Short path gas cells are often used when one wants to measure gases at high concentrations. Beer’s Law simply states that the absorbance signal is proportional the absorptivity (a) of the gas (how strong an IR absorber) times the pathlength (b) times the concentration (c).
A = abc
There are other forms of this equation, but this conceptual equation works well for our discussion. The stronger the IR absorber the more IR signal that is absorbed from the IR beam. The more IR absorber molecules in the path of the more IR signal that is absorbed from the IR beam. The longer the pathlength the more IR signal is absorbed from the IR beam and the lower the signal at the detector.
Symmetric molecules such as fluorine (F2), nitrogen (N2) and oxygen (O2) are not mid-IR absorbers. Symmetric molecules usually exhibit strong Raman lines.
Asymmetric molecule are IR absorbers. In general, the more asymmetric or polar a molecule is, the stronger the IR absorption bands. Molecules such as water (H2O) and hydrogen fluoride (HF) are examples of strong IR absorbers.
The IR energy is removed from the IR beam as it passes through the gas. The higher the concentration of gas for a specific pathlength, the more energy that is absorbed. The longer the pathlength for a given concentration of gas, the more molecules that will interact with the IR beam and the more energy that will be absorbed from that beam.
One wants to keep the absorbance levels of the bands being measured to less than 0.3 absorbance units when doing quantitative analysis using IR techniques. That is true for a number of instrumentation reasons, which we do not have to go into here. The important point is that it is necessary to keep the absorbance levels for the bands used to measure the concentration of an IR absorbing compound to less than 0.3 absorbance units. That can be done by keeping the gas cell short in high gas concentration situations. One can also pick very weak bands free from other interferences when high concentrations are present. One can also dilute the gas stream. Usually the easiest thing to do to keep the absorbances less than 0.3 absorbance units is to use a shorter pathlength cell.
We can provide pathlengths from 25 microns to 15 centimeters and even longer single path gas cells in custom applications. The standard available gas cells come in 1 cm, 5 cm, 10 cm, and 15 cm pathlengths. Custom pathlengths are available.
It is important to pick the materials in the gas cell that are compatible with the gases being measured. The cell body can be made of polymers, glass, nickel coated alumimum, stainless steel, nickel coated stainless steel, or solid nickel. The sample inlet and outlet fittings can be simple hose fittings, Swagelok, or VCR. The windows can be KBr, KCl, ZnSe, CaF2, BaF2, quartz, sapphire, etc. The o-ring can be Viton, Teflon, or Kalrez as standard materials. Some seals can be metal in some applications.
The materials and configuration of the cell is designed to meet your specific application.
10 cm stainless gas cell with VCR fittings, valves, and purge baffles in a sample compartment of a typical FTIR laboratory spectrometer. We can provide gas cells for any IR spectrometer configuration.