ASTM E1982-98(2021)

Standard Practice for Open-Path Fourier Transform Infrared (OP/FT-IR) Monitoring of Gases and Vapors in Air

SIGNIFICANCE AND USE
4.1 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 Practice E1685 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 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.3 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Mar-2021
• ICS: 17.180.30 - Optical measuring instruments
• Technical Committee: E13 - Molecular Spectroscopy and Separation Science
• Drafting Committee: E13.03 - Infrared and Near Infrared Spectroscopy

Relations

• Refers: ASTM E131-10 - Standard Terminology Relating to Molecular Spectroscopy
• Effective Date: 01-Mar-2010
• Refers: ASTM E1421-99(2009) - Standard Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FT-MIR) Spectrometers: Level Zero and Level One Tests
• Effective Date: 01-Mar-2009
• Refers: ASTM E1685-00(2006) - Standard Practice for Measuring the Change in Length of Fasteners Using the Ultrasonic Pulse-Echo Technique
• Effective Date: 01-Dec-2006
• Refers: ASTM E168-06 - Standard Practices for General Techniques of Infrared Quantitative Analysis (Withdrawn 2015)
• Effective Date: 01-Mar-2006
• Refers: ASTM E131-05 - Standard Terminology Relating to Molecular Spectroscopy
• Effective Date: 01-Sep-2005
• Refers: ASTM E1655-04 - Standard Practices for Infrared Multivariate Quantitative Analysis
• Effective Date: 01-Dec-2004
• Refers: ASTM E168-99(2004) - Standard Practices for General Techniques of Infrared Quantitative Analysis
• Effective Date: 01-Feb-2004
• Refers: ASTM E1421-99(2004) - Standard Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FT-MIR) Spectrometers: Level Zero and Level One Tests
• Effective Date: 01-Feb-2004
• Refers: ASTM E131-02 - Standard Terminology Relating to Molecular Spectroscopy
• Effective Date: 10-Sep-2002
• Refers: ASTM E1685-00 - Standard Practice for Measuring the Change in Length of Fasteners Using the Ultrasonic Pulse-Echo Technique
• Effective Date: 10-Oct-2000
• Refers: ASTM E1655-00 - Standard Practices for Infrared Multivariate Quantitative Analysis
• Effective Date: 10-Sep-2000
• Refers: ASTM E131-00a - Standard Terminology Relating to Molecular Spectroscopy
• Effective Date: 10-Sep-2000
• Refers: ASTM E1421-99 - Standard Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FT-MIR) Spectrometers Level Zero and Level One Tests
• Effective Date: 10-Oct-1999
• Refers: ASTM E168-99 - Standard Practices for General Techniques of Infrared Quantitative Analysis
• Effective Date: 10-Oct-1999

Overview

ASTM E1982-98(2021), titled Standard Practice for Open-Path Fourier Transform Infrared (OP/FT-IR) Monitoring of Gases and Vapors in Air, is an international standard developed by ASTM International. This standard specifies procedures for using active open-path Fourier Transform Infrared (OP/FT-IR) monitors to detect and measure the concentrations of IR-active gases and vapors in ambient air. The document provides a comprehensive framework for selecting instrumentation parameters, operating procedures, logistical considerations, site selection, data acquisition, data analysis, and quality control measures in field monitoring scenarios.

OP/FT-IR monitoring is a non-intrusive, real-time sensing method suited for applications such as environmental air quality monitoring, industrial perimeter surveillance, hazardous waste site assessment, landfill emissions tracking, and emergency response to chemical spills. This practice is applicable to any IR-active gas or vapor, offering a flexible and valuable tool for regulatory compliance, workplace safety, and environmental stewardship.

Key Topics

• Instrumental Parameter Selection: Guidance is provided for choosing measurement time, spectral resolution, apodization, and zero-filling to optimize data quality. This includes addressing baseline noise, detector linearity, and influences from internal stray light or ambient radiation.
• Initial Instrument Setup & Testing: Procedures outline essential tests, such as electronic noise measurement, detector saturation distance, linear response checks, and signal strength as a function of path length.
• Logistical & Environmental Concerns: The standard addresses power supply, instrument mounting, vibration isolation, optical alignment, and protection of equipment from weather, moisture, and corrosion. Additional requirements for on-site climate control and meteorological data collection are described.
• Path & Site Selection: General principles for choosing monitoring path location and length are established, considering wind direction, pollutant source proximity, and the physical extent of pollutant plumes.
• Data Acquisition & Analysis: Steps include aligning the system, acquiring spectral data, data file selection, background and reference spectrum generation, and statistical assessment for minimum detection limits. Ancillary measurements such as temperature, barometric pressure, relative humidity, and wind velocity are integrated for comprehensive data interpretation.
• Quality Control: Quality assurance is maintained through routine performance checks, RMS noise measurement, and comparisons against reference data and detection limits.
• Safety & Compliance: The standard emphasizes the responsibility for user safety and adherence to local regulatory requirements.

