Summer days are my favorite – hot, sunny, and full of fun. But one thing that’s really not fun is trying to track down contamination and/or carryover in your headspace-GC (HS-GC) system. I get questions about this topic pretty regularly, so I thought I would post a blog on the most common culprit for contamination and carryover – cold spots.
When performing HS-GC, the rule of thumb is that your sample vial should be the coolest part of your system prior to the head of your column. The reason for this is that HS-GC depends on the use of heat to drive volatile compounds into the headspace of your sample vial, where they are then sampled. If you’re using a loop-style system employing a transfer line, then your sample has to be transported a long way before it gets to the head of the column. If your sample encounters a sudden drop in temperature while in transit, then some of the less volatile (or more abundant) compounds in the sample may condense in that cooler area, which is known as a ‘cold spot’, even though it could be significantly warmer than room temperature. The term ‘cold’ is relative to the other temperatures present in your sampling system.
When working with systems employing transfer lines that must be plumbed into an existing inlet, the most common cold spot is the area between where the transfer line ends and the heated inlet begins. This area is circled in Figure 1.
Figure 1: Cold Spot between Heated Transfer Line and Injection Port
As you can see, there is a good amount of tubing that is exposed to room temperature between the end of the heating jacket for the transfer line and the heated inlet. Even though your analytes may be volatile enough to make it through a cold spot like this, over time contamination from less-volatile matrix components can build up in this area, and it will eventually bleed over into your inlet, causing ghost peaks during blank runs as shown in Figure 2.
Figure 2: Ghost Peaks from Contaminated Transfer Line
The contamination shown above is really severe, and usually looks like just a few persistent peaks in your sample and blank runs. Normally, contamination can be eliminated by curing the cold spot, then running a bunch of solvent blanks (e.g. DMSO or DMA) using a method with increased transfer line, needle, and valve oven temperatures.
But how can we cure a cold spot? Two relatively easy fixes will cure most cold spots in your system. The first is to reduce the size of the cold spot. Figure 3 shows how I re-plumbed the transfer line, making the tubing going to the inlet as short as I could get it. You might also notice that since I was doing plumbing anyway, I installed an EZ-Twist Top injection port, which is much more convenient than the old-style weldment shown in Figure 1.
Figure 3: Transfer line Re-Plumbed to Reduce Size of Cold Spot
Although the size of the cold spot has been reduced, there’s still a significant length of tubing that’s exposed to room temperature. This brings us to the second part of our cold-spot remedy – an easy way to keep exposed tubing warm: glass wool and aluminum foil. Simply cover all exposed tubing and the injection port with the wool and use the foil to keep everything further insulated and to keep those irritating wool fibers contained. The setup is shown in Figure 4, and while it’s not pretty, it is effective.
Figure 4: Cold Spot Insulated with Glass Wool Insulation (inlay) and Aluminum Foil
Remember to wear gloves when handling the wool; just like fiberglass insulation, it will make you itchy! Also remember that the wool I’m describing in this blog isn’t the same as the wool that’s used in areas of the instrument that come into contact with sample. There’s no need for deactivation of the insulating wool.
I hope that this blog was useful, and make sure to get out of the lab and have some fun this summer!