Variable path-length liquid cells for infrared spectros-copy are items of routine application. The literature contains few, if any, references to their gas-phase counterpart, however. One obvious use for a variable path-length gas cell would be as a reference cell for the cancellation of the background spectra of gases during in situ studies of weak adsorption on sample dises of varying thickness.
Ronald L. Musselman and Charles P. Nash, "A Variable Path-Length Infrared Gas Cell," Appl. Spectrosc. 29, 527-528 (1975)