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Cannabis Analysis Cannabis Under (Moisture) Control

From Jeffery Gast*, Adam Darling*, Lloyd Allen*, Michael Jakob** |

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The determination of moisture in cured cannabis is an indicator of microbial contamination, a key variable used to calculate other constituent results on a dry basis and required for the pressing process of extracting CBD oil from cannabis. Thermogravimetric moisture determination can provide valuable services here.

1 Cannabidiol (CBD) is a non-psychoactive cannabinoid from female hemp (cannabis)
1 Cannabidiol (CBD) is a non-psychoactive cannabinoid from female hemp (cannabis)
(Source: ©cendeced -

Moisture determination in plant and food products plays a vital role as a basic parameter for the analysis in a modern laboratory. As an example, the determination of moisture in cured cannabis is discussed here. Besides being an indicator of microbial contamination, it is a key variable used to calculate other constituent results on a dry basis. Additionally, determination of moisture content of cannabis is also required for the pressing process of extracting CBD oil from cannabis. Measurement of moisture using a gravimetric oven drying technique is difficult due to the influence of constituent properties of cannabis including, but not limited to, terpenes, cannabidiols, and THC, which may have low temperature volatility or sublimation.

According to many standards, the moisture content in plants is determined as LOD (Loss on drying) which is performed at temperatures of 105° C. There is a threat that oven drying of plant matter may result in thermal decomposition of the plant tissue resulting in a false high value for moisture content when higher temperatures (such as 105 °C) are used. Importantly a false high value for moisture determination will yield a false high value for any subsequent analytical determination. As a result, it is very important that the proper temperature for moisture determination is selected. In addition, LOD incorporates all the components lost for a given set of conditions, including moisture and the volatile organic compounds (VOCs) that are the result of thermal decomposition or sublimation.

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Thermogravimetric analysis

Thermogravimetric analysis (TGA), also known as thermogravimetry, is a method of thermal analysis in which the change in mass of a sample is measured as a function of temperature and time. For this purpose, the sample is heated in a small crucible made of temperature-stable and inert material (e.g. platinum or aluminum oxide) in an oven to high tempera­tures. The sample holder is often coupled to a balance that registers the changes in mass during the heating process. A thermocouple near the crucible measures the temperature. Modern TGA devices allow the final temperature, heating rate, gas flow or similar to be set via a connected computer.

Moisture Determination

The TGM800 is a thermogravimetric moisture determinator designed to indirectly determine moisture content of materials using a mass loss-on-drying technique. Mass loss of the sample is measured as a function of the oven temperature by controlling the atmosphere and ventilation rate. The instrument consists of a computer, an integrated four-place balance, and a multiple sample oven that allows up to 16 samples to be analyzed simultaneously with a maximum temperature of 175 °C.

Table 1: Moisture and LOD results with Leco TGM800 at 80 °C and at 105 °C respectively.
Table 1: Moisture and LOD results with Leco TGM800 at 80 °C and at 105 °C respectively.
(Source: Leco)

Some key features:

  • Supports 1.5 in diameter aluminum foil crucibles.
  • Supports 2.4 in diameter aluminum foil crucibles.
  • Analysis can be performed in air or nitrogen atmosphere.
  • Automated crucible and sample mass recording
  • Optional fixed time or constancy method parameters.

Moisture determination in cannabis can be a critical factor in expressing accurate analytical results for other constituents on a dry basis. However, due to the presence of low temperature VOCs, obtaining a true moisture value becomes more complicated. Utilizing a special water/carbon determinator (Leco RC612) water can be determined directly and simultaneously with carbon allowing for the comparison of moisture and evolved VOCs. The Leco RC612 is able to perform the analysis at multiple temperatures, starting at a lower temperature and ramping to a higher temperature. This allows for the determination of the proper temperature to optimize moisture determination and minimize VOC evolution.

The data presented in Figure 3 illustrates that the evolution of carbon attributed to VOCs at 80 °C is minimal compared to the carbon at 105 °C for both recreational cannabis and hemp. The water peak at 105 °C is attributed to hydrogen in the VOCs combusting to form water as the water peak coincides with the carbon peak. These plots indicate that the determination of moisture at 80 °C represents a closer approximation to the true moisture content of cannabis.

The dry (cured) cannabis samples were ground using a hand tobacco grinder, and a knife mill was utilized for the wet (uncured) samples. The wet samples were prepared immediately prior to analysis to minimize moisture loss during sample preparation. The following table display the results from the Leco TGM800 determining moisture at 80 °C and 105 °C until constant mass is obtained.


From the methodology and subsequent information obtained for the analysis of cannabis on the Leco RC612, a precise, close approximation of the true moisture in cannabis flower and plant tissue may be obtained using thermogravimetric analysis. Analysis of cannabis above 80 °C results in the evolution and decomposition of VOCs contributing to a high moisture bias as shown in the data table above. The Leco TGM800 is able to analyze up to 16 samples simultaneously, resulting in high throughput with accurate and repeatable results. Furthermore, utilizing the TGM800 mass constancy option the analysis time is reduced from five hours, as stated in standard methods, to approximately two hours.

The purpose of this study was to demonstrate that moisture determination in plant material can be performed using a modern thermogravimetric determinator, such as the Leco TGM800.

* J. Gast*, A. Darling*, L. Allen: LECO Corporation, Saint Joseph, Michigan/USA ** M. Jakob: LECO Europe, 41069 Mönchengladbach/Germany


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