Process Conditions to Set for TFF Concentration and Buffer Exchange

What TFF Concentration and Buffer Exchange Do

In TFF, ultrafiltration (UF) removes solvent and small molecules through the membrane to concentrate the product, and diafiltration (DF) adds new buffer as the permeate leaves to exchange the buffer.[9][10] In biopharmaceutical processes, this step usually follows purification and sets the final formulation concentration and buffer.[12][14]

Choosing the Membrane: Cut-off and Format

• The general rule is to choose a molecular weight cut-off (MWCO) 3 to 6 times smaller than the molecular weight of the molecule to be retained.[5][6][7] For concentration only, the 3X end is usually enough; with extensive buffer exchange, a tighter membrane toward the 6X end is safer.[5][7]
• Molecular weight is only a rough guide. Passage depends on shape, charge and buffer conditions, so confirm retention with your actual product.[5][8]
• Membranes come as flat-sheet cassettes and hollow fibers. Rather than assuming hollow fibers are always gentler, some recommend testing product quality under your actual flow and channel conditions.[21][22]

Operating Pressure (TMP) and Crossflow

As TMP rises, permeate flux first increases with it, but beyond a certain point it barely increases.[1][3] Raising pressure further in this region increases the protein concentration at the membrane surface and the risk of fouling and yield loss.[1][2] The operating point is therefore set with a TMP excursion test, measuring flux at different TMPs for each crossflow rate.[1][2] Because viscosity rises as concentration proceeds and changes flux and pressure, check again at the final concentration and in the new buffer.[2][4]

Number of Diavolumes and Diafiltration Concentration

A diavolume (DV) is the volume of new buffer added divided by the product volume at that time. In constant-volume diafiltration, a freely permeating substance theoretically falls as C/C0 = e^(-N), so about 0.7% remains after 5 DV.[9] In practice, poor mixing, partial retention by the membrane and binding to the protein mean less is removed, so measure what remains and set the number of diavolumes from that.[9][10]

You also need to choose the concentration at which to diafilter. When dilute, flux is high but volume is large; when concentrated, less buffer is needed but flux is low, so there is an optimum concentration in between.[1][11]

pH and Excipient Shifts at High Concentration, and Final Volume

• When concentrating to high levels, charged buffer components (for example histidine) can be pushed away from the protein (the Donnan effect), so the pH and excipient levels in the final product may differ from the buffer you added.[12][13][14] For this reason, the concentration or pH of the diafiltration buffer is sometimes adjusted in advance.[12][14]
• Hold-up volume in the system affects how much you recover, and minimum working volume (MWV) limits how far you can concentrate.[15][16][17] Flushing with buffer at the end increases recovery but dilutes the product, so decide how far to over-concentrate beyond the target to allow for this.[15][16]
• Single-pass TFF (SPTFF), which concentrates in one pass without recirculation, is studied and used to connect continuous processes and reduce hold-up volume.[23][24]

Cleaning and Reuse Checks

• To check that the membrane is clean, compare the normalized water permeability (NWP, water flux ÷ TMP) with its initial value.[18][20]
• Sodium hydroxide (NaOH) cleaning is common, but follow the membrane manufacturer's data for concentration, temperature and time.[19][25]
• After installing the cassette and flushing it with water, perform an integrity test before use.[19][20]

Process Condition Checklist

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