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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsChoose a TFF membrane by matching its cutoff, chemistry and flow-path geometry to the molecule and feed, then establish transmembrane pressure (TMP) and crossflow with a short, controlled scouting study. Treat manufacturer MWCO ratios and operating examples as starting points: confirm retention, recovery, flux and product quality on the actual feed before fixing a process setpoint.
Start with the separation you need
A membrane choice is only meaningful after the separation objective is explicit. Tangential flow filtration (TFF) keeps the retained fraction moving parallel to the membrane while smaller species pass through as permeate. In ultrafiltration (UF) and diafiltration (DF), the product is normally retained and concentrated first, then the buffer or other small solutes are exchanged. The same equipment can be used for other fractionation or clarification tasks, but the acceptance tests change with the objective. Cytiva’s technical guide and Sartorius’ TFF overview describe these applications.
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Write down the target and what must pass
- Identify the target molecule, its approximate molecular size and physical form, and whether it must remain in retentate or be allowed into permeate.
- List the species that should pass, such as salts, buffer components or other low-molecular-weight impurities.
- Set numerical requirements for recovery, purity, concentration factor, buffer exchange and product-quality attributes before screening membranes.
- Record feed composition, expected viscosity, temperature range and whether the material is shear-sensitive or prone to aggregation.
- Decide whether the device will be single-use or cleaned and reused; that decision affects material compatibility, validation and economics.
Separate concentration from diafiltration
UF and DF should be recorded as separate operations. Concentration changes retained-solute and viscosity conditions as volume falls; DF adds new buffer while permeate leaves. Flux, pressure drop and product recovery can therefore differ between the two stages even with the same membrane and hardware.
What MWCO should I use?
Molecular-weight cut-off (MWCO) is a screening parameter, not a sharp retention boundary. The observed result depends on molecular shape, aggregation, solution conditions, membrane structure and the desired yield. Start with a tighter cutoff than the nominal target, then measure the target in both retentate and permeate using the real feed.
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| Device format | Manufacturer starting heuristic | How to use it |
|---|---|---|
| Flat-sheet cassette | Choose a cutoff 2–3 times tighter than the target molecular weight. | Use as an initial screen, then verify retention, recovery, flux and quality experimentally. Sartorius cassette guidance |
| Hollow fiber | Choose a cutoff 3–5 times smaller than the target molecular weight. | This is a hollow-fiber-specific heuristic; confirm performance on the selected fiber and feed. Sartorius hollow-fiber guidance |
Run a retention screen instead of trusting the ratio
- Select two or more candidate cutoffs around the relevant starting range. Include a tighter option if losing product to permeate would be costly.
- Use the same feed, temperature, concentration range and representative membrane area for each candidate.
- Collect retentate and permeate samples and assay the target and critical impurities. Calculate recovery from measured product amounts, not only concentration readings.
- Track initial and final flux, pressure behavior and any visible fouling or aggregation.
- Reject a membrane that meets an initial retention target but causes unacceptable adsorption, poor recovery, slow DF or loss of a required quality attribute.
A cutoff that works at the beginning of a run can behave differently after concentration. Recheck permeate passage and product quality at the final concentration and during DF.
How do I choose membrane chemistry?
Membrane material should be evaluated with the molecule, formulation, cleaning agents and intended lifecycle. Adsorption can reduce recovery even when the nominal MWCO is correct, while chemical incompatibility can shorten membrane life or invalidate a cleaning procedure.
| Material described by the manufacturer | Stated attributes | Questions to verify in your process |
|---|---|---|
| Hydrosart, a stabilized cellulose-based membrane | Sartorius describes it as hydrophilic and non-protein-binding. | Measure actual adsorption and recovery with your molecule and formulation; confirm cleaning and reuse limits for the selected cassette. Source |
| PES (polyethersulfone) | Sartorius describes broad pH and temperature tolerance and high flux for its PES cassette membranes. | Check compatibility with your feed and cleaning chemicals, then verify flux, retention and product quality on the exact device. Source |
Those are manufacturer descriptions, not guarantees for every product or fluid. The selected device’s current compatibility table and operating manual control the allowable chemistry, temperature, pressure and cleaning conditions.
Cassette or hollow fiber: which format fits?
Choose geometry with the feed’s viscosity, allowable shear, available pump and the desired scale-up path in mind. Compare complete flow paths rather than membrane area alone: channel or lumen dimensions determine pressure drop, required recirculation flow and local shear.
Flat-sheet cassette
Cassettes provide parallel membrane channels and are commonly selected when a system already supports plate-and-frame modules or when a defined effective area is convenient for scale-up. Check the cassette’s channel design, pressure rating, hold-up volume, cleaning instructions and whether the supplier supports single-use operation.
