[RASMB] further question ln(c) vs r2

[Walter Stafford]

Dear Heather,
	It looks as though you have a slight offset to the data which 
is influencing the lowest concentration data.  You might consider 
fitting this data with NONLIN both with and without a floating offset 
(it's called delta in NONLIN ) or the BeckmanCoulter equivalent (they 
call it the baseline). Unless your protein has a large excluded 
volume, is high charged and/or you are using a very low ionic 
strength buffer; non-ideality should not be a problem.

If you do not try fitting directly ; try adjusting the off-set 
manually until you get a straight line on your plot, the slope your 
get gives essentially the Z-average molar mass.

Walter



>A follow up to that previous question.  I have removed the data which gave
>a negative concentration. and plotted ln (c) vs r2 which I expect to be a
>straight line. However as the plot shows it is very curved.  Am I doing
>something wrong or is that suggesting my system is non ideal?
>Should I only be ploting values for which ln(c) is >0.0?
>
>many thanks
>
>heather :)
>
>Heather Peto			   Department of Biochemistry
>hp217 at mole.bio.cam.ac.uk	   New Building
>Tel: (lab)    +44 (0)1223 333343,  80 Tennis Court Road
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>				   UK
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>
>
>
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-- 
Walter Stafford
mailto:stafford at bbri.org
617-658-7808