RP 12 - organic

what is RP 12

separation of species by thin layer chromatography (TLC)

analyse medicine samples method

1. crush with pestle and mortar

2. transfer to weighing boat

3. dissolve 0.1g in 0.5cm3 ethanol

4. repeat

dissolving caffeine and anadin tablets

7cm3 ethanol

TLC method

1. draw line 1cm above TLC plate and mark 5 equally spaced spots along it

2. use capillary tube to add tiny drop each solution to a different spot and allow plate to dry

3. add 10cm3 solvent to development chamber

4. place TLC plate into development chamber making sure solvent is below line, add lid

5. when solvent reaches 1cm from top remove and mark solvent front with pencil

6. allow plate to dry in fume cupboard

7. place plate under UV lamp and draw around spots in pencil

8. calculate Rf values

why are gloves worn

prevent contamination from hands to plate

whys a pencil used

does not dissolve in the solvent

whys a tiny drop added

too big will cause the different spots to merge

whys the depth of solvent important

if its too deep it will dissolve the sample spots from the plate

whys a lid used and tightly sealed

1. prevents evaporation of toxic solvent

2. so inside of tank is saturated with solvent vapour

why does solvent line rise near to the top of the plate

get more accurate results - but Rf value can still be calculated if it does not reach top

why is it dried in fume cupboard

solvent is toxic

whys a UV lamp used

if spots are colourless and not visible

what ruler is used to measure Rf values

mm ruler - higher resolution so more precise Rf values calculated

what happens if you use less solvent and have a high baseline

large spots

what happens if your samples are too concentrated

spots overlap

ways to see colourless spots

1. place plate under UV lamp and mark locations using pencil

2. in a fume cupboard place the plate in a beaker containing iodine crystals and cover with a watch glass. iodine is a locating agent causing spots to become brown and visible

3. spray with ninhydrin developing agent in a fume cupboard

how can TLC be used to monitor the course of a reaction

by taking samples from a reaction mixture at regular intervals, spotting them on a TLC plate, and running this alongside controls of the organic reagent and product

different chromatography types

1. TLC

2. CC

3. GC

TLC

quick analysis to identify substances or check if reaction is finished

TLC strengths

• very fast

• uses tiny amounts

• cheap, simple equipment

• easy to calculate Rf and compare against known standards

TLC weaknesses

• purely analytical - cannot use to collect components

• spots can overlap if solvent not chosen carefully

• some compounds are colourless

CC

physical separation and purification of larger quantities of a mixture

CC strengths

• actually separates mixture into fractions you can collect in separate flasks

• can handle relatively large quantities

CC weaknesses

• slow and tedious compared to TLC

• uses more solvent which is wasteful

GC

separating and identifying volatile liquids in extremely complex mixtures (ie drug testing, forensics)

GC strengths

• extremely sensitive

• highly accurate retention times and can measure abundance

• can be directly linked to a mass spectrometer to instantly identify compounds

GC weaknesses

• only works for volatile substances

• expensive, highly specialised machinery

• cannot easily isolate physical, pure samples of components

what else can the solvent be other than toxic

flammable

analyse the purity

not pure - contains 3 components

• spots too close

• they can cross over and contaminate neighbouring spots