Why don't we use Capillary columns in HPLC..?

Hai readers....... how do you do...?

Yesterday, One of my friend asked me a question.......

i.e...

Why don't we use Capillary columns in HPLC.....?

Yes.... come we will discuss about this...........

First of all I would like to tell 'what is Capillary column'



A capillary column is made up of mainly two components
                                                                                         1) Tubing
                                                                                         2) Stationary phase



1) Tubing:

Fused silica and stainless steel are the primary tubing materials.

Fused silica is a synthetic quartz of high purity. A protective coating is applied to the outer surface with polyimide being the most common coating material. The polyimide coating is responsible for the brownish color of fused silica capillary columns. The color of the polyimide coating often varies between columns. 

Column color has no effect on the chromatographic performance of the column. 

Polyimide coated tubing often darkens after prolonged exposure to higher temperatures. The upper temperature limit of standard polyimide coated fused silica tubing is 360°C. High temperature polyimide coated tubing has an upper limit of 400°C.

The inner surface of fused silica tubing is chemically treated to minimize interactions of the sample with the tubing. The reagents and process used depend on the type of stationary phase being coated onto the tubing. A silylation process is used for most columns. Silanol groups (Si-OH) on the tubing surface are reacted with a silane type of reagent. Typically, a methyl or phenyl-methyl silyl surface is created for most columns.

Stainless steel capillary columns are used for applications requiring very high column temperatures. Stainless steel tubing is more robust than fused silica tubing, thus it is also used in situations where the possibility of tubing breakage needs to be virtually eliminated. Stainless steel interacts with many compounds,  It is treated to minimize these undesirable interactions. The inner surface is either chemically treated or it is lined with a thin layer of fused silica. When properly made, the inertness of stainless steel capillary columns rivals those made with fused silica tubing.
     
2) Stationary Phase:                                                                           

Poly siloxanes

Polysiloxanes are the most common stationary phases. 
They are available in the greatest variety and are the most stable, robust and versatile.




Polyethylene glycols

Polyethylene glycols (PEG) are widely used as stationary phases. 
Stationary phases with "wax" or "FFAP" in their name are some type of polyethylene glycol.



These two are most commonly used stationary phases in capillary columns. There are other stationary phases usedbased on requirement like Gas-solid stationary phases, Cross-linked stationary phases etc....

Now... coming to the HPLC packed columns..., 

I think all knows about HPLC packed columns...

HPLC columns are usually packed with pellicular, or porous particles. Pellicular particles are made from polymer, or glass beads. Pellicular particles are surrounded by a thin uniform layer of silica, polystyrene-divinyl-benzene synthetic resin, alumina, or other type of ion-exchange resin. The diameter of the pellicular beads is between 30 and 40 µm. Porous particles are more commonly used and have diameters between 3 to 10 µm. Porous particles are made up silica, polystyrene-divinyl-benzene synthetic resin, alumina, or other type of ion-exchange resin. Silica is the most common type of porous particle packing material.

Partition HPLC uses liquid bonded phase columns, where the liquid stationary phase is chemically bonded to the packing material. The packing material is usually hydrolyzed silica which reacts with the bond-phase coating. Common bond phase coatings are siloxanes. The relative structure of the siloxane is shown in Figure 2.

R group attached to siloxane	Chromatography method application
Alkyl	Reverse phase
Fluoroalkyl	Reverse phase
Cyano	Normal and reverse phase
Amide	Reverse phase
Amino	Normal and reverse phase
dimethylamine	Weak anion exchanger
Quaternary Amine	Strong anion exchanger
Sulfonic Acid	Strong cation exchanger
Carboxylic Acid	Weak cation exchanger
Diol	reverse phase
Phenyl	Reverse phase
Carbamate	Reverse Phase

Now coming to the question.....,

Why Don't we use Capillary columns in HPLC...?

Capillary are more useful in GC whareas Packed columns are generally used in HPLC while some Packed columns are also specifically used in GC based on application.

Simple Answer is  the difference in the separation process in the two techniques used in HPLC and GC.

Here are some reasons.....

Nature of mobile phase – the first reason is nature of mobile phase. HPLC uses a liquid as a carrier of sample through the column whereas in GC a gas stream serves to carry the sample. Liquids have higher viscosity than gases and therefore encounter greater resistance during passage through the column.  Therefore HPLC requires shorter column lengths.

Sample volatility – the sample injected into the HPLC is a liquid whereas in GC it can be either a liquid or a gas. However, before entering the GC column: liquids get converted to vapours due to high temperature in the injector block. A gas encounters less resistance in the GC column which permits use of longer column lengths.

Sample stability – samples analysed by HPLC are generally thermally labile so they are in liquid phase in the HPLC column at room temperatures. On the other hand samples entering the GC column are gases having lower molecular weights and boiling points. Such compounds are easily vaporized and remain as gases during passage through the column.

Further reduction in column length particularly in new UHPLC applications have resulted in accelerated analysis with improved sensitivity. Future trends in analytical applications are bound to reduce HPLC column lengths and analysis time from several minutes to a few seconds.

Finally please note that.......

Science is developing day by day, So now a days Capillary packed columns with shorter lengths are using even in HPLC called "Capillary HPLC". 

I hope my Long explanation gives you the sweet and short answer..........

My next post will be on "How HPLC columns will be packed..?"

Thanks

V.Suresh