Lecture 1: Introduction to Mass Spectrometry & Sample Introduction Techniques CU- Boulder CHEM 5181 Mass Spectrometry & Chromatography Prof. Jose-Luis Jimenez Fall 2006 3 Concept of Mass Spectrometry From Watson Sample Inlet Ion Source Mass Analyzer Detector Recorder 4 Example MS • The mass spectrum shows the mass of the molecule and the masses of pieces from it – Much simpler than other types of spectra 5 History of Mass Spectrometry – Pre-MS • 1886: E. Goldstein discovers anode rays (positive gas ions) in gas discharge • 1897: J.J. Thomson discovers the electron and determines its m/z ratio. Nobel Prize in 1906. • 1898: W. Wien analyzes the anode rays by magnetic deflection, and establishes that they carry a positive charge. Nobel Prize in 1911. • 1909: R.A. Millikan & H. Fletcher determine the elementary unit of charge. 6 History of MS – Early Years • 1912: First Mass Spectrometer (J.J. Thomson) • 1919: Electron ionization and magnetic sector MS (A.J. Dempster) • 1942: First commercial instrument 7 History of Mass Spectrometry – Recent • 1953: Quadrupole and the ion trap (W. Paul and H.S. Steinwedel). Nobel Prize to Paul in 1989. • 1956: First GC-MS • 1968: First commercial quadrupole • 1975: First commercial GC-MS • 1990s: Explosive growth in biological MS, due to ESI & MALDI • 2002: Nobel Prize to Fenn & Tanaka for ESI & MALDI • 2005: Commercialization of Orbitrap MS • More info: “Measuring Mass: From Positive Rays to Proteins”, M. Grayson, Ed., 2002. • http://masspec.scripps.edu/information/history/pdf/nobel2002.pdf 8 Growth of Mass Spectrometry Attendance of the ASMS annual conference: 9 Why is Mass Spectrometry so Succesful? Because of its: A. High Sensitivity – ability to detect very small amounts) B. High Selectivity – Ability to tell molecules apart in a mixture C. High Time Resolution D. Low Cost E. I honestly don’t know 10 Why is Mass Spectrometry so Succesful? II 11 Lectures on MS High Vacuum Sample Inlet Ion Source Mass Analyzer Detector MS1 Today MS2-3 MS4-5 MS5 Recorder Interpretation I 1-6 12 The Need for Vacuum I 1. Why do we need vacuum in a MS instrument? A. B. C. D. E. Ions can only be made under vacuum Ions are lost if not under vacuum Ions can only be mass-analyzed under vacuum All of the above I don’t know 2. How much vacuum is enough for MS? A. B. C. D. E. 10-3 Atm 10-7 Atm 10-10 Atm It depends on the instrument I don’t know 13 The Need for Vacuum II • λ = 0.66 / P – Where λ is in cm and P in pascals – Need λ = 10 to 100 times the ion path to reduce the probability of ion / neutral collision to 10%, or better 1% – Typical ion path: 0.2 m (quad) to 2 m (TOF) – P = 10-4 mbar => l = 0.66 m – P = 10-5 mbar => l = 6.6 m • Note that 1 mbar = 100 Pa; 1 Atm = 1012 mbar 14 The Need for Vacuum Mean Free Path of Gas Molecules vs. Pressure Mean Free Path (cm) 1E+08 1E+06 1E+04 1E+02 1E+00 1E-02 1E-04 1E-06 1E-08 1E-12 1E-10 1E-08 1E-06 1E-04 1E-02 1E+00 1E+02 1E+04 Pressure (mbar) 15 References on Vacuum Technology • We don’t have time to cover vacuum technology but two excellent references are: – J.H. Moore et al., “Building Scientific Apparatus,” Westview Press, 3rd Ed., 2002. ($65) • Chapter on web page (password-protected) – J.F. O’Hanlon, “A Users’ Guide to Vacuum Technology,” Wiley Interscience, 3rd Ed., 2003. ($95) 16 Sample Introduction Techniques • Objective: convert sample into gas-phase molecules – Without loss of vacuum Gas 103 mbar Ion Source 10-6 mbar Liquid Solid • Ions freqently made under vacuum – But trend towards high pressure ion sources (ESI, DESI, DART…) 17 Question • How much gas / liquid sample (e.g. cm3 / min) does a MS need? – What limits the minimum? – What limits the maximum? 