The same molecule has two correct masses: molar and monoisotopic. Where the difference comes from and when it starts to matter.
Articles on methods for identifying chemical reagents and assessing their purity: HPLC, NMR, mass spectrometry, CAS numbers and the certificate of analysis (CoA). What each method measures, its limitations, and how to read its result.
A CAS Registry Number has a check digit you can verify on paper. What it identifies unambiguously, and what it does not tell you at all.
A chromatogram answers the question “how much,” a proton spectrum answers the question “what.” Why NMR is proof of identity, while retention time is only circumstantial evidence.
An HPLC purity result rests on three silent assumptions. Each of them can fail — and always in the direction of overestimation.
A certificate of analysis is a record of the measurements someone chose to perform. Which fields are most often left blank, and why those ones in particular.
Declared purity is a measurement result, not a property of the material. Two valid methods can give different numbers for the same sample.
