Year 11 Chemistry - Chromatography

define chromatography

a separation technique for separating mixtures into components to analyse, identify and purify or quantify the mixture or components

what does chromatography rely on

the affinity of components to the stationary or mobile phase

define the mobile phase

the component which carries components of mixture through the stationary phase

define the stationary phase

the component which does not move.

it can be a solid or a liquid supported by a solid

what are the 3 types of paper chromatography

ascending

descending

ascending-descending

describe the 3 types of paper chromatography

ascending → the solvent travels up the paper

descending → the solvent travels down the paper

ascending-descending → the solvent travels up and then down the paper

what causes the components to move further along the paper

lighter components will move further

components more soluble in the mobile phase move further

how does paper chromatography work

it works on capillary action → the adhesion, cohesion and surface tension forces cause the mobile phase to move within the porous spaces.

the mobile phase moves up because it’s attraction for itself is stronger than the force of gravity

it also works with solubility of components in both phases

what is the retention factor in paper chromatography

for paper chromatography it refers to how far the component moved

the higher the retention factor the further the component moves

what is the retention factor formula for paper chromatography

Rf = distance moved by spot / distance moved by solvent

what must the retention factor be less than in paper chromatography

less than one because no component moves further than the solvent

describe thin layer chromatography

uses a small amount of analyte and a liquid mobile phase

the stationary phase is a thin layer of silica gel or alumina on a glass slip

components separate by how much they absorb on the stationary phase vs dissolve in the mobile phase

what polarity is silica gel

silica gel has hydroxide groups on the outer edge, making it polar

hence polar substances won’t move as far as they will be attracted

how can we see components on TLC which aren’t obvious

UV light

describe HPLC

stands for High pressure/performance liquid chromatography

components are separated by interactions with packing particles

uses silica gel and alumina also

name and describe the components of a HPLC machine

pump → forces liquid solvent through chromatograph at a certain rate

injector → introduces sample into mobile phase by syringe

column → holds the stationary phase that separates the components. the column is tightly packed which causes the high pressure, hence the pump must work hard

detector → detects individual molecules and measures the amount of each component that come out of the column

computer → takes signals from detector to produce a chromatogram

describe reverse phase HPLC

the mobile phase is polar

the stationary phase is non polar

it is cheaper, uses less analyte and less toxic

describe normal phase HPLC

mobile phase is non polar

stationary phase is polar

what must the substance being analysed in HPLC be

non volatile or thermally unstable

high molar mass liquid at room temperature

what does retention time in HPLC reveal

high retention time = moved slower, so more attracted to stationary phase

define a trace compound

a substance which is of interest but in very small concentration

describe gas liquid chromatography

analyses volatile liquids using a gas chromatogram

list the parts of a gas chromatogram

carrier gas

injector

column oven

detector

data collection software

descrieb the steps of gas liquid chromatography

1. a syringe injects a small smaple through a thick rumbber septum which reseals as the syringe is removed

2. injector is in an oven to boil the sample and is carried into the column by an inert carrier gas

3. carrier gas carries the sample through the column and different components move through at differet speeds

4. detector produces a gas chromatogram

describe the requirements of a carrier gas

pure

inert

O2 free

type depends on use

describe the two types of columns

packed → filled with particulate solid for large surface area

capillary → stationary phase coated on pipe inner surface

describe requirements of the stationary phase

non volatile and high boiling point

how does volatility affect retention time

high volatility = low retention time

low volatility = high retenetion time

temeprature increase can also cause retention time decrease