Robin Kielkowski, Certified Aromatherapist

Published August 2019

Quality essential oils are necessary to provide safe and effective aromatherapy. Essential oils (“oils”) are growing in popularity and acceptance as evidenced by the number of aromatherapy products popping up in stores and the increased number of essential oil suppliers who promote their use. Much of this is driven by consumers turning to natural solutions and healthier lifestyles. What is often lacking is an awareness of the range of oil quality and its impact on effectiveness and safety. It is important for those of us trained as aromatherapists to have a grasp of these issues to better serve our clients. Following is an overview of how quality can be defined, some causes of variation from expected quality, a basic description of several tests available and the role we play as aromatherapists.

Stratistics MRC states that the global essential oil market is expected to grow from $5.91 billion in 2016 to $12.85 billion by 2023 (5). Allied Market Research indicates that industries using essential oils are diverse and include Flavor & Fragrance (the largest), Pharmaceuticals, Agrochemicals, Cosmetics, Aromatherapy, and others (1). As the market grows and becomes more lucrative, so do the number and diversity of suppliers and middlemen. Ensuring quality becomes increasingly important as emphasis is placed on meeting ever-growing demand for natural products made with what are often limited resources.

So how do we determine if an essential oil is of high quality? There are several tests available but first, let’s define quality and take a look at what causes poor quality or quality variation.


Defining Quality

Our goal as aromatherapists is to use essential oils therapeutically to support health and wellness, promote healing, reduce pathogens and enhance homeostasis. For this to occur there must be interaction between the oils and our bodies; there must be bioactivity. The American Heritage Medical Dictionary defines bioactivity as “The effect of a given agent, such as a vaccine, upon a living organism or on living tissue”(6). In his book, Aromatica, Holmes proposes four criteria for an oil to be defined as bioactive: biological identity, purity, integrity of the source plants and integrity of the extraction process. Meeting all four criteria helps ensure that an oil is bioactive and of appropriate quality for therapeutic use (2). A brief summary of the criteria follows. 

  • Biological Identity refers to proper identification of the plant from which an oil is extracted. This includes genus, species, chemotype (in some cases), plant part used and where the plant was grown. 

  • Purity indicates an oil is unaltered, that what we receive is the same as that distilled by the producer.

  • Integrity of Source Material considers how and when the plant was harvested, a lack of chemicals used on the crop and careful harvesting of only the plant part that is intended for distillation.

  • Integrity of the Extraction Process describes extraction of the most complete range of components possible by using proper distillation techniques and avoiding those that damage the oils.

Causes of Variation in Quality

What causes variation in quality from what is expected? First, we must remember that natural variation of the source plant can occur as a result of environmental factors such as light, soil, temperature, and weather conditions. These natural variations provide us with wonderfully effective oils with known chemistries. However, variation can result from intentional or unintentional actions that negatively impact an oil.  Adulteration, contamination and degradation as well as distillation technique and the testing process itself can impact quality.  All result in changes to an oil’s chemistry and therapeutic properties.

Adulteration occurs if synthetic fillers or less costly oils are added, typically to increase the amount of an expensive oil. Adulteration also occurs when an oil is “enhanced” with the goal of standardizing the fragrance or flavor to meet consumer expectations. While standardization may be acceptable in some areas of the fragrance and flavor industries, it is unacceptable for aromatherapy because it chemically changes the oil.


Contaminants can be introduced during various phases of oil production: growing and harvesting of crops, extraction and storage. For example, careful harvesting helps ensure that only the intended plant is obtained, reducing the likelihood of contamination with other plant material. Further, some chemicals applied to crops can be retained during the extraction process and concentrated alongside the plant’s oils. Solvents used as part of the extraction process may be retained in the final product and phthalates from improper use of plastic storage containers can leach into an oil (7).

Degradation occurs when an oil is exposed to heat, light or air. Oils are at risk of degradation any time these exposures occur regardless of the phase of production or post-production. Even extraction technique can impact quality. Proper distillation time, steam pressure and temperature help preserve the full spectrum of oil constituents; poor distillation technique can degrade an oil. For example use of high temperatures for steam distillation to shorten extraction time, often for rapid delivery of oil to the marketplace, will negatively impact the oil’s quality.



What can we do to have some degree of confidence about an essential oil? Testing to identify the chemical components is a good start. Ideally, these analyses are performed by an independent lab though a supplier may validate results in-house. Results should be available from the supplier for each batch of oil.

There are multiple tests available to identify essential oil components and detect variation. For each of these tests, results are compared to standards. Standards are established and maintained by several organizations: National Institute of Standards and Technology (NIST), International Organization for Standardization (ISO) as well as others. 

Following is an overview of some test techniques. Note that Gas Chromatography coupled with Mass Spectrometry (GC-MS) are two commonly used tests for essential oils. For this reason, we focus on GC-MS. Some of the other tests described below may be used in conjunction with GC-MS. 


Gas Chromatography is described by St-Gelais (4). This process separates different molecules of a substance such as an essential oil. Gas Chromatography does not identify nor quantify the molecules, it only separates them and serves as the front-end to subsequent analyses. The process is useful only for substances that can be turned into gases (evaporated) and not damaged by the heat necessary for evaporation. Molecules that cannot be evaporated cannot be analyzed using this technique meaning essential oils, which are volatile, can be analyzed but solid plant material cannot. As a simplified overview of the process, essential oils are injected into a chamber for evaporation. Heat is gradually increased with smaller, lighter molecules evaporating first and at lower temperatures than larger molecules with higher boiling points. At this point, the molecules have been separated.

