JR - Separation Science

What are the main stages of a measurement system?

Sampling, separation, transduction and analysis

Why are chemical separations carried out?

• Isolate analyte from other components to avoid possible interferences

• To preconcrete the analyte to raise its concentration above the detection limit, or make it easier to measure.

• To enable the use of a universal or non-selective detector (for e.g. flame ionisation detector in gas chromatography that responds to the presence of C in a molecule)

What is partitioning?

The distribution of an analyte between two phases

What causes partitioning to occur?

Differences in solubility and chemical potential between phases

What happens when chemical potential equilibrates between phases?

Partitioning stops because equilibrium is reached

What is the partition coefficient (K)?

The ratio of analyte concentration/activity between 2 phases at equilibrium

What assumption allows activity to be approximated by concentration?

Low analyte concentration

What is the distribution ratio (D)?

The ratio describing partitioning of all forms of a compound (e.g charged and uncharged)

Nernst’s Law

Concentration ratio between phases is constant at equilibrium

What is solvent extraction?

Separation based on partitioning between 2 immiscible liquids

What measurements may be used to determine analyte concentration?

UV/Vis spec or atomic absorption spec

What is counter current separation?

A process where fresh phases are introduced at each equillibrium step

What is the Craig appararus?

A discrete - vessel implementation of counter current separation

What is a theoretical plate?

An individual vessel

Used as there is a hypothetical zone, volume or distance over which an equilibrium partitioning occurs

What rule of thumb describes solubility?

Like dissolves like

What are dispersion (Van der Waals forces)?

Interactions between transient dipoles caused by electron oscillations

What are dipole dipole interactions?

Interaction between permanent dipoles

What is an ion-dipole interaction?

Attraction between ion and polar molecules

Why is water highly effective at stabilising charges?

Has high dielectric constant and hydrogen bonding

What does the dielectric constant describe?

Solvent’s ability to reduce electrostatic attraction between charges

What is the approximate dielectric constant of water?

80

Why is water’s dielectric constant unusually high?

Hydrogen bonding aligns dipoles into large effective dipole

What the mobile phase?

The moving phase carrying solutes through the system

What is the stationary phase?

The phase that remains fixed inside the column

What are the 4 main components of a chromatographic system?

Mobile phase

Injector

Column

Detector

Why do different solutes separate in chromatography?

They interact differently with the stationary and mobile phases based on their polarity and their affinity to each phase.

What determines solute velocity trhough a chromatographic column?

The time spent in the stationary phase

What is a chromatogram

The detector output showing the separated components over time.