Aseptic technique is the set of handling practices that keep micro-organisms out of a solution which will not be sterilized again after it is made up. For a research peptide stock that window runs from the first opening of the vial to the last aliquot going into the freezer, and whatever enters during it stays in.
Why it matters for a stored solution
Micro-organisms growing in a peptide solution secrete proteases that cleave the sequence, so the compound degrades and the concentration falls below the label. None of that is recoverable.
| Consequence | Cause |
|---|---|
| Peptide loss | Secreted proteases cleave the sequence |
| pH drift | Metabolic acids in an unbuffered stock |
| Turbidity | Cell density scattering light in an absorbance reading |
| Loss of a culture | Material transferred into cell culture carries the organisms with it |
| Endotoxin | Gram-negative lysis releases lipopolysaccharide, which is dissolved and stays |
A stock can be heavily contaminated and still look clear. A bacterial suspension generally has to reach 107–108 CFU/mL before the eye reads it as turbid, so a solution inspected against a dark background can carry a population large enough to have altered it. Inspecting for clarity checks for particulate and precipitation; it says nothing about sterility.
A bacteriostatic diluent does not close that gap. Bacteriostatic water is sterile water containing benzyl alcohol at 0.9 % (w/v); it inhibits the growth of organisms introduced into the solution, does not reliably kill an established population, and will not make a contaminated stock clean. It also puts an organic co-solvent into every downstream dilution. See peptide solubility for what that fraction does to an assay, and reconstituting lyophilized peptides for choosing between diluents.
Cold storage slows growth rather than stopping it, and freezing does not sterilize: a population frozen at −20 °C is still there on thaw. Contamination adds to the hydrolysis, oxidation, deamidation and aggregation covered in storing research peptides rather than replacing them.
Preparing the work area
Clear the bench down to the items in use, wipe the surface with 70 % alcohol, and work in still air. Every object left out is a surface for particles and one more thing to reach over.
The standard bench disinfectant is isopropanol or ethanol at 70 % in water, and the water fraction is what makes it work: protein denatures more readily in the presence of water, so absolute alcohol is the weaker disinfectant. The usual active band is roughly 60–90 %. Wipe firmly, leave the surface wet, and let it evaporate.
| Open bench | Laminar-flow cabinet | |
|---|---|---|
| Air supply | Room air, and whatever is moving through it | HEPA-filtered air swept across the work surface |
| What it protects | Nothing, on its own | An open vessel, from room air |
| Before starting | Clear and wipe the surface | Run 5–15 min, then clear and wipe; leave the grilles unobstructed |
| Usual failure | Traffic, a door opening, a hand passing over an open vial | An item placed between the filter and an open vessel |
Filtered air in a cabinet is clean only until something interrupts it, so nothing goes between the filter face and an open container. An open bench is workable for a stock that will be filtered and frozen, and not adequate for one going into cell culture. On a bench, move slowly, open one container at a time, and pause while a door swings.
Gloves out of the box are clean, not sterile. Swab gloved hands with 70 % alcohol before starting and again after touching anything outside the cleared area.
Vial and septum
The rubber septum is what keeps room air out of a stored stock between withdrawals, and it stops being an intact barrier as soon as the flip-off cap comes away. Swab it with 70 % isopropanol and let the alcohol dry before every entry, not only the first.
A swab works only if the alcohol has time to act. Alcohol denatures protein, which needs wet contact for seconds rather than an instant, and it has finished acting only once it has evaporated. Wiping and piercing immediately carries wet alcohol through the hole and leaves the septum much as it was.
Coring is the other septum failure. A needle driven straight through rubber can punch out a plug that drops into the solution, where it is both particulate and a carrier for whatever sat on the septum face. Entering at about 45° with the bevel up and rotating to vertical as the tip passes reduces it, and so does the finest needle that will carry the volume into the filter or tube.
Count the entries. Every pierce leaves a track, resealing degrades with each one, and a septum entered a dozen times is no longer keeping room air out. Plan how the stock will be divided into aliquots before the first entry, and do not re-pierce the same spot. Pressure balance across a septum, and letting a cold vial reach room temperature before it is opened, are covered in reconstituting lyophilized peptides.
