3.3.16 - chromatography

Question 1

thin layer chromatography (TLC) allows us too…

Answer 1

separate and identify unknown compounds

Question 2

stationary phase of TLC and mobile phase

Answer 2

Stationary phase - doesn’t move - silica or alumina mounted on a glass/metal plate
a pencil base line is drawn and drops of mixtures added

Mobile phase - moves - solvent or eluent

Question 3

what is done with stationary phase of TLC

Answer 3

placed in the solvent - the base line must be above the solvent level
leave until solvent has moved up to near the top of the plate
remove, mark the solvent front and allow to dry

Question 4

how does TLC work

Answer 4

works by mixture spots dissolving in the solvent
some chemicals in the mixture may not dissolve as much and stick to the stationary phase quickly
what we are left with is a chromatogram

Question 5

how can we identify chemicals

Answer 5

using their positions on the chromatogram

Question 6

how do we see a colourless compound/ spot on a chromatogram

Answer 6

colourless compounds can be seen using iodine or fluorescent dues and uv light

Question 7

how are fluroscent dyes and uv light used to see colourless compounds

Answer 7

adding a flurosecent dye to silica/alumina can be seen using a uv lamp
the colourless spots on the chromatogram will block any glow from the fluorescent dye
you can then draw round these spots to mark where they are

Question 8

how is iodine used to see colourless compounds

Answer 8

place the chromatogram in a sealed jar with a few iodine crystals
the iodine vapour sticks to the chemicals on the plate dying them purple

Question 9

iodine vapour when using iodine to see colourless compounds is known as a…

Answer 9

locating agent

Question 10

Size of sample in chromatography and what can be done after separation

Answer 10

Only small sample
After seperation pure components can be further analysed

Question 11

TLC adv

Answer 11

Simple and quick

Question 12

TLC method

Answer 12

a) Wearing gloves, draw a pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along line.
b) Use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry.
c) Add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth
d) Place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal.
e) When the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent level with a pencil. Allow the plate to dry in the fume cupboard.
f) Place the plate under a UV lamp in order to see the spots.
Draw around them lightly in pencil.
g) Calculate the Rf values of the observed spots.

Question 13

Why use a pencil line

Answer 13

Won’t dissolve in solvent

Question 14

Tiny drop used bc

Answer 14

Too big a drop will cause different spots to merge

Question 15

Depth of the solvent why

Answer 15

If solvent to deep it will dissolve the sample spots from the plate

Question 16

Why use a lid

Answer 16

Prevent evaporation of toxic solvent

Question 17

Dry in fume cupboard bc

Answer 17

Solvent is toxic

Question 18

Uv lamp used bc

Answer 18

the spots are colourless and not visible

Question 19

Rf =

Answer 19

Distance traveled by spot
—————————————
Distance travelled by solvent

Question 20

TLC works dep on the

Answer 20

affinity of the component molecules for the mobile phase compared to the stationary phase.

Question 21

The greater the solubility of the component molecules in the solvent…

Answer 21

the further the component molecules move up the TLC plate.

Question 22

Solubility of the component molecules depends on the …

Answer 22

intermolecular forces between solvent and component molecules.

Question 23

After you calc Rf value you …

Answer 23

Compare to a known reference value to identify substance
Known Rf value must be for same solvent

Question 24

2 dimensional chromatography

Answer 24

In order to separate a complex mixture that has components of different solubility in solvents, it may be necessary to do chromatography with two different solvents.
A spot of the mixture on a TLC plate is first separated with one solvent.
Then the TLC plate is rotated 90° and the plate is placed in a second solvent for a second
separation to take place

Question 25

Elute means

Answer 25

When a substance leaves a chromatography column

Question 26

Elutant means

Answer 26

This is the solvent/gas that moves the sample through the chromatography column

Question 27

Column chromatography

Answer 27

Column chromatography is a technique for purifying individual components from mixtures. It uses a glass column filled with a solid adsorbent as the stationary phase.

Question 28

How column chromatography works

Answer 28

1. The mixture is placed at the top of the column filled with the stationary phase. - solvent and silica 2. A solvent is passed through the column, carrying the mixture components at different rates. 3.Components separate based on their solubility in the solvent and their adsorption to the stationary phase. 4. The separated components exit the column and are collected for analysis.

Question 29

Gas chromatography is

Answer 29

Gas chromatography (GC) separates volatile liquid mixtures into individual substances

Question 30

How gas chromatography works:

Answer 30

1. A liquid sample is vaporised in a heated chamber. 2. An inert carrier gas then transports the vaporised sample through a chromatographic column containing the stationary phase, consisting of either an immobilised solid or a solid coated with a non-polar high boiling liquid. 3. The column has a stationary phase that interacts differently with each component of the sample. 4. Components are separated based on how they partition between the mobile gas phase and the stationary phase. 5. As they exit the column, a detector records the separation, producing a chromatogram with distinct peaks for each component.

Question 31

Stationary phase of gas chromatography

Answer 31

Liquid supported on an inert solid

Question 32

Mobile phase of gas chromatography

Answer 32

Unreactive gas like nitrogen or Argon

Question 33

Gas chromatography mass spectrometry what is it

Answer 33

GC-MS - GC-MS combines the separation capabilities of gas chromatography with the identification power of mass spectrometry, making it a highly effective method for analysing complex mixtures.

Question 34

How GC-MS works

Answer 34

1. The mixture is first separated by gas chromatography. 2. Each component is then introduced into a mass spectrometer, where it is ionised. 3. A distinct mass spectrum for each component is generated. 4. Computerised matching to spectral libraries then identifies each substance based on their unique fragmentation patterns.

Question 35

Retention time

Answer 35

time taken for the corresponding substance to travel through the chromatographic column. Can be shown on a a graph splay peaks that represent different substances in a mixture. The position of each peak is determined by its retention time,

Question 36

To identify substances using retention time

Answer 36

To identify the substances in the mixture, compare the retention times of the peaks with those of known standards.

Question 37

To identify substances using retention time

Answer 37

To identify the substances in the mixture, compare the retention times of the peaks with those of known standards.

Question 38

Why add developing solvent to a depth of not more than 1cm in TLC

Answer 38

if too deep it will dissolve the mixture from the plate