# Pipetting Organic Solvents and Oils Air-displacement pipettes fail with organic solvents and oils because volatile vapors expand inside the tip and viscous oils cling to the plastic, so the volume you deliver is not the volume you set. The rule of thumb is simple. For aqueous buffers, use your standard air-displacement pipette. For volatile solvents such as hexane, chloroform, or ethanol, or for viscous oils such as mineral oil, switch to positive displacement or a glass syringe, or at minimum use reverse pipetting with heavy pre-wetting of the tip. This article covers the practical side of pipetting organic solvents and oils in a [molecular biology](/blog/careers/molecular-biology) or analytical laboratory. You will learn why ordinary pipetting technique breaks down with these liquids, which tools exist for the job, how the main options compare, and how pipette calibration changes once water is no longer the liquid in the tip. The guidance draws on peer-reviewed method papers and manufacturer-level best practice, and every specific number is tied to its source. ## Why Solvents and Oils Behave Differently From Water Pipettes are calibrated with water by default. Water has a defined density (0.998 g/mL at room temperature), a moderate surface tension, low vapor pressure at bench temperature, and low viscosity. Those properties make it predictable in a pipette tip. Organic solvents and oils violate at least one of those assumptions every time. **Volatility.** Solvents such as hexane, diethyl ether, acetone, and chloroform evaporate readily. In an air-displacement pipette, the air cushion between the piston and the liquid is supposed to move the liquid like a lazy piston. When a volatile solvent sits in the tip, its vapor diffuses into that air cushion and raises the pressure. If you aspirate and then pause, vapor expansion pushes liquid out of the tip and you under-deliver. If you dispense slowly, liquid dribbles down the outside of the tip. **Density.** Hexane is light (about 0.66 g/mL) and chloroform is heavy (about 1.49 g/mL). A pipette calibrated for water will over-deliver a low-density solvent and under-deliver a dense one, unless the instrument is adjusted or you use a system that handles the liquid directly. **Viscosity and wetting.** Oils, glycerol, mineral oil, and detergents are viscous. They drain slowly from a tip and a film remains on the inner wall. When that film drains after the piston has finished its stroke, the extra volume delivered varies with your dispensing speed. Viscous liquids are also slow to equilibrate in the tip, so fast aspiration entrains bubbles. **Surface tension and wetting behavior.** Some solvents wet polypropylene aggressively and creep up the tip, forming drops that fall unexpectedly. Others bead up and give an unstable meniscus. Recent materials research exploits exactly this behavior at the opposite extreme. Superoleophilic, chemically resistant coatings on meshes and cloth let oils and chlorinated solvents pass through selectively while repelling water, which shows how strongly wetting behavior governs liquid handling with these fluids [1]. **Plastic compatibility.** Most laboratory tips and pipette shafts are polypropylene, which tolerates many solvents briefly but is not universal. Chloroform, toluene, and similar solvents can extract additives from some plastics over time, and aromatic solvents can swell certain polymers. The tip may look fine while it quietly contaminates your sample. The consequence of all of this is the same. The error you see when pipetting organic solvents and oils with a standard air-displacement pipette is not small random noise. It is a systematic, direction-specific bias that grows with volatility, density mismatch, and viscosity. ## Your Pipetting Organic Solvents and Oils Options There are five realistic ways to move a defined volume of solvent or oil. Each has a place, and the best pipetting organic solvents and oils option depends on the volume, the liquid, and how many transfers you need to make. | Option | How it works | Best for | Main weaknesses | |--|--|--|--| | Air-displacement pipette, reverse pipetting with pre-wetting | Air cushion drives liquid, but you aspirate extra and dispense the set volume | Small, occasional transfers of moderately volatile solvents or oils, for example layered phenol-chloroform work or mineral oil overlays | Systematic error remains, vapor can still drip, precision is poor for very volatile solvents | | Positive-displacement pipette | Piston sits directly in the tip and pushes the liquid | Accurate transfer of volatile solvents, viscous oils, and dense liquids such as chloroform | Capillary piston tips are expensive, single-vendor, and single-use | | Glass syringe or microsyringe | A sealed plunger displaces liquid from a glass barrel | Repetitive metering of the same solvent volume, chlorinated