Gas Chromatography with Mass Spectrometry (ADS GC-MS)

AGAGE also use in situ gas chromatography with mass spectrometry detection to measure hydrofluorocarbons and hydrochlorofluorocarbons (e.g. HCFC-22, HCFC-141b, HCFC-142b, HFC-134a), which are interim or long-term alternatives to CFC-11 and CFC-12 now restricted by the Montreal Protocol, other hydrohalocarbons (e.g. H-1211, H-1301), perfluorocarbons (PFCs, e.g. CF4 and PFC-116), and trace CFCs, some of which (e.g. CFC-114, CFC-115) are also affected by either the Montreal or Kyoto Protocols.


The first AGAGE automated gas chromatograph-mass spectrometer (GC-MS), based on a Finnigan Magnum Iron Trap coupled to a custom-built (Bristol University) adsorption-desorption system (ADS), was installed at the Mace Head, Ireland AGAGE station on October 1994. The ADS incorporates all of the electrically actuated values, mass flow controller, air sampling pump, absorbent filled microtrap, thermoelectric cooler, and ancillary electronics and software to enable alternative analyses of standards and air samples. By concentrating a 2-liter air sample onto the absorbent-filled microtrap, detection limits of < 0.2 ppt have been achieved with the ADS GC-MS. The calibration and air analysis are alternated every two hours to provide six calibrated measurements per day. In 1997, two new identical ADS GC_MS instruments incorporating Hewlett Packard 5973 quadrupole mass spectrometers were installed at the Mace Head (October 1997) and Cape Grim (November 1997) stations.

Figure 1. Schematic of the ADS.

Figure 1. Schematic of the ADS (click to enlarge).

The ADS, shown schematically in Figure 1, contains three electrically actuated valves with purged housings to minimize contamination from air leaks. Valve 1, a three-port valve, selects either ambient air or standard gas. This was later upgraded to a six-port multiposition valve (not shown in Figure 1) which automatically selects the stream to be analyzed (which can be ambient air, standards, zero air blanks, etc.). Valve 2 is a six-port two-position valve which allows either ancillary helium (He) or a gas sample to pass through the microtrap. Valve 3 is also a six-port two-position valve, and the microtrap is connected to this valve. Using this valve, the microtrap can be switched to be in-line either with the sample or helium purge or with the analytical capillary column.

The microtrap is constructed from thin-walled (0.068 cm ID by 0.109 cm OD) type 304 stainless steel tubing and filled with three adsorbents, Carboxen 1003 (6 mg) , Carboxen 1000 (4 mg) and Carbotrap (4mg) (later changed to Carboxen 1016 (4mg)). Each end of the tube is packed with glass beads, and a small quantity of glass beads is used to separate the three adsorbents materials. The small internal dimensions and volume of the microtrap ensure that chromatographic peak shape and resolution are retained during the one-step thermal desorption stage. The microtrap is housed in a U-shaped channel between two insulated anodized aluminum blocks at a subambient temperature of -55°C, which is achieved using two 3-stage cascaded thermoelectric devices attached to the back of each of the aluminum blocks. The thermoelectric units are switched off after each analysis and allowed to warm to room temperature to minimize the buildup of water or ice; the thermoelectrics are then switched back on 20 min before the next sampling cycle.

Figure 1a illustrates the valve positions when purging the microtrap with helium, which occurs prior to sampling and allows the microtrap to be preheated and cleaned before sampling begins. This valve configuration also occurs at the end of the sampling period, when helium flows through the microtrap to displace any residual H2O and CO2 and leave the microtrap under an inert helium atmosphere in preparation for backflushing and desorption onto the analytical column. In Figure 1b, air is pumped through the microtrap by a KNF pump at a sampling flow rate of 50 mL/min, which is accurately controlled by a Tylan mass flow controller. The flow rate and time of sampling are logged by the computer to determine the total volume sampled during each analysis. When valve 1 is switched to standard gas or zero air blank, this flow is also controlled by the mass flow controller. The sample is dried by passage through a Nafion permeation drier, which is counterpurged continuously with a flow of 200 mL/min of dry nitrogen from a N2 generator at Mace Head or zero air at Cape Grim. The N2 and zero-air generators also provide flows which circulate around the tops of the cold traps to prevent the accumulation of ice. A flow of ancillary helium controlled by a Nupro valve purges the microtrap. This helium is then recycled, after purification through a charcoal filter, and used to purge the housings of valves 1, 2, and 3. This procedure ensures that the microtrap and all associated plumbing are always under an inert helium atmosphere when not involved in sampling. After a nominal 2 L of air or calibration standard have been trapped, valve 2 is returned to the helium purge position to remove residual air, water, and CO2 from the microtrap. Once the microtrap has been purged with approximately 50 mL of helium, valve 3 is switched to the inject position (see Figure 1c), and the enriched sample is backflushed and thermally desorbed directly onto the capillary column by direct resistance heating of the microtrap. The microtrap is heated from -55°C to 240°C in 4 s. Valve 3 is switched back to the helium purge position after 30 s to enable microtrap cleanup and acquisition of the next sample.

Initially both GC-MS instruments were specially targeted for routine measurements of the principal HFCs and HCFCs, and other key halocarbons. For the gases which are quantitatively trapped, the ADS GC-MS provide high quality of data. Some practical limitations of the ADS GC-MS instrument (i.e. limited trapping temperature and adsorbent capacity of the microtrap), however, make it difficult to measure more volatile and radiatively active perfluorocarbons. To overcome these practical limitations and building on the success and experience gained with the ADS GC-MS, advanced cryotechnologies have been employed to construct the next generation of automated GC-MS instruments with new (Medusa CG-MS) modules replacing the ADS modules.