Walter F. Stafford
BOSTON BIOMEDICAL RESEARCH INSTITUTE
ANALTYICAL ULTRACENTRIFUGATION RESEARCH LABORATORY
64 GROVE STREET, WATERTOWN, MA 02472
Voice       617 - 658 - 7808
E-mail: stafford@bbri.org

Revised November 12, 2000




3. Time interval and Maximum number of scans to analyze.

A rule of thumb for choosing the maximum number of scans to analyze is the 
following:

The time difference, delta(t), between the first and the last scan used
should not exceed the following value: 

             delta(t)max <  [900*t]/[(M^0.5)*(SPEED)]

where t is the average of the times for first and last scan. M is the
molecular weight expressed in Kg/mole and SPEED is the angular velocity of
the rotor expressed in units of (RPM/1000). 

This can also be expressed more handily as 

	delta(w^2t)max <  [90*(w^2t)]/[(M^0.5)*(SPEED)] 

where now w^2t is the value of "omega squared t" from the second line of the 
XL-A/XL-I output files.


#This rule of thumb is based on the idea that the boundary, assumed to be 
gaussian and located in the middle of the cell, should not be allowed to
move more than one standard deviation during the time interval over the
subtraction is to carried out. Empirically, this rule avoids excessive 
artificial broadening. This is not a hard-and-fast rule. You should verify 
that it works in your case by trying smaller and larger numbers of files in 
your data set. In some cases you might be able to tolerate fatter peaks in 
order to get better signal-to-noise by including more scans. The peaks will 
be artificially broadened but the peak positions would be unaffected.

For purposes of curve fitting to get the apparent diffusion coefficient, 
you might try a more stringent criterion, for example,

        delta(w^2t)max <  [40*(w^2t)]/[(M^0.5)*(SPEED)]