Gas Chromatography&

High Performance Liquid Chromatography

Invention of Chromatography

Mikhail TswettRussian Botanist

(1872-1919)

Mikhail Tswett invented chromatography in 1901 during his research on plant pigments.

He used the technique to separate various plant pigments such as chlorophylls, xanthophylls and carotenoids.

Original Chromatography Experiment

Later

Start: A glass column is filled with powdered limestone (CaCO3).

End: A series of colored bands isseen to form, corresponding to the different pigments in the original plant extract. These bands were later determined to be chlorophylls, xanthophylls and carotenoids.

An EtOH extractof leaf pigments is applied to the top of the column.

EtOH is used to flush the pigments down the column.

Comparing Chromatography to the Flow of a River...

Base

Water flowLight leaf

Heavy stone

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Interaction Between Solutes, Stationary Phase, and Mobile Phase

• Differences in the interactions between the solutes and stationary and mobile phases enable separation.

Solute

Stationary phase Mobile phase

Degree of adsorption, solubility, ionicity, etc.

How Does Chromatography Work?In all chromatographic separations, the sample is transported in a mobile phase. The mobile phase can be a gas, a liquid, or a supercritical fluid.The mobile phase is then forced through a stationary phase held in a column or on a solid surface. The stationary phase needs to be something that does not react with the mobile phase or the sample.The sample then has the opportunity to interact with the stationary phase as it moves past it. Samples that interact greatly, then appear to move more slowly. Samples that interact weakly, then appear to move more quickly. Because of this difference in rates, the samples can then be separated into their components.

Chromatography is based on a physical equilibrium that results when a solute is transferred between the mobile and a stationary phase.

A

A

A

A

AA

A

AA

A

A

A

K = distribution coefficient or partition ratio

K C S

C M Where CS is the molar concentration of the solute in the stationary phase and CM is the molar concentration in the mobile phase. Cross Section of Equilibrium in a column.

“A” are adsorbed to the stationary phase.“A” are traveling in the mobile phase.

Elution : always (100%) dilution

What is Chromatography?

sam plein

eluentin

CaCO 3

(adsorption)

colum n

eluantout

detector

chrom atogram(m ass spect. IR

spect. etc)

Chromatography: (Greek = chroma “color” and graphein “writing” ) Tswett named this new technique chromatography based on the fact that it separated the components of a solution by color.

Common Types of ChromatographyTswett’s technique is based on Liquid Chromatography. There are now several common chromatographic methods. These include:

Paper ChromatographyThin Layer Chromatography (TLC)Liquid Chromatography (LC)

High Pressure Liquid Chromatography (HPLC)Ion Chromatography

Gas Chromatography (GC)

Three States of Matter and Chromatography Types

Mobile phase

Gas Liquid Solid

Stationary phase

Gas

Liquid

Solid

GasGaschromatographychromatography

LiquidLiquidchromatographychromatography

Classification based on Mobile Phase

Gas ChromatographyGas Chromatography

Gas - solidGas - solid Gas - liquidGas - liquid

Stationary Phase

Classification based on Mobile Phase

Liquid chromatography (LC)

Column(gravity flow) High performance

(pressure flow)Thin layer

(adsorption)

polar s.p.

Adsorption and Partition Chromatography

for GC & LC for GC

Ion Exchange and Gel Permeation Chromatography

resin-SO3- gel filtration

resin-N(CH3)3+ by size

Paper and Thin Layer Chromatography

Later

The solvent moves up paper by capillary action,carrying mixture components at different rates.

solvent

solvent front

• The retention factor, or Rf, is defined as the distance traveled by the compound divided by the distance traveled by the solvent

For example, if a compound travels 2.1 cm and the solvent front travels 2.8 cm, the Rf is 0.75:

Retention factor, Rf

Gas chromatography is a technique used for separation of volatile

substances, or substances that can be made volatile, from one

another in a gaseous mixture at high temperatures. A sample

containing the materials to be separated is injected into the gas

chromatograph. A mobile phase (carrier gas) moves through a

column that contains a wall coated or granular solid coated

stationary phase. As the carrier gas flows through the column,

the components of the sample come in contact with the

stationary phase. The different components of the sample have

different affinities for the stationary phase, which results in

differential migration of solutes, thus leading to separation.

