In the last six columns (1-6) I have presented three quantities that were used to measure chromatographic separation quality, the capacity factor, the separation factor, and the number of plates. Here, we finally pull all this together into what is called the Master Resolution equation for chromatography. This equation combines all three measures of separation quality, and an understanding of these parameters will go a long way towards allowing you to rationally develop chromatographic methods.

Review of Capacity Factor, Separation Factor, and Number of Plates

Recall (3) that the capacity factor for a chromatographic peak is calculated from its elution time and the void time as given by 

Note that since both the numerator and denominator in equation 1 are in units of time, the units of the capacity factor are unitless.

A way of thinking about the capacity factor, which I perhaps should have mentioned in our earlier discussion (3), is that it is a measure of the affinity for or the strength of the interaction of analyte molecules with the stationary phase. Consider trying to separate a mixture of ethanol and toluene. As a reminder, their chemical structures are shown in 

, with toluene on the left and ethanol to the right.

Imagine we are performing liquid chromatography with a silica stationary phase, which has a large number of polar Si-OH groups on the surface, and a non-polar mobile phase such as hexane. Recall (1-6) that the elution time is determined in part by F

, the fraction of time an analyte molecule spends adsorbed on the stationary phase as seen in 

A non-polar molecule like toluene will have low affinity for the polar stationary phase, a high affinity for the non-polar mobile phase, have a low value of F

, and elute early. Whereas a polar molecule like ethanol, will have a high affinity for the polar stationary phase, a low affinity for the non-polar mobile phase, have a larger value of F

 values and hence elution times for these two analytes leads them to have two different capacity factors as illustrated by the chromatogram in 

In Figure 2 we can imagine toluene elutes first with a capacity factor of 1, while ethanol elutes later with a capacity factor of 3. Guess what, we have finally used all the chromatographic theory we have learned to understand a real world separation!

Finally, recall (3) that the capacity factor resets the 

-axis scale on a chromatogram into void time units. The capacity factor measures how many void times after t

 an analyte elutes. Thus, in Figure 2 toluene elutes 1 void time after t

, while ethanol elutes 3 void times after t

. Also, remember that calculating the capacity factors for a separation helps us compare elution times across separations taken under different conditions, such as at different flow rates (3). Versatile thing that capacity factor, isn’t it?

The restriction in Equation 2 that the retention time of peak b be greater than peak a exists so that the separation factor is a number >=1. Since in Equation 2 the numerator and denominator are both in units of time, like the capacity factor the separation factor is a unitless quantity. Above we saw that the capacity factor was a calculation for a specific chromatographic peak. Whereas, note that the separation factor is for a pair of chromatographic peaks and is a measure of how well two peaks are separated from each other.

We found previously (4) that we can derive that the separation factor for a pair of peaks can be calculated from the ratio of their capacity factors as seen in 

 is the capacity factor for analyte a and k

Thus, for the separation in Figure 2, the separation factor is 3/1 or 3. Note this is an excellent, baseline resolved separation. Note that in 

α for the two peaks is also 3, even though this is a poor separation. This illustrates the problem that separation factors ignore peak widths.

Recall (5) that the separation factor as a measure of chromatographic quality falls apart when peaks are wide. We found (5) that a quantity called the 

 gets around this problem by including peak position and width as a measure of separation quality as given by 

Note in Equation 4 that both the numerator and denominator are in units of time, so the chromatographic resolution, R, is unitless. If two peaks elute at the same time the numerator in Equation 4, (t

), is zero and R is zero. This makes sense since in this case there is no separation. The further apart two peaks are the greater (t

) and R goes up. The wider two peaks are the greater (w

) and R goes down. Thus, to maximize R we need narrow peaks that are far apart. Using the chromatogram seen in Figure 2, R = (8- 4)/0.5(1+2) = 2.67. Note that these two peaks are baseline resolved.

Figure 3 shows a chromatogram where the peaks are neither broad nor far apart.

Using the data in Figure 3 we can calculate for this separation that R = (8-4)/0.5(5+6) = 0.73. Note that these peaks are not baseline resolved and that this is a poor separation. An R value of >1.5 is needed for baseline separation, which should always be the goal of a chromatographic separation. The fact that R gives a good measure of separation quality even in the presence of broad peaks makes it perhaps the best measure of separation quality (5,6).

Finally as we learned last time (6), peak widths are caused, amongst other things, by molecular diffusion while analytes are in the mobile phase. The plate number, N, is measure of peak broadening as seen in 

 is the elution time for a chromatographic peak, and w is the chromatographic peak width measured with the triangulation method.

Recall (6) that N is, in a sense, a measure of the number of times an analyte molecule adsorbs as it works its way down a chromatographic column. The idea is that the more times a molecule adsorbs, the less time it spends mobile where it can diffuse and cause peak broadening. Note in Equation 5 that as t

 goes up so does N, which makes sense since the more times a molecule adsorbs the slower it will be and the later it will elute. Also note from Equation 5, that as peak width goes up N goes down. This makes sense since molecules that do not adsorb often spend a greater fraction of their time mobile hence diffuse a lot leading to peak broadening.

