Optimization of a methylation procedure to obtain chloroform-soluble humic acids

It will be important in future analyses of humic acids (HAs) by various chromatographic and spectrometric methods to obtain high yields of methylated HAs. A derivatization procedure for permethylation of HAs was evaluated in relation to the yield of the chloroform-soluble fraction. The procedure involved a 3 h incubation of HAs (50 mg) with methylsulfinyl carbanion (MSC) obtained by mixing petroleum-purified NaH with anhydrous dimethylsulfoxide (DMSO), followed by incubation with methyl iodide (CH 3 I). Ten HAs isolated from different Japanese and American soils (three Andisols, four Inceptisols, one Histosol, one Alfisol, and one Ultisol) were used to optimize this procedure. Optimum derivatization was achieved within the first 6 h after the addition of MSC and CH 3 I; further incubation (up to 24 h) did not result in significant changes in the yield of the chloroform-soluble fraction (60-100%). For most HAs, the maximal yields of chloroform-soluble fractions were obtained at concentrations of 10 mM for MSC and 13 mM for CH 3 I.