solvents, oils, GC-level aliquots | Slower, needs care to avoid needlestick and plunger wear, not for multichannel work | | Repipettor or bottle-top dispenser | A plunger or pump dispenses from the reagent bottle through a chemically resistant valve | Dispensing the same solvent volume many times, for example hexane additions in extraction workflows | Less accurate at small volumes, valve must be checked for chemical compatibility | | Direct-transfer workflow | Design the protocol so the solvent phase is transferred once, with a tool chosen for that single step | Extraction-based assays where the organic phase feeds an analytical instrument | Protocol redesign effort, still needs a competent transfer step | ### Air-Displacement Pipette With Reverse Pipetting If you only occasionally pipette organic solvents and oils, you can usually make a standard air-displacement pipette work, within limits. Reverse pipetting is the key technique. In forward (normal) pipetting, you press the plunger to the first stop, release to aspirate, and press to the first stop again to dispense. In reverse pipetting, you press the plunger all the way to the second stop, release slowly to aspirate, and then dispense only to the first stop. The extra liquid aspirated compensates for two losses, the film that coats the inner tip wall and the volume lost to vapor behavior. The remaining excess stays in the tip and is discarded or returned. Reverse pipetting only works if you pre-wet the tip first. Pre-wetting means aspirating and dispensing the solvent two or three times before you take the real volume. This saturates the air cushion with vapor and equilibrates the film on the tip wall, so the volume you handle becomes repeatable. Without pre-wetting, the first few deliveries of a volatile solvent are the least accurate of the whole series, because the air cushion is still absorbing vapor. The limits are real. For very volatile, low-boiling solvents such as diethyl ether or pentane, even a well pre-wetted air-displacement pipette drips while you hold it. For dense chloroform, systematic under-delivery persists because the pipette was calibrated for water. Reverse pipetting is a mitigation, not a fix. There is also a context where air-displacement pipetting of an organic solvent is entirely legitimate and well described in the literature. In the direct fatty acid methyl ester (FAME) method of O'Fallon and colleagues, hexane is added to a screw-cap Pyrex tube after methylation, the tube is vortex-mixed and centrifuged, and then the hexane layer, which now carries the FAME analytes, is pipetted into a gas chromatography vial for analysis [2]. That transfer is a low-precision, low-stakes step with respect to volume, because the hexane serves as an extraction and transfer medium rather than a measured reagent. When the volume does not need to be accurate, a standard pipette is the cheapest and fastest tool for the job. ### Positive-Displacement Pipette A positive-displacement pipette looks like an air-displacement pipette but works on a different principle. The disposable tip contains an integrated piston or capillary piston. The piston makes direct contact with the liquid, so there is no air cushion to be compressed or contaminated by solvent vapor. When you press the plunger, the piston physically pushes the liquid out. This design removes the two biggest solvent problems at once. Vapor expansion cannot distort the volume, because there is no vapor space doing the work. Density and viscosity matter far less, because the volume is defined by the piston travel and the capillary bore, not by how the liquid behaves at an interface. Positive-displacement pipettes are therefore the standard recommendation from manufacturers for volatile solvents, viscous liquids, and liquids with vapor pressure above roughly 500 mbar at handling temperature. The trade-offs are cost and workflow. Capillary piston tips are individually wrapped, more expensive than standard tips, and generally vendor-specific. You cannot grab a generic box from the shelf. For a handful of transfers per day this is trivial. For a thousand transfers per week, cost becomes a planning decision. ### Glass Syringes and Microsyringes Gas chromatography made glass microsyringes the classic tool for metering organic liquids. A glass barrel with a fine plunger seal delivers volumes from microliters to milliliters with excellent repeatability, and glass is compatible with essentially every laboratory solvent, including chlorinated solvents that attack some plastics. The same glass-and-inert-material principle appears in modern separation research. The chloroform-permeation prototype described by Prasanthi and Datta uses a glass pipet wrapped with a chemically resistant superoleophilic cloth, through which chloroform diffuses upward out of seawater, acidic water, and alkaline water in 45 seconds with a separation