Mobile Phase
Definition
The solvent or mixture that flows over the stationary phase and carries analytes along. Polarity, viscosity and volatility set the separation; switching from chloroform to ethyl acetate on the same TLC plate can move a spot from Rf 0.2 to Rf 0.8.
- Role
- Carries the analyte through the stationary phase during separation
- In liquid chromatography (TLC, HPLC, HPTLC)
- Solvent or solvent mixture
- In gas chromatography (GC/GLC)
- Inert carrier gas (helium, hydrogen, or nitrogen)
- Key factors affecting separation
- Polarity, viscosity, and volatility of the mobile phase
Common questions
What is the mobile phase in chromatography?+
The mobile phase is the fluid (solvent or gas) that moves through the chromatography system and carries your analyte along with it. In liquid techniques like TLC and HPLC, it is a solvent mixture. In gas chromatography, it is an inert carrier gas like helium or hydrogen.
How does changing the mobile phase affect separation?+
Changing the solvent mixture properties like polarity, viscosity, and volatility directly changes how far analytes travel. For example, switching from chloroform to ethyl acetate on the same TLC plate can move a spot's Rf value from 0.2 all the way to 0.8.
Why use different mobile phases for different chromatography methods?+
Different methods require different carriers. Liquid chromatography needs solvents that dissolve your sample and flow through the stationary phase. Gas chromatography needs inert gases that won't react with the sample, which is why helium, hydrogen, and nitrogen are all used as carrier gases.
Related terms
- Stationary Phase
- The immobile material the mixture flows past. In paper it is water inside cellulose fibres; in classical column work and TLC it...
- Resolution (Rs)
- Rs = 2(tR2 − tR1) / (W1 + W2). Above 1.5 means baseline separation; below 1.0 means peaks overlap and quantitation is...
- Rf Value
- The retention factor, defined as the distance the spot travels divided by the distance the solvent front travels on the same plate....
- ECD
- Electron capture detector. A Ni-63 radioactive source produces a baseline electron current that is reduced when electron-affinic species (halogens, nitro groups, peroxides)...
- FID
- Flame ionisation detector. A hydrogen-air flame ionises C-H bond containing analytes and a collector electrode reads the ion current. Near-universal for organic...
- Isocratic vs Gradient Elution
- Isocratic means the mobile-phase composition stays constant through the run; simple, reproducible, but limited resolving power for wide-polarity mixtures. Gradient means the...
- Limit of Detection (LOD)
- The lowest concentration of analyte that produces a signal reliably distinguishable from the instrument noise, conventionally three times the standard deviation of...
- MSD
- Mass Selective Detector. GC-MS or LC-MS detector that identifies analytes by mass spectrum. The confirmation standard for drug and toxicology casework.
- Partition vs Adsorption
- Partition chromatography separates by differential solubility between two liquid phases (GLC, reversed-phase HPLC). Adsorption chromatography separates by surface binding to a solid...
- Plate Theory
- Model that treats a column as a series of theoretical plates. The Height Equivalent to a Theoretical Plate (HETP) measures column efficiency;...
- Retention Time (tR)
- Time from sample injection to peak maximum at the detector in HPLC or GC. Characteristic of an analyte under a defined column...
- Rf (Retention Factor)
- Distance travelled by the analyte divided by the distance travelled by the solvent front, on a TLC or HPTLC plate. A unit-less...
Explained in these topics
- Chromatographic Techniques in Forensic Science: TLC, GLC, HPLC and HPTLCThe phase that carries the analyte through the stationary phase. A solvent system in TLC and HPLC, an inert carrier gas (nitrogen, helium, hydrogen) in GLC.
- Chromatographic Techniques: TLC, HPLC, HPTLC and GLCThe fluid that carries the sample across the stationary phase. A solvent mixture for TLC, HPLC and HPTLC; an inert gas (helium or hydrogen) for GC.
- Chromatography Fundamentals: Paper, Column and TLCThe solvent or mixture that flows over the stationary phase and carries analytes along. Polarity, viscosity and volatility set the separation; switching from c...