Long path gas cells are used to measure gas concentrations down to the single part per billion level. The longer the pathlength at a fixed concentration the more molecules the IR beam passes through. (See the discussion in the Short Path Gas Cell section.) In theory one will get ever increased IR absorption signals the longer the pathlength. In practice, there is the competing process of getting more noise as the IR signal is reduced by mirror reflection losses and losses due to scattering. The reasonable number of reflections in a multi pass gas cell is about 40 reflections with a standard White cell and 80 reflections using a modified Horn-Pinmentel gas cell design using a corner cube objective lens. The goal is to increase the absorption signal by increasing the pathlength at a rate faster than the noise (associated with IR signal losses) increases.
It is important to note that there becomes a trade off when the increased pathlength does not improve the signal to noise ratio of the IR bands of interest. That limit depends on the spectrometer energy output, gas cell design, gases of interest, and the sensitivity (D*) of the detector. Improved signal to noise ratio with increased pathlength usually requires using larger diameter mirrors and that requires larger volume gas cells. Many times larger volume gas cells is not a problem. Sometimes sample size is limit for a number of reasons such as that’s all there is (output from a GC or TGA) or the gas is very toxic or dangerous to have in large volumes. Othertimes (such as ambient air monitoring) there is plenty of sample volume available.
There are two physical dimensions that increase with longer pathlength. They are base path and diameter of the mirrors. The base path is the distance between the reflecting mirrors at each end of the gas cell. The longer the base path, the larger the mirrors need to be to capture the reflected IR images. Larger mirrors require larger gas cell diameter. These two dimensions (length and diameter) yield the volume of the cell. The larger volume cell requires larger volumes of sample gas. Smaller volume gas cells with longer pathlengths are possible, but the IR energy losses are greater producing higher noise thereby defeating the advantage of the longer pathlength. Well collimated optical beams such as what one obtains with lasers often can take advantage of long pathlengths and smaller volumes better than IR beams from FTIR spectrometers.
Typical White Cell.
The beam comes in one window (left) and reflects back and forth between the field mirror (left) on the input side and the objective mirror (right) on the other end of the gas cell. After each 4 passes the beam migrates across the top of the field mirror on the input side until it misses that mirror and exits through the exit window.
The field mirror on the above shows the migration of 40 passes of the IR beam in the gas cell. The IR beam comes in at the position marked 0 and exits at the position marked 40. The field mirror has the corners cut out to allow the IR beam to input and exit the gas cell.