module 6.3.1-6.3.2 - chromatography and spectroscopy

Question 1

in thin layer chromatography, by what process is a mixture sepatared?

Answer 1

adsorption (NOT absorption), it sticks on the surface

Question 2

how can you use thin layer chromatography in continuous monitoring?

Answer 2

take samples of the reaction mixture at regular intervals and run a TLC alongside the reactants/products to show when the reaction has finished.

Question 3

how is thin layer chromatography used in analysis of a mixture

Answer 3

measure how far each spot travels and calculate the Rf value. match the Rf value to a database of known values

Question 4

what is the formula for Rf

Answer 4

Rf = (distance measured by component)/(distance traveled by solvent)

Question 5

if a spot is lower down on the TLC plate, what does this tell you about the component?

Answer 5

it is more strongly adsorbed to the stationary phase and less soluble in the solvent. it has stronger intermolecular forces with the stationary phase.

Question 6

what are the limitations of thin layer chromatography?

Answer 6

similar compounds have similar Rf values (so spots may not fully separate). new compounds will not be in databases to compare to.

Question 7

what is gas chromatography used for?

Answer 7

to separate volatile liquids (low boiling points)

Question 8

what is the mobile phase in gas chromatography?

Answer 8

an inert gas, like N2 or He, which push the liquid through the colum

Question 9

when the stationary phase is a solid separation is by ____?

Answer 9

separation by adsorption

Question 10

when the stationary phase is a liquid separation is by ____?

Answer 10

seaparation by relative solubility

Question 11

how can separation be improved in gas chromatography?

Answer 11

varying the flow rate of the gas and the temperature

Question 12

what is retention time

Answer 12

time taken from injection to detection

Question 13

how is gas chromatography used in analysis is a mixture

Answer 13

retention times can be compared to a database to identify the compound

Question 14

what does the number of peaks on a gas chromatography chromatogram tell you?

Answer 14

the number of compounds preset

Question 15

what do the relative peak areas on a chromatogram tell you?

Answer 15

peak areas are proportional to the amount of compound present

Question 16

what are he “5 steps” for producing and using a calibration curve for gas chromatography?

Answer 16

1. make up standard solutions at varying concentrations of the compound being investigated.
2. obtain a gas chromatogram for each concentration
3. plot a calibration graph of the peak area against the concentration
4. for the unknown mixture being investigated, obtain agas chromatogram and the peak area
5. use the calibration curve to determine the concentration of the compound in the mixture

Question 17

what are the limitations of gas chromatography

Answer 17

some similar compounds have similar retention times, and new compounds will not have database values to compare to

Question 18

explain gas chromatography - mass spectroscopy

Answer 18

GC first separates the compounds based on their retention times
mass spectroscopy is used to analyse the molecular ion and fragmentation pattern.
mass spectrum can be matched to database valued to identify the compound
GC-MS is used to analyse samples on different planets, analyse samples found on crime scenes, identify pollutants in the atmosphere and detect explosives in airports.