While simulating the Anion exchange column process utilizing cadet, I had a question that came up.
During the simulation, I was wondering if I should include all the chemical components of the buffer in the “component”, which is denoted as “n_comp” in cadet.
For example, the process I try to simulate consists of EQ-Loading-Elution and many buffers are used along the way. For example, Histidine, NaCl, Tris, etc. are used in different concentrations in each step.
In this case, I wonder if I should run the simulation assuming a total of 4 components (n_comp=4): Histidine, NaCl, Tris, and protein. If so, should I estimate the concentration of each component by looking at the conductivity estimated by the column controller?
Definitely a good question! The answer depends entirely on what your goal is for this application.
The most “mechanistically” correct approach would be to treat each species as a component with individual eluent strengths and buffering effects. I think for the majority of applications this is unnecessarily complicated. One example off hand would be if you are simulating pH gradients with a complex mixture of buffer species.
What I recommend is that you use a single salt species that is an effective combination of multiple eluents. Take the example of using a sodium acetate buffer with sodium chloride eluent for a CEX process. While the cationic species are Na for both components, the salts have different elution strengths due to the influence of the co-ion (anion, in this case). Through some retention comparisons (NaCl gradient and NaOAc gradient for the same protein) on CEX, you find a 1.5:1 elution strength equivalence between NaOAc and NaCl. It follows that for a 50 mM NaOAc buffer with 500 mM NaCl, the effective NaCl concentration is 50*(2/3) + 500 mM.
This of course becomes messy when you even more species. So, the proper course of action depends on the level of rigor you are willing to handle. Depending on the concentration, some components may even be negligible. It is a good idea to simplify things whenever possible.
2 Likes
Hey, I want to say a big thanks for your awesome answer! Really helped clear things up for me. Thank you very much
2 Likes
If you want to dive deep into pH modeling, you can read chapter 6 in the PhD thesis of Juliane Diedrich. A follow up publication with further applications is in the making. See also this topic on modeling pH transitions in CADET.
1 Like