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No pressure! Low-Pressure Gas Chromatography Analysis of Cannabis Terpenes

Terpenes are responsible for the characteristic aromas and flavors of different natural and consumer products. They produce a wide range of scents, from citrusy and fruity to earthy and piney. Terpenes can be extracted from plant materials and used in various products, including essential oils, aromatherapy, and even as flavor additives in food and beverages. They are also an important part of cannabis' aroma and flavor.

The specific terpene profile of a cannabis strain is influenced by its genetics and growing conditions. Different strains can have vastly different terpene profiles. In the cannabis industry, terpene testing is often conducted alongside cannabinoid testing to provide consumers with information about a strain's terpene content.

In the analysis of cannabis terpenes, utilizing a conventional GC-MS approach is a commonplace technique. However, analysts can harness a powerful technique to reduce run times, increase sample throughput, and reduce lab costs: low-pressure gas chromatography (LPGC). LPGC is a technique that employs sub-ambient pressure conditions to create a vacuum system, while using a short, narrow restrictor column at the injection port, connected by a union to the actual analytical column. Generally, the analytical column will be 0.32 mm or wider. In this case, the analytical column of choice was the Rxi-624Sil MS column. This column is a great phase choice for this analysis because it is low-bleed, selective, and efficient.

The speed of analysis and increased sample volume are the major benefits of LPGC. Peak shapes are comparable to conventional GC-MS, with LPGC peak heights often being higher. The principal drawback of using LPGC is decreased separation efficiency. This can be problematic in the case of closely eluting isobars (compounds that cannot be resolved using MS spectra).

 

The conventional GC-MS run takes about 16 minutes with the latest compound eluting around 15 minutes. With LPGC, analysis time drops to 11 minutes. An analyst can note by using this method, there is an opportunity for great time and helium savings (as the method is shorter and operating under lower flow conditions.)

LPGC provided excellent peaks for those compounds that were fully resolved. Peaks were taller than those on the conventional chromatogram comparison. Of the compounds in the standard, beta-pinene, beta-myrcene, D-limonene, p-cymene, guaiol, and caryophyllene oxide were the compounds which suffered the most from the lack of separation efficiency. Beta-pinene and beta-myrcene coelution is the most concerning due to being isobars and very abundant in cannabis samples. Ocimene 1 and 2 were not detected in the initial run, however an analyst may identify their presence through mass spectrometry confirmation. Ocimene 1 and 2 seem to coelute with D-limonene and eucalyptol, respectively.

Because of the nature of terpenes in cannabis analysis, a higher chromatographic efficiency is paramount to high-quality results. As part of the same class of compounds, the spectra of terpenes can mimic each other very closely. An analyst could be faced with issues differentiating and quantitating compounds even after extracting chromatogram ions. LPGC is great for fast analysis. Analysts may find that LPGC is a great screening tool for cannabis samples.

Below is a comparison of conventional GC-MS analysis conditions of our cannabis terpenes standards with the LPGC analysis conditions.

LPGC Method – 11 min
Agilent 7890A GC
Oven
°C/min Hold (°C) Hold (min)
  55 1
30 130 3
60 280 2
Inlet (Split/Splitless)
Liner Topaz, Single Taper, 4.0 mm x 6.5 x 78.5 (Catalog #: 23303)
Temperature 280°C
Mode Split
Split Ratio 5:1
Analytical Column
Column LPGC Rxi-624Sil MS (Catalog #: 11804), includes Rxi-624Sil MS, 10 m, 0.32 mm ID, 1.8 μm w/5 m, 0.15 mm ID Rxi restrictor connected via SilTite connector
Mode Constant Flow
Flow 0.9 mL/min
Transfer Line Temperature 310°C
Agilent 5975C MSD
Acquisition Mode Scan
Gain Factor 1
Scan Parameters
Source Temperature 330
Quad Temperature 180
Solvent Delay 1.50 min
Mass Range 35-280 amu
Scan Speed 1,562 [N=2]
Chemicals and Reagents
Analyte Cannabis Terpenes Standard #1 (Catalog #: 34095)
Cannabis Terpenes Standard #2 (Catalog #: 34096)
Source Restek
Solvent (Standard Preparation) Isopropanol
Sample Concentration 25 µg/mL (5 µg/mL on column)

 

Conventional GCMS Method – 16 min
Agilent 7890A GC
Oven
°C/min Hold (°C) Hold (min)
  55 0
25 130 6
30 280 2
Inlet (Split/Splitless)
Liner Topaz, Single Taper, 4.0 mm x 6.5 x 78.5 (Catalog #: 23303)
Temperature 280°C
Mode Split
Split Ratio 5:1
Analytical Column
Column Rxi-624Sil 30m x 0.25mmID x 1.4µm (Catalog #: 13868)
Mode Constant Flow
Flow 1.5 mL/min
Transfer Line Temperature 310°C
Agilent 5975C MSD
Acquisition Mode Scan
Gain Factor 1
Scan Parameters
Source Temperature 330
Quad Temperature 180
Solvent Delay 5.00 min
Mass Range 35-280 amu
Scan Speed 1,562 [N=2]
Chemicals and Reagents
Analyte Cannabis Terpenes Standard #1 (Catalog #: 34095)
Cannabis Terpenes Standard #2 (Catalog #: 34096)
Source Restek
Solvent (Standard Preparation) Isopropanol
Sample Concentration 25 µg/mL (5 µg/mL on column)