Mobile Phase vs Stationary Phase
Key Concepts in Chromatography
By Jon Thompson, extraktLAB
Last updated: October 27, 2026
Short answer: The mobile phase is the fluid that carries a sample through a chromatography system. The stationary phase is the solid, or liquid-coated solid, that stays fixed in the column or on the plate. Compounds that interact weakly with the stationary phase travel with the mobile phase and leave first. Compounds that interact strongly leave later. That difference separates the mixture.
Concepts in Chromatography: Overview
To understand chromatography, you must first understand its two basic functions: the mobile and stationary phase. In this article, we will break down each phase into its components, the mediums and solvents used, and how they are used in the chromatography process.
What Is the Mobile Phase in Chromatography?
The mobile phase is a liquid or gas that carries the sample through the chromatography system, allowing its components to separate based on their interactions with the stationary phase.
In gas chromatography, the mobile phase is an inert gas like helium or nitrogen, while in liquid chromatography, it is a liquid solvent like methanol or acetonitrile.
Solvents Used for Mobile Phase
In chromatography, several common solvents are used as the mobile phase, with some being more effective than others due to differences in how they interact with the stationary phase. Popular options include hydrocarbons like pentane, hexane, and petroleum ether and co-solvent mixtures like ether/hexane and methanol/dichloromethane.
The choice of solvent is critical and depends on the specific molecules being separated, as certain solvents work more efficiently with particular materials.
Mobile and stationary phases by chromatography type
The same two roles appear in every form of chromatography. What changes is the material playing each role.
| Technique | Mobile phase | Stationary phase | Typical use |
|---|---|---|---|
| Reversed-phase HPLC | Water mixed with acetonitrile or methanol, often with a buffer | Silica particles bonded with a nonpolar group such as C18 | Analysis and purification of most small organic molecules |
| Normal-phase HPLC and flash column | A nonpolar solvent such as hexane, often mixed with ethyl acetate | Polar silica or alumina | Separating compounds that dissolve well in nonpolar solvents |
| Gas chromatography (GC) | An inert carrier gas: helium, hydrogen or nitrogen | A thin liquid film or polymer coating on the inside wall of a narrow column | Volatile compounds such as terpenes and residual solvents |
| Thin-layer and paper chromatography | A solvent that moves up the plate or sheet by capillary action | A layer of silica or alumina on a plate, or cellulose paper | Quick checks, teaching labs, tracking a column run |
| Supercritical fluid chromatography (SFC) | Supercritical CO2, usually with a co-solvent such as methanol | A packed column, similar to HPLC | Nonpolar compounds and chiral separations |
| Size-exclusion chromatography | A liquid chosen to dissolve the sample without sticking to the packing | Porous gel beads | Separating molecules by size, such as proteins from small molecules |
For more detail on each method, see Types of Chromatography and the Chromatography Guide.
What Is the Stationary Phase In Chromatography?
The stationary phase in chromatography is a solid or a liquid-coated solid that stays fixed inside the column while the mobile phase moves over it. It enables the separation of compounds based on their interaction with the mobile phase.
In liquid chromatography (LC), the stationary phase is a packed solid in the column, while in gas chromatography (GC), it is often a thin liquid film coated on a solid surface.
The materials used for this phase are often:
- Gel
- Silica
- Alumina
- Sand
These materials allow the mobile phase, carrying the sample, to pass through. The material can either have particles of the same size (monodisperse), or the particles can vary in size (polydisperse). The variation in particle size can aid in separation as some sample components may have a unique affinity for specific particle sizes.
Pore Diameter and Its Role in Separation
Pore diameter matters most in size-exclusion chromatography, where molecules are separated by size. Large molecules such as proteins cannot enter small pores and leave the column first. Small molecules such as THC and CBD enter the pores and leave later. In silica columns used for flash and normal-phase work, the separation is driven mainly by polarity, and pore size mostly sets how much surface is available.
Selectivity of the Stationary Phase
Another key factor is the selectivity of the stationary phase material. This refers to how well this phase interacts with the mobile phase on a molecular level.
The interaction between the stationary and mobile phases determines how well the compounds get separated. Some stationary phases may separate compounds in one solvent better than another.
