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Flashcards in Mass spectrometry Deck (42)
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1

What are OE ions and EE ions?

OE ions have an odd number of electrons. EE ions have an even number of electrons.

2

How are OE ions formed during ionisation?

M + e- → M+● + 2e-

M +e- → M-●

3

How are EE ions formed during ionisation?

M + H+ → [M+H]+

M + Na+ → [M+Na]+

M - H+ → [M-H]-

M + Cl- → [M+Cl]-

4

How do you identify the molecular ion?

It is the peak with the highest m/z value, ignoring isotopes. It is usually at an even m/z. It is usually an OE ion (radical cation).

5

What happens during electron ionisation?

The molecule is ionised when the energy supplied equals the enthalpy of electron attachment. The ionising energy populates vibrational states within the energy well so ions of various energies are formed. The electron of lowest ionisation energy is removed as it is held less tightly (non-bonding > π > σ).

6

Why are fragment ions formed during ionisation?

At 70eV, there is excess energy so the ion can access higher vibrational levels, which can cause bond cleavage to occur. The weakest bonds in the molecule break to form fragment ions. At 16eV, fragmentation is unlikely.

7

Why are soft ionisation methods used?

They produce ions from non-volatile or unstable molecules. They generate ions with less residual energy, so less fragmentation occurs.

8

What is chemical ionisation?

It uses a reagent gas (CH4, NH3, isobutane), which is ionised in the ion source. The ionised reagent gas then transfers a proton to the molecule to be analysed. For example:

CH4 + e- → CH4+● + 2e-

CH4+● + CH4 → CH5+ + CH3

CH5+ + M → CH4 + [M+H]+

Proton transfer occurs is the proton affinity (PA) of the analyte is higher than the PA of the reagent gas. The greater the difference between the PA of the reagent gas and the PA of the analyte, the greater the energy transfer on ionisation.

9

What is atmospheric pressure chemical ionisation?

It is ionisation at atmospheric pressure (105Pa). A liquid is introduced into the source. The heater and N2 gas volatilise and remove the solvent. Ionisation occurs by corona discharge due to the high electric potential on the needle. This generates protonated water clusters, which transfer protons to the molecules to be analysed. A counter current of N2 gas is also used to prevent any neutral molecules from entering the mass analyser.

10

What are isotopologues?

They are isomers that have different isotope combinations. The relative intensities of their peaks reflect the natural abundances of the isotopes.

11

What are the classifications given to elements to identify them in a mass spectrum?

A is an element with one significant isotope. A+1 is an element with two significant isotopes separated by 1Da. A+2 is an element with at least two significant isotopes separated at least once by 2Da.

12

How do you calculate the abundance of a peak?

For each element in the molecule, multiply the number of times is occurs in the molecule by its relative abundance. Add these values together.

13

What are some common isotope profiles?

2 peaks in a 1:1 ratio, separated by 2Da is characteristic of 1 Br atom. 3 peaks in a 9:6:1 ratio, separated by 2Da is characteristic of 2 Cl atoms. 4 peaks in a 1:4:4:1 ratio is characteristic of 3 Br atoms.4 peaks in a 4:6:4:3 ratio is characteristic of 5 Cl atoms.

14

How do you find the probability of an isotope occurring in a molecule?

Multiply the number of times the element occurs in the molecule by its relative abundance. To find the probability of the isotope occurring twice, square its relative abundance and multiply it by the number of time the element occurs.

15

How do you find the number of carbon atoms in a molecule?

Scale the molecular ion cluster so the A peak becomes 100%. Divide the abundance of the A+1 peak by the relative abundance of 13C.

16

How can you estimate for error when calculating the number of carbon atoms?

Calculate the values at +10% and -10% for the abundance of the A+1 peak. Determine if n+1 or n-1 fall within this tolerance. If they do not, then it is a reasonable formula.

17

How do you calculate the degrees of unsaturation?

du (r + db) = x - 0.5y + 0.5z + 1

x = the number of tetravalent atoms

y = the number of monovalent atoms

z = the number of trivalent atoms

An OE ion gives an integer value. An EE ion gives a value ending in 1/2.

18

What is the nitrogen rule?

Nitrogen has an even mass and an odd valence. Compounds with an odd number of N atoms give the molecular ion at an odd m/z. Compounds with an even number of N atoms give the molecular ion at an even m/z.

19

What are the different types of fragmentation?

If one bond is broken, EE ions are formed. For example: M+● → A+ + BC

M+● → A + BC+ → B + C+

If two bonds are broken, rearrangement of the atom's bonds can occur and OE ions are formed. For example: M+● → C+● + D

20

What are the main factors affecting ion abundance?

Cation stability, electron sharing and resonance stabilisation.

21

What is Stevenson's rule?

The most probable fragment is where the positive charge is on the fragment with the lowest ionisation energy. If ions have similar stability, loss of the largest radical is favoured, which may lead to the loss of the least stable ion.

22

How are alkanes ionised and how do they fragment?

Ionisation of alkanes removes an electron from a σ-bond. Usually the molecular ion is weak, and the charge is not localised at any specific site so it can fragment at many position along the chain. Fragmentation occurs by σ-cleavage (the radical in the σ-bond moves to a carbon atom producing an alkyl cation and an alkyl radical). Secondary fragmentation can occur when the cation produces an alkene and another cation. The ion series is CnH2n+1.

23

What is radical site initiation?

The radical site has a strong tendency for electron pairing, which results in a-cleavage. This is homolytic cleavage.

 

24

How do aliphatc alcohols fragment?

They undergo a-cleavage and the loss of the largest alkyl is favoured die to the radical stability. They can also rearrange to remove water.

 

25

What are neautral losses?

Neautral species like radicals or molecules can be lost during fragmentation. They are not detected because they are not charged. They are recognised by the difference in m/z between ions. Only a few can be lost by cleavage of one bond.

26

How are alkenes ionised and how do they fragment?

Alkenes are ionised by the loss of a π-electron from a π-bond. They fragment by allylic cleavage, which is similar to a-cleavage, except the radical is on the bond. This forms a positively charged alkene and an alkyl radical. The alkene is stabilised by resonance so it is favourable. Alkenes are in the CnH2n-1+ ion series, with some CnH2n+1.

27

How are aromatic hydrocarbons ionised and how do they fragment?

Aromatic hydrocarbons are ionised by the loss of a π-electron from a π-bond. They fragment by benzylic cleavage, which is made more favourable by the alkyl group. This is because it can form a tropylium cation (a seven membered, fully aromatic structure).

28

What is charge-site initiation?

The positive charge strongly attracts an electron pair, which causes inductive cleavage. This is heterolytic cleavage. It is used for EE ions. For OE ions, inductive cleavgae competes with a-cleavage.

29

How does fragmentation occur in unsaturated heteroatomic molecules?

This includes carbonyls, amides, carboxylic acids and esters. The main driving force is the formation of a stable carbenium (acylium) ion. For aldehydes, hydrogen loss occurs if the acylium ion is stabilised (e.g. by resonance). For ketones, a-cleavage is more important.

30

How does fragmentation occur in cycloalkanes?

If one bond is broken, a stable OE ion is formed. It is a diastonic radical (there is a distance between the charge and the radical). For fragmentation to occur, two bond must be broken.