Greater THC Sensitivity Did Not Guarantee Hair-Matrix Selectivity in Ambient Mass Spectrometry

A mass spectrometer’s ability to detect small amounts of THC did not guarantee that it could distinguish THC from hair-matrix interference. An ambient-ionization study found that sensitivity and selectivity imposed different demands on the analytical system.

The comparison tested intact hair

Duvivier and colleagues compared four mass-analyzer configurations using direct analysis in real time, or DART, for forensic hair analysis. The objective was to assess performance in the hair matrix, rather than assume that a strong THC signal in a clean standard settled identification.

The triple-quadrupole configuration offered the highest sensitivity, but the monitored transitions required high-resolution product-ion information to achieve the necessary specificity in this application.

Greater resolution addressed a different problem

The investigators reported a need for resolving power of at least 30,000 at the relevant THC mass to avoid an isobaric matrix interference in the studied configuration. That is a finding about this DART application, not a universal requirement for every THC assay.

Ion mobility improved selectivity in another configuration, yet the authors still could not indisputably distinguish THC in authentic hair using that setup. The high-resolution approach also involved a sensitivity tradeoff, with detection demonstrated in heavy users.

What a strong signal cannot answer

A larger peak can improve detection without resolving whether another compound contributes to the signal. Authentic negative hair and matrix-specific interference testing therefore address a question that a neat reference standard cannot answer alone.

The study does not show that every hair THC result is false. It shows why the instrument, preparation and identification criteria matter. A correctly identified hair result also remains a different endpoint from recent intoxication or impairment at a particular event.

Related: How the reference specimen changes a cannabis accuracy claim.

Research cited

Duvivier WF; van Beek TA; Nielen MWF; Rapid Communications in Mass Spectrometry; 2016. Critical comparison of mass analyzers for forensic hair analysis by ambient ionization mass spectrometry.