Separation of analytes
Separation of analytes - contextAnalyte mixtures and separation - 1Analyte mixtures and separation - 2Analyte mixtures and separation - 3Introduction to chromatographyChromatographic columnsPrinciple setup of a chromatographic unitExample of separationChromatographic system - 1Chromatographic system - 2Standard gas chromatograph(Old) liquid chromatography unit(New) liquid chromatography unitHPLC columnsComparison GC and HPLC
Selftest
1) Purpose of chromatographic process Answer 2) Unsuitable compounds Answer 3) HPLC vs. GC Answer 4) Detectors Answer 5) Solvent properties Answer 6) Retention time Answer 7) Analyte identification Answer 8) Irreversible or reversible sorption process Answer 9) Short analysis time Answer 10) Retention factor Answer 11) Breakthrough-curve AnswerProblems
End of lesson9) Short analysis time
Question:
How is it possible to keep the analysis time in (i) a GC and (ii) a HPLC as short as possible, if analytes are separated that differ a lot in their sorption to the stationary phase
(i.e., have drastically different retention factors)?
Answer:
The "eluting ability" of the mobile phase (gas in GC and a liquid in HPLC) is increased steadily during the separation process.
In other words, the "boundary conditions" are changed such that the partition constants of the analytes shift towards the mobile phase.
In GC, this is achieved by increasing the temperature (recall T-dependence of partitioning between a condensed and a gas phase - Vant Hoff Equation).
In HPLC, this is achieved by changing the composition of the mobile phase (e.g. from pure water to MeOH) over the course of the separation.
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