Chromatography Forum

When calculating %Difference in area between 2 injections of the same vial, is this calculated as follows:

[(Larger Area/Smaller Area)*100] -100

Yet when you are doing a standard compliance you calculate the difference as:

Difference between the 2 actives/0.5 X [Sum of the 2 actives] X 100 to give a result in percentage.

Which one is correct when validating a custom field where you need to "calculate the percentage difference in area between 2 injections of the one standard"?

The formula you described for standard compliance is certainly *not* correct. It ends up with units of Area^2.

Assuming I did my algebra correctly, that formula amounts to:
[(AL - AS)/0.5]*(AL + AS)*100, where AL is the area of the larger peak and AS is the area of the smaller peak. That rearranges to:
2*(AL^2 - AS^2)*100, which doesn't make a lot of sense to me.

Hi Tom,

The way we calculate Standard Compliance at work is to calculate a percentage difference and that cant be more than, usually, 2%.

I will break down our formula a bit clearer:

Difference in area between 2 actives (Standard A and Standard B) eg 12000-11500. Then you sum the 2 actives and divide this by 2 eg 12000+11500 = 23500. Divide this by 2 to give 11750. Then to get percentage compliance its 500/11750 X100 = 4%.

What I'm unsure of is if you asked to calculate the percentage Difference between 2 areas, what formula is used.

Tom's misunderstanding is just a matter of it being difficult to show equations in a typical forum editor - wish we could!

Basically the "standard compliance" method expresses the absolute difference between the two measurements as a fraction of the average of the two measurements, and therefore makes no assumption about which measurement is "right".

The ((larger/smaller)*100)-100 method assumes that the smaller measurement is right, and therefore gives the over-cautious percentage difference. If, for example, the measurements are 1000 and 500, it gives an error of 100%, meaning that the higher differs from the lower by 100% of the lower's value.

While generally I like caution, I believe the best estimate of a real value is likely to be the average of all the measurements available, so the "standard compliance" method is the most attractive. It's analogous to stating the standard deviation centred on the mean.

Hi

Worth considering

CF 100*(sTD2_amount/Std2_Weight
C%.%..AVE(Value)
C%.%..AVE(Amount)

Where label C= Control std and U std

Thanks for the suggestions. I found a Custom Field training Empower custom fields on PDF from 2013 and it states within:
% Difference between 2 areas = [Larger Number/Smaller Number*100]-100, so I'm going to go with that. They suggested a really really long CF which uses CCompRef1 and a Label, and it worked.
My coworkers seem to think percentage area difference between 2 injections and %RSD are the same thing but I'm going to trust the definition I found on the Empower custom fields training..

It sounds more like you would want Relative Percent Difference (RPD) as listed in EPA methods.

Relative Percentage Difference = (|Num1-Num2|/((Num1+Num2)/2)) x 100

This gives a relative difference but not assuming either number is correct. It is also different from %RSD since that is based on standard deviation.

RPD is what EPA uses to compare duplicate analysis of the same sample to show reproducibility.