ASTM E1865-97(2013)

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 and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Historical
• Publication Date: 31-Dec-2012
• ICS: 17.060 - Measurement of volume, mass, density, viscosity
• ICS: 71.040.50 - Physicochemical methods of analysis
• Technical Committee: E13 - Molecular Spectroscopy and Separation Science
• Drafting Committee: E13.03 - Infrared and Near Infrared Spectroscopy

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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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: E1865 − 97 (Reapproved 2013)
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
1.1 This guide covers active open-path Fourier transform 3.1 For definitions of terms relating to general molecular
infrared (OP/FT-IR) monitors and provides guidelines for spectroscopy used in this guide refer to Terminology E131.A
using active OP/FT-IR monitors to obtain concentrations of completeglossaryoftermsrelatingtoopticalremotesensingis
gases and vapors in air. given in Ref (1).
1.2 The values stated in SI units are to be regarded as 3.2 Definitions:
standard. No other units of measurement are included in this 3.2.1 background spectrum, n—asingle-beamspectrumthat
standard. does not contain the spectral features of the analyte(s) of
interest.
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the 3.2.2 bistatic system, n—a system in which the IR source is
responsibility of the user of this standard to establish appro- some distance from the detector. For OP/FT-IR monitoring,
priate safety and health practices and determine the applica- this implies that the IR source and the detector are at opposite
bility of regulatory limitations prior to use. ends of the monitoring path.
3.2.3 monitoring path, n—the location in space over which
2. Referenced Documents
concentrationsofgasesandvaporsaremeasuredandaveraged.
2.1 ASTM Standards:
3.2.4 monitoring pathlength, n—the distance the optical
E131Terminology Relating to Molecular Spectroscopy
beam traverses through the monitoring path.
E168Practices for General Techniques of Infrared Quanti-
3 3.2.5 monostatic or unistatic system, n—a system with the
tative Analysis (Withdrawn 2015)
IR source and the detector at the same end of the monitoring
E1421Practice for Describing and Measuring Performance
path.ForOP/FT-IRsystems,thebeamisgenerallyreturnedby
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
a retroreflector.
eters: Level Zero and Level One Tests
3.2.6 open-path monitoring, n—monitoring over a path that
E1655 Practices for Infrared Multivariate Quantitative
is completely open to the atmosphere.
Analysis
3.2.7 parts per million meters, n—the units associated with
the quantity path-integrated concentration and a possible unit
This guide is under the jurisdiction of ASTM Committee E13 on Molecular
of choice for reporting data from OP/FT-IR monitors because
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
mittee E13.03 on Infrared and Near Infrared Spectroscopy. it is independent of the monitoring pathlength.
Current edition approved Jan. 1, 2013. Published January 2013. Originally
3.2.8 path-averagedconcentration,n—theresultofdividing
approved in 1997. Last previous edition approved in 2007 as E1865–97(2007).
the path-integrated concentration by the pathlength.
DOI: 10.1520/E1865-97R13.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
3.2.8.1 Discussion—Path-averaged concentration gives the
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
averagevalueoftheconcentrationalongthepath,andtypically
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
3 4
The last approved version of this historical standard is referenced on The boldface numbers in parentheses refer to a list of references at the end of
www.astm.org. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1865 − 97 (2013)
is expressed in units of parts per million (ppm), parts per extracted, or returned to the laboratory for analysis. Detection
−3
billion (ppb), or micrograms per cubic meter (µgm ). limits in OP/FT-IR depend on several factors, such as the
monitoring pathlength, the absorptivity of the analyte, and the
3.2.9 path-integrated concentration, n—the quantity mea-
presence of interfering species. For most analytes of interest,
suredbyanOP/FT-IRmonitoroverthemonitoringpath.Ithas
detection limits typically range between path-integrated con-
units of concentration times length, for example, ppm·m.
centrations of 1.5 and 50 ppm·m.
3.2.10 plume, n—the gaseous and aerosol effluents emitted
from a stack or other pollutant source and the volume of space NOTE 1—The OP/FT-IR monitor can be configured to operate in two
modes: active or passive. In the active mode, a collimated beam of
they occupy.
radiation from an IR source that is a component of the system is
3.2.11 retroreflector, n—an optical device that returns radia-
transmittedalongtheopen-airpath.Inthepassivemode,radiationemitted
tion in directions close to the direction from which it came. from objects in the field of view of the instrument is used as the source of
IR energy. Passive FT-IR monitors have been used for environmental
3.2.11.1 Discussion—Retroreflectors come in a variety of
applications, such as characterizing the plumes of smoke stacks. More
forms. The retroreflector commonly used in OP/FT-IR moni-
recently these systems have been developed to detect chemical warfare
toring uses reflection from three mutually perpendicular sur-
agentsinmilitaryapplications.However,todate,theactivemodehasbeen
faces.Thiskindofretroreflectorisusuallycalledacube-corner
used for most environmental applications of OP/FT-IR monitoring. In
addition to open-air measurements, extractive measurements can be made
retroreflector.
by interfacing a closed cell to an FT-IR system. This type of system can
3.2.12 single-beam spectrum, n—the radiant power mea-
beusedasapointmonitorortomeasuretheeffluentinstacksorpipelines.
sured by the instrument detector as a function of frequency.
