Internal standard

[phucto]

I am perform assay Methocarbamol in tablet by HPLC as USP 35. The assay based on the C18 (250 mmx 4.6 mm, 5 microm) with caffein as internal standard. But when I jected samples for many replicate, the peak area of caffein and methocarbamol per injection vary from injection to injection but the peak area ratio (methocarbamol/caffein) is constant between injections. Pls help me to clarify this problem. is the RSD of each caffein and methocarbamol between replicate injections of same solution is < 2.0% or only care to the peak area ratio?

Thanks you very much!

[WillyOne]

If you are working with Internal Standard, the data you use for calculation is the area ratio. So small variations in individual areas should not affect the result.
I have never used IS in HPLC techniques but it is mandatory in GC because is near impossible to obtain repetitive areas from a hot needle syringe.

[tom jupille]

that is the whole point to using internal standards: to cancel out correlated errors.