The article on importance of gas leak detection discussed the need for checking gas leaks and adoption of recommended leak detection methods. The present article discusses the practices that should be adopted to avoid gas leaks and save your valuable time for carrying out Gas chromatographic analysis.
Leaks should be checked and eliminated before proceeding for column conditioning and start of analytical work on the Gas chromatograph. You should start leak checking at the gas source regulator and carefully monitor each fitting and connector leading to the GC. It is important to switch off the column oven before checking all column connections, adapters, unions and fittings inside the oven.
How to prevent gas leaks?
It is advisable to adopt simple practices that will eliminate gas leaks and increase your confidence and laboratory sample throughput.
Proper cutting of tubing and columns
It is a good practice to use a tube cutter to obtain a clean tube cut. Unevenly cut ends contribute to leakages. A burr or ridge can result during the cutting process. This must be removed to allow free gas flow and obtain a leak free connection. Always hold the tube open end facing downwards while using a deburring tool to prevent fragments from falling inside the tube.
Choice of ferrule size and material
Choose the right size of ferrule that is compatible with the outer diameter of the capillary tubing.
Graphite ferrules are useful for general and high-temperature applications (up to 400°C). However, graphite is not the ideal choice for columns packed with oxygen sensitive stationary phases or oxygen sensitive detectors such as ECD due to semi- permeable nature of graphite. Over tightened graphite tends to flake and extrude thereby contaminating the columns.
Polyimide ferrules have limited application as they tend to shrink when exposed to heat cycling thereby increasing the need to over tighten but the material does not fragment.
Metal ferrules have the advantage of inertness and freedom from fragmentation but have no flexibility.
Never over tighten fittings
Over tightening can result in leaks due to breakage of columns or damage to fitting threads.
Compression fittings provide gas tight, leak free connections without the use of PTFE tape or adhesives.. For 1/8” tubing hand tighten the nut and follow by 3/4 turn using the wrench. For ¼” tubing turn one and quarter turn past fingered tight position with the wrench. On tightening the back ferrule forces itself onto the front ferrule to form a leak free seal. A properly tightened compression fitting usually shows one thread from the back of the nut. Over tighten fittings show no thread and results in leakage.
It always pays to spend a few minutes before start of analysis to ensure freedom from leaks so that you can avoid loss of high purity gases and save several hours trying to get reproducibility of results.