High energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) can give dramatically improved spectroscopic resolution relative to conventional X-ray absorption spectroscopy (XAS), significantly improving speciation sensitivity for complex samples . Conventional XAS exhibits significant broadening due to the lifetime of the core hole arising from the primary photoexcitation event. HERFD-XAS dramatically reduces lifetime broadening by measuring the X-ray fluorescence with an energy resolution much better than the fluorescence natural linewidth, effectively probing a sub-set of the states that normally give rise to the total fluorescence yield. The remaining contributions to the observed resolution mostly derive from the lifetime of the outer core hole vacated by the fluorescent photon and beamline factors.