Recently, several direct and/or ambient mass spectrometry (MS) approaches have been suggested for drugs of abuse imaging in hair. The use of mass spectrometers with insufficient selectivity could result in false-positive measurements due to isobaric interferences. Different mass analyzers have been evaluated regarding their selectivity and sensitivity for the detection of Δ9-tetrahydrocannabinol (THC) from intact hair samples using direct analysis in real time (DART) ionization.
Four different mass analyzers namely 1) an orbitrap, 2) a quadrupole orbitrap, 3) a triple quadrupole, and 4) a quadrupole time-of-flight (QTOF) were evaluated. Selectivity and sensitivity were assessed by analyzing secondary THC standard dilutions on stainless steel mesh screens and blank hair samples, and by the analysis of authentic cannabis user hair samples. Additionally, separation of isobaric ions by use of travelling wave ion mobility (TWIM) was investigated.
The use of a triple quadrupole instrument resulted in the highest sensitivity, however, transitions used for multiple reaction monitoring were only found to be specific when using high mass resolution product ion measurements. A mass resolution of at least 30,000 FWHM at m/z 315 was necessary to avoid overlap of THC with isobaric ions originating from the hair matrix. Even though selectivity was enhanced by use of TWIM, the QTOF instrument in resolution mode could not indisputably differentiate THC from endogenous isobaric ions in drug user hair samples.
Only the high resolution of the (quadrupole) orbitrap instruments and the QTOF instrument in high resolution mode distinguished THC in hair samples from endogenous isobaric interferences. As expected, enhanced selectivity compromises sensitivity and THC was only detectable in hair from heavy users.
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