ASTM E1865-97(2021)
(Guide)Standard Guide for Open-Path Fourier Transform Infrared (OP/FT-IR) Monitoring of Gases and Vapors in Air
Standard Guide for Open-Path Fourier Transform Infrared (OP/FT-IR) Monitoring of Gases and Vapors in Air
SIGNIFICANCE AND USE
4.1 This guide is intended for users of OP/FT-IR monitors. Applications of OP/FT-IR systems include monitoring for hazardous air pollutants 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 guide covers active open-path Fourier transform infrared (OP/FT-IR) monitors and provides guidelines for using active OP/FT-IR monitors to obtain concentrations of gases and vapors in air.
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 standard 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 standard 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
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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: E1865 − 97 (Reapproved 2021)
Standard Guide for
Open-Path Fourier Transform Infrared (OP/FT-IR) Monitoring
of Gases and Vapors in Air
This standard is issued under the fixed designation E1865; 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 3. Terminology
3.1 For definitions of terms relating to general molecular
1.1 This guide covers active open-path Fourier transform
spectroscopy used in this guide refer to Terminology E131.A
infrared (OP/FT-IR) monitors and provides guidelines for
completeglossaryoftermsrelatingtoopticalremotesensingis
using active OP/FT-IR monitors to obtain concentrations of
given in Ref (1).
gases and vapors in air.
3.2 Definitions:
1.2 The values stated in SI units are to be regarded as
3.2.1 background spectrum, n—asingle-beamspectrumthat
standard. No other units of measurement are included in this
does not contain the spectral features of the analyte(s) of
standard.
interest.
1.3 This standard does not purport to address all of the
3.2.2 bistatic system, n—a system in which the IR source is
safety concerns, if any, associated with its use. It is the
some distance from the detector. For OP/FT-IR monitoring,
responsibility of the user of this standard to establish appro-
this implies that the IR source and the detector are at opposite
priate safety, health, and environmental practices and deter-
ends of the monitoring path.
mine the applicability of regulatory limitations prior to use.
3.2.3 monitoring path, n—the location in space over which
1.4 This international standard was developed in accor-
concentrationsofgasesandvaporsaremeasuredandaveraged.
dance with internationally recognized principles on standard-
3.2.4 monitoring pathlength, n—the distance the optical
ization established in the Decision on Principles for the
beam traverses through the monitoring path.
Development of International Standards, Guides and Recom-
3.2.5 monostatic or unistatic system, n—a system with the
mendations issued by the World Trade Organization Technical
IR source and the detector at the same end of the monitoring
Barriers to Trade (TBT) Committee.
path.ForOP/FT-IRsystems,thebeamisgenerallyreturnedby
a retroreflector.
2. Referenced Documents
3.2.6 open-path monitoring, n—monitoring over a path that
2.1 ASTM Standards:
is completely open to the atmosphere.
E131Terminology Relating to Molecular Spectroscopy
3.2.7 parts per million meters, n—the units associated with
E168Practices for General Techniques of Infrared Quanti-
the quantity path-integrated concentration and a possible unit
tative Analysis
of choice for reporting data from OP/FT-IR monitors because
E1421Practice for Describing and Measuring Performance
it is independent of the monitoring pathlength.
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
eters: Level Zero and Level One Tests
3.2.8 path-averagedconcentration,n—theresultofdividing
E1655 Practices for Infrared Multivariate Quantitative the path-integrated concentration by the pathlength.
Analysis 3.2.8.1 Discussion—Path-averaged concentration gives the
averagevalueoftheconcentrationalongthepath,andtypically
is expressed in units of parts per million (ppm), parts per
−3
billion (ppb), or micrograms per cubic meter (µgm ).
This guide is under the jurisdiction of ASTM Committee E13 on Molecular
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
3.2.9 path-integrated concentration, n—the quantity mea-
mittee E13.03 on Infrared and Near Infrared Spectroscopy.
suredbyanOP/FT-IRmonitoroverthemonitoringpath.Ithas
Current edition approved April 1, 2021. Published April 2021. Originally
units of concentration times length, for example, ppm·m.
approved in 1997. Last previous edition approved in 2013 as E1865–97(2013).
