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We have built a 36-pixel superconducting tunnel junction (STJ) spectrometer for high-resolution soft X-ray spectroscopy at the synchrotron. It has an energy resolution between ~10 and ~20 eV FWHM for X-ray energies below ~1 keV, and can be operated at maximum total count rates of ~106 counts/s. For increased sensitivity, we have modified the detector cold finger the so that the array now subtends a solid angle of Omega/4pi ap0.1%. The spectrometer is currently used at the Advanced Light Source synchrotron for speciation measurements of dilute samples by fluorescence-detected X-ray absorption spectroscopy. We present measurements of the oxidation state of dilute cerium activators in novel scintillator materials, determine the Ce3+/Ce4+ ratio, and discuss the sensitivity of the instrument for quantitative speciation measurements.