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Preventing septum blocked sample needle
Posted: Thu Aug 27, 2020 8:51 am
by Ecivon
Hello everyone,
I am working with a 6890 GC that has been adapted with an automated head space sampler.
The needle which transfers the HS sample from the transfer line to the inlet is blocked. I can see that it is blocked with a piece of septum.
My questions are:
1. How to clean a blocked needle (0.7 mm iø, 5 cm long) - can I burn or chemically remove the piece of septum? Air and poking out with a wire didn’t budge it.
2. How to prevent blocking the needle when inserting it through the septum and into the inlet? Surely this must happen a lot??
I hope there is enough info for someone to help!
Re: Preventing septum blocked sample needle
Posted: Thu Aug 27, 2020 4:29 pm
by James_Ball
Hello everyone,
I am working with a 6890 GC that has been adapted with an automated head space sampler.
The needle which transfers the HS sample from the transfer line to the inlet is blocked. I can see that it is blocked with a piece of septum.
My questions are:
1. How to clean a blocked needle (0.7 mm iø, 5 cm long) - can I burn or chemically remove the piece of septum? Air and poking out with a wire didn’t budge it.
2. How to prevent blocking the needle when inserting it through the septum and into the inlet? Surely this must happen a lot??
I hope there is enough info for someone to help!
When we had needles become clogged on the soil sampling portion of the purge and trap autosampler we would connect it to an HPLC pump and use the high pressure solvent to blow it out. If that didn't remove it nothing was going to.
How much air pressure did you use? Could you possible attach it to a nitrogen line and use 50-100psi to try to remove the plug?
To prevent it plug you could get pre-drilled septa and slide the septum nut on the needle then the septa then install it into the inlet, possibly.
Re: Preventing septum blocked sample needle
Posted: Fri Aug 28, 2020 6:59 am
by Ecivon
James_Ball thanks for the suggestion.
Drilling a hole might be possible, but I just wonder how this isn't a common problem with septa? - it might also reduce the seal strength.
Re: Preventing septum blocked sample needle
Posted: Fri Aug 28, 2020 4:08 pm
by James_Ball
I think you can purchase one that is labeled pre-drilled, which could also mean it is molded into the septa when it is made.
When I do purge and trap I connect the transfer line into the incoming gas line at the top weldment of the inlet, using the line normally going into the weldment to supply the carrier gas in the sample concentrator for desorbing and carrier, not sure how the head space unit works though. If you can tap into that line instead of using the needle then you can avoid the problem all together.
Re: Preventing septum blocked sample needle
Posted: Tue Sep 08, 2020 8:42 am
by Peter Apps
What kind of wire did you use ?, the sort that comes with micro-syringes specifically for clearing needles, or something else ?
If you thread the injector nut over the needle, then stick the needle through the septum, then attach it all to the inlet I would be very surpised if the needle cut a core from the septum - that usually happens when a needle is pushed through a septum with the injector nut screwed too tight.
There are ways of getting even fused silica transfer lines through septa, but they are fiddly and I doubt they would be necessary with a metal needle.
Peter