, the number of plates for analyte a is 64, while N

, the number of plates for analyte b is 1024. This means on average molecules of a adsorbed much more often than molecules of b, which is why b elutes later than a. Using the data in Figure 3, N

= 28, which makes sense because in the separation shown in Figure 3 both analytes did not adsorb many times, spent a large fraction of their time mobile, diffused a lot, and hence gave broad peaks.

The Master Chromatographic Resolution Equation

Over the last several months we have covered a lot of chromatographic theory (1-6), but there has been a method to my madness. All these columns were laying the foundation for what we are about to discuss here, the Master Chromatographic Resolution equation (7-11). This equation pulls together in one place everything we have learned about how to understand and quantify chromatographic separations. The full derivation of this equation is beyond the scope of these columns (7-11), but here is this important equation in its final form which can be seen in 

where R is the resolution, N is the plate number, α is the separation factor, and k is the capacity factor.

Recalling that the resolution, R, is our best measure of the quality of a separation. There are three separate terms that R depends upon in equation 6 involving the plate number, separation factor, and capacity factor. The good news is that all the terms in equation 6 are things we have seen before (1-6). The bad news is that there is a lot to unpack here, and it will take at least two columns to do so.

The first term in equation 6 is the square root of the plate number divided by four. Since N is in the denominator here, as N goes up R goes up. That is, the greater the plate number the better the separation. Recall from above that N is a measure of how many times an analyte molecule adsorbs during a separation. The more times a molecule adsorbs, the smaller the fraction of time it spends mobile, the less band spreading, and the narrower the peak and the better the resolution. On the other hand, the fewer times a molecule adsorbs, the fraction of time the molecule spends mobile increases, band spreading goes up, and R goes down. We saw this earlier in Figures 2 and 3.

The second term in equation 6 involves the separation factor, (α-1/α). As this term increases, R goes up. For example, if a = 2 then (α-1/α) = ½, and if α = 4 then (α-1/α) = ¾. This makes sense because as we have seen, the further apart two peaks are the greater their separation factor (4).

The third term in Equation 6 involves the capacity factor and is (k/k+1). Like the separation factor, as k goes up R goes up. For example, if k = 2 then (k/k+1) = 2/3, whereas if k =4 then (k/k+1) = 4/5. This makes sense since k measures elution time in column volumes, and the greater k is the better it is separated from things that elute earlier.

There is a lot more to say about the Master Resolution equation, but there is so much to say it deserves its own column. So, happy separating until next time.

We reviewed concepts previously developed for capacity factor, separation factor, and plate number. I then presented the Master Resolution equation, that shows that to optimize a chromatographic separation we want N, α, and k to be maximized.

Smith, B., Principles of Chromatography, Part I: Theory 

Smith, B., Chromatography Theory, Part II: How Separations Occur Illustrated and Quantitated,

Smith, B., Chromatography Theory, Part III: Calculating Elution Times and Capacity Factors, 

Smith, B., Chromatographic Theory, Part IV: Measuring Separation Quality with Capacity and Separation Factors, 

Smith, B., Chromatographic Theory, Part V: Chromatographic Resolution, 

Smith, B., Chromatographic Theory, Part VI: Peak Widths, Band Spreading, and Plate Number, 

Skoog, D.A., West, D.M., and Holler, E.J., 

Forensic Applications of High Performance Liquid Chromatography

Forensic Applications of Gas Chromatography

, PhD, is Founder, CEO, and Chief Technical Officer of Big Sur Scientific. He is the inventor of the BSS series of patented mid-infrared based cannabis analyzers. Dr. Smith has done pioneering research and published numerous peer-reviewed papers on the application of mid-infrared spectroscopy to cannabis analysis, and sits on the editorial board of 

. He has worked as a laboratory director for a cannabis extractor, as an analytical chemist for Waters Associates and PerkinElmer, and as an analytical instrument salesperson. He has more than 30 years of experience in chemical analysis and has written three books on the subject. Dr. Smith earned his PhD on physical chemistry from Dartmouth College.
Direct correspondence to: 

Smith, B., Chromatographic Theory, Part VII: The Master Chromatographic Resolution Equation

Articles

Cannabis Analysis 

We finally present the Master Resolution Equation for chromatography, which combines all three measures of separation quality, and an understanding of these parameters will go a long way towards allowing you to rationally develop chromatographic methods.

Chromatographic Theory, Part VII:  The Master Chromatographic Resolution Equation

With price compression infecting most legal and medical markets, many cannabis extractors are turning to large scale distillate production or purchasing white label distillate. Producing distillate at scale requires investment in the right equipment and high quantities of biomass at a low cost. However, many markets have a surplus of material and are ripe for this type of production. This article reviews the rise of distillate in the market, how this has helped and hurt manufacturers, and predictions for the future.

The cannabis industry has seen a rise in the popularity of distillate in recent years. Cannabis distillate is a highly refined cannabis extract that is made by extracting and concentrating the cannabinoids from the cannabis plant. Distillate is often found in a cannabis dispensary or on the retail shelf as a vape cartridge with tetrahydrocannabinol (THC) concentrations of more than 80% for a low price. This article reviews the rise of distillate in the market, how this has helped and hurt manufacturers, and predictions for the future.

Distillate is produced through a process called 

. Distillation is a separation process that involves heating a liquid to its boiling point and then collecting the vapors. The vapors are then condensed back into a liquid, which is the distillate.

