Chromatography Forum

Hi!

Currently, I'm developing a method to detect PAHs and other smaller derivatives of benzene using stir bar sorptive extraction (SBSE). For analysis, I use a thermal desorption unit from Gerstel (TDU) linked to an Agilent GC/MS. The problem is that the efficiency of the column (HP-5MS) decreases very rapidly and I need to cut the first 10-20cm every 2 weeks. Particularly, I regularly experience peak duplication for the most volatile compounds (e.g., benzonitrile, tolunitrile, benzyl nitrile) and tailing for the less volatile (e.g., phenanthrene, pyrene). After column cutting, efficiency of the column is totally restored.

Do you have any idea about what might be causing it? Most importantly, do you have any solutions to prevent this situation? Maintenance takes me a lot of time and, moreover, I must replace the column very often because of the loss of the separation capability.

Thanks

1) what does the air background look like? If you have air leaks in the desorbtion unit, the air could be damaging the column.

2) What is the matrix you are using the SBSE in? If you are workign with a corrosive mixture, some of that could transfer to the column.

Thanks for your reply!

Actually, I'm using SBSE in headspace mode and I think that the problem cannot come from the matrix. Air background is "normal" for a thermal desorption (septumless) instrument and I check it everyday, but I'm afraid that the problem is really that. I cannot totally prevent air to enter in the injector. Anyway, such a rapid degradation is surprising. Is there no solution to slow down it?

How hot do you run the column? If the issue is air, then elevated temperature is part of the problem. Can you get by with a lower final temperature on the oven program?

And even in headspace mode, if you have compunds that are volatile and will damage the column, I would expect the posability of transfer.

Run starts at 40°C and finishes at 280°C, which is kept for 0min. I set the post-run at 310°C for 5min. Using a final temperature below 280°C would be difficult, but I compared these conditions with the literature and I found that they are normal for similar methods with the same instruments and target molecules. Maybe the post-run temperature is too high?

The determining factor for post-run column conditioning is: is there anything on the column to come off? If so, you need to heat the column. For liquid extractions, there are often high boilers. With thermal desorption, you have to move compounds from the stir bar first - so you have some chance at discrimination against high boilers. And you have to transfer compunds to the stir bar throubh the headspace - which is slower for high boilers.

If you can skip the post run conditioning, it may help. 30 degrees is not a really big step, but it may help.

The matrix --- I don't know.

Air: It is worth revisiting doing everythign you can do to tighten the desorbtion unit. We have one in the drawer in our lab. I've heard of air background, but if there was column damage, it has not come up in the discussion. I'd suggest contacting Gerstel to see if they can offer any advice on being sure your TDU is a leak free as possible.