Modern analytical techniques 1 and 2

Question 1

What does the frequency of infared radiation absorbed by a covalent bond depend on?

Answer 1

• atoms that are either side of the bond
• the position of the bond in the molecule.

Question 2

M+1 peak is the same as what?

Answer 2

relative molecular mass

Question 3

What are the steps of Mass spec?

Answer 3

• Ionisation: Vaporised Atoms bombarded with high energy electrons, and form a positive ion and a radical
• Acceleration: positively charged plates accelerate the ions.
• Deflection: done by an electromagnet which creates varying EMFs
• Detection.

Question 4

Which part of the mass spectrometer separates ions of different masses?

Answer 4

Electromagnet

Question 5

) In the first half of the twentieth century, oxygen was used as the standard for
relative atomic mass. The unit of atomic mass was defined as 1∕16 the mass of an
oxygen atom. This was based on samples of oxygen obtained from the air which
consisted of a mixture of oxygen isotopes.
Suggest one reason why the use of this standard was discontinued.

Answer 5

Difficult to obtain pure oxygen from air

Question 6

Would you expect the first electron affinities of 16O and 18O to differ? Justify your
answer.

Answer 6

No.
They have the same electronic configuration

Question 7

Bromine has two isotopes.
Explain what is meant by an isotope in terms of subatomic particles.

Answer 7

• same number of protons and electrons
• different number of neutrons.

Question 8

Explain how ions are produced in the Mass spec

Answer 8

• atoms bombarded by high energy electrons through an electron beam
• which results in loss of an electron, forming a positive ion

Question 9

State what is used to deflect the ions in the deflection stage.

Answer 9

• electromagnet

Question 10

What would be the effect, if any, on the m/e value of the peak if the ion detected
had lost two electrons rather than one electron

Answer 10

• it would be halved

Question 11

One of the uses of mass spectrometers is for the detection of banned substances,
such as anabolic steroids, in a blood or urine sample taken from competitors in
sports events.
(i) Suggest two precautions that are necessary to ensure that the result of any
analysis would be valid.
(2)

Answer 11

• running a control sample
• sterilising the equipment used to handle sample

Question 12

These substances can give competitors an unfair advantage. Suggest why the
use of these substances may be of concern to the user.

Answer 12

• Health concerns

Question 13

Suggest one other use for mass spectrometers

Answer 13

Carbon dating

Question 14

Explain the electrical conductivities of sodium and of sodium oxide in the
solid and liquid states.

Answer 14

• Sodium has delocalised electrons in both states, so can carry charge
• but Sodium oxide doesn’t have free moving ions in the solid state
• NaO2 has mobile ions in the liquid state, which can carry a charge.

Question 15

State how you could find the molecular mass of a substance from its mass spectrum

Answer 15

• The last peak of the spectrum

Question 16

In recent years a particular linen cloth was shown, using mass spectrometry, to
have been made from flax grown in the early 14th century. Suggest how mass
spectrometry can be used to estimate the age of the cloth.

Answer 16

• first find the abundance of C-14
• compare to modern day linen cloths’ C-14 abundance

Question 17

where does the peak move if temp is decreased?

Answer 17

To the left
height is increased

Question 18

where does the peak and Ea move with a catalyst?

Answer 18

• Ea moves to the left
• peak unchanged

Question 19

Where does the peak move if there is a higher conc of reactants?

Answer 19

• higher, does not move to the left or right.

Question 20

what is the m/z ratio

Answer 20

mass of a fragment divided by its charge

Question 21

why is the M+1 peak there?

Answer 21

• small amount of carbon 13 isotope.

Question 22

what is 13C NMR used for?

Answer 22

• info for how carbon atoms are arranged

Question 23

When will a nucleus have a spin?

Answer 23

• when the nucleus has an odd number of nucleons

Question 24

What does NMR detect?

Answer 24

• when the nuclear spin creates a magnetic field, and when effected by an external magnetic field,

Question 25

What does high resolution proton NMR tell us?

