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There are several factors that can be considered here. First, you should ensure that you are using an appropriately sized column/capsule; if the capacity is too small, you could be losing product in the flow through. Second, ionic strength is obviously important to consider for an ion exchange column. The load, wash, and elution ionic strength should be optimized to ensure maximal binding and elution. Third, Lenti is not as stable as AAV or Adenovirus, and thus you should be careful with holding the eluate, for example dilution to get the eluate back to a more neutral pH and/or lower salt concentration can be important for maintaining infectious titer.