Instrumentation
J. C. Wright and F. J. Gustafson Department of Chemistry University of Wisconsin Madison, Wis. 53706
Ultratrace Inorganic Ion Determination by Laser Excited Fluorescence Fluorescence spectroscopy is an analytical method that has been increasing in use because the low detection limits that characterize the method are required in the determination of trace contaminants in our environment, industries, and bodies. The chemistry that has been developed for sample preparation prior to the fluorescence measurement step has been predicated on the assumption that the bandwidth of the excitation source is relatively broad. T h e procedures, organic compounds, and inorganic phosphors have been optimized for maximum fluorescence intensity with this fundamental assumption. Typically, the best materials would have broad and intense absorption bands t h a t could be efficiently pumped by the broad bandwidth excitation sources. Although the breadth of the absorption and emission features results in more intense fluorescence, the selectivity of the method is lowered. Typically, detection limits are determined by background fluorescence of the blank, which cannot be distinguished from analyte fluorescence. Powerful methods have been developed to glean as much selectivity as possible from both excitation and fluorescence spectra (7), but broadband excitation methods remain fundamentally limited by selectivity. T h e development of convenient and reliable dye lasers has provided a potentially important new excitation source that has a narrow bandwidth. It follows, therefore, that the chemistry developed for broadband excitation sources is not necessarily the best for methods using narrow line excitation sources. In fact, it is the analytical chemist's responsibility to evaluate 0003-2700/78/A350-1147S01.00/0 © 1978 American Chemical Society
and if necessary develop the chemical support that would best serve the new excitation sources. In the realm of narrow bandwidths, selectivity and sensitivity become compatible with each other. If the linewidth of absorptive features can be narrowed by modifying the analyte or the environment of the analyte, the peak absorbance for a narrow bandwidth source will increase since the overall oscillator strength for the transition remains constant (assuming the modifications have not noticeably perturbed the character of the electronic or vibrational states). The new laser technology therefore offers the challenge and opportunity to the analytical chemist to explore innovative or radical ideas
that would not be feasible with conventional instrumentation. In this article we present the current status of our research toward developing such a method for the ultratrace determination of inorganic ions. The method utilizes the selective excitation of probe ion luminescence (SEPIL) to provide both quantitative and qualitative information about possible analyte species. In this method, fluorescent probe ions that exhibit sharp line fluorescence and excitation features are used (these ions are generally ones possessing unfilled inner orbitals). The wavelength for these transitions is determined by the environment about the probe ion. In an arbitrary material, there is hétérogène-
Figure 1. Block diagram of system ANALYTICAL CHEMISTRY. VOL. 50, NO. 12, OCTOBER 1978 · 1147 A
Figure 2. Excitation spectrum of CaF2:Er3+ obtained by monitoring: a) All fluorescence [lines from sites shown in b) and c) are marked]; b) fluorescence line from Er ions with fluoride ion in nearest interstitial position; c) fluo rescence line from Er ions with fluoride ion in next-nearest interstitial position
ity at the ionic level and therefore a variety of possible environments. Each kind of environment around a probe ion will produce a unique set of transi tions that contribute to the composite spectra. By S E P I L the spectra of the individual environments (or sites) can be examined selectively. In particular, if a new ion (we shall refer to it as the analyte ion) is incorporated into the probe ion surroundings and produces a new environment, new lines charac teristic of the analyte ion's presence are produced in the probe ion spectra. Instrumentation T h e instrumentation constructed to perform these studies is shown in block diagram form in Figure 1 and is explained in detail in ref. 2. A nitrogen pumped dye laser was used as the ex citation source because of its versatili ty, wide wavelength coverage, reliabil ity, and ease of use. It has a 0.01 -nm bandpass and can be scanned linearly in the wavelength to obtain excitation spectra as a particular fluorescence wavelength is monitored. T h e laser in tensity is measured before and after (if the sample is transparent) a sample by photodiodes. T h e fluorescence is measured with either a 1- or '/i-rn monoehromator and a photomultiplier detector. Precipitate samples are pressed into shallow cavities in a cop per block and mounted on the bottom of a cryogenic refrigerator that, can reach 10 K. T h e entire system is under computer control. This instrumenta tion was designed to allow the maxi mum experimental versatility so that any spectroscopic measurement need ed could be made. If one was inter ested in optimizing the system for a particular procedure, the instrumen tation could be simplified and the per formance could be enhanced, particu larly by choosing a tunable laser with