Cannabis distillate is typically produced in two steps. The first step is to extract the cannabinoids from the plant material. This can be done using a variety of methods, including hydrocarbon extraction, CO

 extraction, or ethanol extraction. The second step is to distill the extract using short path, thin film, or wiped film distillation. The result is a highly concentrated cannabis extract with cannabinoid concentrations of 80–95%.

The cost to produce distillate varies depending on a number of factors, including the quality of the starting material, the method of extraction, and the cost of production. In general, the cost to produce distillate in a mature market is low (for example, $1/g) relative to the cost of extracts produced from CO

 (such as $4/g) or hydrocarbon extraction (such as $2/g). This is due to mature markets having a surplus of biomass available at low costs and the scale at which many distillate producers are operating. This scalability comes from large scale ethanol extraction and distillation equipment. In markets where the cost of cannabis biomass remains high, distillate is closer in cost to the production of extracts using other methods of extraction and refinement. While scalability affords low cost of production, start-up costs for large scale ethanol extraction and distillation equipment are hefty and can range from $100,000 to $500,000 (1).

The price of distillate for consumers depends on the cost to produce distillate and the supply and demand dynamic for the product. Some markets and consumers value higher concentrations of THC and a clear extract color. Other markets and consumers see distillate as a commodity and place higher value on extracts that are full-spectrum and more encompassing of the plant’s compounds.

There are several reasons why distillate has become so popular. The extraction and refinement processes to produce distillate can be done at scale. This makes distillate production an attractive option for processors who are looking to reduce costs or operators who have an excess of cheap cannabis biomass. Many cannabis markets are experiencing steep price reductions or discounts due to either highly competitive retail markets or an excess in cannabis products relative to the consumer population. Price compression is also driven by cannabis consumer buying trends. Consumers have an overwhelming number of choices when purchasing cannabis in most legal markets, and many of them are using THC and price to narrow down their desired options. Distillate can satisfy the perceived cannabinoid “bang for your buck” as a consumer.

The business case for distillate based products is mainly about cost. Producing distillate at scale or purchasing white label distillate significantly reduces manufacturing costs. In addition, distillate can easily be used in small quantities to infuse cannabis edibles or topicals leaving little to no cannabis aroma or flavor in the end product. This is highly attractive to many manufacturers, however, the rise in distillate and commoditization of cannabis has hurt many cannabis business owners over the years. Consumers are being trained to place lower and lower value on cannabis products and higher value on THC alone. Not only are consumers witnessing prices plummet, but consumers are now demanding lower and lower prices. As the race to the bottom infects the cannabis industry and a surplus of cannabis products flood cannabis markets across the nation, many cannabis cultivators and processors have been forced to shut down.

The future of distillate appears bright. It is a highly concentrated product that can be used in many products and continues to be in high demand. As the cannabis industry grows, the demand for distillate is likely to increase. This is good news for extractors who are looking for ways to cut costs and make use of a surplus of biomass. However, this makes product differentiation a serious challenge. What makes one distillate based THC product any different from another if the chemical composition of these products are nearly the same? How can any brand demand a higher price or value for their product if it is fundamentally the same as its neighbor? This will continue to be a challenge as companies face consistent price compression and increased competition within the market.

The most affordable extract option may remain the product that moves the most volume, but if companies cannot maintain a sustainable margin on these products they will continue to fail. Consumers will continue to seek differentiation among their product options and companies may need to produce products that have more distinct differences, including cannabinoid and terpene profiles. This competition is great for consumers and product innovators alike.

Distillate is versatile, potent, and a cost-effective option for extractors. It continues to rise in popularity in most markets for both consumers and operators. In addition to the factors mentioned above, the future of distillate may also be influenced by the development of new technologies. Price is a major factor as consumers evaluate their product options. If new extraction methods surface or improvements to efficiency and cost of current methods arise, other extract types may someday capture the market for offering more than just a low price. Until then, it seems that nearly everything from extracts to edibles may continue to be produced with distillate.

Extraction Lab Startup: Cost & Return On Investment 

https://precisionextraction.com/2017/03/starting-an-extraction-business-cost-and-roi/

 is the founder, CEO, and Chief Extractor of Heylo. With a background in chemistry and clinical research, Lo was inspired to explore cannabis as a medicine and to enter the emerging industry. She joined Eden Labs, a leading CO

 extraction equipment manufacturer to support and expand a Research and Development department. There she managed the development of their latest and greatest CO

 extraction system. In 2017, after working with Eden Labs and another cannabis processor, Lo launched Heylo with a mission to help people get more out of life with cannabis.

Friesen, L., Why Is Everything Distillate?, 

Extraction Science

This piece reviews the rise of distillate in the market, how this has helped and hurt manufacturers, and predictions for the future.

Why Is Everything Distillate?

Purpose-built equipment for precise indoor cultivation climate control is an emerging technology. There are several approaches used by cannabis growers and preferred by designers and installers. Regulators have also crafted policies that require minimum efficiency ratings for HVAC processes, such as dehumidification. In some US states, cannabis regulations call out specific energy-efficient options, such as heat pumps. In most cases, ductless “mini split” or variable refrigerant flow (VRF) systems need supplemental dehumidification equipment to manage grow room heat loads. Energy models demonstrate that integrated systems outperform heat pump systems using standalone dehumidifiers for both annual energy use and life cycle costs.

