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