[RASMB] sample with flotating component

[Kristian Schilling]

In Helmut's name, who is encountering problems with his RASMB account, I 
post the following:
________________________________________

Dear Christine,

 From your attached scans you seem to observe Schlieren peaks in the early
absorbance scans. They are caused by a steep concentration gradient and by
light deviated away from the slit of the absorption optics causing this
apparent absorption evident in the Schlieren peak. This Schlieren peak
occurs at the boundary position and you can also observe it at wavelengths
where your sample does not absorb light. One can even use it to calculate
sedimentation coefficients. At your later scans, where the gradient is less
steep, the peak disappears. If you want to avoid it, try lower speeds in
your case to get a less steep initial boundary.

We have published two papers on this effect. Have a look at:

Cölfen, H.; Harding, S. E., A study on Schlieren patterns derived with  the
Beckman Optima XL-A UV-absorption optics. Progress in Colloid & Polymer
Science 1995, 99, 167-186.

And

Dhami, R.; Cölfen, H.; Harding, S. E., A comparative "Schlieren" study of
the sedimentation behaviour of three polysaccharides using the Beckman
Optima XL-A and Model E analytical ultracentrifuges. Progress in Colloid &
Polymer Science 1995, 99, 187-192.

Best wishes

Helmut


At 23:40 07.04.2008 +0200, you wrote:
>Dear all,
>
>I am investigating a micelle forming polymer system. Unfortunately, the 
>samples
>seem to contain a fraction, which flotates at 40 000 rpm where the main
>fraction sediments, see attached file. Did anybody already analyze something
>similar without previous purification?
>
>The flotation apears in the absorbance scans but not in the interference. 
>At low
>speed the absorbance intensity is constant over the radius, only with the 
>onset
>of sedimentation also the flotation starts.
>
>Could one do a synthetic boundary in order to let the light fraction move out?
>
>Thank you for any advice.
>Best regards,
>Christine