“Optimism is the faith that leads to achievement, nothing can be done without hope and confidence” — Helen Keller
Most of the time a Gas Chromatographer is fortunate as he has to use a column specified for a particular method but there is a tendency to use the same column even though it is not the best choice for other samples. Apart from the basic column characteristics discussed in previous module stationary phase choice is a critical decision for achieving the right analytical conditions.
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Stationary phases are required for both Gas Solid Chromatography (GSC) and Gas Liquid Chromatography (GLC). Gas Solid Chromatography separates components in the carrier gas stream by selective adsorption on solid stationary phase whereas in GLC the components are bound to the liquid layer adsorbed on the solid support.
GLC Solid Supports
Used for separation of permanent gases and highly volatile organic compounds
Porous polymers for low carbon number organics, acids, amines and water:
- Porapak series (Millipore corp)
- Chromosorb Century series (Johns – Manville)
- Haysep (Hayes Separation)
Chromosorb – P – manufactured from hard firebrick. High load capacity.
Chromosorb – W – flux calcined diatomite.
Immobilized Liquid Stationary Phases
Desirable Features : Low volatility, High decomposition temperature and Chemical inertness.
Stationary phases are covalently bonded or cross linked liquid phases -waxes, rubbers, glasses at room temperature
Non polar Temp limit
Apiezon L <300°C
Silicone SE – 30 < 350°C
Squalene < 150°C
Desirable features of Liquid Stationary Phase
• Liquid phase should not permeate too deeply into the fine pores of the support structure as slow diffusion in and out of pores affects column efficiency
• Support should be deactivated before use as undesirable surface impurities can cause decomposition of the sample or stationary liquid layer
• Small particles of support give higher efficiency as HETP is proportional to particle diameter but particle size reduction increases back pressure.
• Liquid phase should have low volatility and high stability at elevated temperatures otherwise they can contribute to interference in analysis.
Typical Liquid Phases
|
Stationary Phase |
Trade Name |
Max Temp |
Common Applications |
|
Dimethyl Polysiloxane |
OV – 1, SE – 30 |
350oC |
Hydrocarbons, Polynuclear aromatics, PCB’s |
|
Poly(phenyl methyl) siloxane |
OV – 17 |
250oC |
Steroids, Pesticides, Glycols |
|
Poly (Trifluoro propyl dimethyl) siloxane |
OV – 210 |
200oC |
Chlorinated Aromatics, Nitro Aromatics, Alkyl substituted Benzenes |
|
Polyethylene Glycol |
Carbowax 20 M |
250oC |
Free acids, Alcohols, Essential Oils, Glycols |
|
5% Diphenyl – 95% Dimethyl polysiloxane |
DB – 5 |
325oC |
Flavors, environmental samples and aromatic hydrocarbons |
Chiral Stationary Phases
Chairal stationary phases separate optical isomers by selective interactions such as hydrogen bonding metal bond coordination and also in cooperation of modified cyclodextrins. The chiral selector is anchored to a polycyloxane back bone. GC column coated with modified cyclodextriens are applied for enantiomer analysis in flavors, essential oils, terpenoids, pheromones, enzymatic reactions and organo chloride pesticides.
P.S
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