# Pipetting Errors in Competency Tests: Causes and Fixes The most common cause of a failed pipetting competency test is a tip that does not seal properly, combined with a skipped pre-wet, and the first thing to check is whether the tip fits the cone without rocking and whether a droplet stays behind on the tip wall after delivery. Most operators who fail a competency check blame the instrument. In practice, a small number of technique faults and a small number of mechanical faults account for nearly every failure, and the two are easy to tell apart if you test in the right order. A pipetting competency test asks a narrow question: can this person, with this instrument, deliver the stated volume accurately and reproducibly under defined conditions? Everything that changes the volume between the stock bottle and the target tube is fair game. That includes tip fit, plunger control, immersion depth, the angle of the tip, the temperature of the liquid, the humidity of the air cushion inside the barrel, and the state of the piston seal. It also includes errors in the test itself, such as a balance that drifts or a weigh boat that evaporates. This guide works from the most common causes to the rarest, explains the physical mechanism behind each one, gives a diagnostic sequence you can run in about twenty minutes, and ends with prevention practices that turn a single test into a lab-wide system. Pipetting technique and pipette calibration are treated as separate problems with separate fixes, because confusing the two wastes time and money. ## What a Competency Test Actually Measures Before diagnosing a failure, be clear about which quantity failed. **Accuracy** is the closeness of the delivered volume to the target volume. Its error is systematic, meaning it repeats in the same direction. A pipette that delivers 98.0 µL every time it is set to 100 µL is inaccurate. **Precision** is the closeness of replicate deliveries to one another. Its error is random and appears as scatter. A pipette that delivers values between 95 µL and 105 µL around a correct mean of 100 µL is imprecise. These two failures have different causes. Systematic error points to calibration drift, a volume setting that does not match the displayed value, a mis-seated tip, a systematic temperature bias, or a consistent technique fault such as always dispensing without the blowout. Random error points to inconsistent technique, an unstable seal, bubbles, variable immersion depth, or a worn piston seal that seals intermittently. ### The Three Test Formats Most labs use one of three formats. Each has blind spots. **Gravimetric testing** weighs ten replicate deliveries on an analytical balance and converts mass to volume. It is the reference method and the basis for calibration. Its blind spot is that it tests water, not the buffer, serum, or master mix the person actually pipettes. **Photometric or colorimetric testing** uses a dye dilution read on a spectrophotometer or plate reader. It is fast, works well for multichannel devices, and captures some liquid-handling behavior that weighing misses. Its blind spot is that absorbance depends on path length, meniscus, and plate quality. **Direct observation with a checklist** scores technique against a written standard. Its blind spot is that a confident operator can perform badly and still look smooth. A strong program uses gravimetric data for the number and observation for the method. Neither one alone proves competence. ### The Test Liquid Is Never the Real Sample Competency tests are usually run with water or a dilute dye, because those are safe, cheap, and reproducible. Real samples are not. Serum has a viscosity several times that of water. Lysis buffers contain detergents that foam. Ethanol-based washes evaporate from the tip within seconds. A pipette that measures water perfectly can still underdeliver a viscous reagent by a noticeable margin, and a test result will not reveal it. Treat a passing gravimetric score as evidence about aqueous volumes within the tested range. Treat it as no evidence at all about volatile solvents, glycerol-containing stocks, or whole blood. ## The First Thing to Check: Tip Seal and Pre-Wet Start with the tip, not the pipette. The tip is the cheapest part of the system, the most frequently defective, and the easiest to rule out. ### The Seal Test Fit a tip as you normally would. Hold the pipette vertically and look at the junction between the tip collar and the tip cone. There should be no visible gap and no rocking when you apply light lateral pressure. If the tip wobbles, the tip is wrong for that model, the cone is worn or contaminated, or the tip was pushed on with a partial twist that deformed the collar. Two quick functional checks confirm the seal. The **dye test** fills the tip with a strongly colored solution such as diluted bromophenol blue or a food dye, holds the pipette horizontally for ten seconds, and looks for