Particle Partitioning in the

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NOVEMBER 2001 VOLUME 14, NUMBER 11 © Copyright 2001 by the American Chemical Society

Perspective A Consideration of the Role of Gas/Particle Partitioning in the Deposition of Nicotine and Other Tobacco Smoke Compounds in the Respiratory Tract James F. Pankow* Department of Environmental Science and Engineering, OGI School of Science & Engineering, Oregon Health & Science University, P.O. Box 91000, Portland, Oregon 97291-1000 Received May 11, 2001

Tobacco smoke is an aerosol that contains both gaseous and suspended particulate material (PM). The particles are largely liquid droplets containing a wide variety of condensed organic compounds. Each compound in the smoke will partition between the gas and PM phases and will always seek a state of gas/particle equilibrium. When tobacco smoke is inhaled, a compound such as nicotine can deposit in the respiratory tract (RT) by four different mechanisms: (1) direct gas deposition (DGD) of the portion of the compound that is initially in the gas phase of the inhaled smoke; (2) evaporative gas deposition (EGD) of PM-phase compound by evaporation to the gas phase, then deposition; (3) particle deposition, evaporation from the deposited particle, then deposition from the gas phase (PDE); and (4) particle deposition with diffusion (PDD) into RT tissue. Three of the mechanisms (DGD, EGD, and PDE) involve volatilization from the PM phase. The relative importance of all the mechanisms is therefore greatly affected by the volatility of the compound from the PM phase as it is set by the compound’s gas/particle partitioning constant Kp through the compound’s vapor pressure p°L. For a largely nonvolatile compound such as benzo[a]pyrene, only PDD will likely be important. For a semivolatile compound such as nicotine, all four mechanisms can be important. Because tobacco smoke alkaloids such as nicotine can exist in protonated as well as free-base form, the fraction Rfb of the compound that is in the neutral free-base form in the PM phase plays a critical, pH-dependent role in determining the relative importance of the four mechanisms. Equations are developed that can be used to ascertain the importance of the DGD and EGD mechanisms. The value of Rfb for nicotine in a tobacco smoke PM is set by pHeff, the effective pH of the PM phase. Historically, a primary method for measuring “smoke pH” has involved the direct exposure of a pH electrode to tobacco smoke. This method cannot yield direct insight into pHeff or Rfb values because (1) problems exist in using such an electrode to measure smoke PMphase pH, and (2) by itself, a measurement of the pH of tobacco smoke PM says nothing about the effects of PM-phase activity coefficients of protonated and free-base nicotine on the nicotine species distribution. The “acidic” values that have typically been measured for cigarette “smoke pH” by the direct pH electrode method are therefore neither reliable nor useful in determining the relative distribution of PM-phase nicotine among the protonated and free-base forms. The 10.1021/tx0100901 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/02/2001

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dependence of the volatility of nicotine from tobacco smoke PM on Rfb means that measuring the gas/particle distribution of nicotine under equilibrium conditions in a tobacco smoke by denuder samplers (or by another method) can yield information about the nicotine Kp for that smoke. Knowledge of Kp,fb, the partitioning constant for nicotine in the free-base form, then allows calculation of Rfb through the relation Kp ) Kp,fb/Rfb. The available data suggest that the smoke PM from some commercial cigarettes can be characterized by Rfb g 0.4, i.e., 40% or more of the nicotine in the free-base form. This conclusion is consistent with (1) the gas-sampling denuder results obtained by Philip Morris in which significant tobacco smoke nicotine was observed to deposit in acid-coated denuder tubes, with more depositing when the cigarette tobacco blend was treated with ammonia; (2) the view that the sensory “impact” exhibited by some tobacco smokes is caused by the deposition of gaseous nicotine in the pharynx; (3) the observed throat irritation caused by nicotine inhalers; and (4) the high overall respiratory tract deposition efficiencies for nicotine of 0.9 and greater that have been reported for some cigarette smokes. The available information combines to create a picture of nicotine in cigarette smoke that contradicts the traditional view that cigarette smoke PM is typically acidic, with little free-base nicotine typically present in the smoke PM phase. Government agencies interested in establishing a framework for the testing and monitoring of nicotine delivery may wish to consider requiring the measurement and publication of the PM-phase Rfb values for the cigars and cigarettes marketed in their jurisdictions. I. Introduction II. Theory III. Acid/Base Chemistry of Nicotine A. Forms of Nicotine B. Chemical Activities and Activity Coefficients C. pH of Tobacco Smoke PM

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IV. Acid/Base Dependence of the Volatility of 1473 Nicotine A. General 1473

V.

