PAT-based in-line buffer dilution
This article examines the operational requirements and design of a process-ready PAT-based in-line buffer-dilution (IBD) system,
which is capable of making reproducible and accurate in-line buffer dilutions in concentrations as high as 100 × of the product
using conductivity or pH feedback control. The technology can be used within stand-alone packaged equipment or engineered
into a process-scale liquid chromatography skid. In the latter option, mobile phases are made on demand and sent directly
to a process column for a sequence of chromatography steps in a purification regime. The comparison of a PAT-based and a mass-flow
control system includes conductivity gradients, which are typical for biochromatographic processes, and the generation of
linear pH gradients with good acidity control.
Figure 2: A process analytical technology-based in-line buffer-dilution skid equipped with conductivity and pH process analyzers
linked with a process feedback-control loop.
The literature includes several examples of diluting buffers from concentrates in processes consistent with QbD (3). Some
biopharmaceutical companies currently use a patented PAT-based IBD skid for buffer preparation and dilution (see Figure 2).
To understand how IBD assists the QbD paradigm recommended by FDA, one should examine the basic technology platform. Figure
3 shows a sample piping and instrumentation diagram (P&ID) from the process-control screen of an IBD system.
Figure 3: Piping and instrumentation diagram screen of a process analytical technology-based in-line buffer-dilution system.
A typical PAT-based IBD skid consists of three pumps labeled P001, P002, and P102. Deionized water for injection (WFI) is
connected to P001, and buffer at concentrations as high as 100 × of the product is connected to P002. WFI and buffer concentrate
are pumped into a blending loop, where the buffer concentrate is diluted to 1 × the product. The diluted buffer has a specific
conductivity that can be analyzed by an in-line process conductivity sensor linked to a process-control feedback loop. The
sensor continuously analyzes the conductivity of the buffer blend in the loop and sends an output signal to the process-control
feedback loop. The loop then compares the current process conductivity value with a user-defined conductivity target set point
for the buffer diluted to 1 × the product. Based on the feedback signal, the concentrate pump speeds can be increased or
decreased. Whenever the acidity of the buffer solution must be adjusted, alkali or acid can be delivered from P102. In this
instance, a pH probe would be installed in the blending loop and linked with its own process-control feedback loop to adjust
the solution's pH to meet the user's specifications.
To assess the ability of a PAT-based IBD system to monitor a process and to actively manipulate it to maintain a desired state,
the authors performed experiments to compare simple mass-flow and PAT-based control blending, and to reveal the true benefits
of directly measuring the quantity of raw materials present during runs. The first experiment was a buffer-preparation scenario
in which errors in the formulation of the concentrate raw material can cause an out-of-specification buffer-dilution product.
The second comparison was made in the context of producing a linear pH gradient with pH adjustment and control not commonly
available in a mass-flow-based dilution system.