Just Because You Don’t Have a Leak Does Not Mean You Don’t Have a Leak.

Identifying common GC inlet leaks chromatographically can be challenging since small leaks may result in subtle changes.  I was able to identify a small leak while running straight chained hydrocarbons (C10-C50) in the lab. While reviewing my chromatograms, I noticed a common phenomenon.  My initial standard injections produced sharp peaks and reproducible retention times for the first few injections.  However, over the course of my sequence I noticed my peak shapes broadening on the higher molecular weight hydrocarbons, particularly C35, C40 and C50.  I also noticed that I was starting to see retention times shift in my peaks as well. I suspected a leak when I began to see a shift in retention times and the peaks becoming broader. Leaks would reduce the actual flow in the column, which extends retention times and broadens peaks. When the leak is fixed, I expect to see sharper peaks and earlier elution times.

I began troubleshooting:  First I checked the method I had been running to ensure that all my parameters were correct. The method had an oven temperature of 60°C (hold 5 mins) to 370°C at 20°C/min. with a pressure range of 19.2psi to 41.2psi to maintain the 8ml/min constant flow throughout the oven program. I looked back over the method for errors, but it all checked out, so I reran the method again and watched the GC for errors.  I noticed that the inlet was not able to maintain head pressure as the oven program began to ramp.  The first thing I tried was replacing the septa.  It just so happened that is where my leak was occurring.  An over-cored septa was the culprit. After replacing the septa, my peak shapes were back to the original height and the retention times returned to within their expected range.

The two clues that I had a leak were the shift in retention times and the shorter broader peaks. I was running a programmable oven method with constant flow, and my retention times were shifting and peaks were becoming broader. I found that the system could not maintain the flows given. Under constant flow conditions, as the oven temperature increases, the head pressure increases as well, so the cored septa was unable to maintain the pressure needed for the constant flow. The system was able to maintain the desired head pressure of 19.2psi at the 60°C starting temperature and therefore no leak was detected.  It wasn’t until the oven program reached 145°C with a pressure of 25.5psi that the septum’s seal began to leak.  The band broadening increased as the temperature, pressure and leak increased.

The overlaid chromatograms shown in Figure 1 illustrate the inability of the system to maintain the necessary flow as the oven temperature and head pressure increased.  Notice the shift in retention as well as the later eluting peaks broadening due to the inlet leak.

Figure 1:

This is one of many examples of a difficult to find leak in the GC system. The Restek Leak Detector is a very useful tool for pinpointing small leaks quickly and accurately before they cause major damage to the GC column or GC instrument, but it is important to watch for clues in your chromatograms. By paying attention to peak shape and retention time, I was able to spot signs of a leak, troubleshoot, and correct this issue. There are many places a leak can occur on the inlet side of a GC. My colleague, Chris English, is writing up a blog series on various causes of GC system leaks.  Be sure to check back soon for more information!