VI.

VII. VIII. IX.

B. Mainstream Tobacco Smoke Particulate Material (PM) C. “Impact” D. Kp and Kp,fb for Nicotine Denuder Studies of Gas/Particle Distributions of Nicotine in Tobacco Smoke A. fg, fg,e, TPM, and Kp B. fdenuder: The Total Denuder-Collected Nicotine Fraction C. “Impact” (Reprise) Deposition of a Tobacco Smoke Compound in the Respiratory Tract A. Overall Deposition Fraction F B. fDGD C. FTr D. fEGD E. Components of fEGD: fEGD,dep-particles and fEGD,exh-particles F. The Case When EGD Operates Similarly on Exhaled and Deposited Particles Conclusions Acknowledgement References

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I. Introduction Tobacco smoke aerosols are highly concentrated mixtures of particles and gases (Figure 1). The particles are

Figure 1. Schematic diagram of a tobacco smoke aerosol with the following characteristics: particle diameter ≈ 0.3 µm, particle number concentration ≈ 109 particles/cm3, and particle mass concentration ≈ 107 µg/m3. Each particle is represented as a sphere. The interparticle distance and particle diameter are presented to scale. For tobacco smoke aerosols, a typical particle composition might be ∼7% nicotine, ∼10% water, and ∼83% tar.

comprised primarily of the many organic compounds that condense (along with some water) to form liquid droplets when combustion zone gases cool as they leave that zone. Foremost in importance among the particle-phase organic compounds is nicotine, specifically “(S)-(-)-nicotine”, which can exist in both “free-base” and protonated forms. All of the many other particle-phase organic compounds are grouped together under the category of “tar”. Each of the molecular constituents in the aerosol, e.g., nicotine, the nitrosamines, other organic compounds, water, ammonia, carbon dioxide, etc., will distribute (i.e., partition) between the gas and particle phases (Figure 2). This “gas/ particle partitioning” is the same type of physical, gas/ liquid distribution process by which an atmospheric gas such as nitrogen dissolves in blood. When a tobacco smoke aerosol is drawn into the respiratory tract (RT),1 a portion of each compound of interest (e.g., nicotine) will initially be in the gas phase, and a portion will be in the particle phase. Both

