inverted that is when the closed cuvette is tumbled while containing the assay mixture and air the micro beads are maintained in suspension and the sample is fully exposed to the beads the ability to pass a bubble from bottom to top during tumbling is enhanced by the use of surfactants even with surfactants however it is desirable that a minimum of 3 5 mm be permitted for passage of the bubble for best mixing after incubation and tumbling the cuvette is returned to an upright position and allowed to settle and fractionate under the influence of gravity before during and after separation into the upper and lower phases is achieved measurements are taken by passage of electromagnetic radiation through the mixture to obtain sensed assay data giving information about the analyte s and about completion of the separation of phases such data may be derived using a two sensor arrangement wherein respective sensors are located at vertically spaced locations adjacent the upper phase zone and the lower phase zone in this type of analyzer incident radiation is directed through the container at the two spaced locations and such is sensed by the individual sensors in a more preferred embodiment a single sensor.