Our principle hardware products are the long path absoprtion cells, which we offer in many models. These cells are designed for general analytical applications, using Fourier transform spectrometers, dispersive spectrometers, or tunable lasers. Special cells and optical components can be designed and manufactured to meet the particular need of the customer.

The cells have transfer optics that allow them to be used in the sample compartments of commercial spectrometers without modification. Installation or removal can be just a matter of lifting the cell in or out of the sample compartment.

In most of the cells, the optical beam is folded by the three mirror "White" system, giving multiple-passing in incremnets of four passes.[1] The pathlengths are variable from 1 meter to more than 100 meters. In a multiple-pass cell, the radiation from the source is first focused in the entrance aperture. From there, the diverging beam passes to the first objective mirror,which focuses an image on the field mirror. The field mirror returns the beam, diverging, to the second objective mirror which then directs the beam either out of the cell (4 passes) or back to the field mirror for further multi-passing.

For most of the cells,operating pressures from vacuum to 4 atmospheres are allowed. Stainless steel vacuum valves and pressure relief valves are used. The larger cells also have a vacuum/pressure gauge and a built-in laser to mark the optical path. There is a provision for flow through operation.

The cells are usable in the ultraviolet, visible, and infrared spectral regions. Available mirror coatings include silver, aluminum, and gold. Windows are normally KBr flats, 25 mm in diameter. Each cell has transfer optics for mating to the sample compartment of the spectrometer..

The recommended material for the cell bodies is borosilicate glass, mainly because of its chemical inertness. One might ask: Why not use stainless steel or other metal ? Before answering that, we can ask another question: Why do chemists store their reagents in glass containers rather than metal ones ? The answer to both questions is that glass is chemically inert; metal is not. Stainless steel reacts with and consumes acidic vapors such as NO, NO2, SO2, HNO3, HCl, HCOOH, HCN, and even CO2. A stainless steel cell will need only a few minutes to completely consume parts-per-million concentrations of such vapors. In glass, however, such vapors will exist without measureable reduction in concentration.
 

[1]. "Long optical paths of large aperture", J.U. White, Journal of the Optical Society of America, vol. 32, pg. 285 (1942).

For further discussion of measurement techniques, detection limits, and other gas analysis topics, see:

"Detection of atmospheric pollutants at parts-per billion levels by infrared absorption spectroscopy", P.L. Hanst, A.S. Lefohn, and B.W. Gay Jr., Applied Spectroscopy, vol. 27, pg. 188 (1973).

"Gas measurement glossary", Philip L. Hanst, published by Infrared Analysis Inc.; free on request (714)817-9303. Coming to this web page soon !

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