1、Designation: E1865 97 (Reapproved 2013)Standard Guide forOpen-Path Fourier Transform Infrared (OP/FT-IR) Monitoringof Gases and Vapors in Air1This standard is issued under the fixed designation E1865; the number immediately following the designation indicates the year oforiginal adoption or, in the
2、case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers active open-path Fourier transforminfrared (OP/FT-IR) monitors and pro
3、vides guidelines forusing active OP/FT-IR monitors to obtain concentrations ofgases and vapors in air.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of thesafety concerns, i
4、f any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E131 Terminology Relating to Molecular Spectro
5、scopyE168 Practices for General Techniques of Infrared Quanti-tative AnalysisE1421 Practice for Describing and Measuring Performanceof Fourier Transform Mid-Infrared (FT-MIR) Spectrom-eters: Level Zero and Level One TestsE1655 Practices for Infrared Multivariate QuantitativeAnalysis3. Terminology3.1
6、 For definitions of terms relating to general molecularspectroscopy used in this guide refer to Terminology E131.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 fea
7、tures of the analyte(s) ofinterest.3.2.2 bistatic system, na system in which the IR source issome distance from 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 whichco
8、ncentrations of gases and vapors are measured and averaged.3.2.4 monitoring pathlength, nthe distance the opticalbeam traverses through the monitoring path.3.2.5 monostatic or unistatic system, n a system with theIR source and the detector at the same end of the monitoringpath. For OP/FT-IR systems,
9、 the beam is generally returned bya retroreflector.3.2.6 open-path monitoring, nmonitoring over a path thatis 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
10、/FT-IR monitors becauseit is independent of the monitoring pathlength.3.2.8 path-averaged concentration, nthe result of dividingthe path-integrated concentration by the pathlength.3.2.8.1 DiscussionPath-averaged concentration gives theaverage value of the concentration along the path, and typicallyi
11、s expressed in units of parts per million (ppm), parts perbillion (ppb), or micrograms per cubic meter (gm3).3.2.9 path-integrated concentration, n the 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
12、gaseous and aerosol effluents emittedfrom a stack or other pollutant source and the volume of spacethey occupy.3.2.11 retroreflector, nan optical device that returns radia-tion in directions close to the direction from which it came.1This guide is under the jurisdiction of ASTM Committee E13 on Mole
13、cularSpectroscopy and Separation Science and is the direct responsibility of Subcom-mittee E13.03 on Infrared and Near Infrared Spectroscopy.Current edition approved Jan. 1, 2013. Published January 2013. Originallyapproved in 1997. Last previous edition approved in 2007 as E1865 97 (2007).DOI: 10.15
14、20/E1865-97R13.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The boldface numbers in parentheses refer to
15、a list of references at the end ofthis standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.11.1 DiscussionRetroreflectors come in a variety offorms. The retroreflector commonly used in OP/FT-IR moni-toring uses reflection fro
16、m three mutually perpendicular sur-faces. 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 spect
17、rum is obtained after a fast Fourier transformof the interferogram.3.2.13 synthetic background spectrum, n a 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 simul
18、atethe instrument response in the absence of absorbing gases orvapors.4. Significance and Use4.1 This guide is intended for users of OP/FT-IR monitors.Applications of OP/FT-IR systems include monitoring forhazardous air pollutants in ambient air, along the perimeter ofan industrial facility, at haza
19、rdous waste sites and landfills, inresponse to accidental chemical spills or releases, and inworkplace environments.5. Principles of OP/FT-IR Monitoring5.1 Long-path IR spectrometry has been used since themid-1950s to characterize hazardous air pollutants (2). For themost part, this earlier work inv
20、olved 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 open path was developed (3). The 1990 amendmentsto the Clean Air Act, which may require that as many as 189compounds be monitored in th
21、e 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 identify and quantify atmospheric gases.These instruments can be either transportable or permanentlyinstalled. An open-path monitor contain
22、s 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 consists of the open atmosphere.In contrast to more conventional point monitors, the OP/FT-IRmonitor provides path-integrated concentr
23、ation data. Unlikemany other air monitoring methods, such as those that usecanisters or sorbent cartridges, the OP/FT-IR monitor measurespollutants in situ. Therefore, no samples need be collected,extracted, or returned to the laboratory for analysis. Detectionlimits in OP/FT-IR depend on several fa
24、ctors, such as themonitoring pathlength, the absorptivity of the analyte, and thepresence of interfering species. For most analytes of interest,detection limits typically range between path-integrated con-centrations of 1.5 and 50 ppmm.NOTE 1The OP/FT-IR monitor can be configured to operate in twomo
25、des: 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. In the passive mode, radiation emittedfrom objects in the field of view of the instrument is used as the source ofIR energy. Passive FT
26、-IR monitors have been used for environmentalapplications, such as characterizing the plumes of smoke stacks. Morerecently these systems have been developed to detect chemical warfareagents in military applications. However, to date, the active mode has beenused for most environmental applications o
27、f 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 canbe 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 prim
