![Mass Spectrometry
Ion Source
• Liquid Samples
• Electrospray Ionization (ESI)
• Liquid is nebulized with sheath gas
• Nebulizer tip is at high voltage (+ or –), producing charged droplets
• As droplets evaporate, charge is concentrated until ions are expelled
• Efficient charging of polar/ionic compounds, including very large compounds
• Almost no fragmentation, but multiple charges possible
• For positive ionization, major peak is M+H peak (most common); or for multiply charged compounds,
peak is [M+n]n+ where n = charge on ion
• For negative ionization, M-1 peak is common
• Adduct formation also is possible e.g. [M+Na]+
Liquid
in
Nebulizing gas
High voltage
+
+
+
+
+
M+H+](https://image.slidesharecdn.com/msbasics-231213162415-42896333/85/MS-basics-16-320.jpg)
MS basics
- 2. Announcements • Homework 2.2 Additional Problems - Due • Quiz Today (after Announcements) • 4/18 Lecture – will go back to rest of NMR (interpretation examples and instrumentation), then continue on Mass Spectrometry • Tokmakoff Lecture (4/20/17, 10-11, Lobby Suite, University Union, 1st Floor) • Today’s Lecture • Mass Spectrometry (Harris Ch. 21)
- 3. Mass Spectrometry Introduction • One of the Major Branches of Analytical Chemistry (along with spectroscopy, chromatography, and electrochemistry) • Roles of Mass Spectrometry • Qualitative analysis (less useful than NMR for true unknowns, but can be applied to very small samples) • Quantitative analysis (often used for quantitative analysis)
- 4. Mass Spectrometry Introduction Main information given in MS analysis: • molecular weight • number of specific elements (based on isotope peaks) • molecular formula (with high resolution MS) • reproducible fragment patterns (to get clues about functional groups and/or arrangement of components or to confirm compound identity)
- 5. Mass Spectrometry Main Components to Instruments 1. Ionization Source (must produce ions in gas phase) 2. Separation of Ions (Mass Filter) 3. Detection of Ions Note: most common instruments run in order 1 → 2 → 3, but additional fragmentation to generate different ions can occur after step 2 (1 → 2 → 1 → 2 → 3) MS very common as chromatographic detector
- 6. Mass Spectrometry Overview of Component Types • Ionization Types Type Phase Fragmentation Inductively Coupled Plasma (ICP) Liquid feed Gives elements Electron Impact (EI) gas lots Chemical Ionization (CI) gas some Electrospray (ESI) liquid very little Atmospheric Pressure Chemical Ionization (APCI) liquid some Matrix Assisted Laser Desorption Ionization (MALDI) solid some Desorption Electrospray Ionization (DESI) Portable Very little
- 7. Mass Spectrometry Overview of Component Types • Separation Types (Ion Filters/Mass Analyzers) Type Speed Basis Cost Magnetic Sector slow Acceleration in magnetic field moderate Double Focusing slow Magnetic plus electric field high Quadrupole fast Passage through ac electric field moderate Ion trap fast Orbit in quadrupole moderate Time-of-Flight very fast Time to travel through tube moderate Newer High Resolution varies Various, usually involving orbits high In addition, there are 2-dimenional MS options (sometimes called MS/MS or tandem MS), such as quadrupole – quadrupole, or MSn
- 8. Mass Spectrometry Overview of Component Types • Detectors Type Internal Amplifications? Uses Faraday Cup No Isotope Ratio MS Electron Multiplier Yes Fairly Common Microchannel plate Yes Higher end instruments Induction No Used in FT-ICR
- 9. Mass Spectrometry-GC Ionization (Gas Phase) Electron Ionization (EI), also known as Electron Impact A heated tungsten filament is used to produce electrons which “bombard” the analyte molecules, causing ionization through loss of an electron (for positive polarity MS): M +e- → M(*)+ • + 2e- • However, M+ typically has extra energy and undergoes further decomposition/fragmentation: M*+ → X+ + Y• (where X and Y are fragments) • We only see the charged fragments, but often if M*+ → X+ + Y •,it also may form X • + Y+
- 11. Mass Spectrometry Ion Source • EI Fragmentation Example: + charged fragment m/z = 43 (16 + 15 + 12) charged fragment m/z = 77 (5*13 + 12) O CH3 C + O CH3 C + C + + O CH3 C note: stable fragments (77 ion), tend not to greatly fragment
