6.3 Analysis

Question 1

what are the two types of chromatography

Answer 1

Gas
Thin layer chromatography

Question 2

How does chromatography work

Answer 2

Works on the basis that components have different affinities for a stationary and mobile phase

Question 3

define stationary phase

Answer 3

fixed in place such as the paper in TLC

Question 4

define mobile phase

Answer 4

moves in a definite direction - solvent in TLC

Question 5

Describe separation by adsorption

Answer 5

component molecules bind to the surface of the solid stationary phase, the stronger the adsorption to the stationary phase the more the components molecules are slowed down

Question 6

describe separation by absorption

Answer 6

components dissolve in the liquid stationary phase, the greater the solubility in the stationary phase the more the components molecules are slowed down

Question 7

What are the stationary and mobile phases in TLC

Answer 7

Stationary - silica gel SiO2
mobile - solvent

Question 8

How would you produce a chromatogram

Answer 8

dissolve the sample
draw a pencil line and spot the sample using a capillary tube and allow it to dry
place the plate in a tank of solvent - which must be below the line
separation is by adsorption so allow the solvent to almost reach the top and draw a line at the solvent front

Question 9

How would you find rf values if the spots are colourless

Answer 9

ninhydrin or UV lamp

Question 10

How do you calculate rf value

Answer 10

distance moved by solvent/ distance moved by solvent

Question 11

what are the limitations of chromatography

Answer 11

similar compounds have very similar rf values
unknown compounds have unknown rf values
if the components have little solubility they will hardly move

Question 12

how does gas chromatography work

Answer 12

the stationary phase is a solid or liquid coating the coiled tube; usually a hydrocarbon with a high boiling point

The mobile phase is an inert gas carrier such as helium or nitrogen

The coiled tube is often connected to a mass spectrometer for further analysis

Question 13

How would you interpret the results from a gas chromatogram

Answer 13

the chart will produce a graph of absorption against time
The x-axis is the retention time of a chemical and the value can be compared against a data book

the area under the absorption peak is proportional to the concentration of each component

Question 14

Define retention time

Answer 14

the time taken for a component to travel from the inlet to the detector in gas chromatography

Question 15

define chemical shift

Answer 15

the scale that compares the frequency of NMR absorption with the frequency of the reference peak of TMS

Question 16

What is TMS

Answer 16

an internal standard for both carbon and proton NMR - tetramethylsilane (CH3)4Si has a value of 0ppm

Question 17

Define deuterium

Answer 17

an isotope of hydrogen and does. not produce a signal in the proton NMR spectrum

Question 18

What is the purpose of NMR

Answer 18

the spectrum can confirm the formula mass of an organic chemical and give clues to the structure of the chemical

Question 19

How does NMR work

Answer 19

H1 and C13 have a property known as spin. These isotopes are put in a large magnetic field and the direction of the spin aligns with the magnetic field.

Energy in the form of radio waves is used to force the spins to flip and change direction. The energy required to change the alignment depends on the environment that the atom is in and can be compared to a standard

Question 20

Why is TMS a good internal standard?

Answer 20

contains both carbon and hydrogen so can be used for both NMRs

hydrogens are all in the same environment so only one peak is produced

inert and non-toxic

low boiling point so it can be removed easily from the sample

Question 21

What would the chemical shift of a molecule similar to TMS look like

Answer 21

more upshifted

Question 22

What solvents are used for NMR

Answer 22

organic solvents that contain hydrogen isotopes that are not NMR active
deuterated solvents such as CH3Cl would become the heavy CDCl3

Question 23

define equivalent protons

Answer 23

hydrogen atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR spectrometer

Question 24

describe spin-spin coupling

Answer 24

The protons on the neighbouring carbon atoms affect the magnetic field and the alignment of protons and causes a split in the signal - different numbers of adjacent protons will cause different numbers of splits

Question 25

how would you figure out the number of peaks in the splitting pattern

Answer 25

number of protons adjacent on carbon atoms + 1

Question 26

describe what impacts rf value or retention time

Answer 26

in gas chromatography separation is by relative solubility whereas in tlc it is adsorption
in gas chromatography if the stationary phase is non-polar, non-polar compounds will interact the most, polar compounds will interact the least and have shorter retention times

Question 27

how would you interpret a gas chromatogram

Answer 27

area of peak/ total area of all peaks
an area of peaks is proportional to the amount of a component in a sample

Question 28

how can concentration of a component be found

Answer 28

by using a calibration curve
prepare standard solutions of compound under investigation
obtain gas chromatogram for each concentration
com pare area under curve for the compound under investigation with calibration curve