18 Allowable Flow Rate into MS: Gas • Gas sample – E.g. direct sampling of atmospheric air, or volatiles from pyrolized sample, or output of GC • Assume 1 cm3 / min at ambient pressure – 107 to 108 cm3 / min at the MS pressure – Pumping speed: 167 to 1670 l/s – Typical range of pumping systems for MS: 50 to 1000 l/s 19 Allowable Flow into MS: Liquid • Liquid sample – E.g. direct analysis of chemical process fluid, or output of LC or CE • Assume H2O, 0.1 cm3 / min – Gas flow = 0.1 / 18 moles * 24,500 cm3 /mol = 136 cm3 / min @ atmospheric pressure – Gas flow into MS = 22,700 to 227,000 l/s • Beyond the range of the pumping systems • Typically the eluent of LC or CE cannot be evaporated into the MS 20 Brainstorming on Sample Introduction • How could we possibly introduce a sample (gas / liquid / solid) into the high vacuum of a mass spectrometer? 21 Types of Sample Introduction Systems 1. Batch inlets – Introduce all components of the sample at once 2. Continuous inlets – Can be used to look at processes as f(t) • • E.g. variability in the atmosphere Also time-resolved output of a chromatograph 22 Batch 1: Heated Reservoir 23 Batch 1: Heated Reservoir - Notes • Used for Liquids or solids • Reservoir is evacuated • Valve to ion source is opened, to check for residues from previous samples • Valve to ion source is closed • Then sample is introduced with microsyringe into septum • Valve is open, sample lasts 15-30 min. – Can heat up to help evaporation – Can heat more to clean up before the next sample • Used for mass calibration compounds such as perfluorokerosene • Advantage: Constant signal for a while • Disadvantage: inefficient -> requires large sample 24 Batch 2: Direct Inlet Probe 25 Batch 2: Direct Inlet - Notes • Used for solids with low vapor pressure • Inlet tip introduced into vacuum through a vacuum lock • Often sample is heated – Sometime micropyrolisis oven, e.g. polymers, bacteria • Advantages: Small distance: efficient, can be used for low vapor pressure • Disadvantages: – Increases risk of venting – Increases risk of contamination (large amounts of sample) – Separation with thermal gradient 26 Batch 2.1: Heated Direct Inlet - Example • Thermogram allows some separation – Much less than in chromatography • Below: work of Paul Ziemann on secondary organic aerosols 27 Continuous 1 (g): Direct Injection 28 Direct Injection Notes • GC: typically 250 um diameter column, 25 m long, 1-2 cm3 / min of carrier gas • Be careful about what you put on MS!! – Particles from column – Outgassing 29 Continuous 2 (l): Particle Beam Interface 30 Particle Beam Notes • Liquid is atomized into droplets (few microns) • Heating to evaporate most of the solvent (down to 100 nm) • Differential focusing of the particles vs. the gas allows concentration of the particles • Differential pumping of solvent – Remember, we can’t put all the flow into the high vacuum due to pumping constraints • Flash vaporization of droplets in the ionization chamber, followed by rapid ionization 31 Continuous 3 (l): Continuous-Flow + FAB 32 CF-FAB Notes • Water with 1% glycerol • Flow ~ 1 µl / min – OK for the pumping system • Water evaporates quickly, glycerol stays and serves as matrix 33 Summary of Sample Introduction Systems Lambert 34 Ion Sources • Neutrals don’t feel electric or magnetic fields • Once molecules are ionized, they immediately feel the forces • Ion sources use electric fields to steer ions into mass spectrometer 35 Magnetic Forces • What is the direction of the magnetic force on this ion (at rest) A. B. C. D. E. Towards left Towards right Upwards Outwards from board There is no force • What is the direction of the magnetic force on this ion (moving) A.Towards left B.Towards right C.Upwards D.Outwards from board E.There is no force r B r B + +r v 36 Example of an EI Ion Source • What will the ion speed vs. position be? From Balzers QMA 410 Manual (Ion source of the Aerosol Mass Spectrometer) 37 An Einzel Lens + r v From Wollnik (web page) 38