a drip or a creeping meniscus. A drop forming at the orifice within ten seconds indicates a leak. The **hold test** aspirates the nominal volume, holds the tip vertically in air for ten seconds, and returns the liquid to the source. A visible volume loss in the tip, or a bubble that appears at the top of the liquid column, means the seal is not holding. Both tests are qualitative, and both catch the same class of problem within a minute. ### The Pre-Wet Pre-wetting means aspirating and dispensing the same liquid three to five times before the measuring aspiration. It is a standard step in most written pipetting protocols and it is the step most often dropped under time pressure. The mechanism is humidity. An air-displacement pipette works by moving a trapped column of air. When a dry tip enters liquid, water vapor from the sample saturates that air cushion, and the evaporation that follows during dispensing slightly reduces the delivered volume. After three cycles the air cushion is already near saturation, so the volume change between the last pre-wet and the measured delivery is small. Skipping the pre-wet leaves the full evaporation loss inside the measured aliquot. The effect is small for a 1000 µL pipette and comparatively large for a 10 µL pipette, because the air cushion is a larger fraction of the delivered volume at small settings. This is one reason small-volume failures are so common. Pre-wetting is irrelevant for positive-displacement pipettes, which have no air cushion. Do not add it to a positive-displacement protocol. ## Pipetting Errors in Competency Tests: Causes, From Most Common to Rarest ### 1. Poor Tip Fit and Worn Tip Cones A tip that does not seal is the leading cause. The failure can originate in four places. **Mismatched tips.** Tips are molded for a specific shaft diameter and cone taper. Universal tips fit many models but not all of them well. A tip from an unverified supplier can sit a millimeter too high on the cone, leaving an annular gap that leaks slowly under load. **Worn or scratched cones.** Repeated tip fitting polishes the cone surface, and aggressive handling scratches it. A scratched cone leaks even with correct tips. **Contamination.** Dried salt from a previous buffer, or protein from a previous serum sample, forms a film that prevents a tight seal. This is common on shared pipettes in teaching labs. **Oversized tips on small cones.** A 200 µL tip forced onto a 10 µL shaft seats too deeply, deforms, and can split at the collar. The fix is straightforward. Verify the tip catalog number against the pipette model's published compatibility list. Clean the cone with 70% ethanol and a lint-free swab, then dry it fully. If the wobble persists with the correct tip, the cone needs replacement, which is a service task, not a bench task. ### 2. Plunger Control: First Stop, Second Stop, Blowout This is the second most common cause and the most common pure technique fault. An air-displacement pipette has two plunger stops. Aspiration happens at the **first stop**, which corresponds to the set volume. Dispensing happens to the **first stop** and then, for volumes that require it, to the **second stop**, which pushes an extra bolus of air through the tip to clear the residual film. Four errors recur. **Aspirating past the first stop.** Pressing to the second stop before aspiration pulls extra air into the barrel, then releases it during dispensing. The delivered volume is high by roughly the blowout volume, and the error is inconsistent because it depends on how far the operator pressed. **Dispensing only to the first stop and skipping the blowout.** A film of liquid remains on the inner wall of the tip. For aqueous solutions this residual is small at large volumes and significant at small volumes. Skipping the blowout biases delivery low, and the bias grows as the volume shrinks. **Releasing the plunger too fast.** A rapid release creates a pressure pulse. Liquid rushes into the tip, splashes onto the inner wall above the intended fill line, and can aerosolize. The measured volume becomes variable. **Resting a finger on the plunger between deliveries.** The plunger drifts slowly under finger pressure, so the volume changes without any visible action. The fix is a scripted plunger sequence that the operator rehearses until it is automatic. Press to the first stop, immerse, aspirate slowly, pause one second, withdraw, dispense to the first stop, pause one to two seconds, press to the second stop, hold, withdraw the tip while still at the second stop. ### 3. Skipping the Pre-Wet Covered above in mechanism. In competency tests specifically, the pre-wet is often omitted because the operator assumes the test begins at the first measured delivery. Write the pre-wet into the test protocol explicitly so that it is scored, not assumed. ### 4. Immersion Depth and Pipette Angle **Depth.