Cannabis: In Situ Fluorescence Spectroscopy for In-Line Distillation Process Monitoring

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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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Benchtop Fluorescence Spectrometer

After the EEMS system was used to determine the best wavelength for excitation, a benchtop fluorometer system (Arometrix) was used to measure fluorescence at a single excitation wavelength. Several configurations of the benchtop system were used to test the fluorescence response. Light sources at 340 nm, 365 nm, and 405 nm were used for the excitation wavelength and a spectrometer with a range from 340 nm to 780 nm was used as the detector. The device was enclosed in a housing that was dyed black to reduce any background fluorescence or reflections from the polymer housing of the sensor. Furthermore, the enclosure was designed to ensure that stray light from external sources did not enter the detector. Various optical edge filters were used in front of the detector to reduce the total excitation light intensity from reaching the detector and maximize the measured signal from the fluorescence.

The benchtop system was used to measure the fluorescence response of cannabis extract (NorCal Labs), distillate (NorCal Labs), and pure cannabinoids: ∆8-tetrahydrocannabinol (∆8-THC), ∆9-tetrahydrocannabinol (∆9-THC), and cannabidiol (CBD) (Cayman Chemical) as well as cannabidiolic acid (CBDA) (Sigma-Aldrich). The extract samples were synthesized from either hydrocarbon or ethanol extraction, and distillate samples were all synthesized via short path distillation. The extract and distillate samples that were tested in this work were not characterized with high performance liquid chromatography (HPLC), but the exact composition of these samples is not needed for the measurements performed in this study. These samples were tested to broadly investigate the diversity of the fluorescence response from different quality extract and distillate samples and to investigate any overall trends in these responses with respect to sample type. More rigorous studies on composition and fluorescence will be the focus of future work. Standard samples were 1 mg/mL solution of ∆8-THC in methanol, 1 mg/mL solution of ∆9-THC in methanol, 1 mg/mL solution CBDA in acetonitrile, and a 10 mg/mL solution of CBD in methanol.

For the extract and distillate, only the 365 nm excitation wavelength with no filter was investigated. The samples were diluted in ethanol at approximately 10:1 ratio of ethanol to product. The sample was then placed in a 1 cm x 1 cm polystyrene cuvette and the fluorescence measured.

For the HPLC standards, 2 mL ∆8-THC solution, 2 mL ∆9-THC solution, 1 mL CBDA solution, and 0.5 mL CBD solution were measured with the benchtop system. Three excitation wavelengths (340 nm, 365 nm, and 405 nm) were studied with 370 nm, 380 nm, 390 nm, and 420 nm edge filters (Edmond Scientific) as well as polycarbonate edge filters at thickness of 1/16-in., 3/32-in., and 5/64-in. (McMaster-Carr).

In-Line Fluorescence Spectrometer

An in-line configuration of the benchtop fluorometer was fabricated utilizing the same components from the original system but with a new form factor. Figure 2 shows a simplified schematic of an SPD system along the location of the fluorometer. During the distillation, approximately 3 L of hydrocarbon-extracted crude was distilled. The crude material was winterized and decarboxylated prior to the distillation. The device was placed on the tube directly after the condenser and immediately prior to the collection flask. The spectrometer detector was placed directly at the bottom of the tube and the light source was placed at a 90° angle from the spectrometer and aligned to the bottom of the tube. The alignment and relative placement of the light source to the spectrometer was designed such that the sensor was able to detect fluorescence at very low fill levels, which is a common case during SPD.

Figure 2


For acquisition, measurements were taken every 0.5 s. During each measurement two separate spectra were measured. The first spectrum was taken with no excitation light. The second spectrum was taken with excitation light. The spectrum taken without excitation light was used as a background to account for the signal from external light sources (overhead lighting, stray light, and so forth). The process was monitored during a complete SPD process.

Results and Discussion

Excitation Emission Matrix Spectroscopy

Figure 3 shows EEMS results from different fractions of an SPD process for the separation of ∆9-THC. The y-axis is the excitation wavelength used to induce fluorescence in the sample. The x-axis is the resulting emission spectra. Figures 3a and 3b show an EEMS spectra of a sample that was collected during the separation of high vapor pressure compounds, colloquially referred to as “heads.” The heads typically comprise latent solvents and a mixture of terpenes and degraded terpenes. Figure 3c shows an EEMS spectra of a sample that was collected from the separation of moderate vapor pressure compounds. This sample was taken under conditions that are ideal for the separation of cannabinoids, such as ∆9-THC or CBD, and is colloquially referred to as the “main body” or the “bodies” of the distillation. Figure 3d shows an EEMS spectra of a sample that was collected from the lowest vapor pressure compounds. This fraction, which generally consists of heavy waxes, paraffins, and colorants, is colloquially referred to as the “tails.” For the sake of clarity and brevity the samples from these measurements will be referred to as the corresponding distillation fraction, that is, “heads”, “bodies”, and “tails,” that the sample was collected from.

Figure 3


The heads EEMS spectra in Figure 3a shows two general regimes with fluorescent response, denoted as R1 and R2 in Figure 3. R1 appears between 350–450 nm with an excitation wavelength between 340 nm and 390 nm. R2 occurs between 450–540 nm with an excitation wavelength between 330 nm and 480 nm. A closer inspection of R2 shows that the region is made up of at least two separate fluorescent responses, as indicated by the multiple local maxima intensity value located in this region. These fluorescence responses are indicated with F1 and F2 in Figure 3b, with F1 centered at an excitation wavelength of 360–370 nm and a fluorescence wavelength of 470–490 nm and F2 centered at excitation source of 420–430 nm with a response at 500 nm. The fluorescence response in this sample was later identified to be due to the presence of both residual tails in the SPD system from the previous distillation as well as the beginning of the bodies fraction. A more detailed discussion of this analysis will be given in the section titled “In-line Fluorescence Spectrometer.”

The bodies EEMS spectra in Figure 3b shows that the shape and relative intensities of R1 and R2 between the heads and bodies samples were significantly different. In the bodies, R1 is induced by excitation wavelengths between 300 nm and 390 nm, extending 40 nm below R1 in the heads sample. Furthermore, the relative intensity of the entire R1 regime increased significantly when compared to the heads sample. R2 in the bodies was significantly contracted compared to R2 in the heads sample. Furthermore, F2 either does not exist in the bodies or has such a low response as to make it unresolvable. In contrast, F1 shows a much higher relative intensity in the bodies sample than the fluorescence response in the heads sample. The changes in R1 and R2 between the heads and bodies samples reflect a change in the chemicals that are eluting into the collection flask. This indicates that in-line fluorescence is a strong candidate for determining the transition between the heads and bodies of an SPD process.

The tails EEMS spectra in Figure 3d shows very stark differences in R1 and R2 when compared to the main body sample. The relative intensity of R1 in the tails sample is significantly diminished compared to R1 in the bodies sample. Conversely, the relative fluorescence in R2 of the tails sample has a significantly higher relative intensity in comparison to R2 of the bodies sample. This difference indicates that an in-line fluorescence system could detect the transition bodies and tails of an SPD process.

EEMS showed that fluorescence could be used to distinguish the changes between the investigated distillation fractions and showed that a 340–400 nm excitation wavelength fluorometer would be able to capture these transitions in-line. Based on this analysis, a fluorometer that could use a 340 nm, 365 nm, or 405 nm excitation wavelength was developed and the details of measurement results from this system are discussed in the next section.


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