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Core 4b > Mass Spectrometry > Flashcards

Mass Spectrometry Flashcards

1
Q

What are the fundamental steps in mass spectrometry?

A

1) Molecules are converted to ions
2) Ions are isolated in a vacuum to prevent ion-molecule interactions
3) Ions are separated based on their mass/charge ratio

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2
Q

What are 3 ionisation methods?

A
  • Electron Ionisation (EI)
  • Electrospray Ionisation (ESI)
  • Matrix assisted laser desorption ionisation (MALDI)
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3
Q

How does electron ionisation (EI) work?

A

Gas phase molecules are bombarded by a beam of high energy electrons. An electron passing close to a molecule extracts an electron. The molecule gains additional energy from the electron beam

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4
Q

What type of ion does electron ionisation generate?

A

A radical cation (odd-electron ion)

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5
Q

What is the molecular ion?

A

The highest m/z peak (ignoring isotopes)

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6
Q

What is the base peak?

A

The peak with the highest intensity scaled to 100%

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7
Q

What information can be deducted from the molecular ion?

A
  • mass of the molecule
  • elemental composition of the ion
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8
Q

What information can be deducted from fragment ions?

A
  • structure of the ion
  • stability of the fragment ions
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9
Q

What occurs to the bond length of molecules during ionisation and why?

A

The bond length increases as many vibrational states are populated

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10
Q

What occurs when the ionisation energy is greater than that of the highest vibrational state?

A

Bond rupture - weak bonds break forming fragment ions

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11
Q

An electron beam is not discrete. What does this mean in relation to the appearance of the molecular ion peak?

A

The electron beam has many different energies. The molecular ion peaks is likely to appear as some electrons will not have enough energy to cause bond rupture

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12
Q

ESI and MALDI lead to the formation of positive even electron adduct ions. Why are these ions not referred to as the molecular ion?

A

The adduct ions contain H or Na and so they are referred to as a protonated or sodiated molecules respectively

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13
Q

What is the appearance potential?

A

The energy in which fragment ions may start appearing/the energy that is larger than the highest vibrational state of the molecule

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14
Q

Name 2 soft ionisation techniques

A
  • Chemical Ionisation (CI)
  • Atmospheric pressure chemical ionisation (APCI)
    ALSO:
  • Electrospray
  • MALDI
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15
Q

What are the advantages of using soft ionisation techniques?

A
  • Suitable for involatile or unstable compounds
  • Generates adduct ions with little residual energy/prominent molecular ion peak and low fragmentation
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16
Q

How does Chemical Ionisation (CI) work?

A
  • A reagent gas (methane, ammonia, isobutane) is ionised by a modified EI source
  • Ionised reagent gas undergoes reactions to form an ionising agent
  • Reagent gas reacts to for an EE adduct ion with the analyte
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17
Q

What are the disadvantages of Chemical Ionisation (CI)?

A
  • High energy need to ionise reagent gas of a high pressure
  • Unwanted side reactions result in residue and so needs to be cleaned regularly
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18
Q

In CI what unique event can occur when using ammonia as a regent gas?

A

Ammonia can become the adduct ion

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19
Q

Does methane supply more or less energy than ammonia to the analyte during CI? Why and what are the consequences in the mass peak spectrum?

A

Ionised methane supplies more energy to the analyte than ammonia as it has a lower proton affinity. More energy to the analyte results in more fragmentation and less intensity in the molecular adduct ion

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20
Q

How does Atmospheric Pressure Chemical Ionisation (APCI) work?

A

1) Warm nitrogen gas convert analyte in a solvent to a mist
2) A heater vaporises the solvent and analyte
3) A corona discharge needle emits electrons and ionises nitrogen gas results in secondary reactions producing reagent ions (protonated water and water clusters)
4) Reagent ions transfer protons to analyte

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21
Q

Where is the most likely site for ionisation to form a protonated adduct ion?

A

The site with the highest proton affinity - usually at nitrogen

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22
Q

What are isotopologues?

A

Molecules that differ only in the isotopic composition in on ore more of their atoms

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23
Q

What is the nominal mass?

A

The value calculated from the most abundant isotope of each element rounded DOWN to the nearest integer

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24
Q

What is the monoisotopic mass?

