Mass Spectrometry

Question 1

What are the main components of a MS

Answer 1

Sample
Ionisation
Mass analyser
Detector
Data acquisition and analysis

Question 2

Which elements of an MS are required to be in a vacuum

Answer 2

Mass analyser and Detector

Question 3

What is the level of pressure needed for an ‘ultra-high’ vacuum

Answer 3

< 10 ^-7

Question 4

What equipment is needed to achieve an ultra-high vacuum

Answer 4

Turbo pumps

Question 5

Why is a vacuum necessary for MS

Answer 5

To prevent species from colliding into an air molecule (?)

Question 6

What are the units of the mass/charge ratio

Answer 6

Thompson (Th)

Question 7

What are the two modes of mass measurement and how are they achieved

Answer 7

Positive-ion mode (proton added)

Negative-ion mode (proton taken away)

Question 8

Define an isotope

Answer 8

An atom with the same number of protons/electrons but a different number of neutrons

Question 9

Why are bromine ions annoying

Answer 9

Because the two isotopes of bromine are equally abundant, so mass spec gives two equal peaks

Question 10

What is the relationship between the distance between isotope peaks and charge state

Answer 10

Inverse relationship.
+1 charge gives peaks 1 Dalton apart.
+2 charge gives peaks 0.5 Dalton apart.
+4 charge gives peaks 0.25 Dalton apart

Question 11

What is the equation to work out resolution

Answer 11

Resolution = Mass of 2nd peak / Resolving power 
R = M / delta M

Question 12

What is better; higher or lower resolution

Answer 12

Higher - allows for better individual peak identification

Question 13

What are the main ionisation techniques

Answer 13

Electrospray ionisation (ESI)
Matrix-assisted laser desorption/ionisation (MALDI)
Electron Ionisation (EI)
Chemical Ionisation (CI)
Fast-Atom Bombardment (FAB)

Question 14

What is the main idea behind Electrospray Ionisation (ESI)

Answer 14

Adding voltage to a liquid forces the atoms (charged) to separate and become nanoparticles (which can be run through the MS)

Question 15

What are the main advantages of Electrospray Ionisation (ESI)

Answer 15

‘soft’ ionisation technique - used in solution. Allows analysis of biological samples that are defined by non-covalent interactions.
Able to ionise samples with large masses.
Can easily be coupled with separation techniques (e.g. nano-LC)

Question 16

What are the main disadvantages of Electrospray Ionisation (ESI)

Answer 16

Not good at analysing mixtures.
Becomes contaminated easily, and difficult to clean.
Extra charges added can give skewed results

Question 17

What is the equation to work out molecular mass from an Electrospray Ionisation-MS graph

Answer 17

n = [M(n+1) - H] / [M(n) - M(n+1)]

where n = molecular mass; M = m/z values for different peaks

Question 18

What does MALDI stand for

Answer 18

Matrix-assisted Laser Desorption/Ionisation

Question 19

How does MALDI work

Answer 19

Sample ‘co-deposited’ with ‘Matrix’. Laser excites matrix. Matrix transfers energy to sample. Produces singly charged species.

Question 20

What is necessary for a MALDI matrix

Answer 20

Strong absorption of laser wavelength
Low sublimation temp
Good mixer/solvent compatibility with sample
Able to participate in a photochemical reaction

Question 21

Give examples of Matrices used in MALDI

Answer 21

DHB
CHCA
HPA
Dithranol
Sinnapinic acid

Question 22

What is the normal ratio of sample/matrix in MALDI

Answer 22

1:10,000

Lots of matrix, very little sample.

Question 23

Name some applications of MALDI

Answer 23

Find mass of an intact protein

Find molecular weight distribution of polymers

Question 24

What are the main advantages of MALDI

Answer 24

'gentle' technique
Samples with a high MW can be analysed
Molecules don't need to be volatile
Easy to get sensitive results
Can give a variety of charge states (1-3; +ve or -ve)

Question 25

What are the main disadvantages of MALDI

Answer 25

Low m/z ions can be obscured
Sample must have low vapour pressure
Pulsed therefore some mass analysers wont work with it
Difficult to couple with chromatography
Can have problems when samples absorb laser light

Question 26

What are the main types of Mass Analysers

Answer 26

Sector instruments (magnetic (B) and electrostatic (E))
Time-of-Flight (TOF)
Quadrupol (Q)
Ion Trap (IT)
Ion Cyclotron Resonance (ICR)
Orbitrap

Question 27

How do sector instruments work

Answer 27

Ionised sample curves through an electrostatic sector. Hits a spectrometer lens which splits the beam. Curves through the magnetic sector and exits.

Question 28

What are the pros and cons of sector instruments

Answer 28

Resolving power up to 100,000
Mass range up to 15,000
Not suitable for ESI or MALDI ionisation.
Expensive

Question 29

What is the main idea behind Time-of-Flight (TOF) (mass analyser)

Answer 29

Analyser tests how quickly ions travel a certain distance. Smaller ions get there first, larger ions take longer.

Question 30

How can the resolution of TOF be increased

Answer 30

Longer flight time = higher resolution. Longer flight time from the use of electrostatic mirrors

Question 31

What is the mass range of TOF

Answer 31

Theoretically unlimited, but realistically >250k

Question 32

What are the pros and cons of TOF

Answer 32

Pros. Fast, sensitive, simple, cheap, unlimited mass range,

Cons. Requires pulsed ionising (MALDI or pulser ESI). Variation within one species can cause problems.

Question 33

What is the main idea behind Quadrupole (Q) (mass analyser)

Answer 33

4 parallel metal rods. Rods oscillate DC potential (charge), which makes the ions oscillate. At a fixed DC potential, only a certain m/z passes through. Varying the DC/Rf allows scanning for ions with various m/z ‘s

Question 34

What are the main advantages of the Quadrupole (Q)(mass analyser)

Answer 34

Small
Light
Portable
robust
No high voltage needed
High dynamic range
Cheap

Question 35

What are the main disadvantages of the Quadrupole (Q)(mass analyser)

Answer 35

Low resolution (~2000)
Mass range (m/z) <3000