ASTM E1865-97(2007)
(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
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
1.1This 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 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E1865 − 97 (Reapproved2007)
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 3.2.2 bistatic system, n—a system in which the IR source is
some distance from the detector. For OP/FT-IR monitoring,
1.1 This guide covers active open-path Fourier transform
this implies that the IR source and the detector are at opposite
infrared (OP/FT-IR) monitors and provides guidelines for
ends of the monitoring path.
using active OP/FT-IR monitors to obtain concentrations of
gases and vapors in air. 3.2.3 monitoring path, n—the location in space over which
concentrationsofgasesandvaporsaremeasuredandaveraged.
1.2 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3.2.4 monitoring pathlength, n—the distance the optical
responsibility of the user of this standard to establish appro-
beam traverses through the monitoring path.
priate safety and health practices and determine the applica-
3.2.5 monostatic or unistatic system, n— a system with the
bility of regulatory limitations prior to use.
IR source and the detector at the same end of the monitoring
path.ForOP/FT-IRsystems,thebeamisgenerallyreturnedby
2. Referenced Documents
a retroreflector.
2.1 ASTM Standards:
3.2.6 open-path monitoring, n—monitoring over a path that
E131Terminology Relating to Molecular Spectroscopy
is completely open to the atmosphere.
E168Practices for General Techniques of Infrared Quanti-
tative Analysis
3.2.7 parts per million meters, n—the units associated with
E1421Practice for Describing and Measuring Performance
the quantity path-integrated concentration and a possible unit
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
of choice for reporting data from OP/FT-IR monitors because
eters: Level Zero and Level One Tests
it is independent of the monitoring pathlength.
E1655 Practices for Infrared Multivariate Quantitative
3.2.8 path-averagedconcentration,n—theresultofdividing
Analysis
the path-integrated concentration by the pathlength.
3. Terminology 3.2.8.1 Discussion—Path-averaged concentration gives the
averagevalueoftheconcentrationalongthepath,andtypically
3.1 For definitions of terms relating to general molecular
is expressed in units of parts per million (ppm), parts per
spectroscopy used in this guide refer to Terminology E131.A
−3
billion (ppb), or micrograms per cubic meter (µgm ).
completeglossaryoftermsrelatingtoopticalremotesensingis
given in Ref (1). 3.2.9 path-integrated concentration, n— the quantity mea-
suredbyanOP/FT-IRmonitoroverthemonitoringpath.Ithas
3.2 Definitions:
units of concentration times length, for example, ppm·m.
3.2.1 background spectrum, n—asingle-beamspectrumthat
does not contain the spectral features of the analyte(s) of
3.2.10 plume, n—the gaseous and aerosol effluents emitted
interest.
from a stack or other pollutant source and the volume of space
they occupy.
This guide is under the jurisdiction of ASTM Committee E13 on Molecular
3.2.11 retroreflector, n—an optical device that returns radia-
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
tion in directions close to the direction from which it came.
mittee E13.03 on Infrared and Near Infrared Spectroscopy.
3.2.11.1 Discussion—Retroreflectors come in a variety of
Current edition approved Dec. 1, 2007. Published December 2007. Originally
approved in 1997. Last previous edition approved in 2002 as E1865-97(2002).
forms. The retroreflector commonly used in OP/FT-IR moni-
DOI: 10.1520/E1865-97R07.
toring uses reflection from three mutually perpendicular sur-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
faces.Thiskindofretroreflectorisusuallycalledacube-corner
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 retroreflector.
the ASTM website.
3 3.2.12 single-beam spectrum, n—the radiant power mea-
The boldface numbers in parentheses refer to a list of references at the end of
this guide. sured by the instrument detector as a function of frequency.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1865 − 97 (2007)
3.2.12.1 Discussion—In FT-IR absorption spectrometry the 6. Description of OP/FT-IR Systems
single-beamspectrumisobtainedafterafastFouriertransform
6.1 Therearetwoprimarygeometricalconfigurationsavail-
of the interferogram.
able for transmitting the IR beam along the path in active
3.2.13 synthetic background spectrum, n— a background
OP/FT-IR systems. One configuration is referred to as bistatic,
spectrum made by choosing points along the envelope of a
while the other is referred to as monostatic, or unistatic.
single-beamspectrumandfittingaseriesofshort,straightlines
6.1.1 Bistatic Configuration—In this configuration, the de-
or a polynomial function to the chosen data points to simulate
tector and the IR source are at opposite ends of the monitoring
the instrument response in the absence of absorbing gases or
path. In this case, the optical pathlength is equal to the
vapors.
monitoring pathlength. Two configurations can be used for
bistatic systems. One configuration places the IR source,
4. Significance and Use
interferometer, and transmitting optics at one end of the path
4.1 This guide is intended for users of OP/FT-IR monitors. and the receiving optics and detector at the other end (Fig.
Applications of OP/FT-IR systems include monitoring for 1(A)). Typically a Cassegrain or Newtonian telescope is used
hazardous air pollutants in ambient air, along the perimeter of to transmit and collect the IR beam. The advantage of the
an industrial facility, at hazardous waste sites and landfills, in
configuration depicted in Fig. 1(A) is that the IR beam is
response to accidental chemical spills or releases, and in modulated along the path, which enables the unmodulated
workplace environments.
ambient radiation to be rejected by the system’s electronics.
