[RASMB] XL/I Absorbance Problem (follow-up)

[Leech, AP]

Hello RASMBers,

Firstly, thanks to all who responded to my previous query about
problems with our XL/I absorbance optics. There were actually
two problems which were resolved as follows:

(1) Excessive noise and spikes in absorbance plots, and incident
reference data (.RNx files) which fluctuated between two discrete
values rather than the expected random noise on a flat baseline.

  - This was a problem with the reference detector in the monochromator
and was cured by replacing the monochomator.

(2) A marked "dip" in radial scan intensity data at r ~ 6.3 cm
independent of wavelength or cell and present in sample and reference
sector data, not removed by lamp alignment and not affected by the
monochromator replacement.

  - About a month after the monochromator replacement, absorbance data
suddenly became extremely noisy and unusable. Intensity data from the
scans showed less than 100 counts.

  - This problem and the "dip" were cured by a photomultiplier tube
replacement (well spotted John!).


Although many people record the lamp wavelength/intensity profile from
time to time e.g. to check wavelength calibration, I would recommend
also taking every so often a radial scan (collect intensity data) on an
empty hole at your favourite wavelength. This will rapidly show up any
developing problems in the lamp/monochromator/PMT system and help to
distinguish them from cell-associated problems. I'm quite happy to
compare "before and after" scans with anyone who thinks they have a
similar problem, just email me.

(Note that Borries Demeler has provided a useful set of wavelength
related checks under Resources->Lamp Diagnostics at the CAUMA website.
http://www.cauma.uthscsa.edu/ )

Needless to say, I am very glad that we have a service contract on the
XL/I!

Vacation regards to all,

Andrew

-- 
Dr Andrew Leech                   *  Laboratory Manager
Technology Facility               *  Molecular Interactions Laboratory
Department of Biology (Area 15)   *  Tel   : +44 (0)1904 328723
University of York                *  Fax   : +44 (0)1904 328804
PO Box 373,  York  YO10 5YW       *  Email : apl3 at york.ac.uk