a higher average power than the nitro gen pumped dye laser. Background on Development T h e method was developed from a series of studies of the point defect equilibria that occur in solids (3). When a trivalent rare earth ion is doped into a crystalline material like CaF-ί where it substitutes for a diva lent Ca, an additional charge compen sation is required. In C a F 2 this com pensation is provided by introducing a fluoride ion in an interstitial position in the lattice. T h e coulombic interac tions between the additional negative charge of the fluoride interstitial and the extra positive charge of the triva lent rare earth ion will promote associ ation of these ions. Since the fluoride interstitial can have different posi tions in the lattice relative to the rare earth ion it associates with, the rare earth ions in the lattice will have dif ferent crystal field splittings of their electronic energy levels depending upon the exact arrangement of nearby ions. An absorption or fluorescence spectrum would therefore contain lines resulting from transitions be tween crystal field levels for all of the rare earth ions in different environ ments or sites. T h e complexity of such spectra can be simplified by using laser techniques to selectively excite a specific absorption transition of one specific kind of site. T h e resulting flu orescence spectrum then contains only the transitions of that site. Examples of this procedure are shown in Figure 2. Figure 2a is the spectrum obtained without selective excitation tech niques for a CaF-2:Er:i+ sample with two possible Er : i + sites—those with nearest and those with next-nearest neighbor fluoride interstitials. Figure 2b is the selectively excited spectrum of only those Er ions with a fluoride
1148 A · ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
ion in the nearest possible interstitial position, and Figure 2c is the spec trum of those Er ions with a nextnearest neighbor fluoride interstitial. T h e lines in the two spectra differ in position because of the difference in crystal fields for these two sites. Two observations t h a t had important im plications for analytical measurements were immediately apparent from this early work. T h e signal levels were ex tremely high for a fluorescence mea surement with laser excitation, and the selectivity for excitation of only one type of rare earth site was excel lent. Could these observations be translated into an analytical method with extremely small limits of detec tion and high selectivity for particular analytes? Trace Lanthanide Analysis T h e first step in the development centered around establishing whether a simple method that would incorpo rate trace analytes into a crystalline lattice could be established. T h e pre vious work in CaF could be created by precipitation from solution. There are several crucial questions that de termine the feasibility of such an idea. If the short-range order of the precipi tate lattice is low, the spectral lines of the rare earths will be broad because there are essentially an infinite num ber of environments that a rare earth will encounter. Is the short-range order of precipitate microcrystals suf ficient to give sharp line transitions? Secondly, can the rare earth be incor porated into the lattice? T h e precipitates obtained by stan dard gravimetric methods where the rare earth ions have coprecipitated
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produce very sharp lines and excellent fluorescence intensities (4). T h e rare earth ions are very favorably incorpo rated into the CaF-> precipitate and allow the precipitation to serve effec tively as a preconcentration and sepa ration step as well as a sample prepa ration step. Typical excitation spectra are shown in Figure 3 for Ho : i + and Er : i + , two rare earth ions that have very similar electronic energy levels. Their lines are sharp and well resolved so that one ion can be easily excited without exciting the other. Since the lines in the fluorescence spectra are equally well resolved, the selectivity of the analysis for a particular rare earth ion is essentially complete. The line positions provide the information for a qualitative analysis of rare earths in a sample. Any of the rare earth ions that have optically accessible energy levels can be determined by this method with out the need to separate individual lanthanides. The lanthanides that can be done include Pr, Nd, I'm, Sm, Eu, Gd, T b , Dy, Ho, Er, T m , and Yb. We have recently found methods that can determine the remaining nonfluorescenl lanthanides as well. Addition ally, it should be possible to perform analysis of actinides using the same ideas since the optical transitions of the actinides are very similar to the lanthanides. T h e detection limits in solution are generally a part per tril lion and correspond to ca. 4 X 10 - ! ) mol % of rare earth in the CaF^ pre cipitate. If the sample placed in the laser beam is 5 mg, there would be 0.4 pg of Er :!+ present. Smaller sample sizes, higher laser powers, and better detection electronics can all lower this value considerably. Linear calibration curves are maintained up to 1 ppm. Full details of the technique will be published shortly (5). Feasibility of Determining Other Ions
The success of these experiments encouraged research on extending