High-quality cannabis flower production requires precise management of target environmental conditions for cultivation spaces and drying rooms. Heating, ventilation, air conditioning, and dehumidification (HVACD) systems are critical for controlling air temperature and humidity levels (1).

A 2021 Southern California study found that HVACD systems account for more than half of the electricity consumed at indoor cannabis cultivation facilities (2). Energy demand from climate control systems is greater than even sole-source horticultural lighting systems (see 

There are diverse types of equipment that can be used to cool and dehumidify grow and dry/cure rooms. The most common approach is a decoupled heating, ventilation, and air conditioning (HVAC) system with independent cooling equipment and dehumidification equipment. While this is the lowest first-cost option, these systems may not be the most energy-efficient and can have higher life-cycle costs than other HVACD options suitable for cannabis production, such as integrated ducted systems or hydronic HVACD systems (3). To address the high energy consumption of typical HVACD equipment for cultivation, policymakers in some states have enacted requirements for HVAC systems serving cannabis cultivation facilities to meet minimum efficiency standards. Some requirements address standalone dehumidification equipment performance in L/kWh, which describes the amount of moisture removed per unit of electricity consumption. Because indoor cultivation involves dynamic environmental conditions, equipment needs to modulate to maintain target setpoints, but the testing standards that rate the efficiency of dehumidification equipment are designed for 80 °F/60% relative humidity (RH) for human-occupied spaces and there are no standards defined for controlled environment agriculture.

In Illinois, HB 1438 requires cannabis grow operations to use “high-efficiency ductless split HVAC units, variable refrigerant flow HVAC units or other more energy-efficient equipment.” But heat pump systems are not necessarily the optimal system for indoor cannabis cultivation. Desert Aire, an HVACD system manufacturer, shares that while the efficiency of cooling equipment may be high, it must run more often to offset the heat generated by dehumidifiers removing moisture from grow rooms (4).

Crafting regulations for process equipment is generally done when the industry has enough data to understand baseline equipment performance and the potential energy savings of high-performance systems. Since research in the cannabis industry has many barriers and process equipment for cultivation is evolving quickly, regulators sometimes do not have enough information to reference for ]efficiency requirements.

To close the gap, expert professionals in the architecture, engineering, and construction community are conducting independent research and sharing findings publicly to inform growers of the life cycle impacts of HVACD system selection and guide the next generation of cannabis regulations affecting these systems.

The Cultivation HVACD System Comparison Study by Anvil Agronomics, Anderson Porter Design, and Zartarian Engineering published in 2023 compares three systems (5):

Ductless "mini split" heat pumps with standalone dehumidification equipment

Variable refrigerant flow (VRF) heat pump systems with stand-alone dehumidification equipment

Packaged purpose-built "integrated" ducted HVACD systems

The HVACD systems were modeled using Carrier Hourly Analysis Program (HAP) V5.11 to simulate a full year of system operation using standard operating procedures for cultivation that include light-emitting diode (LED) lighting, two tiers of flowering canopy, and lights on targets of 82 °F and 58% RH.

The space performance of a decoupled HVACD system (using mini splits) is demonstrated in 

. Figures 2 and 3 show grow room space temperature and relative humidity for one photoperiod. The cycling of the HVACD systems results in dynamic conditions that do not maintain a tight range for environmental setpoints; the study notes that “temperature stability is +/- 2.5 degrees F…and there is a swing of +/- 10% RH throughout the day.” Figure 2 details that modeled decoupled systems’ space temperature ranges from around 80 °F to 86 °F and Figure 3 shows RH ranges from about 42% to 60%.

The space performance of an integrated HVACD system is demonstrated in 

. Figures 4 and 5 show grow room space temperature and relative humidity for one photoperiod. The integrated nature of the HVACD systems results in more stable conditions as they can maintain a tight range of environmental setpoints. The study notes that “temperatures vary by less than +/- 1 degree F and RH varies by +/- 2.5% RH once the room reaches equilibrium after lights off.” Figure 4 shows that modeled integrated systems’ space temperature ranges from around 79 °F to 80 °F and Figure 5 highlights RH ranges from about 68% to 70%.

 show that the integrated HVACD system has the lowest annual energy use of the three systems analyzed, 14–16% better than the heat pump systems. Decoupled systems are also not under a single point of control, which wastes energy. Desert Aire notes that decoupled systems constantly overshoot set points as they try to balance temperature and RH, which leads to extended run time, more energy usage, and constantly changing vapor pressure deficit (VPD) which stresses plants. The integrated system also had the lowest construction costs and the lowest maintenance costs.

The findings of the study suggest that policymakers writing energy regulations for cannabis HVACD systems should consider allowing integrated HVACD systems as a pathway for compliance that offers robust and cost-effective energy savings. The authors of the study shared that (5): “the industry is not mature enough for regulations to be developed around the process of climate control in cultivation spaces. Cultivation tactics are diverse and ever evolving, and new equipment is becoming available as manufacturers address this emerging market. There are several different system configurations that can be successful, and we are still collecting the data on which ones perform best over the long term.”

