Calculate impurity content using linearity

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

7 posts Page 1 of 1
We calculate impurities using regression equation. We don’t Just use 100% standard solution. We prepare LOQ, 100% and 120% solutions in release test and calculate using y=mx+b
Authority asked “Why do you substract intercept value from impurities area? Please show scientific source.”

Impurity content % = (A-b)/m x 100/5 x 100/L

A: Impurity area from test solution
b: intercept value from standard solutions linearity
m: slope from standard solutions linearity
100/5: dilution factor
L: Label claim

How can we response the question?

Thank you
Could you elaborate on what you're trying to achieve? What's the end goal?

Do I get it correctly that:
1. You have another source of pure Impurity and you make a solution with it at LOQ, 100% and 120% relative to the concentration that you expect to find? And in your y=mx + b, y is concentration (%) of the impurity in the injection?
2. You take your test vial with L% impurity and dilute it another 20x?
Software Engineer at
Did "Authority" fail math class?

Once you agree that your calibration line is "A = mx + b", then solving for "x" (concentration) gives "x = (A - b)/m", which is exactly what your formula says. Your "scientific source" could be a basic algebra textbook.

I would be much more concerned about inclusion of the LOQ point.
-- Tom Jupille
LC Resources / Separation Science Associates
+ 1 (925) 297-5374
If you're feeling cheeky, reply with "As far as I know, not all lines pass through the origin. If you have a scientific source indicating otherwise, I'd be happy to consider it."
DR wrote:
If you're feeling cheeky, reply with "As far as I know, not all lines pass through the origin. If you have a scientific source indicating otherwise, I'd be happy to consider it."

Good answer.

Authority=Lawyer=Little knowledge of linear algebra

But Tom's answer is the correct one. Simple linear curve fit math, when solving the equation you get -b. I think you would only not have -b if you are forcing through the origin, which usually leads to bias high results for lower concentrations.
The past is there to guide us into the future, not to dwell in.
Thank you for your good answers:)

It is a finished product. For example, we will calculate the unknown impurity in molecule A. We draw linearity using the working standard of molecule A. We also calculate the unknown impurity with this regression equation. I know it's more accurate to calculate with a 100% standard, but it's a study before me and we're expected to explain it. I answered the question before, and they persistently asked it again. There are many scientific articles that calculate in this way. I don't know if anyone who asks this question will accept these articles:)
Tom's answer plus a "it is very commonly done this way in this industry" should suffice.
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