- 12. Mass Spectrometry Ion Source • Fragmentation Example 2: CH2Cl2 + CH2Cl+ + Cl • CH2Cl• + Cl+ mass peak at 49 (and 51) - observed mass peak at 35 (and 37) - not observed Presence of ions also depends on their stability (Cl is electronegative so hard to form cation)
- 13. Mass Spectrometry Ion Source Gas Phase Sources (cont.) • Chemical Ionization (CI) • “Softer” ionization technique • Results in less fragmentation • Possible in both negative and positive ion modes • Initial ionization like EI but in “reagent” gas • methane (+) mode shown below: CH4 + e- → CH4 + • + 2e- CH4 + CH4 + • → CH5 + + CH3 • CH5 + + M → MH+ + CH4 major ion typically is M mass + 1
- 14. Mass Spectrometry-GC Ionization EI spectrum results in more lower m/z fragments
- 15. Mass Spectrometry-HPLC Ionization Electrospray Ionization The HPLC eluent flows through a nebulizer, ions are in this solution (or that are formed in the solution) are guided into the MS region Another soft ionization technique-produces molecular (or M+H+/M-H-) ions Also may produce “doubly” or greater charged ions (i.e., multiply- charged species), which is more common for larger molecules (i.e., proteins and other biomolecules)
- 16. Mass Spectrometry Ion Source • Liquid Samples • Electrospray Ionization (ESI) • Liquid is nebulized with sheath gas • Nebulizer tip is at high voltage (+ or –), producing charged droplets • As droplets evaporate, charge is concentrated until ions are expelled • Efficient charging of polar/ionic compounds, including very large compounds • Almost no fragmentation, but multiple charges possible • For positive ionization, major peak is M+H peak (most common); or for multiply charged compounds, peak is [M+n]n+ where n = charge on ion • For negative ionization, M-1 peak is common • Adduct formation also is possible e.g. [M+Na]+ Liquid in Nebulizing gas High voltage + + + + + M+H+
- 19. Mass Spectrometry Ion Source • ESI Example: • glycodendrimer core (courtesy of Grace Paragas) • C30H60N14O12 (sorry, no structure) • Mass = 808.451 or for M+H+: 809.459 First Hitachi “high resolution” ESI-MS sample – Full Spectrum M+H+ peak mass error = -2.6 ppm (+/- 5 ppm needed) Internal Standard: used for calibration
- 20. Mass Spectrometry Ion Source • ESI Example: • So if ESI results in no fragmentation, what are the other peaks? M+H and isotope peaks M+41 = M+Na+H2O M+2H/2 peak = (808+2)/2 = 405 13C isotope peaks observed at +1/2 amu 425 peak = (M+H+Na+H2O)/2
- 21. Mass Spectrometry Ion Source • DESI – Desorption Electrospray Ionization • Use of Electrospray focused onto sample to produce ionization • Commonly used for remote MS analysis of untreated surface • Tip with electrospray is pointed toward sample with vacuum pick up line near by • Collisions of electrospray charged drops end up charging surface molecules • Resulting ions are picked up to mass spectrometer entrance Sample plate (electrically conductive) sample Mass Analyzer Electrospray source vacuum line to mass analyzer M+
- 22. Mass Spectrometry Ion Source • Ion Sources • For Liquids (continued) • Atmospheric Pressure Chemical Ionization • Liquid is sprayed as in ESI, but charging is from a corona needle nearby - More restricted to smaller sized molecules • For Solids • Matrix Assisted Laser Desorption Ionization • Ionization from Laser • Samples normally doped with compound that absorbs light strongly (to cause intense heating/ionization)
- 23. Mass Spectrometry Ion Source • For Elemental Analysis • Inductively Coupled Plasma • Produces ions as well as atoms used in ICP-AES • Most sensitive method of elemental analysis skimmer cone to mass analyzer
- 24. Mass Spectrometry Ionization Questions 1. Which ionization method can be achieved on solid samples (without changing phase)? MALDI 2. If one is using GC-MS and concerned about detecting the “parent” ion of a compound that can fragment easily, which ionization method should be used? CHEMICAL IONIZATION (CI) 3. For a large, polar non-volatile molecule being separated by HPLC, which ionization method should be used? ELECTROSPRAY IONIZATION (ESI, good for larger ionizable analytes) 4. When analyzing a large isolated peptide by ESI-MS, multiple peaks are observed. What is a possible cause for this? MULTIPLE PROTONS ATTACHED TO PEPTIDE 5. What ionization method should be used to analyze for lead (Pb) in a sample? INDUCTIVELY COUPLED PLASMA-Want to reduce analyte/sample to elemental components, single form