two components is required fortiming purposes. For example, a bistatic system with thisconfiguration developed for monitoring workplace environ-ments had a maximum monitoring pathlength of 40 m (5). Theother bistatic configuration places the IR source and transmit-ting optics at one end of the pat
31、h and the receiving optics,interferometer, and detector at the other end of the path (Fig.1(B). This is the most common configuration of bistaticsystems in current use. In this configuration the beam from theIR source is collimated by a mirror shaped as a paraboloid. Theconfiguration shown in Fig. 1
32、(B) allows the maximum moni-toring path, in principle, to be doubled compared to that of themonostatic configuration. The main drawback to this bistaticconfiguration is that the IR radiation is not modulated before itis transmitted along the path. Therefore, radiation from theactive IR source and th
33、e ambient background cannot bedistinguished by electronic processing.6.1.2 Monostatic ConfigurationIn monostatic configura-tions, the IR source and the detector are at the same end of themonitoring path. A retroreflector of some sort is required at themidpoint of the optical path to return the beam
34、to the detector.Thus, the optical pathlength is twice the distance between thesource and the retroreflector. Two techniques are currently inuse for returning the beam along the optical path in themonostatic configuration. One technique uses an arrangementof mirrors, such as a single cube-corner retr
35、oreflector, at oneend of the path that translates the beam slightly so that it doesnot fold back on itself (Fig. 2(A). The other end of the paththen has a second telescope slightly removed from the trans-mitter to collect the returned beam. Initial alignment with thisconfiguration can be difficult,
36、and this type of monostaticsystem is normally used in permanent installations rather thanas a transportable unit. Another configuration of the monostaticmonitoring mode uses the same telescope to transmit andreceive the IR beam. A cube-corner retroreflector array isplaced at the end of the monitorin
37、g path to return the beam(Fig. 2(B). To transmit and receive with the same optics, abeamsplitter must be placed in the optical path to divert part ofthe returned beam to the detector. A disadvantage to thisconfiguration is that the IR energy must traverse this beam-splitter twice. The most efficient
38、 beamsplitter transmits 50 % ofthe light and rejects the other 50 %. Thus, in two passes, thetransmission is only 25 % of the original beam. Because thisloss of energy decreases the signal-to-noise ratio (S/N), it canpotentially be a significant drawback of this configuration.7. Selection of Instrum
39、ental Parameters7.1 Introduction and OverviewOne important issue re-garding the operation of OP/FT-IR systems is the appropriateE 18652instrumental parameters, such as measurement time, resolu-tion, apodization, and degree of zero filling, to be used duringdata acquisition and processing. The choice
40、 of some of theseparameters is governed by the trading rules in FT-IR spectrom-etry and by specific data quality objectives of the study.7.2 Trading Rules in FT-IR SpectrometryThe quantitativerelationships between the S/N, resolution, and measurementtime in FT-IR spectrometry are called “trading rul
41、es.” Thefactors that affect the S/N and dictate the trading rules areexpressed in Eq 1, which gives the S/N of a spectrum measuredwith a rapid-scanning Michelson interferometer (6):SN5UvT!uDvt1/2jD*AD!1/2(1)where:Uv(T) = spectral energy density at wavenumber v from ablackbody source at a temperature
42、 T,u = optical throughput of the spectrometric system,Dv = resolution of the interferometer,t = measurement time in seconds,j = efficiency of the interferometer,D* = specific detectivity, a measure of the sensitivityof the detector, andAD= area of the detector element.NOTE 2This equation is correct
43、but assumes that the system isdetector noise limited, which is not always true. For example, sourcefluctuations, the analog-to-digital converter, or mechanical vibrations cancontribute to the system noise.7.3 Measurement TimeAs shown in Eq 1, the S/N isproportional to the square root of the measurem
44、ent time (t1/2).For measurements made with a rapid scanning interferometeroperating at a constant mirror velocity and a given resolution,the S/N increases with the square root of the number ofco-added scans. The choice of measurement time for signalaveraging in OP/FT-IR monitoring must take into acc
45、ountseveral factors. First, a measurement time must be chosen toachieve an adequate S/N for the required detection limits.However, because monitoring for gases and vapors in the air isa dynamic process, consideration must be given to the temporalnature of the target gas concentration. For example, i
46、f theconcentration of the target gas decreases dramatically duringthe measurement time, then there would be a dilution effect. Inaddition, varying signals cannot be added linearly in theinterferogram domain. Nonlinearities and bandshape distor-tions will be observed if the concentrations of gases in
47、 the pathvary appreciably during the measurement time.7.4 ResolutionSeveral factors must be considered whendetermining the optimum resolution for measuring the IRspectra of gases and vapors along a long, open path. Thesefactors include (1) the ability to distinguish between thespectral features of t
48、arget analytes and those of ambientinterfering species in the atmosphere, such as water vapor; (2)the tradeoffs between resolution, IR peak absorbance, and S/N;(3) practical considerations, such as measurement time, com-putational time to process the interferogram, and the size of theinterferogram f
49、ile for data storage; (4) procedural consider-ations, such as the choice of background spectrum and thedevelopment of an adequate water vapor reference spectrum;FIG. 1 Schematic Diagram of the Bistatic OP/FT-IR Configuration Showing (A) a System with the IR Source and Interferometer at One End of the Path andthe Detector at the Opposite End, and (B) a System with the IR Source at One End of the Path and the Interferometer and Detector at the Opposite EndE 18653and (5) logistical considerations, such as the size and the costof the instrument.7.4.1 Eff
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