States encouraging energy-efficient equipment for cannabis cultivation can support research by funding utility studies to investigate the energy savings potential of emerging technologies. For example, energy engineers from Energy Resources Integration (ERI) are studying energy-saving HVACD systems for indoor and greenhouse cultivation for the natural gas utilities in California (6).

If you are planning a new construction project or retrofitting an existing facility’s HVAC equipment, reach out to experts at ERI for design-phase guidance or other energy-related services (7). Stay tuned for the next study from Anvil Agronomics, Anderson Porter Design, and Zartarian Engineering which is comparing hydronic HVACD systems (using chillers and boilers) against integrated ducted HVACD systems (8–10).

Schimelpfenig, G., HVACD Controls for Plant Health, Product Quality, and Energy Efficiency in Cultivation and Drying Processes, 

https://www.cannabissciencetech.com/view/hvacd-controls-for-plant-health-product-quality-and-energy-efficiency-in-cultivation-and-drying-processes

Market Characterization of Indoor Cannabis Cultivation, https://www.etcc-ca.com/reports/market-characterization-indoor-cannabis-cultivation.

Best Practices Guide: HVAC for Cannabis Cultivation & Controlled Environment Agriculture, 

https://catalog.resourceinnovation.org/item/best-practices-guide-hvac-cannabis-cultivation-controlled-environment-agriculture-407516

https://www.desert-aire.com/news/growers-search-hvacd-efficiency

Cultivation HVACD System Comparison Study, 

https://issuu.com/anderson-porter-design/docs/230120_cultivation_system_comparison_study

CEA and Greenhouse HVAC Market & Technology Study, 

https://www.etcc-ca.com/reports/cea-and-greenhouse-hvac-market-technology-study

Zartarian Engineering: Experts in Precision Agriculture, 

, PE, is a Senior Energy Engineer at Energy Resources Integration, LLC where she implements energy efficiency programs and provides direct consulting services. She is the former Technical Director of the Resource Innovation Institute and manager of the organization’s Technical Advisory Council. She authored RII’s Lighting, HVAC, and Facility Design & Construction Best Practices Guides for Controlled Environment Agriculture and the Lighting, HVAC, and Controls Best Practices Guides for Cannabis Cultivation. She has worked with partners like the University of Vermont to optimize commercial and industrial buildings, including greenhouses and controls systems used for academic research on hemp and other cultivars. She has a B.S. in Architectural Engineering from the University of Wyoming and an M.S. in Civil Engineering from Stanford University. She is a licensed Civil Professional Engineer in California and Vermont.

, PE. With over 35 years of experience in the design and construction of building systems, Jim Megerson leads Anvil Agrinomics while keeping current with issues of building systems design, including indoor controlled environments for agriculture, energy efficiency standards, and green building practices.

grew up in an agricultural-based community and developed a deep respect for the people, plants, animals, and systems of small-scale production. He earned his BS and MS in Electrical Engineering from Northeastern University and spent the first portion of his career in product development and research roles in consumer and industrial electronics.

 is a Co-Founder of Anderson Porter Design, which is a leading architecture firm in the cannabis industry. He is a highly experienced architect with a passion for creating innovative and beautiful spaces that work in tandem with complex systems and engineering. Anderson is an expert in this field, having developed a deep understanding of the unique challenges and needs of the cannabis industry. He is experienced in working with local, state, and federal regulations to ensure that projects are built to the highest standards and compliant with all applicable laws. He has worked in the cannabis space for almost a decade designing upwards of 45 cannabis cultivation facilities and approximately 35 retail stores in 23 states. Brian has a master's degree from Harvard and a BFA and B.Arch from RISD.

Schimelpfenig, G, Megerson, J., Anderson, B., Zartarian, M., Integrated HVAC Systems for Cannabis Cultivation Have the Lowest Life Cycle Cost, 

The Cultivation Classroom

This column reviews a study published by cannabis industry design and construction professionals and describes the impact of energy regulations affecting heating, ventilation, and air conditioning (HVAC) system selection in cannabis production facilities.

Integrated HVAC Systems for Cannabis Cultivation Have the Lowest Life Cycle Cost

This article delves into the aspects of horticultural light intensity and spectra as well as their relative influence on cannabis photosynthesis, photomorphogenesis, and overall yield. While the manipulation of lighting spectrum and the utilization of sufficient photon density can govern the expression of both major and minor cannabinoids, as well as secondary metabolites such as an array of terpenes, anthocyanins, and flavonoids; these important environmental factors exert considerable influence on plant transpiration and thus a plant’s physiological response to vapor pressure deficit (VPD). In the rapidly expanding cannabis industry, optimizing cultivation practices to maximize crop quality and yield hinges upon a cultivator’s understanding of the intricate interplay between various lighting modalities and VPD. In turn, correct management of VPD alleviates many of the problems that plague controlled environment agriculturists, such as nutritional deficiencies, mold, mildew, and waste of valuable resources, such as CO

, water, and nutrients—the latter of which also results in undesirable capital expenditures.

