GCI TECH NOTES ©
Gas Chromatography Gas Cleaning System
by
David Gossman
Gas cleaning for GC’s using ECDs is imperative. Even a small amount of contaminant in the gas can change the detection limit, contaminate the detector or alter the response factors. Removal of trace oxygen, moisture and hydrocarbons is all part of these systems. As part of the design of these systems toggle valves are used to isolate individual components so that when maintenance to change filters or gases, or to work on the GC the system is performed, the system is not contaminated more than absolutely necessary by ambient air.
In the past it was common to design two identical systems, one for N2 and the other for He. Supelco now has a specialty He filter that combines the hydrocarbon, oxygen and water filter into one unit. On the N 2 system there is a hydrocarbon filter followed by a high capacity purifier that removes water and O2. This filter is heated and requires an electrical outlet. It can be mounted on the wall. In both systems there is a final combined indicating filter that does not have much capacity but acts both as a backup to the primary filters and as an indicator of breakthrough on an earlier filter in the system indicating the need to change the larger capacity filters. There is also a fine particulate filter and appropriate pressure gauges included in the system.
The whole system is made of 1/8” copper tubing. It is critical that all components be purchased clean and free of any oil that is often found in off the shelf copper tubing and brass fittings. For that reason it is recommended that purchases be made through Supelco (now a Sigma-Aldrich company). It may be worth checking your gas supplier for 2 two stage regulators to use for the He and N2. If you purchase that way instead of through Supelco make sure that you get whatever fittings are need to reduce the regulator to 1/8” swagelok connectors. The list below includes those connectors for the regulators listed from Supelco.
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22131-U |
Figure 1. Helium System
Figure 2. Nitrogen System
There are some important tips to remember when putting together this type of system:
- Buy new parts. Used lab equipment can in many cases save lots of money. One contaminated Swagelok fitting can ruin a column. It is not worth the trouble or the risk.
- Use lots of toggle valves in the system so that contaminated traps can be isolated and changed with a minimum of downstream contamination.
- Always use indicating traps at the end of a series so that you can monitor for breakthrough of earlier higher capacity traps.
- For carrier gases; oxygen, water and hydrocarbons all need to be controlled.
- Assemble the system from the gas bottle/supply to the instrument. Carefully flush and isolate with toggle valves each portion of the system as it is assembled to minimize downstream contamination.
- Connect and flush the GC, then install and flush the column and detector prior to heating the injection port, oven or detector. Hot air (with oxygen) can destroy many GC columns and create oxide coats in detectors that impair performance.