4.3.1. SIL-Stock and Working Solution Preparation

Stock solutions were prepared from the independent weight of compounds and stored at −<sup>20</sup> ◦C. d6-KYN and 13C6-AA were prepared in H2O/DMSO (1/1, *<sup>v</sup>*/*v*) at 1.5 and 5.0 mg/mL, respectively. 15N-13C2-OH-KYN was prepared in H2O/DMSO (1/19, *v*/*v*) at

2.0 mg/mL. 13C15N2-8-OH-dG, 15N4-Hyp, d4-L-Tyr, and 15N4-I were prepared in water at 1.0 mg/mL.

Internal Standard Working Solutions (IS-WS) were prepared by adding appropriate volumes of the stock solutions to 50 mL of ultrapure H2O (ISWS-A) and ACN (ISWS-B) to reach a final concentration of 200 ng/mL for all the standards. The solutions were maintained at 4 ◦C and freshly prepared every week.
