If delta-t is made sufficeintly small, the result is a baseline corrected time derivative pattern containing the desired signal plus purely random noise. After subtraction of pairs of concentration profiles, to eliminate the baseline, the x-axis of each curve must be tranformed to compensate for the different times of sedimentation. This tranformation takes us from the stationary reference frame of the centrifuge cell (c vs. r) to the moving reference frame of the sedimenting boundary (c vs. s*). These patterns collected over appropriate time intervals can then be averaged at constant s* to increase the signal-to-noise ratio.
There are at least two possible sources of systematic error in the interference optical system that have time dependenece and would not be removed by taking the time difference.
One is precession of the rotor and would result in slight radial misallignment of the scans so that they would not be subtracted at corresponding radii.
The other is time dependent variations in the refractive index of components of the optical system. This might be caused by thermal gradients in lenses or mirrors or the air in the optical path.
Neither one of these potential sources of error seems to be significant in the Beckman-Coulter Optima XL-I analytical ultracentrifuge.