Applications

ASTM E1982-98(2021) is applicable to a wide range of practical scenarios, such as:

• Industrial Perimeter Monitoring: Real-time detection of fugitive emissions around manufacturing plants to ensure regulatory compliance and protect public health.
• Hazardous Waste Sites & Landfills: Continuous monitoring of off-gassing and potential contaminant plumes, supporting remediation and risk assessment efforts.
• Emergency Response: Rapid assessment of air quality in incidents involving accidental chemical releases or toxic spills.
• Workplace Air Quality: Monitoring exposure to hazardous vapors in occupational settings, supporting occupational health and safety management.
• Environmental Baseline Studies: Long-term surveillance of atmospheric gases in ambient air for environmental protection and research.

The standard’s general procedures are tailored for flexibility, enabling organizations to develop specific standard operating procedures for unique monitoring programs.

Related Standards

ASTM E1982-98(2021) references and complements several other important standards and guidance documents, including:

• ASTM E131: Terminology Relating to Molecular Spectroscopy.
• ASTM E168: Practices for General Techniques of Infrared Quantitative Analysis.
• ASTM E1421: Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared Spectrometers.
• ASTM E1655: Practices for Infrared Multivariate Quantitative Analysis.
• ASTM E1685: Practice for OP/FT-IR monitoring system fundamentals.
• FT-IR Open-Path Monitoring Guidance Document: Provides detailed operational guidance.
• EPA Compendium Method TO-16: Covers long-path FT-IR monitoring of atmospheric gases.

These related documents provide additional detail and techniques to optimize the application, accuracy, and reliability of OP/FT-IR monitoring as described in ASTM E1982-98(2021).

