2Gas Chromatography-Mass Spectrometry An Hybrid technique which couples the powerful separation potential of gas chromatography with the specific characterization ability of mass spectroscopy.Development of GC (1941) by Martin and Synge
3What is Gas Chromatography? The father of modern gas chromatography is Nobel Prize winner John Porter Martin, who also developed the first liquid-gas chromatograph. (1950)
5The compounds to be separated are considered solutes Separation of molecules by distribution between a stationary phase and a mobile phase.A stationary phase (absorbent) phase the material on which the separation takes place. can be solid, gel, or liquid. Also called matrix, resin, or beads.The mobile phase is the solvent transports the sample and it is usually a liquid, but may also be a gas. Also called eluting bufferThe compounds to be separated are considered solutes
9Depending on its nature 1.Packed column: columns are available in a packed manner2.Open tubular or Capillary column or Golay columnLong capillary tubing M in lengthUniform & narrow d.m of cmMade up of stainless steel & form of a coilDisadvantage: more sample cannot loaded
103.SCOT columns (Support coated open tubular column Improved version of Golay / Capillary columns, have small sample capacityMade by depositing a micron size porous layer of supporting material on the inner wall of the capillary columnThen coated with a thin film of liquid phase
11Column Types Capillary Columns Length: 10m to 100m Diameter: 180um, 250um, 320um & 530um I.dPacked ColumnsLength: <2mDiameter: 1/8” & ¼” OD
26DETECTORS Heart of the apparatus The requirements of an ideal detector are-Applicability to wide range of samplesRapidityHigh sensitivityLinearityResponse should be unaffected by temperature, flow rate…Non destructiveSimple & inexpensive
44Schematic of a Gas Chromatography-Mass Spectrometry (GC-MS) Instrument
45- CI (chemical ionization) - FAB (fast atom bombardment) Ionization techniques- EI (electron impact)- CI (chemical ionization)- FAB (fast atom bombardment)- ESI (electrospray ionization)- MALDI (matrix assisted laser desorption ionization)- APCI (atmospheric pressure chemical ionization)
46Electron Impact Ioniser In an electron-impact mass spectrometer (EI-MS), a molecule is vaporized and ionized by bombardment with a beam of high-energy electrons.The energy of the electrons is ~ 1600 kcal (or 70eV).The electron beam ionizes the molecule by causing it to eject an electron.
49Quadrupole Mass Ion Filter Quadrupoles are four precisely parallel rods with a direct current (DC) voltage and a superimposed radio-frequency (RF) potential. The field on the quadrupoles determines which ions are allowed to reach the detector. Quadrupoles thus function as a mass filter.
52Definition of Terms Molecular ion The ion obtained by the loss of an electron from the moleculeBase peakThe most intense peak in the MS, assigned 100% intensityRadical cation+ve charged species with an odd number of electronsFragment ionsLighter cations formed by the decomposition of the molecular ion. Isotope abundance PeakThese often correspond to stable carbocations.“A” Element—an element that is monoisotopic“A + 1” an element with an isotope that is 1 amu above that of the most abundant isotope
54a, molecular ion and base peak(C10H+8, 100%); b, 13C isotope peak; Mass to charge ratiom/z50100150Relative abundance (%)20406080128102776451abcMass spectrum of CO2. Note that the molecular ion appears at m/z = 44 (C = 12, O = 16). Frag-ment ions appear at m/z values of 28, 16, and 12. These correspond to CO+, O+, and C+, resp-ectively.The mass spectrum of naphthalene with electron impact ionization by 70 eV electrons.a, molecular ion and base peak(C10H+8, 100%);b, 13C isotope peak;c, fragment ion peaks.
55Ionization process in mass spectrometry. TheoryIn mass spectrometry, a small sample of a chemical compound is vaporized, bombarded with high energy electrons to ionize the sample, and the ions produced are detected based on the charge to mass ratio of the ions.Ionization process in mass spectrometry.
64Electron ionization (70 eV) mass spectra of molecular ion region of benzene (C6H6) and biphenyl (C12H10).Intensity of M+1 relative to molecular ion for CnHm :Intensity = n × 1.08% m × 0.012%Contribution from 13C Contribution from 2H
65Applications of GC-MSGC-MS is increasingly used for detection of illegal narcotics marijuana, cocaine, opioids Clinicians oxycodone and oxymorphonePiperazines are not detectable by typical immunoassay testing, but they may be detectable via GC-MSSports anti-doping analysis
66Applications of GC-MSpossible to test a newborn for over 100 genetic metabolic disorders by a urine test at birth based on GC-MSFoods and beverages contain numerous aromatic compounds (identification)Environmental monitoring and cleanupGC-MS is becoming the tool of choice for tracking organic pollutants in the environment
67LimitationOnly compounds with vapor pressures exceeding about 10–10 torr can be analyzed by gas chromatography-mass spectrometry (GC-MS).Determining positional substitution on aromatic rings is often difficult.Certain isomeric compounds cannot be distinguished by mass spectrometry (for example, naphthalene versus azulene), but they can often be separated chromatographically.