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The chromatographic mobile phase carries solutes through the apparatus. In liquid chromatography it is a liquid solvent or chemical solution.

The eluent is the supplied chromatographic mobile phase. Its composition can alter both solute solubility and competition for adsorption sites, so changing the eluent can change retention even with the same chromatographic stationary phase.

Chromatography separates components of a mixture through different migration rates in contact with a chromatographic stationary phase while a chromatographic mobile phase transports them. Liquid chromatography describes the physical state of the transporting phase; size-exclusion chromatography describes a separation mechanism. Classification by phase and classification by mechanism answer different questions. In ideal size-exclusion chromatography, differential access to liquid-filled pores replaces binding as the source of differential migration.

In many other forms of chromatography, a solute repeatedly leaves the chromatographic mobile phase and returns to it. Reversible adsorption holds it at a surface; partition distributes it between the chromatographic mobile phase and an immobilized liquid chromatographic stationary phase. Spending more time stationary means less progress with the moving fluid. Two initially overlapping components can therefore emerge as separate bands without either being permanently trapped. Chromatography is useful both for comparing a sample with reference substances and for collecting separated components; a visible band need not contain just one substance.

Relative retention belongs to the sample together with the chosen chromatographic stationary phase, chromatographic mobile phase, eluent composition, and temperature. Changing these conditions can change the separation order. Chemical polarity helps explain interactions in some systems but is not a universal ranking rule. In thin-layer chromatography, a spot's TLC retardation factor is consequently conditional evidence about identity, rather than a fixed identifier intrinsic to a molecule.

Thin-layer chromatography spreads a thin chromatographic stationary phase on a supporting plate. A small sample spot starts on a pencil origin line. The lower edge contacts the eluent, whose level must stay below that origin; capillary action draws the chromatographic mobile phase upward. Components exchange with the chromatographic stationary phase as the wet region advances. The leading wet edge is the solvent front. Mark it after removing the plate, before solvent evaporation erases it.^[ref]

Juniata College's Science in Motion protocol grinds vegetable tissue with about $5\mathrm{mL}$ of acetone, transfers the extract, adds about $2\mathrm{mL}$ of hexane, and specifies sampling the upper layer after separation. It spots a silica gel plate with a capillary tube, drying between repeated applications; a reference pigment occupies another lane. A closed jar contains about $0.5\mathrm{cm}$ depth of 70:30 hexane–acetone eluent, below the pencil origin. The plate is removed before the front reaches the top; the front is marked immediately. Spot centres and front are measured from the origin.^[ref]

The protocol's standard provides a comparison control. It specifies no added water in this extraction and reports no measured plate positions or temperature. Its tabulated pigment values are guidance, not observations from this run.^[ref]

A useful additional control is an eluent blank, checking contamination. Compare sample and standard on the same plate, and optionally apply both together at a third origin position. Agreement under identical conditions supports a proposed identity but cannot prove it: different substances can migrate together. A second eluent giving different selectivity strengthens the comparison. One spot cannot establish purity, and a missing visible spot can reflect poor detection rather than absence. Overloading, broad spots, or an uneven solvent front also weaken the readout. These are interpretation checks, not extra observations claimed for the protocol.

For thin-layer chromatography, define the TLC retardation factor using distances along the development direction:The origin coordinate is $y_0$; use the centre of a resolved spot and a front marked at the end of development. Both distances begin at the origin, never at the plate bottom. The ratio is dimensionless. An immobile spot has $R_f=0$; a spot at the front has $R_f=1$, where separation from other fast components may be poor. This distance ratio is distinct from the chromatographic partition coefficient and from a column retention time.

For an original stylized plate, choose absolute vertical coordinates $y_0=1\mathrm{cm}$, $y_{\mathrm{front}}=11\mathrm{cm}$, $y_A=4\mathrm{cm}$ and $y_B=8\mathrm{cm}$ above the plate bottom. The solvent distance is therefore $10\mathrm{cm}$, while the spot distances are $3\mathrm{cm}$ and $7\mathrm{cm}$:Using $4/11$ or $8/11$ would wrongly include the unused centimetre below the origin. These chosen positions do not represent measured pigments or the protocol's plate dimensions.

The TLC retardation factor depends on the sample, chromatographic stationary phase, eluent composition, and temperature; plate handling and development conditions must also match when comparing runs. The moving wet front and possible surface adsorption require their own transport assumptions. The equilibrium mobile fraction $f_m$ in the chromatographic partition coefficient model is not unconditionally equal to $R_f$; even that model depends on the phase volume ratio. Coincident TLC retardation factors alone never prove chemical identity.