Analysis of complex mixtures – Cigarette smoke
Introduction
A cigarette consists of a blend of tobaccos surrounded by a paper of a defined specification. Most modern cigarettes are filter tipped and tip ventilated. Tip ventilation means that mainstream smoke is diluted with a defined amount of air during a puff. “American blend” cigarettes, cigarettes that include a blend of Burley, flue cured and oriental tobaccos, are one of the most common types of cigarettes worldwide. Small amounts of additives, like sugars, cocoa, licorice, fruit-extracts and aroma substances, are added to the tobacco in these cigarettes to give a brand its specific taste, smell and aroma. Cigarettes that contain a flue-cured blend of tobacco are also popular in some parts of the world. In some countries, these cigarettes are sold without additives, in keeping with local taste preferences. The tobacco blend, the cigarette paper, the type and efficiency of the filter and the degree of tip ventilation determine the chemical composition of cigarette smoke (Fisher, 1999; Norman, 1999).
When cigarettes are smoked, a complex mixture is inhaled into the respiratory system. Ideally, cigarette smoke should be tested as it is generated and experienced by the smoker. However, during the sequence from lighting a cigarette to inhaling a puff of smoke into the respiratory system, various overlapping chemical, physical and physiological phenomena occur. As fresh, un-aged cigarette smoke is generated, it is an extremely complex, dynamic and reactive system due to its physical properties and its chemical composition. In this regard, it is similar to other extensively studied complex mixtures such as air pollution or diesel engine emissions.
The complex nature of cigarette smoke and potential changes in the material during collection and analysis are key factors that must be considered when evaluating the toxicity of cigarette smoke. There are hundreds of papers in the scientific literature that address the physical nature and the chemical composition of tobacco smoke, especially mainstream cigarette smoke (Baker, 1999; Johnstone and Plimmer, 1959; Wynder and Hoffmann, 1967a; Stedman, 1968; International Agency for Research on Cancer (IARC), 1985; Institute of Medicine, 2001a). The first sections of this paper provide a brief overview of mainstream and sidestream smoke. The definitions and the differences between mainstream smoke, sidestream smoke and environmental tobacco smoke (ETS) are reviewed. Current knowledge about the generation, the physical properties and the chemical composition of the aerosol “cigarette smoke” is summarized and discussed briefly. Emphasis is placed on examples of artifact formation that have been reported when evaluating cigarette smoke and the relevance of artifact formation to biological testing intended to assess the toxicity of cigarette smoke is noted. Special attention is paid to cigarette smoke ageing effects and to specific examples of artifact formation observed during sampling (nitrogen dioxide, methyl nitrite, etc.).
Subsequent sections of the manuscript address smoke generation and testing protocols that are applied to mainstream cigarette smoke. Historically, the generation of cigarette smoke for chemical and biological testing has been based on standard smoke generation procedures that are intended for product comparisons. More recently, emerging global regulations have called for alternative smoke generation methods, with emphasis on results relevant to conditions of product use, e.g., estimates of maximum smoke emissions. However, such regulations have done so without the benefit of a complete understanding regarding the effects of different machine smoking regimes on the chemical composition of smoke. Therefore, the many factors that affect mainstream smoke yield and composition will be discussed, followed by a summation of perceptions and misperceptions regarding current standardized machine-based smoking regimes.
Two areas of current scientific interest and debate follow, including a discussion of test methods intended to estimate mainstream smoke yields under actual conditions of consumer use, and the scope of chemical testing appropriate to characterize the chemical composition of cigarette smoke in regard to its toxicity. Strategies for establishing alternative smoke generation methods will also be addressed, as will the potential effects of alternative smoking conditions on analytical accuracy and precision. Numerous smoke constituents have been reported to be associated with the risks of cigarette smoking. Such analytes are typically referred to as “Hoffmann analytes.” Current regulatory requirements that include Hoffmann analyte analysis are summarized and the potential effect of alternative smoke generation methods on individual constituent yields considered. Finally, a limited critique of emerging regulation that relates to mainstream cigarette smoke measurements, including a discussion of recent WHO recommendations, is offered.
