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Mass Spectrometry Workshop Ionization Methods Three Major Info 7/9/2018 Mass Spectrometry Workshop Ionization Methods Abraham Badu Assistant Professor Chemistry and Biochemistry July 9, 2018 OSU Three major info from MS • Molecular weight • Structural information via fragmentation • Isotope ratios 1 7/9/2018 Advances in Mass Spectrometry vacuum vacuum Ion Mass Ion Mass Detector Detector Source Analyzer Source Analyzer ‐ Electron Ionization (EI) ‐ Atmospheric Pressure Chemical Ionization (APCI) ‐ Chemical Ionization (CI) ‐ Electrospray Ionization (ESI) → For volale compounds → For non‐volatile liquid samples (≈1% of all compound) ‐ MALDI Ambient vacuum → For thermally Mass Detector unstable Ionization Analyte Sampling from their Native Analyzer solid samples Environment without Sample Methods Pre-treatment Some Sample Types Human Tissue Banknotes Whole Plant Drug Tablet Cooks et. al. Science 2004, 306, 471-473; Science 2006, 311, 1566 Badu-Tawiah et. al. Annu. Rev. Phys. Chem. 2013, 64, 481–505 Ionization Methods Neutral species → Charged species • Removal/addition of electron(s) – M + e- → (M+.)* + 2e- • electron ionization • Removal/addition of proton(s) – M + (Matrix)-H → MH+ + (Matrix)- • Chemical ionization (CI) • Atmospheric pressure CI (APCI) • Fast atom bombardment (FAB) • Electrospray/nanospray ionization (nESI) • Matrix assisted laser desorption/ionization (MALDI) • Desorption electrospray ionization (DESI) • Direct analysis in real time (DART) • Native spray 2 7/9/2018 Ion Sources: volatiles; non-volatile liquids; non-volatile solids EI ESI FAB MALDI Fast atom bombardment Electron Impact (EI) Ionization Developed in 1918 by Dempster Still widely used in Forensic Environmental Drug Metabolism, etc. EI is a gas‐phase process, and requires the analyte to be volatile 3 7/9/2018 Electron Ionization (EI) – hard ionization (g) (g) 70 eV Sample inlet Electron collector Repeller To analyzer Source cage filament Fragmentation in Electron Ionization Consider formaldehyde: H2C=O Excess energy is deposited into the ion, which causes fragmentation Why 70 eV? It gives reproducible fragmentation What electron is removed? patterns necessary for the creation of library How much excess energy is unaccounted for? 4 7/9/2018 5 7/9/2018 A Useful Theoretical Insight Excess gained ≈ 4 eV Statistically Distributed OH m/z 77 Direct bond cleavage H - O - H 2 O m/z 76 Rearrangement Phenol MW 94 Da Critical Energy 6 7/9/2018 → hard ionization (g) (g) 70 eV heat M(s) 7 7/9/2018 Chemical Ionization – Soft Ionization • Ion-molecule reaction(s) between a reagent gas and a gas- phase analyte at a relatively high pressure • Most common reagent gases – methane, isobutane, ammonia • Mechanisms - +. + +. –CH4 + e (70 eV) → CH4 , CH3 , CH2 , … + + –CH4 + CH4 → CH5 + CH3 + + –CH3 + CH4 → C2H5 + H2 + + –CH5 + M → [M+H] + CH4 + + Fill chamber with reagent gas –C2H5 + M → [M-H] + C2H6 + + –C2H5 + M → [M+ C2H5] 8 7/9/2018 Types of Reactions Used for CI Protonation is one type of ionization M + AH+ → MH+ + A + + CH3CH2NH2 + NH4 → CH3CH2NH3 + NH3 Reaction proceeds only if it is Thermoneutral or exothermic (i.e., ∆Hr ≤ 0) ΔHr = ∆PA = PA (reagent) – PA (analyte) For negative ∆PA, PA (analyte) > PA (reagent) i.e., energy will be released! -- Proton affinity (PA): M + H+ → MH+ 9 7/9/2018 Proton affinity (PA): PAs of common CI reagents (kcal/mol) methane (131) < water (173) < methanol (185) < CH2=C(CH3)2 (197) < ammonia (205) → energy released is controllable via proper selection of reagents → energy released (≈∆PA) is absorbed by analyte → extend of fragmentation is also controllable CIMS: Two different reagent gases 10 7/9/2018 Major Limitation of EI and CI Requires volatility, good for thermally stable compound (<1% of all compounds) Solution: develop desorption and spray ionization (reagent) (g) (g) 70 eV heat M(s) (not applicable to large molecules) Can we teach elephants to fly? John Fenn How important will this be? 11 7/9/2018 How important? Nobel Price in Chemistry, 2002 John B. Fenn Koichi Tanaka ESI and MALDI for Biological Systems ESI Spray Ionization Soft works well for large molecules MALDI Desorption Ionization 12 7/9/2018 Matrix-Assisted Laser Desorption\ Ionization Pulsed Laser Extraction Grid to TOF Desorbed plume of matrix and analyte ions Analyte Matrix Cation (H+, Na+ etc) Sample Plate MATRIX - A small acidic organic molecule (matrix) is mixed with a low concentration of analyte in a common solvent and allowed to co-crystallize on a sample plate to form a “solid solution” by which the analyte molecules are isolated from each other. ASSISTED –matrix “assists” the desorption and ionization of the analyte(s) LASER – Typically a Nitrogen laser (351 nm) or Yag/Nd laser (334 nm) DESORPTION – Energy from the laser desorbs the matrix into the gas-phase and “carries” the analyte with it. IONIZATON - Detect [M+H]+ by transferring a proton from the matrix to the analyte Choose a matrix based on the molecular weight, solubility and chemical structure of the analyte. Excellent for intact molecular weight determination for both polar and non-polar molecules with mass > 500 amu. Hypothesis/speculations about Ionization • Disintegration of solid: – Statistical charging of clusters in a net neutral plume – Total charge due to • Charge separation of “salts” • Photoionization of matrix • Chemical Ionization – Charged clusters generated in selvedge evaporate and undergo • Ion-neutral reactions, • Ion-ion reactions • Ion-electron reactions 13 7/9/2018 Sample and matrix are spotted on MALDI plate 14 7/9/2018 MALDI instrument MALDI is a vacuum-based ionization method Often coupled to Time-of-Flight Instrument MALDI Matrix Materials 15 7/9/2018 MALDI Matrices Matrix Abbrev Sample Type Condition 2,5-dihydroxybenzoic acid DHB Peptides < 5,000 50% ACN in 0.1% TFA, polymers, dedrimers THF, 2:1 Good universal matrix chloroform:MeOH “cold matrix” 3,5-dimethoxy-4-hydroxycinnamic SA Peptides and Proteins 50-70% ACN in 0.1% acid (Sinapinic acid) > 10,000 TFA “hot matrix” -cyano-4-hydroxycinnamic acid HCCA Excellent for peptides, 50% ACN in 0.1% TFA digestion products and proteins Dithranol Non-polar polymers THF, Methylene chloride Indoleacrylic acid IAA Non-polar polymers THF, methylene chloride 3-hydroxypicolinic acid HPA DNA and negative ion See me for more specific samples procedure Trihydroxyacetophenone THAP DNA and negative ion See me for more specific samples procedure Nor-harmane