In Situ Fluorescence Spectroscopy for In-Line Distillation Process Monitoring

October 24, 2019
Abstract / Synopsis: 

Currently, cannabis distillation requires specialized personnel, which raises costs and lowers yields. Furthermore, process monitoring is dependent on indirect controls, such as temperature, flow, and color. Here, fluorescence spectroscopy was investigated as an in-line process monitoring tool for cannabis distillation to alleviate these challenges. First, excitation emission matrix spectroscopy (EEMS) was utilized to determine optimal excitation wavelengths for various stages of fractional distillation. Based on these results, a benchtop fluorometer that could use various excitation wavelengths was developed. Samples of extract, distillate, and pure laboratory grade cannabidiol (CBD), cannabidiolic acid (CBDA), delta-8 tetrahydrocannabinol (∆8-THC), and delta-9-tetrahydrocannabinol (∆9-THC) standards were measured with the benchtop system. The measurements from the extract and distillate samples exhibited several fluorescence peaks. These measurements depended on the processing conditions and product quality of the tested samples. Measurements of the chemical standards exhibited similar fluorescence to the extract and distillate samples. Finally, an in-line sensor was developed and installed on a short path distillation system (SPD). Measurement from the in-line sensor showed distinct differences between distillation fractions validating its capability as a cannabis distillation process monitoring tool.

The rising commercial interest in cannabis extract and distillate is increasing the need for more rapid and precise extraction and distillation methods. This need is especially critical for more precise dosing of compounds derived from cannabis for medical applications. While many common distillation methods exist (short path distillation, wiped film distillation, column separation, and so on) the techniques are highly technical and often can only be carried out by specialized personnel. This leads to lower production volumes and higher costs. To help reduce costs and increase purity of cannabis distillation, more precise and intuitive process control tools are needed.

Historically, fluorescence spectroscopy has been used for inline process control and quality control in several industries, including pharmaceuticals and food safety (1–8). Furthermore, studies on cannabinoids and their metabolites indicate that many of the compounds derived from cannabis will have unique spectroscopic properties, including fluorescing under ultraviolet (UV) light and Raman signal (9–15). While some literature exists, very little work has been published on using these unique optical properties to provide a process control system that can help improve product safety and purity.

Distillations of cannabis extract are carried out at temperatures reaching over 165 °C and under vacuum pressure. Vacuum distillation is utilized because desired cannabinoids chemically degrade into undesired compounds at temperatures below their boiling point under atmospheric pressure. This degradation is either decelerated greatly or completely halted under vacuum pressure.

The required vacuum pressure and temperature make the process of selecting and adding an in-line sensor challenging. The sensor must be robust enough to function under harsh conditions without creating undue risk of vacuum leaks during operation. This challenge is only exacerbated when considering a sensor that can be retrofitted onto existing distillation systems. Optical metrology methods are a promising approach for process control because they can probe processed material through a sight-glass or glass tube positioned away from the heat source. Specifically, fluorescence spectroscopy is a promising method to investigate the presence or absence of auto-fluorescing compounds within the distillate throughout the process.

In this work, fluorescence spectroscopy is investigated as a process monitoring technique for short path distillation (SPD).


Excitation Emission Matrix Spectroscopy

A portable benchtop excitation emission matrix spectrometer (EEMS) was developed to determine the optimal excitation wavelength that could be used for monitoring fluorescence during the distillation of cannabinoids. Figure 1 shows the EEMS system used to investigate the approach.

Figure 1

For the excitation components of the instrument, the EEMS system used a Lambda LS Xenon Arc Lamp (Sutter Instrument) with a SPG-120-REV monochromator (Shimadzu). The sample holder was designed for a 1 cm x 1 cm quartz optical vial and was printed with a 3D printer (Zortrex M200). A USB 2000+ ultraviolet-visible (UV-vis) spectrometer (Ocean Optics) was set at a 90° angle with respect to the monochromated light source to collect fluorescence emission from the sample, and a STS-UV spectrometer (Ocean Optics) was placed directly across from the monochromated light source to measure the optical absorption of the sample.

For the measurements, 1 mL samples were collected from SPD of cannabinoids at three processing stages, colloquially referred to as heads, bodies, and tails. EEMS measurements were taken by first setting the monochromator to a specific wavelength. The resulting spectrum was then measured for both the fluorescence and absorption geometries. Next, the excitation wavelength was incrementally increased by 10 nm, and the process was repeated for excitation wavelengths from 300 nm to 800 nm.

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Jonathan Kenneth Bunn is the Chief Scientist at Arometrix, Inc., in Rockville, Maryland. Christopher Jason Metting is the Chief Technology Officer at Arometrix, Inc. Hasso von Bredow is the Chief Mechanical Engineer at Arometrix, Inc. Direct correspondence to: [email protected]

How to Cite This Article

J.K. Bunn, C.J. Metting, and H. von Bredow, Cannabis Science and Technology 2(5), 38-46 (2019).