Polarity and Its Impact on Separation
Polarity also affects how well components separate during chromatography. Molecules can be polar (having a charge distribution, like water) or nonpolar (lacking a charge, like oil). Cannabinoids are nonpolar. For extraction, a nonpolar or moderately polar solvent such as ethanol or a hydrocarbon dissolves them well. For chromatography, what matters is the match between the mobile phase and the stationary phase. On silica (normal phase), nonpolar solvents carry cannabinoids through. On C18 (reversed phase), cannabinoids are held strongly and need a mobile phase with a high share of organic solvent such as methanol, acetonitrile or ethanol.
| Mobile Phase vs. Stationary | Mobile Phase | Stationary Phase |
|---|---|---|
| Composition | Liquid or gas | Solid material |
| Material | Solvent: hexane, ethyl acetate, petroleum ether (normal phase); water mixed with methanol, ethanol or acetonitrile (reversed phase) | Porous Solid (silica, alumina, gel) |
| Purpose | Dissolve and carry the mixture through the stationary phase | Separate the mixture into various components as they pass through it |
| Post-processing | Solvent removal needed | Remains in column |
Normal phase vs reversed phase
The two main modes of liquid chromatography differ in which phase is more polar. In both, a compound is held back by the phase it resembles most.
| Normal phase | Reversed phase | |
|---|---|---|
| Stationary phase | Polar, such as silica or alumina | Nonpolar, such as C18-bonded silica |
| Mobile phase | Nonpolar to moderately polar organic solvents | Water mixed with a polar organic solvent |
| Compounds that leave first | The least polar | The most polar |
| How to speed up elution | Make the mobile phase more polar | Raise the share of organic solvent in the mobile phase |
| Common examples | Flash chromatography on silica gel, TLC on silica plates | Most analytical HPLC, many preparative HPLC methods |
Cannabinoids are nonpolar molecules. On a C18 column they are held strongly and leave late, so reversed-phase methods use a mobile phase with a high share of organic solvent. On silica, nonpolar solvents carry them through quickly. The pure99 systems are not tied to one mode: extraktLAB lists normal, chiral and reverse phase operation for both the pure99-x and the pure99-z, so the mode follows from the method and the stationary phase packed in the column.
How the Stationary and Mobile Phase Work in Chromatography
Both the stationary and mobile phases are essential in chromatography, and chromatography separation cannot be accomplished without them.
Let’s break down the chromatography process in simple steps:
- A column is packed with a stationary phase (e.g., silica beads), and a filter is placed at the bottom to retain the packing material.
- The mixture intended to be separated is suspended in a small amount of solvent and added above the stationary phase.
- More packing material is added above the mixture solvent solution, followed by the remaining solvent, which is used as the mobile phase.
- Using simple gravity or some kind of vacuum pump, the mobile phase is passed through the mixture, pulling it through the stationary phase.
- As each component is separated by passing through the stationary phase, they are collected in separate vessels. The chromatography process is then complete.
A gravity column like this is the starting point for flash chromatography, which uses pressure to speed up the run.
Now that we’ve established the basic chromatography steps, let’s get more detailed:
The mobile phase is a solvent or fluid eluent that can move freely through a chromatography system using a pump. As a solvent, it is also important for this phase to dissolve the solution being tested or separated so that the individual components can be identified and remediated. This is why some eluents or solvents work more efficiently than others, depending on the separated substance.
The material used as the stationary phase is usually a porous solid (in gas chromatography, often a thin liquid film coated on the inside wall of the column) that remains fixed in a column that allows the eluent used as the mobile phase to pass through it. This is where the actual chromatography process takes place. As the mobile phase passes through, the various components of the solution will move through the material at various rates depending on their affinity for that material.
If a particular material passes through the stationary phase more quickly, it will be seen on a chromatograph or collected in a containment vessel. Then, the following fastest material will be seen or collected, and the process continues until each component is identified and separated. As they pass through, they can be identified in a chromatogram for testing purposes or collected into separate vessels.
Several factors can cause variations in the efficiency of a chromatography separation process, but the basic principles remain the same. The stationary and mobile phase in chromatography can be as simple as water and a coffee filter or as complicated as a co-solvent solution and a silica gel, but the separation process itself remains the same.
