Pump s Valves and Gradient Formation

Each characteristic is important however the most critical factor for process scale pharmaceutical purification applications is reproducibility. Processes that are required to be validated must yield consistent results between operations. For a chromatography process to be reliable, the gradient profile must be reproduced consistently during each operation. Several approaches are used to control the formation of a gradient profile. As the most important principle valve- or pump-based mixing systems can be identified. 

0 to 2001 valve-based gradients can be mixed effectively within the ratio of 10-90%, while pump-based mixing assures acceptable accuracies only between 40 and 60%. For higher flow rates, both methods offer effective gradient mixing. These differences between valve and pump mixing are valid independently from the flow rate or pump capacity. 

Some applications require gradients to be formed to a prescribed set-point value. The most common parameter used to identify the set-point value is the ionic strength or conductivity of the buffer solution. Another possibility for alcohol or acetonitrile - water mixtures is the use of a near infrared sensor. 

A very reliable, but more expensive gradient mixing unit is based on the use of regulation valves combined with mass flow meters. 

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