Dissociation ofEven-Electron Ions
Andrea RaffaelliCNR – Istituto di Fisiologia Clinica
Via Moruzzi, 1, 56124 Pisa.E-Mail: [email protected]: http://raffaelli.ifc.cnr.it
A …Simple? ESI Spectrum
300 350 400 450 500 550
353
370
375
391 421
437
489
505
Glucosidic CompoundMW 352
[M+H]+
[M+NH4]+
[M+Na]+
[M+K]+
[M+FA+K]+
[M+TFA+K]+
[M+TFA+Na]+
[M+FA+Na]+
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
EI Odd-Electron Ions Fragmentation vs. “Cold” Even-Electron Ions Fragmentation
• EI MS deals with odd-electron, high internal energy ions.
• Fragmentation patterns have been completely described and rationalized.
• High internal energy induces extended fragmentation.
• Soft ionization MS (ESI, APCI, MALDI…) deals with even-electron, low internal energy ions.
• Fragmentation patterns are still waiting a complete rationalization.
• Fragmentation can be completely missing so that we need MS-MS techniques.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
Comparison between the EI-MS and the ESI-MS-MS Spectra of Cocaine
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 3100
50
100
15 32
42
5159 68
77
82
94105
122140 152 166
182
198 272
303
EI70 eV
40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 3100
50
100
44
82
91105 119 150
182
272
304
ESI-MS-MSQqQ, CE 20 eV
Moreover…
• Independently of the instrument type, EI spectra are quite similar.
• The EI source is always the same (at least, very similar).
• Spectra are acquired under the same experimental conditions (70 eV).
• MS-MS instruments can be quite different (quadrupoles, traps, TOFs, …).
• Collision cells can operate at different levels (high energy vs. low energy)
• Collision energy can be, however, adjusted by the operator.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
Effect of the Collision Energy
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 201525 50 75 100 125
CE, Volts
Intensity, cps N
N
O
O N
HN
theophyllin
181
CE 10 eV
181
124CE 20 eV
124
96
69 18142
CE 30 eV6942
94109
CE 50 eV
… Also:Positive vs. Negative Ion Mode
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
50 100 150 200 250m/z, Da
107
135
11991 165 229183
211
50 100 150 200 250m/z, Da
143
185
22715941
11769 93
Positive IonMS-MS
Negative IonMS-MS
… And How About Fragmentation Pattern?
• In contrast with EI fragmentations, even-electron fragmentations have not been completely rationalized yet.
• Owing to the low internal energy, these ions tend to behave in a “more chemical” fashion.
• Even if the factors affecting the fragmentation processes are roughly the same, probably the bond energy here gets more important.
• Considering that the behavior is more like normal organic chemistry, the interpretation of the spectra can be obtained in such a way.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
- C=O
… Let’s Look to an Example: Theophyllin
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
124
96
69 181
42
- CH3-N=C=O
-CHN
Fragmentation of Even-Electron Ions
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
+ Even-Electron positive ions (cations, EE+)
- Even-Electron negative ions (anions, EE-)
1. Even-Electron Cations… Let’s Try to Put Some Order …
• As opposed to radical cations, which are met with almost exclusively in mass spectrometry, the even electron cations are common in “classical” chemistry.
• Their reactions, hence, seem much more familiar to the chemist.
• These molecular species are generally more stable than the radical cations produced by electron ionization.
• The spectra are, usually, much simpler, but the rearrangements are more frequent and more varied.
• As a consequence, these spectra yield less information and are more difficult to interpret than EI spectra.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
Actors in the Fragmentation Plot
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
EE+ Even-Electron Cation
OE+ Odd-Electron Cation
M Neutral Molecule
R Radical
r Rearrangement
Fragmentation Reactions
EI: Odd-Electron Ions Soft: Even-Electron Ions
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
*: This process is usually highly endotermic and thus improbable. A statistical estimate based on many spectra indicate that EE ions yield EE fragments in about 95% of cases.#: These processes, more common, obey the “Parity Rule”
*
#
McLafferty Classification of EE Ions Reactions Obeying the Parity Rule
1. Cleavage of a bond with charge migration2. Cleavage of a bond with cyclization and charge
migration3. Cleavage of two bonds in a cyclic ion with charge
retention4. Cleavage of two bonds with rearrangement and
charge retention
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
McLafferty, F.W. Organic Mass Spectrometry 1980, 15, 114.
Cleavage of a Bond withCharge Migration
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
The reaction occurs more easily when the protonated site is less basic. For instance, for Bu-XR molecules, we can observe:
-XR NH2 SH OH I Br Cl[(M+H)-HXR]+/(M+H)+ 0.04 0.15 19.5 240 >250 >250PA (kcal/mol) 207 175 164 145 141 140
Cleavage of a Bond with Cyclization and Charge Migration
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
It is the same reaction as above, but it is assisted by the presence of a heteroatom in a convenient position. This also provides anchimeric assistance to the expulsion of a neutral fragment. The basicity of the protonation site is important also in this case.
