Calibration curve

[jacoblee]

I am a developer of link removed I am using ED-XRF for geochemical analysis for a geology dissertation. The ED-XRF provides a ppm reading for elements and a 2sigma error, for example 472 +- 5.11ppm Sr.

An example calibration curve made for Sr comprising 37 standards has a linear regression of y=1.26x + 32.1 (R^2 0.9991), which I can easily apply to correct my unknown analyses. I need to plot my corrected readings with their error bars, so how is the 2sigma error associated with this unknown calibrated?

 

Does this error value also simply get calibrated with the curve equation, or just the slope value, or is this error propagation far more complex? I have searched for a solution and keep finding an equation for uncertainty propagation with calibration curves, but this only calculates the uncertainty caused by the curve, and has no relation to the uncertainty of each analysis? So is the total error the original 2sigma error provided by the machine + calibration curve uncertainty?

 

Thanks

 

Edited March 12, 2015 by hypervalent_iodine
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[DrP]

QUOTE: "How would I calculate the concentration?".....

 

You do not need to calculate it do you? You can read the value off of your graph. If you know the absorbance value of the unknown sample then you can read off from the curve what the concentration would be to give that value...... or am I missing something simple?

[John Cuthber]

QUOTE: "How would I calculate the concentration?".....

 

You do not need to calculate it do you? You can read the value off of your graph. If you know the absorbance value of the unknown sample then you can read off from the curve what the concentration would be to give that value...... or am I missing something simple?

Yes, what you are missing is that you can't expect to read three digits of accuracy from a graph.

 

I suspect that the OP needs to read something like this

http://www.bipm.org/en/publications/guides/gum.html