%rsd issue

Discussions about methods, troubleshooting and best practices across both pharmaceutical and biopharmaceutical analysis.

5 posts Page 1 of 1
Hi

Im really hoping that someone could help me with this method we are having constant trouble with.

The method is used to detect the solvent ethylene chlorohydrin (ECH) in pharmaceutical samples and uses a DB-WAX column 30m x 530 x 1um.
The system is agilent 6890n.

inj volume: 1ul (10ul syringe)
flow: 7psi constant pressure, ~ 11ml/min (helium gas)
inlet: splitless 230C, purge on at 1.5min at 25ml/min
detector: FID 245C, makeup: 25ml/min
temp program:, initial 8min at 35C, 10C/min to 200C, hold 11min

The method uses a gooseneck liner 4mm (agilent brand). The standard used is initially diluted with water to give a conc of 28ppm and finally diluted again using methanol to give a conc. of 14ppm. Peak usually comes off at ~16min.

The problem we are having is that has become impossible at times to get this method to pass %rsd system suitability, spec is nmt 4% for 3 injections of the 14ppm working stock. I have pasted an image of a typical chromatogram of the 14ppm working stock overlayed. As you can see there is a significant loss of ECH from injection to injection. Also the peak seems to move slightly in retention time. Is this specific to any problem?

Just to clarify this is not a method im responsible for coming up with so you can pick holes in it as much as you want but if someone has any ideas on this precision problem it would be great. Also there was a period many months ago when we had zero issues with this method so i have my suspicions that it could be consumable related (liner, septum etc?)

Any thoughts are appreciated
https://photos.app.goo.gl/vkKCVA6SqBsRf2Cn6
Have you trimmed the head of the column lately?

Injecting water splitless and holding the temperature at 35c could allow water to condense at the head of the column plus any contaminates could also be building up there.
The past is there to guide us into the future, not to dwell in.
Water is a terrible solvent for GC applications. The problem is the very large expansion volume upon flash heating in the GC inlet. Can you dilute without using water and retry?
I'd inject 0.5ul from a 1ul syringe. Water expands a ton; you didn't state how much you dilute the aqueous with methanol.
RT drifting later could definitely be leak at the septum. If youre doing alot of injections, a month is a long time for a septum.
5 posts Page 1 of 1

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