1、Designation: E1982 98 (Reapproved 2013)Standard Practice forOpen-Path Fourier Transform Infrared (OP/FT-IR) Monitoringof Gases and Vapors in Air1This standard is issued under the fixed designation E1982; the number immediately following the designation indicates the year oforiginal adoption or, in t
2、he 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 practice covers procedures for using active open-path Fourier transform infrared
3、 (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, and
4、 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 procedure
5、s 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 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thiss
6、tandard.1.3 This practice does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility 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. Refe
7、renced Documents2.1 ASTM Standards:2E131 Terminology Relating to Molecular SpectroscopyE168 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 Te
8、stsE1655 Practices for Infrared Multivariate QuantitativeAnalysisE1685 Practice for Measuring the Change in Length ofFasteners Using the Ultrasonic Pulse-Echo Technique2.2 Other Documents:FT-IR Open-Path Monitoring Guidance Document3Compendium Method TO-16 Long-Path Open-Path FourierTransform Infrar
9、ed Monitoring of Atmospheric Gases43. Terminology3.1 For definitions of terms used in this practice relating togeneral molecular spectroscopy, refer to Terminology E131.3.2 For definitions of terms used in this practice relating toOP/FT-IR monitoring, refer to Guide E1685.3.3 For definitions of gene
10、ral terms relating to opticalremote sensing, refer to the FT-IR Open Path MonitoringGuidance Document.4. Significance 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 thefun
11、damental aspects of their operation are given in GuideE1685 and the FT-IR Open-Path Monitoring Guidance Docu-ment. A 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 monitorin
12、g for gases and vapors in ambient air,along the perimeter of an industrial facility, at hazardous wastesites and landfills, in response to accidental chemical spills orreleases, and in workplace environments.5. Instrumental Parameters5.1 Several instrumental parameters must be chosen beforedata are
13、collected with an OP/FT-IR system. These parameters1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy 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.
14、 Published January 2013. Originallyapproved in 1998. Last previous edition approved in 2007 as E1982 98 (2007).DOI: 10.1520/E1982-98R13.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume in
15、formation, refer to the standards Document Summary page onthe ASTM website.3EPA/600/R-96/040, National Technical Information Service TechnologyAdministration, U.S. Department of Commerce, Springfield, VA22161, NTIS OrderNo. PB961704771NZ.4Compendium of Methods for the Determination of Toxic Organic
16、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 ofdifferent thicknesses to test
17、 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 spectrum. Create an absorpti
18、onspectrum 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 spectrum obtained in 6
19、.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 absorbance value of the sel
20、ected 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 known it can be calc
21、ulated by using Eq 1:b 512nNv12 v2!(1)where:b = thickness of the sample,n = refractive index of the sample,E1982 98 (2013)3N = number of interference fringes in the spectral rangefrom v1to v2,v1= first wavenumber in the spectral range over which thefringes are counted, andv2= second wavenumber in th
22、e 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 known amountand are coupl
23、ed 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 background spectrum fo
24、r 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 of the targetgas. Th
25、e 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 Measure the absorbance ma
26、xima 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 other. The ratio of th
27、eabsorbance 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 a different, known m
28、eshsize 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 linearization circuits may not pe
29、rformadequately 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 determined to be resp
30、onding 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 IR beam. Ambient radi
31、ation 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 measurements. The magnitu
32、de 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 IRbeam, point the teles
33、cope 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 eachmonitoring program
34、 or whenever optical components in thesystem are changed or realigned. An example of an internalstray light spectrum is given in Guide E1685.NOTE 5Internal stray light can also be caused by strong sources of IRradiation that are in the field of view of the instrument. For example, thesun may be in t
35、he 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, block or turn off thesourc
36、e 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 spectrum must beacquir
37、ed 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 typical formost appli
38、cations. An example of an ambient radiation spec-trum is given in Guide E1685.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 molecules inthe atmospher
39、e, 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 bedifferent for each site a
40、nd 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 along the path, but diver
41、ges 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 IR source atthe minimum
42、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 retroreflector or IRsource some dist
43、ance away from the receiving telescope, forE1982 98 (2013)4example, 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 determinethe distance at which the magnitude of the signal will reach
44、 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 signal from the active
45、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 to be used during the fiel
46、d 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 not important. Measure t
47、he 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 noise should be determined in
48、 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 forfuture reference.7. Logistical Concerns and Ancillary Measurements atthe Monitoring Sites7.1 Logistical ConcernsSeveral logistical con
49、cerns 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 required 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 ba
