Assessment of the Cobas Bio centrifugal analyser

The Cobas Bio (Roche) is a discrete spectrometric centrifugal analyser which is capable of monitoring :29 reaction cuvettes. The dispensing and analysing systems are contained within one unit and an Intel 8080A microprocessor controls the total system. Two Hamilton syringes are used to pipette samples and reagents. The sample and reagent syringes have nominal capacities of 100 and 400 #1 respectively. A second reagent can be pipetted by the sample syringe at a time interval of between 10 and 999 s. Up to 25 sample cups are housed in coded discs which form the basis ofan identification system. The sample cups are capped to minimize evaporation. They can be placed in one of two positions and the probe will only sample, by piercing the cup, those cups which are in the down position. This provides a way of selectively running different methods on the same sample plate. The reagent boat contains receptacles for the primary and secondary reagents and up to three standards. The reagent pipettor uses disposable plastic tips, whereas the stainless-steel tip of the sample pipettor is automatically rinsed between samples. The disposable rotor constructed from acrylic butadiene styrene polymer has 30 cuvettes, one ofwhich is used to optimize the photometer light-source characteristics. A program allows for the effective use ofempty cuvettes and the rotor need only be removed when all the cuvettes have been used. Ifthe photometer arm is raised or the instrument turned off, this program is reset and the rotor must be replaced. The detector unit consists of a digitally-controlled concave halographic grating monochromator, a central processing unit controlled energy adjustment system and a photodiode detector. The pulsed Xenon flash-tube has a spectral range of285-750 nm. Unlike other centrifugal analysers, the optical path-length is not fixed as the Cobas Bio measures absorbance in the same direction as the centrifugal force (see figure 1). The calculation factor is therefore dependent only on the selected sample volume and not the ratio of sample to reagent volume (Eisenwiener and Keller [l-l). This allows the addition ofa second reagent without altering the calculation factor (Eisenwiener et al. [2]). Space is available for 30 programs, 10 have a key specifically