Liquid chromatography/MS–grade methanol was purchased from VWR (Randor, PA, USA). The following reagents were purchased from Sigma-Aldrich Corp. (St. Louis, MO, USA): ion chromatography–grade heptafluorobutyric acid (HFBA), analytic-grade 6N hydrochloric acid (HCl), sodium borohydride (NaBH4), sodium hydroxide, HPLC-grade butanol, HPLC-grade 1-propanol, sodium acetate trihydrate, citric acid monohydrate, glacial acetic acid (AcOH), Chloramine-T, dimethylaminobenzaldehyde, ACS-grade perchloric acid, HPLC-grade acetonitrile (ACN), and L-hydroxyproline (OH-pro). Necessary standards for collagen cross-link analysis were obtained from various sources. Dihydroxylysinonorleucine (DHLNL) and LNL were purchased from Santa Cruz Biotechnology, Inc. (Dallas, TX, USA). Pyridinoline (PYD)/deoxypyridinoline (DPYD) HPLC calibrator was purchased from Quidel Corp. (San Diego, CA, USA) and Pent was purchased from Cayman Chemical (Ann Arbor, MI, USA). The elastin cross-links desmosine (DES) and isodesmosine (IDE) were also commercially available from MP Biomedicals (Santa Ana, CA, USA). We also obtained as a kind gift from Simon P. Robins (University of Aberdeen, Aberdeen, Scotland, UK), the major trifunctional corneal cross-link HHL as well as the difunctional cross-link hydroxylysinonorleucine (HLNL).