Table 3. Elemental ratios (at.%) for C, N, O components measured by XPS and theoretical values.

	C/N	C/O	O/N
Non-calcified region 1	5.8 ± 1.1	2.2 ± 0.4	2.6 ± 0.1
Mineralized region 1	8.9 ± 1.7	3.3 ± 0.7	2.7 ± 0.3
Global average for natural amino acids 2	4.3 ± 2.1	2.4 ± 1.2	2.1 ± 1.0

¹Means ± 95% CI; N = 3

²Means ± SD.