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of these phases can serve as pathways for the delivery of the compound to RT tissue, be it oral, pharangeal, 1Abbreviations: BaP, benzo[a]pyrene; c , concentration (ng/m3) in g gas phase; cp, concentration (ng/µg) in particulate material phase; cp,fb, concentration (ng/µg) of free-base form of an alkaloid such as nicotine in the particulate material phase; DGD, direct gas deposition; EGD, evaporative gas deposition; ELJ, electrical potential at the external side of the liquid junction to the internal reference electrode of a combination pH electrode; ELJ,buffer, value of ELJ when the pH electrode is in an aqueous buffer; ELJ,smoke, value of ELJ when the pH electrode is exposed to a given tobacco smoke; ETS, environmental tobacco smoke; fdenuder, fraction (dimensionless) of the total smoke amount of a compound that enters a denuder that is captured by the denuder; fDGD, fraction (dimensionless) of the total amount of an inhaled smoke compound that deposits in the respiratory tract by the DGD mechanism; fEGD, fraction (dimensionless) of the total amount of an inhaled smoke compound that deposits in the respiratory tract by the EGD mechanism; fEGD,dep-particles, fraction (dimensionless) of the total amount of an inhaled smoke compound that deposits in the respiratory tract by the EGD mechanism from particles that deposit; fEGD,exh-particles, fraction (dimensionless) of the total amount of an inhaled smoke compound that deposits in the respiratory tract by the EGD mechanism from particles that are ultimately exhaled; fg, fraction (dimensionless) of a compound that is in the gas phase; fg,e, fraction (dimensionless) of a compound that is in the gas phase at equilibrium; fg,init, fraction (dimensionless) of a compound that is initially in the gas phase as cigarette smoke exits the cigarette filter; fg,init,ref, fraction (dimensionless) of a compound that is initially in the gas phase as cigarette smoke from a reference cigarette exits the cigarette filter; fp, fraction (dimensionless) of a compound that is in the particle phase; fp,e, fraction (dimensionless) of a compound that is in the particle phase at equilibrium; fPDE, fraction (dimensionless) of the total amount of an inhaled smoke compound that deposits in the respiratory tract by the PDE mechanism; fPDD, fraction (dimensionless) of the total amount of an inhaled smoke compound that deposits in the respiratory tract by the PDD mechanism; F, fraction (dimensionless) of the total amount of an inhaled smoke compound that deposits in the respiratory tract; Fnicotine, fraction (dimensionless) of the total inhaled smoke nicotine that deposits in the respiratory tract; Fparticles, fraction (dimensionless) of the total initial number of smoke particles that deposits in the respiratory tract; FTr, fraction (dimensionless) of the total amount of a nonvolatile tracer in tobacco smoke particulate material that deposits in the respiratory tract; H+, the proton; [H+], concentration (molal) of the proton; {H+}, activity (molal scale) of the proton; H2A, a diprotic organic acid; HA-, first conjugate base of the diprotic organic acid H2A; Ka, acid dissociation equilibrium constant; Kp, gas/particle equilibrium partitioning constant (m3/µg); Kp,fb, gas/particle equilibrium partitioning constant (m3/µg) for free-base form of an alkaloid such as nicotine; Nic, free-base nicotine; [Nic], concentration (molal) of free-base nicotine; {Nic}, activity (molal scale) of free-base nicotine; NicH+, monoprotonated nicotine; [NicH+], concentration (molal) of monoprotonated nicotine; {NicH+}, activity (molal scale) of monoprotonated nicotine; NicH22 +, diprotonated nicotine; [NicH22+], concentration (molal) of diprotonated nicotine; {NicH22+}, activity (molal scale) of diprotonated nicotine; (NicH+)(HA-)(s), solid salt of monoprotonated nicotine with the anion HA-; (N/Tr)i, mass/mass ratio (dimensionless) of nicotine/ tracer in the inhaled smoke; (N/Tr)e, mass/mass ratio (dimensionless) of nicotine/tracer in the exhaled smoke; MWtar, mean molecular weight (g/mol) of the smoke tar; MWPM, mean molecular weight (g/ mol) of the smoke PM phase; p°L, vapor pressure (Torr) of a compound as a pure liquid; PDD, particle deposition with diffusion; PDE, particle deposition with evaporation; pH, t - log{H+}; pHeff, the effective pH of the tobacco smoke particulate material phase for nicotine; pKa, t log Ka (for monoprotonated nicotine pKa ) 7.98 at 20 °C, 7.80 at 37 °C); PM, particulate material; RH, relative humidity; RT, respiratory tract; T, temperature (K); Tri, per puff or per cigarette mass amount (mg) of a nonvolatile tracer compound in the inhaled tobacco smoke particulate material phase; Tre, per puff or per cigarette mass amount (mg) of a nonvolatile tracer compound in the exhaled tobacco smoke particulate material phase; TPM, concentration (µg/m3) of total particulate material (including water, and therefore equivalent to WTPM); WTPM, concentration (µg/m3) of total wet particulate material (equivalent to TPM); Rfb, fraction of the total alkaloid (e.g., nicotine) that is present in the free base form in the particulate material phase; γ, molal scale activity coefficient (dimensionless, always >0, and equal to 1.0 in dilute water); γNicH+, molal scale activity coefficient of protonated nicotine (dimensionless, always >0, and equal to 1.0 in dilute water); γNic, molal scale activity coefficient of free-base nicotine (dimensionless, always >0, and equal to 1.0 in dilute water); γH+, molal scale activity coefficient of proton (dimensionless, always >0, and equal to 1.0 in dilute water); ζ, mole fraction scale activity coefficient of a compound in the smoke particulate material phase (dimensionless, always >0, and equal to 1.0 in the pure liquid); (1 - fg)/(fgTPM) ) cp/cg (m3/µg) for a compound in the smoke aerosol; (1 - fg,e)/(fg,eTPM) ) cp/cg (m3/µg) at equilibrium for a compound in the smoke aerosol; (1 - fg,init)/(fg,initTPM) ) cp/cg (m3/µg) for a compound in the smoke aerosol as the smoke exits the cigarette filter; (N/Tr)e/(N/Tr)i, tracer-normalized fraction of nicotine that remains in the exhaled particles as compared to the inhaled particles (when ) 1, no off-gassing has occurred; when ) 0, total off-gassing has occurred); γNic/γNicH+ ) activity coefficient ratio (dimensionless) for free-base and monoprotonated nicotine.