A

The lowest isotopologue

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25
What is the equation for resolution?
(m/z) / (Δm/z) Δm/z - the full width of a peak at half its maximum height
26
What is the mass defect?
The difference between the nominal mass of a species and its monoisotopic mass
27
What are isobars?
Chemical species with the same nominal mass but different exact masses
28
At low resolutions, what do the peak intensities of isotope clusters reflect?
The natural relative abundances of the isotopes
29
Nitrogen is an A+1 type element. What does that mean?
That is has two naturally occurring isotopes
30
Chlorine is an A+2 type element which means it has three naturally occurring isotopes, true of false?
False - it only has two but the second isotope is +2u larger than the most abundant isotope and therefore it in an A+2 type element
31
What does the simplification of an ion cluster suggest when moving from right to left?
Loss of an isotope
32
13C is 1.1% abundant, for a molecule with 9 carbons what is the possibility that any one of them will be 13 C?
1.1% * 9 = 9.9% for any carbon to be 13 C
33
What are the steps in estimating the number of carbons in a molecule?
1) Scale the molecular ion peak (M) to 100% 2) Multiply the intensity of the M+1 peak by the same factor 3) Divide M+1 by the isotope abundance (for 13C it is 1.1%) and round DOWN
34
Assuming an error of ±10% how do we determine if calculation of the number of atoms is likely?
- Make an estimate range with ±10% of our calculated number of atoms - See if the value falls within n±1 range - If it does the calculated value is likely - If not then our value is unlikely
35
What are the steps in estimating the number of a certain elements in a molecule?
1) Scale the molecular ion peak (M) to 100% 2) Multiply intensity of other peaks by the same factor 3) Consult data book for elements that have similar abundances and divide down by that abundance 4) Correct remaining peaks for other isotopes of that element
36
Howdo the natural abundances (from the data book) need to be changed?
They must be changed to relative abundances against the most abundant isotope of that element
37
What is the degree unsaturation?
The amount of rings and/or double bonds
38
What is the equation for the degree of unsaturation?
= x - 1/2y + 1/2z + 1 x: number of tetravalent atoms y: monovalent atoms z: trivalent atoms
39
What is the nitrogen rule?
- compounds with odd numbers of N atoms give a molecular ion peak at odd m/z values - compounds with even number of N atoms (or no N atoms) give a molecular ion peaks at even m/z values
40
Ionisation prefers to remove an electron with the lowest ionisation energy. Rank the bond orbitals from lowest ionisation energy to highest
- Non-bonding orbital - π-orbital - σ-orbital
41
What does fragmentation generate and which are detected in mass spec?
Ions, radicals and molecules - only ions are detected
42
How do OE ions gives rise to EE fragment ions?
By cleavage of one bond
43
How do OE ions give rise to OE fragment ions?
By cleavage of two bonds (by rearrangement or ring cleavage)
44
EE ions usually give rise to OE or EE fragment ions? (The even electron rule)
EE - separation of an electron pair is unfavourable
45
What are the two possible effects that can occur to radical cations when undergoing fragmentation?
Charge retention or charge migration
46
What is charge retention during fragmentation?
The fragmentation reaction occurs and the charge remains on the part of the molecule where the ionisation occurs
47
What is charge migration during fragmentation?
During fragmentation there is a redistribution of electron density to form a fragment that has the greatest stabilisation
48
What are the important aspects to identify in mass spectra?
- The molecular ion peak - The primary fragment ion and complementary fragment ions
49
For a molecule AB how would you calculate the m/z for B+ if you knew M+ and A+ ?
B+ = M+ - A+
50
What is Stevensons rule?
(Where cleavage of a single bond generates more than one ion and radical pair:) - The fragment with the higher IE retains the unpaired electron (radical) - The other fragment becomes the ion
51
Using Stevensons rule; If the IE of methane is 12.6EV and the IE for propane is 10.9EV, when butane fragments which is the ion and which is the radical?
Methane has a higher IE and so is the radical and propane has the lower IE and so is the ion. The inverse IS possible however this way round is favoured
52
What is an ion series?
EE ions that differ by 14units that are characteristic of a functional group
53
Why do straight chain alkanes have a loss of 14units between the ion peaks?
The alkane ions undergo secondary fragmentation with the loss of a stable ethene molecule (mass of 14units)
54