DOI: 10.1520/E1865-97R21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The boldface numbers in parentheses refer to a list of references at the end of
the ASTM website. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1865 − 97 (2021)
NOTE 1—The OP/FT-IR monitor can be configured to operate in two
3.2.10 plume, n—the gaseous and aerosol effluents emitted
modes: active or passive. In the active mode, a collimated beam of
from a stack or other pollutant source and the volume of space
radiation from an IR source that is a component of the system is
they occupy.
transmittedalongtheopen-airpath.Inthepassivemode,radiationemitted
from objects in the field of view of the instrument is used as the source of
3.2.11 retroreflector, n—an optical device that returns radia-
IR energy. Passive FT-IR monitors have been used for environmental
tion in directions close to the direction from which it came.
applications, such as characterizing the plumes of smoke stacks. More
3.2.11.1 Discussion—Retroreflectors come in a variety of
recently these systems have been developed to detect chemical warfare
forms. The retroreflector commonly used in OP/FT-IR moni-
agentsinmilitaryapplications.However,todate,theactivemodehasbeen
toring uses reflection from three mutually perpendicular sur-
used for most environmental applications of OP/FT-IR monitoring. In
addition to open-air measurements, extractive measurements can be made
faces.Thiskindofretroreflectorisusuallycalledacube-corner
by interfacing a closed cell to an FT-IR system. This type of system can
retroreflector.
beusedasapointmonitorortomeasuretheeffluentinstacksorpipelines.
3.2.12 single-beam spectrum, n—the radiant power mea-
6. Description of OP/FT-IR Systems
sured by the instrument detector as a function of frequency.
3.2.12.1 Discussion—In FT-IR absorption spectrometry the
6.1 Therearetwoprimarygeometricalconfigurationsavail-
single-beamspectrumisobtainedafterafastFouriertransform
able for transmitting the IR beam along the path in active
of the interferogram.
OP/FT-IR systems. One configuration is referred to as bistatic,
3.2.13 synthetic background spectrum, n—a background
while the other is referred to as monostatic, or unistatic.
spectrum made by choosing points along the envelope of a
6.1.1 Bistatic Configuration—In this configuration, the de-
single-beamspectrumandfittingaseriesofshort,straightlines
tector and the IR source are at opposite ends of the monitoring
or a polynomial function to the chosen data points to simulate
path. In this case, the optical pathlength is equal to the
the instrument response in the absence of absorbing gases or
monitoring pathlength. Two configurations can be used for
vapors.
bistatic systems. One configuration places the IR source,
interferometer, and transmitting optics at one end of the path
4. Significance and Use
and the receiving optics and detector at the other end (Fig.
4.1 This guide is intended for users of OP/FT-IR monitors. 1(A)). Typically a Cassegrain or Newtonian telescope is used
Applications of OP/FT-IR systems include monitoring for to transmit and collect the IR beam. The advantage of the
hazardous air pollutants in ambient air, along the perimeter of configuration depicted in Fig. 1(A) is that the IR beam is
an industrial facility, at hazardous waste sites and landfills, in modulated along the path, which enables the unmodulated
response to accidental chemical spills or releases, and in ambient radiation to be rejected by the system’s electronics.
workplace environments. Themaximumdistancethattheinterferometerandthedetector
can be separated in this configuration is limited because
5. Principles of OP/FT-IR Monitoring
communication between these two components is required for
5.1 Long-path IR spectrometry has been used since the timing purposes. For example, a bistatic system with this
mid-1950s to characterize hazardous air pollutants (2). For the configuration developed for monitoring workplace environ-
most part, this earlier work involved the use of multiple-pass, mentshadamaximummonitoringpathlengthof40m (5).The
long-pathIRcellstocollectandanalyzeairsamples.Inthelate other bistatic configuration places the IR source and transmit-
1970s a mobile FT-IR system capable of detecting pollutants ting optics at one end of the path and the receiving optics,
along an open path was developed (3). The 1990 amendments interferometer, and detector at the other end of the path (Fig.
to the Clean Air Act, which may require that as many as 189 1(B)). This is the most common configuration of bistatic
compounds be monitored in the atmosphere, have led to a systems in current use. In this configuration the beam from the
renewed interest in OP/FT-IR monitoring (4). The OP/FT-IR IRsourceiscollimatedbyamirrorshapedasaparaboloid.The
monitor is a spectrometric instrument that uses the mid-IR configuration shown in Fig. 1(B) allows the maximum moni-
toring path, in principle, to be doubled compared to that of the
spectral region to identify and quantify atmospheric gases.
These instruments can be either transportable or permanently monostatic configuration. The main drawback to this bistatic
configuration is that the IR radiation is not modulated before it
installed. An open-path monitor contains many of the same
componentsasthoseinalaboratoryFT-IRsystem,forexample is transmitted along the path. Therefore, radiation from the
the same types of interferometers and detectors are used, active IR source and the ambient background cannot be
exceptthatthesamplevolumeconsistsoftheopenatmosphere. distinguished by electronic processing.
