ASTM E1982-98(2007)
(Practice)Standard Practice for Open-Path Fourier Transform Infrared (OP/FT-IR) Monitoring of Gases and Vapors in Air
Standard Practice for Open-Path Fourier Transform Infrared (OP/FT-IR) Monitoring of Gases and Vapors in Air
SIGNIFICANCE AND USE
An OP/FT-IR monitor can, in principle, measure the concentrations of all IR-active gases and vapors in the atmosphere. Detailed descriptions of OP/FT-IR systems and the fundamental aspects of their operation are given in Guide E 1865 and the FT-IR Open-Path Monitoring Guidance Document. A method for processing OP/FT-IR data to obtain the concentrations of gases over a long, open path is given in Compendium Method TO-16. Applications of OP/FT-IR systems include monitoring for gases and vapors in ambient air, along the perimeter of an industrial facility, at hazardous waste sites and landfills, in response to accidental chemical spills or releases, and in workplace environments.
SCOPE
1.1 This practice covers procedures for using active open-path Fourier transform infrared (OP/FT-IR) monitors to measure the concentrations of gases and vapors in air. Procedures for choosing the instrumental parameters, initially operating the instrument, addressing logistical concerns, making ancillary measurements, selecting the monitoring path, acquiring data, analyzing the data, and performing quality control on the data are given. Because the logistics and data quality objectives of each OP/FT-IR monitoring program will be unique, standardized procedures for measuring the concentrations of specific gases are not explicitly set forth in this practice. Instead, general procedures that are applicable to all IR-active gases and vapors are described. These procedures can be used to develop standard operating procedures for specific OP/FT-IR monitoring applications.
1.2 This practice does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this practice to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E1982 − 98 (Reapproved2007)
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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope E1685Practice for Measuring the Change in Length of
Fasteners Using the Ultrasonic Pulse-Echo Technique
1.1 This practice covers procedures for using active open-
2.2 Other Documents:
path Fourier transform infrared (OP/FT-IR) monitors to mea-
FT-IR Open-Path Monitoring Guidance Document
sure the concentrations of gases and vapors in air. Procedures
Compendium Method TO-16Long-Path Open-Path Fourier
for choosing the instrumental parameters, initially operating
Transform Infrared Monitoring of Atmospheric Gases
the instrument, addressing logistical concerns, making ancil-
lary measurements, selecting the monitoring path, acquiring
3. Terminology
data, analyzing the data, and performing quality control on the
3.1 For definitions of terms used in this practice relating to
dataaregiven.Becausethelogisticsanddataqualityobjectives
general molecular spectroscopy, refer to Terminology E131.
of each OP/FT-IR monitoring program will be unique, stan-
dardized procedures for measuring the concentrations of spe- 3.2 For definitions of terms used in this practice relating to
cific gases are not explicitly set forth in this practice. Instead,
OP/FT-IR monitoring, refer to Guide E1685.
general procedures that are applicable to all IR-active gases
3.3 For definitions of general terms relating to optical
and vapors are described. These procedures can be used to
remote sensing, refer to the FT-IR Open Path Monitoring
develop standard operating procedures for specific OP/FT-IR
Guidance Document.
monitoring applications.
4. Significance and Use
1.2 This practice does not purport to address all of the
safety concerns, if any, associated with its use. It is the
4.1 An OP/FT-IR monitor can, in principle, measure the
responsibility of the user of this practice to establish appro-
concentrations of all IR-active gases and vapors in the atmo-
priate safety and health practices and determine the applica-
sphere. Detailed descriptions of OP/FT-IR systems and the
bility of regulatory limitations prior to use.
fundamental aspects of their operation are given in Guide
E1685 and the FT-IR Open-Path Monitoring Guidance Docu-
2. Referenced Documents
ment. A method for processing OP/FT-IR data to obtain the
concentrations of gases over a long, open path is given in
2.1 ASTM Standards:
E131Terminology Relating to Molecular Spectroscopy Compendium Method TO-16. Applications of OP/FT-IR sys-
tems include monitoring for gases and vapors in ambient air,
E168Practices for General Techniques of Infrared Quanti-
tative Analysis alongtheperimeterofanindustrialfacility,athazardouswaste
sites and landfills, in response to accidental chemical spills or
E1421Practice for Describing and Measuring Performance
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom- releases, and in workplace environments.
eters: Level Zero and Level One Tests
5. Instrumental Parameters
E1655 Practices for Infrared Multivariate Quantitative
Analysis 5.1 Several instrumental parameters must be chosen before
data are collected with an OP/FT-IR system.These parameters
include the measurement time, spectral resolution, apodization
This practice is under the jurisdiction ofASTM Committee E13 on Molecular function, and zero filling factor. In some cases, the choice of
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
mittee E13.03 on Infrared and Near Infrared Spectroscopy.
