“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.

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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