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What are the specific involved with viral particle concentration?

Why does spinning the viral supernatant down cause the titer to increase but you lose yield?If you have a titer of 1 x 10^6 TU/mL before concentration and you are concentrating 100 mL. your total number of functional viral particles is 1 x 10^8 TUs (1 x 10^6TU/mL X 100mL). If you concentrate the viral particles by ultracentrifugation and resuspend the particles in a total of 1 mL (so a 100-fold concentration), you would expect that the titer to increase to 1 x 10^8 TU/mL. However you do not recover all of the viral particles you started out with. You lose some amount of viral particles during the spins, filtration, pipetting, etc. Also the viral particles can be damaged during the ultracentrifugation. Therefore the titer measured on the concentrated stock will be less, on average about half as much. In this case 5 x 10^7 TU/mL. What if you spin faster in the concentration step? Have we tried this in house?Spinning faster or longer does not increase yield significantly. We have spun virus for 4 hours without seeing an appreciable increase in yield. Spinning faster may actually result in more damage to the viral particles. Have we ever tested for the titer left over in the supernatant?No, but there is probably very little left in the supernatant following centrifugation. Is there a difference in collection at 48h and 72h vs just collection at 72h?Sometimes. We have found that the amount of virus combined from 48 and 72 hours is either the same or slightly more than that when we just collect at 72h.