New ultracentrifuge
A new analytical ultracentrifuge by Spin Analytical Inc. (Durham, NH), the centrifugal fluid analyzer (CFA), is being developed specifically for the Open AUC Project. This base instrument will facilitate significant improvements to the traditional AUC optical systems (absorbance, interference, and fluorescence) and, more importantly, stimulate the development of new optical detectors as outlined above. The CFA serves as a spinning sample holder (Fig. 2). All light sources and detectors are external to the rotor chamber. Three 50-mm diameter optical tracks are positioned at 120° intervals around the rotor chamber. The larger diameter optics will overcome fundamental accuracy limits in the XLI Rayleigh optics (Yphantis et al. 1994). In addition, optical systems, which require multiple lenses and other optical components and thus a long optical path like the ultrasensitive Schlieren optics (Cölfen and Borchard 1994) can be realized. Optical working distances to the sample cells are minimized (~8–10 cm, set by safety considerations) to optimize radial resolution.
Fig. 2 The analytical ultracentrifuge for the Open AUC Project is simply a rotating sample holder that sits in three optical paths simultaneously. Sketches are shown of the CFA (A, B) and fiber composite eight hole rotor (C) being developed for the Open AUC Project by Spin Analytical. The base centrifuge (A, B) simply consists of a vacuum containment chamber (a), a rotor (b), and high-speed motor (c). Since only analytical rotors will be used in this instrument, a shorter rotor chamber may be used. Three optical tracks are arranged at 120° intervals around the chamber. The optical tracks (B) have to use a periscope to avoid the drive motor. While complicating the optical path somewhat, using the periscopes means that the CFA can accommodate different drive motors, thus providing flexibility for future designs. The new rotor (C) being developed by Spin Analytical holds eight samples and may be operated at speeds up to 60,000 rpm. The sample holders are compatible with existing cell components
The CFA uses the newest high-speed drive and drive electronics from Beckman Coulter. However, a different high-speed drive may be used by changing the base plate and drive electronics. The vacuum system is isolated from the rest of the instrument to reduce vibrations and surface contamination that degrade optical performance. The CFA does not require special electrical power and is mounted on a bench with space above and below it for optics.
Resources are provided on the CFA to aid optics development. Power (5, 15, and 72 VDC) and a 48-bit external digital bus are provided on the CFA to operate the positioners, light sources, and detectors that comprise optical systems. A variety of specialized cards (described below) are being produced as part of the Open AUC Project to help interface optical systems to the signal bus. Since 48-bit digital I/O cards are widely available, developers can choose computers and operating systems that fit their requirements. There also is an internal data bus that provides the signals needed to synchronize external electronics to the spinning rotor (described below). Motion control may be provided by servos or by stepping motors. Descriptions, schematics, and timing diagrams for the bus signals will be published, as will the data bus and motion control driver software source code. While these resources are provided to help AUC developers, they are not constrained to use them.