According to ICH guidance,impurities of drug products refer to any component of the drug product that is not the chemical entity defined as the drug substance or an excipient in the drug product. And now HPLC are becoming a valuable tool for determining impurities of drug products. But under normal situation, excipients peaks will be observed on the chromatogram especially when use short detection wavelength.For the purpose of calculating total impurities for the drug products,it is unresonable to consider excipients peaks in calculation.But when the drug selling on international market,how the offical authority identify the excipients peaks and carry out analysis for the total impurities test?

Can anybody help me to clarify this point? Thanks very mach!

During method validation, a placebo preparation (i.e. product with all the excipients but without drug substance) is prepared and injected. If peaks are observed due to these excipients, it should be mentioned in the analytical part of the registration file (method description). Official authorities carrying out impurities tests should stick to this method.
In my experience, common tablet excipients (talc, Mgstearate, cellulose, PVP, ..) rarely disturb impurity profiles because they are often insoluble, not retained and not UV visible. You will observ a larger injection front though. Preservatives in liquid preparations such as parabens you will always see in your chromatograms
Hopes this clarifies.
Jan

Thank Jan!
But when peaks are observed due to these excipients, it is difficuly to be mentioned in the analytical procedure ,because as you said common tablet excipients are often insoluble and not retained,which means the retetion time of excipients will change according to dead volumn . And as we known,dead volumn is a inherent parameter different from instrument to instrument.So it is difficulty to be mentioned in analytical procedure using qualititative analytical properties such as chromatographic relative retention time.

The dead volume is a propertiy of the column. The dwell volume is a property of the instrument.
Non rentained peaks elute just after the dead volume. Unless you change flow rate or column dimensions retention times will be stable.