A comparison of data quality using quartz vs. sapphire cell windows in analytical ultracentrifugation

Analytical ultracentrifugation (AUC) cells use either quartz or sapphire windows as end caps for the cell housing. Current generation sapphire windows are not recommended for absorbance data collection below 235 nm, because the window material shows a precipitous drop in transmittance at low wavelengths due to impurities in the sapphire. Quartz windows can be used below 235 nm as they do not exhibit adverse transmittance at low wavelengths. In this study, we demonstrate the optical properties of new generation sapphire windows and compare them to those of quartz windows across a wide range of wavelengths and present the results of sedimentation velocity experiments on BSA using both types of windows using data collected at both the 280 nm absorbance maxima as well as the 230-240 nm (closer to the peptide bond maximum). Our results show that the quartz and new generation sapphire windows deliver identical results in absorbance mode. We also demonstrate that quartz windows suffer significant mechanical deformation while spinning at very high speeds, while sapphire windows do not. This renders Rayleigh interference mode data collected at high speeds using quartz windows much noisier than with sapphire windows-which we have quantified by measuring how the signal to noise ratio of Fourier transformed Rayleigh interference scans degrades at high speed. Thus, we conclude that new-generation sapphire windows can be used for all AUC experiments through almost the entire mid UV range-obviating the need for quartz windows, unless wavelengths below 220 nm must be accessed.