Figure 2. Equilibrium between the particle and gas phases for a compound represented as an asterisk (*). In the case depicted, the compound can be present in significant amounts in both phases: the compound is neither highly volatile, nor of very low volatilitysthe compound is “semi-volatile”. (Other compounds besides * that are present in the gas and particle phases are not shown.) Kp is the equilibrium gas/particle partitioning constant.

bronchial, or alveolar. Four distinct mechanisms can be identified by which RT uptake can occur. Abbreviated here as DGD, EGD, PDE, and PDD, the four mechanisms are described in Table 1 and Figures 3 and 4. Two of the mechanisms, DGD and EGD, involve only gas deposition. The other two mechanisms, PDE and PDD, involve particle deposition. It is the last step of each of the mechanisms that provides the final, molecular-level delivery of the compound to the tissue. In three of the mechanisms, DGD, EGD, and PDE, the last step is deposition from the gas phase. A single particle that ultimately deposits in the RT can contribute to EGD, PDE, and PDD. Indeed, a portion of a compound of interest in such a particle can evaporate and deposit by EGD before the particle deposits; after the particle deposits,what remains of the compound can deposit by PDE and PDD. For high volatility compounds (e.g., CO and CO2) that reside almost exclusively in the gas phase in tobacco smoke, only DGD will be important. For very low volatility compounds [e.g., polycyclic aromatic hydrocarbons such as benzo[a]pyrene (BaP)], which tend to reside almost exclusively in the particle phase in tobacco smoke (see below), only PDD will be important. For “semivolatile” compounds (e.g., nicotine) that exhibit intermediate volatility from the particle phase, significant amounts of such compounds can be present in both phases, and so all four mechanisms can be important.

II. Theory The theory of gas/particle partitioning has been discussed in detail by Pankow and co-workers (1-6) for a wide range of aerosol types, including environmental and mainstream tobacco smoke. As discussed in that work, one form for expressing the equilibrium partitioning constant for this process is

Kp )

cp cg

(1)

where

cp ) concentration in the particle phase (ng/µg of particle phase) (2) cg ) concentration in the gas phase (ng/m3 of gas phase) (3)

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Figure 3. Four mechanisms by which molecules [each of which is represented as an asterisk (*)] in a tobacco smoke aerosol can come into direct molecular contact with respiratory tissue. The first three mechanisms require that the species represented by an asterisk (*) have some significant ability to vaporize to the gas phase. No ability to vaporize is required for the PDD mechanism. In PDE, the length of time that evaporation continues after deposition will depend on how long a deposited particle remains as a distinct entity on the surface of the tissue.

Figure 4. Gas/particle distribution status for a particle in an aerosol in the respiratory tract for three related stages: (a) initial gas/particle distribution, with initial driving force (f) for deposition from the gas phase; (b) after some DGD, which then drives evaporation from the particulate material phase and EGD; and (c) DGD is complete and EGD continues. Kp is the equilibrium gas/particle partitioning constant. Table 1. Mechanisms of Deposition to Respiratory Tract (RT) Tissue of Compounds of Interest in Tobacco Smoke mechanism

steps

DGD

direct gas deposition

1. A gaseous molecule initially in the gas phase of the inhaled smoke deposits directly to RT tissue

EGD

evaporative gas deposition

1. A molecule initially in the particle phase of the inhaled smoke evaporates to the gas phase 2. The gaseous molecule deposits to RT tissue