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Hollow fiber
Hollow fibers use many small lumens and can be useful when a compact flow path or low hold-up volume is important. Sartorius gives 0.5 mm lumen fibers as an example for standard protein solutions and 1.0 mm lumens for viscous or cold processing. It also advises shorter cartridges for higher-viscosity upstream feeds to manage pressure drop and longer cartridges for lower-viscosity downstream streams. Treat these as product-family guidance and confirm the limits for the chosen module. Hollow-fiber guidance
Selection checks for either format
- Target retention and sieving of all critical components.
- Product recovery and adsorption after the full concentration and DF cycle.
- Feed viscosity, temperature and shear sensitivity.
- Pressure drop, allowable TMP and maximum crossflow in the current manual.
- Fouling behavior and whether the membrane can be cleaned, sanitized and reused as intended.
- Available membrane area, pump capacity, tubing, sensors and scale-up increments.
What is TMP and how should I set it?
TMP is the pressure driving permeation through the membrane. In practice, it is controlled with the feed or inlet pressure, retentate or outlet pressure and the permeate-side condition; the exact calculation and sensor arrangement depend on the system. Record all relevant pressure readings and use the module manual’s limits rather than transferring a number from another device.
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At a fixed crossflow, increasing TMP often raises flux at first. As concentration polarization and a gel layer build at the membrane surface, the extra pressure can produce little additional permeate. Cytiva’s Handbook: Cross flow filtration method describes the useful region as just before gel-layer accumulation and emphasizes that TMP and crossflow must be optimized together. Read the handbook.
Use a TMP step test
- Stabilize the feed temperature, concentration and crossflow at a conservative condition within the module limits.
- Increase TMP in small steps, allowing flux and pressures to reach a steady reading at each step.
- Plot flux against TMP. Mark the point where the slope falls sharply or where product quality, recovery, pressure drop or fouling becomes unacceptable.
- Repeat the sequence at at least one other crossflow that the pump, tubing and module can safely provide.
- Select a working point before the diminishing-return region, not the highest pressure the hardware can tolerate.
Do not quote TMP without the associated membrane format, area, crossflow, feed, temperature and concentration. A pressure that is productive for one channel geometry may be wasteful or damaging in another.
How do I set the crossflow rate?
Crossflow sweeps along the membrane surface and helps carry away the concentration boundary layer. Cytiva states: “At a given TMP, increasing the cross flow rate helps to reduce the concentration gradient layer and increase flux.” The handbook’s guidance also makes clear that the benefit is process-specific.
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More crossflow can improve mass transfer, but it can also increase shear, pump energy, pressure drop and product exposure to interfaces. For a shear-sensitive or fragile product, a lower flow may give better recovery or quality even if its instantaneous flux is lower. For a viscous feed, a geometry that keeps pressure drop manageable may matter more than maximizing nominal flow.
Crossflow scouting procedure
- Choose a low, a middle and a higher crossflow that remain inside the module and pump specifications.
- At each flow, hold TMP and feed conditions as constant as practical and record stabilized flux, inlet and outlet pressures, temperature and product-quality observations.
- Repeat at a second TMP if the first comparison does not reveal whether the process is mass-transfer-limited.
- Compare total process time, recovery, quality, fouling and energy—not flux alone.
- Set the operating range with an upper and lower alarm so that drift in flow or pressure is detected before product loss.
Build a process-specific scouting study
A useful study examines the TMP/crossflow interaction over the whole operation, not just an initial clean-membrane reading.
Record the starting conditions
- Feed identity, composition, temperature and viscosity when available.
- Initial and final volume, target concentration factor and DF volume or exchange plan.
- Membrane chemistry, MWCO, format, effective area and module identifier.
- Pump, tubing and flow-path configuration, including any recirculation restrictions.
- Pressure readings at feed or inlet, retentate or outlet and permeate, plus the sensor calibration status.
- Assays for target concentration, permeate leakage, recovery and critical quality attributes.
Choose the useful operating region
Use the measured flux-versus-TMP curves at each crossflow to identify diminishing returns. The practical setpoint is the combination that delivers acceptable throughput while preserving recovery and quality and avoiding rapid fouling. Confirm that choice at the intended final concentration, because viscosity and retained contaminants can change the pressure and flux response.
Separate UF and DF records
Log UF and DF as distinct stages with their own average flux, elapsed time, pressures and product assays. A DF result should not be used as a promise of UF performance, or vice versa.
Worked manufacturer example: what the numbers do—and do not—mean
A Sartorius application note reports a specific Sartocon Slice 200 Hydrosart 300 kDa cassette trial. The figures below describe that clarified-feed setup only; they are not general performance guarantees.
