Mass Spectrometry

Question 1

What does mass spectrometry measure?

Answer 1

The mass to charge ratio (M/Z), since Z is usually 1 it is usually equal to the weight.

Question 2

What can mass spec show us?

Answer 2

Atomic mass
Molecular weight 
Molecular formula 
Isotope ratio 
Chemical structure

Question 3

What do you need to do to the sample before it can be analysed using mass spec?

Answer 3

Sample needs to be ionised

Question 4

General process of mass spec:

Answer 4

ionisation
acceleration
deflection - by magnetic field according to mass
detection

Question 5

Does mass spec weigh molecules?

Answer 5

Not technically, it measures the mass to charge ratio, since charge is normally one, we can usually take it as weight

Question 6

What does mass spec measure?

Answer 6

The mass to charge ratio (m/z)

Question 7

What is the general process of mass spec?

Answer 7

Sample ionised
accelerated magnetic field applied
deflection by field separates ions.
Detection

Question 8

What species are detected by mass spec?

Answer 8

Only charged species

Question 9

Can all species be detected by mass spec?

Answer 9

No, only charged species

Question 10

What do little peaks next to bigger ones in mass spec mean?

Answer 10

isotopes present

Question 11

Advantages of mass spec?

Answer 11

can be used on lots of substances (polar + non-polar)
Can be integrated with separation techniques (GC and LC)
Small sample size needed

Question 12

Limitations of mass spec?

Answer 12

High purity sample needed
Destructive
Instruments can be expensive

Question 13

Applications of mass spec?

Answer 13

Identification of unknown substance 
radiocarbon dating (abundance of C14 measured)

Question 14

What does the sample need to be for mass spec?

Answer 14

Solid or liquid needs to be heated to vaporisation.

Question 15

List the types of ionisation (4 of them)

Answer 15

Electron impact (hard)
Chemical (soft)
electrospray (soft)
MALDI (soft)

Question 16

What are the soft ionisation techniques? (3)

Answer 16

Chemical
electrospray
MALDI

Question 17

What type of molecules do you analyse with electron impact ionisation?

Answer 17

Small volatile ones

Question 18

What type of molecules do you analyse with MALDI

Answer 18

Non-volatile peptides

Question 19

Is there a lot of fragmentation with hard ionisation?

Answer 19

Yes

Question 20

Is there a lot of fragmentation with soft ionisation?

Answer 20

No

Question 21

Do hydrocarbons work well with soft ionisation?

Answer 21

No

Question 22

What are the 3 types of mass analyser?

Answer 22

Magnetic sector
quadrupole
time of flight

Question 23

What does varying the magnetic field (B) do in mass spec?

Answer 23

Lower B means lower m/z ions directed to detector

Higher B means higher m/z ions directed to detector

Question 24

What are the disadvantages of a magnetic sector analyser?

Answer 24

Cost

size - it is huge!

Question 25

General principle of quadrupole analyser

Answer 25

4 parallel electrodes. Ions attracted to -ve pole, trajectory is different depending on mass. Higher mass attracted less, machine cycles through for spectrum.

Question 26

Advantages and Disadvantages of quadrupole analyser

Answer 26

- Low resolution + 'low' cost + reproducibility

Question 27

Advantages of time of flight

Answer 27

Highest mass range

Question 28

General principle of time of flight analyser

Answer 28

ion pulser accelerates all ions. Velocity depends on mass. Higher mass ions will take longer to reach detector.

Question 29

Advantages and disadvantages of magnetic sector analyser

Answer 29

+ highest resolution - expensive - large

Question 30

What is the most common detector in mass spec?

Answer 30

Electron multiplier (eg beryllium oxide) this amplification means we can have low detection limits

Question 31

What is soft ionisation good for

Answer 31

when you don't want many fragments, eg if you want molecular ion

Question 32

Types of soft ionisation (3)

Answer 32

APCI MALDI Electrospray

Question 33

What analyser usually goes with MALDI?

