Atomic Mass Spectrometry Chap 11 (Cont’d). Types of Atomic Mass Spectrometers Quadrupole MS...
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Atomic Mass SpectrometryAtomic Mass Spectrometry
Chap 11 (Cont’d)Chap 11 (Cont’d)
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Types of Atomic Mass SpectrometersTypes of Atomic Mass Spectrometers
• Quadrupole MSQuadrupole MS
• Time-of-flight (TOF) MSTime-of-flight (TOF) MS
• Double-focusing MSDouble-focusing MS
• Ion trap MS (covered in Chap 20)Ion trap MS (covered in Chap 20)
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Double-Focusing MSDouble-Focusing MS
Fig.11.9Fig.11.9
Advantage: mass resolution > Advantage: mass resolution > ∼10∼1055
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Interferences in Atomic Mass SpecInterferences in Atomic Mass Spec
Fig 11.15Fig 11.15
Optical ICPOptical ICP Mass ICPMass ICP
10 ppm Ce10 ppm Ce100 ppm Ce100 ppm Ce
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Interferences in Atomic Mass SpecInterferences in Atomic Mass Spec
Consider ICP-MSConsider ICP-MS
Two types of interferences:Two types of interferences:
(1)(1) SpectroscopicSpectroscopic
• Isobaric ions Isobaric ions ≡ isotopes of ≡ isotopes of different different elements elements with essentially the same masswith essentially the same mass
e.g., e.g., 5858FeFe++ overlaps the peak for overlaps the peak for 5858NiNi+ +
• Polyatomic ions ≡ ions formed from species Polyatomic ions ≡ ions formed from species interaction in plasma, matrix, or atmosphereinteraction in plasma, matrix, or atmosphere
e.g., e.g., 1414NN22++ with with 2828SiSi++
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• Doubly charged ions Doubly charged ions
e.g., e.g., 118118SnSn2+2+ (m/z = 59) overlaps (m/z = 59) overlaps 5959CoCo++ (m/z = 59) (m/z = 59)
• Oxide and hydroxide formation (most serious) Oxide and hydroxide formation (most serious) ≡ ≡
MOMO++ and MOH and MOH++ formed from any O formed from any O22 and H and H22O presentO present
(2)(2) Nonspectroscopic Matrix EffectsNonspectroscopic Matrix Effects
• Noticeable at high concentrationsNoticeable at high concentrations
• Dilute sample, separate species, use internal standardDilute sample, separate species, use internal standard
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Molecular Mass SpectrometryMolecular Mass Spectrometry
Chap 20Chap 20
Uses:Uses:
• Structures of inorganic, organic, and bio-moleculesStructures of inorganic, organic, and bio-molecules
• Quant and qual analysis of mixturesQuant and qual analysis of mixtures
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Basic Principle:Basic Principle:
• Molecular vapor is bombarded Molecular vapor is bombarded with stream of fast electronswith stream of fast electrons
• Molecules are promoted to Molecules are promoted to high energy excited statehigh energy excited state
• Relaxation occurs via fragmentation:Relaxation occurs via fragmentation:
e.g., ethyl benzenee.g., ethyl benzene
CC66HH55CHCH22CHCH33 + e + e−− C C66HH55CHCH22CHCH33•+•+ + 2e + 2e−−
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Mass Spectrum of Ethyl BenzeneMass Spectrum of Ethyl Benzene
Fig. 20-1Fig. 20-1
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Ion Sources for Mass SpecIon Sources for Mass Spec
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Ion sources for Mass SpecIon sources for Mass Spec
Table 20-1Table 20-1
Gas PhaseGas Phase
(1)(1) Electron ImpactElectron Impact
(2)(2) Chemical IonizationChemical Ionization
(3)(3) Field IonizationField Ionization
Desorption SourcesDesorption Sources
(1)(1) Electrospray IonizationElectrospray Ionization
(2)(2) Matrix-Assisted Laser Matrix-Assisted Laser Desorption/ionization (MALDI)Desorption/ionization (MALDI)
(3)(3) Fast Atom Bombardment (FAB)Fast Atom Bombardment (FAB)
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Electron Impact (EI)
• Electrons from filament accelerated through Electrons from filament accelerated through ∼ 70 V∼ 70 V
• Molecules excited to very high vib and rot levelsMolecules excited to very high vib and rot levels
• Relaxation via fragmentation:Relaxation via fragmentation:
• Molecular ion, MMolecular ion, M•+•+, not always observed, not always observed
• ““Daughter ions” ≡ lower mass ionsDaughter ions” ≡ lower mass ions
• Complex spectra result (much fragmentation) Complex spectra result (much fragmentation)
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““hard source”hard source”
MM++
Base peakBase peak
Electron Impact (EI)
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• AdvantagesAdvantages::
• ConvenientConvenient
• Good sensitivityGood sensitivity
• Extensive fragging Extensive fragging unambiguous identification unambiguous identification
• DisadvantagesDisadvantages::
• Extensive fragging Extensive fragging loss of molecular ion peak loss of molecular ion peak
• Analyte must be volatized Analyte must be volatized thermal decomposition thermal decomposition
• Useful only for analytes < 1000 DaUseful only for analytes < 1000 Da
Electron Impact
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Chemical Ionization (CI)
• Modern instruments allow interchangeable electron Modern instruments allow interchangeable electron impact and chemical ionizationimpact and chemical ionization
• Less fragmentation than with EILess fragmentation than with EI
• Ionization chamber filled with a Ionization chamber filled with a reagent gasreagent gas::
• e.g., e.g., CHCH44 NH NH33 iso-butane iso-butane at P at P ∼ 1 ∼ 1 torrtorr
• Energetic electrons (100 – 200 eV) convert Energetic electrons (100 – 200 eV) convert reagent gas to variety of reactive products:reagent gas to variety of reactive products:
e.g., e.g., CHCH44 + e + e−− CH CH44++ + 2e + 2e−−
Likewise: CHLikewise: CH33++ and CH and CH22
++
CHCH33++ + CH + CH4 4 C C22HH55
++ + H + H22
Reacts withReacts withanalyte to analyte to form MHform MH++
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““soft source”soft source”
Base peakBase peakProtonated moleculeProtonated molecule
Chemical Ionization
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EIEI
CICI
Comparison of EI and CI Mass SpectraComparison of EI and CI Mass Spectra
1-decanol
Fig 20-2Fig 20-2