ASTM E1865-97
(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
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 E 1865; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope concentrations of gases and vapors are measured and averaged.
3.2.4 monitoring pathlength, n—the distance the optical
1.1 This guide describes active open-path Fourier transform
beam traverses through the monitoring path.
infrared (OP/FT-IR) monitors and provides guidelines for
3.2.5 monostatic or unistatic system, n—a system with the
using active OP/FT-IR monitors to obtain concentrations of
IR source and the detector at the same end of the monitoring
gases and vapors in air.
path. For OP/FT-IR systems, the beam is generally returned by
1.2 This standard does not purport to address all of the
a retroreflector.
safety concerns, if any, associated with its use. It is the
3.2.6 open-path monitoring, n—monitoring over a path that
responsibility of the user of this standard to establish appro-
is completely open to the atmosphere.
priate safety and health practices and determine the applica-
3.2.7 parts per million meters, n—the units associated with
bility of regulatory limitations prior to use.
the quantity path-integrated concentration and a possible unit
2. Referenced Documents of choice for reporting data from OP/FT-IR monitors because
it is independent of the monitoring pathlength.
2.1 ASTM Standards:
3.2.8 path-averaged concentration, n—the result of divid-
E 131 Terminology Relating to Molecular Spectroscopy
ing the path-integrated concentration by the pathlength.
E 168 Practice for General Techniques of Infrared Quanti-
3.2.8.1 Discussion—Path-averaged concentration gives the
tative Analysis
average value of the concentration along the path, and typically
E 1421 Practice for Describing and Measuring Performance
is expressed in units of parts per million (ppm), parts per
of Fourier Transform Infrared (FT-IR) Spectrometers:
−3
billion (ppb), or micrograms per cubic meter (μgm ).
Level Zero and Level One Tests
3.2.9 path-integrated concentration, n—the quantity mea-
E 1655 Practices for Infrared, Multivariate, Quantitative
sured by an OP/FT-IR monitor over the monitoring path. It has
Analysis
units of concentration times length, for example, ppm·m.
3. Terminology
3.2.10 plume, n—the gaseous and aerosol effluents emitted
from a stack or other pollutant source and the volume of space
3.1 For definitions of terms relating to general molecular
they occupy.
spectroscopy used in this guide refer to Terminology E 131. A
3.2.11 retroreflector, n—an optical device that returns ra-
complete glossary of terms relating to optical remote sensing is
diation in directions close to the direction from which it came.
given in Ref (1).
3.2.11.1 Discussion—Retroreflectors come in a variety of
3.2 Definitions:
forms. The retroreflector commonly used in OP/FT-IR moni-
3.2.1 background spectrum, n—a single-beam spectrum that
toring uses reflection from three mutually perpendicular sur-
does not contain the spectral features of the analyte(s) of
faces. This kind of retroreflector is usually called a cube-corner
interest.
retroreflector.
3.2.2 bistatic system, n—a system in which the IR source is
3.2.12 single-beam spectrum, n—the radiant power mea-
some distance from the detector. For OP/FT-IR monitoring,
sured by the instrument detector as a function of frequency.
this implies that the IR source and the detector are at opposite
3.2.12.1 Discussion—In FT-IR absorption spectrometry the
ends of the monitoring path.
single-beam spectrum is obtained after a fast Fourier transform
3.2.3 monitoring path, n—the location in space over which
of the interferogram.
3.2.13 synthetic background spectrum, n—a background
This guide is under the jurisdiction of ASTM Committee E-13 on Molecular
spectrum made by choosing points along the envelope of a
Spectroscopy and is the direct responsibility of Subcommittee E13.03 on Infrared
single-beam spectrum and fitting a series of short, straight lines
Spectroscopy.
Current edition approved March 10, 1997. Published July 1997.
or a polynomial function to the chosen data points to simulate
Annual Book of ASTM Standards, Vol 03.06.
the instrument response in the absence of absorbing gases or
The boldface numbers in parentheses refer to a list of references at the end of
vapors.
this guide.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1865
4. Significance and Use interferometer, and transmitting optics at one end of the path
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.
The maximum distance that the interferometer and the detector
5. Principles of OP/FT-IR Monitoring
can be separated in this configuration is limited because
5.1 Long-path IR spectrometry has been used since the communication between these two components is required for
mid-1950s to characterize hazardous air pollutants (2). For the timing purposes. For example, a bistatic system with this
most part, this earlier work involved the use of multiple-pass, configuration developed for monitoring workplace environ-
long-path IR cells to collect and analyze air samples. In the late ments had a maximum monitoring pathlength of 40 m (5). The
1970s a mobile FT-IR system capable of detecting pollutants
other bistatic configuration places the IR source and transmit-
along an open path was developed (3). The 1990 amendments
ting optics at one end of the path and the receiving optics,
to the Clean Air Act, which may require that as many as 189
interferometer, and detector at the other end of the path (Fig.
compounds be monitored in the atmosphere, have led to a
1(B)). This is the most common configuration of bistatic
renewed interest in OP/FT-IR monitoring (4). The OP/FT-IR
systems in current use. In this configuration the beam from the
monitor is a spectrometric instrument that uses the mid-IR
IR source is collimated by a mirror shaped as a paraboloid. The
spectral region to identify and quantify atmospheric gases.
