Gas Sample Handling and Preconcentration

The use of preconcentration is a common strategy to increase the sensitivity of gas sensors [1]. Typically with these methods, the sorbent material is used to collect material in the field for subsequent analysis by laboratory instruments. By integrating a sorbent collection system into the spectroscopic sensor, in combination with low cell pressures, the system can realize sensitivity gains of up to five orders of magnitude over direct collection of the gas into the spectrometer cell. These sensitivity gains are due to the removal of the common atmospheric components such as nitrogen, oxygen, carbon dioxide, argon, and water, which serve only to dilute the trace gases of interest. While as defined by the EPA procedures, the sorbent process in reasonably robust against false positives that result from chemical reactions, the specificity of the sorbent based system does not have the ‘absolute’ (PFA -10) specificity of rotational spectroscopy. Indeed, the specificity of any sensor is limited by reactions, desorption, etc. in its gas handling system.

The demands for high sensitivity and selectivity have a strong impact on the design of the gas handling system. Because Doppler width is proportional to frequency, the optimum sensor pressure, for which the Doppler broadening and pressure broadening are similar [5]. is also proportional to frequency and can be very low (10^-5 – 10^-6 atm) in the SMM/THz, two to three orders of magnitude lower than in the Visible/IR. Therefore, the gas sample handling system requires the use of vacuum technology to achieve ultimate pressures of no more than 1 mtorr. The choice of vacuum technology components for the gas handling system was a critical engineering decision with significant impact on the system size, power consumption, and operational timing sequence. The specifications driving vacuum equipment selection were mainly cell pressure and overall system size (1 ft³).

A particular advantage of SMM/THz systems is that a significantly smaller air sample volume is required to achieve a detectable result than systems that operate in the visible or infrared regions of the spectrum. For technical reasons, sensors in both the SMM/THz [7].often are optimized at pressures that significantly increase their linewidths beyond the Doppler limit. As a result, these sensors typically use 10³ – 10⁵ higher operating pressures than the system described here. Thus, for a cell volume of 1 L (which is typical for these systems) and a preconcentrator gain of 10⁵, a cw electronic SMM/THz systems operating at 10^-5 atm requires the processing of 1 L of atmospheric air, in comparison to the 10³ – 10⁵ L required by the sensors with higher optimal pressures.