A brief description of several common solid supports for Restek GC packed columns

Selecting a GC packed column for a specific analysis can be daunting because there are so many variations possible.   For example, there are dozens of liquid phases.  In addition, there are multiple solid supports which may be coated with one or more liquid phases.  Similarly, there are packings which are not coated with any liquid phase.  As if all of those choices were not enough, you also will need to select from several different types of tubing (material, internal diameter, and deactivation (if required)), its configuration (how the tubing is bent), end plugs, end voids, and end fittings (which need to be ordered separately).

In this post, I will only focus on the common solid supports available from Restek.  Therefore, any packings which are not commonly used as solid supports will not be discussed here.  These uncoated packings include molecular sieves (5A, 13X and ShinCarbon), porous polymers (Hayesep and Porapak), and alumina (aluminum oxide).   Although (in theory) one could coat these particles with a liquid phase, these packings are almost always left uncoated.

Before you read any further, I am going to ask that you take a few minutes to read this post (which I published several years ago) so that you will understand the terminology which I will be using.  Restek - Packed Column information for the beginner

The two sentences below are from this previous post.

• All packed columns contain particles, as mentioned earlier. Particles may be uncoated or coated (with a liquid phase). When the particles are uncoated, they are usually referred to simply as the “packing”.
• When coated, these particles are referred to as a “solid support”. In other words, the particle is the support for the liquid phase.

The solid supports which will be discussed in this post are diatomaceous earth (DiatoSorb W, Silcoport W, and Chromosorb P), graphitized carbon black (CarboBlack B and CarboBlack C), and amorphous silica (Res-Sil B and Res-Sil C).  Although some of these materials can be used uncoated, most often they are coated with a liquid phase (or multiple liquid phases) to provide specific target compound separations.  In addition, graphitized carbon blacks are sometimes coated with a base/alkali (like KOH) or acid (H₃PO₄) to improve target compound peak shapes.

DiatoSorb W & Silcoport W

DiatoSorb W was developed by Restek to replace Chromosorb W.  You can read more about DiatoSorb W here.  Restek - A brief introduction to DiatoSorb-W

Chromosorb W and DiatoSorb W are two very popular solid supports.    They are flux-calcined diatomites with surface areas of approximately 1m²/gm.  The particles are considered “slightly friable”, which means you need to handle the material and packed/micropacked columns with care as they are somewhat fragile compared to other packings (like Chromosorb P).   Rough handling may produce particle fines which will lead to poor chromatography.

Maximum liquid phase loading capacity differs depending upon the liquid phase.  The approximate maximum loading range is 15% to 20%.

There are several different DiatoSorb W packings offered by Restek.

AW = Acid Washed*.  USP S1A.

HP = High Performance**.  USP S1A.

NAW = Non-Acid Washed***.  USP S1NS.

Silcoport W is manufactured from Diatosorb W.  The particles are subjected to a proprietary cleaning process, followed by a proprietary deactivation process.  This packing is considered USP S1A.

 

Since these solid supports are intended to be used coated with a liquid phase, there are no published temperature limits.  Instead, the temperature limits of the liquid phase(s) and/or tubing (or end plugs) should be used to determine the operating temperature limits of your specific packed column.  The same is true for the Chromosorb P solid supports (described below).  Although maximum temperature limits are provided below for the CarboBlack and Res-Sil solid supports, in the majority of situations, the temperature limits (both minimum and maximum) of a packed column are determined by the liquid phase(s) and/or the tubing (or end plugs).

 

Chromosorb P

Chromosorb P is crushed firebrick.  The firebrick is prepared by molding diatomaceous earth into bricks.  These bricks are either calcined or baked at high temperatures, then crushed and screened.

There are several different Chromosorb P packings offered by Restek.

AW = Acid Washed*.  USP S1C.

AW/DMDCS = Acid Washed and Deactivated**.  USP S1C.

