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Derivatization / Reaction Headspace

Derivatization is a technique that can be used to increase sensitivity and chromatographic performance for specific compounds.

Compounds such as acids, alcohols and amines are difficult to analyse because of the presence of reactive hydrogens. When attempting to analyse these types of compounds, they can react with the surface of the injection port or the analytical column and result in tailing peaks and low response.

In addition, they may be highly soluble in the sample phase, causing very poor partitioning into the headspace and low response. Derivatization can improve their volatility, as well as reduce the potential for surface adsorption once they enter the GC system.

Common reagents used to derivatize compounds of interest:  
Compound of Interest Derivatizing Reagent Resulting Derivative
fatty acids methanol with boron trifluoride esterfication
glycerol acetic anhydride with sodium carbonate  acetylation

Derivatization Techniques

Common derivatization techniques used in reaction headspace/GC are:

  • esterfication
  • acetylation
  • silylation
  • alkylation

Any of these derivatization techniques can be performed using the sample vial as the reaction vessel. Although derivatization may improve chromatographic performance and volatility for some compounds, derivatization reactions may introduce other problems into the analytical scheme:

  • Derivatization reagents, as well as the by-products from derivatization reactions, may be volatile and can partition into the headspace along with derivatization compounds. These extra volatile compounds may pose problems by eluting with similar retention times as the compounds of interest, causing either partial or complete coelutions.
  • Derivatization reactions are typically run at elevated temperatures. Pressures inside the sample vial may exceed the pressure handling capabilities of the vial or the septa. Specially designed caps are available that allow excess pressure to be vented during derivatization reactions. Use of the correct vial and cap is important see Vial and Septa Issues.