1、Designation: E 1982 98 (Reapproved 2002)Standard Practice forOpen-Path Fourier Transform Infrared (OP/FT-IR) Monitoringof Gases and Vapors in Air1This standard is issued under the fixed designation E 1982; the number immediately following the designation indicates the year oforiginal adoption or, in
2、 the 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 practice covers procedures for using active open-path Fourier transform infra
3、red (OP/FT-IR) monitors to mea-sure the concentrations of gases and vapors in air. Proceduresfor choosing the instrumental parameters, initially operatingthe instrument, addressing logistical concerns, making ancil-lary measurements, selecting the monitoring path, acquiringdata, analyzing the data,
4、and performing quality control on thedata are given. Because the logistics and data quality objectivesof each OP/FT-IR monitoring program will be unique, stan-dardized procedures for measuring the concentrations of spe-cific gases are not explicitly set forth in this practice. Instead,general proced
5、ures that are applicable to all IR-active gasesand vapors are described. These procedures can be used todevelop standard operating procedures for specific OP/FT-IRmonitoring applications.1.2 This practice does not purport to address all of thesafety concerns, if any, associated with its use. It is t
6、heresponsibility of the user of this practice 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:E 131 Terminology Relating to Molecular Spectroscopy2E 168 Practices for General Techn
7、iques of Infrared Quanti-tative Analysis2E 1421 Practice for Describing and Measuring Performanceof Fourier Transform Infrared (FT-IR) Spectrometers:Level Zero and Level One Tests2E 1655 Practices for Infrared, Multivariate, QuantitativeAnalysis2E 1865 Guide for Open-Path Fourier Transform Infrared(
8、OP/FT-IR) Monitoring of Gases and Vapors in Air22.2 Other Documents:FT-1R Open-Path Monitoring Guidance Document3Compendium Method TO-16Long-Path Open-Path Fou-rier Transform Infrared Monitoring of AtmosphericGases43. Terminology3.1 For definitions of terms used in this practice relating togeneral m
9、olecular spectroscopy, refer to Terminology E 131.3.2 For definitions of terms used in this practice relating toOP/FT-IR monitoring, refer to Guide E 1865.3.3 For definitions of general terms relating to opticalremote sensing, refer to the FT-IR Open Path MonitoringGuidance Document.4. Significance
10、and Use4.1 An OP/FT-IR monitor can, in principle, measure theconcentrations of all IR-active gases and vapors in the atmo-sphere. Detailed descriptions of OP/FT-IR systems and thefundamental aspects of their operation are given in GuideE 1865 and the FT-IR Open-Path Monitoring Guidance Docu-ment. A
11、method for processing OP/FT-IR data to obtain theconcentrations of gases over a long, open path is given inCompendium Method TO-16. Applications of OP/FT-IR sys-tems include monitoring for gases and vapors in ambient air,along the perimeter of an industrial facility, at hazardous wastesites and land
12、fills, in response to accidental chemical spills orreleases, and in workplace environments.5. Instrumental Parameters5.1 Several instrumental parameters must be chosen beforedata are collected with an OP/FT-IR system. These parametersinclude the measurement time, spectral resolution, apodizationfunc
13、tion, and zero filling factor. In some cases, the choice ofthese parameters might be limited by the parameters used to1This practice is under the jurisdiction of ASTM Committee E-13 on MolecularSpectroscopy and is the direct responsibility of Subcommittee E13.03 on InfraredSpectroscopy.Current editi
14、on approved October 10, 1998. Published March 1999.2Annual Book of ASTM Standards, Vol 03.06.3EPA/600/R-96/040, National Technical Information Service Technology Ad-ministration, U.S. Department of Commerce, Springfield, VA 22161, NTIS OrderNo. PB961704771NZ.4Compendium of Methods for the Determinat
15、ion of Toxic Organic Compoundsin Ambient Air, 2nd Ed., EPA/625/R-96/010b, Center for Environmental ResearchInfo., Office of Research using a dual-chambered gas cell; or attenuating thebeam with wire screens of different, known mesh sizes.6.4.1 Polymer FilmsAcquire spectra of polymer films ofdifferen
16、t thicknesses to test the linearity of the OP/FT-IRsystem.6.4.1.1 Collect a single-beam spectrum over the monitoringpath without the polymer film in the beam. Use this spectrumas the background spectrum.6.4.1.2 Insert a polymer film of known thickness into the IRbeam and obtain a single-beam spectru
17、m. Create an absorptionspectrum from this spectrum by using the background spec-trum acquired in 6.4.1.1.6.4.1.3 Replace the first polymer film with another film of adifferent, known thickness and obtain a single-beam spectrum.Create an absorption spectrum from this spectrum by using thebackground s