efficacy of 94% sustained over 20 cycles [1]. The choice of glass as the core liquid-handling element reflects the same reasoning you apply at the bench. Glass tolerates the solvent, and the functional surface handles the wetting behavior. Syringes demand discipline. Inspect the plunger seal, because worn seals leak solvents onto your glove. Never cap a needle with your finger. Dispose of used needles in a sharps container. For oils, warm the barrel gently if the oil is too viscous to draw at room temperature, and confirm that the oil does not contain particles that jam the needle. ### Repipettors and Bottle-Top Dispensers When you need the same solvent volume many times, for example 10 mL aliquots of hexane across a plate of extractions, a repipettor mounted on the solvent bottle is faster and safer than repeated pipetting. The plunger draws directly from the bottle and dispenses through a valve and nozzle, so the solvent spends its life in glass or chemically resistant polymer rather than in a stream of disposable tips. Check the valve material against your solvent. Most manufacturers publish compatibility charts, and a swollen valve seal is both a leak hazard and a source of plastic contamination in the dispensed liquid. Repipettors are generally accurate at the milliliter scale and less precise in the sub-milliliter range, which matches them to extraction and dilution workflows rather than to quantitative transfers. ### Direct-Transfer Workflow Design The cheapest way to improve accuracy is sometimes to reduce the number of times the solvent is handled. The FAME method cited above is a good model. The entire sample preparation, including hydrolysis, methylation, extraction, and clarification, runs in a single screw-cap Pyrex tube, and the only manual liquid handling of the organic phase is one pipetting step of hexane into a GC vial [2]. By keeping water in the reaction mixture rather than drying and re-extracting through multiple vessels, the method removes several solvent-transfer steps that would each have added error and exposure. When you plan a solvent-based protocol, ask three questions. Can the reaction and the extraction share one vessel? Can the organic phase be transferred once instead of twice? Can the transfer be sized so that accuracy requirements are low? Each yes removes a pipetting step that would otherwise need positive displacement or a syringe. ## Comparing the Options on the Criteria That Matter ### Accuracy and Precision For quantitative transfers, positive displacement wins. The piston-in-tip design is largely immune to vapor pressure, density, and viscosity, which are exactly the properties that distort air-displacement performance. Glass microsyringes match or exceed positive displacement for small volumes when handled by a trained operator. Reverse-pipetted air displacement sits last, acceptable for moderately volatile liquids when the volume tolerance is loose. A useful nuance is that error direction depends on the liquid. A water-calibrated air-displacement pipette tends to over-deliver light solvents such as hexane and under-deliver dense solvents such as chloroform, because the calibration assumed the density of water. Viscous oils tend to be under-delivered in forward mode because the film left in the tip never drains. Reverse pipetting biases in the opposite direction on purpose, deliberately aspirating extra so the delivered volume lands close to the set point. ### Chemical Compatibility Glass and polytetrafluoroethylene (PTFE) tolerate nearly everything found in a molecular biology laboratory. Polypropylene, the standard tip material, is acceptable for brief contact with most alcohols, ketones, and alkanes, but extended contact with chloroform, toluene, and other aromatics risks leaching and swelling. If your protocol involves strong solvents in contact with plastic for minutes or hours, choose glass. This is the same materials logic behind the chemically resistant ZIF-7 and ZIF-11 percolation coatings, which were engineered specifically to survive acidic and alkaline marine conditions while maintaining their oil-selective wetting properties [1]. ### Throughput and Ergonomics Multichannel pipettes, electronic pipettes, and repipettors exist because repetition is where error and injury accumulate. If your workflow involves dispensing one solvent volume dozens of times, a repipettor reduces both exposure and hand strain. If it involves plating oil overlays onto many wells, an electronic pipette in reverse mode with pre-wetted tips outperforms a manual pipette held to the same standard. ### Safety and Exposure Volatile solvents evaporate into your breathing zone at the tip. Work in a fume hood or a well-ventilated filtered enclosure for chloroform, ethers, and aromatics. Keep solvent bottles closed except during transfer. Every additional manual transfer step is an additional