The conductivity and pH analyzers used in these experiments were standard components on a 5-L/min IBD skid from Asahi
Kasei Bioprocess (Glenview, IL). The pH probe was an ABB Limit model #AP121 21000 electrolyte-filled glass electrode from
ABB Instrumentation (Cary, NC). The conductivity sensor was an Optek CF60-45 with six electrodes and a Pt1000 platinum resistance-temperature
device used for temperature measurement and compensation from Optek-Danulat (Germantown, WI).
Both probes were mounted in a 1-in. F40 Optek PEEK Flow cell with a PF12 pH electrode adapter. An Optek Control 200 controller
was installed on the skid and used a dual-input electrochemical converter to monitor pH, conductivity, and temperature. Conductivity,
pH, and temperature output signals from the electrochemical converter were processed and trended in a run chart using Proficy
HMI/SCADA-iFIX, Proficy Historian Version 3.1a, and SP1 process-control software from GE Fanuc (Charlottesville, VA) on a
personal computer housed in the skid. Oakton Instruments's (Vernon Hills, IL) pH and conductivity standards were used for
One liter of sodium chloride (1 × the product) was carefully weighed out using a model JK-200 analytical digital balance from
Chyo Balance (Komatsu, Japan) with ± 0.01 mg accuracy. Sodium chloride (~6 × the product) concentrate was weighed out using
a model MK-2000B Petit balance (Komatsu) from Chyo Balance with ± 0.1 g accuracy and dissolved into deionized water in a polycarbonate
drum. Acetic acid and ammonium hydroxide solution were diluted from their respective concentrations. Sodium chloride and glacial
acetic acid were purchased from Amresco (Solon, OH), and ammonium hydroxide was obtained from Mallinckrodt Baker (Phillipsburg,
Comparison of mass-flow and PAT-based control blending
Conductivity control. The carefully prepared 1-L standard of 0.167 M sodium chloride gave a conductivity reading of 17.8 mS/cm, which was used
as the programmed conductivity set point in the process-control software. Initially, the 1-M sodium-chloride concentrate
delivered by the P002 pump was diluted in the blending loop by the WFI (P001) pump. Both pumps' frequencies were adjusted
accordingly to obtain the programmed conductivity set point. The recorded WFI, concentrate, and blending-loop pump speeds
were entered into a new method as pump preset start speeds. Starting from 1-M sodium-chloride concentrate, either mass-flow
or PAT-based blending achieved the desired molarity with the targeted conductivity of 17.8 mS/cm (see Figure 4). Because of
the presence of impurities, mixing inefficiency, and weighing errors, feedstock variability of 5% was expected.
Figure 4: Conductivity of final products from mass-flow and process analytical technology (PAT)-based in-line buffer-dilution
To reveal the state of a process, the authors prepared a sodium-chloride concentrate ± 5% from the original 1-M solution.
Using the same pump frequency and method of mass-flow measurement determined earlier, the conductivity of the final product
was obtained. The average conductivity of the dilution of 0.95-M sodium-chloride concentrate was about 16.8 mS/cm, which was
1 mS/cm or 5.6% below the set point. For 1.05-M sodium-chloride concentrate, the average conductivity was 18.5 mS/cm, or 5%
above the set point. Data indicated that the variability of the starting raw materials determined the outcome, and mass-flow
measurement could not adjust to the desired set point. In contrast, the PAT-based IBD used the in-line conductivity sensor
in the loop to measure and correct the variability of the incoming salt concentrate through the process-control feedback loop.
Whether the starting salt concentrate was 5% above or below the original 1-M concentrate, the conductivity of the product
was well within the range of 17.8 ± 0.082 mS/cm.
pH gradient. The titration curve between acid and alkali, whether strong or weak, is not linear (4). Therefore, protein chemists seldom
use pH gradients during a chromatographic run to resolve proteins with similar pI, especially at production scale. To determine
whether the PAT-based IBD system could produce a linear pH gradient, the authors blended a weak acid with a weak alkali. Two
experiments, one based on mass-flow, and the other PAT-based, were performed.