ASTM E1982-98(2002)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: E 1982 – 98 (Reapproved 2002)
Standard Practice for
Open-Path Fourier Transform Infrared (OP/FT-IR) Monitoring
of Gases and Vapors in Air
This standard is issued under the fixed designation E1982; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope FT-1R Open-Path Monitoring Guidance Document
Compendium Method TO-16–Long-Path Open-Path Fou-
1.1 This practice covers procedures for using active open-
rier Transform Infrared Monitoring of Atmospheric
path Fourier transform infrared (OP/FT-IR) monitors to mea-
Gases
sure the concentrations of gases and vapors in air. Procedures
for choosing the instrumental parameters, initially operating
3. Terminology
the instrument, addressing logistical concerns, making ancil-
3.1 For definitions of terms used in this practice relating to
lary measurements, selecting the monitoring path, acquiring
general molecular spectroscopy, refer to Terminology E131.
data, analyzing the data, and performing quality control on the
3.2 For definitions of terms used in this practice relating to
dataaregiven.Becausethelogisticsanddataqualityobjectives
OP/FT-IR monitoring, refer to Guide E1865.
of each OP/FT-IR monitoring program will be unique, stan-
3.3 For definitions of general terms relating to optical
dardized procedures for measuring the concentrations of spe-
remote sensing, refer to the FT-IR Open Path Monitoring
cific gases are not explicitly set forth in this practice. Instead,
Guidance Document.
general procedures that are applicable to all IR-active gases
and vapors are described. These procedures can be used to
4. Significance and Use
develop standard operating procedures for specific OP/FT-IR
4.1 An OP/FT-IR monitor can, in principle, measure the
monitoring applications.
concentrations of all IR-active gases and vapors in the atmo-
1.2 This practice does not purport to address all of the
sphere. Detailed descriptions of OP/FT-IR systems and the
safety concerns, if any, associated with its use. It is the
fundamental aspects of their operation are given in Guide
responsibility of the user of this practice to establish appro-
E1865 and the FT-IR Open-Path Monitoring Guidance Docu-
priate safety and health practices and determine the applica-
ment. A method for processing OP/FT-IR data to obtain the
bility of regulatory limitations prior to use.
concentrations of gases over a long, open path is given in
Compendium Method TO-16. Applications of OP/FT-IR sys-
2. Referenced Documents
tems include monitoring for gases and vapors in ambient air,
2.1 ASTM Standards:
2 alongtheperimeterofanindustrialfacility,athazardouswaste
E131 Terminology Relating to Molecular Spectroscopy
sites and landfills, in response to accidental chemical spills or
E168 Practices for General Techniques of Infrared Quanti-
2 releases, and in workplace environments.
tative Analysis
E1421 PracticeforDescribingandMeasuringPerformance
5. Instrumental Parameters
of Fourier Transform Infrared (FT-IR) Spectrometers:
5.1 Several instrumental parameters must be chosen before
Level Zero and Level One Tests
data are collected with an OP/FT-IR system.These parameters
E1655 Practices for Infrared, Multivariate, Quantitative
2 include the measurement time, spectral resolution, apodization
Analysis
function, and zero filling factor. In some cases, the choice of
E1865 Guide for Open-Path Fourier Transform Infrared
2 these parameters might be limited by the parameters used to
(OP/FT-IR) Monitoring of Gases and Vapors in Air
2.2 Other Documents:
EPA/600/R-96/040, National Technical Information Service Technology Ad-
ministration, U.S. Department of Commerce, Springfield, VA 22161, NTIS Order
This practice is under the jurisdiction ofASTM Committee E-13 on Molecular
No. PB96–1704771NZ.
Spectroscopy and is the direct responsibility of Subcommittee E13.03 on Infrared
Compendium of Methods for the Determination of Toxic Organic Compounds
Spectroscopy.
in Ambient Air, 2nd Ed., EPA/625/R-96/010b, Center for Environmental Research
Current edition approved October 10, 1998. Published March 1999.
Info., Office of Research & Development, U.S. Environmental Protection Agency,
Annual Book of ASTM Standards, Vol 03.06.
