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Flashcards in Mass Spectrometry Deck (35):
1

What are the main components of a MS

Sample
Ionisation
Mass analyser
Detector
Data acquisition and analysis

2

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

Mass analyser and Detector

3

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

< 10 ^-7

4

What equipment is needed to achieve an ultra-high vacuum

Turbo pumps

5

Why is a vacuum necessary for MS

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

6

What are the units of the mass/charge ratio

Thompson (Th)

7

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

Positive-ion mode (proton added)
Negative-ion mode (proton taken away)

8

Define an isotope

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

9

Why are bromine ions annoying

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

10

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

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

11

What is the equation to work out resolution

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

12

What is better; higher or lower resolution

Higher - allows for better individual peak identification

13

What are the main ionisation techniques

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

14

What is the main idea behind Electrospray Ionisation (ESI)

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

15

What are the main advantages of Electrospray Ionisation (ESI)

'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)

16

What are the main disadvantages of Electrospray Ionisation (ESI)

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

17

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

n = [M(n+1) - H] / [M(n) - M(n+1)]
where n = molecular mass; M = m/z values for different peaks

18

What does MALDI stand for

Matrix-assisted Laser Desorption/Ionisation

19

How does MALDI work

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

20

What is necessary for a MALDI matrix

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

21

Give examples of Matrices used in MALDI

DHB
CHCA
HPA
Dithranol
Sinnapinic acid

22

What is the normal ratio of sample/matrix in MALDI

1:10,000
Lots of matrix, very little sample.

23

Name some applications of MALDI

Find mass of an intact protein
Find molecular weight distribution of polymers

24

What are the main advantages of MALDI

'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)

25

What are the main disadvantages of MALDI

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

26

What are the main types of Mass Analysers

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

27

How do sector instruments work

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

28

What are the pros and cons of sector instruments

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

29

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

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

30

How can the resolution of TOF be increased

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

31

What is the mass range of TOF

Theoretically unlimited, but realistically >250k

32

What are the pros and cons of TOF

Pros. Fast, sensitive, simple, cheap, unlimited mass range,
Cons. Requires pulsed ionising (MALDI or pulser ESI). Variation within one species can cause problems.

33

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

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

34

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

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

35

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

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