opportunity for exposure and for spill, which is why the single-tube, single-transfer design of the direct FAME method is described as saving time and effort as well as being economical compared with multi-step alternatives [2]. ### Cost Standard tips are the cheapest consumable. Capillary piston tips cost more per transfer. Glass syringes cost more up front and last for years with care. Repipettors amortize quickly at high volume. There is no single best pipetting organic solvents and oils answer on cost, only a match between transfer count and tool. A dozen transfers per month never justifies a repipettor. A thousand transfers per month justifies almost anything that reduces exposure and error. ## Pipetting Technique for Solvents and Oils, Step by Step Good pipetting technique with organic solvents and oils is mostly about respecting the physics of the liquid. The following sequence applies to air-displacement pipetting in reverse mode, and the same habits carry over to positive displacement. 1. **Choose the smallest reasonable tip size.** Tips should be rated for the volume you set. A 1000 µL tip used at 50 µL performs worse than a 200 µL tip used at 50 µL. 2. **Work at consistent temperature.** Cold solvent drawn into a warm pipette warms up and expands. Let solvents and samples equilibrate to room temperature before pipetting. 3. **Pre-wet the tip.** Aspirate and dispense the solvent two to three times. For oils, pre-wetting coats the wall and makes the film drain predictably. 4. **Aspirate slowly and vertically.** Immerse the tip only a few millimeters into the liquid. Deep immersion wets the outside of the tip and carries over droplets. Slow plunger release avoids bubbles in viscous liquids. 5. **Dispense against the receiving vessel wall, or into the liquid at an angle.** For volatile solvents, dispense onto a wetted wall so the liquid does not splash or dribble off the tip exterior. 6. **Dispense to the first stop, not the second.** In reverse mode, the second-stop blow-out expels the excess you deliberately aspirated. 7. **Discard or return the excess consistently.** Do not return excess hexane to a stock bottle if the tip touched sample, because you will contaminate the stock. Discard to waste. 8. **Change tips between different liquids.** Residual solvent in a tip will contaminate the next liquid and can attack the pipette shaft if vapor migrates upward. For viscous oils specifically, two extra habits help. First, aspirate and dispense more slowly than feels natural, giving the oil time to flow. Second, if the oil must be delivered into an aqueous system, consider delivering it onto the wall above the liquid so it flows down as a coherent layer rather than emulsifying at the tip orifice. For two-phase systems such as phenol-chloroform extraction, the density ordering matters for which phase is on top. Chloroform is denser than water, so the chloroform phase sits at the bottom. If you need the organic (chloroform) phase, you can either draw it from the bottom with the tip inserted through the aqueous layer, or remove the upper aqueous phase first and then transfer the organic phase. Avoid interfacial material, because protein and DNA collect at the interface and ruin downstream quantitation. This is the context in which the hexane-into-GC-vial transfer of the FAME method is typically done, taking the clear upper phase cleanly after centrifugation [2]. ## Pipette Calibration When Water Is Not the Liquid Pipette calibration is the process of verifying and adjusting the volume a pipette delivers against a traceable standard. Standard air-displacement pipettes are calibrated gravimetrically, by dispensing water into a vessel on an analytical balance and converting mass to volume using the density of water and correction factors for temperature, air buoyancy, and evaporation. Quality systems generally expect routine verification, for example at the start of a critical experiment, plus periodic full calibration at intervals set by your laboratory's quality plan. Calibration becomes more interesting once you pipette organic solvents and oils routinely. **Method choice.** Gravimetric calibration with water remains the reference method, because water's properties are known precisely. You can also perform a performance check with the solvent or oil you actually use. Dispense the solvent onto a balance in a sealed, vapor-tight weighing vessel to prevent evaporative loss during weighing, and apply the solvent's density at the test temperature to convert mass to volume. This tells you the real-world delivery error with that liquid, which is usually larger than the water calibration suggests. **What to adjust.