How to choose the correct GC column – Part 4

In Parts 1-3, I explained the process I use to help select the correct GC column. Hopefully it has provided a foundation of how someone may go about choosing an appropriate column for a particular compound, or group of compounds. For the fourth and final post in my series, I would like to present a few situations which, while not unique, don’t really fit into the normal selection process.

How to choose the correct GC column – Part 1

How to choose the correct GC column – Part 2

How to choose the correct GC column – Part 3

 

Gas sample loops pictured above.

The first situation I would like to discuss is how to select the proper column internal diameter (ID) when the sample introduction involves a gas sample loop. Please note that I will only be discussing pressure-loop systems which use multi-port gas sampling valves, like the one shown below. These systems are very common for natural gas analysis and greenhouse gas analysis.

 

6-Port gas sampling valve pictured above.

The rule-of-thumb I use goes something like this (and yes, I know units are mismatched, but please hear me out); the volume of a sample loop (in mL) should not exceed 1/10th of the carrier gas flow rate (in mL/min) to keep the compound peaks reasonably sharp. For example, if using a 1mL sample loop, in order to keep the compound peaks from becoming excessively broad, the carrier gas flow rate should be at least 10mL/min. Since this is a common flow rate for a 1/16” OD (1mm ID) micropacked column, this is the minimum ID column which should be used.

 

Let’s look at one more example. Let’s say you are using a 0.25mm ID column with a carrier gas flow rate of 1mL/min; then the size of the sample loop should not exceed 0.1mL (or 100µL).

 

Please keep in mind a few things concerning this rule-of-thumb:

1.  This is only for gas sample loops, not gas-tight syringes (which can inject a sample much quicker than a sample loop).

2.  This only applies when the compounds are not able to condense within the column or instrument (and become refocused into a narrow band). If the compounds are able to condense at the head of the column, larger gas volumes can be injected without getting broader peaks.

3.  Do not confuse sample loop volume with sample concentration. The rule above only references sample loop volume, not the compound concentrations in the sample.

4.  Generally speaking, the smaller the sample loop volume, the sharper the peaks.

 

The next situation involves samples which only contain either acidic (& neutral) or basic (& neutral) compounds. Fortunately, there are columns which have either an acidified or basic liquid phase or packing which produces better peak shapes (less tailing) which may result in more compound sensitivity.  To minimize column damage, please remember to keep the pH of what you are injecting between 5 and 9 for most columns, between 5 and 7 for acidified columns and between 7 and 9 for basic columns.  

 

Common acidified capillary columns include the Stabilwax-DA (similar to the liquid phase FFAP for packed columns).

Other acidified packings include any solid support listed with a small % of H3PO4 and/or CarboBlack B-DA.

 

Common basic capillary columns include the Stabilwax-DB, Rtx-5 Amine, Rtx-35 Amine, and Rtx-Volatile Amine.

Basic packings include any support listed with a small % of KOH or HayeSep B.

 

The last situation I will discuss involves very reactive samples (including highly acidic or basic samples); those that are so reactive that they will damage the phase of capillary and packed columns.  For situations when this happens, there are two columns which are more robust than virtually all other columns produced.  They are both packed columns:

5% Krytox on CarboBlack B in Nickel or Hastelloy tubing.  

Chromosorb T in PTFE or Nickel or Hastelloy tubing.

If you simply cannot use one of these packed columns and need to use a capillary column, then try a thick film Rtx-1, as it is the most robust of WCOT (wall-coated open tubular) columns.

 

I hope you have all enjoyed reading my posts on how to select the correct GC column. While I couldn’t cover every conceivable situation that each of you will encounter, I am hoping that I provided you the tools necessary to help you develop your own selection process.