Mass spec

Question 1

Background

Answer 1

The generation, separation and detection of ions
Positive ion mass spec - separation of cations
Analyte molecules that lost an electron - positive charge - molecular ion
Pseudomolecular ions formed as adducts - may fragment - produces daughter ions
Parent and daughter ions detected - recorded on mass spec - ions recorded based on m/Z value

Question 2

Peak resolution

Answer 2

R = m/delta M
M = mass of peak
Delta M = mass difference between peak and next peak

2 peaks referred to as resolved if differences between heights is less that 10%

Question 3

Sample introduction

Answer 3

Injection - samples dissolves in volatile organic solvent
Chromatographic separation - for samples that are a mix of compounds - separated before analysis
Direct insertion - solids and in volatile liquids

Question 4

Ionisation methods

Answer 4

8 methods
Can occur at high or atmospheric pressure
Hard - high energy - significant fragmentation of parent molecule
Soft - low energy - little fragmentation

Question 5

Electron ionisation (EI)

Answer 5

Ionisation occurs at high vacuum (low pressure).
EI is a hard, high energy, ionisation process.
EI generates a fragmentation pattern.
EI is used for routine and rapid identification of organic compounds

Heated filaments give out electrodes - attracted towards anode - sample injected - collides with electron - forms sample ions

Question 6

Electrospray ionisation (ESI)

Answer 6

Occurs at atmospheric pressure
Soft technique - useful for bio molecules
Ions formed in solution by protonation - ion formed depends on pH of solution

Sampled added to capillary - forms aerosol droplets as it exist - droplets ionised by positive voltage on the top of capillary - electrons stripped off - left with positive charges droplets - break up as charge density increases

Positive charged analyte ions repel positive charge droplets - cations accelerated to plate - passes through mass analyser

Question 7

Atmospheric pressure chemical ionisation (APCI)

Answer 7

Soft process - uses chemical ionisation
Reagent gas - N2

Sample enters capillary - forms aerosol droplets as it exist - heater evaporates solvent from sample - corona discharge needle ionises N2 gas - ionised gas ionises sample molecules - cations formed - pass through mass analyser

Question 8

Atmospheric pressure photoionisation (APPI)

Answer 8

Soft process - sample nebulised - liquid turned into spray

Sample enters capillary - forms aerosol as it exits capillary - heater evaporates solvent from sample - left with unionised sample molecules - light from UV ionises sample molecules - forms cations - passes through mass analyser

Question 9

Matrix assisted laser desorption/ionisation (MALDI)

Answer 9

Soft ionisation process
Matrix - crystalline solid
Matrix mixed with sample - placed on target - inserted into mass spectrometer

Sample mixed with matrix - matrix absorbs at wavelength of laser - matrix transfers energy to sample - cation, anions and neutrals sputtered out from matrix - addict ions forms - cations attracted towards negatively charged plate - passes through mass analyser

Single laser system - 1 laser absorbs ands ionises sample in one go
Dual laser system - 2 lasers
Mass limit - 250kda
Requires TOF

Question 10

Desorption electrospray ionisation (DESI)

Answer 10

Related to ESI
Analyte is desorbed from a solid 
Analyte desorbed in solvent droplets 
Analyte is ionised by protonation 
Solvent evaporates 
High mass - 66kda

Question 11

Direct analysis in real time (DART)

Answer 11

Related to APCI
Helium ions generated by corona discharge - He ions and electrons removed - left with energetic helium ions - collide with sample - form sample ions
Highest mass - 1kda

Question 12

Fast atom bombardment (FAB)

Answer 12

Sample mixed with involatile matrix - placed on tip of direct insertion probe - sample bombarded with Xe or Ar atoms - cations, anions and neutrals sputtered out from matrix - cations accelerated towards mass analyser

Question 13

Mass analysers

Answer 13

Separat ions according to m/Z

3 types

Question 14

Quadruple mass analyser

Answer 14

Compact - easy to use - separates cations and anions
Low resolution - mass range up to m/Z 2500

4 rods - opposite pairs connected electrically - AC and DC voltage applied across rods - ions follow oscillating path through mass analyser - ions with 1 m/Z have stable path and pass through detector

Question 15

Ion trap mass analyser

Answer 15

Small - central space 1cm
Ions above given m-Z value retained in space between middle - trapped by voltage
Voltage altered to release ions of increasing m/Z value to the detector

Question 16

Time of flight mass analyser

Answer 16

Ions accelerated to a grid - pass through grid to field free region
Time taken depends on m/Z values
Larger mass = longer time
Higher charge = shorter time

Question 17

Tandem mass spectrometry (MS/MS)

Answer 17

2 or more mass analysers coupled together in sequence

Parent ions separated in first mass analyser - separated ion pass into collision cell - contains low pressure of argon/helium - collides with ions causing collision induced dissociation (CID) - CID causes fragmentation of parent ions - daughter ions separated in second mass analyser

Unseparated mixtures can be analysed

3 configurations:

Triple sector - QQQ - 3 quadrupoles in sequence - Q1 - first mass analyser, separates parent ions - Q2 - CID, generates daughter ions - Q3 - second mass analyser, separates daughter ions

Ion trap - more compact then QQQ - parent ions enter trap - lower m/Z ions allowed to exit - higher ions trapped - helium added - causes CID - daughter ions generated

Time of flight - QQT - Q1 - first mass analyser, separates parent ions - Q2 - collision cell - TOF - second mass analyser - separates daughter ions