Mass Spectrometry: Ionisation, Detection, and Isotope Abundance

Give an equation

including state symbols

State how the detector enables the relative abundance of each ion to be determined

The (relative) abundance is proportional to the size of the current

definition of relative atomic mass

mean mass of 1 atom (of an element) divided by 1/12 mass of one atom of 12 Carbon

During TOF maths question and electrospray is used

what do you need to do at the end

How are ions detected by detector

(ions hit a detector and) each ion gains an electron (generating a current)

How is abundance measured

(the abundance is) proportional to (the size of) the current

Electrospray ionisation is used instead of electron impact for the ionisation of a protein in a mass spectrometry experiment. Suggest why

the protein (ion) does not break up/fragment

State why the isotopes of rhenium have the same chemical properties

same electron configuration
allow same number of electrons
allow same electron structure

State how the relative abundance of 185Re+ is determined in a TOF mass spectrometer

at the detector the ions gain an electron
(relative) abundance depends on the size of the current

Give two reasons why it is necessary to ionise isotopes before they can be analysed in a TOF mass spectrometer

(Ions will interact with and) be accelerated (by an electric field)
Ions create a current when hitting the detector

What defines a transition metal

by having an incompletely filled outer shell

When writing an equation

what should you always add

what to take into account when comparing ionisation energy

number of protons

what should you do to the m/z charge if you use electrospray ionisation

Minus 1 from the m/z value

State which of the elements magnesium and aluminium has the lower first ionisation energy.

Al
(Outer) electron in (3)p sublevel / orbital
Higher in energy / further from the nucleus so easier to remove

Element Q forms a sulfate with formula QSO4

what is the electronic configuration for Q

State and explain the trend in the first ionisation energies of the elements in Group 2 from magnesium to barium

Decrease
more (energy) shells
Weaker attraction of ion to lost electron

Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer

Ions

Outline how the TOF mass spectrometer is able to separate two species to give two peaks

Positive ions are accelerated by an electric field
To a constant kinetic energy
The positive ions have the same kinetic energy
Therefore

Definition of first ionisation energy

amount of energy needed to remove ONE MOLE of electrons from ONE MOLE of gaseous atoms to form ONE MOLE of gaseous unipositive ions

Definition of 2nd ionisation energy

amount of energy needed to remove ONE MOLE of electrons from ONE MOLE of gaseous positive ion to form ONE MOLE of gaseous +2 charge positive ions

Factors affecting ionisation energy

Atomic radius
num of protons/nuclear charge
sheilding=repulsion by electrons in shells between the electron and the nucleus

across a period what is more important

how many protons
unless electron is in a new sub-shell

down a group what is more important

shielding

Tellurium has a relative atomic mass of 127.6
Iodine has a relative atomic mass of 126.9
Suggest one property of tellurium that justifies its position before iodine in the modern Periodic Table

Tellurium has Z = 52 but iodine has Z = 53
Te has one fewer proton than I / I has one more proton
Tellurium has 6 outer shell electrons/valence electrons but iodine has 7

the ionisation energy from ionisation 6 to 7 has a massive increase. why

large jump after the sixth electron is removed due to the 7th electron being removed
electron removed from (second) shell which is closer to the nucleus

For the TOF formula what unit is the mass in

Kg

State how the ions are detected in TOF

(ions hit a detector and) each ion gains an electron (generating a current)

State why the isotopes of rhenium have the same chemical properties

same electron configuration

ionisation energy definition

the minimum amount of energy required to remove one mole of electrons from one mole of gaseous atoms

when explaining ionisation methods what should you always include

equation

what substances is electron impact used for

small molecules. not large as can cause fragmentation

what substance is electrospray used for

large molecules as doesnt cause fragmentation

what do you have to do to the m/z charge for electrospray ionisation

minus 1

Explain the general pattern in the first ionisation energies of the elements from lithium to neon

General Trend (Li → Ne)
1a. 1st IE increases
1b. More protons/increased nuclear charge
1c. Electrons in same energy level / shell
1d. No extra/similar shielding
1e. outer e closer to nucleus (ignore radius decreases)

explain the deviations in the general pattern in the first ionisation energies of the elements from lithium to neon

Deviation Be → B
2a. B lower than Be
2b. Outer electron in (2)p
2c. higher in energy than (2)s
Deviation N → O
3a. O lower than N
3b. 2 electrons in (2)p need to pair
3c. pairing causes repulsion (do not award if it is clear reference to repulsion is in s orbital)

Importance of percentage yield

idea of getting as much product as possible in the reaction / idea of efficient conversion of reactants to products

Importance of percentage atom economy

idea of maximising the mass of reactants / atoms that ends up in desired product or idea of minimising the amount of by-products

why is sodium ions smaller than fluoride ion

Both Na+ and F- same electron arrangement
Sodium (ion) has more protons so attracts (outer) electrons closer

why might there be a difference between the experimental value for x.H20
and the correct value during water of crystallisation. How can this be improved

1. Not heated to constant mass
2. Heat to constant mass
3. You can be sure all / more of the water has been driven off

Some of the liquid injected did not evaporate because it dripped into the gas syringe nozzle outside the oven.
Explain how this would affect the value of the Mr of Y calculated from the experimental results.

If less liquid evaporates

Why might the experimentally determined Mr differ from the actual value (excluding apparatus inaccuracy)?

Gas volume may be wrong (air leak or incomplete vaporisation)