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2.5F: Sample Preparation for Gas Chromatography

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    95702

    • Liquid GC Samples

    1. Fill a GC vial to the \(1.5 \: \text{mL}\) mark (Figure 2.93a) with a low boiling solvent (e.g. methanol, clean acetone, diethyl ether, or dichloromethane). Add one drop of the sample to be analyzed. If you think you possibly only added a half-drop, it's probably enough. Two drops are really too much.
    2. Cap the vial and invert once or twice to dissolve the sample. If it appears like the sample drop did not dissolve fully, prepare another sample using a different solvent.
    3. Alternatively, if you have already prepared a high-field NMR sample, use one-third of this sample and dilute it further with a low-boiling solvent.
    4. Run the GC, as demonstrated by your instructor. Procedures vary at each institution.
    Figure 2.93: a) GC vial, b) GC-MS instrument, c) Small amount of solid in a GC vial.

    Solid GC Samples

    1. Some relatively volatile solids (never ionic solids!) can be analyzed by GC. Add one or two "specks" of solid\(^{16}\) (a pile approximately \(2 \: \text{mm}\) in diameter), or a very small spatula-tip of solid to a GC vial (Figure 2.93c). Then add a low boiling solid (e.g. methanol, clean acetone, diethyl ether, or dichloromethane) to the \(1.5 \: \text{mL}\) mark.
    2. Cap the vial and invert several times to fully dissolve the solid. If the solid dissolves, run the GC.
    3. If the solid does not dissolve, do NOT run the GC anyway! Solids can plug the very small microliter syringes used by the instrument.
      1. If the quantity of solid does not appear to have changed at all after adding the solvent, try making another sample with a different low boiling solvent.
      2. If it appears the solid has decreased in quantity but is not fully dissolved, it is likely enough of it has dissolved to analyze. Use a pipette filter to filter the solution, and run the GC on the filtered liquid.

    \(^{16}\)The quantity of solid may need to be adjusted if the solid is "fluffy". A GC is at the ideal dilution when it produces abundances around one million using a mass spectrometer detector.

    Contributor

    Lisa Nichols (Butte Community College). Organic Chemistry Laboratory Techniques is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Complete text is available online.