Trace Analysis of Nonfluorescent Ions by Selective Laser Excitation of Lanthanide Ions J. C. Wright Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
The feasibility of using selectively excited probe ion luminescence (SEPIL) for detection and measurement of nonfluorescent ions in solution is demonstrated. I n the example described in this paper, a Eu3+ ion acts as a spectroscopic probe of a BaSO, precipitate to detect the presence of coprecipitated PO4 ions. The Eu3+ions whose crystal fleld levels are perturbed by nearby PO4 ions can be excited wlth very high selectivity over other Eu3+sltes In the lattice by a tunable dye laser. The fluorescence intensity that results is a direct measure of PO4 concentrations. The method represents a new way of achieving narrow line widths specific to analyte ions which are required for obtaining the inherent high selectivity and sensitivity of laser excited fluorescence.
T h e use of a laser as an excitation source for fluorescence has shown the promise of performing analytical measurements a t extraordinary small concentration levels below 10' atoms/cm3 ( I ) . At these levels, the problem of selectivity toward the particular species of interest becomes very important. The narrow line width of the laser also offers the potential for excellent selectivity if the species of interest has narrow spectral lines. Typically the narrow lines are achieved by placing the species in the gas phase where excellent selectivity may be achieved. In this paper, we report the feasibility of a different method for trace analysis that achieves the sharp line transitions by placing an optically active species in an ordered material. The optically active species then serves as a probe of the ordered material it is in (the host) since the ligand field splittings of the probe will reflect the local environment it encounters in the host. If another species that is of analytical interest is also present in the host and is nearby the probe, new ligand field splittings will result that are characteristic of the analyte's presence. If a probe is selected that inherently has sharp line transitions, those probes with the analyte species nearby can be selectively excited with a laser and the intensity of their emission can be directly related to the concentration of the analyte. The method should therefore permit the application of the inherent high sensitivity and selectivity of the laser to a wide variety of analytical measurements. If several analytes are incorporated in a single host or if several hosts are each capable of incorporating a single analyte, the method should also be capable of multiple ion analysis. In the preceding paper, we reported the use of this technique for the case where the analyte and probe were the same ion ( 2 ) . This technique was capable of measuring lanthanide ion concentrations at 25 parts in l O I 5 with very high selectivity by coprecipitating the lanthanide ion with CaF2. The research reported here represents the case where the analyte, probe, and host are different. A BaS04 host lattice with a Eu3+probe ion was selected to measure trace concentrations of P043-.The Eu3+ probe ion has an unfilled 4P shell which is well-shielded from external fields by outer 5s25p6 orbitals. T h e free Eu3+ energy level structure is shown in Figure 1. Each level is characterized by a J value and possesses a (W+ 1)degeneracy. 1690
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
When the Eu3+ is placed in a low symmetry crystalline environment, the ligand fields remove the (2J + 1)degeneracy and each manifold splits into a series of crystal field levels. The splittings obtained are only 100-300 cm-' because of the shielding effects of the outer orbitals. For the same reasons, the homogeneous line widths are less than 0.1 cm-' unless there are single phonon relaxation processes that rapidly depopulate the level. An example of the crystal field splittings for one manifold of one B&04 site is shown on the right part of Figure 1.
EXPERIMENTAL Reagents. The BaS04precipitates were generally formed from analytical grade BaC12 (J. T. Baker Chemical Co.) and Na2S04
(Mallinckrodt Chemical Works). The hydrated salt of 99.9% EuCl, (ROC/RIC) was used for the source of Eu3+and reagent grade Na3P04.12H20(Mallinckrodt Chemical Works) was used for the source of Pod3-.Deionized water was used for preparing solutions, dilutions, or washings. Apparatus. A detailed description of the apparatus for selective laser excitation was published in the previous issue ( 3 ) . A block diagram of the essential features is shown in Figure 2. A 300-kW nitrogen laser is used to excite a tunable dye laser of the Hhsch design (4). The laser bandwidth is generally 0.014.02 nm. The wavelength can be scanned linearly with time by a stepping motor and a sine-bar drive. Samples are mounted in a copper holder which attaches to a cryogenic refrigerator that cools them t o 13 K. The fluorescence is dispersed with either a '/,-meter or a 1-meter monochromator and the fluorescence is detected with a photomultiplier. After passing through a current-to-voltage converter, the signal is sent t o a gated integrator which provides the output for a strip-chart recorder. Procedure. A solution of Na2S04,Na,PO,, and EuC13 was prepared by mixing the three stock solutions together. Unless otherwise stated, the Na2S04concentration was 0.100 M and the EuC13concentration was 5 X lo-' M. A 0.10 M BaC12 solution was slowly added to this solution to precipitate 80% of the SO, as BaS04. The rate and order of addition and the percent of precipitation did not influence the results. The same results were also obtained if the EuC13was in the BaC12solution. The resulting precipitate was then set aside to age. The precipitate was separated from the supernate by either suction, filtering, or decantation. There were no differences measured in the relative site distribution attributable to the different methods. The precipitate was then dried at room temperature and ignited in a furnace. The mass was finely powdered and a small portion ( - 5 mg) was pressed into small cylindrical holes in the copper sample holder. The sample holder was attached to the cryogenic refrigerator and either excitation or fluorescence spectra were obtained. If the '/,-meter monochromator is used with a 6.6-nm bandpass to monitor fluorescence from a specific manifold while the dye laser is scanned over all wavelengths in which absorption lines are possible, a composite excitation spectrum of all the sites is obtained. However if the 1-meter monochromator is used with sufficiently narrow slits (