** You can adjust an air-displacement pipette to a specific solvent, but then it is no longer correctly calibrated for water. Some laboratories keep a dedicated pipette adjusted for a high-use solvent, for example a pipette permanently set up for isopropanol in nucleic acid precipitation workflows. Label it clearly. Positive-displacement pipettes are calibrated with the capillary pistons installed, and because the piston defines the volume, the calibration holds across liquid types much better than for air displacement. **When calibration matters most.** If your protocol measures the solvent as a reagent, for example 500 µL of chloroform that defines a reaction stoichiometry or an extraction ratio, systematic error changes your chemistry. If the solvent is a carrier, for example the hexane that ferries FAME analytes into a GC vial, volume accuracy matters little because the analytical instrument quantifies the analytes independently [2]. Decide which case you are in before you spend effort on calibration. **Maintenance after solvent use.** Vapor migrates into the shaft of an air-displacement pipette and can corrode or grease-wash the piston seal over time. If you regularly pipette volatile solvents with an air-displacement pipette, inspect and service it more often, and store it upright with the tip removed. Positive-displacement and glass tools are largely immune to this failure mode. ## Worked Scenarios ### Scenario 1. Mineral Oil Overlay for a PCR Plate You need to overlay 50 µL of mineral oil on each well of a 96-well PCR plate, 4,800 µL total. Mineral oil is viscous and nonvolatile. The best pipetting organic solvents and oils choice here is a multichannel air-displacement pipette used in reverse mode with pre-wetting, or an electronic pipette with a slow aspirate and dispense profile. Volume accuracy hardly matters, since the oil is a physical barrier against evaporation, not a reagent. What matters is coverage and avoiding bubbles. Pre-wet each tip, aspirate slowly, dispense slowly down the side of each well. ### Scenario 2. Chloroform Transfer in Nucleic Acid Purification You need 300 µL of chloroform, added as a measured reagent for phase separation. Chloroform is dense, volatile, and hard on some plastics. The best option is a positive-displacement pipette with a capillary piston tip rated for chlorinated solvents. If none is available, use reverse pipetting with an air-displacement pipette, pre-wet the tip three times, work in a fume hood, and accept a modest systematic under-delivery. If the volume is not critical, for example when you simply need an excess of chloroform to form a phase, the standard pipette with reverse mode is adequate. ### Scenario 3. Hexane Transfer After FAME Methylation You are running the direct FAME protocol. After hydrolysis in 1 N KOH in methanol at 55 °C for 1.5 hours and sulfuric-acid-catalyzed methylation at 55 °C for 1.5 hours, you add hexane to the screw-cap Pyrex tube, vortex, and centrifuge. The hexane layer containing the fatty acid methyl esters is then pipetted into a GC vial [2]. Here, the transfer volume is not a measured reagent, so a standard air-displacement pipette is the right tool. Take the clear upper phase, avoid the interfacial pellet or tissue debris, and transfer promptly so volatile losses do not concentrate the analytes unexpectedly. A glass Pasteur pipette or a syringe works equally well. ### Scenario 4. Repeated 10 mL Hexane Additions Across a Sample Batch You are processing 40 feedstuff or tissue samples through the FAME workflow and each needs a hexane addition for extraction [2]. A repipettor on the hexane bottle dispenses each aliquot with one thumb stroke, keeps the bottle closed between transfers, and keeps hexane out of your breathing zone. Volume precision at the milliliter scale is sufficient, because the hexane is a carrier phase. This is the option with the best combination of speed, exposure control, and cost at this transfer count. ### Scenario 5. Solvent-Resistant Device for Chloroform Recovery From Water You are separating chloroform from contaminated water under acidic or alkaline conditions. The FG-ZIF-11-CC system, a superoleophilic chemically resistant cotton cloth wrapped around a glass pipet, pulls chloroform upward out of seawater, acidic water, and alkaline water in 45 seconds with 94% separation efficacy maintained over 20 cycles, and the related FG-ZIF-7 stainless steel mesh achieved 96 to 94% oil separation efficiency over 10 cycles while tolerating folding, tape peeling, and 250 cycles of sandpaper abrasion without losing its water-repellent behavior [1]. The relevant lesson at the bench is that glass plus a purpose-chosen wetting surface outperforms plastic in chemically harsh, solvent-rich conditions, and that reusable solvent-contact materials are feasible when their chemical resistance is engineered deliberately. ## Common Mistakes and Limitations **Skipping pre-wetting.** The most common error in pipetting organic solvents and oils. The first delivery from a dry tip is the worst delivery. Pre-wet every time the liquid is volatile, viscous, or different in density from water. **Using forward pipetting on a viscous oil.