The next step is to quantify and identify what has been separated by capturing the molecules from the Gas Chromatography process. St-Gelais describes two techniques used for this analysis: Mass Spectrometry (MS) and Flame Ionization Detection (FID) (4).

  • Mass Spectrometry identifies and quantifies the molecules that are separated during Gas Chromatography. The molecules are passed through an electric current, splitting them into fragments which are then passed through a detector where they are sorted based on atomic weight. The fragments are then measured, yielding the proportions of each. There is an expected fragmentation pattern for each of the molecules. Mass spectrometers typically interface with software that contains the standards against which these spectra are compared to identify the components of the oil.

  • Flame Ionization Detection is another technique that can be used to identify components separated during Gas Chromatography. This process passes a carrier gas containing essential oil molecules through a controlled flame. The flame oxidizes the oil and generates charged ions for each of the oil’s components. Electrodes measure the electrical currents with results plotted on a graph and compared to standards just as they are for Mass Spectrometry.


Optical Rotation and Refractive Index are described by Price and Price (3). In both cases, the results are compared to known standards to verify the authenticity of an oil.

  • Optical Rotation passes plane-polarized light through an oil. Optically active molecules contained within the oil rotate the light and the angle of rotation helps identify the oil. 

  • Refractive Index utilizes light of a known wavelength that is passed through an essential oil and refracted. The refractive index is measured and compared to standards.  


            All testing has limitations. For instance, as GC-MS reports often show, constituents do not always add up to 100%. This is because some oil components have not been identified and are not in databases for comparison. Other limitations exist when components are similar in structure or when an unknown component is close in structure to known components and incorrectly identified. Nongaseous components cannot be identified. This includes certain pesticides and heavy metals.


Essential oil suppliers must consider the quality, reliability and experience of independent labs that perform testing. Some considerations include:

  • Is the lab performing the tests trustworthy? Testing is fairly expensive and must be performed on each batch of oils. 

  • Are the lab’s procedures thorough and designed to ensure there are no contaminants introduced during sampling and testing?

  • Is equipment calibrated?

  • Is the technician properly trained on test procedures and analysis?


Will the average aromatherapist observe harvesting and distillation or research the lab analyzing their supplier’s oils? Probably not. Some of what we have discussed is not readily available or not practical for the individual. However, we should understand that these processes exist in case something seems not quite right with an oil.

Role of the Trained Aromatherapist

                As aromatherapists, our role in ensuring quality goes beyond review of test results. Though critical to ensuring quality and proper identification of oils, testing is only part of the story. We are in a unique position to understand synergies that exist between oils but for which no test is available. We are familiar with the oils and have awareness of what impacts quality.

We know our suppliers. A chain is only as strong as its weakest link and there can be many entities handling an oil: distillers, processors, distributors, wholesalers, retailers and those of us providing aromatherapy to clients. Suppliers come in a variety of sizes. While corporate size does not determine quality, there can be more steps between grower and consumer in a large operation. A small producer may have fewer steps between the source and destination and offer an opportunity to understand their processes and have confidence in their team. A larger organization may have more resources for testing. Both ends of the spectrum have advantages and disadvantages. While I do not argue for one or the other, we must be aware of the possibilities.

It is prudent to go beyond relying exclusively on test results by taking a wider perspective. We need to understand that, though GC-MS testing is what we typically rely on, there are other tests available. If our supplier performs multiple tests, so much the better.


Test results are a snapshot of a point in time and do not guarantee that quality is unchanged between testing, delivery to us and, ultimately, use by our clients. For instance, were the oils stored properly at the supplier? Were delivery conditions acceptable? Have we stored and handled the oils properly after receipt? As trained aromatherapists, we have a basic familiarity with many oils. Is the fragrance “off?” Is there cloudiness? Is the color what it should be? Are there particulates in the bottle? I.e., is there deviation from what is expected? Answering these questions offers another set of tools in obtaining the highest quality essential oils possible to provide effective and safe aromatherapy to clients.



  1. Allied Market Research (2016). Essential Oil Market.

  2. Holmes P. (2016). Aromatica A Clinical Guide to Essential Oil Therapeutics, Volume I: Principles and Profiles. London: Singing Dragon. pp. 62-79

  3. Price S, Price L. (2014). Aromatherapy for Health Professionals, 4th ed. London: Churchill Livingstone. p. 3

  4. St-Gelais A. (2017). The Highs and Lows of GC-MS in Essential Oil Analysis. Tisserand Institute.

  5. Stratistics MRC (2019). Essential Oil – Global Market Outlook (2017-2023).

  6. The American Heritage Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company

  7. Tisserand R., Young R. (2014). Essential Oil Safety A Guide for Health Care Professionals, 2nd ed. Churchill Livingstone Elsevier, London. p. 9

  8. Zero Breast Cancer. Phthalates - The Everywhere Chemical.


This article was originally published in the October 2019 issue of AromaCulture Magazine ( and has been adapted for use here with permission from the publisher.