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| Parameter | Reported value |
|---|---|
| Device | Sartocon Slice 200 Hydrosart cassette |
| Membrane MWCO | 300 kDa |
| Effective filtration area | 0.2 m² |
| Pressure difference (ΔP) | 0.8 bar |
| Target inlet pressure | 1.0 bar |
| TMP | 0.5–0.6 bar |
| Average UF flux | 131 LMH |
| Average DF flux | 58 LMH |
| UF time | 16 minutes |
| DF time | 20 minutes |
131 LMH UF and 58 LMH DF average flux — Sartorius, 2026 (application-note publication date shown approximately six months before 2026-10-13; exact date not present in the search result). The complete context is in the Sartorius application note. Differences between the UF and DF values illustrate why each stage needs its own measured baseline.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Monitor the run and define stop criteria
Flux should be trended against time, volume processed and retained concentration. A falling flux accompanied by rising pressure can indicate concentration polarization, gel formation or fouling; a sudden change can indicate a leak, air, pump problem or sensor issue. Investigate the cause before increasing TMP.
Stop or intervene when
- Additional TMP produces little flux improvement.
- Pressure approaches the module, cassette, tubing or pump limit.
- Recovery or a critical quality attribute moves outside its acceptance range.
- Permeate contains more target than the retention study allows.
- Crossflow increases shear-related degradation or unacceptable aggregation.
- Temperature, viscosity or concentration leaves the validated operating window.
At the end of concentration and DF, sample both retentate and permeate. Confirm final recovery, concentration, buffer composition and product quality rather than relying on the starting flux.
Troubleshoot common operating problems
Flux rises with TMP, then levels off
This is consistent with a surface concentration or gel-layer limitation. Return to the last TMP that gave a meaningful gain, test whether a permitted increase in crossflow improves flux, and check concentration and viscosity. Do not keep raising pressure simply because the hardware allows it.
Flux falls throughout the run
Check whether retained solids, product concentration or viscosity is increasing. Compare pressure drop and permeate flow with the clean-start baseline, inspect for air or pump instability, and verify that the membrane has not fouled. Reassess the chosen cutoff and feed pretreatment if the decline is reproducible.
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Recovery is lower than expected
Measure target in permeate and retentate to distinguish passage from adsorption or hold-up. Review membrane chemistry, rinsing and system dead volume, then repeat with a tighter cutoff or a lower-binding material if the data support that change.
Product quality deteriorates at higher flow
Reduce crossflow and compare quality, not just flux. A lower-shear condition may be the correct process point if the higher-flow setting causes aggregation or other damage.
Pressure drop is excessive
Check viscosity, temperature, tubing restrictions, channel or lumen size and cartridge length. Sartorius’ hollow-fiber guidance associates shorter cartridges with higher-viscosity upstream feeds and larger lumens with viscous or cold processing; verify the recommendation for the exact module before changing hardware. Source
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Scale membrane area and flow path using the supplier’s current design rules, then recheck pressure, flux and product quality at the new geometry. Keep the critical ratios and limits defined by the equipment manufacturer, but do not assume that the same TMP or volumetric flow will transfer unchanged. Channel dimensions, pump behavior, hold-up volume and feed viscosity can all alter the result.
For a robust scale-up package, retain the cutoff and chemistry decision, membrane area calculation, crossflow/TMP scouting curves, concentration and DF endpoints, pressure alarms, sampling plan, cleaning or disposal procedure and recovery data. The Cytiva guide and Sartorius cassette guidance both emphasize evaluating process conditions for each application and cassette type.
Concrete laboratory cassette example
For product discovery, Sartorius lists a Vivaflow 200 cassette with a 200 cm² PES membrane, 50 kDa MWCO, maximum operating pressure of 4 bar, recommended feed volume of 1,000–2,500 mL and recommended flow rate of 200–400 mL/min. These specifications describe that product page, not a universal choice for every 50 kDa target or feed. Confirm current regional specifications, compatibility and instructions before use. View the product specification.
Quick Recap
Practical decision checklist
- Define whether the operation is concentration, DF, fractionation, clarification or a combination.
- Specify the target, pass-through species, recovery and product-quality limits.
- Screen a cutoff using the format-specific starting heuristic, then measure retention on the actual feed.
- Compare membrane chemistry for adsorption, compatibility, cleaning and single-use or reuse policy.
- Choose cassette or hollow fiber using viscosity, shear, pressure drop, area and scale-up constraints.
- Scout at multiple TMP and crossflow combinations and identify the diminishing-return region.
- Verify flux, recovery, permeate leakage and quality at both initial and final concentration and during DF.
- Document the selected range, alarms, module limits and corrective actions in the batch or development record.
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