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analytical chemistry > Mass Spectrometry > Flashcards

Mass Spectrometry Flashcards

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1
Q

What type of techniques does GCMS combine?

A

Chromatographic and spectral

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2
Q

where is the column directly inserted into?

A

MS ionization chamber

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3
Q

Why do all interface devices contain a heat source?

A

To prevent analyte condensation within the transfer line

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4
Q

What must this heat not cause?

A

Thermal decomposition of analyte

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5
Q

Name 3 advantages of direct capillary interface?

A

Low cost, no dead volume, no selectivity

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6
Q

What are 3 disadvantages of direct capillary interface?

A

limits flow range that column can use, limits ID of column that can be used, part of column is ‘lost’ - serves as a flow restrictor

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7
Q

volatile substances can be ionised by electron impact ionisation by interacting with what?

A

an electron beam generated by a heated filament in the ion

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8
Q

why are internal standards necessary in GC?

A

due to the need for normalisation due to volume changes

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9
Q

What does adding a fixed concentration of IS to each standard mean for calibration?

A

calibration can be carried out using peak area ratio and thus ignores changes in injection volume

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10
Q

What is the injection volume of LC?

A

10-50uL

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11
Q

Why is the precision better in LC than GC?

A

a fixed volume loop is filled with sample and only this amount goes on every time

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12
Q

why is it common in both LC and GC to extract samples prior to analysis?

A

due to a need for preconcentration or because the sample matrix is not appropriate for chromatographic system

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13
Q

What is commonly done prior to extraction to compensate for the sample being lost?

A

addition of an internal standard

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14
Q

What is the advantage of considering peak area ratios instead of peak area?

A

normalisation is achieved and precision and accuracy are better

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15
Q

In LC and GC, why must the internal standard be separated from the analyte?

A

For an internal standard to be of use otherwise you could not distinguish area representing each

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16
Q

why is this one of the most difficult tasks for a chromatographer?

A

because the internal standard must have similair chromatographic properties as the analyte and be able to be detected in the same way

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17
Q

what is deuterium

A

a stable isotope of hydrogen depicted as 2H, has a mass of 2

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18
Q

what are 2 important characteristics a deuterated standard should have

A

should coelute with compound to be quantified, should also contain enough mass to show a signal outside the natural mass distribution of the analyte

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19
Q

what are the 2 most common interfaces used in HPLC?

A

electrospray ionisation and atmospheric pressure chemical ionisation

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20
Q

what is the more recently used interface in HPLC?

A

Atmospheric pressure photo ionisation

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21
Q

in electrospray ionisation, what flow rate is the analyte typically intoduced at?

A

1uL min-1

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22
Q

The analyte solution flow passes through the electrospray needle that has high potential difference applied to it - what does this cause?

A

This forces the spraying of charged droplets from the needle with a surface charge of the same polarity to the charge on the needle

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23
Q

what are the droplets repelled towards from the needle?

A

source sampling cone on counter electrode

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24
Q

What occurs as the droples traverse the space between the needle tip and cone?

A

solvent evaporation

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25
Q

What happens to the droplet as solvent evaporation occurs?

A

it shrinks until it reaches the point that surface tension can no longer sustain the charge (rayleigh limit)

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26
Q

This causes a ‘coulombic explosion’ to occur, what does this do to the droplet?

A

it is ripped apart, causing smaller droplets which can repeat the process

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27
Q

what makes this method of ionisation very soft?

A

very little residual energy is retained by the analyte upon ionisation

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28
Q

what is a major disadvantage to electrospray ionisation?

A

very little fragmentation is produced

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29
Q

how might this be overcome?

A

with the use of tandem mass spectrometric techniques such as MS/MS

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30
Q

What is Atmospheric pressure chemical ionisation suitable/not suitable for?

A

suitable for low mass compounds, not suitable for thermally liable compounds.

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31
Q

In APCI, what is the analyte solution introduced to?

A

a pneumatic nebulizer

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32
Q

What is the analyte then desolvated in?

