calibrations will almost certainly vary from instrument
to instrument. This occurs for several reasons,
including (a) variations in both abscissa and ordinate
scale accuracy from instrument to instrument, (b)
infinitesimal variations in spacer thickness and
window curvature at different points within the
cell cavity, and (c) variations in exact location
of the interface of the beam and the cell from instrument
Figure 3, the pathlength is computed as follows:
(Fig. 4), are fringes from an empty cell mounted
in an FTIR spectrophotometer. Applying the pathlength
formula for wavenumbers (Fig. 2), V1
is 1235.67 cm-1,
V2 is 806.98 cm-1
and n is 9 as we again count only maxima (not minima),
and ignore the first maxima.
Figure 4, the pathlength is computed as follows:
are many reasons why fringes may be difficult to
produce. For example, cells with pathlengths of
1 mm and larger and cells with wedged windows (designed
to reduce fringing), do not produce dramatic fringes.
In these cases, calibration against a standard is
useful. At ICL we use Benzene, which produces a
useful peak at about 2650 cm-1.
To calculate the pathlength of a cell with an unknown
pathlength, the Benzene absorbance at 2650 cm-1
of the cell with a known pathlength is compared
to the Benzene absorbance of a cell with the unknown
pathlength at that wavenumber. A cell with a known
pathlength of 1.0340mm produces the Benzene absorbance
shown in Fig. 6 at 2650 cm-1.
The Benzene absorbance (0.7635) is the difference
between the absorbance units shown on the ordinate
scale at the trough of the peak (0.2315) and the
absorbance units shown on the ordinate scale at
apex of the peak (0.9950). The cell with the unknown
pathlength is shown in Fig. 7 and the Benzene absorbance
at 2650 cm-1 is
0.7885. Applying the formula in Fig. 5, the pathlength
of the unknown cell is 1.0679mm.