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Long-path gas cells

From Specac

Long-path gas cells work according to the White principle, in which IR light is reflected several times within a three-mirror-system in order to increase the path length. The cells are available in various basic sizes.

Features

Cell body from glas or metal
Path lengths from 0,5 m to 20 m
Optional electrical heating jacket up to 200 °C
Fixed path length
Vacuum up to 9 bar (with metal body and ZnSe windows)

Further information

Which pathlength should I choose?

Due to the low density of gases compared to liquids, special gas cells are used for IR measurements. For trace analysis of gaseous samples, path lengths of up to several meters must be used. Lambert-Beer's law helps to estimate the required path length.

The absorbance of a gas depends on the distance travelled by the IR light beam through the gas sample. The relationship between absorbance A, concentration c, and pathlength d, is given by Beer’s Law:

A = -log10 (I/I₀) = ε∙c∙d

Atmospheric concentrations of gases are usually expressed in c∙d units of ppm∙m - the number of molecules that would be encountered by the infrared beam across a 1.0 m path.

A gas at 0.1 ppm atmospheric concentration will absorb as much IR light over a pathlength of 100 m as would the same gas at 1 ppm over 10 meters or at 10 ppm over 1 meter.
Accordingly, the pathlength of cell should be chosen to give absorbance values within the spectrometer’s linear range for a given concentration. The following table may be taken as a guide:

Gas name	formula	ν(cm^-1)	ppm∙m	absorbance
Carbon dioxide	CO₂	2360	100	0.40
Carbon monoxide	CO	2170	100	0.04
Methane	CH₄	3020	100	0.10
C₂ to C₆ n-alkanes		2960	100	0.10
Nitrogen dioxide	NO₂	1630	100	0.15
Nitric oxide	NO	1900	100	0.015
Sulfur dioxide	SO₂	1370	100	0.09
Hydrogen sulfide	H₂S	1300	1000	0.002
Ammonia	NH₃	960	100	0.12
Hydrogen chloride	HCL	2940	100	0.04
Water	H₂O	1650	1000	0.20
Vinyl chloride	CH₂CHCl	950, 900	100	0.06
Acetaldehyde	CH₃CHO	2750	100	0.015
Benzene	C₆H₆	670	10	0.09
Toluene	C₆H₅CH₃	730, 690	100	0.10
Methanol	CH₃OH	1040	100	0.40
Ethanol	CH₃CH₂OH	1050	100	0.05
Carbonyl sulfide	COS	2070	100	0.40
Nitrous oxide	N₂O	2235	100	0.15
Sulfur hexafluoride	SF₆	950	10	0.40