Poor reproducibility HS/GC-ECD

[Stangatang]

Hello all,

I have been running a perkin elmer headspace autosampler 110 vial rack with a clarus 500 gc-ecd for thm's in water matrix and been experiencing some serious problems with reproducibility. I have searched through the forums for advice but couldn't find anything relating to the perkin elmer itself.

From advice I have seen given on this forum based on other HS instruments I have a feeling the solenoids may be malfunctioning. The reproducibility is completely random, so I do not believe it to be a problem with timings. The instrument has worked well in the past and has slowly deteriorated to the point where area counts for any particular quality check may be 40% of what is expected and perfectly fine the next time.

We have had problems with correct crimping, but I do not think that human error could be responsible for the amount of anomalous results we are seeing with occasionally 6 in 10 results being half the response we expect.

Any help would be greatly super great greatly appreciated.
Chris.

[thohry]

There can be a leak in the sampling valve of the HS or in the tubings.

[Stangatang]

Hey Thohry,

Thanks for the reply mate. We suspected that a gas flow problem would cause this and tried to use a H2 detector to find any major leaks, but have been unable to pinpoint anything near the sample needle. Do you think there are any lines in particular that a leak is more likely to occur at?

Cheers,
Chris.

[cbwhipp]

Have you changed the needle o-rings? (Most likely cause - needs to be replaced every thousand injections or so) And then maybe tried a new needle? Have you plugged the end of the transfer line and done a leak check using the HS display? There is also a valve leak check diagnostic to try.

[Stangatang]

Hi Cbwhipp,

Indeed, we have looked at the needle o-rings, needle and tried a leak check. We found a leak as one of the hydraulic solenoids was burned out and after replacing that and we have not been able to trace another leak.

The problem is really baffling us. We really are not sure what else to try to check and perkin elmer have not been able to offer any advice. We have thought that the erratic response might be due to a faulty carrier gas solenoid, in your opinion do you think this is possible? we have not had to change any of them in the past and the instrument has been in operation for approximately 4 years.

Thanks,
Chris.

[cbwhipp]

Does it pass the leak check and valve leak check. If it passes these, then the HS more than likely does not have a leak.
It is very rare for the solenoids to fail or leak - check all the fittings to make sure they are tight around all the valves/tubing.
You said that you looked at the o-rings and needle - but have you replaced them? The #1 reason for poor repeatability is the o-rings need to be replaced. You may also want to replace the transferline fused silica. Make sure that you are not able to twist the caps on the vials when they are crimped - if you can turn them at all, they will leak. ( i know you said you checked this already)