** The tip should enter just below the surface. Common guidance is 1 to 2 mm for small volumes (1 to 100 µL), 2 to 3 mm for mid volumes (100 to 1000 µL), and 3 to 6 mm for large volumes above 1 mL. Deeper immersion does two harmful things. It raises hydrostatic pressure at the orifice, which forces extra liquid into the tip, and it wets the outside of the tip, which carries a film that later drips or runs down the barrel. **Angle.** Keep the pipette within about 20 degrees of vertical during aspiration. Tilting changes the hydrostatic head and the geometry of the air path. During dispensing, a common and acceptable practice is to deliver against the vessel wall at a slight angle, tip touching the wall, so the liquid flows down rather than forming a hanging drop. **Withdrawal.** Withdraw the tip before releasing the plunger, or release while still immersed and risk drawing a droplet back onto the tip exterior. ### 5. Aspiration and Dispense Speed A smooth aspiration over roughly one to two seconds is standard for aqueous liquids. Snapping the plunger up produces turbulence. Bubbles form at the orifice, get drawn into the tip, and compress during dispensing. The result is a low delivery with high scatter. The same applies to dispensing. Pushing the plunger down hard creates a jet that splashes out of the tube or sprays up the vessel wall. A steady push with a one to two second pause at the first stop before the blowout is more accurate. ### 6. Temperature Mismatch and Hand Heat Two thermal effects matter. **Liquid temperature.** Water expands by roughly 0.02% per degree Celsius near room temperature. A 1 mL aliquot taken from a refrigerator stock and then pipetted before equilibration delivers a slightly different volume than the balance-based calibration assumes. The error is small per degree but accumulates across a large dilution series. **Hand heat.** Holding a pipette in a closed hand for minutes warms the barrel and the trapped air cushion. The air expands, the plunger creeps, and drops appear at the tip. Operators who keep a pipette in hand while talking, or who hold it horizontally during a cell culture workflow, see drop formation that they cannot explain. Keep reagents at room temperature unless the protocol says otherwise, hold pipettes loosely and only when delivering, and set the instrument down between uses. ### 7. Calibration Drift and Mechanical Wear A pipette that was accurate a year ago may not be accurate today. Wear is progressive, so a slow drift is invisible unless you measure it. Common wear points are the piston seal, the O-ring, the spring, and the volume-setting mechanism. Seals harden and take a compression set. Springs lose tension, so the first stop becomes soft and the operator cannot feel it reliably. The volume screw can drift on models without a locking mechanism, particularly after repeated adjustment over a narrow range, such as constant switching between 2 µL and 5 µL. The clue is a bias that appears on the balance but not in the technique checklist. A pipette that is consistently 2% low across ten replicates with tight scatter has a calibration or wear problem. A pipette with a correct mean and wide scatter has a technique problem. ### 8. Liquid Properties: Volatile, Viscous, Foaming, and Dense Water is the calibration standard and the worst possible proxy for some reagents. **Volatile liquids** (ethanol, methanol, chloroform, acetone) evaporate from the tip during aspiration and dispensing. The loss is continuous from the moment liquid enters the tip. Delivered volume is low, and the size of the error depends on how long the operator holds the tip before dispensing. Use reverse pipetting or a positive-displacement pipette, work quickly, do not pre-wet with the volatile solvent unless the protocol says so, and never blow out (the blowout pushes vapor out and encourages further evaporation). **Viscous liquids** (glycerol stocks, 50% sucrose, some mounting media) drain slowly from the tip wall. The liquid keeps moving after the plunger stops, so the delivered volume depends on how long the operator waits. Use a slow aspiration, a two to three second pause after aspiration, and a longer pause at the first stop before the blowout. **Foaming liquids** (detergent-containing lysis buffers, protein-containing solutions) trap air. Aspirate slowly and never from the foam layer. Let the foam settle before pipetting, or spin the tube briefly. **Dense liquids** (high-salt buffers, CsCl solutions) create a larger hydrostatic effect at the tip orifice. Reduce immersion depth. ### 9. Multichannel Failures A multichannel pipette adds failure modes that a single-channel test will never catch. Channels leak independently, so one bad channel out of eight produces a plate with one bad column and seven good ones. Tips seat unevenly because the tip tray is warped or the operator presses on one corner. Channel spacing drifts after a drop, and tips no longer align with wells. Test each channel. Load a full rack of tips, aspirate a dye solution from a reservoir, and dispense into a flat-bottomed tray or a plate. Photograph the result. Uneven intensity across channels identifies the leaking or mis-seated channels directly. Repeat the gravimetric test with a single tip position if the instrument allows it, or weigh a pooled delivery from all channels and then from channels individually. ### 10. Balance and Recording Errors in the Test Itself Some competency failures are measurement failures, not pipetting failures. **Balance readability.