Most of the methods on the net are on ion chromatography. It does not give reproducible results, some are GC methods but don't work. If you know any authentic method please share it.
It can be done by static headspace GC (FID or MS), using if possible a column specially designed for amines. The "trick" is to alkalinise your solution using a tertiary amine that deactivates the system without causing chromatographic interference: N-methylmorpholine has been used for that; pyridine is also available very pure. If you have a choice of injector, best avoid the complicated type that has a 6-port valve with a loop.
Concerning the choice of solvent among those approved or not by the pharmacopeias, an observation that didn't get published is that an impurity (degradation product) of DMI* reacts (or catalyses a reaction) with diethylamine and pyrrolidine to form the tertiary amines, which have much better chromatographic properties. I don't know whether one could rely on that for a regulatory method!
That lead me to consider using (deliberately) reductive alkylation in a reaction headspace method, because trace GC analyses of secondary amines can be challenging in a busy lab, whereas tertiary amines don't usually need a special column. Reductive alkylation can't lead to the formation of (involatile) quaternary amines.
We had previously developed a reaction headspace method for some alkylating agents in drug substances and were impressed by the simplicity and reliably of the approach, particularly when a method is needed only intermittently.
Finally, I'm not up to date on this aspect of genotoxic impurities, but guess you could find enough information on the occurrence of these simple secondary amines as natural metabolites to set a limit higher than the default 1.5µg/d.
* DMI: 1,3-dimethyl-2-imidazolidinone. A freshly vacuum distilled cut didn't react, but some fractions did.
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