What cause the bracket standard RSD% failure?

[baobaobei]

Question:
HPLC condition: mobile phase A - 25mM ammonium formate buffer pH5.0 (using formic acid to adjust pH), mobile phase B- AcN; Gradient; Sometimes the RSD% of bracket standards are higher than 0.8% which fail the System suitability. What are the causes? Something wrong with the methods?

[danko]

What is the relevant parameter here - content, retention time, purity, or.....?
0.8% is not too bad, but of course it depends on what you're testing.
By the way, your buffer is actually no buffer at pH 5 - out of capacity range.

Best Regards

[baobaobei]

The compound has the functional group - NH2.
The standard is pure as 99.7% potency.
Is that because the pH5.0 out of the ammonium formate buffer capacity and the pH changes when mixing the buffer with AcN at gradient run? Thus it impacts the repeatability of the peak area?

[danko]

I don't think the pH matters in this context, but you still need to share the infrmation I asked for:
What parameter are you testing for - content? With other words what varies with more than 0.8%.

Best Regards

[baobaobei]

I am testing an API compound. I can only tell it is a small molecular contains amine group. The compound is stable at room temp and under light.The requirement for the method system suitability is %RSD of the bracket standard (6injections, 3 injections before and 3 injections after the sample) not more than 0.67%. But the repeatability (%RSD of standard) seems not good, varies from 0.1% to 0.8% of a few trial runs. The peak area of each injection of standard is not very consistent and it happened by using another HPLC also. So I suspect the method is not robust enough due to some reasons.

Thank you for the input.

[danko]

I forgot to ask - sorry - but do you se a tendency, like falling peak area or increasing ditto, for each injection? What about retention time and peak shape?

Best Regards