A Protein A capture cycle is the same sequence everywhere: prepare the feed, equilibrate, load, wash, wash again at higher ionic strength, condition, elute at low pH, then either neutralise immediately or hold the pool for virus inactivation, strip, clean — and re-equilibrate or store. The buffer table below is the whole protocol. The same steps run on a MonoCore™ Protein A capsule (1.8 mL or 5.4 mL, UNF 10/32) at about 8 column volumes per minute, which puts the residence time in seconds rather than minutes; everything after the table explains the decisions inside it.
The cycle at a glance
| Step | Buffer | Volume | Time on a 1.8 mL capsule | Purpose |
|---|---|---|---|---|
| Equilibration | 20 mM sodium phosphate + 150 mM NaCl, pH 7.4, or TBS | 3–10 CV | ≈ 25 s – 1 min | Set the baseline; UV and conductivity flat before loading |
| Load | Clarified feed, pH 7.0–7.4 | To 70–80 % of measured capacity | Set by feed volume: 1 L ≈ 65 min | Bind the antibody through its Fc |
| Wash 1 | Equilibration buffer, or 20 mM phosphate + 0.25–0.5 M NaCl, pH 7.0–7.4 | 5 CV | ≈ 35 s | Remove what is merely in the liquid, and loosely bound host-cell protein |
| Wash 2 — high salt or arginine | 20 mM phosphate + 0.25–1 M NaCl, pH 7.4, or 0.5–1 M arginine, pH 7.0–8.9 | 3–5 CV | ≈ 20–35 s | Remove host-cell protein that is bound, not free |
| Wash 3 — conditioning | 50 mM sodium acetate, pH 5.5–6.0, or equilibration buffer without salt | 1–2 CV | ≈ 7–15 s | Displace the salt and pre-lower the pH, so the elution peak stays sharp |
| Elution | 50 mM sodium acetate or citrate, pH 3.4–3.6; 0.1 M glycine-HCl pH 3.0 where that is not enough | 3–5 CV | ≈ 20–35 s | Release the antibody at the highest pH that still desorbs it |
| Then either: neutralise | 1 M Tris-HCl pH 8.5–9.0 — 30–50 µL per mL of fraction for a glycine pool, 120–250 µL for citrate or acetate | In the collection tube | Immediate | Return the pool to pH 6.5–7.5 |
| …or: low-pH virus inactivation | Adjust the pool to pH 3.4–3.6 — with acid if it eluted higher, with base if it eluted lower — then neutralise | Pool volume | 30–60 min hold | Inactivate enveloped virus: a validated hold, not an accident of collection |
| Strip | 0.1 M acetic acid pH 2.9, or glycine-HCl pH 2.5–3.0, below the elution pH | 2–3 CV | ≈ 15–20 s | Remove what did not elute, before the caustic step fixes it in place |
| Cleaning in place | Sodium hydroxide per the medium's specification — published regimes run from 10–20 mM for native ligands to 0.1–1.0 M for alkali-stabilised ones | 2–3 CV | 10–15 min contact | Remove denatured product, host-cell protein, DNA and lipid |
| Re-equilibration / storage | Equilibration buffer, 3–10 CV; or the storage solution per the datasheet, 5 CV, 2–8 °C | 3–10 CV / 5 CV | ≈ 25 s – 1 min | Ready for the next run, or safe while idle |
Preparing the feed
Protein A binds through the Fc, not through charge, so the conductivity of the feed hardly matters: harvested cell culture supernatant can usually be loaded as it is, without dilution or buffer exchange. That is the single most useful property of this step and the reason it sits first in almost every antibody process.
What does matter is particles — though how much depends on the medium. Centrifuge or depth-filter the harvest so the feed is clarified. A packed bed, whose interstitial gaps are the narrowest part of the flow path, is the case where a 0.22 µm filtration immediately before loading pays for itself; the 15 µm channels of a MonoCore™ capsule tolerate what is left after clarification, which is the point of the format. Set the pH to 7.0–7.4 — anything above pH 6 binds, but the wash is cleaner at neutral.
If the column has been stored in 20 % ethanol, displace it with equilibration buffer before any protein reaches it. Protein meeting residual ethanol precipitates, and it precipitates at the inlet.