18、pectrum obtained in 6.4.1.1.6.4.1.4 Measure the absorbance maxima of selected bandsin the two absorption spectra acquired in 6.4.1.2 and 6.4.1.3.Choose absorption bands that are not saturated. Perform thistest on several absorption bands in different regions of thespectrum.6.4.1.5 Compare the absorb
19、ance value of the selected bandin the spectrum of one polymer film to that measured in theother. The ratio of the absorbance values of the two differentfilms should be equal to the ratio of the film thicknesses.NOTE 3If the thickness of the polymer film used to test the linearityof the system is not
20、 known it can be calculated by using Eq 1:b 512nNv12v2!(1)where:b = thickness of the sample,n = refractive index of the sample,N = number of interference fringes in the spectral rangefrom v1to v2,v1= first wavenumber in the spectral range over which thefringes are counted, andE 1982 98 (2002)3v2= se
21、cond wavenumber in the spectral range over whichthe fringes are counted.6.4.2 Dual-Chambered Gas CellUse a dual-chamberedgas cell containing a high concentration of the target gas to testthe linearity of the system. This cell should be designed withtwo sample chambers that differ in length by a know
22、n amountand are coupled so that each chamber contains the sameconcentration of the target gas (3).6.4.2.1 Fill the dual-chambered cell with dry nitrogen atatmospheric pressure and insert it into the IR beam.6.4.2.2 Acquire a single-beam spectrum along the monitor-ing path. Use this spectrum as the b
23、ackground spectrum for thechamber that is in the IR beam.6.4.2.3 Reposition the cell so that the other chamber is in theIR beam, and acquire a single-beam spectrum along themonitoring path. Use this spectrum as the background spec-trum for that chamber.6.4.2.4 Fill the cell with a high concentration
24、 of the targetgas. The absolute concentration of the target gas does not needto be known with this method.6.4.2.5 Acquire single-beam spectra alternatively with eachchamber positioned in the IR beam. Create absorption spectraby using the appropriate background spectrum for each cham-ber.6.4.2.6 Meas
25、ure the absorbance maxima of selected bandsin the two spectra created in 6.4.2.5. Choose absorption bandsthat are not saturated. Perform this test on several absorptionbands in different regions of the spectrum.6.4.2.7 Compare the absorbance value measured with onechamber to that measured with the o
26、ther. The ratio of theabsorbance values measured with the two separate chambers inthe beam should be equal to the ratio of the lengths of thechambers.6.4.3 Wire Mesh ScreensInsert a wire screen of a knownmesh size in the IR beam and record the signal. Remove thiswire screen, insert another screen of
27、 a different, known meshsize in the beam, and record the signal. The ratio of the signalsobtained with the two different screens should be equal to theratio of the mesh sizes of the screens.NOTE 4Linearization circuits are available to minimize the problemof detector nonlinearity. These linearizatio
28、n circuits may not performadequately for all detectors.6.5 Measure the Signal Due to Internal Stray Light orAmbient RadiationSingle-beam spectra recorded with anOP/FT-IR monitor can exhibit a non-zero response in wave-number regions in which the atmosphere is totally opaque. Ifthe detector has been
29、determined to be responding linearly tochanges in the incident light intensity, this non-zero responsecan be attributed to either internal stray light or ambientradiation. Internal stray light is most likely to be a problem inmonostatic systems that use a single telescope to transmit andreceive the
30、IR beam. Ambient radiation mostly affects bistaticsystems in which an unmodulated, active IR source is sepa-rated from the interferometer and detector. The presence ofinternal stray light or ambient radiation causes errors in thephotometric accuracy and, ultimately, errors in the concentra-tion meas
31、urements. The magnitude of the instrument responsedue to internal stray light or ambient radiation determines theminimum useful signal that can be measured with the OP/FT-IR system.6.5.1 Measure the Internal Stray LightIn monostatic sys-tems that use a single telescope to transmit and receive the IR
32、beam, point the telescope away from the retroreflector or movethe retroreflector out of the field of view of the telescope andcollect a single-beam spectrum. This spectrum represents theinternal stray light of the system and is independent of thepathlength. Record this spectrum at the beginning of e
33、achmonitoring program or whenever optical components in thesystem are changed or realigned. An example of an internalstray light spectrum is given in Guide E 1865.NOTE 5Internal stray light can also be caused by strong sources of IRradiation that are in the field of view of the instrument. For examp
34、le, thesun may be in the instruments field of view during sunrise or sunset andcause an unwanted signal from reflections inside the instrument.6.5.2 Measure the Ambient RadiationIn bistatic systems,which use an unmodulated, active IR source that is separatedfrom the interferometer and detector, bloc