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Keywords: ASTM E1982-98, OP/FT-IR, open-path FTIR, gas monitoring, vapor detection, air quality, environmental monitoring, industrial emissions, hazardous waste, infrared spectroscopy, atmospheric surveillance, standard operating procedures, environmental standards.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E1982 − 98 (Reapproved 2021)
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 2. Referenced Documents
1.1 This practice covers procedures for using active open- 2.1 ASTM Standards:
path Fourier transform infrared (OP/FT-IR) monitors to mea- E131Terminology Relating to Molecular Spectroscopy
sure the concentrations of gases and vapors in air. Procedures E168Practices for General Techniques of Infrared Quanti-
for choosing the instrumental parameters, initially operating
tative Analysis
the instrument, addressing logistical concerns, making ancil-
E1421Practice for Describing and Measuring Performance
lary measurements, selecting the monitoring path, acquiring
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
data, analyzing the data, and performing quality control on the
eters: Level Zero and Level One Tests
dataaregiven.Becausethelogisticsanddataqualityobjectives
E1655 Practices for Infrared Multivariate Quantitative
of each OP/FT-IR monitoring program will be unique, stan-
Analysis
dardized procedures for measuring the concentrations of spe-
E1685PracticeforMeasuringtheChangeinLengthofBolts
cific gases are not explicitly set forth in this practice. Instead,
Using the Ultrasonic Pulse-Echo Technique
general procedures that are applicable to all IR-active gases
2.2 Other Documents:
and vapors are described. These procedures can be used to
FT-IR Open-Path Monitoring Guidance Document
develop standard operating procedures for specific OP/FT-IR
Compendium Method TO-16Long-Path Open-Path Fourier
monitoring applications.
Transform Infrared Monitoring of Atmospheric Gases
1.2 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
3. Terminology
standard.
3.1 For definitions of terms used in this practice relating to
1.3 This practice does not purport to address all of the
general molecular spectroscopy, refer to Terminology E131.
safety concerns, if any, associated with its use. It is the
3.2 For definitions of terms used in this practice relating to
responsibility of the user of this practice to establish appro-
OP/FT-IR monitoring, refer to Practice E1685.
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
3.3 For definitions of general terms relating to optical
1.4 This international standard was developed in accor-
remote sensing, refer to the FT-IR Open Path Monitoring
dance with internationally recognized principles on standard-
Guidance Document.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Barriers to Trade (TBT) Committee.
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
EPA/600/R-96/040, National Technical Information Service Technology
This practice is under the jurisdiction ofASTM Committee E13 on Molecular Administration,U.S.DepartmentofCommerce,Springfield,VA22161,NTISOrder
Spectroscopy and Separation Science and is the direct responsibility of Subcom- No. PB96–1704771NZ.
mittee E13.03 on Infrared and Near Infrared Spectroscopy. Compendium of Methods for the Determination of Toxic Organic Compounds
Current edition approved April 1, 2021. Published April 2021. Originally in Ambient Air, 2nd Ed., EPA/625/R-96/010b, Center for Environmental Research
approved in 1998. Last previous edition approved in 2013 as E1982–98(2013). Info., Office of Research & Development, U.S. Environmental Protection Agency,
DOI: 10.1520/E1982-98R21. Cincinnati, OH 45268, Jan. 1997.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1982 − 98 (2021)
4. Significance and Use collecting or analyzing the data. For example, the spectral
resolution affects the type of background spectrum that can be
4.1 An OP/FT-IR monitor can, in principle, measure the
used, the method for generating a water vapor reference
concentrations of all IR-active gases and vapors in the atmo-
spectrum, and the choice of analysis method. The following
sphere. Detailed descriptions of OP/FT-IR systems and the
stepscanbetakentochoosethebestresolutionforaparticular
fundamental aspects of their operation are given in Practice
application.
E1685 and the FT-IR Open-Path Monitoring Guidance Docu-
5.3.1 Examine reference spectra of the target gases and
ment. A method for processing OP/FT-IR data to obtain the
potential interfering species. If possible, acquire or obtain
concentrations of gases over a long, open path is given in
reference spectra of these gases at various resolutions. Deter-
Compendium Method TO-16. Applications of OP/FT-IR sys-
minethelowestresolutionthatresolvesthespectralfeaturesof
tems include monitoring for gases and vapors in ambient air,
interest. Use this resolution as a starting point for future
alongtheperimeterofanindustrialfacility,athazardouswaste
measurements.
sites and landfills, in response to accidental chemical spills or
5.3.2 If the appropriate facilities are available, develop
releases, and in workplace environments.
calibration curves of the target gases at different resolutions. If
an inadequate resolution is used, the relationship between the
5. Instrumental Parameters
peak absorbance and concentration will not be linear. This
5.1 Several instrumental parameters must be chosen before
relationship is also affected by the apodization function (see
data are collected with an OP/FT-IR system.These parameters 5.4).Ifthecompoundofinterestdoesnotrespondlinearlywith
include the measurement time, spectral resolution, apodization
respect to concentration, a correction curve must be applied to
function, and zero filling factor. In some cases, the choice of the data during quantitative analysis.
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-
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 1min
to 5 min are typical for ambient monitoring, whereas shorter
NOTE 1—Most volatile organic compounds of interest in OP/FT-IR
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
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
