Rare earth containing complex compounds for the determination of inorganic ions

The use of rare earth ions as spectroscopic probes of inorganic ions in inorganic host crystals is well established. The narrow line excitation and emission features of these ions result in both selective and sensitive analyses. The atomic character of these spectra are a result of intra-4f transitions where the 4f electrons are strongly shielded by outer 5s and 5p electrons. Although this shielding preserves the sharpness of the lines, the positions of the lines are determined by the numbers, kinds, and geometries of neighboring atoms in the crystal matrix. The line position shifts are ordinarily small (on the order of a few nm) but due to the sharpness of the lines (FWHM 0.05 nm at 10°K) many different situations can be resolved.

The previous use of inorganic crystals as hosts for rare earth - analyte ion pairs required laborious and time consuming procedures involving precipitation and annealing. The process of annealing is required to allow the crystals to arrange themselves in such a manner as to give the rare earth probe a homogeneous micro-environment or site. The production of a few such sites often concentrates the emitting fluorophore into a sensitive indicator of the analyte ion. The process precludes the use of the method of standard addition for quantitation and has therefore resulted in relatively poor precision. The difficulties encountered in these crystal production experiments may be circumvented by the use of charged rare earth chelate compounds containing inorganic counter ions which provide a suitable micro-environment without the need for complicated annealing procedures. Rare earth chelates, under suitable conditions, allow for ion exchange of the counter ions. To enhance the stability of the compounds toward degradation, chelating ligands will be used. The use of an ion exchanger allows standard additions to be performed.

Laser excited fluorescence spectroscopy is performed at low temperatures (10°K or 77°K). The low temperature narrows the spectral lines by elimination of vibrations, which reduces the background and improves selectivity. By using a flashlamp pumped scannable dye laser both excitation and emission spectra are observed. In the event of a small diluent ion signal being overwhelmed by the large emission signal of a dominant ion the excitation can be tuned such that only the ion of interest is excited. When this problem is absent a UV source can be used for excitation of all species as the ligands can be made to absorb in the UV and transfer their energy to the rare earth ion for emission.

This investigation describes the rare earth chelates employed along with the instrumentation used. Spectra of the compounds are presented and ion exchange is described.