Why Mass Spectrometry? Introduction to GC/MS A powerful analytical technique used to: 1.Identify unknown compounds 2. Quantify known materials down to trace levels 3. Elucidate the structure of molecules (mol. wt.) Presented with permission of H.M. McNair. Also, needs only micrograms of samples 26, Pierce Biotechnology, Inc. Mass Spectra of Carbon Dioxide Relative Abundance 1 9 8 7 6 5 4 3 2 1 C+ 12 O+ 16 CO+ 28 CO 2 + [M+] 44 5 1 15 2 25 3 35 4 45 5 A mass spectrum is a graph of ion abundance vs. mass-to-charge ratio Why Gas Chromatography? Capable of resolving complex volatile mixtures (>45 peaks for coffee aroma) Uses small sample size (micrograms to picograms) Fast analyses usually a matter of minutes Good quantitation 1-2% RSD normally
Capabilities of GC/MS Combines advantages of both techniques: 1. High resolving power of GC 2. Positive identification of MS Requires only small samples (micrograms to picograms) Quantitative trace analysis ~ ppm, ppb Mass range of 1-8 Limitations of GC/MS Expensive ($4 K to $25 K) Bench-top systems $4 K to $8 K Complex to operate (improving) Must know GC, MS, vacuum and interpretation Only volatile samples GC 76 torr Heated Output Combining GC and MS Types of interfaces: Requires Interface jet separator packed columns MS High Vacuum Heated Input direct interface capillary columns Jet Separator Column Flow Interfaces Capillary Direct Interface GC Oven Fused Silica Column To Vacuum Heated Transfer Line To MS source (separate Vacuum) MS Source Separate Vacuum
Ionization Sources Electron Impact Chemical Ionization Negative Chemical Ionization Repeller GC Column Electron Impact Source Source Filament Accelerator Ionization Current Focusing Electrodes MASS ANALYZER Eluent Ion Beam Collector (Anode) Entire source is at high vacuum Chemical Ionization Produces ions by gentle process of proton transfer from an ionized reagent gas (e.g., CH 5 +) Ionization is made by collision of sample molecule with reagent ions Leads to simpler and more easily interpretable mass spectra Comparison of CI and EI Spectra Relative Abundance Relative Abundance 1 8 6 4 2 1 8 6 4 2 C. I. E. I. ORTAL MW 24 5 1 15 2 25 M/E 156 14 24 5 1 15 2 25 M/E
Mass Analyzers Single-Focusing Magnetic Sector Double-Focusing Electrostatic/Magnetic Quadrupole Ion Trap Time of Flight Single Focusing Magnetic Sector Mass Analyzer Acceleration Region Ion Source Magnetic Field Detector Design of a Double-Focusing Separator Quadrupole Mass Analyzer Electrostatic Field Slit 2 Magnetic Field Detector Slit 1 Slit 3 Source From Ion Source To Detector Quadrupole
Ion Trap Mass Analyzer End Cap RF Ring Electrode Filament Electron Current Ions Source Mass Analyzer Time of Flight (T.O.F.) M+ M+ M+ M+ M+ M+ M+ M+ Collector Electron Multiplier End Cap Different Masses Give Different Flight Times Ion Detectors Discrete-Dynode Electron Multiplier Continuous-Dynode Electron Multiplier Ion Sensitive Photoplates Channel Electron-Multiplier Array Discrete-Dynode Electron Multipliers Ion Beam* A B C A. Conversion dynode (emits electrons) B. Secondary electrons (ions to electrons) C. Second dynode emits electrons (electrons to electrons) Amplifier *Dynode is a metal plate (copper-beryllium)
Electron Multiplier Continuous-Dynode Version Semi-Conductive Surface Summary MS System The Mass Spectrometer Single Ion Semi-Conductive Surface Cascade of Ions 1 2 3 4 Inlet System Ion Source Mass Analyzer Detector GC or LC or SFC Produces Ions M M + Separates (m/e) Discrete Dynode Detector E. I. C. I. Magnetic Sector T.O.F. Quadrupole I.T.D. Continuous Dynode GC Analysis of a Hydrocarbon Mixture Response.5 sec 1 2 3.5 1. 1.5 Time (min.) 4 1. sec 5 1. 2-Methylbutane 2. Pentane 3. Cyclopentane 4. Nexane 5. Benzene Fragmentation of Hexane in a Mass Spectrometer CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 Hexane e [CH CH CH CH CH CH ] + 3 2 2 2 2 3 Molecular ion, M + (m/z. = 86) CH CH CH CH CH + CH CH CH CH + CH CH CH + CH CH + 3 2 2 2 2 3 2 2 2 3 2 2 3 2 m/z.: 71 57 43 29 Relative abundance (%): 1 1 (base peak) 75 4
Mass Spectrum of Hexane C 6 H 14 ; Mol. Wt. = 86 Mass Spectrometry of Isomers N-Butane Relative Abundance % 1 8 6 4 2 29 43 57 M + = 86 abundance 12 1 8 6 4 2 H 3 C CH 2 CH 2 15 CH 3 29 27 39 41 43 58 2 4 6 8 m/z 1 12 14 1 2 3 4 5 6 m/e Iso-Butane Why High Vacuum Systems? abundance 12 1 8 6 4 CH 3 CH 3 C H CH 3 41 43 Low pressure is essential for production of free electrons and ions Permit free passage of the electrons, sample ions and fragments Mean free path of ions is increased high vacuum 2 27 29 39 58 To minimize ion-molecule collisions 1 2 3 4 5 6 m/e
Total Ion Chromatogram Universal Response Scans a specified mass range (say 4 to 24 AMU) Not as sensitive as SIM Slower acquisition rates can compromise with quantitation Permits retrieving mass spectra Best for qualitative analysis Selective Ion Monitoring Monitors several discrete m/z values (target compounds) Increases S/N ratio Permits greater acquisition rate Best for quantitative analysis TIC vs. SIM For More Information SIM: m/z = 18 Water Watson, J.T. Introduction to Mass Spectrometry. Raven Press, New York, 1985. (ISBN -88167-81-2) TIC: 1-2 AMU Water Carbon Dioxide Phenol McLafferty, F.W. Interpretation of Mass Spectra ed. 3. University Science Books, Mill Valley, California, 198. (ISBN -93572-4-) March, R.E. and Hughes, J.R. Quadrupole Storage Mass Spectrometry. Wiley-Interscience Publication, New York, 1989. (ISBN 471-85794-7)