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Ultrasonic energy, broadly defined as sound above the range of human hearing (approximately 20,000 cycles/s or 20 kHz), can be used in several ways during the production of cannabis to ensure that the product is of the highest quality. In addition to providing a tutorial on how ultrasonic equipment works, this article describes how the equipment is applied in laboratory practices and manufacturing. Examples include extraction procedures, cleaning ion targets in mass spectrometers, preparing oil emulsions for edible or drinkable products, degassing oil (removing trapped air) to maintain stable oil volume during selling, degassing high performance liquid chromatography (HPLC) solvents, bubbling off ethanol before oil distillation, and removing gums and waxes from glassware used in production.
Ultrasonic energy is commonly associated with ultrasonic cleaning. As explained by Edward W. Lamm in his article “The Development of Ultrasonic Cleaning” (1), its history dates to the early 1930s and work done at Radio Corporation of America (RCA) laboratories in New Jersey. The first practical applications, according to Lamm’s article, were introduced in the 1950s, and were operated at 18–40 kHz. “Up until the late 1980s most of the commercially available systems operated at 25–40 kHz,” Lamm stated.
Today, ultrasonic cleaners are available in several frequencies, including 25, 45, 80, and 130 kHz. Units are also available offering dual-frequency options.
Ultrasonic energy is used in research, product development, and manufacturing operations. Typically, these involve homogenizing, emulsifying, dispersing, dissolving or mixing difficult samples, and degassing liquids to remove trapped air.
Ultrasonic energy is a proven technique to achieve fast, safe extraction. For example, it is a method often specified in United States Pharmacopeia (USP) monographs to extract active pharmaceutical ingredients from carriers for content uniformity and potency assay tests.
In cannabis production, most regulated markets require all cannabis products to be tested for efficacy (active ingredients, such as cannabinoids and terpenoids), as well as for contaminants (such as pesticides, mycotoxins, heavy metals, microbes, and residual solvents).
Cannabis products include plant material (mostly flowers and trimmed leaves), concentrated extracted essential oils (concentrates, waxes, and oils), and infused products (edibles such as candies, chocolates, baked goods, transdermal patches, suppositories, and beverages).
Clearly, accurate test results depend on efficient, reproducible extraction from these often complex matrices, and sonication is one way many cannabis laboratories seek to achieve those goals. Sonication is valuable because it deposits energy into the solvent–matrix system, effectively speeding the process of extraction and dissolution.
Mass spectrometers are typically the workhorse instruments in a cannabis laboratory. Many laboratories have inductively coupled plasma–mass spectrometry (ICP-MS), liquid chromatography–tandem mass spectrometry (LC–MS/MS) and gas chromatography (GC)–MS/MS instruments all in the same laboratory.
Since cannabis contains viscous oils and resinous compounds of moderate to high molecular weight, ion sources and associated components can get contaminated with organic residues that are difficult to remove. In particular, contamination of electrodes that steer the ions leads to defocusing and loss of signal.
Sonication is often the most efficient method to clean these parts. Likewise, the chromatography injectors and inlets can also become contaminated and clogged with the resins and residues and sonication in a nonpolar solvent is often the method of choice to clean these components as well.
For cleaning applications, ultrasonic energy is used to remove contaminants from the surfaces of virtually any product that can be safely immersed in a water-based biodegradable ultrasonic cleaning solution. Cleaning solution formulas, dilution recommendations, and operating procedures are available for specific cleaning tasks.
There are multiple manufacturers of ultrasonic cleaning and processing equipment. Regardless of the manufacturer, common components include
A tank, usually stainless steel, to hold the cleaning solution or water that is typically mixed with a surfactant
Selection criteria then can move to
One might ask about the need for a more sophisticated unit. The answer is simple. It provides cannabis processors with the ability to develop and customize optimum processing steps to achieve consistent, high-quality product from a variety of sources.
When activated, the equipment’s generator powers the transducers to vibrate at their designed ultrasonic frequency. This vibration causes the tank bottom to vibrate as a membrane that produces countless microscopic vacuum bubbles.
In applications such as cleaning glassware, these bubbles implode with tremendous force in a process called cavitation. This cavitation quickly and safely blasts loose contaminants and carries away even the most tenacious residue. Products are cleaned with a solution formulation designed for the application.
In a processing application, products are contained in Erlenmeyer flasks, test tubes, or beakers. These containers are lowered, but not fully immersed, into a water or surfactant solution. Ultrasonic energy passes through the glass walls of the containers to act on the contents.
This approach achieves the homogenizing, emulsifying, degassing, and other cannabis processing steps in a fast, efficient, and environmentally friendly way.