How much to load
Load to roughly 70–80 % of the dynamic binding capacity you measured at your own residence time. A datasheet headline is not a loading instruction — ours states the capacity together with the conditions it applies to, and it is those conditions, not the number, that decide whether it transfers to your feed. The margin absorbs the capacity drift every medium shows over its lifetime and keeps the flow-through clean enough that UV there is a meaningful alarm. Measure it on your own feed where the datasheet conditions do not match (how to read a capacity figure), and record the residence time alongside: on a packed bed usually several minutes, on a MonoCore™ capsule a few seconds.
Elution and neutralisation
The working default in process work is a mild acid at the highest pH that still desorbs the product: 50 mM sodium acetate or citrate at pH 3.4–3.6, over 3–5 CV. Glycine-HCl at pH 3.0 is the option for molecules that do not come off there, not the starting point — every tenth of a pH unit you give back reduces the aggregation risk and the ligand exposure. The comparison, with the trade-offs per buffer, is in Protein A elution buffer: glycine, citrate or acetate.
Neutralise in the collection tube, not after pooling: 1 M Tris-HCl at pH 8.5–9.0 — about 30–50 µL per mL of fraction for a 0.1 M glycine pool, 120–250 µL for 50–100 mM citrate or acetate, pre-loaded, aiming for a pooled pH of 6.5–7.5. Verify that target once with a buffer-only run and once on real eluate — the product buffers too. Aggregation at low pH depends on how long the molecule sits there as much as on the pH itself, which is why the collection scheme matters more than the choice of acid.
The exception: low-pH virus inactivation
In a bioprocess, the Protein A eluate is also the low-pH virus inactivation step: the pool is held at about pH 3.5–3.6 at ambient temperature for 30–60 minutes and only then neutralised. It is an alternative to immediate neutralisation, not a step after it. Note the direction of travel — a pool that eluted at pH 3.0 has to be brought up into the hold window with base, one that eluted at pH 3.6 may need nothing at all, which is one more argument for eluting with acetate or citrate close to the hold condition.
The hold is a validated requirement, and it is the one case where time at low pH is deliberate. The two goals — short acid exposure for the product, long enough exposure for the virus — are reconciled by validating the mildest pH that still inactivates for your molecule, not by shortening the hold. For analytical and preclinical work, where there is no virus claim, neutralise immediately as above.
Host-cell protein in the eluate
If host-cell protein carries over, lengthening the plain wash rarely helps, because what carries over is bound rather than free. The salt wash is what does the work — and two things are worth knowing before you reach for the salt. Its effect largely saturates: published work across several antibodies found HCP clearance proportional to sodium chloride up to roughly 200–250 mM and little further gain up to 1 M, see the study on the adequate amount of sodium chloride. And for the host-cell proteins that survive a salt wash, an arginine wash is the more effective route, including at elevated pH, because the interactions holding them are not purely electrostatic.
Leached Protein A, and how to see it
Every cycle releases a small amount of ligand into the eluate, and the amount rises as the medium ages and as the elution pH is lowered. Published process work puts a fresh medium in the region of a few nanograms of Protein A per milligram of antibody — see the process development review. It is quantified by a Protein A ELISA, and the sample has to be acid-dissociated first, because leached ligand travels bound to the Fc and is otherwise under-reported.
Clearance happens downstream rather than at capture: anion exchange and hydrophobic interaction polishing routinely take it into the low picogram-per-milligram range. Two practical consequences: do not elute at a lower pH than you need, and track leachate against cycle number in the same study in which you track capacity.
Cleaning, regeneration and service life
Strip after every cycle, and clean in place as your process requires — in a platform mAb process the caustic step runs in every cycle, in low-throughput lab work less often. It is the cleaning, not the binding, that mainly consumes a Protein A medium, which is why service life is counted in cleaning cycles rather than runs. Two practical points that are easy to get backwards: the strip protects the cleaning step, because product left on the medium is what the caustic bakes on; and on a packed bed the cleaning step is commonly run in reverse flow, because a packed bed fouls at its inlet and vendors specify up-flow CIP to lift that material off rather than push it through the bed.
The full picture — what the four procedures actually are, what limits lifetime, and how to measure the decline — is in Protein A column regeneration, CIP and service life.
For MonoCore™ Protein A and Protein G, the cleaning conditions ship with the product and are available on request — the datasheet is the fastest route to them. What you set yourself in any case is concentration, contact time, frequency and flow direction; what tells you whether it worked is back pressure, capacity and leachate over cycles. The MonoCore™ capsule is specified for pH 4–13 in long-term use and pH 2–14 short term, and the housing is the limiting component there, not the matrix and not the ligand — on an affinity capsule the ligand is the sensitive part, and its conditions are in the datasheet. The pH 2.5 strip belongs to the short-term window: keep it to the 3–5 CV it needs and re-equilibrate straight after.
