[RASMB] Abs optics design question

[John Philo]

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
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