e-Liquid Testing

Production and Quality Control of e-Liquids

Quality control (QC) of e-liquids is critical to ensuring the quality and, to some extent, the safety of e-liquids. During the development of new e-liquid blends and for assuring the quality of known formulations, a variety of laboratory QC analyses are essential from both a regulatory compliance and consumer safety standpoint. As e-liquid production and regulation continue to evolve, advances in laboratory testing will no doubt be critical to companies that create and manufacture these products.

e-Liquid Testing in Production

1.0% ethanol

1.9% nicotine

9.1% flavor

10.0% glycerol

48.0% propylene glycol

e-Liquid manufacturing has created a new value chain for flavor development, blending, and distribution. Producers may be subject to manufacturing guidelines and compliance requirements from the Tobacco Products Directive (TPD), the European Community Identification (ECID or EC-ID), or others. e-Liquid quality testing occurs at multiple points along the distribution chain, and these tests should be conducted both internally and by third-party testing labs.

e-Liquids are subject to quality testing throughout the various phases of production:

Raw materials, which are generally pharma- or food-grade, are characterized for formulation.
Intermediate testing ensures that e-liquid blends deliver desired characteristics.
Final product testing helps ensure that e-liquid products meet environmental, toxicity, and safety regulations as well as permissible exposure limits according to Material Safety Data Sheets (MSDS).

Delivering on desired properties and meeting labeling requirements depends on staying within critical testing parameters, including nicotine content, propylene glycol/vegetable glycerin (PG/VG) ratio, water content, boiling point, refractive index, density, specific gravity, and pH.

Characterization and Quality Control of e-Liquids

Standard lab techniques, such as refractive index, density, boiling point, UV Vis spectrophotometry, and titration, are useful for determining the quality and concentrations of e-liquid ingredients. Often, combining analytical techniques into a single, multiparameter workflow can allow for swift grading of components to ensure the quality and compliance of e-liquids.

Properties That E-Liquid Labs Typically Test For Include:

Nicotine

Nicotine, the highly addictive component of the tobacco plant, is arguably the most important e-liquid ingredient for products aimed at replacing cigarettes. In most countries, e-liquid nicotine content is regulated, and concentration thresholds must meet prescribed standards.

PG/VG Ratio

PG/VG ratio is important for a vapor’s sensory properties, including mouthfeel and sweetness. A higher ratio (as PG increases) delivers higher flavor intensity, and a heavier "throat hit,” while a lower ratio (as VG increases) provides a characteristic sweetness yet a duller flavor overall. Ratio modulation also helps to fine-tune vapor cloud thickness and density, as VG’s higher viscosity requires more time and battery power to achieve optimal vaporization temperature. In most countries, PG/VG content must be declared.

The Quality of E-liquid Ingredients

The quality of e-liquid ingredients (e.g., PG, VG) can be ascertained by examining their physical characteristics, such as boiling point. Since e-liquids are vaporized during the e-smoking process, the temperature of the e-cigarette coil needs to approach the boiling point of the e-liquid. As the boiling point of the e-liquid depends on its composition, this metric can also be used as a quality indicator. Boiling point determination is also used to assess the ideal temperature for vaporization and to ensure appropriate water content, both of which affect the overall mouthfeel of the e-liquid.

The Gardner Index

The Gardner Index is a 1-dimensional color scale used to characterize the yellowness of a liquid. As nicotine containing e-liquids age, they yellow due to oxidation. The Gardner Index is a widely used method for measuring the degree of this color change and can help determine the freshness and quality of the e-liquid. The higher the Gardner Index, the more yellow the e-liquid has become, indicating that it may have degraded in quality.

