Detection limits of calcium by EELS, TEM-EDX and X-ray absorption spectroscopy for biomineralization studies

To investigate the mechanisms of biomineral formation it is necessary to determine Ca concentrations in the vicinity of growing minerals at the highest spatial resolution. All techniques that use ionizing radiation will be limited by the onset of radiation damage. Limits to the minimum concentrations of Ca and minimum number of Ca atoms that can be detected are determined for EELS in TEM using the Ca L₂₃ signal, and EDX in TEM using the Ca Kα line. A similar approach is used to set limits on what can be achieved via soft X-ray absorption spectroscopy detecting the Ca L₂₃ features. Due to the large background, the lowest concentrations that can be detected via EELS are about 1 mM. Although the collection efficiency for EDX is inferior to that of EELS, the much lower background signal means that concentrations of about 0.05 mM could be detected, 20 times better than EELS. The spatial resolution of soft X-ray absorption spectroscopy is much lower, about 20 nm, but since the Ca L₂₃ lines are at lower energies than the oxygen K edge that dominates the spectrum, the detection limits for Ca in an aqueous environment can be as low as 35 nM.