Repeatability of GC, Multiple injections per vial

[Tony_Montanari]

I have had trouble getting my new Varian 3800 to yield good RSDs for multiple injections from the same vial. If I limit injections to 1-2 injections per vial, there is no problem. This could be expensive for standards.

Q: Is there a fix for this?

[chromatographer1]

There are common issues with multiple sampling of a vial.

The vial must no undergo changes in temperature as this changes the pressure inside the vial and can affect the amount of liquid sampled by the autosampler.

The pierced septum must not absorb the analytes of interest or allow them to disperse into the lab atmosphere.

Yet the septum must not seal so well that a partial vacuum or change in internal pressure in the vial is created after sampling. This can affect the amount of sample taken and injected into the GC injection port.

Do any of these possibilities appear to be the case concerning your problem?

Using inserts to hold 50µL of std instead of 1mL of std in a vial is one possible solution.

Good luck

Rod

[Tony_Montanari]

Thanks for the quick reply.

All that is possible. We are using 2 mL HPLC autosampler vials with thick silicone septa. I believe they have PTFE on the inside. You are right, we can go to using inserts as well. I haven't seen any 50 mcL yet, but I am willing to look. If I can get to 50 mcL inserts, filtering will be an issue.

I want to try an experiment soon if I can. I would like to inject the same sample vial over and over again, and change the septa inbetween injections. It will only cost me 10 screw caps, but I can learn a bunch.

Thanks for all the tips. I will keep you posted on my progress.

[mattsol]

If you can use an internal standard, this problem will be alleviated.

[chromatographer1]

I did not intent for you to understand that I suggested inserts to hold a sample that you reject over and over again from the SAME vial.

No no please.

Use multiple vials each holding 50µL and pierce each septa only ONCE.

FYI

Heating of samples in a HP autosampler is a problem that I and many others have dealt with over the years. Some companies even sell coolers to keep samples at the same temperature.

best wishes,

Rod

[stappg]

Tony,
What is the solvent you are using? If it is highly volatile with high vapour pressure you may have to use separate vials for each injection. Another thing to check may be the fill volume of your vials - if you fill to the max prior to capping you may get vacuum problems when you inject, especially if your injection volume is large. Maybe also try a sellection of different septa? What is the ambient temp in your lab? Hope this helps?
Peter