[RASMB] Abs optics design question

Hayes, David HayesD at MedImmune.com
Thu Jun 3 09:55:55 PDT 2010


Hi John,

Just a quick note of explanation:
My actual question about the sapphire windows is why we have to test
them rather than the manufacturer testing them.  I am glad to hear that
all of the windows you have purchased are clear enough to use down at
230:  maybe the warnings about absorbing sapphire are out of date now.
My impression was not to trust sapphire and the buying instructions also
state that sapphire is not guaranteed to work down to 230.
About machine design questions, my questions are somewhat rhetorical,
but I wanted to ask them because the next analytical centrifuge will
also probably be based on a preparative base and maybe the designers can
ask themselves if making a few changes might be worth the effort.
Improved temperature control and measurement is another feature where
analytical and preparative desired specifications differ quite a bit.
I think something might be added to the bottom of an AUC cell housing to
help hold the cell steady in its hole, but only if one were willing to
make the whole cell bit taller.  The sectors leave room on each side for
a partial arc:  and though it is certainly easier to manufacture a
simple round hole in the bottom, it seems possible to put a small
tightening screw there.

David
-----Original Message-----
From: rasmb-bounces at rasmb.bbri.org [mailto:rasmb-bounces at rasmb.bbri.org]
On Behalf Of John Philo
Sent: Thursday, June 03, 2010 12:13 PM
To: rasmb at rasmb.bbri.org
Subject: Re: [RASMB] Abs optics design question

David is right that it is the pulsed nature of the lamp that precludes
Andrew's approach, and that the pulse rate of the lamp limits the
scanning
rate, but actually the lamp can flash 100 times per second rather than
60 as
David said. My understanding is that there is now a lamp available that
could go 200 flashes per second, which would be a big help especially
for
using 7-hole rotors, but whether Beckman will ever switch to that
remains to
be seen.

Regarding David's other questions, the only answer to the first three of
them is "these are design elements of the underlying preparative XL
centrifuge and are held fixed in the analytical".

With regard to a cell tightening mechanism, it seems to me there is
physically no room for such a system in any 12 mm path analytical cell.
The
screw ring on the top automatically precludes a tightening adjustment
screw
on that end. It might be possible to add such a mechanism in the bottom
end
of a special housing for shorter pathlength cells, but there is no room
with
12 mm centerpieces (without raising the center of the cell).

I am puzzled by David's comment about sapphire windows. All of the ones
I
have purchased in the last 20 years (directly from Meller Optics, who
supplies them to Beckman) have had low enough absorbance for use at 230
nm,
and more recently many would be usable even at 210 nm.

John

-----Original Message-----
From: rasmb-bounces at rasmb.bbri.org [mailto:rasmb-bounces at rasmb.bbri.org]
On
Behalf Of Hayes, David
Sent: Thursday, June 03, 2010 8:12 AM
To: rasmb at rasmb.bbri.org
Subject: Re: [RASMB] Abs optics design question

Hi Andrew,
My understanding is that it is not just one cell scanned at a time, but
one
sector of one cell scanned at each time.  There is one flash of the lamp
for
the reference sector and one flash for the lamp for the sample sector.
At
rotor speeds above about 6000 RPM, it is the flash lamp maximum flash
rate
(60 flashes per second without overheating) that is the time limiting
factor
in collecting absorbance data.  

Besides engineering reasons for choosing the flash lamp back when the
XLA
was designed, John Philo did raise the question once if there might be a
molecular downside to having a continuous lamp shining UV on your sample
for
the entire time of the experiment (hours).
Tom Laue is the expert on this topic of instrument design, but your
basic
idea that collecting absorbance data from every cell at once should be
possible has been confirmed in prototype form by Tom.

Although I know Tom has experienced or heard just about everything, I
thought it would be fun and useful to have a "reminder" thread about
inexplicable design decisions for the XLA and add a few questions of my
own.


I would also like to know why the XLA engineers placed all the Peltier
heating/cooling plates on a circuit board with uninsulated wire traces,
so
that when the board heats or cools too fast it warps and short circuits
(Yes
this really happened).  Why couldn't the plates be connected
individually to
the metal can or why couldn't there be some insulation between the metal
can
and the circuit board just in case it warps more than the thickness of
the
Peltier plates?  

Why do we have to run the XLA at 0 speed just to get the diffusion pump
going and get a reliable low vacuum?  Couldn't there just be a switch?

Why does the XLA "wait for diffusion pump cooling" even after we
replaced
the diffusion pump with a turbo-molecular pump?

Why can't the interference optics laser be physically connected to the
interference camera to reduce vibration noise?(sorry Tom, you have
apologize
about this before, but maybe some newer AUC people haven't
heard)

Why can't analytical cell housings have a built in tightening system
like
the counterweight? (People have used hair, white-out "paint", and
kimwipe
strips to hold the cells a little tighter in the rotor)

Why can't the manufacturer test the sapphire windows for Low UV
absorbance
before selling them to us?

David Hayes


-----Original Message-----
From: rasmb-bounces at rasmb.bbri.org [mailto:rasmb-bounces at rasmb.bbri.org]
On Behalf Of Leech, AP
Sent: Thursday, June 03, 2010 10:39 AM
To: RASMB
Subject: [RASMB] Abs optics design question

Hello all,

This is an idle question about the design of the XL/A (or I) absorbance
optics. Why are the cells scanned one at once?
Would it not be possible to scan them all "simultaneously"
by gating the output from the photomultiplier into a suitable number of
different channels?

Andrew

-- 
Dr Andrew Leech                   *  Laboratory Head
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
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