Figure 1. Comparison between calcium, sulfur, and nitrogen peaks as observed by nanoscale secondary ion mass spectrometry.

(A) Nanoscale secondary ion mass spectrometry is a chemical-imaging technique which uses an ion source (Cs⁺ or O⁻) to produce a charged primary ion beam. When the beam strikes the surface of the sample, secondary ions of the opposite charge are created and measured. To analyze both ¹⁴N and ⁴⁰Ca in our samples, it was necessary to first analyze the samples using the Cs⁺ source. Once all samples were analyzed using Cs⁺, the instrument was switched over to a radio-frequency RF source, and ⁴⁰Ca was measured in a second run. (B) The intensity of the ⁴⁰Ca peak at various points from each image is compared with the intensity of the ¹⁴N peak at the same location. The data are divided into two groups: a Ca²⁺-binding group (⁴⁰Ca > 2, N = 104, P = 0.0123), and a non-Ca²⁺-binding group (⁴⁰Ca < 2, N = 462, P = 1.68 × 10⁻⁹). (C) The intensity of the ⁴⁰Ca peak at various points from each image is compared with the intensity of the ³²S peak at the same location. The data are divided into two groups: a Ca²⁺-binding group (⁴⁰Ca > 2, N = 104, P = 0.0001), and a non-Ca²⁺-binding group (⁴⁰Ca < 2, N = 462, P = 0.0317).

Source data are available for this figure.