DETERMINATION OF DISTRIBUTION COEFFICIENTS FOR CATION EXCHANGE RESIN AND OPTIMISATION OF ION EXCHANGE CHROMATOGRAPHY FOR CHROMIUM SEPARATION FOR GEOLOGICAL MATERIALS

Abstract

Catherine Mary Davies - The University of Manchester - Master of PhilosophyDetermination of Distribution Coefficients for Cation Exchange Resin and Optimisation of Ion Exchange Chromatography for Chromium Separation for Geological Materials.ABSTRACTPresented in this study is an improved separation technique for the purification of chromium from geological materials. Variations in the stable isotope ratios of chromium (Cr) provide a formidable means of tracing and quantifying Cr(VI) contamination either from anthropogenic impact or natural attenuation from redox reactions of Cr from mineral constituents. In order to detect slight changes in the isotopic composition of Cr, high precision analysis by MC-ICPMS requires careful isolation of Cr from elevated concentrations of matrix elements present in geological materials. It is essential that interfering elements with isotopes of similar masses, such as Ti, Fe and V, must be removed from sample dissolutions. In comparison with previous studies, a two-step cation exchange separation procedure has been optimised for the isolation of Cr, for subsequent isotope analysis by Multiple Collector - Inductively Coupled Plasma Mass Spectrometry, with improved recoveries and purity. The cation exchange procedure relies on Cr within the digested materials being trivalent; therefore, a hexavalent speciation assay was employed to confirm the absence of Cr(VI) in dissolutions of chosen geological materials and standards, prior to cation exchange. In order to evaluate improvements made to the separation procedure, numerous geological materials were digested and processed through the cation exchange separation chemistry. Effective dissolutions of USGS silicate basalt standards: BHVO-2 and BCR-2, Cody Shale (SCo-1); Chromite from the Bushveld Igneous Complex and the carbonaceous chondrite Allende, were achieved by microwave assisted methods. Chromium was separated from matrix elements and analysed by MC-ICPMS, in conjunction with isotopically certified Cr standards (NIST 3112a and SRM 979). The chosen samples possess diverse matrices combined with varying Cr concentrations (16 - 764,000 ppm) in order to assess the limits of the exchange procedure. Modifications were effected to twelve sequential two-step cation exchange procedures, in order to improve the purity and yield of the separated Cr. Through this fast separation method, low procedural blanks (typically < 20 ng) are achievable in conjunction with improved purity and procedural yields of up to 97% for chemically separated Cr.In addition, to facilitate improvements to the ion exchange procedure, extensive series of distribution coefficients were determined for 38 elements, across a range of nitric acid molarities with Bio-Rad AG® 50W-X8 cation resin. Precise distribution coefficients, with improved detection limits, were obtained providing an invaluable insight for the sorption properties of Cr and matrix elements as a function of nitric acid molarity with Bio-Rad AG® 50W-X8 cation resin.

Date of Award	31 Dec 2012
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Maria Schonbachler (Supervisor) & James Gilmour (Supervisor)