GAS CHROMATOGRAPHY

PRESENTD BY- GUIDED BY- Mr.Mayur R. Wagh Prof. Mr. M. T.

Mohite (M.Pharm First Sem.) -Pharmaceutics

Department OF PharmaceuticsDr. D. Y. Patil College Of Pharmacy, Akurdi, Pune-44

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Contents:- Introduction

Principle

Instrumentation

Detectors

Applications

References

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Introduction:-

Modern gas chromatography (GC) was invented by Martin and James in 1952.

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Principle of GC- The organic compounds are separated due to

differences in their partitioning behavior between the mobile gas phase and the stationary phase in the column.

The sample solution injected into the instrument enters a gas stream which transports the sample into a separation tube known as the "column.“

The detector measures the quantity of the components that exit the column.

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Mobile Phase in GC-In gas chromatography, the mobile phase (or "moving phase") is a carrier gas, usually an inert gas.such as helium, argon or an unreactive gas such as nitrogen.

Stationary Phase in GC-Here, the principle of “like dissolves like” applies, where “like” refers to the polarities of the solute and the immobilized liquid. e.g. Dimethyl Polysiloxane, Polyethylene Glycol.

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• Instrumentation-

1.Gas source (carrier gas)2. Injector or sample application system (sample inlet)3. Chromatographic column (with oven for temperature

control)4. Detector & recorder

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Flow meter

Gas supply

Pressure regulator

Flowcontroller

Septum Detector

Oven

Column

InjectorGC Chart

Recorder

1.Carrier gas-supply- Carrier gases serve to produce wind through the

column to move our solutes forward.  Carrier gases, which must be chemically inert. E.g. Helium, nitrogen, and hydrogen.

2.Sample Injection System-The most common method of sample injection

involves the use of microsyringe to inject a liquid or gaseous sample.

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3.ColumnsColumns vary in length from less than 2 m to 50 m or more.They are constructed of stainless steel, glass, fused silica.Two general types of columns are-1.Packed column-.

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Packed columns are made of a glass or a metal tubing which is densely packed with a solid support like diatomaceous earth.  Due to the difficulty of packing the tubing uniformly, these types of columns have a larger diameter than open tubular columns and have a limited range of length.  As a result, packed columns can only achieve about 50% of the efficiency of a comparable WCOT column.  Furthermore, the diatomaceous earth packing is deactivated over time due to the semi-permanent adsorption of impurities within the column.

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2.Open tubuler column- When the stationary phase is uniformly distributed on the interior

surface of column it is called an open tubular (capillary) column. Open tubular columns are longer, smaller in diameter, and more efficient than packed columns. Open tubular columns have less flow resistance which allows for them to be longer and have a lot of theoretical plates. Capillary columns are between 3 and 100 meters long and form a helical shape. The most common stationary phases used for open tubular columns are polysiloxanes.

Types of Open tubular column- 1.WCOT

2.SCOT

3.PLOT

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1.Wall-coated open tubular column-:In 1957, Golay demonstrated the superiority of wall-coated open

tubular (WCOT) columns (a 100-fold or higher increase in efficiency) relative to packed columns; yet, it took a quarter century before this efficiency was realized in practice. In WCOT columns, the wall is directly coated with the stationary-phase layer at a film thickness of 0.05–3 μm.

2. Support-Coated Open Tubular Column-:These columns contain an adsorbed layer of a very fine solid

support (such as Celite) coated with the liquid phase. SCOT columns can hold more liquid phase and have a higher sample capacity than the thin films of early wall-coated open tubular (WCOT) columns had. With the introduction of cross-linking techniques, the use of stable, thick films in WCOT columns has become possible, thereby making SCOT columns redundant.

