GAS CHROMATOGRAPHY-MASS SPECTROMETRY (GC-

MS)

Presented By,Pradeep s jaiswal.Msc.part-2.semester-4.paper-2.

CONTENTS :•Introduction to GC-MS.•Gas chromatography.•Mass spectrometer.•coupling of GS to MS.•Types of Interfaces.•Ionization sources and techniques.•Mass analyzers.•Limitation of GC-MS.•Application of GC-MS.•References.

Gas ChromatographyIt separates components of sample

InterfaceCombines both techniques by removing pressure incompatibility problem between GC and MS

Mass spectrometryIonise eluted component and separate, identify it according to its mass to charge ratio

INTRODUCTION TO GC-MS

Gas chromatography–mass spectrometry (GC-MS, or alternatively

HPLC-MS) is an ADVANCED ANALYTICAL

INSTRUMNTAL technique that combines the physical

separation capabilities of GAS CHROMATOGRAPHY with the

mass analysis capabilities of MASS SPECTROMETER.

GAS CHROMATOGRAPHY• Gas chromatography leads to

Separation of volatile organic compounds.

• Separation occurs as a result of unique equilibrium established between the solutes and the stationary phase (the GC column).

• An inert carrier gas carries the solutes through the column.

Distribution Coefficient :

Concentration of component A in stationary phase.

Concentration of component A in mobile phase.

Different affinity of any 2 components to stationary phase causes the separation.

Principle of GCIn GC the main principle of separation is partition.

The chromatographic process – partitioning.

MOBILE PHASE (gas or liquid)

Samplein

Sampleout

\STATIONARY PHASE

(solid or heavy liquid coated onto a solid or support system)

Component OF GC61. Carrier Gas.

2. Injector.3. Pneumatic controls.4. Oven.5. Column.6. Detectors:Flame Ionization Detectors (FID)• Electron Capture Detectors (ECD)• Electron impact/chemical ionization (EI/CI)

Mass spectrometry

Mass Spectrometer

Diagram of mass spectromter

Molecular ion peakBase peak

Fragment ions

• BASE PEAK:The most intense peak in the mass spectrum is called the base peak. Base peak is the highest peak it is assigned a relative intensity of 100%.

• MOLECULAR ION PEAK:The ion formed from a molecule by removal of one electron of lowest ionization potential is known as molecular ion.

• FRAGMENT IONS:The ions produced from the molecular ion by cleavage of bonds are called fragment ions.

• METASTABLE IONS:Mass spectrum of molecule shows sharp peaks at m/z integrals. But some show diffuse, broad low intensity peaks at non integral m/z values these are called metastable ions.

MASS SPECTRUM

GC-MS •It’s a Hyphenated Technique.•Invented By James & Martin in 1952.

Gas chromatography

Mass spectrometry GC-MS

Separates mixture of

components into individual

Identifies molecules

based on their mass

A chemical analysis technique

combining two instruments to

provide for powerful separation

& identification.

Coupling of GC to MS

GC Atmospheric

density heated (200-300 ∘C)

Interfaces

MSHigh vacuum

(10-6 torr)heated

The interface b/w the GC&MS is an important role to play in the overall

efficiency of the instrument.

Both system are heated (200 -300 C) both deal with compounds in the ∘vapor state.

Only one problem is that the atmospheric pressure output of the GC must

be reduced to vacuum of

10-5 – 10-6 torr for the MS inlet.

Types of interfaces:

Capillary direct interface

Jet separator(column packed)

Watson-Biemann effusion separator

12

Capillary direct interface:Today most GC-MS systems use capillary columns &

fused silica tubing permits an inert,high efficiency,direct

transfer between the 2 systems.

Flow rates is 5ml/min.

Jet separator (packed column)

•The separator consist of two glass tubes aligned with a

Small distance between them.

•Carrier gas entering from the GC column is pumped

away by a separate vacuumed system.

•The larger sample molecules maintain their momentum

&pass preferentially in to the second capillary.

•Sample enrichment occurs & the initial atmospheric

pressure is reduced.

• It consists of a sintered glass tube .

• The carrier usually Helium, passes preferentially through the

sintered glass tube & the effluent in concentrated by a factor of

up to 100.

• The gas flow rates in the order of 20-60ml/min.

Watson & Biermann effusion separator:

It converts the components of a sample into ions by

bombardment with electrons, ions, molecules.

IONIZER;

CH3OH + 1e CH3OH+ + 2e

molecular ion or radical ion

The gas molecules exiting the GC are bombarded by a high energy

electron beam.

