Vapour Phase Chromatography

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Vapour Phase Chromatography

INTRODUCTION

Vapour phase chromatography (VPC) is an advanced technique similar to gas chromatography. But it differs from the gas chromatography as both the stationary phase and mobile phase are gases in gas chromatography.

In conventional VPC, there is a stationary liquid/solid phase (generally high boiling liquid coated upon an inert granulated support) and a mobile gas phase, and the relative different interaction of the components of the mixture with these two phases result in desired separation. The stationary phase is liquid which is converted to vapour by increasing the temperature gradient moves in opposite direction that is to the gaseous phase. The main difference between the gas chromatography and the VPC is that both the phases are mobile where in the gas chromatography the stationary phase is immobile and the mobile phase is mobile.

THEORY

The most distinctive element of this method is that both phases are mobile and move in opposite directions through a temperature gradient. In this method, the solid phase used is the normal chromatographic materials such as alumina, silica or active carbons. This is further coated with the volatile liquids such as chloroform, hexane and acetone. The solid is coated with the volatile liquid which is mobile after converting into vapour by increasing the temperature. The mobile phase is used by the inert gases such as hydrogen, helium and nitrogen gas.

The relative different interaction of the components of the mixture with these two phases results in the desired separation.

INSTRUMENTATION

vapour phase chromatography

Flow chart of the VPC instrument

The essential components of VPC instrument are the following:

1. Carrier gas
2. Sample injector
3. The column
4. The detector
5. The recorder

1. Carrier gas: The carrier gas is mainly used as the mobile phase to carry the sample into the column. The most widely used carrier gases in the gas chromatography are the following:
2. Hydrogen: which have more advantages.
3. Helium: this is expensive but used most commonly because of it thermal conductivity, inertness and low density.
4. Nitrogen gas: it is inexpensive but has reduced sensitivity.
5. The ideal requirements of the mobile phase or carrier gas are the following:
1. It should be dry and pure.
2. It should be compatible with the type of the detector used.
3. It should be free from impurities.
4. It should be regulated.
5. It should be non-toxic.
6. It should not be expensive.
6. Among the all carrier gases, hydrogen and nitrogen fulfil the all above requirements. Hence these two are mostly employed as carrier gases. A gas reservoir is used to hold the carrier gas which is connected with the pressure regulators to control the flow of the gas. Commonly pressure gas cylinder is used in which the carrier gas is filled in a compressed form.
7. The injection port: This is mainly used to introduce the sample into the continuous flow of a carrier gas. The commonly used inlet types are the following:
9. S/SL (split/split less) injector: This contains the syringe which contains the septum. It introduces the sample into a small heating chamber. The heat chamber is used to volatilise the sample and sample matrix. The carrier gas forcibly pushes the portion or the entire sample into the column. The split mode injector is mainly used and the carrier gas mixture in the injection chamber is introduced through the split vent. These types of split injections are mainly used to handle the analytes with the high concentrations. The split less injection is used for analysis of the low analyte concentration.
1. On-column inlet: This is same as the split less injector, only difference is that the sample is introduced with out heating.
2. PTV injector: This injector is first introduced by Vogt. The PTV inlet functions same as the split/split less inlet. These are mainly used to separate the light analytes from the heavy matrix.
3. Gas source inlet or gas switching valve: This is mainly based on the sample introduced by the six-port valve. The carrier gas is flowed continuously to avoid the interference. The switching of the column allows the contents of the sample loop which are then inserted into the carrier gas stream. The samples are injected through the PTV inlet at low temperatures. For large volume of the samples, the following steps are required:
• Injection elimination
• Solvent elimination
• Sample is introduced into the column
• Chromatographic separation
4. P/T (purge-and-trap) system: The volatile chemicals are purged from the matrix by the formation of the inert gas bubble through an aqueous sample. These purged volatiles are trapped on the adsorbent column which is commonly known as the trap or concentrator at appropriate temperatures. Then this trap is heated and is introduced into the carrier gas which is subjected onto the column. This method is mainly used for the microlevel analysis.
10. Column: The chromatography column is constructed of metal, glass and/or ceramic materials. In general, stainless steel is the material of choice.
1. Inertness
2. Wear resistance
3. Strength
4. Thermal properties
11. The columns are of the following types:
1. Open tubular columns: The column's internal diameter is 25–60 m. These are made up of liquid stationary phase coated on the inner walls of the glass or fused silica column. These are of lengths 10–100 m and the diameter is of 0.2–0.5 mm. These are mainly of two types:
2. - Support-coated open tubular columns: These support-coated open tubular columns are made up of long capillaries which are coated with the support materials such as silica or alumina. This support coating is coated with the very thin film of the volatile liquid. These columns are of more sample capacity.
3. - Wall-coated open tubular columns: These are made up stainless steel of the long capillary tubes. The inner wall of the capillary tube is coated with the stationary phase liquid in the form of a thin film. These offer more resistance to the carrier gas flow.
5. 
7. Packing of the columns
9. Glass columns: These are less expensive than other columns. These are composed of the small amount of metallic ions to increase the retention of the polar components.
10. Fused silica columns: These are composed of high purity glass with SiO2. These columns are more expensive than other columns but the advantages are easily handled and purity is more.
11. Micro-packed columns: These are mainly used in the capillary column instruments. This is composed of column packed in the column. Thus the outer space of the column is packed with one supporting material and the inner space of the column is packed with another supporting material.
12. The ideal column diameter is 0.2-6 in. The shape of the columns is oval and rectangular. The length of the column effects the efficiency of the column. This is insulated by the insulating materials such as the asbestos, fibreglass, clays and mineral salts. In some cases, the insulating materials used are epoxies, phenolics and silicone.
13. The mobile phase is introduced through the column by the conveyer. The mostly employed conveyer is the auger type screw conveyer. This consists of a porous tube containing the substrate which moves with the column or porous cellular substrate is a rod shaped for the conveying.
14. The cross-linking of the any material with the cotton, glass or polymer helps in the construction of the web. These are reinforced into the matrix liquid or beams of materials having high tensile moduli. The web is impregnated with the volatile liquids such as poly ethers, poly esters, poly glycols, silicones, aliphatic hydro carbons and poly amides.
12. Detector: The most commonly used detectors in the VPC are the following:
1. Thermal conductivity detector: This detector is a heated wire by means of the electrical source and positioned in the gas stream at the end of the column. This wire is cooled by means of the flow of the carrier gas.