Themaximumdistancethattheinterferometerandthedetector
5. Principles of OP/FT-IR Monitoring
can be separated in this configuration is limited because
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-
spectral region to identify and quantify atmospheric gases.
toring path, in principle, to be doubled compared to that of the
These instruments can be either transportable or permanently
monostatic configuration. The main drawback to this bistatic
installed. An open-path monitor contains many of the same
configuration is that the IR radiation is not modulated before it
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
monitor provides path-integrated concentration data. Unlike
6.1.2 Monostatic Configuration—In monostatic
many other air monitoring methods, such as those that use
configurations, the IR source and the detector are at the same
canistersorsorbentcartridges,theOP/FT-IRmonitormeasures
end of the monitoring path. A retroreflector of some sort is
pollutants in situ. Therefore, no samples need be collected,
required at the midpoint of the optical path to return the beam
extracted, or returned to the laboratory for analysis. Detection
to the detector. Thus, the optical pathlength is twice the
limits in OP/FT-IR depend on several factors, such as the
distance between the source and the retroreflector. Two tech-
monitoring pathlength, the absorptivity of the analyte, and the
niques are currently in use for returning the beam along the
presence of interfering species. For most analytes of interest,
optical path in the monostatic configuration. One technique
detection limits typically range between path-integrated con-
uses an arrangement of mirrors, such as a single cube-corner
centrations of 1.5 and 50 ppm·m.
retroreflector, at one end of the path that translates the beam
slightly so that it does not fold back on itself (Fig. 2(A)). The
NOTE 1—The OP/FT-IR monitor can be configured to operate in two
other end of the path then has a second telescope slightly
modes: active or passive. In the active mode, a collimated beam of
radiation from an IR source that is a component of the system is
removed from the transmitter to collect the returned beam.
transmittedalongtheopen-airpath.Inthepassivemode,radiationemitted
Initial alignment with this configuration can be difficult, and
from objects in the field of view of the instrument is used as the source of
this type of monostatic system is normally used in permanent
IR energy. Passive FT-IR monitors have been used for environmental
installations rather than as a transportable unit. Another con-
applications, such as characterizing the plumes of smoke stacks. More
recently these systems have been developed to detect chemical warfare figuration of the monostatic monitoring mode uses the same
agentsinmilitaryapplications.However,todate,theactivemodehasbeen
telescope to transmit and receive the IR beam. A cube-corner
used for most environmental applications of OP/FT-IR monitoring. In
retroreflector array is placed at the end of the monitoring path
addition to open-air measurements, extractive measurements can be made
toreturnthebeam(Fig.2(B)).Totransmitandreceivewiththe
by interfacing a closed cell to an FT-IR system. This type of system can
beusedasapointmonitorortomeasuretheeffluentinstacksorpipelines. same optics, a beamsplitter must be placed in the optical path
E1865 − 97 (2007)
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 Interferom-
eter and Detector at the Opposite End
to divert part of the returned beam to the detector.Adisadvan- where:
tage to this configuration is that the IR energy must traverse
U (T) = spectral energy density at wavenumber v from a
v
this beamsplitter twice. The most efficient beamsplitter trans-
blackbody source at a temperature T,
mits 50% of the light and rejects the other 50%.Thus, in two
θ = optical throughput of the spectrometric system,
passes, the transmission is only 25% of the original beam. ∆ v = resolution of the interferometer,
Because this loss of energy decreases the signal-to-noise ratio t = measurement time in seconds,
ξ = efficiency of the interferometer,
(S/N), it can potentially be a significant drawback of this
D* = specific detectivity, a measure of the sensitivity of
configuration.
the detector, and
A = area of the detector element.
7. Selection of Instrumental Parameters
D
NOTE 2—This equation is correct but assumes that the system is
7.1 Introduction and Overview—One important issue re-
detector noise limited, which is not always true. For example, source
garding the operation of OP/FT-IR systems is the appropriate
fluctuations, the analog-to-digital converter, or mechanical vibrations can
instrumental parameters, such as measurement time,
contribute to the system noise.
resolution, apodization, and degree of zero filling, to be used
7.3 Measurement Time—As shown in Eq 1, the S/N is
during data acquisition and processing. The choice of some of
1/2
proportional to the square root of the measurement time (t ).
these parameters is governed by the trading rules in FT-IR
For measurements made with a rapid scanning interferometer
spectrometry and by specific data quality objectives of the
operating at a constant mirror velocity and a given resolution,
study.
the S/Nincreases with the square root of the number of
7.2 Trading Rules in FT-IR Spectrometry—The quantitative
co-added scans. The choice of measurement time for signal
relationships between the S/N, resolution, and measurement
averaging in OP/FT-IR monitoring must take into account
time in FT-IR spectrometry are called “trading rules.” The
several factors. First, a measurement time must be chosen to
factors that affect the S/N and dictate the trading rules are
achieve an adequate S/N for the required detection limits.
expressedinEq1,whichgivesthe S/Nofaspectrummeasured
However,becausemonitoringforgasesandvaporsintheairis
with a rapid-scanning Michelson interferometer (6):
adynamicprocess,considerationmustbegiventothetemporal
1/2
S U T ·θ·∆v·t ·ξ·D*
~ ! nature of the target gas concentration. For example, if the
v
5 (1)
1/2
N A concentration of the target gas decreases dramatically during
~ !
D
E1865 − 97 (2007)
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 Same Optics to Transmit and Receive the IR Beam
themeasurementtime,thentherewouldbeadilutioneffect.In root of the measurement time, the measurement time required
...
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