Evaluated Infrared Reference Spectra
This collection of infrared spectra was originally
edited and published by the Coblentz Society.
A detailed discussion of the Coblentz IR spectral collection, acquired
primarily on prism and grating instruments, has been given in the
Deskbook of Infrared Spectra[1,
text]. A brief discussion of issues is presented below
concerning the comparison of spectra in this collection with
spectra acquired on modern FTIR instruments.
For prism-based spectrometers, the spectral resolution of a given
prism material depends directly on the rate of change of its
refractive index versus wavelength, which in turn depends on
wavelength. Materials used for prism manufacture tend to show a
general decline in resolution with decreasing wavelength over the
conventional IR region. This noticeably broadens IR spectral features
in the 3,000 cm-1 region. See, for example, tables of refractive index
for crystals and glasses in the American Institute of Physics
Handbook. Changes in resolution over a
spectrum for grating instruments are less noticeable, partly because
they occur repetitively over each of several grating changes needed to
acquire a complete spectrum. In both grating and prism instruments,
resolutions in the low frequency (fingerprint) region are usually not
very far from the “natural” resolution of the
sample. However, very sharp features can be noticeably broadened in
- Pen Widths and Irregularities:
In the digitization process spurious variations can occur due to
varying clarity of spectral lines and overlapping grid lines. If a
minor feature is of interest, it is may be necessary to visually
examine the original image to determine whether it is real or
- Absorbance Scale:
Spectra were selected for inclusion in the Coblentz collection based
on a consensus of evaluators. In some cases, in order to clearly show
weak features, spectra were recorded at levels where the maximum
absorbance was high. Under these conditions relative absorbances of
major bands become less uncertain, since accuracy depends on the
distance between the band maximum and the maximum absorbance limit,
which can become very small. This must be kept in mind when comparing
these spectra to those acquired under higher levels of transmission.
- Sample Conditions:
Many of the spectra in the Coblentz collection were painstakingly
measured in solution in “split solvent” cells. It is
important to keep in mind that the spectrum of a compound in solution
can depend on its concentration in solution and features may be
significantly different as a pure liquid or solid, and can differ
drastically from gas phase spectra. Solvents used in split solvent
measurements were selected to minimize solvent absorptions over the
complete IR spectra range.
- Instrument Class:
Spectra in the Coblentz collection were obtained primarily on
dispersive instruments (prism, grating and combinations) and were
digitized long after their measurement. For reasons described above,
care is needed when comparison spectra to those acquired on FTIR
instruments. Differences in resolution, sample state, impurities and
dynamic range need to be kept in mind when examining these spectra.
- Time of Spectral Acquisition/Evaluation:
For spectra where a date was not provided on the spectrum, an estimate
was made based on the approximate sequence number in the collection.
- Smith, A.L., The Coblentz Society Desk Book of
Infrared Spectra in Carver, C.D., editor, The Coblentz
Society Desk Book of Infrared Spectra, Second Edition, The
Coblentz Society:Kirkwood, MO, 1982, pp 1-24. A PDF file of this article is available
(reproduced with permission of the Coblentz Society).
- D. E. Gray, ed., American Institute of
Physics Handbook, Third Edition, McGraw Hill:New York, 1972.