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Home , Faraday cup, Whole number rule

Spectrometry (MS) • Technique for studying the masses of or molecules or fragments of molecules • Provides information about … o Elemental composition o Molecular structure o Composition of complex mixtures o Structure & composition of solid surfaces o Isotopic ratios of atoms • Discriminate between 79Br & 81Br Applications

Skoog, Table 20-5 Mass Spectrometry Beginnings • Aston (Cambridge) o 1919 – Discovered two of neon (20Ne & 22Ne) • 1922 – Nobel Prize in Francis William Aston o “for his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole-number rule" • Discovered 212 of 281 naturally occurring isotopes

www.nobel.se Obtaining a • Gaseous molecules desorbed from condensed phases are ionized • are accelerated by an electric field • separation by mass-to-charge ratio (m/z)

ethyl benzene

Skoog, Fig. 20-1 The Instrument • Sample ionization o Gas-phase o Desorption • Mass analyzer ~ monochromator o Quadrupole (Q) o Time-of-flight (TOF) o Double-focusing (B, E) o • Ion transducer o multiplier o cup

Skoog, Fig. 11-1 Ion Transducers • o Analogous to a photomultiplier tube (PMT) o Rugged & reliable o Current gain ~ 107

• Faraday cup o Inexpensive o Simple mechanically & electrically o Less sensitive than electron multiplier

Skoog, Fig. 11-2(b), 11-3 Ion Sources

Basic Type Name and Acronym Ionizing Agent

Gas phase Electron impact (EI) Energetic

Chemical ionization (CI) Reagent gaseous ions

Field ionization (FI) High-potential electrode

Desorption (FD) High-potential electrode

Electrospray ionization (ESI) High electrical field

Matrix-assisted desorption/ionization (MALDI) Laser beam

Plasma desorption (PD) Fission fragments from 252Cf

Fast bombardment (FAB) Energetic atomic beam

Secondary ion mass spectrometry (SIMS) Energetic beam of ions

Thermospray ionization (TS) High temperature

Skoog, Table 20-1 (EI) • Electrons accelerated through potential of 70 V and interact with incoming molecules M + e– → M+• + e– + e– 70 eV Molecular ~55 eV 0.1 eV ion

• Interaction with 70-eV Formaldehyde electron will likely remove electron with lowest ionization energy o n < < 

Harris, 6th ed., Fig. 22-3 Electron Ionization (EI)

• Path of electrons & molecules are at right angles o Collide to produce mostly singly-charged positive ions • Inefficient process

Skoog, Fig. 20-3 (CI) • Ionization source is filled with a reagent gas

o CH4,C4H10, NH3,H2, CH3OH, NO

• Energetic electrons (100 – 200 eV) convert CH4 to a variety of reactive products:    CH 4 e  CH 4 2 e    CH 4 CH 4  CH 5 CH 3    CH 4  CH 3 H   CH 3 CH 4  C 2 H 5 H 2

  CH 5 M  CH 4 MH   C2H 5 M  C2H 4 MH EI vs. CI Mass Spectra BASE PEAK

• Hard source • Soft source o More fragmentation o Less fragmentation • Structural information • Molecular weight o Functional groups information

Harris, Fig. 21-14 EI vs. CI Mass Spectra • 1-Decanol mass spectra • Hard source o More fragments o Structural info • Soft source o Less fragments o MW info

Skoog, Fig. 20-2 (ESI) • Sample typically in form of solution (organic or aqueous) • Excess solvent must be removed before entering MS o Large increase in pressure from solvent vaporization • Differential solvent removal o Solution passed through stainless steel capillary tube o Apply high electric potential (3 – 5 kV) o Solvent evaporates rapidly from droplet surface and droplets get smaller and smaller o Solvent molecules diffuse away Electrospray Ionization (ESI)

Harris, 6th ed., Fig. 22-16 (b); (Skoog, Fig. 20-8) Laser Desorption Ionization (LDI) • Molecular system exposed to laser beam has its internal energy greatly increased o Melting o Vaporization o Ionization o Decomposition • Process of beaming laser light onto small area of sample specimen to desorb ions