By adding a solution to a mobile phase and passing it through a stationary phase, you can effectively separate the individual components in a solution so that they can be identified, remediated, and isolated altogether.
How the mobile phase controls separation
The stationary phase is usually chosen first and stays the same during a run. The mobile phase is what the operator adjusts. Its composition decides how long each compound is retained and how sharp the peaks are.
Solvent strength and polarity
A stronger mobile phase pushes compounds off the stationary phase sooner. In reversed-phase work, raising the organic share (for example, from 60% methanol to 80% methanol in water) shortens retention times. In normal-phase work, adding a more polar solvent such as ethyl acetate to hexane does the same.
Isocratic and gradient elution
In an isocratic run the mobile phase composition stays the same from start to finish. In a gradient run the composition changes over time, for example from 40% to 95% organic solvent. A gradient pulls strongly retained compounds off the column sooner and keeps their peaks narrow, so it suits samples that contain compounds with very different polarity.
pH and buffers
Acids and bases gain or lose charge as the pH changes, and that changes how strongly they are retained. A buffer in the water portion keeps the pH steady so retention times repeat from run to run.
Flow rate
A faster flow shortens the run but gives compounds less time to settle between the two phases, so resolution can drop. Flow rate is balanced against the pressure the column and pump can handle.
Solvent quality and degassing
Use solvents graded for chromatography and filter them before use. Dissolved air forms bubbles in the pump and detector, so mobile phases are degassed before a run.
Common mobile phase solvents
| Solvent | Polarity | Typically used for | Handling |
|---|---|---|---|
| Water | Very high | Base of reversed-phase mobile phases | Use purified water; add a buffer if pH matters |
| Methanol | High | Organic part of reversed-phase mobile phases | Flammable and toxic |
| Acetonitrile | High | Organic part of reversed-phase mobile phases | Flammable and toxic |
| Ethanol | High | Organic part of reversed-phase mobile phases in some preparative work | Flammable; lower toxicity than methanol and acetonitrile |
| Ethyl acetate | Medium | Polar component of normal-phase mixtures | Flammable |
| Dichloromethane | Medium to low | Normal-phase mixtures where a stronger solvent is needed | Hazardous vapor; regulated in many places |
| Hexane | Very low | Base of normal-phase mobile phases | Flammable; affects the nervous system with long exposure |
Safety data sheets and local rules decide which of these a lab can use. Ventilation, ignition-source control and solvent storage belong in the plan before the first run.
Mobile phase at preparative scale
Analytical HPLC uses a column that holds a few grams of packing and a mobile phase flow measured in milliliters per minute. A preparative column holds liters of packing, and the mobile phase used per run grows with it.
At that scale the mobile phase is often one of the largest running costs and a main part of the safety plan. What happens to it after the column (recovered, re-proofed to the right composition and reused, or disposed of) changes the cost of each kilogram purified.
The pure99 systems include closed-loop solvent recycling for this reason. Related reading: solvent recycling methods, how to calculate solvent reuse and loss costs, and solvent removal with the fracTron.
Is preparative chromatography the right tool for your lab?
- If you need to measure what is in a sample, such as potency, purity or residual solvents, you need an analytical HPLC or GC. Our systems are preparative: they separate and collect compounds, they do not measure them.
- If you need to separate a compound and collect it, such as minor cannabinoids, THC removal from hemp extract, or an active ingredient from a botanical extract, a preparative system is the tool. extraktLAB builds the pure99-x for automated purification and the pure99-z for research and method development, with stand-alone columns from 1.5 L (0.4 US gal) to 13 L (3.4 US gal) of packing.
- If you are not sure which group you are in, describe the separation and we will tell you plainly whether a preparative system fits.
Prefer to read first? The white paper Separation in Preparative Chromatography with On Line Solvent Recovery & Reproofing covers the process in detail.
Chromatography terms used on this page
- Eluent: The mobile phase when it is used to move compounds through the column.
- Eluate: The liquid that leaves the column, carrying the separated compounds.
- Analyte: The compound being separated or measured.
- Retention time: The time a compound takes to travel from injection to the detector or collection point.
