Impurity Calculation: Weight%
Posted: Fri Jan 24, 2014 5:05 pm
I would like some feedback on the difference between calculating impurities using area normalization vs. external standard.
Example 1, calculating via area%:
Related Substance,w/w%=(%Area of Related Substance)/RRF
where:
%Area = (impurity peak's area) * 100 / (total peak areas from same chromatogram)
RRF = (slope of impurity linearity curve) / (slope of main component linearity curve)
Example 2, calculating via external standard:
Related Substance,w/w%=(AreaImp/AreaStd)*(StdWt/StdDil)*StdPurityFactor*(SampleDil/SampleWt)*(100%/RRF)
where:
AreaImp = Impurity peak area from individual chromatogram
AreaStd = Average area of main component from standard injections
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Ok so assuming all other things being equal the questions are:
1. Is it true that in Example 1 above, the impurity w/w% calculated is relative to the weight of the total material with impurities inlcuded whereas in Example 2, it is relative to the actual weight of the main component only?
2. Is it true that Example 2 is the more accurate and accepted method of quantifying impurities? (i.e. relative to the actual weight of the main component rather than to the total weight of material with impurities included?)
Example 1, calculating via area%:
Related Substance,w/w%=(%Area of Related Substance)/RRF
where:
%Area = (impurity peak's area) * 100 / (total peak areas from same chromatogram)
RRF = (slope of impurity linearity curve) / (slope of main component linearity curve)
Example 2, calculating via external standard:
Related Substance,w/w%=(AreaImp/AreaStd)*(StdWt/StdDil)*StdPurityFactor*(SampleDil/SampleWt)*(100%/RRF)
where:
AreaImp = Impurity peak area from individual chromatogram
AreaStd = Average area of main component from standard injections
---
Ok so assuming all other things being equal the questions are:
1. Is it true that in Example 1 above, the impurity w/w% calculated is relative to the weight of the total material with impurities inlcuded whereas in Example 2, it is relative to the actual weight of the main component only?
2. Is it true that Example 2 is the more accurate and accepted method of quantifying impurities? (i.e. relative to the actual weight of the main component rather than to the total weight of material with impurities included?)