Current edition approved Dec. 1, 2007. Published December 2007. Originally EPA/600/R-96/040, National Technical Information Service Technology
approved in 1998. Last previous edition approved in 2002 as E1982–98(2002). Administration,U.S.DepartmentofCommerce,Springfield,VA22161,NTISOrder
DOI: 10.1520/E1982-98R07. No. PB96–1704771NZ.
2 4
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Compendium of Methods for the Determination of Toxic Organic Compounds
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM in Ambient Air, 2nd Ed., EPA/625/R-96/010b, Center for Environmental Research
Standards volume information, refer to the standard’s Document Summary page on Info., Office of Research & Development, U.S. Environmental Protection Agency,
the ASTM website. Cincinnati, OH 45268, Jan. 1997.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1982 − 98 (2007)
these parameters might be limited by the parameters used to 5.3.3 Determine the effect of resolution on the other proce-
acquire and process the available reference spectra. Use the dures involved with generating OP/FT-IR data, such as the
following procedures to select the instrumental parameters for creation of a synthetic background spectrum (see 10.3) and a
each OP/FT-IR monitoring study. water vapor reference spectrum (see 10.6.1) from the field
spectra. These procedures rely on a series of subjective
5.2 Measurement Time—Determine the measurement time
judgements, which require a visual inspection of the field
requiredtoachievethedesiredsignal-to-noiseratio(S/N)atthe
spectra. The use of a higher resolution generally facilitates the
selected resolution (see 5.3 and 6.7). Verify that this measure-
ability of the operator to visualize the pertinent features of the
ment time is appropriate for capturing the event being studied.
field spectra.
Ifthemeasurementtimeislongerthantheresidencetimeofthe
5.3.4 Assess the resolution requirements of the analysis
plume in the path, the interferograms collected after the plume
method. If the comparison (see 10.8.1) or scaled subtraction
has exited the path will not contain spectral information from
(see 10.8.2) method is used, the resolution should be sufficient
the target gas. Adding these signals in the interferogram
to separate the spectral features of the target gases from those
domain to signals that contain information from the target gas
of the interfering species. If classical least squares (CLS) is
will result in a dilution effect and can cause band distortions −1
used (see 10.8.3), a resolution higher than 4 cm is generally
and nonlinearities. The variability in the water vapor concen- 5
required (1). Ifpartialleastsquares(PLS)isused(see10.8.3),
tration along the path can also limit the use of extensive signal −1
a resolution as low as 16 cm may be sufficient (2).
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
measurement times may be required for plume modeling
−1
heights (FWHHs) of approximately 20 cm . This observation would
studies.
indicate that low-resolution spectra would be adequate for OP/FT-IR
measurements. However, each OP/FT-IR spectrum will also contain
5.3 Resolution—The choice of what spectral resolution to
featuresduetoambientgases,suchaswatervapor,carbondioxide,carbon
−1
use while collecting OP/FT-IR data depends on the spectral
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
analysis method must be able to handle the spectral overlap and
required to observe the pollutant plume, the concentrations of
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
5.4 Apodization—Use the same apodization function that
monitoring study. This choice might be limited by the capa-
was used to process the reference spectra. If a choice of
bilities of the specific OP/FT-IR monitor used to collect data.
apodization function can be made, the Norton-Beer-medium
Most commercially available, portableOP/FT-IR monitors are
function typically yields the best representation of the true
capableofproducingspectraatamaximumresolutionof0.5or
−1
absorbanceascomparedtoHapp-Genzelortriangularapodiza-
1cm
, although instruments are available that will produce
−1 tion.
spectraat0.125-cm resolution.Thereiscurrentlynoconsen-
5.5 Zero Filling—Assuming that the field spectra were
sus as to the optimum resolution to use while collecting field
data.Mostcurrentpractitionersusearesolutionofeither0.5or acquiredatthesameresolutionasthereferencespectra,choose
−1
zero-filling parameters that allow the data point density of the
1.0cm ,althoughrecentadvancesininstrumentationanddata
analysis techniques provide for the potential of using much field spectra to match that of the reference spectra. In general,
the original interferogram should be zero filled to the degree
lowerresolutions.Thechoiceofresolutioncanalsoaffectother
that the number of data points used in the Fourier transform is
decisions that the operator must make before collecting or
twicethatintheoriginalinterferogram.Noadvantageisgained
analyzing the data. For example, the spectral resolution affects
by zero filling by more than a factor of two for most
the type of background spectrum that can be used, the method
applications.
for generating a water vapor reference spectrum, and the
choiceofanalysismethod.Thefollowingstepscanbetakento
6. Initial Instrument Operation
choose the best resolution for a particular application.
