# Internal Standard Curve in Sample

This is a question for the Empower 3 software. I have a method that requires an internal standard curve to be generated from the samples where the first sample is endogenous but the next 3 injections are the sample spiked at a 20%, 50% then 100% of the standard concentration and I was trying to figure out how to have a curve be generated off those levels so a line of best fit is generated and determine the relative concentration in the endogenous sample. Has anyone here had a similar method to work without the need to have a custom field?

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## Answers

You are using a standard additions approach. You need to plot your signal (peak area) vs. the amount of standard added. Your sample with no standard added will be plotted on the y-axis.

You then extrapolate to the point at which this line crosses the x-axis. The absolute value of the x-intercept will be the original concentration in the sample.

As far as I am aware, Empower will not be able to provide this result to you by default. You will need to calculate external to Empower or add a custom field.

Simply stated, the y-intercept divided by the slope gives you the x-intercept. In Empower variables, I believe that is B (y-intercept) divided by A (slope) for a linear regression type line.