[RASMB] averages from SV

[John Philo]

Actually these averages are normally NOT calculated as Bo suggests, with
weighting based on molecular masses. By convention the weight, z-, and z+1
averages are calculated with the g(s) values weighted by s, s^2, or s^3,
respectively, rather than M, M^2, and M^3. Because of that convention there
is no way to calculate the corresponding number average.

These are what are called "formal" averages rather than "true" averages. See
P.O. Kinell (1959), Arkiv Kemi 14, 327-336.

All three are calculated directly by my program DCDT+. The formulae
corresponding to those definitions, and formulae for the error bars on those
quantities, are given in the Help file (search the index for 'z average').
You can download the trial version to see the Help file if you don't have
it.

John

-----Original Message-----
From: rasmb-bounces at rasmb.bbri.org [mailto:rasmb-bounces at rasmb.bbri.org] On
Behalf Of Borries Demeler
Sent: Monday, January 23, 2006 8:29 AM
To: Holger Strauss
Cc: rasmb at server1.bbri.org
Subject: Re: [RASMB] averages from SV


> 
> Hi everybody,
> 
> we're looking for the pertinent reference for calculating number- and
> z-averages from dc/dt-distributions - any hints? Furthermore, are 
> there formula for getting these averages from c(S)- and vHW-distributions?
> 

Holger,

to get number averages, I presume you want number of molecules, right?
I guess in that case you need to make some assumptions about how the
measured s-value relates to mass so you can make the conversion to
numbers.  The exact conversion of course would then of course also depend
on the measurement mode (i.e, interference, or particular wavelength
and absorbance properties). All molecules in your mix better have the
same signal properties, for example, if you have a mixture of molecules
that have a different ratio of trp/MW, and you use absorbance at 280 nm
as mass measure, then your conversion is going to be wrong. Otherwise,
standard formulas for calc. Mw, Mn and Mz apply.

-Borries
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