Color Measurement

The pH of an E-liquid Product

The pH of an e-liquid product can impact its flavor, aroma, and overall quality. Testing e-liquid pH helps to ensure consistent taste, safety, product stability, and batch-to-batch consistency.

pH Measurement of e-Liquids

Multiparameter Measurements

Given the proper instrumentation and multiparameter measurement application, the density, refractive index, pH, and color of e-liquid products can be assessed simultaneously. This type of check is ideally suited for verifying the final quality of e-liquid products, as any impurities or shifts in ingredient quantities can be accurately and reliably detected in a single analysis.

Multiparameter Measurements of E-liquids

e-Liquid manufacturing and compliance benefits from implementing product testing that is accurate, reliable, and highly automated. METTLER TOLEDO’s smart weighing sensors, for example, increase throughput by enabling shorter settling times and faster, more stable weighing results. These sensors’ simple “plug and communicate” design allows for rapid characterization, real-time diagnostics, and secure data backup. Our specialized scales with continuous airflow under the weighing pan prevent dust and humidity from entering the weigh module interior, eliminating these sources of inaccuracy. We have solutions specifically designed for small-volume filling. Our small-footprint weigh modules are integrated into e-liquid machines to enable precise filling of small volumes, preventing over- or under-filling to meet labeling requirements. Enabled for statistical quality control (SQC), these sensors can also adjust fill quantities so that e-liquid producers work within the tightest tolerances and save money.

FAQs

What Is an e-Liquid?

e-Liquid, also called e-Juice or vape juice, refers to a solution crafted for use in electronic cigarettes (e-cigarettes) or other “vaping” products. e-Cigarettes simulate smoking using an electronically heated coil that vaporizes the fluid so it can be inhaled. e-Liquid generally consists of PG, VG, water, glycerol, and flavoring, with PG, VG, and water comprising the majority of the blend. As a combustion cigarette substitute, most e-liquids also contain nicotine.

Why Test e-Liquids?

e-Liquid manufacturers must stay abreast of and adhere to regulatory requirements, so QC testing is essential both during new product development and manufacturing of established products. Although the health data related to actually inhaling a vaporized e-liquid are incomplete, the constituent parts of an e-liquid must meet quality standards to be considered safe for ingestion. Thus, e-liquid testing is essential for the quality, batch-to-batch consistency, safety, and regulatory compliance of e-liquids. Regulatory guidelines for e-liquid manufacturers and labs vary among countries.

Which e-Liquid Tests Are Most Important?

A series of laboratory analyses help e-liquid producers develop the desired e-liquid sensory properties and also ensure the quality of final products. The most important tests tend to be nicotine content, PG/VG ratio, boiling point, water content, refractive index, density, specific gravity, pH, and Gardner Index. Multiparameter measurements are essential for e-liquids. Two products that could differ drastically in quality, flavor, and freshness may show similar results for only 1 or 2 properties, so comprehensive testing is necessary to verify the overall quality and safety of a product.

What Regulations Guide Testing and Quality Control of e-Liquids?

In 2014, the EU directive 2014/40/EU, which concerns the manufacture, presentation, and sale of tobacco and related products, included e-cigarettes and e-liquids for the first time. Guideline implementation and enforcement is the responsibility of each individual EU country. For example, in Germany, producers and importers of e-liquids must receive authorization for new e-liquids by submitting a complete list of all ingredients with corresponding weight percentages. [1], [2]
In the United States, e-liquids have been regulated by the FDA since 2016. Manufacturers and importers of e-liquids must register their products with a complete ingredient list that includes harmful and potentially harmful constituents. [3]

[1] EUR-Lex
[2] German Federal Ministry of Food and Agriculture
[3] U.S. Food & Drug Administration (FDA)

What Are the Typical Nicotine Levels of e-Liquids?

Nicotine is a basic alkaloid that is naturally present in the tobacco plant. A psychoactive substance with both stimulant and relaxant effects, it is a key component of most e-liquids. Commercially available e-liquids have nicotine concentrations from 3 to 18 milligrams per milliliter. The 2 most common tests used to accurately measure the amount of nicotine present in e-liquids are potentiometric titration and spectroscopy.

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