** For a 10 µL delivery, the weighed mass is about 10 mg. A balance reading to 1 mg cannot resolve a 0.2 mg difference, so the scatter you see is the balance, not the operator. Choose a balance whose readability is at least ten times finer than the smallest difference you need to detect. For volumes below 100 µL, that generally means a readability of 0.01 mg. **Evaporation from the weigh vessel.** Water evaporates from an open vessel at a rate that depends on temperature, humidity, and air movement. A 20 mg loss over a long series is common in a dry room. Use a covered vessel with a small entry port, work quickly, and keep the balance draft shield closed. **Density and buoyancy.** A milligram of water is not a microliter of water. At 20 °C, water density is 0.9982 g/mL, and the air displaced by the water also has mass. The combined correction is usually expressed as a Z factor of about 1.003 µL per mg near 20 °C at sea-level pressure. Use the Z table that matches your measured temperature and pressure rather than assuming 1.000 µL/mg. **Recording errors.** Transposed digits, weighing the tip instead of the vessel, zeroing the balance with the vessel missing, and failing to tare between replicates all appear in real competency files. ### 11. Rare Mechanical and Environmental Causes **Static electricity.** In dry winter air, plastic tips and plastic tubes accumulate charge. Droplets cling to the tip wall instead of falling. An ionizing bar or a humidified room solves it. **Vibration.** A centrifuge, a vortexer, or a pump on the same bench makes the balance unstable and makes the operator's hand unsteady. Move the test to a stable bench. **Cracked tip cone or barrel.** A hairline crack admits air. It is invisible on inspection and obvious on the leak test. **Contaminated barrel.** Aspirated liquid that reached the barrel grows organisms or dries into crystals that interfere with the piston. A pipette that has taken up liquid into the barrel must be decontaminated and serviced, not just wiped. **Autoclave damage.** Only autoclavable models survive a cycle. Others deform quietly and lose accuracy. **Volume setting beyond the range.** Turning the adjustment knob past the maximum volume on a mechanical pipette can damage the mechanism or decalibrate the screw. Set volumes by dialing down from a higher value, not up from a lower one. ## Troubleshooting Table | Symptom | Likely cause | How to confirm | Fix | |--|--|--|--| | Mean volume low at all settings, tight scatter | Calibration drift, worn seal, or skipped blowout | Compare error across ten replicates at nominal and at 10% of nominal. If both biased low, it is systematic | Service or recalibrate. Retrain the blowout step | | Mean volume high, tight scatter | Volume setting wrong, tip seated too deep, aspirating past first stop | Check the displayed volume against a second pipette. Observe the plunger during aspiration | Reset the volume, refit the tip, rehearse the first-stop sequence | | Correct mean, wide scatter | Unstable seal, bubbles, variable immersion, rushed plunger | Dye test and hold test. Watch for bubbles entering the tip | Correct tips, slow the aspiration, standardize depth | | Drops form at the tip while idle | Leaking seal, hand heat, volatile liquid | Hold test at nominal volume for ten seconds | Replace tip or seal, set the pipette down between deliveries, use reverse pipetting for volatiles | | One channel of a multichannel is off | Leaking channel, warped tip tray, mis-seated tip | Dye delivery into a flat tray, one row per channel | Replace the channel O-ring, use a fresh tip rack, service the head | | Passes at nominal, fails at 10% of nominal | Technique fault amplified at small volumes, or wear that only shows at low travel | Run both volumes in the same session with the same operator | Retrain small-volume technique. Service if both volumes fail | | Weight drifts downward across the series | Evaporation from the weigh vessel | Weigh the same sealed vessel repeatedly with no pipetting | Cover the vessel, shorten the series, control the room | | Weight drifts upward across the series | Balance drift or draft | Zero the balance and reweigh a fixed check weight mid-series | Re-zero, close the draft shield, move off the vibrating bench | | Volume changes when the same operator repeats the test an hour later | Hand heat, fatigue, or room temperature change | Log room temperature and time on task | Shorten sessions, control the room, schedule tests early | | Operator passes with a borrowed pipette but fails with their own | Instrument-specific fault | Swap instruments and repeat | Service the failing instrument. Do not assume the operator is at fault | ## A Diagnostic Sequence You Can Run in Twenty Minutes Follow the steps in order. Stop as soon as the cause is identified. 