Analytical methods · step-by-step guide
Peptide signal drops after filtration: is the filter retaining it?
A drop in signal after filtration is not necessarily all peptide adsorbed by the membrane. First align sampling, volume, and detection conditions to know at which step the sample was lost.
First distinguish signal, concentration, and recovered amount
Instrument signal is a measurement reading, concentration is the amount per unit volume, and recovered amount also requires multiplying by the actual collected volume. Comparing only peak heights before and after filtration easily mistakes detection response or volume changes for material loss.
Sample may remain in the membrane or housing, may be adsorbed onto surfaces, may precipitate during handling, or may show a weaker response due to detection conditions. The interactions among membrane material, sample, and solvent need specific verification.
A set of hypothetical data: concentration recovery and total recovery are not the same
Before filtration: 1.00 mL, concentration 100 μg/mL, total 100 μg. After filtration, actually collected 0.80 mL, concentration 80 μg/mL, total 64 μg. Compared by concentration it is 80%, but compared by total it is 64%.
This data only demonstrates how to calculate; it does not represent the true performance of any membrane. If concentration is converted from a detection method, the method's own calibration and applicability must also be reasonable.
How do you make a useful before-and-after comparison?
Split a control portion and a filtered portion from the same homogeneous sample. The control should undergo as much as possible the same waiting time, container transfers, and dilution, omitting only the filtration step under investigation. Record the filtration device, batch, processed volume, actual collected volume, and time.
Keep methods and injection conditions comparable when measuring before and after samples. Detection blanks can help rule out interference from the membrane or solvent. Whether spiked recovery or other controls are needed should be decided by the method and research purpose.
Once results are in, locate the cause first, then change conditions
If the collected volume is low, first check device holdup and operation records; if concentration drops, then look at adsorption, precipitation, or changes in detection response. Both can happen at the same time, and a single percentage cannot pin down the cause.
After changing membrane material, processed volume, or other steps, recovery and analysis performance need to be re-verified. Do not infer from one improved signal that the membrane is suitable for all peptides and all concentrations.
Write “effective recovery” as a record that can be rechecked
The report should clearly state whether recovery is calculated by concentration, peak area, or total, and whether volume and dilution were corrected. This lets others recalculate the same result and connects loss troubleshooting with follow-up experiments.
| Stage | Concentration | Volume | Total |
|---|---|---|---|
| Before filtration | 100 μg/mL | 1.00 mL | 100 μg |
| After filtration | 80 μg/mL | 0.80 mL | 64 μg |
Try judging for yourself: looking only at concentration, is reporting 80% recovery complete?
Not complete. If what matters is how much peptide you got back, the collected volume should also be counted; in this hypothetical example the total recovery is 64%.
Download the Chinese worksheet (TXT; copy and edit as needed)
Avoid assigning every loss to the membrane
Compare a direct aliquot, an aliquot transferred without filtration, and a filtered aliquot. Record collected volumes. Knowing that 100 μg was present initially and 64 μg was recovered identifies a 36 μg shortfall; it does not allocate that amount among containers, the device, precipitate or other steps.
What does an actual adsorption study show?
A 2015 HPLC study of mastoparan X, melittin and magainin 2 reported losses of 90% or more under some container conditions. It studied those peptides and containers; the percentage is not a recovery rate for a particular filter, LL-37 or an NHD product.
A useful filtration record states sample identity, processed and collected volumes, the concentration basis and the control results. “Good recovery” alone cannot be recalculated or applied confidently to another sample.
Adsorption of Cationic Peptides to Solid Surfaces of Glass and Plastic · 2015
Original sources and further reading
Sources checked on 2026-10-13. Findings apply to the models and materials studied. Batch quality requires records for the actual batch.