Flow

As a material travels through the column, it assumes a Gaussian concentration profile as it distributes between the stationary packing phase and the flowing mobile gas or liquid carrier phase.

In a chromatography column, flowing gas or liquid continuously replaces saturated mobile phase and results in movement of A through the column.

Column is packed with particulatestationary phase.

Good for volatile samples (up to about 250 oC)

0.1-1.0 microliter of liquid or 1-10 ml vapor

Can detect <1 ppm with certain detectors

Can be easily automated for injection and data analysis

Gas Chromatography

Components of a Gas Chromatograph

Gas Supply: (usually N2 or He)

Sample Injector: (syringe / septum)

Column: 1/8” or 1/4” x 6-50’ tubing packed with small uniform size, inert support coated with thin film of nonvolatile liquid

Detector: TC - thermal conductivityFID - flame ionization detector

Schematic of a Commercial Gas Chromatograph

A carrier gas should have the following properties:

1. Highly pure (> 99.9%)2. Inert so that no reaction with stationary phase or

instrumental components can take place, especially at high temperatures.

3. A higher density (larger viscosity) carrier gas is preferred.4. Compatible with the detector since some detectors require

the use of a specific carrier gas.5. A cheap and available carrier gas is an advantage.

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InjectorsSeptum type injectors are the most common. These are composed of

a glass tube where vaporization of the sample takes place. The

sample is introduced into the injector through a self-sealing

silicone rubber septum. The carrier gas flows through the injector

carrying vaporized solutes. The temperature of the injector should

be adjusted so that flash vaporization of all solutes occurs. If the

temperature of the injector is not high enough (at least 50 degrees

above highest boiling component), band broadening will take

place.

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Column Configurations and Ovens

The column in chromatography is undoubtedly the heart of the

technique. A column can either be a packed or open tubular.

Traditionally, packed columns were most common but fast

developments in open tubular techniques and reported

advantages in terms of efficiency and speed may make open

tubular columns the best choice in the near future. Packed

columns are relatively short (~2meters) while open tubular

columns may be as long as 30-100 meters.

Column Ovens• Column temperature is an important variable that

must be controlled to a few tenths of a degree for precise work. Thus, the column is ordinarily housed in a thermostated oven. The optimum column temperature depends upon the boiling point of the sample and the degree of separation required.

• Roughly, a temperature equal to or slightly above the average boiling point of a sample results in a reasonable elution time (2 to 30 min). For samples with a broad boiling range, it is often desirable to employ temperature programming, whereby the column temperature is increased either continuously or in steps as the separation proceeds.

26

Detection SystemsSeveral detectors are available for use in GC. Each

detector has its own characteristics and features as well as drawbacks. Properties of an ideal detector include:

1. High sensitivity2. Minimum drift3. Wide dynamic range4. Operational temperatures up to 400oC5. Fast response time6. Same response factor for all solutes7. Good reliability 8. Nondestructive9. Responds to all solutes (universal)

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a. Thermal Conductivity Detector (TCD)This is a nondestructive detector which

is used for the separation and collection of solutes to further perform some other experiments on each purely separated component. The heart of the detector is a heated filament which is cooled by helium carrier gas. Any solute passes across the filament will not cool it as much as helium does because helium has the highest thermal conductivity. This results in an increase in the temperature of the filament which is related to concentration. The detector is simple, nondestructive, and universal but is not very sensitive and is flow rate sensitive.

Heated wire

TCD characteristics include:

1. Rugged

2. Wide dynamic range (105)

3. Nondestructive

4. Insensitive (10-8 g/s)

5. Flow rate sensitive

29

Flame Ionization Detector (FID)This is one of the most

sensitive and reliable destructive detectors. Separate two gas cylinders, one for fuel and the other for O2 or air are used in the ignition of the flame of the FID. The fuel is usually hydrogen gas. The flow rate of air and hydrogen should be carefully adjusted in order to successfully ignite the flame.

