From: John Philo <jphilo@mailway.com>
  To  : Holger Strauss <strauss@fmp-berlin.de>
  Date: Fri, 6 Apr 2001 11:25:42 -0700

RE: reliability at high speeds

Holger,

I do extensive velocity work at 60K rpm and have never seen any
speed-dependent degradation of the absorbance data.

For the interference data on the other hand there is generally a significant
increase in fringe "waggling" at high speeds. For some machines they may
kick in as low as 40K, on others not until above 50K. I was told by Beckman
that at least some of that problem was due to laser timing issues caused by
the software, and that those have been corrected in the latest version of
the firmware ROMs in the instrument but I have no direct information on how
much that helps. The extent of that problem also depends strongly on the
optical alignment.

Regarding your absorbance problems, the problem you describe of the
absorbance jumping up and down from scan to scan is due to shifts in the
wavelength, and has been discussed a number of times in this forum in the
past (see the archive at UTHSC). The apparent changes in timing between
scans is really just an illusion; it is the variation in amplitude that is
the underlying cause.

With regard to what might cause all the scans to be very noisy in one
particular run, this can happen if you accidentally rotate the wavelength
cut-off filter away from the correct position. If the metal arm is not
exactly vertical, the filter blocks much of the light and the data are very
very noisy.

'Hope this helps,

John Philo

-----Original Message-----
From: Holger Strauss [strauss@fmp-berlin.de]">mailto:strauss@fmp-berlin.de]
Sent: Friday, April 06, 2001 11:09 AM
To: rasmb@bbri.harvard.edu
Subject: reliability at high speeds


Dear RASMBers,

this is a question concerning the reliability of the instruments, both
XL-A and -I, at high speeds.

Recently, we had some trouble with the absorption optics of the XL-I
(extremely noisy radial scans, OK for wavelengths and intensities),
which now seem to have subsisted, because of reasons currently not very
well understood by us (some problems just disappear...?). However, we did
a sedimentation velocity run at 60k, 3.5hs, 5mins between scans.

a) some scans show less absorption than the following ones.
b) the spacing between the scans is not even (some are nearly overlaid,
but at least in one case, a scan seems to have been utterly omitted

There seem to be two possibilities:

- problems just pop up again, then our optics are faulty
- or there is a problem because we are operating at the upper limit of
what is certified

As we don't have much experience at the maximum speed of the
instrument, we are wondering how well the reproduceability of radial
position and lamp firing actually is at such high speeds, and wether it is
sustained over an extended period of time (however, 3.5hs is not much).
Based on trial and error, some scans do give reasonably good results when
using them in programs based on the Lamm equation; there is however a
problem using methods like vanHolde-Weischet, 2nd moment and dc/dt.

Although the instrument is certified up to 60k, is the quality of the
data as good as, say, 50k or 45k?
Is there a difference between interference and absorption optics at
high speeds?
How much stress is put on the material? This is difficult to estimate, but
something like doubling operating speed reduces lifespan of
windows/cells/centerpieces/rotors/centrifuge by x would be helpful.

Thanks a lot in advance.

Holger

- - - - - - - - - - - - - - - - - - - - - - - - - - - - -

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


- - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Die Situation des Haustieres zieht die des Schlachttieres nach sich.