Section snippets
Smoke stream definitions and smoke formation processes
“Mainstream smoke” is the smoke emerging from the mouth end of a cigarette during puffing; “sidestream smoke” is primarily the smoke emerging into the environment from the lit end of the cigarette between the puffs. Sidestream and exhaled mainstream smoke diffuse into the atmosphere, become diluted by ambient air and, after various physical and chemical changes, including reactions with chemical substances not generated by tobacco, become “ETS” (environmental tobacco smoke). Exhaled,
The dynamic nature of mainstream cigarette smoke and potential for artifact formation
A chemist can easily imagine what reactions are likely to take place between the different classes of components in the dynamic, complex system that we have called “fresh, un-aged tobacco smoke”. In addition to a time- and environment-dependent increase of the size of the smoke particles, new substances will be generated as artifacts, and/or disappear, due to interaction of the various smoke components. Artifact formation may occur also in the collected smoke. The chemical nature of the smoke
The physical properties and chemical composition of sidestream smoke
In general, the same substances present in mainstream smoke are also present in sidestream smoke. However, the relative yield per cigarette and the mainstream/sidestream ratios found for different components are highly dependent on cigarette construction and on the specific component considered. For most components there is no correlation between the amounts of tobacco burnt during the puffing and smoldering periods. As first shown by Wenusch (Wenusch and Schöller, 1938) in the 1930s, the
The generation and testing of mainstream cigarette smoke
“Yield” and “composition” measurements are the two types of mainstream smoke testing typically applied to investigate mainstream cigarette smoke. Smoke yield measurements include the determination of “tar,” nicotine and carbon monoxide as generated under standardized conditions. Smoke yield measurements gauge the relative amount of smoke produced by a cigarette under specific smoking conditions, without regard for smoke composition. When investigating the composition of cigarette smoke, the
Standardized test methods applied to determine mainstream smoke yields
The development and validation of standardized smoking methods for testing cigarettes has been of interest for many decades. For example, in 1933, Pfyl recognized the need to standardize “the artificial smoking of tobacco products” (Pfyl, 1933). To that end, considerable effort has been expended since the 1960s to establish national and international standard testing protocols (e.g., the FTC (US Federal Trade Commission), ISO (International Organization for Standardization), CORESTA
Test methods intended to estimate mainstream smoke yields under conditions of use
The implementation of a cigarette test method to estimate mainstream smoke yields under conditions of use has been suggested for many years. For example, in 1981, the US Surgeon General's Report recommended establishing a “maximum-yield assay” for the determination of “tar,” nicotine and carbon monoxide (US Department of Health and Human Services, 1981). The report suggested a strategy for devising the machine testing parameters, recommending that machine puffing parameters be based on an upper
Tests that explore the chemical composition of cigarette smoke
Tests that explore the chemical composition of cigarette smoke have been conducted for many decades and for many reasons. Extensive research efforts have been conducted to characterize the general composition of cigarette smoke, often with emphasis on compounds that may impart desirable taste characteristics to smoke. Such studies have been “open-ended” in nature. The study objective has been to identify as many smoke constituents as possible. The studies have also been largely qualitative in
Emerging regulation and mainstream cigarette smoke measurements
National and international regulations that require the testing and reporting of mainstream smoke “tar,” nicotine and carbon monoxide yields have existed for many decades in many countries. Since the late 1990s, the testing of various other smoke emissions has also been required by some countries. When considered collectively, it is apparent that substantial differences have existed in both the scope of testing required and in the technical execution of the tests prescribed by different
Conclusion
Fresh, un-aged cigarette smoke is an extremely complex and reactive mixture. To achieve correct results when testing cigarette smoke, special care must be taken to avoid sample ageing effects and artifact formation. This is valid for chemical analytical evaluation and also for in vitro and in vivo testing. For instance, the change in particle size due to ageing may influence the results of inhalation tests with small rodents. Artificial changes in smoke condensate composition may also have an
Acknowledgments
The authors wish to express their appreciation to Ms. Hilda Foster and Ms. Helen Chung of the RJRT Library for their helpful insight and support in gathering literature citations. The authors are also grateful to Ms. Joy Bodnar for helpful discussions and technical support during the preparation of this manuscript, and to Dr. Ryan Potts for his thoughtful suggestions.
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