How to choose the correct GC column – Part 2

My last post looked at how to use published boiling points of compounds to help select a GC column. In this post, I will be discussing the role of compound polarity when selecting a GC column.

To briefly summarize my column selection process (discussed in How to choose the correct GC column – Part 1 )

 

My four categories of narrowing down column selection:

1.  I look at the published boiling points of the compounds of interest.

2.  I consider the polarity of the compounds (and polarity of the sample/standard solvent, if applicable).

3.  I consider compound concentrations, column capacities, and detector used.

4.  I look at any other details which may help narrow down the choice to just one, or a handful, of column catalog numbers.

I use the four categories listed above to help narrow down possible column choices into one of the four categories listed below.

 

My four categories of GC columns:

A. Gas analysis columns (carbon & zeolite molecular sieves, porous polymers, silica gel, etc.).

B. Volatile columns (Rtx-VMS, Rxi-624Silms, Rtx-502.2, etc.).

C. Semi-volatile columns (Rxi-5Silms, Rxi-35Silms, Rxi-17Silms, etc.).

D. High-temperature columns (MXT-Biodiesel, MXT-1HT, MXT-2887, etc.).

For those unfamiliar with chemical (molecular) polarity, in very generic terms, think of straight-chain hydrocarbons (with their symmetrical C-H bonds and equal sharing of electrons) at one end of the scale (non-polar), and alcohols/water (with their unsymmetrical O-H bonds and unequal sharing of electrons) at the other end (polar).

Chemical polarity

Structure of (non-polar) methane (from Wikipedia)

Structure of (polar) methanol (from Wikipedia)

You can also think of polarity another way; remember the statement “like dissolves like”? If so, the solvent used to prepare your standards may be all you need to know since, for solubility and stability, compounds are typically prepared in a solvent of similar polarity. For example, this is why straight-chain hydrocarbons (non-polar) will be prepared in a non-polar solvent (like hexane, toluene or carbon disulfide).

I prefer to use tables for polarity information on many compounds and solvents.

Properties of organic solvents

Physical properties of common solvents

So now that you see there are resources available, how do you use this information to help narrow down column selection? Generally speaking, choosing a column with a phase of similar polarity as the compounds (& solvent) will provide the most compound retention and have proper wettability ( Wetting ). Without proper retention, compounds with similar chemical structures may not separate, and without proper wettability, chromatographic issues like split peaks may occur.

While we are on the topic of solvents for samples/standards, to minimize the possibility of chromatographic issues like split or tailing peaks, I suggest using (whenever possible) GC-friendly solvents like acetone, methylene chloride and hexane and to avoid problematic solvents like chloroform, acetonitrile and water.

Above I provided information to help you determine the polarity of compounds (and solvents). Below I’ll provide examples of different polarity GC columns.

Gas analysis columns

Non-Polar/Slightly Polar: Rt-Q-Bond, Rt-QS-Bond, Alumina/KCl, HayeSep Q & D, Chromosorb T Mid-Polarity: Rt-S-Bond, HayeSep C, R & S, Alumina/Na2SO4 & MAPD Polar: Rt-U-Bond, HayeSep B, N & T

Volatile columns

Non-Polar/Slightly Polar: Thick-film Rtx-1 & Rtx-5, Rtx-Volatile Amine Mid-Polarity: Rtx-VMS, Rtx-502.2, Rtx-624, Thick-film Rtx-1301 & Rtx-200 Polar: Stabilwax (including DA & DB) and Rtx-Wax

Semi-Volatile columns

Non-Polar/Slightly Polar: Thin-film Rxi-1ms, Rxi-5ms, Rxi-5Silms, Rxi-XLB Mid-Polarity: Rxi-17 (& 17Silms), Rxi-35Silms, Rtx-50, Thin-film Rtx-200 Polar: NA

High Temperature columns

Non-Polar/Slightly Polar: Rxi-1HT, Rxi-5HT, MXT-Biodiesel, MXT-1HT, MXT-1 SimDist Mid-Polarity: MXT-65TG Polar: NA

So using this post with the previous post, I am hoping that you can see how one can quickly narrow down column selection. My next post will focus on how one can use column capacity (along with detector selection) to help narrow down GC column choices even further. Keep watching for Part 3.