28、ary 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 the other is referred to as monostatic, or unistatic.6.1.1 Bistatic ConfigurationIn this configuration, the de-tector and the IR source
29、are at opposite ends of the monitoringpath. In this case, the optical pathlength is equal to themonitoring pathlength. Two configurations can be used forbistatic systems. One configuration places the IR source,interferometer, and transmitting optics at one end of the pathand the receiving optics and
30、 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 depicted in Fig. 1(A) is that the IR beam ismodulated along the path, which enables the unmodulatedambient radiation to be rejected by the
31、 systems electronics.The maximum distance that the interferometer and the detectorcan be separated in this configuration is limited becausecommunication between these two components is required fortiming purposes. For example, a bistatic system with thisconfiguration developed for monitoring workpla
32、ce 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 path and the receiving optics,interferometer, and detector at the other end of the path (Fig.1(B). This is the most common configuration o
33、f 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(B) allows the maximum moni-toring path, in principle, to be doubled compared to that of themonostatic configuration. The main drawback
34、 to this bistaticconfiguration is that the IR radiation is not modulated before itis transmitted along the path. Therefore, radiation from theactive IR source and the ambient background cannot bedistinguished by electronic processing.6.1.2 Monostatic ConfigurationIn monostaticconfigurations, the IR
35、source and the detector are at the sameend of the monitoring path. A retroreflector of some sort isrequired at the midpoint of the optical path to return the beamto the detector. Thus, the optical pathlength is twice thedistance between the source and the retroreflector. Two tech-niques are currentl
36、y in use for returning the beam along theoptical path in the monostatic configuration. One techniqueuses an arrangement of mirrors, such as a single cube-cornerretroreflector, at one end of the path that translates the beamslightly so that it does not fold back on itself (Fig. 2(A). Theother end of
37、the path then has a second telescope slightlyremoved from the transmitter to collect the returned beam.Initial alignment with this configuration can be difficult, andthis type of monostatic system is normally used in permanentE1865 97 (2013)2installations rather than as a transportable unit. Another
38、 con-figuration of the monostatic monitoring mode uses the sametelescope to transmit and receive the IR beam. A cube-cornerretroreflector array is placed at the end of the monitoring pathto return the beam (Fig. 2(B). To transmit and receive with thesame optics, a beamsplitter must be placed in the
39、optical pathto divert part of the returned beam to the detector. A disadvan-tage to this configuration is that the IR energy must traversethis beamsplitter twice. The most efficient beamsplitter trans-mits 50 % of the light and rejects the other 50 %. Thus, in twopasses, the transmission is only 25
40、% of the original beam.Because this loss of energy decreases the signal-to-noise ratio(S/N), it can potentially be a significant drawback of thisconfiguration.7. Selection of Instrumental Parameters7.1 Introduction and OverviewOne important issue re-garding the operation of OP/FT-IR systems is the a
41、ppropriateinstrumental parameters, such as measurement time,resolution, apodization, and degree of zero filling, to be usedduring data acquisition and processing. The choice of some ofthese parameters is governed by the trading rules in FT-IRspectrometry and by specific data quality objectives of th
42、estudy.7.2 Trading Rules in FT-IR SpectrometryThe quantitativerelationships between the S/N, resolution, and measurementtime in FT-IR spectrometry are called “trading rules.” Thefactors that affect the S/N and dictate the trading rules areexpressed in Eq 1, which gives the S/N of a spectrum measured
43、with a rapid-scanning Michelson interferometer (6):SN5UvT!vt1/2D*AD!1/2(1)where:Uv(T) = spectral energy density at wavenumber v from ablackbody source at a temperature T, = optical throughput of the spectrometric system, v = resolution of the interferometer,t = measurement time in seconds, = efficie
44、ncy of the interferometer,D* = specific detectivity, a measure of the sensitivity ofthe detector, andAD= area of the detector element.NOTE 2This equation is correct but assumes that the system isdetector noise limited, which is not always true. For example, sourcefluctuations, the analog-to-digital
45、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 measurement time (t1/2).For measurements made with a rapid scanning interferometeroperating at a constant mirror velocity and a given resolutio
46、n,the S/Nincreases 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 accountseveral factors. First, a measurement time must be chosen toFIG. 1 Schematic Diagram of the Bistatic OP/FT-IR Configuration Showing
47、(A) a System with the IR Source and Interferometer at OneEnd of the Path and the Detector at the Opposite End, and (B) a System with the IR Source at One End of the Path and the Interferom-eter and Detector at the Opposite EndE1865 97 (2013)3achieve an adequate S/N for the required detection limits.
48、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, if theconcentration of the target gas decreases dramatically duringthe measurement time, then there would be a dilution eff
49、ect. Inaddition, varying signals cannot be added linearly in theinterferogram domain. Nonlinearities and bandshape distor-tions will be observed if the concentrations of gases in 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 target analytes and those of ambientinterfering species in the a
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