From: Jack Correia <jcorreia@umsmed.edu>
  To  : rasmb@bbri.eri.harvard.edu
  Date: Mon, 17 Jul 1995 10:21:48 -0500

Re: gaskets and alignment

Greg,

     On the subject of parallel light, my understanding is that one of the 
reasons each XLA is so different is that the light coming out of the 
monochromitors is not necessarily parallel to begin with.  This may in part 
explain why stray light is so bad as you rotate the lamp to achieve constant 
illumination.  It is also my understanding that focus at a 2/3's point is 
randomly achieved by monochromators and thus mine may give false association 
at high gradient and yours may give false nonideality (see brief discussion 
in Correia et al (1995) Biochemistry 34: 4898-4907).  The discussion on this 
service last year by John Philo and myself on sloping plateaus is related to 
variations in the light source, although I believe John found replacing the 
monochromator did not fix the problem.

     We change the gaskets after each run, although two velocity runs on the 
same day may get the same gaskets.  For equilibrium runs we definitely 
change them out after each run.  In fact, we have found a centerpiece 
dependent problem with the gasket spreading out into channel A giving rise 
to a slope at the meniscus that must be editted out.  Old gaskets are worse, 
although we have selected for centerpices where the problem is minimal.  It 
is not clear to me why different centerpieces cause this problem, but it 
must be slight differences in machining.  I see the question of cost for 
changing out gaskets every time differently; the samples are often more 
valuable than any other consideration.  I have collaborators that can make 
enough DNA by PCR for one days worth of velocity runs every one to two 
weeks.  Add in the time lost doing an experiment that leaked and the cost of 
gaskets is minimal, outrageous at $52/100, but minimal.  This week we will 
run 30 samples by sed. velocity so I appreciate the problem - I just ordered 
400 gaskets last week!   

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| Dr. Jack Correia              |
| Univ. of Miss Medical Center  | 
| 2500 N. State St.             |
| Jackson, MS, 39216            |
| (601) 984-1522                | 
| fax (601) 984-1501            |
| correia@fiona.umsmed.edu      |
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 -------------------------------
| Dr. Jack Correia              |
| Univ. of Miss Medical Center  | 
| 2500 N. State St.             |
| Jackson, MS, 39216            |
| (601) 984-1522                | 
| fax (601) 984-1501            |
| correia@fiona.umsmed.edu      |
 -------------------------------