VERY strange problem with GCMS columns

[BZK]

I think that I get it. This is a possible mechanism to generate a higher concentration of siloxanes towards the downstream end of the column, the leading edge of the "peak" then corresponds to the slowly rising vapour pressure of the band of siloxanes as the oven temperature increases - NB that there are contaminant siloxanes all the way to the end of the column so this is not a chromatographic process. The back edge of the "peak" is then the trailing edge of the zone of previously accumulated siloxanes, which does move chromatographically. If this is how it works then a slower temperature programme will give a slower rise on the leading edge of the "peak", you might even be able to generate a stepped front edge by using a step gradient in the temperature programme.

Peter

Yes. Your explaination could be the right answer. I will try to decrease slowly the temperature program. If your hypothesis is rigth at least changement on peak shape must be observed.

Thanks Peter and Japp.

Robertino

[Peter Apps]

Hi Robertino

Please post the results here when you have them - if apparent chromatographic peaks can be generated by uniform columm bleed it will make a difference to troubleshooting contaminant problems.

Peter

[BZK]

Hi Peter, unfortunately GC2010 doesn't perform negative temperature ramp. I have tried to partially closed the oven IN/OUT air without result. The cooling from 280 to 180 °C remain the same and very fast (10-12 seconds). Delaying on cooling could be observed only for temperature less than 120°C. The oven was design for a very fast cooling. The only way thath i can see to overcome the problem is contact Shimadzu factory for negative temperature ramp implementation.

[Peter Apps]

Hi Robertino

If your model is correct there might be an intermediate temperature at which the contaminants are generated more slowly than they move along the column. If you hold the column at that temperature for several minutes, and then complete the cooling to the usual programme start temperature (what is your initial temperature ?) then you should see less contamination. As a start try cooling to 200 C and holding for 30 min, then cooling the rest of the way.

Of course it might just be easier to use a different brand of column ! - but not as interesting !

Peter

[BZK]

Hi Robertino

If your model is correct there might be an intermediate temperature at which the contaminants are generated more slowly than they move along the column. If you hold the column at that temperature for several minutes, and then complete the cooling to the usual programme start temperature (what is your initial temperature ?) then you should see less contamination. As a start try cooling to 200 C and holding for 30 min, then cooling the rest of the way.

Of course it might just be easier to use a different brand of column ! - but not as interesting !

Peter

I have yet done some tests at 180 and 200 °C but not with long time of holding at high temperature.
About column changing I have changed from zebron to agilent with similiar results. DB5MS-UI is little better in bleeding.
Due to budget reason I can't change column anymore.
Well, now is friday afternoon and stop the job. On the next monday I restart with extensive temperature tests.

A good weekend.

Robertino