Every flow through apparatus has to fulfill the method requirements it has been designed for.

To solve dosage forms' solubility problems a USP 4 Open System is used. "Open" means that a certain volume of media passes through the 7 flow cells continuously in an open loop. The eluate is filtered upon leaving the cell and is either collected as a fraction "Off-line Open or directly analyzed :"On-line systems". A pH or media change can be easily performed automatically with a media selector at selected time intervals. This is useful for a pH sensitive release product, enteric coated tablets, and IVIVC studies. It is even possible to perform a complete media change on a suspension or powder formulation. The results are shown as a differential curve or rate of release profile.

Alternatively, a small or fixed volume of media can pass through the cells and is recirculated in a closed loop. This is useful for products with a small amount of active to improve sensitivity or as an alternative to 900 or 500 ml paddles methods. The total media volume used in the closed loop configuration can range from as low as 5 ml to as much as needed such as 900 ml, 1 L, 2L or 4L. In this USP 4 Closed System, the eluate can be analyzed directly:"On-line systems" samples can be taken from the loop by a fraction collector:"Off-line Closed" or even a manual sampling syringe could be used. The results are shown as a cumulative curve.

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