the target gases, the presence of interfering species, the choice nonlinearities caused by an inadequate resolution of these atmospheric
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, portableOP⁄FT-IR monitors are
apodization function can be made, the Norton-Beer-medium
capable of producing spectra at a maximum resolution of 0.5
function typically yields the best representation of the true
−1 −1
cm or1cm , although instruments are available that will
absorbanceascomparedtoHapp-Genzelortriangularapodiza-
−1
producespectraat0.125cm resolution.Thereiscurrentlyno
tion.
consensus as to the optimum resolution to use while collecting
5.5 Zero Filling—Assuming that the field spectra were
field data. Most current practitioners use a resolution of either
acquiredatthesameresolutionasthereferencespectra,choose
−1 −1
0.5cm or 1.0 cm , although recent advances in instrumen-
tation and data analysis techniques provide for the potential of
using much lower resolutions. The choice of resolution can
The boldface numbers in parentheses refer to a list of references at the end of
also affect other decisions that the operator must make before this standard.
E1982 − 98 (2021)
zero-filling parameters that allow the data point density of the 6.3.5 If nonphysical energy is observed in the single-beam
field spectra to match that of the reference spectra. In general, spectrum obtained at the initial pathlength, increase the path-
the original interferogram should be zero filled to the degree length until the instrument response below the detector cutoff
that the number of data points used in the Fourier transform is frequency is flat and at the baseline. This distance represents
twicethatintheoriginalinterferogram.Noadvantageisgained
the minimum operating pathlength.
by zero filling by more than a factor of two for most
6.3.6 If the instrument response below the detector cutoff
applications.
frequency is flat and at the baseline in the single-beam
spectrum obtained at the initial pathlength, decrease the path-
6. Initial Instrument Operation
length until nonphysical energy is observed in the single-beam
spectrum. This distance represents the minimum operating
6.1 Several tests should be conducted before the OP/FT-IR
pathlength.
monitor is deployed for a field study. These tests include
measuring the electronic noise, the distance at which the 6.3.7 If nonphysical energy is observed at the desired
monitoring pathlength and the pathlength cannot be increased,
detector saturates, the linearity of the system, the signal due to
internal stray light or ambient radiation, the signal strength as attenuatetheIRsignalbyplacingafinewiremeshscreeninthe
modulated,collimatedbeam.Changingthegainofthedetector
a function of distance, and the random baseline noise. Use the
instrumental parameters chosen in 5.2 through 5.5 for these preamplifier to lower the magnitude of the signal is not useful
because the detector nonlinearity does not depend on gain.
tests.
6.2 Measure the Electronic Noise—Place a piece of opaque
NOTE 2—Determining the distance at which the detector becomes
material in front of the detector element while the detector is saturated is particularly important for MCT detectors. Detector saturation
is not as severe a problem for thermal detectors, such as deuterated
operational, for example after the mercury-cadmium-telluride
triglycine sulfate detectors.
(MCT) detector has been cooled and has equilibrated. Record
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
of the system can be checked by using one of the following
example, 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
test for determining the ratio of the nonphysical energy to the 6.4.1.3 Replace the first polymer film with another film of a
maximum energy in the single-beam spectrum is given in different, known thickness and obtain a single-beam spectrum.
Practice E1421. An example of an OP/FT-IR spectrum that Createanabsorptionspectrumfromthisspectrumbyusingthe
exhibits nonphysical energy is given in Practice E1685. background spectrum obtained in 6.4.1.1.
E1982 − 98 (2021)
NOTE 4—Linearization circuits are available to minimize the problem
6.4.1.4 Measure the absorbance maxima of selected bands
of detector nonlinearity. These linearization circuits may not perform
in the two absorption spectra acquired in 6.4.1.2 and 6.4.1.3.
adequately for all detectors.
Choose absorption bands that are not saturated. Perform this
test on several absorption bands in different regions of the 6.5 Measure the Signal Due to Internal Stray Light or
spectrum. Ambient Radiation—Single-beam spectra recorded with an
6.4.1.5 Compare the absorbance value of the selected band OP/FT-IR monitor can exhibit a non-zero response in wave-
in the spectrum of one polymer film to that measured in the number regions in which the atmosphere is totally opaque. If
the detector has been determined to be responding linearly to
other. The ratio of the absorbance values of the two different
films should be equal to the ratio of the film thicknesses. changes in the incident light intensity, this non-zero response
can be attributed to either internal stray light or ambient
NOTE 3—If the thickness of the polymer film used to test the linearity
radiation. Internal stray light is most likely to be a problem in
of the system is not known it can be calculated by using Eq 1:
monostatic systems that use a single telescope to transmit and
1 N
receive the IR beam.Ambient radiation mostly affects bistatic
b 5 (1)
2n v 2 v
~ !
1 2 systems in which an unmodulated, active IR source is sepa-
rated from the interferometer and detector. The presence of
where:
internal stray light or ambient radiation causes errors in the
b = thickness of the sample,
n = refractive index of the sample, photometric accuracy and, ultimately, errors in the concentra-
N = number of interference fringes in the spectral range from v to v ,
1 2 tion measurements. The magnitude of the instrument response
v = first wavenumber in the spectral range over which the fringes are
due to internal stray light or ambient radiation determines the
counted, and
minimum useful signal that can be measured with the OP/
v = second wavenumber in the spectral range over which the fringes
FT-IR system.