1. **Confirm the test conditions.** Room temperature recorded, balance leveled, draft shield closed, check weight within tolerance, fresh weigh vessel, Z factor matched to the day's temperature and pressure. 2. **Confirm the tips.** Catalog number matches the pipette model's compatibility list. Open a fresh rack. Discard any tip with a visible split at the collar. 3. **Run the dye test at the nominal volume.** Ten seconds horizontal. Any drip is a seal failure. Fix the tip first, then the cone. 4. **Run the hold test.** Ten seconds vertical. A bubble at the top of the liquid column is a leak. 5. **Observe one delivery in slow motion.** Watch the plunger. Confirm first stop for aspiration, first stop then second stop for dispensing, and a pause at the first stop before the blowout. 6. **Weigh ten replicates at nominal volume.** Compute mean, standard deviation, and percent error. Compare against the published maximum permissible error for that model and volume. 7. **Weigh ten replicates at 10% of nominal, or at the smallest volume the operator routinely uses.** This is the step that catches most failures. A pipette can be inside specification at nominal and well outside it at the low end. 8. **Repeat the low-volume series with a deliberate pre-wet and a slow aspiration.** If the error drops into range, the original failure was technique. 9. **Repeat with a second pipette of the same model, using the same operator and the same tips.** If the second instrument passes, the first one needs service. 10. **Repeat with a second operator on the first instrument.** If the second operator passes, the first one needs retraining. Steps 9 and 10 separate instrument faults from operator faults in about five minutes and are the two steps most often skipped. ## How to Fix Pipetting Errors in Competency Tests: Solutions That Hold ### Fixes at the Instrument Level **Recalibrate or service.** Any pipette with a systematic bias outside the published tolerance at either tested volume should be removed from service and either adjusted in-house with a documented procedure or sent to the manufacturer. Do not apply an arithmetic correction factor to routine deliveries. A pipette that needs a correction factor is a pipette that needs service. **Replace seals on schedule.** Seals are consumables. High-use pipettes in a busy molecular lab benefit from seal replacement at each full service, not only when a leak appears. **Retire overlapping instruments.** Using a 200 µL pipette for 10 µL deliveries is a common cause of low-volume error because the instrument is operating at the very bottom of its range, where the relative error is largest. Assign a dedicated small-volume pipette to the small-volume steps. **Label every pipette.** A unique identifier on the barrel, matched to a calibration record, lets you trace which instrument produced which result. Unlabeled pipettes cannot be audited. ### Fixes at the Technique Level **Write the technique down.** A one-page standard operating procedure that names each step, the first and second stop, the pre-wet, the immersion depth for each volume range, the angle, the pause times, and the tip type. Train against that document, not against a colleague's habits. **Rehearse with a practice dye.** Aspirating colored water and dispensing into a clear tube teaches the blowout and the pause far faster than weighing does, because the operator can see the residual film. **Standardize the pause.** One second after aspiration, one to two seconds at the first stop before the blowout. The pause is not decorative. It allows the film on the tip wall to drain to the orifice. **Separate aspiration from dispensing in time.** Operators who aspirate a full rack of tips and then dispense slowly across a plate accumulate evaporation loss in the early tips. Aspirate and dispense in matched sets. **Correct the small-volume habits.** At volumes below 20 µL, the tip should touch the vessel wall during delivery so the droplet transfers by contact rather than falling. The pause before withdrawal matters more, not less. ### Fixes at the Test Level **Test the volumes actually used.** If the operator routinely pipettes 2 µL of template and 18 µL of master mix, test at 2 µL and 20 µL, not only at 100 µL. **Test with the tips used in the assay.** A passing score with a premium low-retention tip and a failing score with the standard tip used in production tells you which tip to buy. **Include a liquid class beyond water.