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3.Porous Layer Open Tubular Column-:Porous layer open tubular (PLOT) columns, first suggested by Golay in the late 1950s, have been successfully developed and commercialized. PLOT columns contain a porous layer of a solid adsorbent such as alumina, molecular sieves, or Porapak. PLOT columns are well suited for the analysis of light, fixed gases, and other volatile compounds

4.Column Ovens- The optimum column temperature depends upon the boiling point of the sample and the degree of separation.

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5.Detectors- Characteristics of the Ideal Detector:

1. Adequate sensitivity

2. Good stability and reproducibility.

3. Nondestructive of sample

4. High reliability and ease of use.

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Types Of Detectors- 13

1.Flame Ionization Detectors (FID) The flame ionization detector is the most widely used detectors.

ADV.- Responds to all organic compounds

The effluent from the column is mixed with hydrogen and air and then ignited

electricallyOrganic compounds burning in the flame

produce ions & electrons which can conduct electricity through the flame

Ions attracts towards electrode and ionisation of sample takes place.

The current resulting from the pyrolysis of any organic compounds is amplified by amplifier& the output fed to a data to

recorder.

Working-

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2.Thermal Conductivity Detectors(TCD)

TCD is based upon changes in the thermal

conductivity of the gas stream brought about by the presence of analyte molecules.

The heated element may be a fine platinum, gold, or tungsten wire or a semiconducting thermist.

The device contains an electrically heated source whose temperature at constant electrical power depends on the thermal conductivity of the surrounding gas.

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The thermal conductivities of helium and hydrogen (commonly used carrier gases for TCD) are roughly 6~10 times greater than those of most organic compounds. Thus, even small amounts of organic species cause relatively large decreases in the thermal conductivity of the column effluent, which results in a marked rise in the temperature of the detector.Advantages:• Simplicity, large linear dynamic range, nondestructive.Disadvantages: •Low sensitivity (precludes their use with WCOT columns with small amounts of sample).

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3.Electron-Capture Detectors(ECD)

Radioactive decay-based detector. Selective for compounds containing

electronegative atoms, such as halogens, peroxides, quinones, and nitro groups.

The sample effluent from a column is passed over a radioactive β emitter, usually 63Ni. An electron from the emitter causes ionization of the carrier gas (often N2) and the production of a burst of electrons.

In the absence of organic species, a constant standing current between a pair of electrode results from this ionization process. The current decreases significantly in the presence of organic molecules containing electron negative functional groups that tend to capture electrons.

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Advantages: useful for environmental testing detection of chlorinated pesticides or herbicides. aromatic carcinogens, organometallic compounds selective for halogen- (I, Br, Cl, F), nitro-, and sulfur-

containing compounds.

Disadvantages: It is insensitive to functional groups such as amines,

alcohols, and hydrocarbons.

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4.Nitrogen-phosphorus Detectors 19

The nitrogen–phosphorus detector (NPD) is also known as thermionic specific detector.

In which thermal energy is used to ionize an analyte.

In this method, nitrogen and phosphorus can be selectively detected with a sensitivity that is 104 times greater than that for carbon.

Working-: A concentration of hydrogen gas is used such that it is

just below the minimum required for ignition. A rubidium or cesium bead, which is mounted over the

nozzle, ignites the hydrogen (by acting catalytically), and forms a cold plasma.

Excitation of the alkali metal results in ejection of electrons, which in turn are detected as a current flow between an anode and cathode in the chamber.

As nitrogen or phosphorus analytes exit the column, they cause a reduction in the work function of the metal bead, resulting in an increase in current.

ADV.- High sensitivity, specific towards nitrogen and phosphorus.

Disadv.- Performance deteriorates with time.

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Applications of GC-1.Qualitative Analysis- Gas chromatograms are widely used as criteria of

purity for organic compounds. Contaminants, if present, are revealed by the appearance of additional peaks.

The technique is also useful for evaluating the effectiveness of purification procedures.

Gas chromatography provides an excellent means of confirming the presence or absence of a suspected compound in a mixture.

2.Quantitative Analysis- -Calibration of standards -The internal standard method

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THANK YOU…

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