6.1 Several tests should be conducted before the OP/FT-IR
5.3.1 Examine reference spectra of the target gases and
monitor is deployed for a field study. These tests include
potential interfering species. If possible, acquire or obtain
measuring the electronic noise, the distance at which the
reference spectra of these gases at various resolutions. Deter-
detector saturates, the linearity of the system, the signal due to
minethelowestresolutionthatresolvesthespectralfeaturesof
internal stray light or ambient radiation, the signal strength as
interest. Use this resolution as a starting point for future
a function of distance, and the random baseline noise. Use the
measurements.
instrumental parameters chosen in 5.2 through 5.5 for these
5.3.2 If the appropriate facilities are available, develop
tests.
calibration curves of the target gases at different resolutions. If
6.2 Measure the Electronic Noise—Place a piece of opaque
an inadequate resolution is used, the relationship between the
material in front of the detector element while the detector is
peak absorbance and concentration will not be linear. This
operational, for example after the mercury-cadmium-telluride
relationship is also affected by the apodization function (see
5.4).Ifthecompoundofinterestdoesnotrespondlinearlywith
respect to concentration, a correction curve must be applied to
The boldface numbers in parentheses refer to a list of references at the end of
the data during quantitative analysis. this standard.
E1982 − 98 (2007)
(MCT) detector has been cooled and has equilibrated. Record 6.4 Linear Response—There are two types of nonlinearity
the signal either as the interferogram or as a single-beam that can affect OP/FT-IR data: detector nonlinearity and non-
spectrum with the detector blocked. This signal represents the linearity in absorbance. Evidence of detector nonlinearity can
electronic noise of the system. The magnitude of this signal be observed by conducting the tests described in 6.3, although
should be less than 0.25% of the signal without the detector
theabsenceofnonphysicalenergyinthesingle-beamspectrum
blocked, remain relatively constant over time, and decrease doesnotguaranteethatthedetectorisoperatinglinearly.Some
with the square root of the measurement time. If this signal is
MCT detectors exhibit nonlinear response even when there is
uncharacteristically large, an electrical component is most no evidence of detector saturation. The OP/FT-IR system can
likely producing spurious noise. When this is the case, service
also exhibit nonlinearity in the change in absorbance with
of the system is indicated. respect to changes in concentration due to the convolution of
theinstrumentallineshapefunctionwiththespectraldata.The
6.3 Measure the Distance to Detector Saturation—The
choice of apodization function affects the severity of this
distance at which the detector becomes saturated determines
nonlinearity. If a multipoint calibration is used in the data
the minimum pathlength over which quantitative data can be
analysis, this type of nonlinearity can be accounted for.
obtained without making changes to the instrument. Evidence
However, many OP/FT-IR systems rely on a single-point
of detector saturation indicates that the detector may not be
calibration. When this type of calibration model is used, the
responding linearly to changes in the incident light intensity.
absorbance of the reference spectra should match the absor-
6.3.1 Set up the OP/FT-IR system with the retroreflector
bance of the field spectra as closely as possible. The linearity
(monostatic configuration) or external, active IR source (bi-
of the system can be checked by using one of the following
static configuration) at some predetermined distance, for
methods: analyzing polymer films of different, known thick-
example, 25 m, from the receiving telescope.
nesses; using a dual-chambered gas cell; or attenuating the
6.3.2 Align the system to maximize the detector output,
beam with wire screens of different, known mesh sizes.
which can be measured either as the peak-to-peak voltage of
6.4.1 Polymer Films—Acquire spectra of polymer films of
the interferogram centerburst or the intensity of a specific
different thicknesses to test the linearity of the OP/FT-IR
wavenumber in the single-beam spectrum. If the intensity of
system.
the single-beam spectrum is used, choose a wavenumber
6.4.1.1 Collect a single-beam spectrum over the monitoring
region that does not contain any absorption bands due to the
path without the polymer film in the beam. Use this spectrum
target gases or atmospheric gases, such as water vapor.
as the background spectrum.
6.3.3 Obtain a single-beam spectrum.
6.4.1.2 InsertapolymerfilmofknownthicknessintotheIR
6.3.4 Examine the single-beam spectrum in the wavenum-
beamandobtainasingle-beamspectrum.Createanabsorption
ber region below
...
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