The art of cannabis and hemp cultivation is reliant on achieving a balance between utilizing stable genetics and the meticulous optimization of various environmental parameters. These factors synergistically contribute to the cultivation outcome, influencing the production of optimal yield and phytochemical composition of the plant varietals involved. To this point, vapor pressure deficit (VPD) emerges as a metric of paramount importance to the overall success of the cultivation environment. VPD is a measurement that represents the deficit between the amount of moisture in the air and the maximum moisture the air can hold (saturation). This metric exerts a profound impact on the transpiration rate within cannabis plants. Transpiration is a physiological process involving the transport of water (along with CO

 and dissolved nutrients) from the roots through the xylem to the leaf (stomata), where it undergoes a phase change into vapor and is subsequently released into the atmosphere.

Although transpiration is primarily driven by the vapor pressure within the plant, it’s important to note that the internal atmosphere of the leaf is not saturated. However, the stomata, which are tiny pores on the leaf, play a crucial role in pressure regulation and thermoregulation. The stomata serve as a gateway between the highly humid internal environment of a plant and the surrounding atmosphere. When humid and dry air come into contact, they attempt to balance each other out, with vapor pressure flowing from high humidity to low. Therefore, if the atmosphere is very hot and dry, water vapor will escape from the leaves at a faster rate, increasing the transpirational demand on cannabis plants and potentially leading to wilting and nutrient deficiencies (see 

). Conversely, in humid grow rooms where atmospheric conditions are not significantly different from those of the leaf, less water vapor will be able to escape—giving rise to various issues, including the development of fungal diseases, mold, and root rot (see 

). When the VPD is at an optimal level, plants can more effectively absorb and distribute the nutrients they need for growth and yield (see 

). Scientifically-minded growers favor VPD as a measure that provides insight in the ever-changing conditions of the air’s moisture-holding capacity at any given temperature.

How to Calculate for VPD in Your Cultivation Environment

The Alduchov and Eskridge equation is an improved form of the Magnus’ formula and is commonly used to estimate the saturation vapor pressure (SVP) and VPD in meteorology. The VPD represents the difference between the saturation vapor pressure (es) and the actual vapor pressure (ea) of the air.

The Alduchov and Eskridge equation for calculating es is as follows in 

where es is the saturation vapor pressure in kilopascals (kPa) and T is the temperature in degrees Celsius (°C).

To calculate the VPD, you need to know both the saturation vapor pressure (es) and the actual vapor pressure (ea) of the air. The VPD is then given by 

For example, assume the temperature (T) is 25 °C and the actual vapor pressure (ea) is 2.5 kPa. We will calculate the saturation vapor pressure (es) and the VPD. Using the Alduchov and Eskridge equation, we can calculate the saturation vapor pressure (es)
as follows:

es = 0.6108 * exp[(17.27 * 25) / (25 + 237.3)]

Now, we can calculate the vapor pressure deficit (VPD) using the given actual vapor pressure (ea):

In this example, at a temperature of 25 °C, theSVP is approximately 3.17 kPa, and the VPD is approximately 0.67 kPa.

Understanding VPD is crucial for optimizing plant growth and determining appropriate environmental conditions in various applications, including greenhouse cultivation, where maintaining an optimal VPD range is critical for maximizing crop productivity
and quality.

 illustrates the recommended VPD levels for cannabis and hemp plants in vegetative stages. Within this chart, the dark green region (0.9-1.1 kPa) represents the most favorable VPD for cultivating cannabis plants. The light green area represents VPD ranges (0.7-0.9 kPa and 1.1-1.3 kPa) that are slightly outside of optimal range yet remain highly desirable. The yellow region indicates VPD levels that require adjustment to achieve optimal plant growth. Lastly, VPD values falling within the white range, lower than 0.5 kPa and higher than 1.7 kPa, are considered detrimental to the overall growth of the plants and require immediate correction.

 represents recommended atmospheric VPD values for cannabis or hemp plants in the flowering stage. The green section is the VPD sweet spot and represents the most desirable atmospheric conditions for cannabis cultivation. Purple indicates VPD fringe ranges, but is still more than acceptable. Orange represents VPD conditions requiring adjustment. VPD falling within the white range is detrimental to plant function.

Conversely, the borderline transpiration zone (orange) denotes both elevated and low VPD values ranging from (1.7–2.3 kPa and 0.7–1.0 kPa), respectively. In the elevated orange zone, plants may experience accelerated water loss, increased stress, and lower transpiration rates. In the lower orange zone, plants may experience low transpiration, higher humidity, and may experience mold and mildew issues. The danger zone (white) represents extremely high or low VPD values outside the recommended ranges. Values below 0.4 kPa or above 2.3 kPa indicate potential severe transpirational issues such as excessive humidity levels or nutrient deficiencies due to high transpiration rates and should be corrected immediately.

Why a Lower VPD in Vegetative versus Flower?

For plants in vegetative stages, root mass and stem structure are immature and have relatively little mass. The plant is focusing almost all of its resources to build these two components. Leaves with stomata are few, at least in the early stages of vegetation. A smaller root system contributes less root pressure to the plant system. Lower VPD exhibits more control over water and nutrient uptake for this stage of growth. A more controlled transpiration rate and less water loss occurs, minimizing water and nutrient stress.

During the flowering stage, the root system should be quite large and robust and root pressure should be high. Overall, plant mass during flower is many times that of plants in the vegetative stage of growth. Leaf area will be extensive and as flowers develop, water, CO₂, and nutrient demand will be significant. A higher VPD will provide for greater transpiration to satisfy the plant’s needs. However, it is important to note that extremely high VPDs can result in lower nutrient absorption and many growers increase nutrient levels to compensate. This can result in higher nutrient costs. In addition, higher transpiration may make plants more susceptible to mold, mildew, and root rot. Extremely high VPD may cause stomata to close to preserve resources, which demonstrates the undesirable side-effect of stunting growth.