Answer 33

Time of flight analyser

Question 34

What analyser usually goes with electrospray?

Answer 34

quadrupole

Question 35

What kind of molecules are good for soft ionisation + example of what wouldn't be good

Answer 35

Non-volatile, polar components Not good for hydrocarbons Good for big proteins

Question 36

What is ACPI ionisation?

Answer 36

Firing electrons at atmospheric pressure with a reagent gas

Question 37

What is ESI bad for?

Answer 37

Hydrocarbons and non-polar compounds

Question 38

Does ESI cause a lot of fragmentation?

Answer 38

No

Question 39

What is chromatography?

Answer 39

Separation due to affinity between two phases

Question 40

What is normal phase liquid chromatography?

Answer 40

Non polar mobile phase Polar silica phase (polar phase retained)

Question 41

What is reverse phase chromatography?

Answer 41

Polar mobile phase Non-polar silica phase (non-polar phase retained)

Question 42

What is the retention time controlled by in gas chromatography

Answer 42

Heat

Question 43

What is GC-MS suitable for?

Answer 43

small, volatile, not too polar compounds

Question 44

What is GC-MS unsuitable for?

Answer 44

Big biomolecules will decompose with the heat

Question 45

Advantages of mass spec

Answer 45

Can be used on lots of substances (polar + non-polar - high + low mw) Doesn't rely on functional groups can be integrated with separation techniques small sample size neeeded

Question 46

Disadvantages of mass spec

Answer 46

High purity sample needed Destructive technique Instruments can be expensive

Question 47

What atoms do things we use soft ionisation for contain?

Answer 47

O , N, S

Question 48

What is soft ionisation not good for?

Answer 48

Hydrocarbons

Question 49

What is electron impact ionisation good for?

Answer 49

Volatile organic molecules

Question 50

What happens in electron impact ionisation?

Answer 50

Sample heated electrons driven off a heated filament and attracted to an anode collides with molecule, knocking off an electron and leaving a radical cation.

Question 51

Is electron impact ionisation soft or hard?

Answer 51

Hard - can cause lots of fragmentation

Question 52

Why is the molecular ion more stable in chemical ionisation than electron impact

Answer 52

The molecular ion isn't a radical.

Question 53

What happens in chemical ionisation?

Answer 53

Ionised reagent gas protonates the molecule (MH+) so m/z is one higher than molecular weight

Question 54

What is the most common detector in mass spec?

Answer 54

an electron multiplier eg beryllium oxide releases electrons when hit - so amplifies number of electrons

Question 55

why is an electron multiplier commonly used in mass spec for detection?

Answer 55

It amplifies the signal meaning we can have low detection limits

Question 56

Soft ionisation methods:

Answer 56

APCI MALDI Electrospray (ESI)

Question 57

Normal phase liquid chromatography

Answer 57

polar silica phase, non polar mobile phase (eg hexane) polar things retained

Question 58

Reverse phase liquid chromatography

Answer 58

hydrophobic stationary phase that non polar compounds have high affinity to. Polar mobile phase Polar things lowest retention time

Question 59

What is retention time controlled by in gas chromatography?

Answer 59

Heat

Question 60

What is GC-MS suitable for ?

Answer 60

Small, not too polar volatile compounds

Question 61

What ionisation method is GC-MS usually paired with?

Answer 61

Electron impact ionisation

Question 62

What does relative abundance show about the fragment in mass spec?

Answer 62

the relative stability of the fragment

Question 63

What is heterolytic cleavage?

Answer 63

Pair of electrons moved onto one fragment (full arrow)

Question 64

What is homolytic cleavage ?

Answer 64

One electron moves onto each fragment, forming a radical (single head arrow)

Question 65

Do these cations increase or decrease in stability? | Primary - secondary - tertiary

Answer 65

Increase

Question 66

Will noncharged species be seen in mass spec?

Answer 66

No