** A dye-and-spectrophotometer check, or a gravimetric test with a viscosity-adjusted correction, will catch failures that water cannot. **Record everything.** Date, operator, instrument identifier, tip lot, room temperature, pressure, balance identifier, volumes tested, all ten replicate weights, mean, standard deviation, [coefficient of variation](/blog/research-skills/how-to-calculate-coefficient-of-variation), percent error, pass or fail, and the corrective action if any. ## Pipette Calibration: What Belongs in Your Program Calibration and competency testing are different activities with different owners. Calibration establishes that the instrument delivers the stated volume. Competency testing establishes that the person can operate it. **In-house gravimetric checks** are inexpensive and should be routine. Ten replicate weighings at nominal volume and at 10% of nominal, run at the temperature and pressure actually present, converted with the Z factor for those conditions. The documentary standard for piston-operated pipettes, ISO 8655, defines the maximum permissible errors that most manufacturers publish, and the published table for the exact model is the correct acceptance criterion. An internal acceptance limit tighter than the published one, such as half the published error, is a defensible policy for critical assays. **Full service** by the manufacturer covers seal replacement, lubrication, and adjustment beyond what a bench check can do. Base the interval on use. A pipette used daily for hundreds of deliveries needs attention far more often than one used weekly. **Schedule** the checks so that no instrument goes longer than a defined interval without a gravimetric record. Quarterly checks with an annual full service is a common framework, adjusted for how much the instrument is used and how tight the assay tolerance is. **Documentation** should be able to answer three questions for any date in the past year: which pipette delivered the volume, was it inside specification on that date, and who performed the check. ### Working Through the Numbers A worked example makes the arithmetic concrete. Two series were weighed on a balance with 0.01 mg readability at 20 °C and 1013 hPa, using a Z factor of 1.003 µL/mg. | Setting | Mean mass (mg) | Converted volume (µL) | Percent error | SD (mg) | CV | |--|--|--|--|--|--| | 100 µL | 99.80 | 100.10 | +0.10% | 0.258 | 0.26% | | 10 µL | 9.551 | 9.58 | -4.2% | 0.046 | 0.48% | The same instrument, the same operator, the same tips. At 100 µL the result is excellent. At 10 µL the precision is still good (0.48% CV) but the accuracy fails, because the bias is 4.2% low. Commonly published tolerances at 10% of nominal volume for a 100 µL pipette sit around 2 to 3 percent, so this instrument is outside specification at the low end and should be serviced or the technique corrected before retesting. The lesson generalizes. Precision without accuracy is a systematic fault, and systematic faults at low volume usually mean wear, a mis-seated tip, or a skipped blowout. Scatter without bias means technique. ## Prevention: Turning a Single Test into a System **Train on the first day, not the first failure.** New staff should demonstrate technique with a dye before they touch a critical reagent. A five-minute observation prevents months of quiet error. **Reassess on a fixed interval.** Many accredited laboratories reassess pipetting competence at defined intervals, commonly every 6 to 12 months, and after any documented outlier, instrument change, or long absence from the bench. **Run a monthly mini-check.** Ten weighings at the smallest routine volume takes ten minutes and catches drift before it reaches a result. **Keep a pipette map.** A wall chart listing each instrument, its volume range, its assigned bench, its last calibration date, and its next due date removes the guesswork. **Match instrument to volume.** Never use a pipette below 10% of its maximum volume for a critical delivery. A 1000 µL pipette set to 20 µL is operating at 2% of range, where relative error is largest and least predictable. **Control the environment.** Stable room temperature, no direct sunlight on the balance, no vibration from adjacent equipment, and humidity in a comfortable range for both people and static control. **Audit the records.** A calibration file that lists pass or fail without raw replicate weights cannot be used to diagnose a trend. Keep the weights. **Address ergonomics.** Elbow support, a relaxed grip, and a bench height that keeps the shoulder low reduce the fatigue tremor that shows up as scatter late in a long session. ## Common Mistakes and Limitations **Testing only at nominal volume.** The most common design flaw in a competency program. Most real errors live at the low end of the range. **Using a single replicate.** One weighing cannot distinguish accuracy from precision and cannot detect an unstable seal. **Assuming a service fixes technique.