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

c:\star\examples\level4.run File:

Channel:

Last recalc:

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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

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1

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3

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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.