1. Atomic Mass Spectrometry
Chap 11 (Cont’d)

2. Types of Atomic Mass Spectrometers
Quadrupole MS
Time-of-flight (TOF) MS
Double-focusing MS
Ion trap MS (covered in Chap 20)

3. Double-Focusing MS
Fig.11.9
Advantage: mass resolution > ∼105

4. Interferences in Atomic Mass Spec
Fig 11.15
Optical ICP
Mass ICP
100 ppm Ce
10 ppm Ce

5. Interferences in Atomic Mass Spec
Consider ICP-MS
Two types of interferences:
Spectroscopic
Isobaric ions ≡ isotopes of different elements with essentially the same mass
e.g., 58Fe+ overlaps the peak for 58Ni+
Polyatomic ions ≡ ions formed from species interaction in plasma, matrix, or atmosphere
e.g., 14N2+ with 28Si+

6. Doubly charged ions
e.g., 118Sn2+ (m/z = 59) overlaps 59Co+ (m/z = 59)
Oxide and hydroxide formation (most serious) ≡
MO+ and MOH+ formed from any O2 and H2O present
Nonspectroscopic Matrix Effects
Noticeable at high concentrations
Dilute sample, separate species, use internal standard

8. Molecular Mass Spectrometry
Chap 20
Uses:
Structures of inorganic, organic, and bio-molecules
Quant and qual analysis of mixtures

9. Basic Principle:
Molecular vapor is bombarded
with stream of fast electrons
Molecules are promoted to
high energy excited state
Relaxation occurs via fragmentation:
e.g., ethyl benzene
C6H5CH2CH3 + e−  C6H5CH2CH3•+ + 2e−

10. Mass Spectrum of Ethyl Benzene
Fig. 20-1

11. Ion Sources for Mass Spec

12. Ion sources for Mass Spec
Table 20-1
Gas Phase
Electron Impact
Chemical Ionization
Field Ionization
Desorption Sources
Electrospray Ionization
Matrix-Assisted Laser Desorption/ionization (MALDI)
Fast Atom Bombardment (FAB)

13. Electron Impact (EI)
Electrons from filament accelerated through ∼ 70 V
Molecules excited to very high vib and rot levels
Relaxation via fragmentation:
Molecular ion, M•+, not always observed
“Daughter ions” ≡ lower mass ions
Complex spectra result (much fragmentation)

14. Electron Impact (EI)
Base peak
“hard source” M+

15. Electron Impact Advantages: Convenient Good sensitivity
Extensive fragging  unambiguous identification
Disadvantages:
Extensive fragging  loss of molecular ion peak
Analyte must be volatized  thermal decomposition
Useful only for analytes < 1000 Da

16. Chemical Ionization (CI)
Modern instruments allow interchangeable electron impact and chemical ionization
Less fragmentation than with EI
Ionization chamber filled with a reagent gas:
e.g., CH4 NH3 iso-butane at P ∼ 1 torr
Energetic electrons (100 – 200 eV) convert reagent gas to variety of reactive products:
e.g., CH4 + e−  CH e−
Likewise: CH3+ and CH2+
CH3+ + CH4  C2H5+ + H2
Reacts with
analyte to
form MH+

17. Chemical Ionization
Protonated molecule
Base peak
“soft source”

18. Comparison of EI and CI Mass Spectra
Fig 20-2 EI
1-decanol CI