are counted.
6.5.1 Measure the Internal Stray Light—In monostatic sys-
6.4.2 Dual-Chambered Gas Cell—Use a dual-chambered
tems that use a single telescope to transmit and receive the IR
gascellcontainingahighconcentrationofthetargetgastotest
beam,pointthetelescopeawayfromtheretroreflectorormove
the linearity of the system. This cell should be designed with
the retroreflector out of the field of view of the telescope and
two sample chambers that differ in length by a known amount
collect a single-beam spectrum. This spectrum represents the
and are coupled so that each chamber contains the same
internal stray light of the system and is independent of the
concentration of the target gas (3).
pathlength. Record this spectrum at the beginning of each
6.4.2.1 Fill the dual-chambered cell with dry nitrogen at
monitoring program or whenever optical components in the
atmospheric pressure and insert it into the IR beam.
system are changed or realigned. An example of an internal
6.4.2.2 Acquire a single-beam spectrum along the monitor-
stray light spectrum is given in Practice E1685.
ingpath.Usethisspectrumasthebackgroundspectrumforthe
chamber that is in the IR beam.
NOTE5—InternalstraylightcanalsobecausedbystrongsourcesofIR
radiation that are in the field of view of the instrument. For example, the
6.4.2.3 Repositionthecellsothattheotherchamberisinthe
sun may be in the instrument’s field of view during sunrise or sunset and
IR beam, and acquire a single-beam spectrum along the
cause an unwanted signal from reflections inside the instrument.
monitoring path. Use this spectrum as the background spec-
6.5.2 Measure the Ambient Radiation—In bistatic systems,
trum for that chamber.
which use an unmodulated, active IR source that is separated
6.4.2.4 Fill the cell with a high concentration of the target
from the interferometer and detector, block or turn off the
gas.The absolute concentration of the target gas does not need
to be known with this method. source and collect a single-beam spectrum. This spectrum is a
record of the IR radiation emitted by the objects in the field of
6.4.2.5 Acquire single-beam spectra alternatively with each
viewoftheinstrument.Becausethisspectrumdependsonwhat
chamber positioned in the IR beam. Create absorption spectra
objects are in the field of view, it also depends on the
by using the appropriate background spectrum for each cham-
pathlength. Thus, the ambient radiation spectrum must be
ber.
acquired each time the pathlength is changed or whenever
6.4.2.6 Measure the absorbance maxima of selected bands
different objects come into the field of view. A recommended
in the two spectra created in 6.4.2.5. Choose absorption bands
schedule for recording the ambient radiation spectrum has not
that are not saturated. Perform this test on several absorption
been determined for all situations. However, recording an
bands in different regions of the spectrum.
ambient radiation spectrum once every half hour is typical for
6.4.2.7 Compare the absorbance value measured with one
most applications. An example of an ambient radiation spec-
chamber to that measured with the other. The ratio of the
trum is given in Practice E1685.
absorbancevaluesmeasuredwiththetwoseparatechambersin
the beam should be equal to the ratio of the lengths of the
NOTE 6—The ambient radiation spectrum recorded by an OP/FT-IR
chambers.
monitorisacompositeofthevariousIRsourcesinthefieldofviewofthe
6.4.3 Wire Mesh Screens—Insert a wire screen of a known instrument,suchasgraybodyradiators,emissionbandsfrommoleculesin
the atmosphere, and the instrument itself. Because the ambient radiation
mesh size in the IR beam and record the signal. Remove this
spectrum is temperature dependent, its relative contribution to the total
wire screen, insert another screen of a different, known mesh
signal will vary. This variation will most likely be greater than any other
size in the beam, and record the signal.The ratio of the signals
source of instrumental noise. The ambient radiation spectrum will be
obtained with the two different screens should be equal to the
different for each site and can also change with varying meteorological
ratio of the mesh sizes of the screens. conditionsthroughouttheday.Forexample,cloudcovercanattenuatethe
E1982 − 98 (2021)
atmospheric emission bands.
portable12V power supply, such as a car or marine battery.
The output of the battery must be stabilized for quantitative
6.6 Measure the Signal Strength as a Function of
measurements.
Pathlength—In OP/FT-IR systems, the IR beam is collimated
7.1.2 Mounting and Support—For short-term field studies,
before it is transmitted along the path, but diverges as it
thespectrometer,theretroreflector,ortheremoteIRsourceare
traversesthepath.Oncethediameterofthebeamislargerthan
typically mounted on transportable tripods with swivel heads
the retroreflector (monostatic system) or the receiving tele-
that allow for vertical and horizontal adjustments. For perma-
scope (bistatic system), the signal strength will diminish as the
nent installations, a more rigid mounting system can be used.
square of the pathlength.
In either case, the OP/FT-IR monitor should be isolated from
6.6.1 StartwiththeretroreflectorortheexternalIRsourceat
vibrations.
the minimum pathlength as determined in 6.3. Record the
7.1.3 Climate Control—Although some OP/FT-IR systems
magnitude of the signal either as the peak-to-peak voltage of
might be designed to withstand the elements, some effort
the interferogram centerburst or as the intensity of the single-
shouldbemadetoprotecttheopticalandelectricalcomponents
beam spectrum at a specific wavenumber. Once the initial
of the system from rain and other forms of moisture, corrosive
measurement has been recorded, move the retroreflector or IR
gases, and extreme cold or heat.
source some distance away from the receiving telescope, for
7.1.3.1 Spectrometers with hygroscopic internal optics,
example, 25 m, and record the magnitude of the signal.
such as KBr beamsplitter, must be purged with a dry, inert gas
Continuethisprocedureuntilthesignaldecreasesasthesquare
or hermetically sealed to prevent moisture from damaging the
ofthemonitoringpathlength.Extrapolatethedatatodetermine
