Sorbent Tubes and Sorbent Material Selection

Although sorbents are well established for quantitative use in analytical systems and sensors, our application requires that they be used in a non-standard mode. For most sensors, such as gas chromatograph/mass spectrometer (GC/MS) systems, a carrier gas at or above 1 atm is used to carry the analyte out of the sorbent region and into the analytical instrument. However, in our system any sorbed compounds are thermally desorbed from the sorbent tube and evolve directly into the evacuated spectrometer cell.

The presence of even a spectroscopically transparent gas such as helium in the spectrometer cell reduces the sensitivity of our system because it pressure broadens and dilutes the analyte. For example, if the volume of the sorbent tube used in our system is 1 mL and the spectrometer cell has a volume of 500 mL, the expansion of 1 mL of carrier gas at STP into the cell would result in a cell pressure of ~1.5 torr. A pressure broadening coefficient of 5 MHz/torr, would result in a linewidth of ~7.5 MHz. Since this is about 20 times the Doppler width, the pressure broadened analyte line would be 20 times broader and consequently have a peak absorption 20 times smaller than if the helium were not present. If the pressure in the cell were subsequently reduced by a factor of 20, this would reduce the linewidth back to the Doppler limit (and restore the specificity of the sensor), but the line would remain reduced in peak absorption by the factor of 20.

The sorbent Carboxen 1000 was selected for the demonstration system, though many others (Tenax, HayeSep, etc.) are available that provide adequate functionality. Carboxen 1000 can selectively trap a wide range of compounds, with varying sorption/desorption efficiencies. Importantly, Carboxen 1000 does not trap water very efficiently, making it an excellent candidate because water removal is critically important to reduce dilution of the trace analytes inside the spectrometer cell.