Universal 50% ACN, THF, chloroform MALDI Matrices: common features? Alpha-cyano-4-hydroxy cinnamic acid 16 7/9/2018 MALDI Matrices: aromatic to absorb UV Alpha-cyano-4-hydroxy cinnamic acid Protein Analysis by MALDI-TOF MS Why are singly charged ions predominantly formed in MALDI? “Lucky survivor model” 17 7/9/2018 High m/z can be observed with MALDI - TOF Resolution modest, no sequence info 1.20E+009 Alkaline Phosphatase 1.00E+009 + [M+H] 47155.22 8.00E+008 2+ [M+2H] 23445.54 + 6.00E+008 [2M+H] 94472.97 Intensity 4.00E+008 2.00E+008 0 25000 50000 75000 100000 m/z MALDI Spectrum from Nicotinic Acid Matrix of Monoclonal Antibody (IgG) 18 7/9/2018 MALDI of peptide mixture (Also works for these “lower” MW) 19 7/9/2018 MALDI works well for polymers http://www.psrc.usm.edu/mauritz/maldi.html 20 7/9/2018 Synthetic Polymer Analysis by MS (MALDI-TOF) Intens. [a.u.] Intens. [a.u.] 2000 4000 1816.292 1772.260 1660.310 1500 1728.231 1684.205 44 3000 1000 44 44 44 500 2000 0 1675 1700 1725 1750 1775 1800 1825 1850 1875 m/z 1000 0 500 1000 1500 2000 2500 3000 3500 m/z Repeating unit masses of polymers http://www.polymerprocessing.com/polymers/alpha.html Polystyrene (PS) Poly(ethylene terephtalate) (PET) C8H8(104.062600) C10H8O4 (192.042259) Poly(methyl methacrylate) (PET) C5H8O2 (100.052430) Poly(vinyl alcohol) (PVA) C2H4O (44.026215) - CH2-CH2-O- Poly(ethylene glycol) (PEG) 21 7/9/2018 Molecular weight distribution defines polydispersity of polymers Zhu, H.; Yalcin, T.; Li, L. JASMS, 1998, 9, 275-281. Polystyrene with Ag+ PD≈1.01 Polydispersity (PD) = Mw/Mn x105 829.987 * NB_89_M4\0_M16\1\1SRef, "Baseline subt." Intens. [a.u.] 991.986 Truncated Vectra Polymer 2.0 1123.212 (Somogyi et. al., Macromolecules, 1143.205 1.5 2005, 38, 4067-4071 1313.231 1433.246 PD > 1.05 1.0 1553.257 1723.288 2013.333 0.5 2303.405 2593.467 2933.604 0.0 1000 1500 2000 2500 3000 3500 4000 4500 m/z Problems with polymer analysis • Sample preparation – Several polymers are not well soluble in conventional solvents – Solventless technique – Cationizing with Na+, K+, and Ag+ by spiking – Synthesis of tailor (home) made matrices • Degradation during ionization (especially with MALDI) – Photodissociation in MALDI (MALDI/ESI comparison desirable) 22 7/9/2018 Some Examples for Tailor-Made Matrices F F F F F F F F HO N HO F N COOCH N F N COOH 3 F F F F F F F F M-2 M-3 M-1 F F F F F F F F F F F F F HO N HO F N OH OH F F F F F F F F F F F F M-6 M-4 M-5 F F F F F F F F H CO N F HO N 3 N OCH COOCH3 N COOCH 3 3 F F F F F F F F M-7 M-8 M-9 Somogyi, et al., Macromolecules, 2007, 40, 5311-5321. ESI and MALDI for Biological Systems ESI Spray Ionization Soft works for large molecules MALDI Desorption Ionization 23 7/9/2018 Spray Ionization Techniques Technique Pressure Flow Rate Comment Electrospray (ESI) Atm =1 μL/min No assistance Ion spray Atm >1 μL/min Pneumatic assistance Nano spray (nESI) Atm nL/min No assistance Sonic spray (SSI) Atm <1 μL/min No voltage Pneu. assit. Electrosonic spray (ESSI) Atm =1 μL/min Pneumatic assistance Thermospray 1-10 Torr Electrohydrodynamic Ionization (EHI) <10-3 Torr Electrospray Ionization • Formation of spray • Desolvation and breakup of droplets • Emergence of ions
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