Sterile filtration
Passing a stock through a 0.22 µm membrane into sterile labware is sterilizing-grade against bacteria and fungi, and it is the last point at which organisms can be removed from a solution that will not be sterilized again.
| Entity | Retained | Why |
|---|---|---|
| Bacteria | Yes | The organism class the sterilizing-grade rating is defined against |
| Fungi and yeast | Yes | Larger than the rating |
| Mycoplasma | Not reliably | 0.2–0.3 µm, no cell wall, deform through the membrane routinely |
| Viruses | No | Far below the rating; they pass freely |
| Endotoxin | No | Dissolved lipopolysaccharide, not a particle at this scale |
| Degradation products, counter-ion, solvent | No | Anything dissolved passes with the filtrate |
Because nothing dissolved is retained, filtration cleans a solution up without improving the material in it. A filtered stock is not a stock of known purity; that figure comes from reversed-phase HPLC on the lot, covered in peptide purity by HPLC and MS.
Membrane chemistry decides how much peptide survives the step. Nylon and cellulose esters bind peptides appreciably; PES and PVDF sold as low protein binding are the sensible default. Pre-wetting with a small volume of the same diluent saturates part of the membrane’s binding capacity before the stock arrives, which matters most at low concentration, where the fixed quantity adsorbed is a larger fraction of the preparation. Hold-up volume compounds that loss; the reconstitution article works the arithmetic for 4 mm and 25 mm devices.
The filtrate is only as clean as the vessel it lands in, so filter into a sterile tube that is already labeled and open only long enough to receive the liquid, never back into the vial the stock came from. Push slowly and steadily: resistance that climbs sharply means the membrane is loading with particulate, and a membrane forced past its rated pressure can pass what it would otherwise retain. Change the device rather than pushing harder.
Aliquoting
Divide a filtered stock into single-use volumes in sterile labware, in one pass, and never return withdrawn liquid to the parent vial. Every entry after the first is another chance to introduce an organism.
Sterile labware means tubes supplied sterile in sealed packaging, usually gamma-irradiated cryovials or screw-cap microtubes, or glassware autoclaved and dried before use, since damp autoclaved labware dilutes what goes into it by an unmeasured amount. Low-binding polypropylene loses less peptide to the tube wall than untreated polypropylene, and the difference shows at low concentration.
- One tube open at a time. Hold the cap rather than resting it on the bench; if it must be set down, threads upward.
- A fresh tip per tube. A tip that has entered one tube is no longer clean for the next one.
- One direction only. Anything that has passed through a pipette tip or a needle has left the sterile path, and returning it puts whatever it picked up into the whole stock. The same goes for topping up an old aliquot, which leaves the tube with no single preparation date.
- Leave headspace. Aqueous solutions expand on freezing, and a cryovial that cracks in the freezer is a contaminated aliquot by the time it thaws.
Single-use volumes also remove repeated freeze–thaw, the largest avoidable loss in frozen storage, covered in storing research peptides.
Labeling and records
Label a tube at the moment it is filled, not at the end of the session. Five fields make it usable later: compound, lot number, concentration, diluent, and date.
| Field | What it records |
|---|---|
| Compound | Which material is in the tube |
| Lot number | Which manufactured batch it came from |
| Concentration | mg/mL, so a dilution can be calculated without opening anything |
| Diluent | What it is dissolved in, which decides assay compatibility |
| Date | When the solution was made up; its stability clock runs from here |
The lot number is the field that has to travel with every aliquot. Concentration and date describe the tube; the lot describes the material, and it is the only thing connecting a tube at the back of a freezer to the Certificate of Analysis carrying its purity, water content and counter-ion figures. Enter it at verify to check that certificate against our records; reading a Certificate of Analysis covers what each field means.
Aseptic technique leaves no evidence on the tube. Nothing about a frozen aliquot shows whether the septum was swabbed, which membrane the stock passed through, or how many times the parent vial was entered, so the written record is the only place those answers exist. Record the membrane and device size, whether the stock was filtered, the number of entries, and who prepared the aliquots.
Use freezer-rated labels and a solvent-resistant marker. Ordinary marker lifts under the 70 % alcohol used to wipe the bench, and an unlabeled tube is waste regardless of what is in it.
For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure, or prevent any disease.