IONIZATION SOURCE

1 • ELECTRON IMPACT

2 • CHEMICAL IONIZATION

3 • NEGATIVE CHEMICAL IONIZATION

IONIZATION TECHNIQUES

• The most common form of ionization is EI.

• Electrons are produced by tungsten filament.

• These electrons accelerated towards the ion source chamber.

• The electrons require an energy equal to the voltage B/W the

filament & ion source chamber.

• A proportion of electron beam will strike the electron trap

producing trap current.

ELECTRON IMPACT (EI)

• In CI a reagent gas methane or ammonia or isobutene are

introduced into the mass spectrometer.• The reagent gas will interact with the electron to produce

radical electrons.

• R + e R+ + 2e

CHEMICAL IONIZATION (CI)

Negative ion chemical ionization(NICI):

In NICI a reagent gas is used & the electrons collide with it so that their

energies are reduced to 10Ev.

Molecules with a high affinity for electrons are able to

capture these low energy thermal electrons.

This is known as NICI but it does not involved in the

formation of a chemical adduct.

Ionization method

electron impact Chemical ionization

Typical analyses Relatively small volatile

Relatively small volatile

Sample introduction GC (or) liquid/solid probe

GC/Liquid /solid probe

Mass range 1-1000 Dolton's 1-1000 Daltons

Method highlights Hard method versatile

provides structure information

Soft method molecular ion peak

(M+H)+

Mass analyzersQuadrupole Ion trap Time of flight

Mass Analyzers

They deflects ions down a curved tubes in a magnetic fields based on

their kinetic energy determined by the mass, charge and velocity. The

magnetic field is scanned to measure different ions.

25

• A set of four rods are arranged parallel to the direction.

• Here a DC current and radio frequency RF is applied to generate

oscillating electrostatic field in between the rods.

• Based on this only m/z is been determined.

Quadrupole

26

TIME OF FLIGHT

13/9/2014 VIGNAN PHARMACY COLLEGE

• The velocities of two ions are created by uniform electromagnetic force applied

to all the ions at same time, causing them to accelerate down a flight tube.

• Lighter ions travels faster and strike the detector first so that the m/z ratio of ions

is detected.

ION TRAP MASS ANALYSER: • The ion trap mass analyser operates by

similar principles where it consists of

circular ring electrode

• Plus two end caps that form a chamber. Here

AC or DC power along RF potential is

applied between the cups and the ring

electrode.

• There the ions entering into the chamber

are trapped by electromagnetic fields and

they oscillates in concentric trajectories.

This process is called resonant ejection.

• All the mass spectrometers now

employ computer control of same

functions and also use a

computerised display and output.

• The amount of data generated

even by a fairly modest mass

spectrometer is very large indeed,

a single run may store data for

upto 100 fragments from each

type of molecule.

DATA HANDLING

•Only Compounds with Vapour Pressure exceeding about 1010 torr

can be analyzed by Gas chromatography –mass spectrometry.

•Certain isomeric compounds cannot be distinguished by mass

spectrometry (EG : naphthalene vs. azulene).

limitation of GC-MS

Environmental monitoring &Clean up• GC-MS is becoming the tool of choice for tracking organic

pollutants in the environment.

Criminal forensics• GC-MS can analyze the particles from a human body in order

to help link a criminal to a crime.

• GC-MS especially useful here as samples often contain very

complex matrices &results used in court.

Law Enforcement GC-MS used for detection of illegal narcotics & may

eventually supplant drug-sniffing dogs.

It’s also commonly used in forensic toxicology to find drugs

&poisons in biological specimens of victims .

Sports antidoping analysis GC-Ms is main tool used in sports anti doping laboratories to

test athletes urine samples for prohibited performance

enhancing drugs.

EG : anabolic steroids.

Food ,Beverage & Perfume Analysis• Food & beverage contain numerous aromatic compounds ,

some naturally present in the raw materials &some forming

during process.

• GC-MS is extensively used for the analysis of these

compounds which include ester, fatty acids , alcohols,

aldehydes, terpenes etc……

Astrochemistry GC-MS 2 were brought to mars by the Viking program.

Venera11&12 pioneer Venus analyzed the atmosphere of

Venus with GC-MS.

The material in the comet 67p will be analyzed by the

rosetla mission with a chiral GC-MS in 2014.

MEDICINE• In born errors of metabolism are now detectable by new born

screening tests, especially the testing using GC-MS .• It can determine compounds in urine even in minor

concentration.

• The measurement of c13-c12 ratio with an isotope ratio mass

spectrometer.

• Fundamental of analytical chemistry, D.A skoog,5th edition.

• Instrumental method of analysis,H.H. willard 6th edition.

References :