The Case of -Aminoalcohols
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
n 2 3 4 5 6[(M+H)-H2O]+/(M+H)+ 0.42 0.21 0.20 0.15 0.07
It must be noted that the other possible fragmentation, loss of ammonia, uccurs only when n = 4 or 5.
Cleavage of Two Bonds in a Cyclic Ion with Charge Retention
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
This rearrangement reaction, in contrast with most of the odd-electron ones, is observed especially when it involves a four-centre transition state.
Cleavage of Two Bonds with Rearrangement and Charge Retention
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
Also this reaction is observed especially when it involves a four-centre transition state.
Cleavage of Two Bonds with Rearrangement and Charge Retention: Examples
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
*
*: Note that in CI a ketone can lose water, which never occurs under EI conditions
Fragmentation Reactions not Obeying the Parity Rule
• The production of odd-electron ions from even-electron ions is, as stated before, much rarer and more difficult to predict.
• They are observed sometimes in ions with extended systems.
• The driving force is commonly the formation of very stable species, where the unpaired electron can be extensively delocalized.
• The occurrence of these reactions is, however, lower than 5%.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
44 7286
113130
148306
321
363
381
399510
528
50 100 150 200 250 300 350 400 450 500m/z, amu
10
20
30
40
50
60
70
80
90
% In
tens
ity
ESI-MS-MS Spectrum of Daunorubicine
O
OH
H3N
OH
-15
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
2. Even-Electron Anions… Even Harder to Put Some Order …
• Even-Electron anions are normally more stable than the cations.
• Fragmentation reactions, hence, require usually higher collision energy values with respect to positive ion mode.
• The parity rule does not apply: both homolytic and heterolytic breakages can occur:
• Even-Electron anions can yield, hence, both Even-Electron and Odd-Electron fragments.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
Even-Electron Anions Fragmentations
• Simple homolytic cleavage reactions where loss of a radical forms a stable radical anion
• Reactions that occur by initial formation of an anion complex which may then undergo a variety of reactions involving the bound anion, including direct displacement of the anion, and reactions involving the bound anion, including deprotonation, elimination processes, etc.
• Reactions that are not specifically directed by the first-formed deprotonated species, but where proton transfer to that species forms a new anion that may then fragment as above.
• Rearrangement reactions, including internal nucleophilicsubstitution/displacement and skeletal rearrangement reactions.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015Bowie, J.H. Mass Spectrom. Rev. 1990, 9, 349.
… Some Examples
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
H Loss
R Loss
Formation of anion-neutral complex
… More …
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
Rearrangement involving cyclyzation:
… And so on
… A Few Words about Adducts
• Soft ionization techniques provide often adduct ions, i.e. ions produced by ion-molecule reactions.
• Sometimes these adduct are predominant of even exclusive.
• Adduct ions are even-electron ions as well.• Their use for getting structural information by MS-
MS fragmentations need to face some peculiar behavior.
• In particular, their composition includes a strong and stable pre-formed ion, not related to the structure of the analyte.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
MS-MS of Adduct IonsPositive Adducts
• NH4: OK. They fragment with loss of ammonia affording the protonated ion.
• Na, K: Almost useless: they fragment badly, in an unpredictable mode and very often the only fragment is the metal ion.
• Other adducts and supramolecular assemblies: structurally dependent.
Negative Adducts
• In general they can be somehow useful.
• Their fragmentation tends to provide the original ion (Cl-, HCOO-, CH3COO- and so on), but also some useful fragments.
• The only exception is the trifluoroacetate anion: TFA adducts provide TFA anion as the only product ion.
• This as another counter-indication to the use of TFA in the mobile phase.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
Conclusions• Fragmentation of even-electron ions can be different
with respect to odd-electron ions.• Α-cleavage, very important for odd-electron ions is
completely missing for even-electron ones.• Positive and negative ions behave differently.• The fragmentation patterns are not been completely
rationalized yet.• They occur, however, in a very similar way with respect to
traditional solution organic reactions.• A good knowledge of organic chemistry and a good
practice with calculators allow in general a good spectral interpretation in the case of known analytes.
• For unknown analytes the situation is different…19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
Some Help from Libraries and Fragmentation Pattern Interpretation Software?
• Recent releases of the most common mass spectral libraries begin to include MS-MS even-electron fragmentation spectra.
• For instance, NIST 14 MS library package contains 234,284 Spectra (121,586 in the NIST 11) of 45,298 ions (15,180 previously).
• Most MS instrumentation vendors can provide specific library and search algorithms for their data systems.
• Several MS fragmentation software packages are available on the market (Mass Frontier), and they can take care also of even-electron ions reactions.
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015
Molte grazie per la cortese attenzione e…
Arrivederci a…
Tutti!
19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015