Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

PSFC2 > Mass Spectrometry 2 > Flashcards

Mass Spectrometry 2 Flashcards

1
Q

The rest of the spectrum: Fragmentation

A

EI: AB + e- → BA●+ + 2e- most common

-Odd-electron ion = free radical = relatively unstable
- Fragmentation and/or Rearrangement may occur
• Relative energy differences of potential fragments (parent / daughter) crucial
• Kinetics also has a role to play: will ions fragment before they reach the detector?
– Same applies to possible rearrangements

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Fragmentation

used in most databases: EI

A

• Not all 70eV transferred on e- impact
• Ions have distribution of internal energies
• If no collisions (vacuum in mass spectrometer), then uni-molecular processes to revert to lower energy
– Collisions can help when using soft ionisation – Sometimes you can cause collisions to occur on purpose
• Metastable ions: fragmentation after leaving source
– appear at non-integral mass
– Position reveals fragmentation process and hence structural features of the original molecule
• But we mostly rely on stable ions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Electron ionisation (EI)

A

• Hard ionisation: 70 eV 6700 kJ mol-1
– Extensive fragmentation
– Not all energy passed-on from electrons to molecules
– Not always a molecular ion

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Fragmentation (EI)

A 
- No need to apply energy as there is no barrier to overcome. However this depends on the fragment to be made 
• noEa -> no M+remains
• high Ea( >= 4eV =390 kJmol-1)
  -> little fragmentation
  -> abundant M+.
• Fragm. with loss of stable mol.:
 -> (substantial) reverse Ea
• Fragm. with single scission:
->  little or no reverse Ea
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Single bond cleavage

A

Preferred fragmentation path way: Thermodynamics ∆fH(products) - ∆fH(reactant(s))

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Fragmentation (EI)

A

• Energy considerations: predict which decomposition route most favourable
• Statistics -> relative peak intensities
– several pathways can occur at same time
• Single bond cleavage may not be favoured:
– Fragmentation with (multiple) H-rearrangement(s)
– Skeletal reorganisation incl. formation of stable neutrals
• Geometry also important
• Reactions involving bond formation unlikely if NOT to H
– Exception is in case of unsaturation
• Lower energy promotes H-rearrangement over single-bond cleavage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

General “rules”

A
  1. Formation of very small ions unlikely
  2. Ease of ion formation (most stable ion):
    tertiary > secondary > primary
  3. If ions are stable in solution, same applies to gas
    (e.g. delocalised cations, acylium, oxonium, imminium ions)
  4. Radicals more stable with increased substitution
  5. Vinyl cations: high ∆fH (C=C+ +C-C=C)
  6. Vinyl radicals unstable
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Molecular ions

A

• If peak below supposed M+/- with
difference 4 - 13 u: – Not M+/- after all
– Mixture
• Peak 14 u below M+/- suggests homologue as ∆fH(:CH2) = very high
– “Same” molecule, but differs in length by –CH2-
• Compounds with C, H, O and # N = 0, 2, … have even mass

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

EI: Fragmentation of Aromatics

A
  • Ar-X : in general loss of X / part of X
  • Tables list ease of fragmentation (e.g. Williams T4.8)
  • Competitive losses possible leading to more complex spectrum
  • Rearrangements can compete with decomposition, leading to different fragmentation
  • Resonance effects may be important
  • Ortho-substituted: consider proximity of side groups which may interact
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

EI: Fragmentation of Aliphatics

A
  • Tables: primary single-bond cleavages for common functional groups
  • If primary fragmentation leads to competition then loss of larger radical normally dominant
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

EI Fragmentation: Example 1 Ketones

A

• Loss of alkyl group attached to C=O readily predicted
• Acyllium ions formed
C2H5C=O+; C4H9C=O+ • Rearrangement?
see Williams T4.11
• High resolution:
resolve between C4H9+ and C2H5C=O+ at m/z = 57

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

EI Fragmentation: Example 2 Amines

A

M+●: Odd molecular weight → odd number of N
• Check tables for ion series (Williams T4.10: 30, 44, 58, 72)
• How? Rearrangement allows for loss of largest radical

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Summary of major fragmentation routes

A

diagram 21

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Common Impurities

in lab or in production…

A

m/z 149, 167, 279
Plasticizers (phtalic acid derived)

129, 185, 259, 329
Plasticizer (tri-n-butyl acetyl citrate)

133, 207, 281, 355, 429
Silicone grease

99, 155, 211
Plasticizer (tributyl phosphate)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

EI Fragmentation: using losses

A

After identification of M+/- look for neutral losses
• Tables for common losses (e.g. Williams T4.13)
• Use your chemistry knowledge

M - 1 -> loss of H
M - 2 -> loss of H2
M - 15 -> loss of CH3
M - 16 -> loss of O

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Fingerprinting

A
  • Compare to database of spectra
  • Needs to be established first
  • Spectra specific for specific conditions
  • EI mainly
  • Other MS techniques much smaller databases
  • Make own dedicated database of limited size
17
Q