Cincinnati, OH 45268, Jan. 1997.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 1982 – 98 (2002)
acquire and process the available reference spectra. Use the creation of a synthetic background spectrum (see 10.3) and a
following procedures to select the instrumental parameters for water vapor reference spectrum (see 10.6.1) from the field
each OP/FT-IR monitoring study. spectra. These procedures rely on a series of subjective
judgements, which require a visual inspection of the field
5.2 Measurement Time—Determine the measurement time
spectra. The use of a higher resolution generally facilitates the
requiredtoachievethedesiredsignal-to-noiseratio(S/N)atthe
ability of the operator to visualize the pertinent features of the
selected resolution (see 5.3 and 6.7). Verify that this measure-
field spectra.
ment time is appropriate for capturing the event being studied.
5.3.4 Assess the resolution requirements of the analysis
Ifthemeasurementtimeislongerthantheresidencetimeofthe
method. If the comparison (see 10.8.1) or scaled subtraction
plume in the path, the interferograms collected after the plume
(see 10.8.2) method is used, the resolution should be sufficient
has exited the path will not contain spectral information from
to separate the spectral features of the target gases from those
the target gas. Adding these signals in the interferogram
of the interfering species. If classical least squares (CLS) is
domain to signals that contain information from the target gas
−1
used (see 10.8.3), a resolution higher than 4 cm is generally
will result in a dilution effect and can cause band distortions
required(1). Ifpartialleastsquares(PLS)isused(see10.8.3),
and nonlinearities. The variability in the water vapor concen-
−1
a resolution as low as 16 cm may be sufficient (2).
tration along the path can also limit the use of extensive signal
averaging to improve the S/N. Measurement times from 1 to 5
NOTE 1—Most volatile organic compounds of interest in OP/FT-IR
min are typical for ambient monitoring, whereas shorter
monitoringapplicationshaveabsorptionenvelopeswithfullwidthsathalf
−1
measurement times may be required for plume modeling heights (FWHHs) of approximately 20 cm . This observation would
indicate that low-resolution spectra would be adequate for OP/FT-IR
studies.
measurements. However, each OP/FT-IR spectrum will also contain
5.3 Resolution—The choice of what spectral resolution to
featuresduetoambientgases,suchaswatervapor,carbondioxide,carbon
use while collecting OP/FT-IR data depends on the spectral −1
monoxide, and methane, which have FWHHs on the order of 0.2 cm at
characteristics of the target gases, the measurement time
atmospheric pressure. If low resolution measurements are made, the
required to observe the pollutant plume, the concentrations of analysis method must be able to handle the spectral overlap and
nonlinearities caused by an inadequate resolution of these atmospheric
the target gases, the presence of interfering species, the choice
gases.
of analysis method, and the data quality objectives of the
monitoring study. This choice might be limited by the capa-
5.4 Apodization—Use the same apodization function that
bilities of the specific OP/FT-IR monitor used to collect data.
was used to process the reference spectra. If a choice of
Most commercially available, portable OP/FT-IR monitors are
apodization function can be made, the Norton-Beer-medium
capableofproducingspectraatamaximumresolutionof0.5or
function typically yields the best representation of the true
−1
1cm , although instruments are available that will produce
absorbanceascomparedtoHapp-Genzelortriangularapodiza-
−1
spectraat0.125-cm resolution.Thereiscurrentlynoconsen-
tion.
sus as to the optimum resolution to use while collecting field
5.5 Zero Filling—Assuming that the field spectra were
data.Mostcurrentpractitionersusearesolutionofeither0.5or
acquiredatthesameresolutionasthereferencespectra,choose
−1
1.0cm ,althoughrecentadvancesininstrumentationanddata
zero-filling parameters that allow the data point density of the
analysis techniques provide for the potential of using much
field spectra to match that of the reference spectra. In general,
lowerresolutions.Thechoiceofresolutioncanalsoaffectother
the original interferogram should be zero filled to the degree
decisions that the operator must make before collecting or
that the number of data points used in the Fourier transform is
analyzing the data. For example, the spectral resolution affects
twicethatintheoriginalinterferogram.Noadvantageisgained
the type of background spectrum that can be used, the method
by zero filling by more than a factor of two for most
for generating a water vapor reference spectrum, and the
applications.
choiceofanalysismethod.Thefollowingstepscanbetakento
choose the best resolution for a particular application. 6. Initial Instrument Operation
5.3.1 Examine reference spectra of the target gases and
6.1 Several tests should be conducted before the OP/FT-IR
potential interfering species. If possible, acquire or obtain
monitor is deployed for a field study. These tests include
reference spectra of these gases at various resolutions. Deter-
measuring the electronic noise, the distance at which the
minethelowestresolutionthatresolvesthespectralfeaturesof
detector saturates, the linearity of the system, the signal due to
interest. Use this resolution as a starting point for future
internal stray light or ambient radiation, the signal strength as
measurements.
a function of distance, and the random baseline noise. Use the
instrumental parameters chosen in 5.2 through 5.5 for these
5.3.2 If the appropriate facilities are available, develop
calibration curves of the target gases at different resolutions. If tests.
