the case of detection of viruses second upon dissolution of the lipid envelope with such agents the increase in the virus density is significant the density of the viral core which is the virus minus its lipid envelope is in general significantly higher than that of the enveloped virus in the case of hepadnaviridae for example this may be from about 1 25 to 1 36 both effects serve to further differentiate viruses from particularly microsomes in the virus window plot the first by acting to eliminate the microsomes and the second by increasing the difference in density between the viruses and the background microsomes third release agents enhance the desorption of viruses from solid matter which is particularly important in the detection of airborne viruses release agents can also break up aggregates of viruses especially aggregates of encapsulated viruses the present invention minimizes this aggregation problem in other ways besides the use of release agents the centrifugation can be performed without pelleting consequently buoyant density and thus isopycnic banding is not greatly affected by aggregation under these circumstances indeed banding times are favorably reduced in the case of aggregation and techniques can be applied that take advantage of this within the broad context of the present invention any aggregation will generally produce only a small shift in and or broadening of resulting virus bands the portion of this exiting stream that contains the virus window is pumped to the purification stage 103 with the position of a particle along this stream giving the density of that particle the useful part of the stream in the case of general virus detection where the range 1 175 1 46 is passed to the next stage is in the preferred embodiment.