Except for some special cases (e. g. enantiomer separation of amino acids with PERMABOND® L-CHIRASIL-VAL) in GC generally methylation is the only type of alkylation used.
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| Methylation reagents
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N,N-dimethylformamide dimethylacetal
m.w. 119.17 Bp 106 – 107 °C (760 mm Hg),
density d20°/4° = 0.897
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DMF-DMA
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Methylation with DMF-DMA can be applied for fatty acids, primary amines and (partially) amino acids forming N-dimethyl-aminomethylene amino acid methyl esters. DMF-DMA is a poor solvent, for this reason it is necessary to use a mixture of DMF-DMA with pyridine, THF, acetone (barbiturates) or another solvent.
For a working protocol click here.
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Trimethylsulphonium hydroxide (0.2 M in methanol)
m.w. 94.06
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TMSH |
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Methylation with TMSH is recommended for free acids, chlorophenoxycarboxylic acids, their salts and derivatives as well as for phenols and chlorophenols. One great advantage is simplification of the sample preparation. Lipids or triglycerides can be converted to the corresponding fatty acid methyl esters (FAMEs) by a simple transesterification. Isomerisations of multiple unsaturated fatty acids have not been observed.
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This reaction is very elegant and convenient, because it is just
necessary to add the reagent (0.2 M in methanol) to the sample
solution. Removal of excess reagent is not required, since in the
injector of the gas chromatograph at 250 °C pyrolysis to volatile
methanol and dimethylsulfide will occur. Due to the high reactivity,
complete derivatisation is often obtained at ambient temperature.
However, heating (e. g. 10 min at 100 °C) in a closed sample vial may
be necessary. For a working protocol click here.
For GC separation of FAMEs from natural butter fat after derivatisation with TMSH see Appl. 201680.
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Methylation with methanol/TMCS
A 1molar solution of TMCS in methanol is suited for the esterification of free carboxylic acids and transesterification of glycerides. Formation of HCl catalyses the reaction. TMCS and silyl ether remove water and thus drive the reaction to completion. The mixture should be prepared fresh. For a working protocol click here.
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