linearity of impurities method

[umoshplc]

when establishing linearity of impurities method how critical is to measure the linearity of each available known impurity? If we use parent compound as external standard, is that ok to establish linearity of parent compound only and find RRFs for rest of the impurities in the validation. Or every impurity needs passing criteria of linearity?

[prepcolumn]

yes you have to check linearity for impurities (although i never faced with a non-linear compound at low concentrations)..
besides, the best way to get the correct RRF for impurities is the ratio of the slopes of active substance and the impurity from linearity studies (instead of single concentration ratios).

[umoshplc]

Thanks for your response. The point is you still see residual sum of squares, correlation coefficient, %y-intercept besides slopes for all impurities or just for parent compound(because we use it as an external standard for quantification)?

[prepcolumn]

we search all linaerity parameters for impurities

[gtma]

I would certainly use a risk base approach to assess Linearity in cases where there are several known impurities. Does the impurities have similar UV? If so, evaluate an early and late eluting impurity (esp for gradient method).....provides info on the LOD/LOQ also. If the RRFs or responses at a given conc. are significantly different, evaluate the two extremes. These are two rationales for minimizing the workload while providing critical info about your method.