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CH506 Chemical Identification Techniques > Mass Spectrometry > Flashcards

Mass Spectrometry Flashcards

1
Q

What does mass spectrometry measure?

A

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

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2
Q

What can mass spec show us?

A 
Atomic mass
Molecular weight 
Molecular formula 
Isotope ratio 
Chemical structure
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3
Q

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

A

Sample needs to be ionised

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4
Q

General process of mass spec:

A

ionisation
acceleration
deflection - by magnetic field according to mass
detection

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5
Q

Does mass spec weigh molecules?

A

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

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6
Q

What does mass spec measure?

A

The mass to charge ratio (m/z)

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7
Q

What is the general process of mass spec?

A

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

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8
Q

What species are detected by mass spec?

A

Only charged species

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9
Q

Can all species be detected by mass spec?

A

No, only charged species

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10
Q

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

A

isotopes present

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11
Q

Advantages of mass spec?

A

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

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12
Q

Limitations of mass spec?

A

High purity sample needed
Destructive
Instruments can be expensive

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13
Q

Applications of mass spec?

A 
Identification of unknown substance 
radiocarbon dating (abundance of C14 measured)
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14
Q

What does the sample need to be for mass spec?

A

Solid or liquid needs to be heated to vaporisation.

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15
Q

List the types of ionisation (4 of them)

A

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

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16
Q

What are the soft ionisation techniques? (3)

A

Chemical
electrospray
MALDI

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17
Q

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

A

Small volatile ones

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18
Q

What type of molecules do you analyse with MALDI

A

Non-volatile peptides

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19
Q

Is there a lot of fragmentation with hard ionisation?

A

Yes

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20
Q

Is there a lot of fragmentation with soft ionisation?

A

No

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21
Q

Do hydrocarbons work well with soft ionisation?

A

No

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22
Q

What are the 3 types of mass analyser?

A

Magnetic sector
quadrupole
time of flight

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23
Q

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

A

Lower B means lower m/z ions directed to detector

Higher B means higher m/z ions directed to detector

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24
Q

What are the disadvantages of a magnetic sector analyser?

A

Cost

size - it is huge!

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25
Q

General principle of quadrupole analyser

A

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

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26
Q

Advantages and Disadvantages of quadrupole analyser

A
  • Low resolution
    + ‘low’ cost
    + reproducibility
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27
Q

Advantages of time of flight

A

Highest mass range

28
Q

General principle of time of flight analyser

A

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

29
Q

Advantages and disadvantages of magnetic sector analyser

A

+ highest resolution

  • expensive
  • large
30
Q

What is the most common detector in mass spec?

A

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

31
Q

What is soft ionisation good for

A

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

32
Q

Types of soft ionisation (3)

A

APCI
MALDI
Electrospray

33
Q

What analyser usually goes with MALDI?

A

Time of flight analyser

34
Q

What analyser usually goes with electrospray?

A

quadrupole

35
Q

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

A

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

36
Q

What is ACPI ionisation?

A

Firing electrons at atmospheric pressure with a reagent gas

37
Q

What is ESI bad for?

A

Hydrocarbons and non-polar compounds

38
Q

Does ESI cause a lot of fragmentation?

A

No

39
Q

What is chromatography?

A

Separation due to affinity between two phases

40
Q

What is normal phase liquid chromatography?

A

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

41
Q

What is reverse phase chromatography?

A

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

42
Q

What is the retention time controlled by in gas chromatography

A

Heat

43
Q

What is GC-MS suitable for?

A

small, volatile, not too polar compounds

44
Q

What is GC-MS unsuitable for?

A

Big biomolecules will decompose with the heat

45
Q

Advantages of mass spec

A

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

46
Q

Disadvantages of mass spec

A

High purity sample needed
Destructive technique
Instruments can be expensive

47
Q

What atoms do things we use soft ionisation for contain?

A

O , N, S

48
Q

What is soft ionisation not good for?

A

Hydrocarbons

49
Q

What is electron impact ionisation good for?

A

Volatile organic molecules

50
Q

What happens in electron impact ionisation?

A

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.

51
Q

Is electron impact ionisation soft or hard?

A

Hard - can cause lots of fragmentation

52
Q

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

A

The molecular ion isn’t a radical.

53
Q

What happens in chemical ionisation?

A

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

54
Q

What is the most common detector in mass spec?

A

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

55
Q

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

A

It amplifies the signal meaning we can have low detection limits

56
Q

Soft ionisation methods:

A

APCI
MALDI
Electrospray (ESI)

57
Q

Normal phase liquid chromatography

A

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

58
Q

Reverse phase liquid chromatography

A

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

59
Q

What is retention time controlled by in gas chromatography?

A

Heat

60
Q

What is GC-MS suitable for ?

A

Small, not too polar volatile compounds

61
Q

What ionisation method is GC-MS usually paired with?

A

Electron impact ionisation

62
Q

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

A

the relative stability of the fragment

63
Q

What is heterolytic cleavage?

A

Pair of electrons moved onto one fragment (full arrow)

64
Q

What is homolytic cleavage ?

A

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

65
Q

Do these cations increase or decrease in stability?

Primary - secondary - tertiary

A

Increase

66
Q

Will noncharged species be seen in mass spec?

A

No

CH506 Chemical Identification Techniques (3 decks)

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