** Forward mode leaves the wall film in the tip, so you under-deliver by the film volume, which varies with dispensing speed. Reverse pipetting exists for this case. **Blow-out confusion.** In reverse mode, pressing to the second stop dispenses the excess you intentionally drew. Operators trained only in forward mode often blow out, erasing the correction. Train everyone who shares the pipette. **Ignoring density in two-phase systems.** Newcomers sometimes try to pipette the upper chloroform phase when chloroform is actually the lower phase, or vice versa with hexane. Confirm the density ordering before you insert the tip. **Letting solvent vapor into the pipette shaft.** Holding a volatile solvent in the tip for more than a few seconds sends vapor toward the piston seal. Aspirate and dispense promptly, and never leave a loaded tip standing in a rack. **Assuming plastic is always fine.** Brief polypropylene contact with most solvents is acceptable, but chlorinated and aromatic solvents deserve glass. When in doubt, check the manufacturer compatibility chart for the specific tip and pipette materials. **Calibrating only with water when the workflow is solvent-heavy.** A water calibration certifies performance with water. If your daily reality is chloroform or mineral oil, run a gravimetric check with the actual liquid in a sealed weighing vessel so you know the delivery error where it counts. **Treating carrier volumes as reagent volumes.** The reverse error also exists. Spending time on calibration for a hexane transfer whose volume does not affect the analytical result wastes effort. Match the accuracy requirement to the role of the liquid. **Transferring volatile liquids too slowly.** A deliberate, slow dispense is right for viscous oils and wrong for ether or pentane, which drip off the tip while you wait. Adjust your pace to the liquid's volatility. **Safety assumptions.** A pipette is not a substitute for a fume hood, eye protection, and compatible gloves. Any limitation listed here concerns technique and equipment, and individual safety decisions should follow your institution's chemical hygiene plan and, where health questions arise, a qualified occupational health provider. ## Frequently Asked Questions ### Why does my pipette drip when I aspirate hexane? The vapor of the hexane expands in the air cushion of an air-displacement pipette and pushes liquid out of the tip. Pre-wet the tip several times, work quickly, use reverse pipetting, or switch to a positive-displacement pipette for high-volatility solvents. ### What is reverse pipetting and when should I use it? Reverse pipetting means pressing the plunger to the second stop to aspirate and dispensing only to the first stop, leaving excess liquid in the tip. Use it for viscous liquids, oily samples, and moderately volatile solvents, always with pre-wetting, and discard the excess after each delivery. ### Can I pipette chloroform with a normal pipette tip? Brief contact of chloroform with polypropylene is usually tolerated, but chloroform can extract components from some plastics over longer exposure and can affect accuracy through its high density and volatility. For measured, quantitative transfers, a positive-displacement pipette with a solvent-rated capillary tip or a glass syringe is the better choice. ### How do I pipette viscous oils accurately? Pre-wet the tip with the oil, aspirate and dispense slowly, hold the tip a few millimeters into the liquid, and use reverse pipetting so the wall film is accounted for. If accuracy is critical and volumes are small, a positive-displacement pipette or a glass microsyringe removes the viscosity problem almost entirely. ### Does pipette calibration change for solvents and oils? The reference calibration is still performed gravimetrically with water, but you can run an additional performance check by dispensing your actual solvent onto a balance in a sealed vessel and applying that solvent's density. If a pipette is adjusted for a solvent, label it and do not use it for water without recalibration. ### Do I need a special pipette for mineral oil over PCR plates? Usually not. Because the oil overlay is a physical barrier rather than a measured reagent, a standard multichannel pipette in reverse mode with pre-wetted tips and slow dispensing is sufficient. Electronic pipettes with adjustable speed profiles make this more comfortable across a full plate. ## Sources 1. [Three in One: Superoleophilic, Chemically and Mechanically Resistant ZIF-7 and ZIF-11 Percolation Networks for Selective Permeation of Oils and Chlorinated Solvents.](https://pubmed.ncbi.nlm.nih.gov/37850868/) 2. [A direct method for fatty acid methyl ester synthesis: application to wet meat tissues, oils, and feedstuffs.](https://pubmed.ncbi.nlm.nih.gov/17296772/)