ASTM E1865-97(2002)
(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
SCOPE
1.1 This guide describes 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: E 1865 – 97 (Reapproved 2002)
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 (e) 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 Thisguidedescribesactiveopen-pathFouriertransform
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
3.2.3 monitoring path, n—the location in space over which
gases and vapors in air.
concentrationsofgasesandvaporsaremeasuredandaveraged.
1.2 This standard does not purport to address all of the
3.2.4 monitoring pathlength, n—the distance the optical
safety concerns, if any, associated with its use. It is the
beam traverses through the monitoring path.
responsibility of the user of this standard to establish appro-
3.2.5 monostatic or unistatic system, n—a system with the
priate safety and health practices and determine the applica-
IR source and the detector at the same end of the monitoring
bility of regulatory limitations prior to use.
path.ForOP/FT-IRsystems,thebeamisgenerallyreturnedby
2. Referenced Documents a retroreflector.
3.2.6 open-path monitoring, n—monitoring over a path that
2.1 ASTM Standards:
is completely open to the atmosphere.
E131 Terminology Relating to Molecular Spectroscopy
3.2.7 parts per million meters, n—the units associated with
E168 Practice 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
E1421 PracticeforDescribingandMeasuringPerformance
it is independent of the monitoring pathlength.
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
3.2.8 path-averaged concentration, n—the result of divid-
eters Level Zero and Level One Tests
ing the path-integrated concentration by the pathlength.
E1655 Practices for Infrared, Multivariate, Quantitative
3.2.8.1 Discussion—Path-averaged concentration gives the
Analysis
averagevalueoftheconcentrationalongthepath,andtypically
3. Terminology
is expressed in units of parts per million (ppm), parts per
−3
billion (ppb), or micrograms per cubic meter (µgm ).
3.1 For definitions of terms relating to general molecular
3.2.9 path-integrated concentration, n—the quantity mea-
spectroscopy used in this guide refer to Terminology E131.A
suredbyanOP/FT-IRmonitoroverthemonitoringpath.Ithas
completeglossaryoftermsrelatingtoopticalremotesensingis
units of concentration times length, for example, ppm·m.
given in Ref (1).
3.2.10 plume, n—the gaseous and aerosol effluents emitted
3.2 Definitions:
from a stack or other pollutant source and the volume of space
3.2.1 backgroundspectrum,n—asingle-beamspectrumthat
they occupy.
does not contain the spectral features of the analyte(s) of
3.2.11 retroreflector, n—an optical device that returns ra-
interest.
diation in directions close to the direction from which it came.
3.2.11.1 Discussion—Retroreflectors come in a variety of
1 forms. The retroreflector commonly used in OP/FT-IR moni-
This guide is under the jurisdiction of ASTM Committee E13 on Molecular
toring uses reflection from three mutually perpendicular sur-
Spectroscopy and is the direct responsibility of Subcommittee E13.03 on Infrared
Spectroscopy.
faces.Thiskindofretroreflectorisusuallycalledacube-corner
Current edition approved March 10, 1997. Published July 1997.
retroreflector.
Annual Book of ASTM Standards, Vol 03.06.
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.
E 1865
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,
ments had a maximum monitoring pathlength of 40 m (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
6.1.2 Monostatic Configuration—In monostatic configura-
monitor provides path-integrated concentration data. Unlike
many other air monitoring methods, such as those that use tions, the IR source and the detector are at the same end of the
canistersorsorbentcartridges,theOP/FT-IRmonitormeasures monitoringpath.Aretroreflectorofsomesortisrequiredatthe
midpoint of the optical path to return the beam to the detector.
pollutants in situ. Therefore, no samples need be collected,
extracted, or returned to the laboratory for analysis. Detection Thus, the optical pathlength is twice the distance between the
source and the retroreflector. Two techniques are currently in
limits in OP/FT-IR depend on several factors, such as the
monitoring pathlength, the absorptivity of the analyte, and the use for returning the beam along the optical path in the
monostatic configuration. One technique uses an arrangement
presence of interfering species. For most analytes of interest,
detection limits typically range between path-integrated con- of mirrors, such as a single cube-corner retroreflector, at one
end of the path that translates the beam slightly so that it does
centrations of 1.5 and 50 ppm·m.