these ideas to other analytes. If other analytes can be incorporated into the crystal lattice in association with a rare earth ion (the probe ion), the crystal field experienced by that rare earth ion will be different from the fields experienced by the other rare earth ions that do not have the analyte nearby. Since the spectral transitions of rare earth ions are sharp, the differ ent crystal field levels can be resolved from each other and selectively excit ed with the same techniques discussed earlier. Since there is a 1:1 correspon dence between the analyte and a rare earth ion, the method should have comparable selectivity and sensitivity with the rare earth analysis studies. T h e feasibility of one method for achieving the required association be tween analyte and rare earth was shown for trace analysis of PO4 by formation of a BaSO., precipitate (6'). In this work, new lines were produced in the Eu spectrum that were charac teristic of the presence of ΡΟ:ί~ and can be used for its quantitative analy sis. If BaCT is added to a solution of Na^SO.i that also contains trace amounts of PO;; - and a rare earth ion like Eu : i + , a precipitate of BaSO, forms with PO',' - and Eu , , + impurities. Both of these ions require a charge compensation when they enter the BaSO, lattice. T h e charge compensa tion can be provided by defects in the lattice, or the two ions can compensate each other. T h e nature of the B a S 0 4 defects is unknown, but one might speculate the most favorable defects might be oxygen (Ov) and barium vacancies (Ba v ). Since the actual char acter of the defects is not important for this discussion, we will assume these are the defects for illustrative purposes. Eu ;}+ impurities could then be charge compensated by having one Ba v for every two Eu : ! + ions, and PO:,1impurities could be compensated by having one O v for every two PO:,1- ions. T h e opposite charges on an ion (either Eu :1+ or P O j - ) and its compensating
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Figure 3. Excitation spectrum of C a F 2 : H o 3 f , E r 3 +
1150 A · ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
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defect (either Ba v or 0 V ) again promote association ( E u - B a v pairs or P 0 4 - O v pairs). In the same way, the Eu : ! + ion can be compensated with a PO4" ion, and pairing between Eu : , + and PO:,,_ can result from the coulombic interactions. One can write this situation as a series of competitive equilibria.
At any finite temperature, defects (Ba v and O v ) will be present in the BaSC>4 lattice. These defect concentrations can be changed by aliovalent dopants which will affect all of the solid state equilibria. T h e equilibrium forming EuPO.i is the analytically important one because it represents the associated E u - P O ( pair. At the Eu ion the PO4 will produce crystal fields that are unique to the presence of PO4 in the pair. These crystal field levels of Eu can be selectively excited and provide an analysis t h a t is selective for PO4. To function effectively as an analytical method, the equilibrium for Eu and PO4 association must be very favorable. This equilibrium can be controlled by the position of all the competing equilibria. T h e situation is analogous to the familiar cation-chelate equilibria of liquid solutions. T h e perfect BaSC>4 lattice plays the role of the undissociated water solvent, the defects Ba v and O v play the same role as the "water defects" H + and O H - , Eu dopants compensated with Bay, are analogous to cations cornplexed with O H - , PO4 dopants compensated with O v are analogous to chelates complexée! with H+, and the EU-PO4 association is analogous to the cation-chelate complex. T h e key to a favorable conditional formation constant for cation-chelate equilibria is pH control. In the same way, one would expect the key to effective use of the ideas presented here lies in control of the native defect equilibrium, i.e., the concentrations of Ba v and O v . This picture of our method has only recently been formulated and remains to be substantiated. We have been quite successful in using a solid state buffer in the CaF^ system to control possible interferences that act to change the equilibria, but work on extending these concepts to the systems with associated rare qarth-analyte pairs is only just beginning. T h e work described in ref. β did not include such a buffer; this meant that addition of PO4 to the BaSO.i both changed the
Figure 4. Excitation spectrum of B a S 0 4 : E u 3 + , Ρ 0 4 _ obtained by monitoring: a) All fluorescence; b) fluorescence line at 576.9 nm; c) fluorescence line at 577.0 nm; d) fluorescence line at 578.2 nm corresponding to Eu ions with nearby P0 4
native defect equilibrium (or in more familiar terms, addition of a chelate to an unbuffered solution can change the pH) and joined in association with the Eu. This effect leads to nonlinear cali bration curves and unfavorable equi libria for E u - P O i associates at low POi concentrations as observed exper imentally. An example of the spectra that are obtained from the B a S 0 4 system is shown in Figure 4. The excitation spectrum in Figure 4a was obtained by monitoring fluorescence with a wide bandpass so that, the excitation transi
tions from Eu ions in all possible crystallographic sites could be observed. T h e spectrum is relatively complex because there are five important sites in BaSO.,:Eu :1+ , PO,", each of which can have five transitions. T h e com plexity can be eliminated by using a narrow bandpass (
