Prospects and limitations of a clay-enabled pre-concentration method for spectrophotometric quantification

Abstract

Listen

Translate article

The width of applicability of a pre-concentration protocol for quantifying analytes from dilute aqueous solutions, relying on adsorbate loading onto Bentone LT and subsequent clay delamination, was studied. The applicability of the protocol was investigated on seven organic (cetyltrimethylammonium bromide (CTAB), diclofenac, topotecan, methyl red) or inorganic (iron(ΙΙΙ) chloride, cobalt(ΙΙ) nitrate, copper(II) acetate) molecules. The study verified that Bentone LT adsorbed all studied analytes from dilute aqueous solutions. The relationship between adsorbate concentration in loading medium and adsorbate availability in supernatant upon delamination was investigated. A behaviour approximating a linear relationship with either a single or two linear ranges was observed for most adsorbates at low loading concentrations. For diclofenac and Fe(III) this relationship was described by a logistic regression model. Concentrations in the linear portion of the quasi-desorption curves correspond to environmental levels for certain analytes tested. High pre-concentration factors were achieved for many adsorbates (about 71-fold, 34-fold, and 29-fold for Co(II), Cu(II), and methyl red, respectively). Lower pre-concentration factors (about 12-fold, 8-fold, and 2-fold) were obtained for topotecan, Fe(III), and CTAB, respectively. For those adsorbates where pre-concentration was significantly successful, the lowest adsorbate concentration that could be reliably detected decreased by a factor ranging between 46- and 5-fold, when compared to the limit of quantitation (LOQ) of the hyphenated quantification assay not supplemented by a pre-concentration step. Pre-concentration of specific analytes in the presence of mixtures of chemicals, at environmentally-relevant levels or above, was equally successful. Limitations of the pre-concentration methodology were studied: The methodology works poorly when hyphenated to assays that depend on negatively charged compounds for ion-pair formation. Low pH values that may be required to favour adsorption, limit quasi-release efficiency. Adsorbate release, in the latter cases, is effective though upon pH increase, without delamination.