Posted in | Chemical Analysis | FTIR Accessories

Heatable Long-Pathlength Gas Cells for FTIR Gas Testing

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The standard long pathlength gas cell range available from Specac is now the latest Atmos™ series of metal-bodied fixed-pathlength gas cells.

These cells have been improved using new optical modeling tools to provide the highest signal-to-noise.

Model	A2.5	A5	A10	A20
Pathlength (m)	2.5	5.0	10	20
Base pathlength (mm)	104	139	250	455
Number of passes	24	36	40	44
Volume (L)	0.27	0.63	2.12	3.68

The Atmos™ series can be heated to 200 °C with the inclusion of a temperature controller and heating jacket.

Why Atmos™?

Low cell volume to facilitate rapid gas exchange
Improved optical path for unparalleled signal-to-noise
Retrofittable heating jacket option for up to 200 °C
Maximum cell pressure of 125 psi (8.6 bar)
Includes inert stainless-steel mirror substrates and does not contain glues, thus preventing outgassing
Ni-coated Al cell bodies enable better thermal transfer from heating jackets; the Atmos™ series also holds higher gas pressure when compared to glass

Flexible Specification Long-Pathlength Gas Cells—Cyclone™ and Tornado™

Cyclone™ and Tornado™ cells are available with metal or glass cell bodies and can be configured to a range of pathlengths.

Model	C2	C5 or T5	C10 or T10	C20 or T20
Pathlength settings (m)	0.5—2.5 in 0.5 m steps	1.0—8.0 in 1.0 m steps	2.1—10.6 in 1.06 m steps	2.0—20.0 in 2.0 m steps
Base pathlength (mm)	125	250	264	500
Number of passes	4—20	4—32	8—40	4—40
Volume (L)	0.19	1.33	2.60	4.30

Cyclone™ Versus Tornado™

Tornado™ cells are a stripped-back offering, delivered as standard without outlet or inlet tap valves; they cannot be heated.
Cyclone™ cells can be delivered with a fitted heating jacket for analyses of heated gases of up to 200 °C (unavailable for model C20)
A modifiable mirror carriage can be installed into the Cyclone™ cells to allow users to change the pathlength throughout the given range.

Which Pathlength to Choose?

Gas absorbance relies on the distance traveled by the IR light beam across the gas sample. The association between pathlength (L), concentration (C), and absorbance (A) is given by Beer’s Law, as shown below:

A = -log₁₀(I/I₀) = a.C.L

The concentrations of gases in the atmosphere are generally expressed in C.L units of ppm⋅m—the number of molecules encountered by the infrared beam across a 1.0-m path.

A gas at 0.1 ppm atmospheric concentration will be able to absorb as much IR light over 100-m pathlength as would the same gas at 10 ppm over 1 m or at 1 ppm over 10 m.

Therefore, the cell’s pathlength should be selected to give absorbance values within the linear range of the spectrometer for a specified concentration. The table given below can be considered as a guide:

Gas name	formula	v (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	0.10
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