Thin, initially transparent films made of variants of a polymeric cation-exchange material have been found to be useful for facilitating the luminescence detection and quantitation of several rare-earth ions dissolved in aqueous solutions. A film of this type is prepared in an acid form, then converted to a Ca2+salt form. The acid form of the material consists of polyacrylic acid entangled in a matrix of insolubilized or further cross-linked polyvinyl alcohol. The conversion to the salt form is effected by exposing the film to Ca(OH)2.
Fluorescence Spectra of Tb3+ Ions were obtained from aqueous solutions and from an ion-exchange film in which ions had been collected from one of the solutions. At a wavelength of 488 nm, for example, the spectral intensity from the film specimen was 4,100 times that from the corresponding solution. The increase in intensity is a result of both increased concentration and the displacement of coordinated water molecules by carboxylate groups of the ion-exchange material.
The film is placed in an aqueous solution that one seeks to analyze. By virtue of the cation-exchange function of the film material, rare-earth ions from the solution become concentrated in the film. The film is then mounted in a luminescence spectrometer apparatus for analysis of its rare-earth-ion content by fluorometry and/or phosphorimetry. The concentration of the ions in the film increases the fluorometric and/or phosphorimetric response beyond that achievable through spectrophotometric analysis of the solution (see figure), thereby effectively increasing the sensitivity of measurement of concentrations of the dissolved ions.
This approach to spectrometric analysis is denoted generally as solid-phase spectrophotometry (SPS). As practiced heretofore, SPS has involved (1) the use of ion-exchange resins and (2) the enhancement of selectivity and sensitivity by use of chromophoric agents as is done in conventional spectroscopy. Unfortunately, many of the ion-exchange resins used heretofore in SPS are not transparent; on the contrary, they are highly absorbing in the spectral regions of interest and are highly scattering at all wavelengths. In contrast, the present ion-exchange films are sufficiently transparent that they do not interfere appreciably with spectrophotometry throughout the visible and most of the ultraviolet spectrum of interest. Furthermore, because the present ion-exchange material is not particulate and its index of refraction matches that of water, the light-scattering problems associated with prior ion-exchange resins are eliminated.
Another advantage arises in connection with fouling by Ca2+ions: These ions, which are often present in natural waters, compete with the metal ions of interest for sites on ion-exchange resins, thus rendering the resin beads less effective. Because it is in the Ca2+ form, the present ion-exchange material resists fouling by Ca2+.
One drawback of the present ion-exchange material is that it emits a large amount of background fluorescence. However, this is not a major drawback, inasmuch as the luminescence from the metal ions of interest lasts much longer than does the luminescence from the film; one can suppress the response to the luminescence from the film by use of instrumentation with a temporal-discrimination capability. The only major drawback is slowness of uptake of ions because of the slowness of diffusion of ions to the ion-exchange material.
This work was done by Kenneth W. Street, Jr., of Glenn Research Center and Stephen P. Tanner of the University of West Florida.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Refer to LEW-17074.