Tornado Series Long Pathlength Gas Cell

Atmos | Heatable long pathlength gas cells


Better value long pathlength gas cell | AtmosTM

Download the free datasheet in 'Documents' below.

Specac’s standard long pathlength gas cell range is now the new Atmos™ series of metal-bodied fixed-pathlength gas cells. They have been optimized using the latest optical modelling tools to give maximum 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™ range of cells are heatable to 200 °C with the addition of a heating jacket and temperature controller.

Why Atmos™?

  • Optimized optical path for unmatched signal-to-noise
  • Low cell volume for fast gas exchange
  • Ni-coated Al cell bodies for improved thermal transfer from heating jackets. Also holds higher gas pressure than glass.
  • Inert stainless-steel mirror substrates and avoidance of glues to prevent outgassing
  • 125 psi (8.6 bar) maximum cell pressure
  • Retrofittable heating jacket option for up to 200 °C

Which pathlength should I choose?

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, L, is given by Beer’s Law:

A = -log10 (I/I0) = a.C.L

Atmospheric concentrations of gases are usually expressed in C.L 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 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

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Read More
  • Atmos
  • Documents
Atmos Gas Cell
  • Atmos A2.5 AtmosTM A2.5 FTIR Gas Transmission Cell (2.5m Pathlength) GS24602
  • Atmos A5 AtmosTM A5 FTIR Gas Transmission Cell (5m Pathlength)  GS24605
  • Atmos A10 AtmosTM A10 FTIR Gas Transmission Cell (10m Pathlength) GS24610
  • Atmos A20 AtmosTM A20 FTIR Gas Transmission Cell (20m Pathlength) GS24620
Atmos Heating Jacket
  • A2.5 Heating Jacket Atmos A2.5 Heating Jacket and Controller GS24651
  • A5 Heating Jacket Atmos A5 Heating Jacket and Controller GS24652
  • A10 Heating Jacket Atmos A10 Heating Jacket and Controller GS24653
  • A20 Heating Jacket Atmos A20 Heating Jacket and Controller GS24654
Data Sheets
User Manuals


  • Wide pathlength range (2.5 to 20m)
  • Vacuum to 125 psi operation
  • Up to 200 °C with heating jacket
  • Ni-coated aluminium body
  • Gold mirrors (protected)
  • Viton or Kalrez 'O' ring seals
  • CaF2 or ZnSe windows
  • Purgeable transfer optics box
  • Leverlock series baseplate mounting
  • Optional Features

  • Vacuum / gas inlet & outlet taps
  • Pressure gauge
  • Desiccant storage caps
  • Purge bellows