optics.Asanalternative,ZnSeopticalcomponentscanbeused.
thedistanceatwhichthemagnitudeofthesignalwillreachthat
7.1.3.2 Water vapor can condense on optical components,
of the random noise (see 6.7), internal stray light, or ambient
such as the retroreflector, that are exposed to the atmosphere.
radiation.Thisdistancerepresentsthemaximumpathlengthfor
Somemethodtopreventthiscondensation,suchasheatingthe
that particular OP/FT-IR monitor.
componentslightlyabovethedewpoint,mustbeimplemented.
NOTE 7—In bistatic systems, the relative contribution of the ambient 7.1.3.3 If exposure of the optical components to a corrosive
radiation to the total signal increases as the signal from the active IR
environmentcannotbeavoided,devisesometypeofsystemto
sourcedecreases.AsthesignalfromtheactiveIRsourceapproacheszero,
purge the surface of the optical components to minimize this
there may be apparent shifts in the peak intensity of the single-beam
exposure.
spectrum.
7.1.3.4 The spectral response of the spectrometer can be
6.7 Determine the Random Baseline Noise of the System—
sensitive to changes in ambient temperature. In some
Set up the instrument at a pathlength that is representative of
instruments, the interferometer will not scan at ambient tem-
that to be used during the field study. Collect two single-beam
peratures below 5°C. In permanent installations, the tempera-
spectra sequentially. Do not allow any time to elapse between
ture inside the shelter that houses the spectrometer should be
the acquisition of these two spectra. Create an absorption
controlled and monitored. For short-term field studies con-
spectrum from these two spectra by using one spectrum as a
ducted in cold-weather climates, the spectrometer should be
background spectrum. Which spectrum is used for the back-
covered with some type of heated, insulating material.
ground is not important. Measure the random noise as the
7.2 Ancillary Measurements—Make continuous, real-time
root-mean-square (RMS) noise (4). The actual wavenumber
measurements of the following parameters: temperature, rela-
range over which the noise should be calculated will vary with
tive humidity, barometric pressure, and wind velocity. These
the number of data points per wavenumber in the spectrum.A
measurementsshouldberecordedandarchivedwithsometype
range of 98 data points is optimum for the RMS noise
ofautomateddatalogger.Guidanceforselectingandsettingup
calculation. The RMS noise should be determined in wave-
the instruments for making meteorological measurements is
number regions that are not significantly impacted by water
−1 −1 −1
given in a United States Environmental Protection Agency
vapor,forexample,958cm to1008cm ,2480cm to2530
−1 −1 −1
(USEPA) handbook (5). Although this handbook does not
cm , and 4375cm to 4425 cm . Record the value of the
directly address open-path measurements, it provides useful
RMS noise for future reference.
information about meteorological instrumentation and mea-
surements.
7. Logistical Concerns and Ancillary Measurements at
the Monitoring Sites
NOTE8—Ameasurementofrelativehumidityisnotsatisfactoryforuse
in OP/FT-IR monitoring. The actual partial pressure of water vapor must
7.1 Logistical Concerns—Several logistical concerns must
bedetermined.Ifrelativehumidityismeasured,thenthetemperaturemust
be addressed at each monitoring site before the OP/FT-IR
also be recorded so that the partial pressure of water can be calculated by
consulting the Smithsonian psychrometric tables. These tablescan be
monitor is deployed in the field. Consideration must be given
found in the Handbook of Chemistry and Physics (6).
to power requirements, mounting and support requirements,
and climate control. Some ancillary measurements should also
8. Selecting the Monitoring Path
be made.
7.1.1 Power—Supply the required electrical power to the 8.1 Themonitoringpathcanbeselectedoncethelocationof
spectrometer. In bistatic systems with a remote IR source, an the pollutant source is known, pertinent meteorological data
additional source of power must be provided if an electrical areavailable,andspecifictargetgaseshavebeenchosenforthe
outlet is not available. Some IR sources can operate off a monitoring program.
E1982 − 98 (2021)
8.2 Orient the Path—Determinethedirectionoftheprevail- is interrogated by the IR beam. The pathlength should be
ing winds. Set up the monitoring path downward of the nominally longer than the width of the plume to account for
pollutant source and perpendicular to the wind field. Unless variations in the plume over time. For homogeneously distrib-
there is a specific need to do otherwise, the path should be uted gases, the path can be made longer, if needed, to increase
horizontal to the ground because the concentration contours of the measured absorbance. For plumes of finite extent, making
the target gases can vary with altitude. An example of a the path longer than the width of the plume is detrimental
possible orientation of the monitoring path relative to the because the OP/FT-IR monitor measures the path-averaged
pollutant source area is given in Fig. 1. concentration. If part of the path has zero concentration, then
there is a dilution effect. In some applications, the pathlength
NOTE 9—The USEPAhas amended Part 58 of Chapter 1 of Title 40 of
might be determined by logistical concerns, such as the
the Code of Federal Regulations (40 CFR58) that define ambient air
availability of electrical power and suitable sites to accommo-
monitoring criteria for open-path monitors (7). These amendments de-
scribehowthepathistobechosenwithrespecttoobstructionsandheight
date the instrument and peripherals.
above the ground. They also describe the appropriate positioning of the
NOTE 10—The actual dimensions of the plume are difficult to define.
path in relation to buildings, stacks, and roadways.
Some models assume that the concentration profile of the plume can be
8.3 Select the Pathlength—Choose the pathlength to maxi-
described by a Gaussian function.The boundaries of the plume, however,
mizethepercentageoftheplumefromthepollutantsourcethat
may not be known prior to selecting the monitoring path.
FIG. 1 Possible Orientations of the Monitoring Paths Relative to the Direction of the Prevailing Wind and the Pollutant Source for
