Chromatography Forum

Firstly, hello folks
I am pretty new to this amazing world of chromatography. I have been setting up a GCMS system with an OI analytical purge and trap including autosampler. I am noticing inconsistent sample uptake when running and a few other things I am not sure about but I have very little information about exactly how the system works. The system was set up for me around 6 months ago but there does not appear to be support here in the UK for it (Scotland to be specific). If any one on here has experience with this instrument it would be great to pick your brains about it.
thanks
Rory

so the first of many queries.

When the sample needle pierces the septum and draws up the sample, it does this by pressurising the vial with the carrier gas rather than using a pump to suck the sample out. Would i be correct in this?

This uptake of sample is set to run for a specified time (0.15mins) which fills the 5 ml calibrated loop and from there it is transferred to the purging chamber.

I would assume that it is vitally important it purges 5mls each and every time?

thanks

so the first of many queries.

When the sample needle pierces the septum and draws up the sample, it does this by pressurising the vial with the carrier gas rather than using a pump to suck the sample out. Would i be correct in this?

This uptake of sample is set to run for a specified time (0.15mins) which fills the 5 ml calibrated loop and from there it is transferred to the purging chamber.

I would assume that it is vitally important it purges 5mls each and every time?

thanks

With this type of sample loading, where you fill a loop, you need to make sure it removes more than the loop volume from the sample vial. A 5ml loop should be loaded with about 10ml of sample from the vial that way you are sure all the air and rinse water is flushed out and the loop is full of actual sample. Easy way to check is to fill a vial with water and weigh it, then have the sampler take the sample from the vial then weigh it again and the difference would be the volume removed. If it is less than 5 ml(5g) then you are not fully loading your sample loop.

If not loading the loop fully, either there is a restriction in the valves or the gas pressure is too low. If you can adjust the loading time then you may be able to compensate for either of those conditions.

ahh! i already weight the contents it uptakes each time so this explains why it is around 10 grams and not 5 as i was expecting.

Sometimes it fully fills the sample loop and sometimes it only 3/4 fills it. I have noticed that if i run a duplicate sample it often doesn't fill it fully on the second run. I have been assuming this could be something to do with the vial already being pierced and losing pressure when it pressurises it for a second run?

The other thing that happens every single run is bubbles constantly enter the loop when filling. Im am not sure if this is a problem or effects the volume. To be honest I can't get my head around how filling this loop delivers exactly 5mls each time. I get that it is a calibrated piece of glassware and if it is completely full it should contain 5mls, but as it fills from the bottom pushing the excess into the waste and is then reversed emptying it does it not have liquid in the lines that will also make up part of the volume thus making it not exactly 5 mls?

thanks very much for the reply it is very much appreciated.

so the first of many queries.

When the sample needle pierces the septum and draws up the sample, it does this by pressurising the vial with the carrier gas rather than using a pump to suck the sample out. Would i be correct in this?

This uptake of sample is set to run for a specified time (0.15mins) which fills the 5 ml calibrated loop and from there it is transferred to the purging chamber.

I would assume that it is vitally important it purges 5mls each and every time?

thanks

With this type of sample loading, where you fill a loop, you need to make sure it removes more than the loop volume from the sample vial. A 5ml loop should be loaded with about 10ml of sample from the vial that way you are sure all the air and rinse water is flushed out and the loop is full of actual sample. Easy way to check is to fill a vial with water and weigh it, then have the sampler take the sample from the vial then weigh it again and the difference would be the volume removed. If it is less than 5 ml(5g) then you are not fully loading your sample loop.

If not loading the loop fully, either there is a restriction in the valves or the gas pressure is too low. If you can adjust the loading time then you may be able to compensate for either of those conditions.

figured out today how to adjust the loading time...thanks for that:)