6.2 Measure the Electronic Noise—Place a piece of opaque
an inadequate resolution is used, the relationship between the
peak absorbance and concentration will not be linear. This material in front of the detector element while the detector is
operational, for example after the mercury-cadmium-telluride
relationship is also affected by the apodization function (see
5.4).Ifthecompoundofinterestdoesnotrespondlinearlywith (MCT) detector has been cooled and has equilibrated. Record
respect to concentration, a correction curve must be applied to
the data during quantitative analysis.
5.3.3 Determine the effect of resolution on the other proce-
The boldface numbers in parentheses refer to the references at the end of this
dures involved with generating OP/FT-IR data, such as the practice.
E 1982 – 98 (2002)
the signal either as the interferogram or as a single-beam 6.4 Linear Response—There are two types of nonlinearity
spectrum with the detector blocked. This signal represents the that can affect OP/FT-IR data: detector nonlinearity and non-
electronic noise of the system. The magnitude of this signal linearity in absorbance. Evidence of detector nonlinearity can
should be less than 0.25% of the signal without the detector be observed by conducting the tests described in 6.3, although
blocked, remain relatively constant over time, and decrease theabsenceofnonphysicalenergyinthesingle-beamspectrum
with the square root of the measurement time. If this signal is doesnotguaranteethatthedetectorisoperatinglinearly.Some
uncharacteristically large, an electrical component is most MCT detectors exhibit nonlinear response even when there is
likely producing spurious noise. When this is the case, service no evidence of detector saturation. The OP/FT-IR system can
of the system is indicated. also exhibit nonlinearity in the change in absorbance with
respect to changes in concentration due to the convolution of
6.3 Measure the Distance to Detector Saturation—The
theinstrumentallineshapefunctionwiththespectraldata.The
distance at which the detector becomes saturated determines
choice of apodization function affects the severity of this
the minimum pathlength over which quantitative data can be
nonlinearity. If a multipoint calibration is used in the data
obtained without making changes to the instrument. Evidence
analysis, this type of nonlinearity can be accounted for.
of detector saturation indicates that the detector may not be
However, many OP/FT-IR systems rely on a single-point
responding linearly to changes in the incident light intensity.
calibration. When this type of calibration model is used, the
6.3.1 Set up the OP/FT-IR system with the retroreflector
absorbance of the reference spectra should match the absor-
(monostatic configuration) or external, active IR source (bi-
bance of the field spectra as closely as possible. The linearity
static configuration) at some predetermined distance, for ex-
of the system can be checked by using one of the following
ample, 25 m, from the receiving telescope.
methods: analyzing polymer films of different, known thick-
6.3.2 Align the system to maximize the detector output,
nesses; using a dual-chambered gas cell; or attenuating the
which can be measured either as the peak-to-peak voltage of
beam with wire screens of different, known mesh sizes.
the interferogram centerburst or the intensity of a specific
6.4.1 Polymer Films—Acquire spectra of polymer films of
wavenumber in the single-beam spectrum. If the intensity of
different thicknesses to test the linearity of the OP/FT-IR
the single-beam spectrum is used, choose a wavenumber
system.
region that does not contain any absorption bands due to the
6.4.1.1 Collect a single-beam spectrum over the monitoring
target gases or atmospheric gases, such as water vapor.
path without the polymer film in the beam. Use this spectrum
6.3.3 Obtain a single-beam spectrum.
as the background spectrum.
6.3.4 Examine the single-beam spectrum in the wavenum-
6.4.1.2 InsertapolymerfilmofknownthicknessintotheIR
ber region below the detector cutoff frequency.The instrument
beamandobtainasingle-beamspectrum.Createanabsorption
response in this region should be flat and at the baseline. An
spectrum from this spectrum by using the background spec-
elevated baseline in this wavenumber region is due to non-
trum acquired in 6.4.1.1.
physical energy and indicates that the detector is saturated. A
6.4.1.3 Replace the first pol
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