Atomic fluorescence spectrometry via photofragmentation of volatile beta-diketonate chelates

Author

Priscilla Johnson Gannicott

Date of Award

12-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Earl L. Wehry

Committee Members

Kenneth R. Wathem, William Bull, Gleb Mamantov

Abstract

Laser-based atomic spectroscopic methods for trace metals analysis offer many advantages of potential use to the analytical chemist; namely, extremely high sensitivity and selectivity. The key to achieving high sensitivity Is efficient sample atomization. An atomization reservoir that is highly efficient with minimal background interferences is desirable in the achievement of ultra-trace metals analysis and is the focus of the research described herein. Multiphoton ultraviolet laser photolysis was the method chosen to generate gas-phase metal atoms from volatile metal chelates. The resulting emissive fragments were detected via atomic fluorescence spectrometry. The performance of photofragmentation of volatile metal chelates as an alternative to more conventional atomization reservoirs for atomic fluorescence spectrometry was investigated. Fluorinated beta-diketonate complexes of Fe and Ga and carbonyl complexes of Mo and Fe were volatilized into a high vacuum chamber and subsequently photolyzed by a 193 nm ArF excimer laser to generate metal atoms, which were then detected by atomic fluorescence spectrometry. Measurements of analytically significant parameters (limit of detection, linear dynamic range, precision and dependence of the atomic fluorescence signal on photolysis laser pulse energy) were performed with each of these metal complexes. Good precision, wide dynamic range and low limits of detection were observed in each case. In cases in which predominantly ground state atoms were presumed to form in the photofragmentation process, probe laser excitation of these ground state atoms was performed in an effort to Improve even further the sensitivity of the technique. Measurable signal enhancement over the use of just the photolysis laser (193 nm) was observed in the cases of bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedione)diaquo copper (II), iron pentacarbonyl and tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedione) iron (III). Probe laser excitation was found to be only minimally useful for tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedione) gallium (III). No measurable signal enhancement was observed with tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedione) chromium (III). The usefulness of the second (probe) laser in terms of sensitivity improvement was determined to be dependent on the compound under study, at least under the experimental conditions employed here. For those metal complexes which benefited from probe laser excitation, analogous figures of merit to those performed for the one-laser (193 nm) experiments (dependence of laser-induced fluorescence (UP) signal on sample pressure, dependence of UP signal on photolysis laser pulse energy and precision of the LIP signal) were measured. The utility of laser photolytic atomic fluorescence spectrometry for superior limits of detection of metal-containing "real" samples in both solution and solid form was investigated. Determination of the concentration of metal ions in an aqueous solution via photofragmentation fluorescence spectrometry involved direct complexation of the analyte metal ion(s) in a sample with a beta-diketone ligand (1,1,1,5,5,5-hexafluoro-2,4-pentanedione or 1,1,1-trifluoro-2,4- pentanedione), volatilization of the resulting metal chelate into a high vacuum chamber, and laser photolysis of the gaseous chelates resulting in emissive fragments which were detected via their fluorescence. It was found that aqueous solutions (specifically Fe³⁺, Cu²⁺ and Cr³⁺) containing metal ion concentrations lower than ca. 10^-5M were incapable of detection, presumed to be as a result of the unfavorable characteristics of the chosen ligands when directly reacted with aqueous solutions. Solid samples containing trace amounts of metals (specifically, NIST standard reference materials: aluminum casting alloy SRM 855a, silicon metal powder SRM 57a and bovine liver SRM 1577) were subjected to direct complexation with the ligand hfa (1,1,1,5,5,5-hexafluoro-2,4- pentanedione). The resulting volatile metal chelate(s) were volatilized and photofragmented via 193 nm laser radiation. The resulting metal atoms were detected via their fluorescence. Detectable atomic fluorescence signals generated from the 193 nm photolysis of an aluminum casting alloy and a silicon metal powder sample after being treated with hfa were observed. Detectable atomic fluorescence signals were not observed from the 193 nm laser photolysis of the gaseous vapor produced as a result of complexation of hfa with metallic constituents of bovine liver.

Recommended Citation

Gannicott, Priscilla Johnson, "Atomic fluorescence spectrometry via photofragmentation of volatile beta-diketonate chelates. " PhD diss., University of Tennessee, 1993.
https://trace.tennessee.edu/utk_graddiss/10677