[RASMB] question for XLA experts

[Kristian Schilling]

Borries,

how dare I reply to a question addressed to XLA "experts"...

I think it's basically a good idea to use the XL for measuring the 
positions as you are getting the same precision as in measurements (and 
probably same errors). I would not use interference optics, as the 
algorithm tends to continue the transformed scan into the direction it last 
went even though there might be no intensity any more... So you would find 
the bottom distance too high. With absorbance, you are right, the bottom 
positions are often difficult to determine.

The cell bottoms for reference & sample are almost never exactly identical 
which causes the calculated Abs to exhibit enormous jumps in the vicinity 
of the cell bottom (and top).

We have turned to scan Abs in the transmission mode, getting counts both 
for reference and sample. In these scans, you can read the bottom position 
in high precision.

As a matter of fact, we have turned to acquire all scans in the intensity 
mode in order to recognize any errors in the raw data that might look like 
absorbance signals later. We then transform the data into Abs units after 
import into evaluation software. This has proved extremely helpful 
especially with density gradients.

Hope this helps,

Kristian



At 15:08 15.11.01 -0600, you wrote:
>Hi,
>I am trying to calibrate our rotors for speed-dependent rotor stretching
>and the channel positions of our centerpieces. The problem I am facing is
>that it seems to be very difficult to assign the exact bottom of a channel
>to a peak from an absorption scan.
>
>Attached is a typical absorbance scan of an empty 6-channel centerpiece.
>For the first channel, I labeled the peaks 1-10 and found the radial
>positions for them, shown below:
>
>1 = 5.741
>2 = 5.768
>3 = 5.790
>4 = 6.125
>5 = 6.138
>6 = 6.186
>7 = 6.234
>8 = 6.249
>9 = 6.273
>10 = 6.286
>
>I then scanned the centerpiece on a 3200 DPI scanner and came up with
>these dimensions (believe it or not, a half-way decent scanner does a
>pretty good job at measuring small parts, good to within +/- 0.01 mm!)
>
>Cell dimensions:
>
>first channel: 3.83 mm, darkspace: 1.27 mm (channel width: 3.29 mm)
>
>Now can someone tell me which peaks correspond to the channel bottom?
>Or won't I be able to find it correctly with absorbance optics, should
>I use interference optics? I guess IF has a better radial resolution,
>is it also as accurate as the absorbance radii?
>
>Any suggestions would be appreciated.
>
>-Borries



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Gesellschaft fuer Kolloidanalytik mbH
Dr.  Kristian Schilling

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