Quantitative Infrared Database

P.M. Chu, F.R. Guenther, G.C. Rhoderick, and W.J. Lafferty

Introduction

The NIST Quantitative Infrared Database currently consists of a collection of absorption coefficient spectra of volatile organic compounds. These spectra were measured from primary gas standards prepared and verified at NIST. Spectra are provided in 5 resolutions from 2.00 cm⁻¹ to 0.125 cm⁻¹ using 5 apodization functions. The data file format conforms to the standard JCAMP-DX format and should be readable by commercially available analysis programs. Also included is an uncertainty estimate for the absorption coefficient data, which includes the estimated uncertainties associated with compound purity, primary standards, computation and instrumental performance.

About the Spectra

These spectra were measured at the National Institute of Standards and Technology (NIST) using Fourier-transform infrared (FTIR) spectrometry. At each wavenumber of the provided spectrum the value represents the absorption coefficient a, as defined by the equation:

I_t(ν) = I₀(ν)10 ^−acl

where I_t and I₀ are the transmitted and incident light intensities, c denotes the sample amount-of-substance fraction in μmol/mol, and l is the path length in meters. The uncertainty is expressed as an expanded uncertainty U = ku_c, with the combined standard uncertainty u_c determined from experimental standard uncertainties, and a coverage factor of k = 2. The associated expanded uncertainty for absorption coefficients greater than 1 × 10⁻⁴ (μmol/mol)⁻¹m⁻¹ has been listed as the relative uncertainty. For absorption coefficients less than 1 × 10⁻⁴ (μmol/mol)⁻¹m⁻¹ the expanded uncertainty U, for each absorption coefficient a, can be calculated using the equation:

U ≈ 2(Ba² + Ca +D)^½

where the coefficients B, C, and D are listed under the uncertainty. The true value for the absorption coefficient at a given wavelength is asserted to lie in the interval defined by (a ± U) with a level of confidence of approximately 95 %[1]. The absorption coefficients are not certified in regions of the spectra where water (1325 – 1900) cm⁻¹ and (3550 – 3950) cm⁻¹, carbon monoxide (2050 – 2225) cm⁻¹ and carbon dioxide (2295 – 2385) cm⁻¹ absorb.

The absorption coefficient spectrum was calculated with the Beer's Law relationship from nine transmittance spectra using NIST primary gas standards. The data were processed to produce spectra at five resolutions using five apodization functions. Since instrumental line functions will vary from instrument to instrument, it is highly recommended that integrated spectral features be used when comparing data with NIST spectra in the database. Results obtained by comparing point intensities may be misleading. An absorbance spectrum for a given concentration and path length may be calculated by multiplying the absorption coefficient data by the desired concentration in μmol/mol, and path length in meters. For the calculated absorbance spectrum to be accurate, the user must be assured that the spectrum is applied within a linear absorbance region. The absorbance levels where non-linearities become dominant depend on the resolution and apodization of the spectrum, as well as the natural width of the absorption feature.

The primary gas standards used for these measurements were prepared and verified by G.C. Rhoderick; purity analysis of the volatile organic compounds was performed by F.R. Guenther, C. Mack, and S. Margolis; all of the NIST Analytical Chemistry Division. Statistical consultation was provided by N.F. Zhang of the NIST Statistical Engineering Division.

The FTIR data acquisition and analysis was performed by P.M. Chu of the NIST Analytical Chemistry Division with consultation from W.J. Lafferty of the NIST Optical Technology Division. The overall direction and coordination of the technical work was performed by P.M. Chu and F.R. Guenther of the NIST Analytical Chemistry Division.

A detailed discussion of the database, has been presented in the Journal of Research of the National Institute of Standards and Technology [2].

References

1. Guide to the Expression of Uncertainty in Measurement, 1st Ed. ISO: Geneva, Switzerland, (1993), ISBN 92-67-10188-9; see also B.N. Taylor, and C.E. Kuyatt, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, NIST Technical Note 1297, U.S. Government Printing Office: Washington, D.C. (1994).
2. P.M. Chu, F.R. Guenther, G.C. Rhoderick, and W.J. Lafferty, The NIST Quantitative Infrared Database, J. Res. Natl. Inst. Stand. Technol., 104, 59, (1999). [doi:10.6028/jres.104.004]

Contents of the Database