What Role Does Lighting Play in the VPD Equation?

Horticultural lighting, such as full-spectrum light emitting diode (LED) fixtures impact VPD indirectly by affecting the temperature and humidity in the growing environment. It is a common misconception that LEDs produce very little, or no, heat at all. This is incorrect. LEDs apply less heat to the plants than other technologies if the luminaire and facility thermal management systems are well designed (1). Most LEDs used for illumination purposes convert approximately 22–28% of input power into light that can be used by the plant. To the contrary, high intensity discharge (HID) lighting, such as high pressure sodium (HPS), ceramic metal halide (CMH), and metal halide (MH) lamps convert only 5–10% of their energy consumption into usable light. The rest of the input power for both HID and LED lighting is converted into infrared light (IR), which is felt as heat. Interestingly, HID lighting converts this energy into IR at the lamp (light bulb) itself, while LED waste heat is produced at the silicon junctions of the diodes (1). A properly ventilated LED luminaire will move this thermal radiation away from the plant as the heat rises, whereas HID lighting radiates this IR directly to the plant leaves. Additionally, HID horticultural lighting typically uses non-flammable metallic “hoods” to reflect light towards the plants, which further collects this IR and directs it at the leaves themselves. The bottom line here is that both technologies produce heat that greatly affects VPD, albeit LEDs present significantly less impact on transpiration issues due to lower leaf temperatures and lower VPD than their HID counterparts.

The Influence of Lighting Spectrum on VPD

Lighting spectrum also affects VPD. Red light (~630–760 nm) is known to promote stomatal opening, increasing transpiration rates and potentially elevating VPD. Blue light (wavelengths around 400–500 nm), on the other hand, can have a suppressive effect on stomatal opening, which may reduce transpiration rates and subsequently lower VPD. However, both blue and red light used simultaneously and in the correct ratios can increase VPD dramatically (2).

White LEDs present a unique variable to VPD manipulation. Most LED luminaires on the global market use “full spectrum” white LEDs (

). This full spectrum is created by coating a discrete high-intensity blue LED, commonly peaking at 450 nm, with a specially formulated phosphor. When this phosphor is excited by the output of the blue LED, it fluoresces and produces light across a broad spectrum. Since this blue LED’s base frequency (450 nm) is the source of most of the spectral energy produced, its amplitude is usually significant compared to the rest of the spectral radiation produced by the LED. Addition of any supplemental red (discrete) LEDs must take this into account to optimize crop yield and phytochemical production (1).

In plants, red light is primarily detected by phytochrome Pr (phytochrome red) and Pfr (phytochrome far red) photoreceptor proteins, whereas blue light is detected by the photoreceptor proteins cryptochromes and phototropins. The interaction of these phytochromes initiate a signaling cascade within the plant cells, ultimately leading to convergence of these protein signaling pathways, activating ion channels, which changes guard cell turgor pressure (3).

Guard cells are specialized and found in the epidermis of plant leaves and stems. When they are filled with water, or turgid, they stretch the epidermis and create openings in the stomata. When they deflate, the pressure on the plant epidermis is lessened and the stomata close. This opening and closing regulates gas exchange in transpiration and allowing CO

 to enter via the stomata. Guard cell water pressure is controlled by ion movement, specifically, potassium and chloride ions, across the cell membranes. Pumping these ions out of the guard cells causes water to also escape, deflating the guard cells (4,5).

Ion channels are protein channels residing in the cell membranes and they come in two configurations: gated channels, which are triggered by specific stimuli, such as voltage, chemicals, force, and temperature; and open channels, which as you might have guessed, stay open and serve as baseline or “leakage” channels. Upon exposure to red light, phytochrome photoreceptors activate signaling pathways that open potassium ion channels, allowing potassium ions to increase cell pressure and open stomata. The addition of blue light to this red light stimulates ion channel activity, thus increasing stomatal response (6).

Ion pumps use adenosine triphosphate (ATP, the cell’s energy currency) to transport ions across cell membranes. The pumps help maintain cell function, signaling, pressure balancing, and other physiological cell functions. Both red and blue light stimulate changes in ion concentrations and membrane potential that help manage cellular processes (7).

Collectively, the optimal ratio of red to blue light for stomatal opening can vary among different plant species, as well as stage of growth. Some plants exhibit a higher sensitivity to red light especially during flowering, while others have a greater requirement for blue light, especially during propagation and vegetative stages. Determining the ideal balance of red and blue light for specific plant species can maximize the stomatal response, thus having a profound impact on finding the optimal VPD window.

By and large, the ongoing advancements in lighting technology continue to revolutionize indoor cannabis cultivation. Specifically, the importance of lighting spectrum and VPD cannot be overstated in aiming to achieve optimal growth and yield results. Fine tuning the spectral composition of light and maintaining an appropriate VPD within the cultivation environment are vital considerations for growers aiming to maximize their crop's potential. The interplay between red and blue light wavelengths, for instance, impacts the overall VPD, which in turn influences transpiration rates, stomatal conductance, and overall plant health. In simpler terms, maintaining consistent temperature and relative humidity in the cultivation space directly impacts the VPD, which in turn affects plant transpiration and growth. Consequently, a comprehension of the intricate relationship between lighting factors and VPD dynamics is indispensable for cultivators seeking to achieve exceptional outcomes in their cannabis production endeavors.

