Why do we need a new ultracentrifuge? Problems and restrictions with detector development for the XL ultracentrifuge platform
Since the introduction of the XL-I and the digitalization of experimental data, tremendous progress has been made in analysis software and method development. These software advances have enabled researchers to analyze AUC data with much higher quality than was available a decade ago. Sedimentation coefficient distributions (Stafford 1992; Schuck 2000; Demeler and van Holde 2004), molar mass distributions (Brookes et al. 2006; Brown and Schuck 2006; Brookes and Demeler 2007, 2008), interaction constants (Cao and Demeler 2008; Stafford and Sherwood 2004; Schuck 1998), particle size distributions with Angström resolution (Cölfen and Pauck 1997) and the simultaneous determination of size and shape distributions from sedimentation velocity experiments (Brookes and Demeler 2006; Brookes et al. 2006) are examples of sophisticated data analysis methods that are used routinely by AUC laboratories. Powerful software packages that combine these methods, such as Ultrascan (Demeler 2005), Sedfit/Sedphat (Schuck et al. 1998; Vistica et al. 2004) and Sedanal (Stafford and Sherwood 2004) are available free of charge. In particular, the source code for UltraScan is licensed under the GNU Public License (GPL) (http://www.gnu.org/copyleft/gpl.html), making it possible for other groups to make improvements or advances. These analysis programs are supported by workshops.
In contrast to the tremendous advances made in data analysis, hardware development has not progressed significantly. The commercially available hardware is based on 20-year-old detector technology, and while new detection systems have been developed in research laboratories, they are not commercially available. One reason for this lag is the cost and effort associated with the development of an AUC detector. Designing the optical, mechanical, and electrical components, as well as writing reliable real-time data acquisition software, makes detector development challenging. However, the overriding causes of slow detector development for the AUC are the geometric and vacuum constraints of the XLI platform. Constructing the custom hardware is time consuming and expensive. Furthermore, the geometric constraints imposed by the XLI result in compromised optical performance. Worst of all, insurance and liability concerns have made it cost-prohibitive for Beckman Coulter or other companies to commercialize any optics that require changes to the heatsink or in the vacuum/containment chamber.
In most cases, a light source or its associated detector must be introduced into the vacuum chamber, which places severe material-compatibility constraints on components. The vacuum also poses significant cooling issues for lasers and mechanical positioners. Optical and electrical conduits, which require new holes in the heatsink (Fig. 1a), must be added to the vacuum chamber, creating warranty and safety issues for Beckman Coulter and the user (Fig. 1b). Although no problems have ever surfaced in machines with modified heatsinks, modification of a safety barrier in such an intrusive manner is certainly not desirable. To circumvent this problem, the standard XL-I heatsink may be converted for use with new optical systems (Fig. 1c). This heatsink has three holes, which can be used for conduits. As can be seen in Fig. 1c, the hole for the flash lamp and monochromator mount serves as cable and fiber conduit, leaving two more holes (photomultiplier mount and window in interference optical path) available for mounting a second and possibly a third optical system in the rotor chamber. Each of these additional optical systems, however, must fit in the remaining space. Furthermore, the introduction of new hardware in the vacuum chamber raises concerns about rotor explosion containment.
Fig. 1  a Xl-heatsink with six additional holes (indicated by arrows) drilled in as optical channels or for mounting optical or electric vacuum feeds. b The same heatsink mounted in a XL preparative ultracentrifuge with multiwavelength detector. c Multiwavelength detector mounted on an XL-I heatsink
The XL-platform also suffers from a lack of publicly available software drivers to operate the instrument. System commands are issued to the XL via an RS232 serial port, but the command sequence is not published and has changed over the years. While the commands have been reverse-engineered (e.g. Aviv AU_AOS fluorescence detector operating software), the process is time consuming, and software development by a third party is very difficult.
From the above considerations, it becomes clear that the XL-platform is ill-suited for future AUC developments, and the requirement for an entirely new platform designed specifically for the analytical tasks of the AUC is apparent. The new AUC platform must:Have open source communication protocols to allow for third party detector module development.
Provide mounts for light sources and detectors outside of the vacuum chamber and provide windows for all optical paths to illuminate the spinning AUC cell. This principle hearkens back to the Model E AUC series (1950–1972) by Beckman Instruments.
Offer multiple optical paths and support simultaneous data acquisition by different optical systems.
Support a standard docking mechanism to allow for convenient swapping of multiple detector designs and allow the user to recombine those optical systems which may best meet the requirements for a particular application.
Offer the specialized hardware and software drivers needed to synchronize data acquisition with the spinning rotor. The hardware and software must be open source to encourage optical system development.
By meeting these specifications, the new AUC platform will encourage the development and commercialization of detectors. Because the difficult task of data acquisition synchronization is taken care of, and the open source communications modules relieve developers from writing software to operate the centrifuge, research laboratories will be able to develop new and specialized detectors. Likewise, it is hoped and anticipated that this “open platform” concept will encourage diverse optical instrument manufacturers to provide commercial detectors for the new AUC.