Methodology: The sulfur standards were ground using an agate mortar and pestle to the consistency of a fine talcum powder (approximately 10 um). A cellulose acetate filter was then gently dredged through a small quantity (less than 1 mg) of powder placed on a microscope slide. This procedure produced a thin and almost imperceptible coating on the filter in order to limit the thickness and thus self-absorption. S-NEXFS spectra of sulfate standards were collected in bulk mode. Self-absorption must be carefully controlled when measuring fluorescent X-rays from thick specimens; however, the effects of self-absorption are limited to the region of the spectrum above the K-edge. In our repeated measurements, the post-edge features were consistent and reproducible, which allows discrimination of different sulfate standards.

Sampling and analytical procedures: S compounds and minerals were obtained from chemical supply houses or mineral dealers. See papers cited below for details.

Instruments: X-ray fluorescence microscope located at beamline 2-ID-B at the Advanced Photon Source, Argonne National Laboratory. The beamline is optimized to examine samples over a 1–4 keV energy range using a focused X-ray beam with a spot size of approximately 60 nanometers squared . The energy was calibrated using an elemental sulfur standard. The whiteline energy of the elemental sulfur standard was aligned to 2472 eV.

Data processing: S-NEXFS data were normalized to create a relative intensity value of approximately 1 for post-edge area of the spectra. The data were also processed using a three-point smoothing algorithm built into the software package Athena to remove high-frequency noise (Ravel and Newville, 2005).