Herbert, C. G.; Johnstone, R. A. W.; Mass Spectrometry Basics; 2003, p. 8 Matrix-Assisted Laser Desorption Ionization (MALDI) • Aqueous/alcohol solution of sample is mixed with radiation-absorbing matrix to TOF-MS material o Matrices (Skoog, Table 20-4) • Solution evaporated on metallic probe surface • Solid mixture is exposed to pulsed laser beam o Analyte is sublimed as ions • Useful for obtaining accurate molecular weights of biopolymers

Source: http://www.srsmaldi.com ESI & LDI Pioneers • Fenn (Virginia Commonwealth) & Tanaka (Shimadzu) • 2002 – Nobel Prize in Chemistry o "for the development of methods for John B. Fenn identification and structure analyses of biological macromolecules" o "for their development of soft desorption ionization methods for mass spectrometric analyses of Koichi Tanaka biological macromolecules"

Source: http://www.nobel.se (FAB)

• Focus a high primary current beam of neutral atoms or molecules on sample • Sample dissolved in non-volatile liquid matrix • Inert gas atoms are ionized to give positive ions • As ions collide with other inert gas atoms (He, Ar, Xe), charge exchange occurs o Fast-moving ions become fast- moving atoms

Source: http://www-methods.ch.cam.ac.uk/meth/ms/theory/fab.gif Secondary Ion Mass Spectrometry (SIMS) • Focus a high primary current beam of ions on sample • Sample dissolved in non- volatile liquid matrix • Dynamic SIMS o Current beam high enough to damage surface o Elemental and isotopic information obtained • Static SIMS o Dedicated to analysis of top monolayer of surface o Fresh layer of new ions continuously brought to surface

Source: http://www.chemistry.wustl.edu/~walker/sims_exp.gif; http://www.ulb.ac.be/sciences/cpmct/images/logosims.gif Magnetic Sector (B) • Ions deflected according to their mass • Spectrum obtained by changing the field strength m B2r 2  z 2V

m/z = mass-to-charge ratio B = magnetic field strength r = radii of curvature (trajectory) V = accelerating voltage

Harris, 6th ed., Fig. 22-2; (Skoog, Fig. 20-12) Electrostatic Sector (E) • Ions deflected according to their kinetic energy o KE = ½mv2

Harris, 6th ed., Fig. 22-12 Double-Focusing • Combination of magnetic & electrostatic sectors o Improved resolving power • Resolution of 105 o Compatible with chromatographic columns o Compact • Configurations o Mattauch-Herzog • Skoog, Fig. 11-9 o Nier-Johnson

• Skoog, Fig. 20-13 Mattauch-Herzog

Source: http://www.oup.com/images/booksites/higson/higson_fig9.8.jpg Double-Focusing Ion Optics

Forward Geometry EB Configuration (Nier-Johnson)

Reverse Geometry BE Configuration

Herbert, C. G.; Johnstone, R. A. W.; Mass Spectrometry Basics; 2003, p. 178-179 Time-of-Flight (TOF)

Harris, 6th ed., Fig. 22-14; (Skoog, Fig. 11-8) Quadrupole (Q)

Harris, Fig. 21-13; (Skoog, Fig. 11-4) Ion-Trap

Harris, 6th ed., Fig. 22-15; (Skoog, Fig. 20-15) Hyphenated MS Methods • Mass = detector for other analytical techniques o Mass spectra collected as compounds exit • Chromatography/MS o Gas Chromatography/MS (GC-MS) o Liquid Chromatography/MS (LC-MS) • Capillary Electrophoresis/MS (CE-MS) Chromatography/Mass Spectrometry • MS requires high vacuum o Avoid molecular collisions during ion separation • Chromatography is high-pressure technique o Must remove huge excess matter between the chromatograph and the spectrometer • For GC, narrow capillary column connected directly to inlet of the mass spectrometer • For LC, liquid from column creates huge volume of gas when vaporized o Pneumatically assisted electrospray o Atmospheric pressure chemical ionization (APCI) APCI

• Uses heat and coaxial flow of N2 to convert eluate into a fine aerosol mist • Creates new ions from gas-phase reactions between ions & molecules • High voltage is applied to needle in the path of the aerosol

Harris, 6th ed., Fig. 22-18 Liquid Chromatography/MS (LC-MS)

