Selective Detection of Oxygenates, Including Cannabis Analytes

Over several decades, DETector Engineering and Technology, Inc. (DET, Walnut Creek, CA) has explored surface ionization as an element-specific detection technology for gas chromatography. Column effluent passes over a heated ceramic bead that produces surface ions, which are detected electronically. Often the ionization process is catalytic, which can increase the current and hence signal.

Early models focused on detection of nitrogen and phosphorous compounds. Recently, DET discovered that, by changing the detector gases and increasing the polarization voltage to –45 V or higher, the detector becomes very selective for oxygen containing hydrocarbons (oxygenates). This is significant, since oxygen is the most common heteroatom in organic chemistry. Nitrogen is probably the second, but nitrogen detection is decades old.

The firm just issued DET Report #76 (July 2017), which describes the conditions for the selective detection of oxygenates, with a particular emphasis on cannabinoids.

Figure 1 shows the structure of cannabidiol (CBD) and Δ-9-tetrahydrocannabinol (THC). Note that all contain two oxygen atoms, which renders them suitable for detection with thermionic beads.

Figure 1 –Structure of cannabidiol (left) and Δ-9-tetrahydrocannabinol (right). (Figures 1–3 reproduced with permission from DETector Engineering and Technology, Inc.)

Detection of the three cannabis analytes separated by GC using a flame ionization detector (top) and TID-10 (bottom) is shown in Figure 2. Note the methanol peak is reduced in the chromatogram for the TID-10 compared to the FID chromatogram.

Figure 2 – Three cannabis analytes separated by GC using a flame ionization detector (top) and TID-10 (bottom).

Increasing the polarization voltage on the bead to –90 V (Figure 3) increases the signal-to-noise response by 20-fold compared to the TID-10 in Figure 2.

Figure 3 – Increasing the polarization voltage on the bead to –90 V increases the signal-to-noise response by 20-fold compared to the TID-10 in Figure 2.

The take-home message is that one can improve gas chromatography with selective detectors other than mass spectrometers. TID-10 is much less expensive and more robust than mass spectrometers.

Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; email:  [email protected].

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