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schmops

Albania
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Posted - 04/12/2013 :  1:55:05 PM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic


Hi, I am trying to fit these curves with the ExpDec2 function to be able to compare the fast and slow decaying rates. I also would like to compare the time when 50% of the compound is degraded (I assume this is also called the half-life). When I use the ExpDec2 function, I get values like this (pH3 in the graph):

y0 56.40753
A1 19.91272
t1 47.78739
A2 23.67953
t2 1.6236

Why is A1 smaller than A2 if its supposed to be the upper amplitude? An vice versa, why is the time t1 bigger than t2? In other graphs with a similar curve I even get negative A1 and t1. Also, what is the difference between ExpDec2 and ExpDecay2?

Thanks a lot in advance,

Sascha

Sam Fang

291 Posts
Posted - 04/15/2013 :  05:31:05 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
A combined or total half life for ExpDec2 function may not make sense. In most cases it is used in the function:
y=A*exp(-k1*x)*exp(-k2*x)

Origin provides individual half life parameters tau1 and tau2 in Derived Parameters for ExpDec2 function.

We will improve it to constrain t1<t2 in the future. Thanks for your suggestion.

You can set bounds for A1 and t1, e.g. A1>0 or t1>0 in the dialog's Bounds tab.

ExpDecay2 is a general form of ExpDec2. If the x offset is known, you can use ExpDecay2 function, set the initial parameter x0 and fix it.



Sam
OriginLab Technical Services
Edited by - Sam Fang on 04/15/2013 10:15:59 PM
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schmops

Albania
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Posted - 04/21/2013 :  11:15:24 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Thank you Sam!
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