Bioequivalence and Bioavailability Forum

Dear Hemant!

❝ [...] reason or Consequence for difference in the retention time of Analyte

❝ and Internal Standard. As per regulatory requirment is there any specific

❝ criteria.

Reason: lipophilicity, pk, degree of ionisation of both analyte and IS; pH, buffer capacity and percentage of organic modifier of the mobile phase, type of stationary phase, column length/diameter, temperature, flow rate,...
Consequence: Retention times per se are irrelevant! You are interested in good separation.
FDA recommends:

R_s of >2 between the peak of interest and the closest potential interfering peak (impurity, excipient, degradation product, internal standard, etc.) is desirable.

See this post for the reference.