Harris, 6th ed., Fig. 22-16 (a) Gas Chromatography/MS (GC-MS) • Must remove most of the carrier gas from the analyte • Quadrupole or ion trap mass analyzers used

Skoog, Fig. 27-13, 27-14 Capillary Electrophoresis/MS (CE-MS) • Capillary effluent is passed into an electrospray ionization device • Products enter • Detection limits: tens of femtomoles (10–14 M)

Skoog, Fig. 30-7 Chromatography/MS Spectra

GC

CE

Herbert, C. G.; Johnstone, R. A. W.; Mass Spectrometry Basics; 2003, p. 264 (MS/MS) First MS Second MS • Isolates molecular ions • Fragments ions • Soft ionization source o Collisions between ions & He atoms cause o Molecular ions or protonated molecular ions further fragmentation • “Daughter” ions • “Parent” ions • Analogous to • Provides series of mass chromatographic column spectra for each molecular ion produced o Provides pure ionic species for second spectrometer QQQ Tandem Instrument

• Q1 &Q3 are regular quadrupole filters

• Q2 is a collision focusing chamber o Helium pumped into chamber & collides with parent ions o Operates in rf-mode only • Focuses scattered ions but does not act as a mass filter Skoog, Fig. 20-24 The Mass Spectrum

• Molecular ion (M+•) = unknown molecular mass • M+• breaks apart efficiently with EI o Fragments provides clues about structure • CI mass spectrum has strong MH+ peak o Molecular mass information • Nitrogen Rule o Odd nominal mass for M+• • Odd # N atoms o Even nominal mass for M+• • Even # N atoms

Harris, Fig. 21-14 Molecular Ion & Patterns • M+●peak is base peak for aromatic compounds o EI spectra • Next higher mass peak provides elemental composition info o M + 1 peak • Carbon Intensity of M + 1 relative to M+●for C H : o 98.92 % 12C n m Intensity = n × 1.08 % + m × 0.012 % o 1.08 % 13C • 13C 2H o 0.012 % 2H Benzene: Intensity = 6 × 1.08 % + 6 × 0.012 % = 6.55 % Biphenyl: Intensity = 12 × 1.08 % + 10 × 0.012 % = 13.1 %

Harris, Fig. 21-18 Molecular Ion & Isotope Patterns

Harris, Table 21-1 Rings and Double Bonds • Rings + double bonds (R + DB) formula o Used if composition of a molecular ion is known h n R DB c   1 2 2

c = # of 14 atoms (e.g., C, Si) [make 4 bonds] h = # of (H + ) atoms [make 1 bond] n = # of Group 15 atoms (e.g., N, P) [make 3 bonds]

22 11 11 R DB 14 1  15 rings & double bonds 2 2 Harris, 6th ed., p. 526 figure Identifying the Molecular Ion (M+●) Peak • Highest m/z value of any “significant” peaks o ~ 5 – 20% of base peak intensity • Isotopic peak intensity (M+1, M+2, etc.) must be consistent with proposed chemical composition • Heaviest fragment ion must correspond to a probable mass loss o Loss in 3 – 14 or 21 – 25 Da range rare o Common mass losses

● ● 15 Da ( CH3) 29 Da ( C2H5) ● ● 17 Da ( OH or NH3) 31 Da ( OCH3) ● ● 18 Da (H2O) 43 Da ( C3H7 or CH3CO ) Fragmentation Patterns

Harris, Fig. 21-26, 21-17 Interpreting Fragmentation Patterns

• Highest peak of “significant” intensity = m/z 100 ● • Next highest peak at m/z 85 (loss of CH3) • M+●has an even mass o Nitrogen rule  cannot be an odd number of N atoms in molecule

Harris, 6th ed., Fig. 22-10 Interpreting Fragmentation Patterns

observed (M 1)/M intensity 6 % Number of C atoms   6 contributionper carbonatom 1.08 %

Intensity = 6 × 1.08 % + 12 × 0.0012 % + 1 × 0.038 % = 6.7 % of M+●

13C 2H 17O

R + DB = c – h/2 + n/2 + 1 = 6 – 12/2 + 0 + 1 = 1 ring or double bond

Harris, 6th ed., Fig. 22-10 Fragmentation of 2-Hexanone

Harris, 6th ed., Fig. 22-11