The concentration of the IS should be such that it gives a signal of the same order of magnitude of the analyst of interest (at the concentrations of interest). Practical issue. Make it of a similar area of your middle point of your calibration curve for your analyte of interest. you MUST ensure that you add the same amount of IS to all of your samples (calibration and incognites).
Then, the possibility of using a higher concentration than those of my calibration curve is unviable?
Since you use the peak area ratio in internal standard method in HPLC, the mid concentration of the callibration curve is suitable to select as it makes easy for calculation
Its depend on many factors! Assuming you using stable label internal standard having similar response and optimization like analyte on interest then generally you can use IS concentration that lies in between your curve ( might be 25 or 50 times of LLOQ).
If you having interference from IS to analyte channel then you should lower your IS concentration. And, if you IS plot suffering suppression at higher analyte concentration then you should consider to either lower your analyte concentration range or increase your IS concentration of compensate the suppression.
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