In 2015, the National Hemp Association published an article in Hemp News titled “Five Major Types of Cannabis Extraction” (2). In the article, Rien Havens, PhD, CTO, Really Helping, PBC, stated that in the winter of 2014 he began research to develop the optimal methods of hemp extraction. “It was quite a ride. I had in mind three main goals,” said Havens. “No use of fossil fuels, low energy footprint, and cost effectiveness.”
Here, we paraphrase Havens’ findings as published in the article (2). Readers may wish to access the full article for additional details.
Ultrasonic cleaner tanks are available in multiple sizes in terms of length, width, and depth. When processing in flasks and beakers, a shallow-depth tank is a good choice with a length and width that allows the processing of several containers at once.
The following sections provide a more detailed illustration of how the process works. This example describes the use of a 37-kHz ultrasonic cleaner based on its tank configuration and operating features.
Remember that the transformation or extraction process avoids chemical degradation that can be caused by excessive heat or mechanically induced damage.
Extraction and Processing Steps
Product is placed in flasks along with a recommended solvent. Flasks are partially immersed in a sonicator bath containing a surfactant.
The tank configuration of the ultrasonic unit used in this process is especially designed to quickly and safely accomplish extraction and further processing. The inside dimensions of the shallow basket, 17.9 x 9.8 x 2.2 in. (LxWxH), facilitate positioning of multiple smaller containers or larger beakers. Flask clamps are used to affix flasks to the mesh-bottom basket; test tube holders are also available.
The equipment described was also selected because of its high ultrasonic power per unit volume. This feature permits the preparation process to be completed before heat buildup, a natural result of ultrasonic energy, which can degrade product. If heat is a concern, a useful accessory is a cooling coil to prevent temperature increase. The cooling coil must be attached to a source of recirculating cold liquid such as a laboratory chiller.
Another suggestion for producers is to look for an ultrasonic unit equipped with a microprocessor-controlled ultrasonic generator that adjusts to the load; a degas mode to remove trapped air, and a timer that displays set and remaining time.
Other useful features include the ability to operate in a fixed frequency (also called normal) mode that is ideal for breaking up product and a sweep mode that provides uniform distribution of ultrasonic energy when it is used to clean glassware and other processing equipment (see below). The sweep mode delivers a small positive and negative fluctuation in ultrasonic frequency throughout the bath.
An Extraction Sequence
Water and a surfactant are added to the fill line of the sonicator tank. The unit is turned on and the degas function is activated to both mix the solution and drive off trapped air. This step should take about 10 min.
The product is lowered into the bath and the unit is set to operate in the normal mode. The generator provides ultrasonic energy in the bath that passes through flask walls. This step mixes, disperses, emulsifies, homogenizes, and dissolves the samples. The unit will shut down at the end of the timed cycle.
Operators will soon develop their own “techniques” or “standard operating procedures” for their processing cycles.
Substantial investments may be made in cannabis processing glassware and other tools. Because of the nature of the process, difficult-to-remove deposits adhere to the inside of flasks, test tubes, and beakers. Overall, cleaning is also recommended to ensure a quality product.
Cleaning internal surfaces can be accomplished by filling the container with a suitable biodegradable cleaning solution and, as with the extraction and processing steps, placing the container in the water–surfactant solution and activating the degas mode and ultrasound. Cavitation passes through the glass walls to loosen and remove the strongly adhering residues. These residues are then discarded and the containers can be rinsed for further use.
Small instruments can be placed in the mesh tray. In this case, the water–surfactant solution is removed and replaced with a degassed biodegradable formulation designed for glassware. In this instance, cleaning should be accomplished using the sweep mode to provide more-uniform cleaning.
To thoroughly clean internal and external surfaces of processing equipment, a larger ultrasonic cleaner is required with a suitable depth to enable full immersion of the equipment.
Biodegradable concentrates for labware are available in acidic, basic, and neutral formulations depending on the nature of the contaminants to be removed. All of the formulations come with material safety data sheets and use instructions including dilution recommendations and cleaning temperatures.
The authors acknowledge with thanks the contributions of Donald P. Land, PhD, to this article. He is Chief Scientific Consultant for Steep Hill, a leading cannabis testing and research and development (R&D) company with locations across the world.
Bob Sandor is a director at Tovatech LLC. Kirsten Blake is Director of Business Development and Global Sales at Emerald Scientific, a distributor of cannabis-related scientific supplies including reference materials, chemicals, reagents, and equipment. Direct correspondence to: firstname.lastname@example.org and email@example.com.
B. Sandor and K. Blake, Cannabis Science and Technology1(2), 49-51 (2018).