NAW = Non-Acid Washed***.  USP S1D.

 

As you may have guessed, Chromosorb P is a very sturdy solid support.  The approximate range for the particle’s average surface area is 4 to 8m²/gm.  Maximum liquid phase loading capacity differs depending upon the liquid phase, but has an approximate range of 20% to 30%.

 

* Acid washing helps to remove mineral impurities from the surface of the particle.  Removing the surface metals/impurities helps to minimize catalytic surface activity.  Deionized water rinsing brings the particles back to neutrality.

**Particles are acid washed and deactivated.

***Particles are untreated.

 

CarboBlack B & C

CarboBlack B & C are graphitized carbons which are produced by heating natural gas, or a liquid petroleum product (like mineral oil), in the absence of oxygen.  Surface area is approximately 100m²/gm for CarboBlack B and 12m²/gm for CarboBlack C.  The maximum temperature of the uncoated/untreated particles is 500°C.

Because the particle’s surface is typically not as inert as other solid supports, or appropriate for polar compounds (the particle’s surface is very non-polar), this solid support is often deactivated with phosphoric acid when the target compounds are acidic or potassium hydroxide when the target compounds are basic.  It is also common to coat the particles with a small % of a polyethylene glycol or other non-silicone liquid phase to improve peak shape of not only polar compounds (such as alcohols) but also compounds such as fluorocarbons.  Because of its higher surface area, CarboBlack B has a maximum loading capacity of up to 10% for non-silicone liquid phases while the lower surface area of CarboBlack C has a maximum loading capacity of up to 1% for a non-silicone phase.

USP S7 = CarboBlack C.

USP S11 = CarboBlack B (80/120 mesh) coated with 3% Rt-1500 (or similar liquid phase).

USP S12 = CarboBlack B.

 

Res-Sil B & Res-Sil C

Res-Sil supports were developed by Restek to replace several Porasil (and Unibeads S) supports.  These supports are spherical porous silica beads.  Res-Sil B is similar to Porasil B (and Unibeads 3S).  Res-Sil C is similar to Porasil C.  The surface area of Res-Sil B has a range of approximately 150 to 300m²/gm while Res-Sil C is approximately 50 to 100m²/gm.  The average pore diameters range from 100 to 200 Angstroms (10 to 20 nanometers) for Res-Sil B and 200 to 400 Angstroms (20 to 40 nanometers) for Res-Sil C.  The maximum temperature of the uncoated particles is 300°C for both Res-Sil B and Res-Sil C.

Of the solid supports listed in this post, Res-Sil B & C are the most likely to be used uncoated.  However, these Res-Sil supports are often coated, with the most popular used for light hydrocarbon analysis (see link below).

Restek - Res-Sil C Bonded GC Packings for Analyses of Light Hydrocarbons

 

Conclusion

You may be asking yourself; how would you know which solid support to select for a given analysis?  Just like with liquid phase selection, you should search for a packed column method, or application chromatogram, to decide which column(s) may be worth trying.  In addition, you should search for any GC column, including capillary columns, capable of a similar analysis.  In some cases, this liquid phase is available for both capillary and packed columns; in some cases, it is not.  You will need to check.

One notable difference is that for a packed column, the liquid phase is on the surface of the particles instead of on the inside of the capillary tubing.  Because capillary columns have many more theoretical plates than most packed columns, you may be able to use a 15-meter x 0.53mmID x 1µmdf (specific liquid phase thickness) capillary column to replace the typical 6-foot (2-meter) x 1/8-inch packed column for an analysis (as long as you do not overload the capillary column).

One last note, the availability of certain packed columns seems to be decreasing as both liquid phases and solid supports become discontinued by the manufacturer/supplier.  When that happens, you should consider switching to a suitable capillary column.  But before you do, you may want to review this blog post to help you select your next column.  Restek - What to do when your packed-micropacked column is no longer available

I hope you have found this post informative.  Good luck with your analysis.