Chromatography Forum

Dear all

A question about carryover.

We turn our GC off daily after testing - detector off, oven off, inlet off, leaving only makeup gas on.

Each morning on start up we run a water blank and we see 2 large peaks coming out at the end of the sequence as the temperature ramps. It is normal to see baseline disturbance but these are large well defined peaks.

However, we have observed that by leaving the inlet flow and inlet temperature on overnight (oven and detector still off) we don't see these peaks.

Also, these peaks only persist for the initial blank injection. They are gone by the second injection of the day.

Column flow is 4.58ml/min
Purge flow 5ml/min
Split flow 18ml/min

Column is ZB-624.

Septum and liner have been changed. This issue has persisted for around 2 months with the retention time of these ghost peaks very stable.

Any ideas for how and why this seems to be building up without flow would be appreciated.

Hmm - I worked with GCs over 4 decades, and only turned them off to service/repair....

Agree - 2 things GC users generally like to avoid -
1) shutting off the GC
2) injecting water

Pilger,

without wanting to sound like a broken record here.... What type of GC?

I have a slightly different view on shutdown. Sleep can be used to reduce flows both out the split and at the detector. Sleep can also be used to swap the carrier from helium to hydrogen for example. But... what Sleep should not do is turn off the heaters or reduce the carrier flow. The "big money" if you want to call it that is in the gases, not the electricity.

The garbage can be any number of things in your inlet or even septum bleed since you are no longer sweeping that.

Best regards,

AICMM

We shut down for the purpose of gas conservation - however the makeup gas is left on permanently. Make up is 25ml/min. Shut down is not ideal but is lab procedure.

It is a TRACE1310. Column is ZB-624. I believe this column is meant to be good for column bleed. I agree injecting water into a GC is not ideal but this is lab procedure.

Oven temp is 35C and ramps to 190C. We see bleed as the temp ramps. Baseline is 4.2pA @ 35C and 6.2pA @ 190C and this is normal.

Septum and liner have been changed so I don't believe the bleed is coming from there.

Ghost/carryover peaks are about 0.05pA*min so not huge. I was more wondering about the mechanism that is causing this to happen as it seems to be a particular compound that is persistent.

I guess my question is, if the GC is turned off, wouldn't this mean that there is some accumulation of material at room temperature? I thought bleed was associated with temperature.

Hi Pilger

Your observation is a useful insight into the reality of practical Gas Chromatography.

In 40 years of supporting GC customers, what you see is pretty much normal.
The accumulation of higher m.w. material while the column flow and temperature are reduced is to be expected.

Since your laboratory protocol requires instrument shutdown overnight and a blank injection the next day, then no problem.

In fact a blank injection is mandatory prior to any analysis.

We shut down for the purpose of gas conservation - however the makeup gas is left on permanently. Make up is 25ml/min. Shut down is not ideal but is lab procedure.

It is a TRACE1310. Column is ZB-624. I believe this column is meant to be good for column bleed. I agree injecting water into a GC is not ideal but this is lab procedure.

Oven temp is 35C and ramps to 190C. We see bleed as the temp ramps. Baseline is 4.2pA @ 35C and 6.2pA @ 190C and this is normal.

Septum and liner have been changed so I don't believe the bleed is coming from there.

Ghost/carryover peaks are about 0.05pA*min so not huge. I was more wondering about the mechanism that is causing this to happen as it seems to be a particular compound that is persistent.

I guess my question is, if the GC is turned off, wouldn't this mean that there is some accumulation of material at room temperature? I thought bleed was associated with temperature.

Sounds like your lab procedures need a bit of an overhaul...