35、k or turn off thesource and collect a single-beam spectrum. This spectrum is arecord of the IR radiation emitted by the objects in the field ofview of the instrument. Because this spectrum depends on whatobjects are in the field of view, it also depends on thepathlength. Thus, the ambient radiation
36、spectrum must beacquired each time the pathlength is changed or wheneverdifferent objects come into the field of view. A recommendedschedule for recording the ambient radiation spectrum has notbeen determined for all situations. However, recording anambient radiation spectrum once every half hour is
37、 typical formost applications. An example of an ambient radiation spec-trum is given in Guide E 1865.NOTE 6The ambient radiation spectrum recorded by an OP/FT-IRmonitor is a composite of the various IR sources in the field of view of theinstrument, such as gray body radiators, emission bands from mo
38、lecules inthe atmosphere, and the instrument itself. Because the ambient radiationspectrum is temperature dependent, its relative contribution to the totalsignal will vary. This variation will most likely be greater than any othersource of instrumental noise. The ambient radiation spectrum will bedi
39、fferent for each site and can also change with varying meteorologicalconditions throughout the day. For example, cloud cover can attenuate theatmospheric emission bands.6.6 Measure the Signal Strength as a Function ofPathlengthIn OP/FT-IR systems, the IR beam is collimatedbefore it is transmitted al
40、ong the path, but diverges as ittraverses the path. Once the diameter of the beam is larger thanthe retroreflector (monostatic system) or the receiving tele-scope (bistatic system), the signal strength will diminish as thesquare of the pathlength.6.6.1 Start with the retroreflector or the external I
41、R source atthe minimum pathlength as determined in 6.3. Record themagnitude of the signal either as the peak-to-peak voltage ofthe interferogram centerburst or as the intensity of the single-beam spectrum at a specific wavenumber. Once the initialmeasurement has been recorded, move the retroreflecto
42、r or IRsource some distance away from the receiving telescope, forexample, 25 m, and record the magnitude of the signal.Continue this procedure until the signal decreases as the squareof the monitoring pathlength. Extrapolate the data to determineE 1982 98 (2002)4the distance at which the magnitude
43、of the signal will reach thatof the random noise (see 6.7), internal stray light, or ambientradiation. This distance represents the maximum pathlength forthat particular OP/FT-IR monitor.NOTE 7In bistatic systems, the relative contribution of the ambientradiation to the total signal increases as the
44、 signal from the active IRsource decreases. As the signal from the active IR source approaches zero,there may be apparent shifts in the peak intensity of the single-beamspectrum.6.7 Determine the Random Baseline Noise of the SystemSet up the instrument at a pathlength that is representative ofthat t
45、o be used during the field study. Collect two single-beamspectra sequentially. Do not allow any time to elapse betweenthe acquisition of these two spectra. Create an absorptionspectrum from these two spectra by using one spectrum as abackground spectrum. Which spectrum is used for the back-ground is
46、 not important. Measure the random noise as theroot-mean-square (RMS) noise (4). The actual wavenumberrange over which the noise should be calculated will vary withthe number of data points per wavenumber in the spectrum. Arange of 98 data points is optimum for the RMS noisecalculation. The RMS nois
47、e should be determined in wave-number regions that are not significantly impacted by watervapor, for example, 9581008 cm1, 24802530 cm1, and43754425 cm1. Record the value of the RMS noise for futurereference.7. Logistical Concerns and Ancillary Measurements atthe Monitoring Sites7.1 Logistical Conce
48、rnsSeveral logistical concerns mustbe addressed at each monitoring site before the OP/FT-IRmonitor is deployed in the field. Consideration must be givento power requirements, mounting and support requirements,and climate control. Some ancillary measurements should alsobe made.7.1.1 PowerSupply the r
49、equired electrical power to thespectrometer. In bistatic systems with a remote IR source, anadditional source of power must be provided if an electricaloutlet is not available. Some IR sources can operate off aportable 12-V power supply, such as a car or marine battery.The output of the battery must be stabilized for quantitativemeasurements.7.1.2 Mounting and SupportFor short-term field studies,the spectrometer, the retroreflector, or the remote IR source aretypically mounted on transportable tripods with swivel headsthat allow for vertical and horizontal adjus
