I have searched the forum for guidance on the use of isotopically labeled internal standards (ISTDs) during sample preparation, but have not found a dedicated discussion on this topic.
If you have experience working with isotopically labeled ISTDs, I would greatly appreciate it if you could share your insights. What key factors should be considered when selecting a suitable ISTD? I have heard that 15N-labeled standards are generally preferred, although I am uncertain as to the specific reasons. Additionally, I came across a publication indicating that deuterium-labeled ISTDs may be susceptible to deuterium-hydrogen exchange under certain conditions.
Yes, typically chromatographically optimal compared to deuteration and requiring less rethinking of synthesis as NaNO2-15N can be used instead of NaNO2.
If nitrosamine impurity standards are synthesized using NaNO2-15N, standards will have one mass unit difference compared to the actual impurities present in the drug substance or drug product.
Thank you for your answers!
Just for clarity - from your experience, would you advise against using deuterated ISTDs and only use an ISTD where either the carbon(s) or nitrogen(s) is/are the isoptopes?
Deuterated internal standards are very commonly used. Hydrogen-deuterium exchange can happen only if the deuterium is attached to a hetero atom. If the hydrogen substituted by deuterium are attached to the carbon atom then there should be no issues.
The deuterated standard should be ideally be same as the nitrosamine except that some of the hydrogens can be replaced by deuterium. Hence the physicochemical properties would be similar but the molecular weight will be different and hence can be separated on your LCMS or GCMS system using any mass based scan or MRM modes.