(Pearl-Omnicell Transmission) AN18-04: Determining Pathlength in Parallel window Liquid Cells Using Interference Fringes

For high accuracy pathlength determination parallel window cells are the gold standard.

When using an empty infrared cell with parallel windows (such as the Omni-Cell or Pearl) a fringing pattern is observed in the IR spectrum [1]. It consists of a series of regularly repeating fine peaks and troughs of a sinusoidal wave pattern superimposed over the spectrum of the window material is observed (Figure 1). This fringing pattern can be used to determine an exact cell pathlength.

Filling the cell with a solvent greatly suppresses this effect; As a result, accurate fringe free IR spectra can be recorded with a parallel cell by recording a background with the same solvent as in the sample spectrum.

For applications, such as quantification or calibration, where the pathlength must be known with precision, a parallel cell may be used in this way to give an accurate measure of pathlength while avoiding fringes in the sample spectrum itself.

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