A

heated quartz tube

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33
Q

What does the analyte then interact with? What does this create?

A

The corona discharge - creates ions

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34
Q

what does the corona discharge replace in Cl?

A

the electron filament

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35
Q

what does this produce?

A

Primary ions; N2 and N4 by electron ionisation

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36
Q

What do these primary ions collide with to form secondary reactant gas ions; H3O and (H2O)nH+?

A

the vaporized solvent

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37
Q

What causes the formation of analyte ions

A

reactant gas ions colliding with analyte

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38
Q

What does high frequency of collisions result in? 2

A

High ionisation efficiency and thermalisation of the analyte ions

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39
Q

What will the resulting spectra look like?

A

predominently molecular species and adduct ions with very little fragmentation

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40
Q

What do the ions enter after forming? is this similair to ESI?

A

the pumping and focussing stage in a similair way as ESI

41
Q

In Atmospheric pressire photo ionisation (APPI), what does the discharge lamp generate?

A

photons in a narrow range of ionization energies

42
Q

why is the range of energies carefully chosen?

A

to ionize as many analyte molecules as possible while minimising ionisation of solvent molecules

43
Q

out of electrospray and APPI, which is more complicated in terms of ion production?

A

APPI

44
Q

What are the 2 ways that radical ions can be produced in APPI?

A

direcctly or via an intermediate using a dopant

45
Q

what is the fundamental process in photoionisation?

A

the absorption of a high energy photon by the molecule and subsequent ejection of an electron

46
Q

In direct APPI, this process occurs for the analyte molecule, forming what?

A

molecular radical cation M+

47
Q

The analyte radical cation can be detected as M+ or……?

A

it can react with surrounding molecules and be detected as another ion

48
Q

What is the most common reaction?

A

The abstraction of a hydrogen atom from the abundent solvent

49
Q

what does this reaction form?

A

form the stable [M+H]+ cation, which is usually the observed ion

50
Q

In dopant APPI (or photoionization-induced APCI) a quantity of photoionizable molecules are introduced to what?

A

the sample stream to create a source of charged carriers

51
Q

Give 2 examples of a photoionizable molecule?

A

toluene or acetone

52
Q

what can be used to give the same effect?

A

A photoionizable solvent

53
Q

What do the dopant or solvent ions react with? and via what reactions?

A

neutral analyte molecules via proton transfer or charge exchange reactions

54
Q

How can APPI produce negative ions? 2

A

By creating alot of thermal electrons from dopant or solvent ionization, or by photons striking metal surfaces in the ionization source

55
Q

How do mobile phase additives affect the analysis? (2)

A

purity and composition

56
Q

what do polymers, including proteins and DNA, form adducts with?

A

inorganic salts

57
Q

What does this lead to?

A

complicated mass spectra and a broad distribution of multiply-charged sodium, potassium and chloride adducts

58
Q

Reagents, chemicals, sample preparation and post column additives always present what risk to the analysis?

A

contamination

59
Q

Why are alkali ions, plasticizers and surfactants particularly problematic?

A

As they are widespread and interfere strongly with LC-MS by causing higher background noise and the formation of adducts

60
Q

give 4 examples of additives used in LC-MS?

A

Formic acid, Acetic acid, ammonium formate, ammonium acetate, ammonium hydroxide solution

61
Q

what are small organic acids commonly added to influence?

A

analyte ionization

62
Q

Name a reason for formic and acetic acid’s widespread use in terms of separation?

A

separation benefits in terms of retention and/or peak shape under acidic conditions

63
Q

why is this?

A

because any silanol activity is suppressed

64
Q

Most mass spectrometric measurements are done in positive ion mode, which is accomplished by the addition of a proton to form the molecular ion [M+H]+. Why can formic and acetic acid readily fulfil this purpose?

A

They have necessary acidity and volatility to provide an excess of cations for this purpose

65
Q

why do mobile phases for HPLC of proteins and peptides usually contain trifluoroacetic acid? 2

A

to contol pH and improve peak shape and resolution

66
Q

How does it enhance retention and improve peak shape?

