Chromatography Forum

Hi all,

I'm totally new to this forum - some very interesting topics for me to delve into but firstly I was hoping for a little help from someone far wiser than I!

I'm working on a Shimadzu QP2010. I'm having a strange problem where, the first run after any down time is plagued with siloxane peaks at the end of a temp programmed run. An identical injection immediatly after this is clean. My baseline curves up at the end of every run regardless. I'm running at 30oC for 0.5min then 45oC/min up to 340oC with a hold of 1 min at the end.

It confusing me as to why this is only appearing in the first run. So far I've tried new column (baked at isothermal temp), new septum, liner, cleaned ion source. I'm thinking either the temp programme is a problem or the carrier gas (helium).

It's worse on the first run after being down overnight but have noticed that even an hour of sitting, I am still getting these peaks.

So I guess my questions are:

What is causing this?
Any thoughts on how to get rid.

I realise you may need more info so please just ask. Many thanks in advance.

Always disregard the first run of the day. With GC or HPLC.

The common sources for siloxanes like this are the septum and the column. Septum crumbs fall into the inlet and bleed siloxanes and degredation of the column results in siloxanes as well.

When I am doing work where silane background is an issue, I normally push the start run button on the GC before beginning a sample sequence - just to run the termperture on the column up - and get rid of silanes.

To avoid silane peaks
- be sure the carrier gas has no oxygen in it. Use copper tubing and include an oxygen trap close to the back of the GC. I use an indicating trap to ensure the trap is working.
- leak check the instrument. When carrier gas leaks out of a fitting - air leaks in. (Diffusion runs both ways.)
- Change colun manufacturers. The bleed level of columns differs beteen manufacturers - and the life of a column varies as well. And, this may be subject to the application as well.

I'm working on a Shimadzu QP2010. I'm having a strange problem where, the first run after any down time is plagued with siloxane peaks at the end of a temp programmed run. An identical injection immediatly after this is clean. My baseline curves up at the end of every run regardless. I'm running at 30oC for 0.5min then 45oC/min up to 340oC with a hold of 1 min at the end.

It confusing me as to why this is only appearing in the first run. So far I've tried new column (baked at isothermal temp), new septum, liner, cleaned ion source. I'm thinking either the temp programme is a problem or the carrier gas (helium).

It's worse on the first run after being down overnight but have noticed that even an hour of sitting, I am still getting these peaks.

So I guess my questions are:

What is causing this?
Any thoughts on how to get rid.

I realise you may need more info so please just ask. Many thanks in advance.

It's normal.You leave the GC oven at low temperature for many hours. Heavy impurity as siloxane from septum (but not only) are continuosly trapped into the first part of column. When the first run start all trapped sustances are eluted. If you need to verify this try to put GC and injector at 220-240°C overnight. The first run must NOT show any particular contamination.

Anyway..... discard the first run (or better inject solvent blank) or, before, put the GC at high temperaure for at least 30-60'.

Robertino Barcarolo, Italy

Yeah, the siloxanes are 'cold trapping' at the head of the column, and then at some point during your temperature program they elute. This phenomenon is much easier to diagnosis with MS, as you see huge multimillion counts of siloxane the first run, then maybe a million the 2nd, then 200k the 3rd etc. Depends on the amount of siloxane.

It is possible to fix the problem and still have bleed problems until the column is replaced, however that is only in severe cases.

I have seen many causes, including a severely contaminated split vent line.