Q.4. (b)Discuss about carrier gases, column and solid support stationary phases in GLC
Ans.4. (b) Carrier gas: For selecting a carrier gas following considerations should be taken into account:
(i) It should be inert. In other words it should not react with the sample, stationary phase or contacted hardware.
(ii) It should be suitable for the detector employed and the type of sample analyzed.
(iii) It should be readily available in high purity.
(iv) It should give best column performance consistent with required speed of the analysis.
(v) It should be cheap.
(vi) It should not cause the risk of fire or explosion hazard.
The various carrier gases used in GLC along with their characteristic features are stated below:
Hydrogen (H2): It has a distinctly better thermal conductivity and lower density. Demerits are its reactivity with unsaturated compounds and hazardous explosive nature.
Helium (He): It has an excellent thermal conductivity, low density, inertness and it permits greater flow rates. It is highly expensive.
Nitrogen (N2): It offers reduced sensitivity and is inexpensive, and
Air: it is employed only when the atmospheric O2 is beneficial to the detector separation.
Importantly, the operating efficiency of a chromatograph is directly dependent on the maintenance of a highly constant carrier gas flow rate.\
Columns:
Separation Column
It is also known as the ‘chromatographic column’. In really the heart of a GC is the column duly packed or capillary in which the separation of constituents is materialized. The packed column is usually tubing having an internal diameter of 4.0 mm and made up of stainless steel, copper, cupronickel or glass either bent in U-shape or coiled. Its length varies from 120 cm to 150 m. Three types of analytical columns are generally used in gas chromatography. These are packed, open tubular and support coated open tubular.
Packed column:
These columns are prepared by packing metal or glass tubing’s with granular stationary phase. For GLC the packing is prepared by coating the liquid phase over a size graded inert solid support. The advantages of porous polymers as packing material are:
1. There is no column bleed. Most of the porous polymers are stable up to 250°c and cause no base line drift. It, therefore, allows the use of highly sensitive detector.
2. There is no adsorption of polar compounds such as water, alcohols or acids and they are eluted rapidly as sharp symmetrical peaks.
3. Sample overload recovery is rapid and without tailing. Thus it is quite useful in preparative and trace analysis work.
4. Porous polymer beads are mechanically strong and can be easily packed on columns.
5. Retention data are highly reproducible.
6. Some of the separations provided are unique.
Open Tubular Columns:These columns are also referred to capillary or Golay columns and are made of long capillary tubing (30-90 meters) having uniform and narrow internal diameter (0.025-0.075 cm). They are made of stainless steel, copper, nylon or glass etc. The inside wall of the capillary tubing is coated with the liquid phase in the form of a thin (0.5-1 micron) and uniform film. The carrier gas flow faces least resistance because there is no packing in the column.
Support Coated Open Tubular Columns:
These columns are made by depositing a micron size porous layer of support material on the inside wall of a capillary column and then coating with a thin film of liquid phase. These columns have more sample capacity and an inlet splitter may not be required. SCOT columns are preferred for trace analysis.
Stationary phase:
The interior of GLC column contains an inert solid support coated with a layer of liquid phase. The particles of the packing material are small (80- to 120- mesh) to minimize void volume, while at the same time providing a large surface area for interaction with the solutes. Alternatively, in a column of capillary dimension (<0.75 mm), the liquid phase may be coated directly on the wall. For GLC, the most commonly used solid support material is diatomaceous earth, which is treated with acid and base to remove impurities and then calcined to activate the surface. Non porous supports such as glass micro beads also have been used. The liquid phase is coated uniformly on the surface of the solid support usually at levels of 1% to 5% by weight. For the separation of compounds that are retained only slightly, amounts as high as 40 % have been used. The liquid must be chemically stable, have a low vapour pressure at operating temperatures and have specific solvent properties towards the compounds to be analyzed
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