
 Posts: 6
 Joined: Tue Mar 20, 2012 1:09 pm
I have fit it with a "power" fit, but it's not as good at all.
Any ideas?
Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.
lmh wrote:
I'm sorry, I'm not totally clear what you're doing. Can I just check this is the situation:
You have a calibration curve of Peak Area (Y) versus Amount (X) which works well as a quadratic fit.
You therefore have an equation Area = a*Amt^2 + b*Amt + c where you know a, b, c
You are now trying to find Amt in unknown samples from Area.
This should be fairly straightforward in that you can rearrange the equation to
0 = a*Amt^2 + b*Amt + (cArea)
and solve this using the wellknown
Amt = (b PlusOrMinus squareroot[b^2  4ac])/2a
99% of the time this ought to give you the right answer. If it isn't doing so, there are two things that can be going wrong.
Firstly, obviously, your lowconcentration points may actually be fractionally a long way from the calibration curve but you don't see it when you look at the curve because all the values are small. This will give big percentage errors in any measured samples at low concentrations. This is a real problem in the analysis side of things, and not a mere matter of maths.
Secondly, if you're really unlucky (but it's never happened to me) the standard equation for solving quadratics as above isn't the most numerically stable. Textbooks recommend you calculate:
q = 0.5*[b + sign(b)*squareroot(b^24ac)]
after which the solutions are q/a and c/q (of which of course only one is relevant in your case; the other will probably be negative).
In my hands, the most likely thing to go wrong is me mistyping the equations in Excel.
Hollow wrote:
Excel seems to have a "problem" when interchanging the axis.
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