prime needle wash diluting sample?

[tilly]

How could it be possible that after performing the prime needle wash function before a run that the peak responses decrease dramatically (only 3% recovery of expected response). If the sample is injected before performing 'prime needle wash' function responses are normal.
Does the needle wash just clean the outside of the needle or could it be going through into the needle(side load) and diluting the sample?
Any thoughts/suggestions would be much appreciated. Thanks

[freemab222]

Please elaborate.
What instrument are you using?
What volume are you injecting?
What are your sample solvent and wash solvent?

[cody84]

I know on our old Varians that the needle wash cleans out the INSIDE of the needle, sometimes a drop was left on the tip when it punctured the next vial.

I'm guessing you can't see the needle as it injects?

[tilly]

Thanks for the interest.
The instrument is a Waters 2695. The drop in response is very dramatic after prming the needle wash and is not specific to the method conditions, It also occurs with other method with different samples and solvent and injection vol.
I have read about how a blocked pressure bypass can cause a change in the splitflow and would dilute the sample. I will be speaking withthe waters engineer to see if this applies to this model. As far as i know the needle wash does not wash inside the needle in this instrument. If anybody knows more about where the needle wash goes in this model or has more info on the pressure bypass i would appreciate it very much. Thank you

[DR]

Perhaps your priming activities actually depleted the needlewash solution, leaving the line full of air?
This usually results in erratic injection volumes.

[tilly]

Good comment, thanks. There was plenty of needle wash solution acn/water available and also a test was performed to verify the injection volume and this passed.
This issue is still unresolved. However, If the autoinjector is purged after the nedle prime the responses return to normal. This could help in troubleshooting the cause of the problem - have you any further thoughts? Very frustrating but all part of the learning curve. Appreciate all your help. Thanks

Kind Regards