From: Walter Stafford <STAFFORD@bbri.harvard.edu> To : PETER.ROSSMANITH@MSM.BASF-AG.DE Date: Mon, 1 Jun 1998 7:33:38 -0400 (EDT)

Dear Peter and RASMBers, This "problem" arises from the properties of the optics (less than one fringe vertical jitter) and the fourier analysis itself (integral fringe shifts). Both of these effects are taken into account in all three of the available software packages that I know about. The fix for this has been well known and was always in the original dcdt software. I started this analyis on the Model-E Rayleigh system where both effects had to be taken into account. It was not necessary for absorbance data. The MS-DOS versions and the MacIntosh versions both allow you to align the fringes above the meniscus. This removes the vertical optical/mechanical jitter that is less than 1 fringe. Then it lets you select a region in the solution column in which to remove the remaining intergral fringe shift that may occur across the meniscus. The very latest Beckman version allows both of these corrections. It is important for my dcdt version that the entire array of all 2048 pixels be taken each time. The software, by the two operations just mentioned, takes care of the problems described by Peter. I urge you *NOT* to take the data the way Beckman originally described; you will get unuseable results with that method. The data must include the meniscus region at the very least. Remember, you have to able to select the meniscus from the screen. Another *VERY* important point: the Interference Optics, in its current incarnation, cannot be used reliably above 50,000 rpm. There are timing problems above that speed that result in random spurious deviations in the fringes. Beckman is close to having a solution to this problem as well. Hope that helps. Download the latest version of the software and you should be OK. Walter Stafford

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