Storage and bioburden
Between runs on the same day, equilibration buffer is fine. Overnight and longer, displace it and store cold at 2–8 °C in the solution your medium specifies — for MonoCore™ affinity capsules that is 20 % ethanol with 10 % glycerol, 50 mM Tris and 150 mM NaCl at pH 7.6, about 5 CV; many packed media use 20 % ethanol alone. A capture column left in neutral buffer with residual feed protein is a growth medium, and bioburden ruins more columns than chemistry does. Flush the storage solution out completely before the next run and re-equilibrate until UV and conductivity are flat.
What to measure
Yield from A280 with the product's extinction coefficient; aggregate by size exclusion, both after neutralisation and at the end of any low-pH hold; host-cell protein and residual DNA by their ELISAs; leached Protein A by the acid-dissociated assay above. Recording back pressure once per cycle at a fixed flow rate costs nothing and gives the earliest warning of trouble.
When the run goes wrong
Low yield is two different failures that look alike on the chromatogram. First establish where the product went: re-read the UV trace of the flow-through and the wash. Product there means the load exceeded the capacity at that residence time. Product nowhere means it is still bound — then measure the actual pH of the elution buffer, because a nominal pH 3.0 that measures 3.4 will not release everything, extend the elution to 5 CV, and look at the strip: a large peak at pH 2.5 is the signature of an elution that was too mild.
The other symptoms — aggregate after elution, HCP carryover, rising back pressure, capacity loss over cycles, carryover between runs and leaching — are laid out with the confirming check for each in Protein A column troubleshooting.
Running the cycle on a monolith capsule
The steps do not change; their duration does. MonoCore™ Protein A capsules are run at about 8 MV/min — 15 mL/min on the 1.8 mL format, 45 mL/min on the 5.4 mL — which puts the residence time in the range of seconds rather than minutes, at up to 0.8 MPa (8 bar) before the capsule. Convert that to your own volumes with the residence time calculator.
The capsule has UNF 10/32 coned ports for 1/16″ tubing and runs on a standard FPLC system without re-plumbing, which makes a side-by-side against your current column straightforward. Background on the format is in monolith chromatography; binding capacity, with the conditions it applies to, is in the MonoCore™ Protein A datasheet.
Protein A or Protein G?
Protein A is the default for human IgG1, IgG2 and IgG4 and for Fc-bearing formats. It binds human IgG3 and mouse IgG1 weakly, and for those — as for rat, goat and sheep antibodies — Protein G is the better ligand. The species and subclass picture, and what it means for a hybridoma supernatant, is in Protein G capture for murine IgG1.
One practical note: native Protein G also binds serum albumin. Recombinant Protein G ligands have the albumin-binding domain removed, which matters whenever the feed contains serum.
Step six is the one most often questioned, because the acid is what damages sensitive formats. If your molecule does not survive it, the alternatives — milder ligands, amino-acid and arginine buffers, salt or chelation, detergent, light — are set out side by side in how to elute from Protein A without acid.
Frequently asked questions
Do I need to adjust the feed before loading a Protein A column?
Set the pH to 7.0–7.4. Clarify the harvest by centrifugation or depth filtration; whether a 0.22 µm filtration is needed right before the column depends on the medium — it matters most for a packed bed and least for an open-channel capsule. You do not normally need to adjust conductivity or exchange buffer, because Protein A binds the Fc rather than charge.
How much antibody can I load?
About 70–80 % of the capacity you measured on your own feed at the residence time you intend to run. A capacity figure transfers between media only when the conditions behind it do — which is why the MonoCore™ datasheets state molecule, buffer, breakthrough level and residence time alongside the number, and why the margin is worth keeping.
Can I load at a higher flow rate?
On a packed bed only to a point: the antibody has to diffuse into the bead, so shortening the residence time costs capacity, and the bed has a pressure limit of its own. In a monolith the target is carried past the ligand by the flow, which is why capsules run at residence times of seconds. Either way, confirm it with a breakthrough curve instead of assuming.
Still an open question?
Tell us the molecule, the feed volume and the system you run it on, and we will say whether a MonoCore™ capsule is the right starting point — or whether it is not.