An Integrated Approach to Buffer Dilution and Storage

This is one of the underlying benefits of using concentrated buffers. Unless the solubility limit is approached, it is easy to scale up buffer capacity by simply increasing the concentration and re-programming the dilution skid. We settled on a conservative 10x concentration factor because we wanted to gather real-world experience before aggressively concentrating the buffers since that puts more challenge on the blend precision required.

Construction of a Prototype Inline Dilution System

Now that we had supporting solubility data, our challenge was to establish proof of concept for inline dilution at near-manufacturing-scale. With Biokinetics (Emeryville, Calif.) we designed and built a custom automated inline dilution system capable of multi-component blends, which we called the “prototype system.” Our design strategy was to equip the system with extremely accurate and precise pumps, valves (Baumann), and inline flow meters that could reliably control the flow rate of both water and the concentrated buffer solutions. If we could guarantee precise flow control over an acceptable range, we felt confident that inline dilution would be a success at our new facility⁵.

The prototype system was capable of a total blend flow rate of 40 L/min. A schematic and picture of the prototype inline dilution system are shown in Figures 1 and 2.

(Click to enlarge image) Figure 1.	Figure 2.

The system was designed so that either a rotary lobe positive displacement pump (Viking DuraLobe) or centrifugal pump could be connected to the inlet of any of the four inlet lines. The pumps were used to drive liquid through the system and multiple pumps could operate simultaneously to create multi-component blends, a function required for performing gradients with concentrates. Flow rate was controlled by direct feedback from Coriolis mass flow meters (Endress + Hauser or MicroMotion) on each individual line to either the rotary lobe pump or valve.

The valve could also be used to control backpressure in the line as the rotary lobe pump simultaneously controlled flow rate. This helps to make “pump slip” less of a problem. Pumping into higher pressure makes PD pumps perform more consistently and over a wider dynamic range. If the centrifugal pump was used, the valve controlled flow rate while the pump supplied constant high-pressure flow at 50-80 psig. Various flow control strategies were tested to determine the best option, which is discussed below. The system was also equipped with a filter housing, a divert-to-drain line, and analytical instruments for pH, conductivity, and optical density measurements.

Lessons Learned from the Prototype System