- Chromatogram: A plot of detector signal against time. Each compound appears as a peak.
- Rf value: In thin-layer and paper chromatography, the distance a compound travels divided by the distance the solvent travels. It is a number between 0 and 1.
- Resolution: How well two neighboring peaks are separated from each other.
- Column volume: The volume of the packed bed in the column, used to scale solvent amounts and flow rates.
Mobile phase and stationary phase: frequently asked questions
What is the mobile phase in chromatography?
The mobile phase is the fluid that moves through the chromatography system and carries the sample with it. It can be a liquid (as in HPLC and flash chromatography), a gas (as in gas chromatography) or a supercritical fluid (as in SFC). Its composition controls how fast each compound travels.
What is the stationary phase in chromatography?
The stationary phase is the solid, or the liquid held on a solid, that stays fixed in the column or on the plate. Examples are silica gel, alumina, C18-bonded silica, cellulose paper and the thin coating inside a gas chromatography column. Compounds are separated by how strongly they stick to it.
What is the difference between the mobile phase and the stationary phase?
The mobile phase moves and carries the sample. The stationary phase stays in place and holds compounds back to different degrees. A compound that favors the mobile phase leaves the column early. A compound that favors the stationary phase leaves late.
Can the mobile phase be a gas?
Yes. In gas chromatography the mobile phase is an inert carrier gas, usually helium, hydrogen or nitrogen. The sample is vaporized, carried through a heated column, and separated by how it interacts with the coating on the column wall.
What is the mobile phase in paper chromatography?
It is the solvent. The solvent rises through the paper by capillary action and carries the components of the sample with it at different speeds. The paper, which is cellulose holding a thin layer of water, is the stationary phase.
What is the most common mobile phase in HPLC?
In reversed-phase HPLC, the most common mobile phase is water mixed with acetonitrile or methanol, often with a buffer or a small amount of acid to control pH. Normal-phase HPLC uses organic solvents such as hexane mixed with ethyl acetate.
What is the difference between isocratic and gradient elution?
In isocratic elution the mobile phase composition stays the same during the run. In gradient elution the composition changes over time, usually by raising the share of the stronger solvent. Gradients suit samples with compounds of very different polarity.
How do you choose a mobile phase?
Start from the stationary phase and the sample. A polar stationary phase such as silica pairs with a nonpolar mobile phase, and a nonpolar one such as C18 pairs with a polar mobile phase. The solvent must dissolve the sample, suit the detector, be pure enough, and be safe to handle. A thin-layer plate is a quick way to test options.
Why does polarity matter in the mobile phase?
Compounds are held back by the phase they resemble most. If the mobile phase is too weak, compounds stay on the stationary phase and the run takes too long. If it is too strong, they leave together and are not separated. Adjusting polarity moves compounds between those two extremes.
Is CO2 used as a mobile phase?
Yes. In supercritical fluid chromatography (SFC) the mobile phase is CO2 above its critical point of 31 °C (88 °F) and 73.8 bar (1,070 psi), usually mixed with a co-solvent such as methanol. See pure99 chromatography compared with CPC and SFC.
Final Thoughts
Understanding the mobile and stationary phases is essential for an efficient chromatography process. These two components work together to ensure effective testing and purification, and they are crucial in the extraction industry.
pure99x
If you want to upgrade your separation process with high-end chromatography equipment, look no further. extraktLAB’s pure99-x and pure99-z are preparative chromatography systems with closed-loop solvent recycling and precise process controls. Talk to an expert about a separation project.
Related guides and equipment
Guides
- Types of Chromatography: GC, TLC, size-exclusion and HPLC compared, with pros and cons.
- HPLC Applications in the Extraction Industry: where liquid chromatography fits in extraction work.
- Flash Chromatography: pressure-driven column chromatography on silica, and how it compares with preparative HPLC.
- Chromatography Methods: THC remediation methods compared.
- pure99 vs CPC vs SFC: how three preparative approaches handle solvent and sample solubility.
Equipment and resources
- Preparative chromatography systems: the pure99 line and its column options.
- pure99-x and pure99-z: automated purification and R&D platforms.
- White paper: separation in preparative chromatography with on-line solvent recovery.
- Talk to an expert about a separation project.
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