%CV between Bracketing Standards

[StephenO]

Hi All

I am currently having problems with high %CV in bracketing standards on Waters 2495 & 2489. I make up my Standard and carry out two injections from one vial position run the samples and bracket them with another two injections of the same Standard from a different vial position.

We are seeing that there is a significant difference in the areas depending on which vial position the injection was taken from for the Standards even though it is the same solution i.e the first 2 bracketing standards give lower areas that the Standards in the second position.

Has anyone experienced this problem before we use this method all the time but this seems to be a new problem. It doesnt happen all the time and seems to be intermittent. The run time for each injection is 6 minutes.

Example sample set below

Std x 2inj
Sam1 x 2inj
Sam2 x 2inj
Sam3 x 2inj
Std x 2inj

Regards

[paulw]

I have seen a few instances over the course of my career where the vials were the culprit in a situation such as you just described. This has always been when working with low levels of analyte. I would suggest trying a different supplier of vial, type of vial (i.e. clear instead of amber), or as a last resort, prewashing the vials a few times with your solution. Might not be the answer in your case, but it sure doesn't sound like the autosampler if you can get reproducibility from a single vial.

[JGK]

If the sample compartment is running below room temperature you may see this effect if you fail to equilibrate your vials for long enough.

[MaryCarson]

You can also see a vial effect if the caps are on very tight and the autosampler has to draw against a vacuum. If you can, remove the caps and see if you still have a position effect.

[DR]

You can also see a vial effect if the caps are on very tight and the autosampler has to draw against a vacuum. If you can, remove the caps and see if you still have a position effect.

In other words, have you overfilled the vials?

[StephenO]

Thanks for your help Ill try this