Chromatography Forum

Dear all,

I am working with a Varian 3400 and I have injected three times the same sample and I always get a different retention time. The largest variation has been of almost one minute!!!
This has started to occur recently and, because I don't have that much experience, I don't know what to do.

Could anybody shed some light on this issue please?

Regards

Initially, I would suspect two things:

1. There is a leak somewhere in the system.

2. The carrier flow and split are too high for the restrictor in the instrument. You need to get a restrictor that allows for greater flow and replace the one you are currently using.

As you can see, both my guesses are going to result in a poorly controlled flow through the column, which will effect the retention time of your analytes.

You probably have a leaky septum - change it for a new one. It is probably a good idea to change the inlet liner as well.

Peter

I changed the septum and it did work!!
Thanks very much!!

I also would like to ask how good/old my instrument is (Varian 3400) and what your opinion is about it.

Regards

I also would like to ask how good/old my instrument is (Varian 3400) and what your opinion is about it.

Gas chromatographs are like automobiles. If they are maintained regularly and not abused, they will last for a long time. The Varian 3400 was a good instrument.

Developments in GCs since the HP 5890, Varian 3400/3600 generation have been towards more and more software control, and more sophisticated data processing. Apart form the wider availability of PTV inlets the hardware itself has changed remarkably little. Leaks, dirty inlets and deteriorated columns have exactly the same inpact in the very latest generation of instruments as they do in their predescessors.

Peter

Thanks for helping me so much!

I am learning a lot!