Waters H-Class variable standard areas

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Can anyone provide any insight into this issue I am having please?

I run a gradient Reverse Phase method on a Waters H-Class instrument. column is C18 2.1 x 50mm, ph of sodium phosphate buffer is 3.1 and the method works very well for assay samples but periodically when I run a standard for system suit, the second or third injection has higher areas than the rest of them (for example, 5 injections of area 100000 then a 6th of 104000) and this can also happen for standards throughout the run, so in a run with 10 standard injections, one of the injection can be higher by up to 4000 and often cause the RSD % to fail specification.

Strangely, if I run the samples and standard again without using fresh standards or putting a fresh cap on the vials, the run works out fine but the random nature of this occurrence means its hard to pinpoint the cause. I use ANSI48vial 2ml vials from Waters and I don't tighten the caps too hard. Could it be something to do with pre/post inject wash? The most common time this error happens is on the 2nd injection of the system suit. Any ideas?
I wonder if a 4% error on the injection volume is outside the specs of the instrument for the volume you're injecting. It sounds reasonable. I suggest to submit a ticket to Waters, personally I have good experiences with their iRequest system.

What is the injection volume and is this fixed-loop or flow-through mode?

Is there any indication of carry-over (are solvent blanks baseline)?

An easy fix might be to run a blank or 2 before your SST.
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