Primary Data Collection and for an Upwind Background Spectrum
E1982 − 98 (2021)
8.3.1 The Longest Pathlength—Thelongestpathlengthfora 8.3.2.5 Calculate the minimum pathlength by using Eq 3,
particular OP/FT-IR system was determined in 6.6.1 as the andthevaluesoftheabsorptivity,minimumconcentration,and
distance at which the total signal approaches the signal due to minimum detectable absorbance found in 8.3.2.2 through
the system noise, internal stray light, or ambient radiation. For 8.3.2.4, respectively.
target gases and interfering species that are distributed homo-
8.4 Estimate Detection Limits—The method detection limit
geneously along the path, the atmosphere is optically dense at
(MDL) in units of the concentration—pathlength product, for
some pathlength. This distance represents the maximum path-
example ppm-m, can be estimated by using Eq 4.
length for that gas and can be determined as follows.
bc 5 A /a (4)
~ !
min
min
8.3.1.1 Measuretheabsorbanceoftheanalyticalbandofthe
targetgasorinterferingspeciesfromareferencespectrum.See
where:
10.2forproceduresforchoosingananalyticalband.Recordthe
A = minimum detectable absorbance, for example, three
min
concentration—pathlength product at which the reference
times the RMS baseline noise, and
spectrum was taken.
a = the absorptivity, as calculated in 8.3.1.2.
8.3.1.2 Calculatetheabsorptivity, a,forthisgasbyusingEq
To obtain the MDL of homogeneously distributed gases in
2.
units of concentration, for example ppb, divide the value of
a 5 A /b C (2)
ref ref ref
(bc) by the pathlength. Examples of estimated detection
min
limits for several hazardous air pollutants and common atmo-
where:
spheric gases are given in Annex A1 (see Table A1.1). This
A = absorbance of the reference spectrum at a specified
ref
table can be used during the planning phase of a field study to
wavenumber,
determineifmeasurementsofselectedtargetgasesarefeasible
b = pathlength at which the reference spectrum was
ref
at a particular monitoring site for a given monitoring path-
measured, and
length. This procedure is also applicable to estimating the
c = concentration of the reference standard.
ref
MDLfor the comparison (see 10.8.1) or the scaled subtraction
8.3.1.3 Estimate the concentration of the target gas or
(see 10.8.2) analysis methods. Lower estimates of the MDL
interfering species from preexisting monitoring data or from
may be obtained when mulivariate analysis methods (see
ancillary measurements.
10.8.3) are used by calculating the standard error of measure-
8.3.1.4 Select a maximum allowable absorbance value,
ment for the target gas in a spectrum in which the target gas is
based on the requirements of the analysis method.
not present.
8.3.1.5 UseEq3toestimatethepathlengththatwouldyield
the maximum allowable absorbance value at the estimated
9. Data Acquisition
concentration.
9.1 Perform the following steps to acquire the OP/FT-IR
b 5 A /ac (3)
max max est spectral data once the instrumental parameters have been
chosen (see Section 5), initial performance tests have been
where:
completed (see Section 6), logistical concerns have been
A = the maximum allowable absorbance selected in
max
addressed (see Section 7), and the monitoring path has been
8.3.1.4,
selected (see Section 8).
c = the concentration estimated in 8.3.1.3, and
est
a = the absorptivity calculated in 8.3.1.2.
9.2 Align the Instrument—Allow the system to equilibrate.
Adjust the vertical and horizontal position of the receiving
Thevalueof b calculatedinEq3isthelongestallowable
max
telescope, the retroreflector, or the external IR source to
pathlength for measuring that particular target gas or interfer-
maximize the peak-to-peak voltage of the interferogram cen-
ing species.
terburst or the intensity of the single-beam spectrum at a
8.3.2 The Shortest Pathlength—The shortest pathlength
specificwavenumber.Recordthevalueofthemaximumsignal.
may be dictated by the distance at which the detector becomes
saturated as determined in 6.3. If the instrument is operating
9.3 Determine the Random Baseline Noise of the System—
linearly at any potential pathlength, the shortest pathlength for
Record the magnitude of the RMS noise as described in 6.7.
the target gas can be calculated as follows.
Compare this value with historical data to determine that the
8.3.2.1 Measuretheabsorbanceoftheanalyticalbandofthe instrument is performing within the data quality objectives of
target gas from a reference spectrum. Record the the study.
concentration—pathlength product at which this spectrum was
9.4 Choose the Type of Data File—Select the type of data
taken.
file that is to be collected, for example, either a single-beam
8.3.2.2 Calculate the absorptivity, , for this gas by using Eq
a
spectrum or an interferogram.
2.
NOTE 11—The interferogram should be the type of raw data that is
8.3.2.3 Estimate a minimum concentration that will be
collected to allow for more choices in post-data acquisition processing.
measured.
8.3.2.4 Set the minimum detectable absorbance at three 9.5 Acquire the Spectral Data—Choose the number of data
times the RMS baseline noise as measured under normal files to be collected and the intervals at which they are to be
operating conditions (see 6.7). acquired, then start acquiring the data.
E1982 − 98 (2021)
10. Data Analysis obtained in the absence of the sample of interest. The single-
beam sample spectrum is divided by this background, or I ,
10.1 The steps that are required to analyze OP/FT-IR field
spectrumtocreateatransmittancespectrum.Thisoperation,in
spectra include selecting the spectral region over which the
effect, nulls out the spectral features due to the detector, IR
analysiswillbeperformed;generatingabackgroundspectrum;
source, beamsplitter, and other optical components. In OP/
correcting the field spectra for internal stray light or ambient
FT-IR monitoring it is not possible to obtain the I spectrum
radiation; generating an absorption spectrum from the inter-
directly because the target gas cannot be removed from the
ferogram; obtaining the appropriate reference spectra; correct-
atmosphere.The following methods can be used to produce an
ing the field spectra for wavenumber shifts; and choosing the
