GC-MS
Gas 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
Gas Chromatography-Mass Spectrometry
What 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)
Chromatography
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 buffer
The compounds to be separated are considered solutes
GC Step by Step • Carrier Gas• Injector• Column
– Capillary– Stationary Phase
• Detectors– Mass Spectrometer
Depending on its nature
1.Packed column: columns are available in a packed manner
2.Open tubular or Capillary column or Golay column
Long capillary tubing 30-90 M in length Uniform & narrow d.m of 0.025 - 0.075 cm Made up of stainless steel & form of a coil Disadvantage: more sample cannot loaded
3.SCOT columns (Support coated open tubular column
Improved version of Golay / Capillary columns, have small sample capacity
Made by depositing a micron size porous layer of supporting material on the inner wall of the capillary column
Then coated with a thin film of liquid phase
Column Types
Packed Columns
Length: <2m
Diameter: 1/8” & ¼” OD
Capillary Columns
Length: 10m to 100m
Diameter: 180um, 250um, 320um & 530um I.d
Columns
• Packed
• Capillary
Cross section
Phases
Broadening can be minimized with:1- decreasing particle size2- decreasing column diameter
What Does GC/MS Data Look Like?Reviewing of Mass Spectra
*
m/z 78
*
Abun
danc
e (S
igna
l)
Retention Time ------>
mass/charge ------>
6.77 min.
1,1-dichloropropene/carbon tetrachloride
Example Chromatogram (Capillary)
1 2 3 4 5
Minutes
-87
0
250
500
750
mVolts0.
541
0.75
4
1.11
3
1.47
4
2.03
8
2.85
3
3.21
0
4.46
3
5.32
0
5.56
2
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A = TCD Results
25/07/1993 18:35
WI:2 WI:4
Time
Inject Point
Detector Response
DETECTORSHeart of the apparatus The requirements of an ideal detector are- Applicability to wide range of samples Rapidity High sensitivity Linearity Response should be unaffected by temperature,
flow rate… Non destructive Simple & inexpensive
Flame Ionization Detector
Thermal Conductivity Detector
Electron Capture Detector
Application of GC
•
1
2
4
6
3
5
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8
10
9
11
12
13
14
15
16 17 18
1. -HCH2. -HCH3. -HCH4. Heptachlor5. -HCH6. Aldrin7. Heptachlor epoxide8. Endosulfan I9. 4,4’-DDE
10. Dieldrin11. Endrin12. 4,4’-DDD13. Endosulfan II14. 4,4’-DDT15. Endrin aldehyde16. Endosulfan sulfate17. Methoxychlor18. Endrin ketone
Analysis of Halogenated Pesticides
2ppb in Water
Schematic of a Gas Chromatography-Mass Spectrometry (GC-MS) Instrument
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)
Electron 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.
Quadrupole 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.
Molecular ion The ion obtained by the loss of an electron from the molecule
Base peak The most intense peak in the MS, assigned 100% intensity
Radical cation +ve charged species with an odd number of electrons
Fragment ions Lighter cations formed by the decomposition of the molecular ion.
Isotope abundance Peak
These 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
Definition of Terms
Mass Spectrum of Methane (CH4)
Mass 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.
Mass to charge ratio m/z
50 100 150
Rel
ativ
e a
bund
ance
(%
)
0
20
40
60
80
100
128
1027764
51
a
b
c
bb
c
b
c
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.
Mass Spectrometry
Theory
In 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.
Fragments Produced by Benzamide
Interpretation of Mass Spectra(1)
Interpretation of Mass Spectra
Isotope Patterns 2,Chloropropane
1,Bromopropane
Electron 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
• GC-MS is increasingly used for detection of illegal narcotics marijuana, cocaine, opioids Clinicians oxycodone and oxymorphone
• Piperazines are not detectable by typical immunoassay testing, but they may be detectable via GC-MS
• Sports anti-doping analysis
Applications of GC-MS
possible to test a newborn for over 100 genetic metabolic disorders by a urine test at birth based on GC-MS
Foods 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
Applications of GC-MS
Limitation
• Only 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.