Stationary Phase
Definition
The immobile material the mixture flows past. In paper it is water inside cellulose fibres; in classical column work and TLC it is silica gel or alumina; in ion-exchange it is a charged resin. The chemistry of this surface decides which analyte moves fast and which lags.
- Role
- Fixed, immobile medium that the sample moves past
- Common materials
- Silica gel, alumina, C18-bonded silica, charged resins, cellulose
- Key function
- Controls separation by selectively retaining or releasing sample compounds
Common questions
What is stationary phase in chromatography?+
The stationary phase is the immobile medium that stays fixed in place while the sample moves through it. It can be silica gel on a TLC plate, chemically bonded silica in an HPLC column, or a liquid film coating the inside of a capillary in gas chromatography. The chemistry of this surface determines which compounds move quickly and which ones move slowly.
Why does stationary phase matter for separating compounds?+
Different compounds interact differently with the stationary phase material. Some bind strongly and move slowly, while others bind weakly and move quickly. This difference in interaction is what allows chromatography to separate a mixture into its individual components.
What materials are used as stationary phase?+
Common stationary phases in forensic work include silica gel (TLC and HPTLC), alumina (column chromatography), water inside cellulose fibers (paper chromatography), and C18 bonded silica (reverse-phase HPLC). Ion-exchange chromatography uses charged resins as the stationary phase.
Related terms
- Mobile Phase
- The solvent or mixture that flows over the stationary phase and carries analytes along. Polarity, viscosity and volatility set the separation; switching...
- 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 fixed phase across which the sample moves. Silica gel on TLC/HPTLC plates, a packed or capillary column coating in GLC, a bonded silica packing in HPLC.
- Chromatographic Techniques: TLC, HPLC, HPTLC and GLCThe immobile medium through which the sample moves. Silica gel for TLC and HPTLC, chemically bonded C18 silica for reverse-phase HPLC, a liquid film coated on...
- Chromatography Fundamentals: Paper, Column and TLCThe immobile material the mixture flows past. In paper it is water inside cellulose fibres; in classical column work and TLC it is silica gel or alumina; in io...