3. 

   vapour phase chromatography
5. Katharometer
7. Flame ionisation detector: It is a more sensitive detector. Here a small quantity of the eluent from the column is fed to hydrogen or normal fame. The change in the conductivity is converted to voltage which is determined by connecting to the potentiometer.
9. 
11. Flame ionisation detector
13. The following are the ideal requirements of the detectors used in the VPC:
• It should have high sensitivity.
• It should have high stability.
• It should have high reproducibility.
• It should be feasible with the wide temperature range.
• It should be easy to handle.
• It should not cause any decomposition of the sample.
13. Recorders: Two devices are used to record the VPC traces/areas under peaks:
1. Integrating recorders
2. Computer program
14. This recorder is used to interpret and draw the data from the detectors as the peaks. Then the retention time and the area under curve are calculated.

PARAMETERS CHECKED FOR THE VPC

• Appropriate selection of the column.
• Flow rate of the gas should be regulated by the pressure regulator.
• Sensitivity should be high.
• Base line should be properly drawn.
• Recorder speed should be regulated.

PROCEDURE

The VPC column is placed in the thermal gradient which contains the two reservoirs through which the carrier gas flows in the fractional proportions. The temperature gradient is composed of the heat exchangers which are maintained at higher temperatures and is placed in the insulated column. The column is insulated by using the insulating materials such as asbestos or fibreglass. Appropriate thermal gradient can be maintained by controlling the heat exchangers and the column temperature. The temperature gradient slope depends on the distance between the heat exchangers and the temperatures maintained.

A pair of collection ducts are placed in the entrance and the exit. The entrance is generally called as the cold end and the exit is said to be hot end. The ducts are combined with that of the high volume condenser and the flow regulator.

The carrier gas is flowed through the column in the fractional portions to carry the volatile compounds and the less volatile compounds are condensed out side of the main stream of the column. The liquid or solid phases pass from the entrance that is from the cold end to the exit that is hot end and from the hot end to the cold end.

The sample mixture is introduced as a liquid or hot gas at the cool end of the column and is transported by the liquid or solid phase towards the hot end. The temperatures are increased gradually and converted into vapour. The vapour is flows in one direction and the mobile phase consists of the inert gases such as hydrogen, helium or argon gas which flows in the opposite direction. The separation of the mixture is completed from the bands of the individual components.

These bands are mainly formed because of the affinities of the compounds to flow with the mobile phase. The bands are well defined and dispersed along breadth and length of the column. The components which show greater affinity towards the gas phase are located near to the column entrance. The compounds with lesser affinity towards the gas phase are located far to the column entrance.

FACTORS EFFECTING ELUTION TIME AND RESOLUTION POWER

The following are the factors affecting the retention time and the resolution power:

• The retention time is inversely proportional to the inert gas flow rate or the carrier gas flow rate. Hence, the flow rate is regulated by the flow regulator.
• The column length is directly proportional to the elution time. Hence, the column length is insulated by the insulating materials such as asbestos and the fibreglass.
• The column temperature and the heat exchangers temperature are directly proportional to the elution time. This is overcome by controlling the temperature by the thermal gradient.
• The resolution power is inversely proportional to the column temperature because the sample is voltalised and is allowed to flow through the column. If the column temperature is not maintained properly, then the sample mixture is not efficiently separated.
• The resolution power is maximum when the flow rate of the carrier gas is increased.

APPLICATIONS

• Used in the separation of glycosides.
• Example: Trimethylsilyl derivatives were 2-O-(α-d-mannopyranosyl)-d-erythritol, the β-anomer (p-nitro benzoate, mp 235–238 °C); i-O-(β-d-mannopyranosyl)-erythritol (p-nitro benzoate, mp 239–243 °C) and the α-anomer.
• Used in the detection of tritium labelled compounds is done by VPC.
• Example: In radiotherapy in the cancer treatment.
• Used in the determination of the purity of the compounds.
• Example: Used in the determination of impurities in the crude extracts.
• Used in the determination of steroids.
• Example: Estrone determination.
• Used in the estimation of the active components in the unknown compounds.
• Used in the separation of the mixture of the components into the individual compounds.
• Used in the separation of the volatile compounds.
• Used in the determination of the organo metallics.
• Used in the detection of the organic impurities present in the compounds.
• Example: Aminoacids mixtures into individual aminoacids such as tyrosine, tryptophan.
• Used in the determination of the unknown compound concentrations.
• Used in the determination of the fuel mechanics.
• Used in the analysis of the crude oils.