In HPLC assays, the lack of reference substance for a compoundto be determined implies that its amount can be estimated by theuse of a different external standard. This is a common practice in theEuropean Pharmacopeia especially in tests for related substances.In this case, the peak area of the analyte is compared to that ofthe external standard. To correct for any significant differences inresponse factors, the peak area of the substance to be determinedis multiplied by a correction factor.Correction factors used in this method were calculated as fol-lows. For each compound, the slope of the regression lines obtainedin the linearity study was divided by the slope obtained for SIL andthe reciprocal of the fraction was taken as the corresponding cor-rection factor (listed in Table 2). For practical purposes, the resultswere rounded to one decimal place.