Chromatography Forum

A value of NMT 2.0% of %RSD for five injections of standard preparation is indicated as per USP. We usually do it by injecting one vial six times. Is it really necessary that we are going to consider the five consecutive injections? Let say, we have six injections (1 to 5 or 2 to 6) in order to test the suitability of the HPLC system? Thanks.

Hello

In my opinion it is better to inject standard from different vials one by one instead of injections from the same vial. Possible problems you can avoid:
- micro-dilution - small amount of solvent outside the needle can dilute standard in the vial
- if your solvent is volatile and injection volume is quite big , you can have headspace phase in the vial and concentration of your standard can be higher after couple of injections


Regards

Tomasz Kubowicz

Hello

In my opinion it is better to inject standard from different vials one by one instead of injections from the same vial. Possible problems you can avoid:
- micro-dilution - small amount of solvent outside the needle can dilute standard in the vial
- if your solvent is volatile and injection volume is quite big , you can have headspace phase in the vial and concentration of your standard can be higher after couple of injections


Regards

Tomasz Kubowicz

Okay so let say we are doing what you suggested and I have vial number 1 to 6 and all of a sudden my vial number 5 has a problem. Should I not consider vial 1 to 4 plus vial number 6 for the calculation of %RSD for my System Suitability Test? USP mentions five replicates at NMT 2.0%. Does it mean consecutive vial replicates? Thanks.

Hello

Try to imagine situation...
You do what you mentioned (you skip vial no5 and you take 1-4 plus 6). You calculate RSD and then you run your sample.
What if you'll have the same problem as for vial no5 (wrong volume injected) ? You won't be able to tell if your sample was injected properly...You'll calculate sample concentration and it'll be wrong.
So if one of your consecutive injection is causing problem for RSD calculations, I would say that there is something wrong with your LC system (sampler).
I assume of course that peak integration is out of the question...

Regards

Tomasz Kubowicz

We inject five from the same vial before samples are injected, plus another injection at the end. Both initial SST and on-going SST are calculated, both must pass.

Thank You all for the responses