A

By ion-pairing with the peptide and reducing silanol interactions

67
Q

what does TFA’s high surface tension prevent?

A

efficient spray formation

68
Q

TFA ions in gas phase ion-pair with peptide’s basic group, what does this then suppress?

A

Their ionisation and reduces the MS signal

69
Q

If TFA cant be avoided, what can be added to reduce its effects?

A

small organic acids known as triple blends

70
Q

what might the addition of a triple blend improve (1) and weaken (2)?

A

signal improves, resolution and sensitivity poorer

71
Q

why might it be necessary to use more neutral conditions in some circumstances?

A

may be because the analytes are sensitive to acids or do not exhibit optimal resolution at low pH

72
Q

Give 2 examples of volatile salts that can be used when acids are not suitable?

A

ammonium formate or ammonium acetate

73
Q

Their use is much more complicaed than acids, what are the 3 issues they may cause?

A

Limited solubility of salts in organic solvents, changing pH during a gradient run, the acidic pH provided by salts permits both + and - ion mode detection

74
Q

the main issue concerns the solubility. Why is its lack of solubility in acetonitrile a problem?

A

acetonitrile is the organic solvent of choice for most separations

75
Q

What is used to stabilise the special blend containing 0.1% w/v of ammonium acetate in acetonitrile?

A

acid

76
Q

what are the 3 desirable effects of this?

A

salt is kept in a solution, solution is stable against decomposition and keeps the pH in the mildly acid range

77
Q

What is tandem mass spectrometry

A

hyphenated technique with more than one mass spectrometer used together

78
Q

What is the first MS used for?

A

to separate or isolate molecular ions of various analytes in a mixture

79
Q

What is the second MS called and used for?

A

its known as collision cell and molecular ions are fragmented

80
Q

What is the 3rd MS used for?

A

Scans for fragments of the molecular ion

81
Q

What is the first quadrouple (MS1) in a tandem situation usually equipped with?

A

a soft ionisation technique (eg electrospray
ionisation)

82
Q

Not much fragmentation takes place here, resulting in what?

A

the production of only the molecular ion or protonated molecular ions

83
Q

When the ions are passed to the second quadrouple, they enter a field free collision chamber. What gas is pumped through?

A

Helium

84
Q

What does the term field free mean?

A

The quadrouple is operated in radio-frequency-only-mode and no dc potential is applied across the rods

85
Q

This mode provides and excellent method of focussing the ……… ….. but does not act as an ….. …….?

A

Scattered ions, ion filter

86
Q

The collisions of the fast-moving parent ions and argon atoms cause what?

A

Further fragmentation of parent ions or precursor ions

87
Q

What does this give?

A

daughter ions

88
Q

What happens in the third quadrouple

A

The spectrum of daughter ions is then scanned

89
Q

For quantitative analysis of target compounds, what mode is the third MS operated in?

A

the selected-reaction monitoring mode

90
Q

What is selected reaction monitoring?

A

a non-scanning MS technique performed on tandem mass spectrometers

91
Q

Collision-induced dissociation is used as a means to increase what?

A

selectivity

92
Q

In SRM, what are the 2 mass analyzers used as?

A

static mass filters

93
Q

What are these used to monitor?

A

a particular fragment ion of a selected precursor ion

94
Q

What are the specific pair of m/z values associated to the precursor and fragment (daughter) ions selected referred to as?

A

a transition and can be written as a parent m/z > fragment m/z

95
Q

In SRM analysis, there is generally no mass spectra recorded. Instead the detector acts as a counting device for what?

A

ions matching selected transition thereby returning an intensity value over time

96
Q

How can multiple reaction monitoring and selective reaction monitoring be measured at the same time on the chromatographic time scale?

A

by rapidly toggling between the different precursor/fragment (daughter) pairs

97
Q

How does the method allow for additional selectivity?

A

By monitoring the chromatographic coelution of multiple transitions for a given analyte

98
Q
A

analytical chemistry (8 decks)

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