Mass Spectrometry - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Mass Spectrometry - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Mass Spectrometry  Mass Spectrometry (a.k.a. MS or “mass spec”) – a method of separating and analyzing ions by their mass-to- charge ratio  MS does not involve a specific region of the electromagnetic spectrum (because it is not directly interested in the energies of emitted photons, electronic or vibrational transitions, nuclear spin transitions, etc…) Ion abundance Up to m/z = 100000! m/z Ion Ion Ion docsity.com History of Mass Spectrometry  J. J. Thomson at Cambridge reported the first MS experiment in 1913 and discovered isotopes.  F. W. Aston built the first MS in 1919 and studied isotopes, winning the 1922 Nobel Prize in Chemistry.  In the 1930’s, Ernest Lawrence invented the calutrons used in WW2 to separate 235U.  Nobel Prize in Physics (1989) to Wolfgang Paul for the ion trap.  Nobel Prize in Chemistry (2002) to John Fenn (electrospray ionization) and Koichi Tanaka (MALDI). Calutron at the Y-12 Plant at Oak Ridge, Tennessee, used during the Manhattan Project J. J. Thomson F, W, Aston docsity.com Ionization Sources  Electron Ionization (EI)  Chemical Ionization (CI/APCI)  Photo-ionization (APPI)  Electrospray (ESI)  Matrix-assisted Laser Desorption (MALDI)  Field Desorption (FD)  Plasma Desorption (PD)  Fast atom bombardment (FAB)  High-temperature Plasma (ICP) See also Table 20-1 in Skoog, et al. Desorption Gas Phase Ionization Source Mass Analyzer Detector docsity.com EI: Electron Ionization/Electron Impact  The electron ionization (EI) source is designed to produce gaseous ions for analysis.  EI, which was one of the earliest sources in wide use for MS, usually operates on vapors (such as those eluting from a GC) Heated Incandescent Tungsten/Rhenium Filament Accelerate! e- Vaporized Molecules 70 eV Ions To Mass Analyzer docsity.com EI: Electron Ionization/Electron Impact  How EI works: – Electrons are emitted from a filament made of tungsten, rhenium, etc… – They are accelerated by a potential of 70 V – The electrons and molecules cross (usually at a right angle) and collide – The ions are primarly singly-charged, positive ions, that are extracted by a small potential (5V) through a slit Diagram from F. W. McLafferty, “Interpretation of Mass Spectra”, 3rd Ed., University Science Books, Mill Valley, CA (1980). docsity.com CI: Chemical Ionization  Chemical ionization (CI) is a form of gas-phase chemistry that is “softer” (less energetic) than EI – In CI, ionization occurs via proton transfer reactions  A gas (ex. methane, isobutane, ammonia) is introduced into the source at ~1 torr.  Example: CH4 reagent gas CH4 EI CH4+ CH4+ + CH4 CH5+ + CH3 AH + CH5+ AH2+ + CH4 Strong acid See B. Munson, Anal. Chem., 49, 772A (1977). docsity.com CI: Hard and Soft Sources  The energy difference between EI and CI is apparent from the spectra:  CI gases: – harshest (most fragments): methane – softest: ammonia docsity.com APCI: Atmospheric-Pressure Chemical Ionization  APCI – a form of chemical ionization using the liquid effluent in a spray chamber as the reagent  APCI is a form of API (atmospheric pressure ionization or ambient ionization) - these are a range of ionization techniques that operate at higher pressures, outside the vacuum MS regions, and sometimes at normal pressures and temperatures  Examples of ambient ionization methods to be discussed later in this lecture: DESI, MALDI docsity.com APCI: Chemical Ionization  An APCI mass spectrum: Diagram from Agilent Technologies docsity.com Electrospray Ionization (ESI)  The ESI process: – Electrospray ionization (ESI) is accomplished by flowing a solution through an electrically-conductive capillary held at high voltage (several keV DC). – The capillary faces a grid/plate held at 0 VDC. – The solution flows out of the capillary and feels the voltage – charges build up on nebulized droplets, which then begin to evaporate – Coulombic explosions occur when the repulsion of the charges overcomes the surface tension of the solution (holding the drop together) – known as the Rayleigh limit. – Depending on whose theory you believe  the analyte ion is eventually the only ion left  or…the analyte ion is evaporated from a small enough droplet docsity.com Electrospray Ionization (ESI)  A picture of two ideas for the electrospray process: El Aneed, et al. , Applied Spectroscopy Reviews, 44: 210–230, 2009. Jet image from http://www.newobjective.com/electrospray/electrospray.html Note – ions which are surface-active will be preferentially ionized – this can lead to ion suppression!  The Taylor cone – the shape of the cone that shoots from the needle when surface tension is overcome by electrostatic forces, and forms a jet docsity.com Typical ESI Spectra  An ESI mass spectrum: Diagram from Agilent Technologies docsity.com Typical ESI Spectra  An ESI mass spectrum of a 14.4 kDa enzyme: Diagram from http://www.nd.edu/~masspec/ions.html docsity.com ESI and APCI  ESI and APCI are complementary techniques for solution- phase analytes: Figure from Agilent Instruments docsity.com Atmospheric Phase Photo-ionization  APPI can ionize things that ESI and APCI can’t: docsity.com Comparison of Ionization Methods  How to choose an ionization technique: Figure from Agilent Instruments docsity.com MALDI: Matrix-Assisted Laser Desorption/Ionization  A method for desorbing a sample with a laser, while preventing thermal degradation  A sample is mixed with a radiation-absorbing “matrix” used to help it ionize  MALDI is heavily used for large biomolecules and polymers. Diagram from Koichi Tanaka (Nobel Lecture), 2002 docsity.com FAB: Fast Atom Bombardment  A soft ionization technique – Often used for polar, higher-mwt, thermally labile molecules (masses up to 10 kDa) that are thermally labile.  Samples are atomized by bombardment with ~keV range Ar or Xe atoms. – The atom beam is produced via an electron exchange process from an ion gun. K. L. Rinehart, Jr., Science, 218, 254 (1982) K. Biemann, Anal. Chem., 58, 1288A, (1986). Xe e - Xe+ + 2e-  Advantages: – Rapid sample heating – reduced fragmentation – A glycerol solution matrix is often used to make it easier to vaporize ions Xe+ accel Xe+ (high KE) Xe+ (high KE) + Xe Xe (high KE) + Xe+ docsity.com SIMS: Secondary Ion MS  Focused Ion Beam – 3He+, 16O+, 40Ar+ – Beam energy 5 to 20 keV – Beam diameter – 0.3 to 5 mm  Beam Hits Target – A small % of the target material is “sputtered” off and enters the gas phase as ions (usually positive)  Advantages: – Imaging of ions (characteristic masses) on a surface or in biological specimens – Surface analysis using beam penetration depth/angle – Can be used for both atomic and molecular analysis – Sensitive to low levels, picogram, femtogram and lower  Will discuss more in surface analysis/microscopy talk… docsity.com Desorption Electrospray: DESI  Desorption- electrospray ionization (DESI) is an ambient ionization technique  A new technique for desorbing ions using supersonic jets of solvents (charged like in electrospray) From Z. Takats et al., Science, 2004, vol 306, p471. docsity.com