Soft Ionisation

A
  1. Only add charge to molecule for easy identification of molecular ion -> molecular mass
  2. If you know the precise mass very well you can calculate # C, H, O, N, S, …
  3. But different molecules may have the same molecular formula!
  4. Different molecules fragment differently
  5. Control what functionality is ionised
18
Q

Electron ionisation (EI)

A

M+ can lose:

  • 1 radical only
  • Any number of neutrals
  • Once radical lost, only neutrals can be lost
19
Q

CI Fragmentation

A 
• Determine molecular weight = great strength 
– M+/-
– (M+H)+ / (M-H)-
– (M-X)+/- / (M+X)+/-
• High mass resolution: molecular formula
• Tailor ionising agent to
– Control level of fragmentation
 – Detect specific functionalities
• Well-suited to MS-MS
• Compare to database spectra
20
Q

Chemical ionisation (CI) table

A

slide 29

Reagent gas
Reagent ion
Analyte ion
Comment

H2
H3+
(M+H)+
Very energetic

CH4
CH5+
(M+H)+
Energetic

i-C4H10
C4H9+
(M+H)+
Mild, protonates all N-bases

NH3
NH4+
(M+NH4)+
Selective, little fragm.

NH3-CH4
NH4+
(M+H)+
Selective

Biacetyl
CH3CO+
(M+CH3CO)+
Acetylating

Ar
Ar· +
M ·+
Energetic

CS2
CS2·+
M ·+
Mild

CH3ONO-CH4
CH3O-
(M-H)-
Mild

NF3
F-
(M-H)-
Medium

CHCl3-CH4
Cl-
(M+Cl)-
Cl addition

21
Q

Electrospray Ionisation (ESI)

A
  • “Soft” ionisation: (quasi-)molecular ion
  • Polar molecules which have poor vapour pressure
    • & -ion spectra
  • Very good for large (bio-) molecules
  • Multiple charging helps with large biomolecules
  • Couples readily with chromatographic separation of mixtures

• Large, polar molecules
• Solvent: mainly MeOH / H2O mixtures
• Additives, e.g. + ions: 0.1% organic acid (CF3COOH) as H+ donor
• +/- droplets
• Desolvation aided by Coulombic repulsion
–&raquo_space; 1 molecule per droplet typical
– Separation of charged molecules aided by repulsion

22
Q

ESI “Fragmentation”

A

• (quasi-) Molecular ion intense
• Add ionising agent to solvent
• Induce fragment by collisions: MS-MS
• Sensitivity: down to ~ femto (10-15) moles for proteins
– for a typical 10,000 D (10 kD) protein this means 200 pg!

23
Q

Step 1/3a: hyphenated techniques

A

Separate mixtures before MS:
• GC-MS
• LC-MS

Select component during MS: MS-MS
• Select ion
• Fragment
• Identify

24
Q

(GC or ) LC coupled to MS

A

• Mixtures
– Natural
– Synthetic organic
• GC or (HP)LC gives some idea of chemical character
– GC if easy to volatilise, mostly LC in pharmaceutical applications
– MS ideal to further analyse minute amounts of eluent

• HPLC:
– Separate complex mixtures
– Especially if comparable polarities
– Remove solvents before MS
– Use solutes with charge (e.g. -NH2; -COOH) or volatile buffer (e.g. NH4OAc; HCO2H)
– Separate ionisation not always necessary

• LC-ESI-MS especially good if only value for [M+H] needed

25
Q

Tandem MS: MS-MS (MSn)

A
  • n >= 2
  • Similar function to other hyphenated techniques
    1. Separate interesting ions (molecular or fragments)
    2. Study each ion separately
  • (Mixtures)
  • Complex compounds: check “fragmentation pattern”
  • Physical coupling of different MS-systems (mass filters), e.g. Q3 : Special type “Ion trap” for MS-MS with single mass filter

-HPLC-MS-MS often used in healthcare applications
• Biomedical, pharmaceutical
• High complexity
• Low concentration of key molecules

26
Q

Key new developments in MS

A

• More practical high mass resolutio ninstruments (£££)
• Compact mass spectrometers (resolution, sensitivity)
• Direct Analysis/ Ambient MS
– Ambient conditions
– Desorption ElectroSpray Ionisation (DESI)
– Direct Analysis in Real Time (DART)
– Plasma Assisted Desorption Ionisation (PADI) Very gentle plasma to generate gas-phase ions Under development at Keele!

27
Q

EI: Fragmentation of Aliphatics - Amine

Simplest ion type?

A

CH2=N+H2

m/z 30

28
Q

EI: Fragmentation of Aliphatics -Ether alcohol

Simplest ion type?

A

CH2=O+H

m/z 31

29
Q

EI: Fragmentation of Aliphatics - Ketone

Simplest ion type?

A

CH3C(TRIPLE BOND)O+

m/z 43

30
Q

EI: Fragmentation of Aliphatics - HC

Simplest ion type?

A

C2H5+

m/z 29

PSFC2 (31 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions