The standard method for sterilizing pipette tips is autoclaving at 121°C (250°F) for 15 to 20 minutes at 15 psi of pressure. This kills bacteria, viruses, fungi, and bacterial spores, leaving tips ready for sterile work. But the details matter: how you load the autoclave, which cycle you choose, how you dry the tips afterward, and whether your application demands more than basic sterility all affect whether your tips are truly safe to use.
Autoclaving: The Standard Method
Most polypropylene pipette tips are designed to withstand autoclave conditions without warping or degrading. The standard protocol uses a gravity cycle at 121°C (252°F) and approximately 1 bar (15 psi) of relative pressure for 15 to 20 minutes. Gravity cycles rely on natural steam displacement and work well for tips because steam can circulate around and through tip boxes without needing a vacuum to pull air out of dense materials.
If your lab uses a pre-vacuum (high-speed) autoclave, you can run a faster cycle at a higher temperature, up to 135°C (275°F). Pre-vacuum autoclaves actively remove air from the chamber before injecting steam, which improves penetration into wrapped goods and densely packed loads. Either cycle type works for pipette tips, but gravity cycles are the more common choice.
One manufacturer-specific recommendation sets the exposure at just 10 minutes at 121°C, followed by a 5-minute drying phase at 110°C. If you’re using branded tip boxes with specific instructions, follow those. Otherwise, 20 minutes at 121°C is the conservative, widely accepted standard.
How to Load Tips in the Autoclave
Proper loading is the difference between sterilized tips and tips that just got warm. Steam needs direct contact with every surface to kill microorganisms, so overcrowding the chamber blocks the airflow that makes autoclaving work.
Keep tip boxes loosely arranged on the autoclave tray with space between them. If your boxes have lids, leave them slightly ajar or use autoclave-compatible tape to hold them cracked open so steam can enter. Stacking boxes directly on top of each other traps air pockets where steam can’t reach. If you need to run a large batch, it’s better to split it across two cycles than to pack everything in at once.
Place a piece of autoclave tape on at least one box per load. The tape changes color when exposed to the correct temperature, giving you a quick visual confirmation that conditions were met. For higher confidence, place a chemical indicator strip (sometimes called a sterigage) inside the load, not just on the outside. External tape only tells you the surface got hot enough. Internal indicators confirm the steam reached the center of the load.
Validating That Sterilization Worked
Chemical indicator tape is a useful daily check, but it doesn’t prove sterility. The gold standard is a biological indicator containing spores of Geobacillus stearothermophilus, a heat-resistant bacterium that serves as the benchmark for autoclave performance. These spores are killed by a 16-minute exposure at 121°C and 15 psi. If the spores in your indicator vial are inactivated after the cycle, your autoclave reached and maintained true sterilization conditions.
MIT’s autoclave validation program, which is representative of best practices across research institutions, recommends placing a biological indicator vial along with a chemical indicator directly inside the load. You don’t need to run biological indicators with every cycle. Most labs validate their autoclaves on a regular schedule (weekly or monthly) and rely on chemical indicators for routine runs.
Drying and Storage After Autoclaving
Moisture left inside tip boxes after autoclaving is a common source of contamination. Residual water creates an environment where microorganisms can grow, effectively undoing what the autoclave accomplished.
After the cycle finishes, leave tip boxes in the autoclave with the door cracked open for 10 to 15 minutes to let steam escape. Then move them to a clean, dry surface and allow them to cool to room temperature. Don’t seal the lids tightly until the tips are completely dry. Wrapping tips in autoclave bags can slow drying, so if sterility during cooling is not a concern, skip the bag. For applications that require keeping tips wrapped, use autoclave pouches with built-in steam vents. Some protocols specify a dedicated drying cycle at around 110°C for 5 minutes immediately after the sterilization phase, which significantly reduces residual moisture.
Store sterilized tip boxes with lids closed in a clean, low-traffic area. Avoid storing them near sinks, chemical fume hoods, or high-humidity zones. Properly dried and sealed tips remain sterile for weeks under normal lab storage conditions.
Filter Tips May Not Survive Autoclaving
Filter tips (also called aerosol barrier tips) contain a small hydrophobic filter near the top of the tip that prevents aerosols and liquids from contaminating your pipette. These filters are often made of materials that degrade or lose their barrier function when exposed to autoclave temperatures. Some filter tips are explicitly labeled “not autoclavable,” including several common models from major suppliers like Fisher Scientific.
Before autoclaving any filter tips, check the manufacturer’s specifications. If the packaging doesn’t clearly state the tips are autoclavable, assume they are not. Autoclaving a non-compatible filter tip won’t necessarily look different to the eye, but the filter may lose its ability to block aerosols, defeating its purpose. Most filter tips come pre-sterilized from the manufacturer for exactly this reason.
When You Need More Than Sterility
Standard autoclaving kills living organisms, but it does not destroy nucleases, the enzymes that degrade DNA and RNA. If you’re working with RNA, even a fingerprint on a tip can introduce enough RNase to ruin an experiment. Autoclaving won’t help because RNases are remarkably heat-stable.
For nuclease-sensitive work, you need tips that are certified RNase-free and DNase-free. Manufacturers like Thermo Fisher achieve this through e-beam (electron beam) sterilization during production and rigorous lot testing to confirm the absence of nuclease contamination. These tips come pre-sterilized and certified. You cannot replicate this level of decontamination in a standard lab autoclave.
If your application also requires pyrogen-free tips (common in cell culture and endotoxin-sensitive assays), those must also be purchased pre-certified. Autoclaving does not reliably remove endotoxins from plastic surfaces.
Chemical Alternatives to Autoclaving
When an autoclave isn’t available, chemical disinfection can reduce microbial contamination on pipette tips, though it falls short of true sterilization.
Ethanol at 70% concentration kills most bacteria, fungi, and viruses on contact within seconds to minutes. However, alcohol cannot kill bacterial spores and doesn’t penetrate organic debris effectively. The CDC notes that alcohols are not recommended for sterilization precisely because they lack sporicidal activity. A 70% ethanol rinse is fine for reducing contamination in non-critical applications, but it won’t make tips sterile.
Bleach (sodium hypochlorite) is more potent. A 1:10 dilution of standard household bleach provides roughly 5,000 to 6,000 ppm of available chlorine, which can inactivate even hardy spores like Clostridium difficile within 10 minutes. For general bacterial contamination, concentrations as low as 100 ppm kill common pathogens in under 10 minutes. The drawback is that bleach residue can interfere with downstream experiments, particularly PCR and cell culture. Any tip soaked in bleach needs thorough rinsing with sterile, nuclease-free water before use.
Neither ethanol nor bleach is a substitute for autoclaving when true sterility is required. They’re useful as backup methods or for decontaminating tips before disposal, not for preparing tips for sterile procedures.
How Many Times Can You Autoclave Tips?
Polypropylene pipette tips are designed to handle autoclaving without warping or changing dimensions. According to Eppendorf, autoclavable plastic materials do not swell or change configuration during heat sterilization, so volume accuracy is unaffected. You don’t need to recalibrate pipettes after autoclaving them, and the same principle applies to the tips themselves: a single autoclave cycle won’t alter their fit or performance.
That said, most manufacturers recommend autoclaving tips only once. Repeated cycles can eventually degrade the plastic, and each handling increases the risk of contamination before the tips reach the rack. In practice, pipette tips are inexpensive consumables. Re-autoclaving used tips to save money introduces more risk than it’s worth for most applications. If you’re autoclaving tips, start with fresh, unused tips and treat them as single-use after sterilization.