P.M. Chu, F.R. Guenther,
G.C. Rhoderick, and P.A. Johnson
Objective: To
develop the NIST Standard Reference Database 79 (SRD 79) “Quantitative Infrared
Database” to support optical-based measurements of chemical emissions and
hazardous air pollutants (HAP).
Problem: Over the last decade,
growing concerns about the environment in general and air quality in particular
have stimulated the development of improved, cost-effective field monitoring
methods. With the recent advances in optical-based technologies, methods such
as open-path Fourier transform infrared (FT-IR) spectroscopy are now becoming
viable tools for field measurements. The in-situ
and real-time nature of this approach offers several advantages over
traditional point source monitoring techniques for applications such as
determining fugitive emissions and chemical contaminants from industrial
processing plants, hazardous waste and municipal landfills, water treatment
plants, oil refineries, and chemical plants. Following successful testing of FT
infrared methods during remediation of several Superfund sites, the US
Environmental Protection Agency (EPA) has issued a protocol (TO-16) for FT
infrared open-path remote sensing. Since molar absorptivity data in the
literature widely differ, a validated quantitative database traceable to
national measurement standards is a critical part of the infrastructure
required for establishing emerging infrared-based monitoring technologies. New infrared-based technologies coupled with
the NIST spectral database provides both industry and EPA with a tool for
assessing regulatory compliance that is both cost-effective and less invasive.
Approach: SRD 79
data are based on NIST primary gravimetric standards prepared with starting
materials of assessed purity and procedures that minimize contamination. For
each compound, the absorption coefficient spectrum was calculated using nine
transmittance spectra and the Beer’s law relationship. The uncertainties in the
absorption coefficient data are estimated from the data analysis and
considerations of other error sources such as the non-linear detector
response.
The primary effort of this project has focused on acquiring
data at 25 ºC. To address the data
needs of communities measuring industrial stack emissions by passive FT-IR and
other infrared-based technologies, we have measured absorption coefficients for
industrially important target gases, sulfur dioxide and methanol, at
temperatures ranging from 25 ºC to 200 ºC.
This project included studies to quantify sample emission contributions
to the measured single-beam spectra.
Results
and Future Plans: Currently, absorption coefficient data for 40 HAPs on a US
EPA priority list is available as the NIST Standard Reference Database (SRD)
79, Version 1.1 with Data Release 3.0. The data are stored in the standard
JCAMP-DX format to enable universal access to the data. The 0.12 cm–1
resolution data were processed to generate data at a number of different
resolutions and apodizations to provide users with data that closely match
their experimental parameters. A
digital signature accompanies each data file, allowing users to ensure the
integrity and source of the data file and traceability to NIST.
Data comparisons with Battelle Pacific Northwest
laboratories and the higher temperature data project have helped us
characterize two additional FT-IR intensity artifacts. Both of these artifacts
are associated with the single limiting aperture design in most commercial
FT-IRs. The first artifact is attributed to the detector viewing the warm
annulus of the aperture in higher resolution measurements (0.2 cm‑1
or higher). The off-axis rays distort the line shapes and increase the observed
band intensities. The second artifact is due to the light exiting the
interferometer, which is directed towards the source, can be reflected off the
aperture back into the interferometer. This double modulated light produces a
2f alias throughout the spectrum. Reducing the reflectivity of the aperture
wheel minimizes this artifact.
Adding an additional aperture system or a multipass cell
after the interferometer minimizes the off-axis rays that reach the detector
and in turn the contributions from these artifacts. For SRD 79, all of the data was acquired with a multipass
cell and the potential biases due to these artifacts are within the stated
uncertainties in the finger print region and no biases are expected at higher
frequencies. The higher temperature absorption coefficient data required
additional measurements with the infrared source off to appropriately account
for sample emission issues. At 200 ºC,
the sample emission correction is approximately 10 % for bands in the spectral
region near 1000 cm-1 for the instrument and optical configuration
used.
This quantitative infrared database is an ongoing project
at NIST. Additional spectra will be
added to the database as they are acquired and updates will be available over
the internet. Plans include continued data acquisition for the compounds listed
in the 1990 US EPA Clean Air Act Amendment, as well as for those compounds that
are of concern in global warming and emissions trading. Additionally,
comparisons of NIST primary standards and molar absorptivity data with National
Metrology Institutes and other laboratories will be expanded to facilitate the
use of this database in issues of global interest and impact.
Recent
Publications:
“Infrared Absorptivity Temperature Dependence
of Gas Phase Methanol and Sulfur Dioxide”, P.M. Chu, G.C. Rhoderick, P.A.
Johnson, NISTIR 6773, National Institute of Standards and Technology,
Gaithersburg, MD 20899, August 2001.
“Removing Aperture-Induced Artifacts from
FTIR Intensity Values”, T.J. Johnson, R.L. Sams, T.A. Blake, S.W. Sharpe,
and P.M. Chu, submitted to Applied Optics.
Data Release 3.0 for NIST
Standard Reference Database 79 CD, P.M. Chu, F.R. Guenther, G.C. Rhoderick, National
Institute of Standards and Technology, Gaithersburg, MD 20899-8393, August
2001.