Atomic Mass Spectrometry Chap 11 (Cont’d)

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

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

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

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+

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

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

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−

Mass Spectrum of Ethyl Benzene Fig. 20-1

Ion Sources for Mass Spec

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)

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)

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

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

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−  CH4+ + 2e− Likewise: CH3+ and CH2+ CH3+ + CH4  C2H5+ + H2 Reacts with analyte to form MH+

Chemical Ionization Protonated molecule Base peak “soft source”

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