Example 1
A TLC spot travels 3.6cm while the solvent front travels 6.0cm. Find RF.
RF=6.03.6=0.60. Both distances are measured from the original baseline; the ratio has no unit.
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Year 12 · SACE Stage 2
Separation, retention and ion exchange
Explain TLC, GC, HPLC and ion-exchange evidence using molecular interactions.
Method and worked examples
Chromatography uses stationary and mobile phases. Stronger stationary-phase interaction generally means slower travel under matched conditions. In normal-phase TLC, polar silica often retains polar components more strongly. RF= spot-centre distance / solvent-front distance, measured from the same baseline. It is dimensionless and depends on the phases and conditions.
GC uses a carrier gas for suitable volatile, thermally stable analytes; HPLC uses a pressurised liquid mobile phase. Retention times support identification against matched standards, but co-eluting compounds can share a peak. Ion-exchange resins have fixed charged sites and exchange counter-ions. Competing-ion concentration or pH can change retention by shifting exchange equilibria.
Example 1
Example 2
A negatively charged resin initially holds Na+. Explain capture of Ca2+.
Two negative sites can bind one Ca2+ while releasing two Na+ into solution. Exchange depends on equilibrium and ion concentrations; total electrical charge is conserved.
Watch out: RF depends on both phases and operating conditions; matched standards provide supporting, not uniquely conclusive, identification.