I spectrum.
analysis method.
10.3.1 Synthetic Background Spectra—A software package
10.2 Select the Analysis Region—Perform the following
that allows individual data points to be selected, deleted, or
steps to determine the optimum region of the spectrum over
moved along the ordinate must be available to generate a
which to perform the data analysis. This determination will be
synthetic background spectrum. An example of a synthetic
influenced by the choice of analysis method (see 10.8).
background spectrum. An example of a synthetic background
10.2.1 Find the most intense absorption band in a reference
spectrum is given in Practice E1685. To create a synthetic
spectrum of the target gas. If the comparison (see 10.8.1)or
background spectrum, perform the following steps.
scaled subtraction (see 10.8.2) method is used to analyze the
10.3.1.1 Select a single-beam spectrum with an intensity
data, choose this absorption band as the analytical band. If a
profilethatmatchestheprofileofthefieldspectraandthatwas
multivariateanalysismethod,suchasCLSorPLS(see10.8.3),
acquired when the concentrations of the target gases and
is used, then select the wavenumber region that encompasses
interfering species were at a minimum.
the entire envelope of the most intense absorption band.
10.3.1.2 Select data points along the envelope of this
10.2.2 Measure the absorbance maximum of the band cho-
single-beam field spectrum, matching the instrument response
sen in 10.2.1. Use Eq 2 to calculate the absorptivity of the
curve as closely as possible. Do not select data points on an
target gas. Estimate the concentration of the target gas that is
absorption band or on the continuum produced by unresolved
expected to be present at the monitoring site, then estimate the
absorption bands.
absorbance of the analytical band by using Eq 5:
10.3.1.3 Fit a series of short, straight lines or some other
A 5 abc (5)
est est appropriatefunctiontotheselectedpointstogenerateasmooth
curve that follows the profile of the original single-beam field
where:
spectrum. Do not introduce any distortions, artificial dips, or
A = estimated absorbance of the target gas,
est
peaks into the intensity function.
a = the absorptivity of the target gas,
10.3.1.4 An automated procedure that fits a series of seg-
b = the monitoring pathlength, and
c = the estimated concentration of the target gas. mented polynomial curves to a single-beam field spectrum can
est
also be used as an alternative to producing a synthetic
If the estimated absorbance is lower than three times the
background spectrum manually (8).
RMS noise (see 6.7), this absorption band may be too weak to
−1
measure the target gas at the monitoring site by either the
NOTE 12—A resolution of 2 cm or better is generally required to
comparison or scaled subtraction methods. The use of weaker develop a synthetic background spectrum. At lower resolutions, the
unresolved water vapor continuum interferes with the visualization of the
absorption bands might be appropriate when multivariate
true instrument response curve (see Fig. 2). A synthetic background
analysis methods are used because these methods have dem-
spectrum is most effective when analyzing for target gases with narrow
onstrated the ability to extract quantitative information from
absorption features.This type of background spectrum is more difficult to
apparent baseline noise.
developfortargetgaseswithbroadabsorptionbands,especiallywhenlow
concentrations are to be measured.
10.2.3 If the most intense absorption band is in a region of
the OP/FT-IR field spectrum that is optically opaque due to
10.3.2 Short-Path Background Spectra—Ashort-path back-
absorptionbyatmosphericwatervapororcarbondioxide,then
ground spectrum can be used when a synthetic background
return to 10.2.1 and select the next most intense absorption
spectrum is not suitable, for example, during low-resolution
band.
measurements or when analyzing for target gases with broad
10.2.4 Determine if the absorption band chosen in 10.2.3 is
absorptionbands.Theshortpathlengththatisusedforthistype
optically dense, or saturated at the monitoring pathlength. If
of background spectrum effectively eliminates the absorption
thisisthecase,returnto10.2.1andselectthenextmostintense
causedbythetargetgasesandminimizestheabsorptioncaused
absorption band.
byinterferingatmosphericspecies.Anexampleofashort-path
10.2.5 Determine if an interfering species other than water
background spectrum is given in Fig. 2(C). Perform the
vapor or carbon dioxide is present that would prohibit the
following steps to produce a short-path background spectrum.
absorptionbandchosenin10.2.4frombeingused.Ifthisisthe
10.3.2.1 Position the retroreflector or external IR source
case, return to 10.2.1 and select the next most intense absorp-
close to the receiving telescope, and obtain a single-beam
tionband.Proceedtotheotherdataanalysisproceduresoncea
spectrum.
suitable absorption band has been found.
10.3.2.2 Inspect the short-path background spectrum in the
10.3 Produce a Background Spectrum—In conventional spectral region below the detector cutoff frequency for non-
FT-IR spectrometry, a background single-beam spectrum is physical energy.
E1982 − 98 (2021)
−1
FIG. 2 Single-beam OP/FT-IR Spectra Collected at 2 cm Resolution Representing: (A) a Field Spectrum Acquired Over a 200 m Path;
(B) a Synthetic Background Spectrum; and (C) a Short Path Background Spectrum. The positions of the spectra are offset slightly on
the ordinate for clarity.
problem can be overcome by placing a field stop in the instrument so that
10.3.2.3 Compare the intensity profile of the short-path
it uses a smaller field of view than the smallest anticipated from the
spectrum with that of the field spectra.
retroreflector.
10.3.2.4 Determine if wavenumber shifts or resolution
changes have occurred between the field spectra and the
10.3.3 Upwind Background Spectra—If the area of the
short-path background spectrum.
pollutant source is relatively small and its upwind side is
10.3.2.5 If any anomalies are detected in 10.3.2.2 through
accessible,anupwind I spectrumcanbeacquired.Anexample
10.3.2.5, do not use th
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