The Adsorption Desorption System or ADS was the original pre-concentration system developed at the ACRG for the trapping and analysis of trace gas species. The ADS was one of the first systems developed which did not require the use of liquid cryogens (N2) for the cooling of adsorbent traps (Simmonds et al., 1995). This is a fundamental component of instruments which can be run continuously in remote field locations.
The core research of the group focuses on the use of custom developed and built front-end preconcentration devices called Adsorption Desorption Systems (ADS). The ADS is illustrated below attached to a commercial GC-MS instrument used for tracer dispersion studies.
ADS-GCMS, ADS circled
In the adsorption/desorption system a large (up to 3litre) air sample is drawn through a microtrap containing adsorbent material at ambient or sub-ambient temperatures before being thermally desorbed onto the GC column. The carrier gas (helium) is cleaned by passing over charcoal to remove volatile organics (VOCs) and a molecular sieve (to remove water vapour and oxygen) before it enters the ADS. Air samples and calibration standards, which enter though valve one, are dried using a Nafion prior to trapping. Samples are trapped on a U shaped microtrap packed with several carbon based (Carboxen) adsorbents. The sample interacts with the weakest adsorbents first and the strongest last thus preventing ‘sticky’ compounds becoming stuck to strong adsorbents. Desorption flow occurs in the opposite direction. This microtrap is held at -50oC whilst trapping occurs using a Peltier cooler, avoiding the use of liquid cryogens. Desorption of trace gas species occurs by rapid heating of the microtrap by passing current through attached wires resulting in rapid heating. The species are desorbed onto the GC column in one sharp plug.
Schematic Diagram of the adsorption desorption system.
ADS microtrap attached to selector valve. Trap is within Peltier cooler module U groove.
Inside the ADS system we can see the U-shaped microtrap with the multi-position valve above. One side of the Peltier cooling unit has been removed so the microtrap can be seen, this Peltier unit can be seen on the left with its’ cooling fins visible.
Once the sample has been separated on the GC column the quantity of each species exiting the column s measured using one of two methods:- mass spectrometry (MS) or electron capture detector (ECD).
Simmonds, P.G., O'Doherty, S., Nickless, G., Strurrock, G.A., Swaby, R.,Knight, P., Ricketts, J., Woffendin, G. and Smith, R., (1995). 'Automated gas chromatograph/mass spectrometer for routine atmospheric field measurements of the CFC replacement compounds, the hydroflurocarbons and hydrochloroflurocarbons'. Anal. Chem., 67:717-723.