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 compounds, ammonia, 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). D3606 Application Column Instruction Sheet (PDF)  Column #1 holds up the fuel matrix allowing only the compounds of interest to pass to Column #2.

83606-800L

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.

 

5 Responses to “GC columns – when one is not enough”

  1. Dear Alan, very nice piece!
    When it comes to analysis of fixed/permanet gases, “you have to play the game” .. :-) I have some recommodations too….. 1) Learn to build your own packed columns. It is surprisingly easy. And fun also! You really feel the history of gas chromatography and the hart of GC analysis. MS5A, MS13X, HayeSep N, Q, T, and Chromosorbs are the main stationary phases, try and use them all..! 2) From start on, buy a gas chromatograph, equipped with two 10-port valves (injection and backflush of ore columns) and two 6-port Valves (column isolation), one split/splitless injection port, one or two TCD’s and one FID equipped for both capillary and packed column installation. You now have an instrument you can configure for almost every possible fixed/permanent gas application. 3) Prepare your gas system, so you easily can switch gases for your carrier gas EPC modules. Both Helium, Argon, Nitrogen and Hydrogen are all very usefull as carrier gases. 4) If light hydrocabons also are going to be analyzed, use PLOT columns! 5) Control carefully the gas sample inlet flows and temperature. Gases are injected in volumes, differences in sample gas flows and pressures are main causes to wrong results. 6) Tracing of sample gas lines are mandatory, if injecting any condensing gases.

    With kind regards –
    Lars Kurstein, Copenhagen

  2. Alan Sensue says:

    Thank you Lars, not only for the kind words, but for your time to provide such a detailed comment. Both are much appreciated.

  3. Stefan Voinea says:

    Thank you Alan and Lars for your comments and advices. On this topic I was wondering how common is the use a temperature ramped column to solve both, the permanent gases and the heavier compounds.
    Best Regards, Stefan.

  4. Alan Sensue says:

    Hello Stefan. Thank you for reading my post.

    I have seen both isothermal GC oven temperatures if the “heavies” will elute at this temperature, and I have seen ramped GC oven temperatures to make sure these heavies will elute. In many cases these individual columns are installed into separate small, isolated GC ovens (they are also called oven compartments) which allow individual column (oven) temperature control. There also may be individually heated “valve” boxes just for the switching valve because these are commonly operated at isothermal temperature.

    I hope this helps.

    Regards, Alan

  5. Albert says:

    Dear Sir and Madam, Someone can help us for Back Flush ???.

    For Dirty Matrix Sample Testing, which system is better and efficient to compare with Back Flush and Guard Column System for Heavily dirty matrix sample test ???. Ple. Help. Albert

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