From: Borries Demeler/Biophysics <demeler@selway.umt.edu>
  To  : JOHN PHILO <JOHN_PHILO@amgengate.amgen.com>
  Date: Mon, 9 May 1994 15:55:41 -0600 (MDT)

Re: more on sloping plateau

>
>        Reply to:   RE>>more on sloping plateau
>As my follow up to RASMB will hopefully have made clear, I disagree that the
>problem Jack and I see has to do with the wavelength.  I have done what you
>suggest and gone on both sides of the 280 peak (or wherever the xl-a says the
>peak is), and the slope DOES NOT CHANGE SIGN.  Also, as I noted, the effect
>is there even with the KNO3 calibration solutions, which have very broad
>absorption maxima.  I fully realize and understand that the effects we are
>describing make no sense IF YOU ASSUME THE PHOTOMETRIC SYSTEM WORKS PROPERLY.
> That is the whole point:  there is strong experimental evidence that at
>least some XL-As have serious photometric problems, but Jack and I have had
>tremendous trouble convincing people that this is true, and that we have done
>all the controls to prove it.
>
>
>
John + Jack.

It seems to me that you have instrumental problems that go beyond a small
change due to shoulder effects. Re-reading a previous message I realize that 
your plateau absorbance change with the length of the cell is around .1 OD.
That is way more than what I see. Simply going on the other side of the
shoulder won't compensate for such a large effect.
My suggestion is to bug Beckman until they fix it. Do not accept their
standard excuse: "it is within specs" as an answer. Clearly, you have
something majorly wrong with the absorption optical system. I don't have a clue
what could cause such a large effect. Is your optical path longer at the 
bottom of the cell than at the top? Doesn't make sense.

I was just trying to help with something that I have seen before, but that 
large effect has never occured here. Fortunately, we haven't seen these
problems on our machines.

Let me know if you have the solution. BTW, it is Beckman's job to figure
out what is wrong with your XL-A, not mine. You don't need "tremendous
effort" to convince me that there is a problem, I believe it in a sec.

Regards, -Borries