** A freshly calibrated pipette in the hands of an operator who skips the blowout will still fail the next test. **Testing on a different tip than the one used in production.** The result then describes a system nobody uses. **Ignoring the balance.** Reported failures that turn out to be balance drift, evaporation, or a missing Z correction are common and waste time. **Treating a pass as permanent.** Pipetting skill degrades with disuse, and instruments drift with use. A single result is a snapshot. **Confusing the test with the assay.** A gravimetric pass on water does not certify performance with serum, ethanol, or detergent-containing buffers. If the assay uses those liquids, add a liquid-specific check. **Overlooking the multichannel head.** A multichannel instrument that passes a pooled gravimetric test can still have a single leaking channel. Channel-by-channel verification is required. **Failing to record corrective action.** A file that shows a failure and a later pass, with no note about what changed, cannot demonstrate that the problem was fixed. **Limitations.** Individual results need interpretation by a supervisor or quality officer who knows the assay tolerance, the instrument history, and the operator's training record. A general guide can point to the likely cause and the right order of checks, but the acceptance criteria come from the instrument's published specification and the laboratory's own policy. ## Safety Notes **Aerosols and infectious material.** Never pipette by mouth. Aspirate and dispense slowly to avoid splash and aerosol, particularly with serum, respiratory samples, and any material handled under containment. Perform aerosol-generating steps in a biosafety cabinet. **Volatile and hazardous solvents.** Methanol, chloroform, phenol, and guanidinium-based lysis buffers carry serious inhalation and skin hazards. Work in a fume hood, wear gloves rated for the solvent, and use positive-displacement or reverse pipetting to reduce evaporation. Read the safety data sheet before the first use, not after a spill. **Never let liquid enter the barrel.** If a sample is drawn into the barrel of an air-displacement pipette, stop using the instrument. Contaminant inside the barrel is an aerosol and a cross-contamination risk. Decontaminate the exterior with a fresh 10% dilution of household bleach (about 0.5% sodium hypochlorite) followed by 70% ethanol, then send the instrument for service. **Autoclaving.** Only autoclavable models survive a sterilization cycle. Autoclaving a non-rated pipette deforms the seal and the barrel quietly, and the instrument will fail a later check without any obvious cause. **Chemical compatibility.** Tips and pipette bodies have different resistance to acids, bases, and solvents. Check compatibility before aspirating anything corrosive. ## Frequently Asked Questions ### What is the most common cause of pipetting errors in a competency test? A tip that does not seal properly, usually because it is the wrong model for the pipette or because the tip cone is worn or contaminated. A skipped pre-wet makes the same failure worse. ### Can a pipette pass calibration and still fail a competency test? Yes. Calibration describes the instrument on water at defined conditions, and competency describes the operator. A perfectly calibrated pipette fails in the hands of someone who aspirates past the first stop or skips the blowout. ### How many replicates should a gravimetric competency test include? Ten is conventional. Fewer replicates cannot separate random scatter from a systematic bias, and they cannot reveal an intermittent seal fault. ### Should pre-wetting be scored during the test? Yes. Write it into the protocol as a required step so that it is observed and recorded rather than left to habit. Pre-wetting is irrelevant for positive-displacement pipettes. ### Why does my volume come out high on the balance? Common reasons are aspirating past the first stop, a volume setting that does not match the display, a tip seated too deeply, a missing Z-factor correction, or a liquid that was colder than the balance area and warmed after delivery. ### How often should pipettes be checked? Base the interval on use. A monthly or quarterly gravimetric check with an annual full service is a common framework, tightened for instruments on critical assays and relaxed for occasional-use pipettes. ### Do I need to test at more than one volume? Yes. Test at nominal volume and at the smallest volume the operator routinely uses. Many instruments sit inside specification at the top of their range and fail at 10% of nominal. ### What is the difference between forward and reverse pipetting in a test? Forward pipetting aspirates at the first stop and dispenses with a blowout. Reverse pipetting aspirates past the first stop and dispenses only to the first stop, leaving residual liquid in the tip. Reverse pipetting is preferred for volatile, foaming, and viscous liquids and should be tested as its own technique.