The Vaping Post to the LFEL team: We are interested in your comments regarding this study (see below our embedded article for more details about the original scientific publication)?
LFEL: LFEL shares with the authors of this paper the rationale of using reference material to characterize e-liquids properties. Some differences in certain factors like in e-liquid composition or in heating elements, or differences in vaporization itself can lead to contradictory results. Nowadays, scientific publications on e-cigarette studies are suffering from this problematic. In response to the issue, the proposition that is made to homogenize analytic conditions, analytical protocols and materials is certainly a good solution.
The article refers to British and French norms, PAS 54115 :2015 and XP D90-300-2, respectively that propose rules for e-liquid composition.
The Vaping Post: If I don’t mistaken, LFEL is also participating in the normalization effort, especially with this French norm XP D90-300, aren’t you?
LFEL: Yes indeed, LFEL has been an actor in the work behind the French norm XPD 90-300-3 that proposes two reference materials and standard vaporizing conditions to homogenize testings, a necessary step to be able to compare the results. The LFEL is also developing a vaping machine called “U-SAV” that allows monitoring and controlling physical conditions in vaping. Emissions generated by our vaping machine are controlled, accurate and reproducible. Such characteristics do interest some scientists and manufacturers.
We very positive about the release of such a protocole that should reassure the vaping community. However, the article suffers from some caveats and cannot be used as a reference for the study of any type of e-liquid in this form.
The Vaping Post: Could you develop a little bit more about the caveats you identified?
LFEL: The study lacks clarity on the aims and goals of the development of a Reference e-liquid. Is the goal to create an “average e-liquid” to get closer to real vaping conditions or is it a solution to standardize and homogenize scientific operations of research teams that work on vaping issues?
In both cases, an e-liquid containing only PG, VG and nicotine is too simple to take into account the whole complexity of vaping characteristics and of reactions of thermal degradation that occurs during the vapoization of a mixture containing several tens of different molecules.
The Vaping Post: Are you meaning to say that their protocol is too simple?
LFEL: To understand the different forcings, physical (flow rate, power, temperature…), behavioral (puff duration, DL vs MTL inhalation pattern, retention time of the aerosol in the lungs…) and chemical (influence of the chemical composition of the e-liquid on that of the aerosol), achieving a simplified e-liquid mixture is attractive. This issue has been widely discussed during the elaboration of the XP D90-300 norms. Our discussions lead us to define two reference e-liquid formulas containing flavorants and thinners (water and ethanol) and differing only from the composition of the matrix (Liquid A : 80/20 PG/VG (v/v) – Liquid B : 50/50 PG/VG (v/v)). Those two liquids are then more complex than in the publication and closer to commercial e-liquids we analyse.
The Vaping Post: Because of the implementation of the EU TPD, there is a real need for quality testing for both e-liquids and e-cigarette emissions. In what does your experience with e-liquid analytics may help making quick progress?
LFEL: Making the use of reference materials (e-liquids or heating elements) is a step forward but also presents several limitations, especially if one takes into account all vaping conditions, the diversity of hardware and the geometry of resistance coils.
From an analytical point of view, the authors of the article propose to produce and characterize the reference e-liquid according to criteria like:
- The use of analytical grade versus pharmacological grade ingredients: The advantage will be that purity should not depend on the stocks of ingredients, this will increase the homogeneity of analytical results from one laboratory to another. However, e-liquids available on the marketplace are using pharmacological grade ingredients and using analytical grade ingredients (much more expensive) would limit the volume of reference e-liquid for financial reasons.
- Homogeneity: The protocol recommends 5 rpm (rounds per minute), a centrifugation speed that is somewhat mild, which would explain the issues with high-VG liquids. With the many measurements we carried out in the lab, we are well aware of homogeneity issues, especially with high-VG content. To us, the use of a mechanical shaking (Vortex) for some tens of seconds seems enough. We warn about the danger of using nicotine as a proxy for the homogeneity of an e-liquid because an analytical precision and accuracy are mandatory to correctly estimate this bias. The authors do not provide analytical uncertainty estimates on nicotine measurement, hence, it is irrelevant, to us, to evaluate homogeneity. In addition, the authors find two phases but do not reveal which one was sampled. The presence of different phases evokes a lack of thinner and points out directly to the formulation of their reference material.
- Stability: Incubating the e-liquid at 50°C is debatable because such a temperature is not representative to what happens for e-liquids contained in a clearomizer during vaporization. This stabilized temperature does not take into account the heating kinetic that, for us, is an important factor of e-liquid stability/degradation.
- E-cigarette testing device and aerosol generation: The use of a DC power supply is an asset of this protocol compared to batteries. We noticed with U-SAV that differences appeared between values obtained at the beginning and at the end of 100-puff series when relying on batteries; such differences were not observed with a DC power supply. The conditions of generation used in the article (3.6 V – 2.8 Ω) are now obsolete.
- Metals in and particle size of the aerosol: LFEL is not currently carrying out such measurements. To our best knowledge, the size spectrum observed by researchers in this study looks higher than in the literature [1-4]. In such analytical conditions, the treatment of the aerosol is crucial because of phenomenons of coalescence (increase of the size of particles) or evaporation (reduction of the size of particles). We acknowledge that simply defining a heating element and a reference e-liquid are not sufficient to satisfactorily conduct such physical characterization, more importantly the generation mode of the aerosol and its sampling are also highly relevant for the reproducibility of the results.
The Vaping Post: Except a few things that remain to be done, I have the feeling that something is about to emerge for the safety of users.
LFEL: Absolutely, we [LFEL] welcome the introduction of a reference e-liquid in vaping sciences since we have been defending this approach for about two years in French normalisation committees and now at the European and International levels. But rather than an e-liquid, the scientific community rather needs a “Handbook of Best Practices” to include also recommendations about generation of aerosol and samplings (aerosol and e-liquid). Similarly to what have done the authors of this study, we believe that the definition of a heating element appears also necessary to avoid any variability linked to manufacturing quality of commercial hardware.
[1] Alderman S.L., Song C., Moldoveanu S.C., Cole S.K. Particle Size Distribution of E-Cigarette Aerosols and the Relationship to Cambridge Filter Pad Collection Efficiency. Contributions to Tobacco Research, 2014, Vol. 26 (4), pp. 183-190.
[2] Ingebrethsen B.J., Cole S.K., Alderman S.L. Electronic cigarette aerosol particle size distribution measurements. Inhalation Toxicology, 2012, Vol. 24 (14), pp 976-984.
[3] Pichelstorfer L., Hofmann W., Winkler-Heil, Yurteri C.U., McAughey J. Simulation of aerosol dynamics and deposition of combustible and electronic cigarette aerosols in the human respiratory tract. Journal of Aerosol Science, 2016, Vol. 99, pp 125-132.
[4] Sundahl M., Berg E., Svensson M. Aerodynamic particle size distribution and dynamic properties in aerosols from electronic cigarettes. Journal of Aerosol Science, 2017, Vol.103, pp 141-150.
A Certified Reference Material to test and improve the quality of e-liquids