configuration shown in Fig. 1(B) allows the maximum moni-
These instruments can be either transportable or permanently
toring path, in principle, to be doubled compared to that of the
installed. An open-path monitor contains many of the same
monostatic configuration. The main drawback to this bistatic
components as those in a laboratory FT-IR system, for example
configuration is that the IR radiation is not modulated before it
the same types of interferometers and detectors are used,
is transmitted along the path. Therefore, radiation from the
except that the sample volume consists of the open atmosphere.
active IR source and the ambient background cannot be
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 configura-
many other air monitoring methods, such as those that use
tions, the IR source and the detector are at the same end of the
canisters or sorbent cartridges, the OP/FT-IR monitor measures
monitoring path. A retroreflector of some sort is required at the
pollutants in situ. Therefore, no samples need be collected,
midpoint of the optical path to return the beam to the detector.
extracted, or returned to the laboratory for analysis. Detection
Thus, the optical pathlength is twice the distance between the
limits in OP/FT-IR depend on several factors, such as the
source and the retroreflector. Two techniques are currently in
monitoring pathlength, the absorptivity of the analyte, and the
use for returning the beam along the optical path in the
presence of interfering species. For most analytes of interest,
monostatic configuration. One technique uses an arrangement
detection limits typically range between path-integrated con-
of mirrors, such as a single cube-corner retroreflector, at one
centrations of 1.5 and 50 ppm·m.
end of the path that translates the beam slightly so that it does
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
modes: active or passive. In the active mode, a collimated beam of then has a second telescope slightly removed from the trans-
radiation from an IR source that is a component of the system is
mitter to collect the returned beam. Initial alignment with this
transmitted along the open-air path. In the passive mode, radiation emitted
configuration can be difficult, and this type of monostatic
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
as a transportable unit. Another configuration of the monostatic
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
agents in military applications. However, to date, the active mode has been
used for most environmental applications of OP/FT-IR monitoring. In
placed at the end of the monitoring path to return the beam
addition to open-air measurements, extractive measurements can be made
(Fig. 2(B)). To transmit and receive with the same optics, a
by interfacing a closed cell to an FT-IR system. This type of system can
beamsplitter must be placed in the optical path to divert part of
be used as a point monitor or to measure the effluent in stacks or pipelines.
the returned beam to the detector. A disadvantage to this
configuration is that the IR energy must traverse this beam-
6. Description of OP/FT-IR Systems
splitter twice. The most efficient beamsplitter transmits 50 % of
6.1 There are two primary geometrical configurations avail-
the light and rejects the other 50 %. Thus, in two passes, the
able for transmitting the IR beam along the path in active
transmission is only 25 % of the original beam. Because this
OP/FT-IR systems. One configuration is referred to as bistatic,
loss of energy decreases the signal-to-noise ratio (S/N), it can
while the other is referred to as monostatic, or unistatic.
potentially be a significant drawback of this configuration.
6.1.1 Bistatic Configuration—In this configuration, the de-
tector and the IR source are at opposite ends of the monitoring
7. Selection of Instrumental Parameters
path. In this case, the optical pathlength is equal to the
monitoring pathlength. Two configurations can be used for 7.1 Introduction and Overview—One important issue re-
bistatic systems. One configuration places the IR source, garding the operation of OP/FT-IR systems is the appropriate
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
1/2
instrumental parameters, such as measurement time, resolu- proportional to the square root of the measurement time (t ).
tion, apodization, and degree of zero filling, to be used during For measurements made with a rapid scanning interferometer
data acquisition and processing. The choice of some of these operating at a constant mirror velocity and a given resolution,
parameters is governed by the trading rules in FT-IR spectrom- the S/N increases with the square root of the number of
etry and by specific data quality objectives of the study. co-added scans. The choice of measurement time for signal
7.2 Trading Rules in FT-IR Spectrometry—The quantitative averaging in OP/FT-IR monitoring must take into account
relationships between the S/N, resolution, and measurement several factors. First, a measurement time must be chosen to
time in FT-IR spectrometry are called “trading rules.” The achieve an adequate S/N for the required detection limits.
factors that affect the S/N and dictate the trading rules are However, because monitoring for gases and vapors in the air is
expressed in Eq 1, which gives the S/N of a spectrum measured a dynamic process, consideration must be given to the temporal
with a rapid-scanning Michelson interferometer (6): nature of the target gas concentration. For example, if the
concentration of the target gas decreases dramatically during
1/2
S U ~T!·u·Dv·t ·j·D*
v
5 (1)
the measurement time, then there would be a dilution effect. In
1/2
N
~A !
D
addition, varying signals cannot be added linearly in the
interferogram domain. Nonlinearities and bandshape distor-
where:
tions will be observed if the concentrations of gases in the path
U (T) 5 spectral energy density at wavenumber v from a
v
vary appreciably during the measurement time.
blackbody source at a temperature T,
u5 optical throughput of the spectrometric system, 7.4 Resolution—Several factors must be considered when
Dv 5 resolution of the interferometer,
determining the optimum resolution for measuring the IR
t 5 measurement time in seconds,
spectra of gases and vapors along a long, open path. These
j5 efficiency of the interferometer,
factors include (1) the ability to distinguish between the
D* 5 specific detectivity, a measure of the sensitivity
spectral features of target analytes and those of ambient
of the detector, and
interfering species in the atmosphere, such as water vapor; (2)
A 5 area of the detector element.
D
the tradeoffs between resolution, IR peak absorbance, and S/N;
(3) practical considerations, such as measurement time, com-
NOTE 2—This equation is correct but assumes that the system is
detector noise limited, which is not always true. For example, source
putational t
...
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