The investigation into the 1932 death of Phar Lap is still ongoing. The famous Australian racehorse died suspiciously while on tour in the United States and there is no shortage of theories. In a recent publication (DOI) Ivan Kempson and Dermot Henry report on an 2008 investigation of hairs from the horses preserved hide (kept at Museum Victoria) for arsenic: poisoning is one of those popular theories. A synchrotron X-ray fluorescence microprobe allowed them to detect the element for each 10 micrometer section of hair and a single intense band was indeed found. This shows an arsenic distribution and chemistry consistent with ingestion of a large dose of arsenic just prior to death according to the investigators and not some sort of chronic exposure to arsenic from herbicides in feed or performance-enhancing arsenic-based tonics. Even the arsenic formulations used in the taxidermy process are ruled out.
But what form of arsenic? Some additional work was done but nothing conclusive. XANES is another X-ray technique that was used in the investigation. It can discriminate between different forms of arsenic two of which were found: As(III) and As(V) arsenate. The As(III) fraction is bound to thiol possibly from the presence of pheomelanin (the horse was a chestnut) or from arsenic as ingested. In a hair cross-section As(III) was also found to build up in the cysteine-rich parts. Analysis gets even more complicated because As(V) can be rapidly photoreduced to As(III) during measurements.