Mass Spec Intro

Question 1

Why Mass Spec ? (MS)

Answer 1

• Very high sensitivity; used to detect small amounts of samples (pg-pikogram)
• To determine; Elements present
  > Molecular weight of molecules (this required soft ionisation)
  > Molecular formula (High resolution MS may be required)
  > Molecular structure (of biomolecules, sequence peptides and proteins) - this process may require fragment patterns which requires MS/MS process
  > High spatial resolution label-free imaging

Question 2

State some limitations of mass spec

Answer 2

• Complex equipment required (although portable/handheld versions becoming available)
• Not all molecules are easily susceptible to MS
  > MS requires molecules to be in vapour phase although not all molecules easily transition to this phase)
  > MS requires molecules to form ions but not all molecules easily form ions
• Not inherently quantitative
• Mixture analysis can be challenging

Question 3

What are the 5 basic steps of MS. State them

Answer 3

1) Generation of Molecules in gas phase
2) Ionisation of molecules
3) Mass separation of ions
4) Ion detection
5) Data intrerpretation

Question 4

Some molecules are already in Gas Phase. TRUE or FALSE

Answer 4

TRUE

Question 5

Describe the generation of molecules into the Gas Phase

Answer 5

Molecules is introduced and heated until evaporation occurs. The molecule has now entered the gas phase and can be analysed using MS

Question 6

State a problem that can occur in the gas phase and give examples

Answer 6

Lots of molecules will react or break down before they evaporate.
e.g: Sugars will caramelise
Upon heating, starchy foods will form Acrylamide

Question 7

What is the most common type of ionisation ?

Answer 7

Electron Ionisation (EI)

Question 8

During the ionisation process, when electrons bump into gas phase molecules and exchange energy, what is the energy of the gas phase molecules. Give answer in eV and kjmol-1

Answer 8

Energy of gas phase molecules = 70eV = 6700kjmol-1

Question 9

How much energy is required to break a C-C bond ?

Answer 9

about 350kjmol-1

Question 10

EI-MS is only used for molecules that can readily be in gas phase without decomposing. TRUE or FALSE

Answer 10

TRUE

Question 11

State 2 properties of molecules that tend to survive the gas phase without decomposing

Answer 11

1) Size limited molecules

2) Polarity limited molecules

Question 12

In EI-MS, electron beams interact with the evaporated molecules (and produce ions). TRUE or FALSE

Answer 12

TRUE

Question 13

In EI-MS, ions are put into a mass filter then in a detector (in order to count charged particles). TRUE or FALSE

Answer 13

TRUE

Question 14

EI mostly forms anions (negatively charged ions). TRUE or FALSE

Answer 14

FALSE, cations (positively charged ions) are formed

Question 15

Where is the preferred location to knock off an electron in gas phase molecule ?

Answer 15

From a lone pair

e.g: ether with charge (positive!) and radical located on O

Question 16

State some locations of missing elcetrons

Answer 16

• Bonding electron e.g: CH4
• Lone pair
• Alkene: pi orbital is HOMO orbital therefore loses e-: with resonance structures that can switch position of charge and radical

Question 17

If there is fragmentation of the molecular ion, what happens to the ion ?

Answer 17

• one part of ion has even no of e- and positive charge
• other part has odd no of e- and no charge
  Therefore, charge ad electron separate on fragmentation

Question 18

In Mass separation, ions should be accelerated to move in a straight line into a ‘mass filter’. TRUE or FALSE

Answer 18

TRUE

Question 19

The mass filter region is in a Vacuum, therefore there re o collisions of ions. TRUE or FALSE

Answer 19

TRUE

Question 20

State and explain the 2 basic methods used in mass separation

Answer 20

1) Deflection of ions in electromagnetic field;
Ions deflect more if charge (z) higher or mass (m) lower

2) Time-of-flight:
Ions fly faster if charge (z) higher or mass (m) lower

In common: m/z is measured rather than just m
(especially important for “soft” ionisation techniques)

Question 21

Describe Electromagnetic field mass filter

Answer 21

– Oldest type, conceptually easiest

– Detector in fixed position which ions can only reach if they are deflected into it

– Vary magnetic field
Only specific m/z deflected precisely into detector

– Maths: convert strength magnetic field → m/z value

-– Graph: m/z on x-axis and number of detected ions (Intensity) on y-axis: the mass spectrum

Question 22

Quadrupole filter only detects ions that carry straight on. TRUE OR FALSE

Answer 22

TRUE

Question 23

Quadrupole filter detect 1 m/z value at a time. TRUE or FALSE

Answer 23

TRUE

Question 24

Quadrupole Analyser

Answer 24

• Mass filter: only 1 mass at a time reaches the detector
• Low mass resolution
• Cheap, robust
• Works well for ions up to 1000m/z
• Diff systems can goup to m/z ~ 70,000
• Can be used to analyse complex (biological) mixtures (when combined with a separation technique)
• very sensitive (~2.5 10-18 mol peptide)
• Couple to chromatographic separation (GC, LC)

Question 25

Name an alternative method to analyse ions

Answer 25

ToF (Time of flight)

Question 26

Describe ToF

Answer 26

• All ions reach the detector!
• But separated by time
• Give all ions same kinetic energy (Ekin) at the same time and place
• Basic physics: Ekin = mv2
• So if mas) is different, but Ekin the same, v must be
  different.
• v is velocity of the moving ions
  -Heavier ions take longer to reach the detector
  – Depends on charge, so again linked to m/z rather than m
• Convert time to m/z, counted ions at each time on y-axis
• Higher mass resolution possible

Question 27

Time-of-Flight analyser is based on what equation ?

Answer 27

E kinetic = 1/2 m v2

Question 28

Electron Ionisation is known as ‘Hard ionisation’. TRUE or FALSE

Answer 28

TRUE

Question 29

State 2 problems associated with EI

Answer 29

1) Diffuclt to identify fragile ions

2) Difficult to analyse composition of mixtures

Question 30

The most intense peak in a mass spec is known ?

Answer 30

Base peak