When a drug is present at low concentration, the way a mass spectrometer produces ions can affect how readily the lab can measure it. A 1980 study compared positive and negative methane chemical ionization for several benzodiazepines and applied the negative-ion approach to nordiazepam.
For some of the compounds, the negative ion formed by loss of hydrogen gave a signal about 20 times as intense as the positive protonated ion. The researchers used selected ions from nordiazepam and an isotopically labelled internal standard to build a gas chromatography–mass spectrometry assay for rat blood and brain.
The authors described a sensitivity of 1 ng or less of nordiazepam in 1 mL of rat blood or 1 g of rat brain. That performance belongs to the method and matrices tested. A laboratory reporting human results would need to establish performance in its own specimen type and distinguish detection from validated quantification.
Garland and Miwa, Environmental Health Perspectives · Full text