Specac are pleased to introduce its latest range of heatable long pathlength gas cells, the Atmos™ range of fixed pathlength, metal bodied cells. Specac are a long-standing supplier of accessories for infrared analysis, having been in business for almost 50 years, and regularly work with scientists and chemists all over the world. Specac was recognised for its global scale and ambition in 2018 with the prestigious Queen’s Award for Enterprise.
Why Atmos™?
The new Atmos™ long pathlength gas cells from Specac are ideally suited to IR measurement of gaseous species at low concentrations.
The principal advantage of the Atmos cell is its high temperature, high pressure capability: its metal-bodied construction contains gas pressures up to 125 psi (Note: up to 80psi for the 20M Atmos) and allows for efficient heating of the cell up to 200 °C using a separate heating jacket. This enables heated gases (such as combustion gases) to be studied while avoiding the condensation of vapours such as water.
In addition, the geometry of the cells has been carefully optimised to maximise signal-to-noise, boosting the limits of detection, and to reduce the overall volume of the cell for rapid gas exchange.
The Atmos™ is available in four factory-set and pre-aligned pathlength configurations:
- Atmos™ A2.5 cell is 2.5 m
- Atmos™ A5 cell is 5.0 m
- Atmos™ A10 cell is 10.0 m
- Atmos™ A20 cell is 20.0 m
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) | Concentration-Pathlength Product (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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