An aircraft-borne chemical ionization – ion trap mass spectrometer ...
Chemical Ionization
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Transcript of Chemical Ionization
Survey of Mass Spectrometry
David SmithEmeritus Professor of ChemistryUniversity of NebraskaLincoln, NE 68588 USA
OverviewElectron Impact Ionization and FragmentationChemical IonizationFast Atom Bombardment IonizationChemical DerivativesElemental CompositionAnalyses of Natural Products: AcetogeninsElectrospray Ionization; LCMS; Matrix Assisted Laser Desorption IonizationCollision Induced Fragmentation; CID MS/MSQuantitation
Former Director of the Nebraska Center for Mass Spectrometry
Course Topics
Chemical Ionization
Similar to EI (Electron Ionization)
Use to analyze same types of compounds (must be volatile)
Use same sample inlets (solids probe or GC)
Requires only minor changes to the ion source
Different from EI
Gives less fragmentation, charge-induced fragmentation only Greatly simplified spectra
Use to determine/verify MW
Determine the number of labile hydrogens
Can selectively ionize amines
Chemical Ionization (CI)
Sample plusreagent gas
-500 V
Emission filament where “White Hot” wire emits electrons
0 V
Trap +20 V
Reagent gas pressure 0.5 Torr
Small apertures are required to achieve high pressure
Analyzer pressure ~10-6Torr
Chemical Ionization (CI)Small apertures are required to achieve high pressure
-500 V
0 V
Trap +20 V
Reagent gas pressure 0.5 Torr
Analyzer pressure ~10-6Torr
EI: Ions drift from electron beam to exit aperture with no collisions
CI: Reagent ions formed by EI
Ions undergo thousands of collisions with reagent gas
Methane Chemical Ionization
Ions formed by EI of methaneReagent gas pressure is sufficiently highin the ionization chamber that each ion undergoes 100-1000 collisions beforeexiting.
Isobutane Chemical Ionization
Less energetic, less fragmentation than methane CI
Formed by EI
Ammonia Chemical Ionization
Less energetic, less fragmentation than isobutane CI
Selectively ionizes compounds that contain N
Formed by EI
Chemical Ionization Reactions
Proton Transfer (PT)
RH+ (Reagent ion) + M (Sample) MH+ + R
Proton Affinity (PA): H+ + M MH+ Proton Affinity = -H
Excitation energy of MH+ = PA (M) – PA (R)
Excitation energy in MH+ facilitates fragmentation
CH5+ + CH3CH2CH2CH2OH CH3CH2CH2CH2OH2
+ + CH4base 1 base 2
PT is an acid/base reaction. The stronger base keeps the proton.
Proton transfer occurs only when PA (M) > PA (R)
Chemical Ionization Reactions (Cont)
Higher source pressure and lower source temperature favor formation of adducts
HA occurs most often with long chain saturated hydrocarbons
Hydride Abstraction (HA)
Adduct Formation (A)
The adduct likely dissociates if the excitation energy is not removedby a third molecule
Chemical Ionization Reactions (Cont)
PT Proton Transfer HA Hydride Abstraction A Adduct
Predict Reactions From Proton Affinities
Compound Proton Affinity kcal/mol
CH4 127C2 H6 142C2 H4 153iC4 H8 195CH3 Br 163CH3 OH 183C2 H5 OH 189
CH3 OCH3 191CH3 COCH3 195CH3 CO2 H 189
NH3 203C2 H5 SC2 H5 204
HCON(CH3 )2 211Aniline 208
NH(CH3 )2 219N(CH3 )3 223
Methane PT PA > 127
Isobutane PT PA > 195
Ammonia PTPA > 203
NH4+ Adduct PA > 184
Use CI to Simplify Spectra
Citronellol
CI Isobutane
EI
Formed by PT
Change reagent gas to CH4 ? To NH3 ?
EI and CI of Ephedrine
EI CI
HA PT
5858
MH+ - H2O
EI spectrum is very simple, but does not give the MW
Together, these three peaks show that MW = 165
CI of Aspartic Acid MW 133
CH4 i-C4H10
NH3 ND3
134MH+
134MH+
134MH+
151m =17MNH4
+
139M(D4)D+
Using ND3 as the reagentgas allows one to determine the number of exchangeable H
159 m = 8
m = 5 4 exchangeable H
Use ND3 to Determine the Number of Exchangeable HMass shift due to charging:
M + ND4+ MD+ MW + 2 MND4
+ MW + 22
Mass shift due to exchangeable H:
ROH/RNH2/RNH/RSH ROD/RND2/RND/RSDND3
Consider 3 structures for C3H9N
MD+ MND4+
CH3CH2CH2NH2 MW + 4 MW + 24
CH3CH2NHCH3 MW + 3 MW + 23
(CH3)3N MW + 2 MW + 22
Ammonium Adducts May Lose Water
Ammonium adducts (M + 18) of some compounds may lose water (M + 18 – 18) to give what appears as M+.
The exact mass of the apparent M+. is not consistent with the elemental composition of the unknown.
Same Nominal Masses Different Exact Masses
Chemical Ionization of Cholesterol MW 386
369 MH+-18
369 MH+-18
385 M-H+
385 M-H+
369369MNH4
+-H2O-NH3MDND4
+-D2O-ND3386
389MNH4
+-H2O MDND4+-D2O
404409
MNH4+ MND4
+
CH4i-C4H10
ND3NH3
New Terms And Concepts
Compare CI to EI; Source modifications required to increase pressureWhy is high reagent gas pressure required?Reagent gas; Common gases; Reagent ions; Common reactionsProton transfer; Hydride abstraction; Adduct formationProton Affinity; How does it relate to ion excitation energy and fragmentationUse proton affinity to predict PT or A in ammonia CIUse ammonia CI to determine the number of exchangeable HExplain apparent M+ in ammonia CI spectra