title: Determination of insoluble soap in agricultural soil and sewage sludge samples by liquid chromatography with ultraviolet detection.
abstract: We have developed a new analytical procedure for determining insoluble Ca and Mg fatty acid salts (soaps) in agricultural soil and sewage sludge samples. The number of analytical methodologies that focus in the determination of insoluble soap salts in different environmental compartments is very limited. In this work, we propose a methodology that involves a sample clean-up step with petroleum ether to remove soluble salts and a conversion of Ca and Mg insoluble salts into soluble potassium salts using tripotassium ethylenediaminetetraacetate salt and potassium carbonate, followed by the extraction of analytes from the samples using microwave-assisted extraction with methanol. An improved esterification procedure using 2,4-dibromoacetophenone before the liquid chromatography with ultraviolet detection analysis also has been developed. The absence of matrix effect was demonstrated with two fatty acid Ca salts that are not commercial and are never detected in natural samples (C₁₃:₀ and C₁₇:₀). Therefore, it was possible to evaluate the matrix effect because both standards have similar environmental behavior (adsorption and precipitation) to commercial soaps (C₁₀:₀) to C₁₈:₀). We also studied the effect of the different variables on the clean-up, the conversion of Ca soap, and the extraction and derivatization procedures. The quantification limits found ranged from 0.4 to 0.8 mg/kg. The proposed method was satisfactorily applied for the development of a study on soap behavior in agricultural soil and sewage sludge samples.
authors.affiliation: Faculty of Sciences, Department of Analytical Chemistry, University of Granada, E-18071 Granada, Spain.
authors.forename: Samuel
authors.initials: S
authors.lastname: Cantarero
authors.forename: Alberto
authors.initials: A
authors.lastname: Zafra-Gómez
authors.forename: Oscar
authors.initials: O
authors.lastname: Ballesteros
authors.forename: Alberto
authors.initials: A
authors.lastname: Navalón
authors.forename: José L
authors.initials: JL
authors.lastname: Vílchez
authors.forename: Guillermo
authors.initials: G
authors.lastname: Crovetto
authors.forename: Coral
authors.initials: C
authors.lastname: Verge
authors.forename: Juan A
authors.initials: JA
authors.lastname: de Ferrer
mesh_terms: D002118:Calcium / Q000032:analysis; D002853:Chromatography, Liquid; D008274:Magnesium / Q000032:analysis; D012722:Sewage / Q000737:chemistry*; D012915:Soaps / Q000032:analysis*; D012989:Soil Pollutants / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D012722:Sewage; D012915:Soaps; D012989:Soil Pollutants; D008274:Magnesium; D002118:Calcium