topic 19: spectroscopy and chromatography Flashcards
what are the different types radiation in a molecule
infrared in analysis
microwaves for heating
radiowaves in NMR
ultraviolet in initiation of reactions
when does the NMR spectroscopy involve
involves interaction of materials with the low- energy radiowave region of the electromagnetic spectrum
what so the radiowaves used in the proton NMR cause
causes the hydrogen nucleus to change its spin state
NMR use the same technology as what
magnetic resonance imaging (MRI)
in an hydrogen NMR spectrum what does one signal represent
each set of equivalent H atoms
eg. ethanol has 3
groups of different
hydrogen atoms
in H NMR, what is the intensity (integration value) of each signal proportional to
the number of the equivalent H atoms it represents
in H NMR, what are the solvents used
solvents without any 1H atoms
eg. CCl4, CDCl3
the solvent used dont have 1H atoms what does this mean
this means that in the H NMR the solvent will not give any peaks
state the use of CCl4 as a solvent
a non polar compound compound that is a good solvent for non-polar organic molecules
what the use of CDCl3
a polar covalent molecule that is a good solvent for polar organic molecules
a small amount of what solution is added to the sample to calibrate the spectrum
TMS (tetramethylsilane)
why is tetramethylsilane (TMS) used
*its signal is away from all the others
*it only gives one signal *gives strong signal so only a small amount needed
*it is non-toxic
*it is inert
*it has a low boiling point and so can be removed
from sample easily
the spectra is recorded on what scale and what does it do
the chemical
shift (δ), which is how much the field has shifted away from the
field for TMS
what is δ a measure in
parts per million (ppm)
what is the δ a scale of
a relative scale of how far the frequency of the proton signal has shifted away from that for TMS
what does the δ depend on
what other atoms/groups are near the
H – more electronegative groups gives a greater shift
splitting of peaks
= number of inequivalent H’s on neighbouring C atoms + 1
describe singlet
- 1 splitting peak
- 0 neighbouring inequivalent H atoms
describe doublet
- 2 splitting peaks
- 1 neighbouring inequivalent H atoms
- 1:1 —> relative size
describe triplet
- 3 splitting peaks
- 2 neighbouring inequivalent H atoms
- 1:2:1 —> relative size
describe quartet
- 4 splitting peaks
- 3 neighbouring inequivalent H atoms
- 1:3:3:1 —> relative size
describe quintet
- 5 splitting peaks
- 4 neighbouring inequivalent H atoms
- 1:4:6:4:1 —> relative size
what is it called when there is more than 5 splitting peaks
multiplet
what is chromatography
an analytical technique that separates
components in a mixture between a mobile phase and a stationary phase
what does separation by column chromatography depend on
the balance between solubility in the moving phase and retention in the stationary phase
how does solid stationary phase separate by
adsorption
how does liquid stationary phase separate by
relative solubility
what is the state of the mobile phase
liquid or gas
what is the state of the stationary phase
- may be a solid (as in thin layer chromatography, TLC)
- a liquid or solid on a solid support (as in gas
chromatography, GC)
if the stationary phase was polar……
the moving phase was non- polar
- e.g. hexane
then non- polar compounds would pass through the column more quickly than polar compounds as they would have a greater solubility in the non-polar moving phase
what does HPLC stand for
high performance liquid chromatography
what is the stationary phase of HPLC
a solid silica
what is the mobile phase of HPLC
a liquid
what is the mobile phase in gas - liquid chromatography
a inert gas such as nitrogen, helium, argon
what is the stationary phase in gas - liquid chromatography
a liquid on an inert solid
what is gas - liquid chromatography be used for
be used to separate mixtures of volatile liquids
what is retention time
The time taken for a particular compound to
travel from the injection of the sample to where
it leaves the column to the detector
what is GC-MS used for
in analysis, in forensics, environmental
analysis, airport security and space probes
if gas liquid chromatography is added to IR or NMR machine what does it enable
enabling all the components in a mixture to be identified
what is the method of TLC
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
what are the precautions of using TLC
- wear plastic gloves to prevent contamination
from the hands to the plate - pencil line –will not dissolve in the solvent
- tiny drop – too big a drop will cause different
spots to merge - depth of solvent– if the solvent is too deep it
will dissolve the sample spots from the plate - lid– to prevent evaporation of toxic solvent
- will get more accurate results if the solvent is allowed to rise to near the top of the plate but
the Rf value can be calculated if the solvent
front does not reach the top of the plate - dry in a fume cupboard as the solvent is toxic
- UV lamp used if the spots are colourless and
not visible
if using amino acids, what can be used instead of a UV lamp
ninhydrin spray to locate spots
equation of Rf
distance moved by the solvent
The dipeptide formed is hydrolysed under acidic conditions and the resulting mixture is analysed by column chromatography. the column uses a polar stationary phase. explain why lysine leaves the chromatography column after alanine
lysine ion has 2 positive charges whereas alanine has 1 so lysine has greater attraction for the stationary phase
explain why different compounds will have different retention times in the same
column, under the same conditions
- depends on the solubility of the component of the stationary phase. the greater the solubility of the component of the stationary phase, the greater the retention time
- depends on the boiling temp of the compounds. high boiling temp compounds spend less time in the gas phase/ mobile phase so have longer retention time
in the tripeptide, giving a reason for the
lack of a third spot on the TLC
one amino acid is present twice
OR
another amino acid has the same Rf value as the other amino acids
give two reasons why different amino acids have different Rf values
amino acids have different solubility to the stationary phase
amino acids have different solubility in the mobile phase
