[RASMB] XLA intensity cutoff?

[Holger Strauss]

Hello everybody,

recently, I'm using more and more intensitiy detection and I've come
across the following problem: 

when doing a SedVel-exp. in intensity mode, it sometimes happens that the
data seem to have a sort of "cutoff" in them, i.e. the intensities to the
left of the meniscus (air/air) have the same value as to the right of the
meniscus. This phenomenon does not change after extensive centrifugation
at sufficient speed for the sample to deplete the meniscus. Further down
the solution column, the material sediments as expected, and the traces
look fine.

Furthermore, the value for the intensities in the air/air region does not
reach the value measured from a wavelength spectrum at that position.

To investigate this in more detail, I performed wavelength scans at
different radial positions and compared these intensities with the
intensities from a radial scan over the region to the left and right of
the meniscus, same cell, temperature, same everything, time between the
exps. ca. 2 mins. The values for the intensities (x10e3) at 279 nm
are as follows (radial position/sample/reference):

wavelength: 5,9/6,8/7        radial: 5,901/3,98/3,99
            5,949/7,26/5,85          5,95/3,98/4
            5,999/6,32/6,6           6/4,03/3,96
            6,049/5,85/6,13          6,048/3,95/3,99
            6,098/4,88/5,32          6,095/4,02/3,98

I understand that the current for the photomultiplier tube is adjusted at
6,5 cm for a radial scan (but obviously not for a wavelength scan), which
would explain the cutoff observed (but what happens if you use columns
which do not reach up to 6,5 cm?). 

Is there any possibility to change that setting? What is the linear range
in terms of intensities for the photomultiplier (and thus a practical
range for the use of pseudo-absorbance)? And what happens when
absorption data are recorded, if these are only converted intensities? 

Any comments and technical details are greatly welcome!

Cheers, Holger

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Holger Strauss 

Forschungsinstitut fuer Molekulare Pharmakologie (FMP)
Robert-Roessle Strasse 10

13125 Berlin/Germany

Tel: +49 (0)30 94793 - 223 (office)
                     - 316 (lab)

Fax: +49 (0)30 94793 - 169 


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Science is spectrum analysis; art is photosynthesis.

                                                    Karl Kraus