Atomic Mass SpectrometryAtomic Mass Spectrometry

Chap 11 (Cont’d)Chap 11 (Cont’d)

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)

Double-Focusing MSDouble-Focusing MS

Fig.11.9Fig.11.9

Advantage: mass resolution > Advantage: mass resolution > ∼10∼1055

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

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++

• 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

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

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−−

Mass Spectrum of Ethyl BenzeneMass Spectrum of Ethyl Benzene

Fig. 20-1Fig. 20-1

Ion Sources for Mass SpecIon Sources for Mass Spec

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)

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)

““hard source”hard source”

MM++

Base peakBase peak

Electron Impact (EI)

• 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

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++

““soft source”soft source”

Base peakBase peakProtonated moleculeProtonated molecule

Chemical Ionization

EIEI

CICI

Comparison of EI and CI Mass SpectraComparison of EI and CI Mass Spectra

1-decanol

Fig 20-2Fig 20-2