What are the 3 factors that effect ion stabilisation?
- Cation stability - Lone pair resonance stability - Pi electron resonance stability
55
If ion abundance increases in reverse order to stability of the cation, what is the driving force?
The size of the radical - the loss of the largest radical is favoured
56
What is Fields rule?
EE decompositions forming the same product ion eject the neutral molecule with lowest proton affinity
57
EE ions can eject both radicals and neutral molecules, true or false?
False - EE ions can only eject neutral molecules
58
Define homolytic and herterolytic cleavages
Homolytic cleavages involve 1e (α-cleavage) Heterolytic cleavages involve 2e (i-cleavage)
59
What is radical site initiation?
α-cleavage occurs due to a strong preference for radical electron to be paired
60
When effects radical site initiation?
How good the radical site is at donating electrons (the heteroatom)
61
What is the formula for an acylium ion?
R-C≡O+
62
What is the formula for an allyl ion?
H2C+-CH=CH2
63
What is charge site initiation?
(i-cleavage) Heterolytic bond cleavage due to a strong attraction of an electron pair towards a positive charge
64
For OE ions which is preferred, radical site initiation or charge site initiation?
Radical site initiation (α-cleavage)
65
For EE ions which is preferred, radical site initiation or charge site initiation?
Only charge site initiation is possible (i-cleavage)
66
What effects charge site initiation?
How electronegative the atom obtaining the electrons is (the heteroatom)
67
What type of cleavage occurs during the decomposition of cycloalkanes?
α-cleavage (radical site initiation)
68
What is the Diels-Alder reaction?
An alkene reacting with a diene to produce a cycloalkene
69
What are the two pathways in the Retro-Diels-Alder reaction and which is preferred for cyclohexene?
1) Ionised diene + alkene (secondary α-cleavage) - preferred 2) diene + ionised alkene (secondary i-cleavage)
70
In the retro-diels-alder reaction, how can the ionised alkene be the preferred route?
Resonance can lower the IE of the ionised alkene formed - it is stabilised and the i-clevage is preferred
71
What is the McLafferty rearrangement?
γ-H rearrangement to an unsaturated group (a double bond) with β-cleavage
72
For the McLafferty rearrangement what influences if α or i-cleavage occurs?
The R group: methyl groups favour α and benzene groups favour i
73
What fragment ion is common for substituted benzene molecules and why is it stable?
Tropylium ion (m/z= 91) - it obeys Huckel's rule (it is aromatic)
74
What are produced when an OE undergoes i-cleavage?
A cation and a radical
75
If the m/z loss from molecular ion peak to fragment peak is EVEN, is the fragment species OE or EE?
OE
76
If the m/z loss from molecular ion peak to fragment peak is ODD, is the fragment species OE or EE?
EE
77
What is a hydrogen rearrangement (rH)?
A radical site on an unsaturated heteroatom can accept a H
78
What is a rearrangement displacement (rd)?
Formation of a bond between a radical site and a distant atom can lead to cyclisation and displacement of the radical
79
Is rearrangement displacement charge retention or
80
Does charge site rearrangements prefer OE or EE ions?
EE ions
81
Where does the charge site move to in charge site rearrangements?
It doesn't move
82
What are two solutions for complicated analysing complex mass spec?
- Chemical Modification - Tandem Mass spectrometry
83
What is the isotope method and what does it help identify?
- Identifies double and triple bonds - Deuterium and Wilkinson's catalysts 2 Deuterium added is a double bond 3 Deuterium added is a triple bond
84
What is the chemical labelling method and what does it help identify?
- Identifies double bonds - Iodine and dimethyl disulfide - Fragmentation occurs between the two groups
85
In a triple quadrupole mass spectrometer, which quadrupole(s) perform mass analysis?
1st and 3rd
86
What is CID?
Collision induced dissociation - collisions between ions and collision gas transfers energy to the ions causing new ions to be produced
87
What are the three Tandam MS operating modes and what are there applications?
1) Product ion scan: investigate sub-structures of precursor ions 2) Neutral loss scan: identify compounds having common neutral loss 3) Precursor ion scan: identify origins of ions in the spectrum
88
For a product ion scan in Tandem MS, what does each quadrupole do?
Q1: single m/z value Q2: CID Q3: scan m/z range
89
For a neutral loss scan in Tandem MS, what does each quadrupole do?
Q1: scan m/z range Q2: CID Q3: offset scan m/z range
90
For a precursor ion scan in Tandem MS, what does each quadrupole do?
Q1: scan m/z range Q2: CID Q3: sing m/z range

Core 4b (4 decks)

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