not fold back on itself (Fig. 2(A)). The other end of the path
NOTE 1—The OP/FT-IR monitor can be configured to operate in two
then has a second telescope slightly removed from the trans-
modes: active or passive. In the active mode, a collimated beam of
mitter to collect the returned beam. Initial alignment with this
radiation from an IR source that is a component of the system is
configuration can be difficult, and this type of monostatic
transmittedalongtheopen-airpath.Inthepassivemode,radiationemitted
from objects in the field of view of the instrument is used as the source of system is normally used in permanent installations rather than
IR energy. Passive FT-IR monitors have been used for environmental
asatransportableunit.Anotherconfigurationofthemonostatic
applications, such as characterizing the plumes of smoke stacks. More
monitoring mode uses the same telescope to transmit and
recently these systems have been developed to detect chemical warfare
receive the IR beam. A cube-corner retroreflector array is
agentsinmilitaryapplications.However,todate,theactivemodehasbeen
placed at the end of the monitoring path to return the beam
used for most environmental applications of OP/FT-IR monitoring. In
(Fig. 2(B)). To transmit and receive with the same optics, a
addition to open-air measurements, extractive measurements can be made
beamsplittermustbeplacedintheopticalpathtodivertpartof
by interfacing a closed cell to an FT-IR system. This type of system can
beusedasapointmonitorortomeasuretheeffluentinstacksorpipelines. the returned beam to the detector. A disadvantage to this
E 1865
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
configuration is that the IR energy must traverse this beam-
where:
splittertwice.Themostefficientbeamsplittertransmits50%of
U (T) = spectral energy density at wavenumber v from a
v
the light and rejects the other 50%. Thus, in two passes, the
blackbody source at a temperature T,
transmission is only 25% of the original beam. Because this
u = optical throughput of the spectrometric system,
Dv = resolution of the interferometer,
loss of energy decreases the signal-to-noise ratio (S/N), it can
t = measurement time in seconds,
potentially be a significant drawback of this configuration.
j = efficiency of the interferometer,
D* = specific detectivity, a measure of the sensitivity
7. Selection of Instrumental Parameters
of the detector, and
7.1 Introduction and Overview—One important issue re-
A = area of the detector element.
D
garding the operation of OP/FT-IR systems is the appropriate
instrumental parameters, such as measurement time, resolu-
NOTE 2—This equation is correct but assumes that the system is
tion, apodization, and degree of zero filling, to be used during detector noise limited, which is not always true. For example, source
fluctuations, the analog-to-digital converter, or mechanical vibrations can
data acquisition and processing. The choice of some of these
contribute to the system noise.
parametersisgovernedbythetradingrulesinFT-IRspectrom-
etry and by specific data quality objectives of the study.
7.3 Measurement Time—As shown in Eq 1, the S/N is
7.2 Trading Rules in FT-IR Spectrometry—The quantitative 1/2
proportional to the square root of the measurement time (t ).
relationships between the S/N, resolution, and measurement
For measurements made with a rapid scanning interferometer
time in FT-IR spectrometry are called “trading rules.” The
operating at a constant mirror velocity and a given resolution,
factors that affect the S/N and dictate the trading rules are
the S/N increases with the square root of the number of
expressedinEq1,whichgivesthe S/Nofaspectrummeasured
co-added scans. The choice of measurement time for signal
with a rapid-scanning Michelson interferometer (6):
averaging in OP/FT-IR monitoring must take into account
1/2
S U ~T!·u·Dv·t ·j·D*
several factors. First, a measurement time must be chosen to
v
5 (1)
1/2
N
achieve an adequate S/N for the required detection limits.
~A !
D
E 1865
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
However,becausemonitoringforgasesandvaporsintheairis be constant for measurements made at both high and low
adynamicprocess,considerationmustbegiventothetemporal resolution,the S/Nwillbehalvedupondecreasingthequantity
nature of the target gas concentration. For example, if the Dv by a factor of 2 (for example, changing the resolution from
−1
concentration of the target gas decreases dramatically during 1 to 0.5 cm ). Because the S/N is proportional to the square
themeasurementtime,thentherewouldbeadilutioneffect.In root of the measurement time, the measurement time required
addition, varying signals cannot be added linearly in the to maintain the original baseline noise level must be increased
interferogram domain. Nonlinearities and bandshape dist
...
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