Answer 25

• how hydrogen atoms are arranged.

Question 26

describe how NMR works

Answer 26

• Radio waves fired
• energy from radio waves is absorbed by the nuclei, making them shift against the spin of the external magnetic field
• those with higher energy can also drop to lower energy and emit radio waves
• NMR measures the amount of energy that is absorbed.

Question 27

What is TMS used for?
why is it good for its purpose?

Answer 27

• to calibrate spec for C-13 spec, used as a comparison for all other peaks.
• volatile, inert, non toxic, and has 12 Hs in identical environments, so produces one peak at 0

Question 28

What is delta?

Answer 28

• chemical shift for C-13 NMR.

Question 29

What does the height represent on C-13 NMR?

Answer 29

• ratio of Carbons.

Question 30

What is the relationship between shielding and energy absorbed in C-13 NMR?

Answer 30

• higher the shielding, the less energy absorbed.

Question 31

How does electronegativity difference effect peaks in C-13 NMR?

Answer 31

• higher electronegativity
• less shielding on the C
• so more energy absorbed
• so chemical shift will be higher 9further to the left)

Question 32

What we look for with cyclic compounds in C-13 NMR

Answer 32

• symmetry, can use to see similarity and diffs of different carbon environments

Question 33

Give the method of how a C-13 NMR would be analysed

Answer 33

• chemical shifts would be noted down
• number of carbon environments would be noted down (number of peaks)
• note down possibilities of functional groups using data booklet.
• deduce compound (also using other data)

Question 34

What does proton NMR tell us? (2)

Answer 34

• how many different Hydrogen environments there are.
• how many hydrogens in each environment in the sample

Question 35

what do the number above the peak tell you in proton NMR

Answer 35

• ratio of the areas under the peaks
• allows us to work out the relative number of hydrogens in each environment,

Question 36

What does a triplet peak tell us about the hydrogens on the neighbouring carbon?

Answer 36

2 Hs on the neighbouring carbon.

Question 37

What happens in a proton MNR if the substance is dissolved in D2O

Answer 37

• NH2 groups
• OH groups
  both don’t produce peaks

Question 38

Describe the purpose of elemental analysis.

Answer 38

• to determine the empirical and molecular formula of a compound.
• helps determine percentage composition or mass of elements that make up a compound
• this can help us figure out the structure of the compound.

Question 39

Describe the method of Thin Layer chromatography

Answer 39

• draw pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along the line
• use a capillary tube to add a tiny drop of each solution to a different spot ad allow the sample to air dry.
• add solvent to a chamber with a lid so that the depth is no more that 1cm.
• place the TLC plate into the chamber and replace the lid.
• when the level of solvent reaches about 1cm from the top of the plate, remove the plate and mark the solvent line with pencil, allow to dry in a fume cupboard.
• place plate under UV lamp/ use ninhydrin spray for amino acids to see the spots - drawing their positions in pencil
• calcualte rf value.

Question 40

Give the equation for calculating Rf value

Answer 40

distance by substance/distance moved by solvent.

Question 41

How would you deduce a certain substance using TLC?

Answer 41

• calculate its Rf values to a known database.

Question 42

State the mobile and stationary phase in HPLC

Answer 42

mobile phase: liquid
Stationary phase: solid silica

Question 43

State the mobile and stationary phase in Gas chromatography

Answer 43

• mobile phase: inert gas, eg nitrogen
• Stationary phase: viscous liquid on an inert solid.

Question 44

What is the purpose of Gas chromatography?

Answer 44

• used to separate mixtures of volatile liquids.

Question 45

Describe how gas chromatography works

Answer 45

• carrier gas, eg nitrogen is passed through column
• sample is vaporised and passed through column.
• the retention time is measured 0 the time taken for the sample to travel from the injection site to the detector.
• the retention times are then compared to a known database of values.

Question 46

Name 2 practical uses of gas chromatography

Answer 46

• airport security
• forensics