Simple question: we are trying to compare absolute and relative exponential decay rates of a chromogenic decay assay. In this assay the chromogenic compound (and thus absorption) decays over time in a biphasic way. I am trying to fit the data to the ExpDec2 function in Origin (y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0). I would like to compare the fast and slow decaying rates under different conditions. In one example where I can clearly see a two-step decay with a continued negative slope, I get a negative t1 and a positive t2 (y0 94.27757, A1 -0.28311, t1 -30.39113, A2 6.00467, t2 8.08285). How can A1 be negative? Is it common practice to write the respective reaction rates (k1 = 1/t1 and k2 = 1/t2) as absolute numbers (positive) for comparison? I'm not sure what the best way is to compare the rates in this case.
Maybe each decay process has a time domain in which it is applicable or dominant. So rather than making the effective decay equal to the sum of the two processes, make it a function in which there are different domains. For example, in the first time domain, only process A is applicable; then in a second domain, perhaps use the sum of the two processes, A+B, and in a third domain only proces B is applicable. See if this makes sense mathematically and in your scientific application.
Thanks Antonio! I will try that. I think it makes sense in my case since I cannot fit the whole curve with a simple biphasic decay function. I will break it up into domains as you said and see how it fits. Thanks!
To compare process 1 and process 2 that you are modelling using a two phase exponential equation you can also use the process 'half-life' concept...
See link for a snap-tutorial and schematic
http://www.graphpad.com/guides/prism/5/user-guide/prism5help.html?reg_exponential_association_2phase.htm
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