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
most part, this earlier work involved the use of multiple-pass, configuration developed for monitoring workplace environ-
mentshadamaximummonitoringpathlengthof40m (5).The
long-pathIRcellstocollectandanalyzeairsamples.Inthelate
1970s a mobile FT-IR system capable of detecting pollutants other bistatic configuration places the IR source and transmit-
ting optics at one end of the path and the receiving optics,
along an open path was developed (3). The 1990 amendments
to the Clean Air Act, which may require that as many as 189 interferometer, and detector at the other end of the path (Fig.
1(B)). This is the most common configuration of bistatic
compounds be monitored in the atmosphere, have led to a
renewed interest in OP/FT-IR monitoring (4). The OP/FT-IR systems in current use. In this configuration the beam from the
IRsourceiscollimatedbyamirrorshapedasaparaboloid.The
monitor is a spectrometric instrument that uses the mid-IR
spectral region to identify and quantify atmospheric gases. configuration shown in Fig. 1(B) allows the maximum moni-
toring path, in principle, to be doubled compared to that of the
These instruments can be either transportable or permanently
installed. An open-path monitor contains many of the same monostatic configuration. The main drawback to this bistatic
configuration is that the IR radiation is not modulated before it
componentsasthoseinalaboratoryFT-IRsystem,forexample
the same types of interferometers and detectors are used, is transmitted along the path. Therefore, radiation from the
active IR source and the ambient background cannot be
exceptthatthesamplevolumeconsistsoftheopenatmosphere.
In contrast to more conventional point monitors, the OP/FT-IR distinguished by electronic processing.
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
E1865 − 97 (2013)
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 the detector. Thus, the optical pathlength is twice the Because this loss of energy decreases the signal-to-noise ratio
distance between the source and the retroreflector. Two tech- (S/N), it can potentially be a significant drawback of this
niques are currently in use for returning the beam along the configuration.
optical path in the monostatic configuration. One technique
uses an arrangement of mirrors, such as a single cube-corner 7. Selection of Instrumental Parameters
retroreflector, at one end of the path that translates the beam
7.1 Introduction and Overview—One important issue re-
slightly so that it does not fold back on itself (Fig. 2(A)). The
garding the operation of OP/FT-IR systems is the appropriate
other end of the path then has a second telescope slightly
instrumental parameters, such as measurement time,
removed from the transmitter to collect the returned beam.
resolution, apodization, and degree of zero filling, to be used
Initial alignment with this configuration can be difficult, and
during data acquisition and processing. The choice of some of
this type of monostatic system is normally used in permanent
these parameters is governed by the trading rules in FT-IR
installations rather than as a transportable unit. Another con-
spectrometry and by specific data quality objectives of the
figuration of the monostatic monitoring mode uses the same
study.
telescope to transmit and receive the IR beam. A cube-corner
7.2 Trading Rules in FT-IR Spectrometry—The quantitative
retroreflector array is placed at the end of the monitoring path
relationships between the S/N, resolution, and measurement
toreturnthebeam(Fig.2(B)).Totransmitandreceivewiththe
time in FT-IR spectrometry are called “trading rules.” The
same optics, a beamsplitter must be placed in the optical path
factors that affect the S/N and dictate the trading rules are
to divert part of the returned beam to the detector.Adisadvan-
expressedinEq1,whichgivesthe S/Nofaspectrummeasured
tage to this configuration is that the IR energy must traverse
with a rapid-scanning Michelson interferometer (6):
this beamsplitter twice. The most efficient beamsplitter trans-
1/2
mits 50% of the light and rejects the other 50%.Thus, in two S U ~T!·θ·∆v·t ·ξ·D*
v
5 (1)
1/2
passes, the transmission is only 25% of the original beam. N A
~ !
D
E1865 − 97 (2013)
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
where: addition, varying signals cannot be added linearly in the
interferogram domain. Nonlinearities and bandshape distor-
U (T) = spectral energy density at wavenumber v from a
v
tionswillbeobservediftheconcentrationsofgasesinthepath
blackbody source at a temperature T,
vary appreciably during the measurement time.
θ = optical throughput of the spectrometric system,
∆ v = resolution of the interferometer,
7.4 Resolution—Several factors must be considered when
t = measurement time in seconds,
determining the optimum resolution for measuring the IR
ξ = efficiency of the interferometer,
spectra of gases and vapors along a long, open path. These
D* = specific detectivity, a measure of the sensitivity of
factors include (1) the ability to distinguish between the
the detector, and
spectral features of target analytes and those of ambient
A = area of the detector element.
D
interfering species in the atmosphere, such as water vapor; (2
NOTE 2—This equation is correct but assumes that the system is
) the tradeoffs between resolution, IR peak absorbance, and
detector noise limited, which is not always true. For example, source
fluctuations, the analog-to-digital converter, or mechanical vibrations can S/N;(3) practical considerations, such as mea
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