30

Characteristics of FID• Rugged• Sensitive (10-13 g/s)• Wide dynamic range (107)• Signal depends on number of carbon atoms in

organic analytes which is referred to as mass sensitive rather than concentration sensitive

• Weakly sensitive to carbonyl, amine, alcohol, amine groups

• Not sensitive to non-combustibles – H2O, CO2, SO2, NOx

• Destructive

31

Electron Capture Detector (ECD)This detector exhibits high intensity for halogen

containing compounds and thus has found wide applications in the detection of pesticides and polychlorinated biphenyls. The mechanism of sensing relies on the fact that electronegative atoms, like halogens, will capture electrons from a emitter (usually 63Ni). In absence of halogenated compounds, a high current signal will be recorded due to high ionization of the carrier gas, which is N2, while in presence of halogenated compounds the signal will decrease due to lower nitrogen ionization.

32

Characteristics of ECD

Simple and reliableSensitive (10-15 g/s) to electronegative groups

(halogens) Largely non-destructive Insensitive to amines, alcohols and

hydrocarbons Limited dynamic range (102)Mass sensitive detector

How do we describe a chromatogram?

1) Chromatogram :A graph showing the detectors response as a function of

elution time : band’s shapes, position, resolution.

2) For individual band :a) Retention time (tr) :

The time needed after injection for an individual solute to reach detector.

b) An ideal chromatographic peak Gaussian shape. w½ = 2.35σ, w = 4σ

How do we describe a chromatogram?

Theoretical plates (N): (from distillation)the more plates on a column, the moreequilibration steps, and the better the separation. Number of plates on column : N = 5.55(tr/w½)2

Plate height : H = L/NThe smaller plate height narrower peaks better separation

How do we describe a chromatogram?

Why do bands spread ?

1) Why broadening?a) diffusionb) slow equilibration of solute between the

m.p and s.p.c) irregular flow paths.

Why do bands spread ?2) Longitudinal

diffusion : the faster the flow

the less a band spends in column.

the less time for diffusion.

broadeningu1

3) solute requires time to equilibrate between phases.

(s.p.m.p.) with temp. broadening u

Can’t equilibrate rapidly enough.

Why do bands spread ?

m.p.

s.p.

Substances that vaporize below 300°C can be measured

quantitatively.

In assuring the quality of products in the chemical industry

Measuring toxic substances in soil, air or water

Gas Chromatography is used extensively in forensic science.

Since the samples have to be volatile, human breathe,

blood, saliva and other secretions containing large amounts of

organic volatiles can be easily analyzed using GC.

In food industry

Applications of Gas Chromatography

Advantages of Gas Chromatography

• Requires only very small samples with little preparation

• Good at separating complex mixtures into components

• Results are rapidly obtained (1 to 100 minutes)

• Very high precision

• Only instrument with the sensitivity to detect volatile organic mixtures of low concentrations

• Equipment is not very complex (sophisticated oven)

From Liquid Chromatography to High Performance Liquid Chromatography

• Higher degree of separation! Refinement of packing material (3 to 10 µm)

• Reduction of analysis time! Delivery of eluent by pump Demand for special equipment that can withstand high pressures

The arrival of high performance liquid chromatography!

HPLC Separation Modes

• Adsorption (liquid-solid) chromatography• Partition (liquid-liquid) chromatography

– Normal phase partition chromatography– Reversed phase partition chromatography

• Ion exchange chromatography• Size exclusion chromatography

Pump

Sample injection unit(injector)

Column

Column oven(thermostatic column

chamber)

Detector

Eluent (mobile phase)

Drain

Data processorDegasser

Flow Channel Diagram for High Performance Liquid Chromatograph

Advantages of High Performance Liquid Chromatography

• High separation capacity, enabling the batch analysis of multiple components

• Superior quantitative capability and reproducibility• Moderate analytical conditions

– Unlike GC, the sample does not need to be vaporized.

• Generally high sensitivity• Low sample consumption• Easy preparative separation and purification of

samples

• Biogenic substances– Sugars, lipids, nucleic

acids, amino acids, proteins, peptides, steroids, amines, etc.

• Medical products– Drugs, antibiotics, etc.

• Food products– Vitamins, food

additives, sugars, organic acids, amino acids, etc.

• Environmental samples– Inorganic ions– Hazardous organic

substances, etc.

Applications of High Performance Liquid Chromatography

• Organic industrial products– Synthetic polymers,

additives, surfactants, etc.