what happen to my Head space GC

[ketone]

I am using HS-GC by auosampling benzene dissolved in water by continuous mode.

the peak area for serial sampling is first climbing but increasing all the way.

I did the just benzene gas sample( there is no water) the peak area first climb to a maxima then slow drop.

So what is happening to my acquous sample?

I am mad at this. and tried change a new DB-5ms column but turned to be same behavior.

by the way, the column temperature is 140 degree C. and sample transfer line temperature is 140 degree C.

Any suggestion will be appreciated!

[Schmitty]

What autosampler is this?

Can you explain what the continuous mode does differently than non-continuous? Is it perhaps a headspace flush onto the column or a tenax trap?

Do you have a temperature gradient on your GC? Or, are you just running isothermally at 140°C?

Maybe you are getting a buildup of something that is causing carryover or contamination. Is your detector an FID or an MS?

[ketone]

Hi, Schmitty,


I built the autosampler which consists of 3 valves. essentially, there are 3 stages for automatic operation: vaccumm the ballist, filling the sample in ballist and injection sample into GC system.

my GC column hold isothermally at 140 C degree and detector is FID.

probabaly some things like water molecule built up in the valve system which messed up my experiment, but how?

Thank you for your suggestion!

[Schmitty]

If you built the valve system, where do you have it located in relation to your GC? Is it a heated system in a separate oven? Is just the transfer line heated?

[ketone]

the 3 valves are kept at 200 C degree by valve oven and the transfer line is kept at 140 C degreen by heating tapes.

[Schmitty]

Well, if your "ballist" is your loop, you may have too high of a temperature in relation to your transfer line. The air at 200°C in the valve system can hold more water, and when moved through the transfer line it may be condensing inside the tubing.

I usually increase the temperature throughout my samplers, or at least keep them all the same, i.e:

sample @ 60°C
needle @ 80°C
loop @ 85°C
transfer line @ 105°C

or something like that. It depends on my analytes, matrix and the system I am using. Also, be aware of putting a hot needle inside of a cooler vial. This may throw off your reproducibility by heating your headspace beyond your initial sample temperature.