Inoue, S. and Kinoshita, T., Blue Light Regulation of Stomatal Opening and the Plasma Membrane H+-ATPase. 

Assmann, S.M. and Jegla, T., Guard Cell Sensory Systems: Recent Insights on Stomatal Responses to Light, Abscisic Acid, and CO

Shimazaki et al., Blue light activates the plasma membrane H+-ATPase by phosphorylation of the C-terminus in stomatal guard cells, 

Mott et al., Blue light inhibition of guard cell anion channels mediated by abscisic acid, 

Alberts et al., Molecular Biology of the Cell, 2002, 4th ed.

is a research Professor at the University of Texas at El Paso, the principal founder of Inform Environmental, a partner of Medusa Analytical, and is a director of the Curtis Mathes Corporation (OTC:CMCZ). 

 is an MS Chemistry student at the University of Texas at El Paso. 

is with Curtis Mathes Corporation.
Direct correspondence to: zlhildenbrand@utep.edu.

Mendoza-Dickey, H., Manes, R., Hildenbrand, Z., Intensity and Spectrum: The Role of Lighting in Vapor Pressure Deficit, 

Feature

This article delves into the aspects of horticultural light intensity and spectra as well as their relative influence on cannabis photosynthesis, photomorphogenesis, and overall yield.


Intensity and Spectrum: The Role of Lighting in Vapor Pressure Deficit

Editor's Note: This agenda was accurate at the time of publication in early August 2023. Speaker names and specific time slots are subject to change.

Knowledge and Art of Combining Cannabinoids for Treatment Success

FIRESIDE CHAT DISCUSSION, CULTIVATION FOCUS


Moderator: Zacariah Hildenbrand, PhD, Chief Technical Officer, ZED Therapeutics
Speaker: Adam Jacques, AgSense LLC Oregon

Coffee Break/Posters/Networking- Exhibit Hall

Analytical Track: Is it the time to pay more attention to flavonoids in cannabis products


Speaker: Dr. Harry Behzadi, President and CEO of AccuScience Labs

Medical Track: Unprescribed Film Screening


Speaker: Steve Ellmore, Filmmaker and veteran, The Unprescribed

Psychedelics Track: Next Generation of Scientists Examining How to Mitigate Chronic Disease with Psychedelics


Moderator:Dr. Denise Vidot, University of Miami
Speakers: Brea-Necole Diggs, PhD Student, University of Miami - Miller School of Medicine; Dr. Amrit Baral, PhD Student, University of Miami - Miller School of Medicine; and Owen Berke, Student, University of Miami

Analytical Track: Analysis of Cannabis Products for As, Cd, Hg, Pb by ICP-OES: Meeting the DL Challenge


Speaker: Sergei Leikin, Texas Scientific Products and Autumn Phillips, Inorganic Ventures


Moderator: Cherissa Jackson, RN, WeDecode
Speakers: Steve Ellmore, Filmmaker and veteran, The Unprescribed; Ellen Brown, Founder; Green Path Training; Stephen Mandile, Veteran Advocate and Co-Founder, Cannabis Advancement Series; and Wayne Smith, Former Combat Medic

Psychedelics Track: Preparation and Analysis of Psychoactive Alkaloids and Their Metabolites in Multiple Matrices


Speaker: Dan DeLurio, Business Development Manager, Restek

Analytical Track: Cannabinoid Extraction Efficiency for Potency Analysis: An in Depth Look of Multiple Techniques


Speaker: Melinda Urich, Advanced Scientist, LC Solutions, Restek

Analytical Track: Quality control in Cannabis microbiology: How to comply and go further than industry standard


Speaker: Anthony Repay, Laboratory Director, Method Testing Labs

Medical Track: Exploring cannabinoid therapy as a safer alternative for dementia symptom management


Speaker: Blake Pearson, Co-Investigator, Pearson Health and Mikelle Bryson-Campbell, co-investigator

Cultivation Track: The Updated Soil Food Web


Speaker: Jeff Lowenfels, Author: Teaming with Microbes Series, lord of the roots

Analytical Track: The Curious Case of Cannabis: Understanding the Microbiology of Cannabis


Speaker: Patrick Bird, Sr. Manager Scientific Affairs, bioMerieux

Cultivation Track: Microbial Consultation Guidelines for Common Reservoirs of Contamination in Cultivation Facilities


Speaker: Frankie Claunch, Microbiologist, PuEr Lab, LLC

See the program overview for the upcoming Cannabis Science Conference Fall in Providence, Rhode Island September 20-22nd.

July/August 2023

Chromatographic Theory, Part VII: The Master Chromatographic Resolution Equation

Why Is Everything Distillate?

Integrated HVAC Systems for Cannabis Cultivation Have the Lowest Life Cycle Cost

Intensity and Spectrum: The Role of Lighting in Vapor Pressure Deficit

2023 Cannabis Science Conference Fall Agenda