GC columns – when one is not enough

Often we get asked for a column recommendation for a complex GC analysis.   This is especially true for gas analysis.  Probably the most common request is for separation of fixed (permanent) gases (like O2, N2, CO, CO2, N2O, etc…) plus separation of larger (higher-boiling point) compounds (light hydrocarbons, moisture, sulfur gases, ammonia, solvents, etc.).  Unfortunately, there is seldom a single column solution for this type of analysis.  So what is the solution?  A two (or more) column set-up with valve switching may be needed to separate all the compounds.

Just to be clear, the two-column set-up/solution mentioned above is not dual-column analysis, where two different columns are used in parallel (most of the time these columns are referred to as the primary analytical column and the secondary, or confirmation column).  Nor is it two different columns connected in series using a union-type fitting.

Instead, it is (generally speaking) two different columns where the outlet of the first column is connected to some type of valve or other switching device, and the inlet of the second column is also connected to this same switching device.  For example, the first column (let’s call it Column #1, or a pre-column, or a stripper column) has the sample injected into it.  The outlet of Column #1 is connected to a switching device.  A second column (let’s call it Column #2) is connected to a different port of this same device.  When the last component/compound of interest elutes from Column #1 onto Column #2, the carrier gas flow is redirected by this switching device so that no other compounds/components are sent onto Column #2, but directed elsewhere (a detector, another column, back-flushed to vent, etc.).

An example of (rotary) valve switching can be seen on the instruction sheet for our D3606 column set (which separates benzene from ethanol in spark ignition fuels).  Column #1 holds up the fuel matrix allowing only the compounds of interest to pass to Column #2.

Note that these aren’t the only types of valves used; there are also other options available for both packed and capillary columns.  Several common terms you may hear include Multiport Valves, Capillary Flow Technology, Deans Switching System, Multidimensional Switching Systems, Multi Column Switching, and Fluidic Switching Devices (among others). 

 

Additional valve applications can be found here.  Valve Applications – Valco Two Position Injectors and Valves

Note that these aren’t the only types of valves used; there are also other options available for both packed and capillary columns.  Several common terms you may hear include Multiport Valves, Capillary Flow Technology, Deans Switching System, Multidimensional Switching Systems, Multi Column Switching, and Fluidic Switching Devices (among others).  To read more about these terms, I suggest reviewing the links below, or typing them into Google.

Valves for Gas Chromatography: Fundamentals

Valves for Gas Chromatography, Part II: Applications

Valves for Gas Chromatography Part III: Fluidic Switching Applications

 

In summary, when there is no single GC column solution available, ask yourself “Are there two (or more) columns which can perform the necessary separations?"  If so, this application may be a candidate for multiple-columns and a switching system.  I suggest contacting your instrument manufacturer to see what hardware and/or software upgrades will be needed and contacting us for your GC column needs.  Thank you.