In contrast to more conventional point monitors, the OP/FT-IR 6.1.2 Monostatic Configuration—In monostatic
monitor provides path-integrated concentration data. Unlike configurations, the IR source and the detector are at the same
many other air monitoring methods, such as those that use end of the monitoring path. A retroreflector of some sort is
canistersorsorbentcartridges,theOP/FT-IRmonitormeasures required at the midpoint of the optical path to return the beam
pollutants in situ. Therefore, no samples need be collected, to the detector. Thus, the optical pathlength is twice the
extracted, or returned to the laboratory for analysis. Detection distance between the source and the retroreflector. Two tech-
limits in OP/FT-IR depend on several factors, such as the niques are currently in use for returning the beam along the
monitoring pathlength, the absorptivity of the analyte, and the optical path in the monostatic configuration. One technique
presence of interfering species. For most analytes of interest, uses an arrangement of mirrors, such as a single cube-corner
detection limits typically range between path-integrated con- retroreflector, at one end of the path that translates the beam
centrations of 1.5 and 50 ppm·m. slightly so that it does not fold back on itself (Fig. 2(A)). The
E1865 − 97 (2021)
FIG. 1 Schematic Diagram of the Bistatic OP/FT-IR Configuration Showing (A) a System with the IR Source and Interferometer at One
End of the Path and the Detector at the Opposite End, and (B) a System with the IR Source at One End of the Path and the
Interferometer and Detector at the Opposite End
other end of the path then has a second telescope slightly 7.2 Trading Rules in FT-IR Spectrometry—The quantitative
removed from the transmitter to collect the returned beam. relationships between the S/N, resolution, and measurement
Initial alignment with this configuration can be difficult, and
time in FT-IR spectrometry are called “trading rules.” The
this type of monostatic system is normally used in permanent
factors that affect the S/N and dictate the trading rules are
installations rather than as a transportable unit. Another con-
expressedinEq1,whichgivesthe S/Nofaspectrummeasured
figuration of the monostatic monitoring mode uses the same
with a rapid-scanning Michelson interferometer (6):
telescope to transmit and receive the IR beam. A cube-corner
1/2
S U T ·θ·∆v·t ·ξ·D*
~ !
v
retroreflector array is placed at the end of the monitoring path 5 (1)
1/2
N ~A !
D
toreturnthebeam(Fig.2(B)).Totransmitandreceivewiththe
where:
same optics, a beamsplitter must be placed in the optical path
to divert part of the returned beam to the detector.Adisadvan-
U (T) = spectral energy density at wavenumber v from a
v
tage to this configuration is that the IR energy must traverse
blackbody source at a temperature T,
this beamsplitter twice. The most efficient beamsplitter trans-
θ = optical throughput of the spectrometric system,
∆ v = resolution of the interferometer,
mits 50% of the light and rejects the other 50%.Thus, in two
t = measurement time in seconds,
passes, the transmission is only 25% of the original beam.
ξ = efficiency of the interferometer,
Because this loss of energy decreases the signal-to-noise ratio
D* = specific detectivity, a measure of the sensitivity of
(S/N), it can potentially be a significant drawback of this
the detector, and
configuration.
A = area of the detector element.
D
7. Selection of Instrumental Parameters
NOTE 2—This equation is correct but assumes that the system is
detector noise limited, which is not always true. For example, source
7.1 Introduction and Overview—One important issue re-
fluctuations, the analog-to-digital converter, or mechanical vibrations can
garding the operation of OP/FT-IR systems is the appropriate
contribute to the system noise.
instrumental parameters, such as measurement time,
7.3 Measurement Time—As shown in Eq 1, the S/N is
resolution, apodization, and degree of zero filling, to be used
1/2
proportional to the square root of the measurement time (t ).
during data acquisition and processing. The choice of some of
these parameters is governed by the trading rules in FT-IR For measurements made with a rapid scanning interferometer
operating at a constant mirror velocity and a given resolution,
spectrometry and by specific data quality objectives of the
study. the S/Nincreases with the square root of the number of
E1865 − 97 (2021)
FIG. 2 Schematic Diagram of the Monostatic OP/FT-IR Configuration Showing (A) a System with a Retroreflector that Translates the
Return IR Beam to Separate Receiving Optics, and (B) a System that Uses the
...
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