PDE

particle deposition with evaporation

1. A particle containing a molecule of a compound of interest deposits on RT tissue 2. The molecule evaporates to the gas phase 3. The gaseous molecule deposits to RT tissue

PDD

particle deposition with diffusion

1. A particle containing a molecule of a compound of interest deposits on RT tissue 2. The molecule diffuses from the particle into RT tissue

The partitioning of gases into blood serum can be expressed in the units of Kp so that for N2 at 37 °C (7):

ng of N2/µg of serum ng of N2/m3of gas

) 1.2 × 10-14

(4)

N2 exhibits a very small equilibrium constant for partitioning to blood because N2 is a highly volatile compound. A fundamentally important parameter in aerosols is the concentration of total particulate material (TPM (µg/m3), Table 2). For tobacco smoke, all of the condensed

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By definition,

Table 2. Some Representative Levels TPMa (µg/m3) clean outdoor air urban air, slightly to very contaminated indoor air, very contaminated with “environmental” tobacco smoke (ETS) mainstream tobacco smoke

∼3-20 ∼30-500 ∼100-1000 106-108

a

In air pollution publications, the term TSP (total suspended particulate material) is frequently used.

nicotine, water, and tar are included in the measurement of TPM; the synonymous term “WTPM” (“wet TPM”) is sometimes used to emphasize the inclusion of water in the TPM measurement. In many of the extant documents on tobacco smoke, the term “TPM” refers to the total mass of PM produced by a single cigarette or cigar. However, since each such mass amount is directly associated with a specific volume of smoke aerosol, TPM may equally well be viewed as carrying units of mass per volume, as will be the convention in this paper. We will assume that a tobacco smoke particle is mostly liquid. For that case, it is known (1) that Kp values for neutral organic compounds depend inversely on the compound-dependent vapor pressure p°L:

Kp ∝

1 p°L

(5)

Kp values decrease with increasing temperature because p°L values increase with temperature. Since we have

TPM (µg/m3) ) mass of particles per volume of aerosol system (6) then

cpTPM ) mass of compound in particle phase per volume of aerosol system (ng/m3) (7) Because cg essentially gives the mass of compound in the gas phase per volume of aerosol system (the aerosol particles occupy only a very small fraction of the volume of the aerosol system), the fraction of the compound of interest that is in the gas phase is given by (5)

fg )

cg 1 ) cg + cpTPM 1 + (cp/cg)TPM

(8)

When there is equilibrium between the gas and particle phases so that cp/cg ) Kp, then the equilibrium fraction in the gas phase fg,e is given by

1 1 + KpTPM

(9)

(cp/cg)TPM cpTPM ) cg + cpTPM 1 + (cp/cg)TPM

(10)

KpTPM 1 + KpTPM

(11)

fg,e )

and

fp,e )

(12)

fg,e + fp,e ) 1

(13)

and

For mainstream tobacco smoke PM at 37 °C, Pankow et al. (5b) have estimated that the neutral free-base form of nicotine exhibits a partition coefficient of Kp,fb ) 10-5.54 (the dependence of Kp for total nicotine on the fraction of the PM phase-nicotine that is in the free-base form is given by eq 37.) Thus, if essentially all of the nicotine in the PM phase of a given mainstream tobacco smoke aerosol was in the neutral free-base form, then for a mainstream TPM value of ∼4 × 107 µg/m3 ()12 mg in eight 35 mL puffs), eq 9 would predict for nicotine at 37 °C that fg,e ≈ 0.008 (i.e. 0.8% in the gas phase). As a comparison, at 37 °C the p°L of BaP [a compound which has been implicated as a tobacco smoke carcinogen (8)] is only about 10-6.3 Torr, which is a factor of ∼105.3 lower than the p°L for free-base nicotine (10-1.0 Torr) at the same temperature (9). By virtue of the inverse dependence of Kp on p°L in relation 5, we therefore obtain the rough estimate that Kp ≈ 10-0.2 for BaP in tobacco smoke at 37 °C so that under the same mainstream smoke conditions just considered, fg,e for BaP is exceedingly small, only ∼4 × 10-8. BaP is thus a very low volatility compound in tobacco smoke, and as suggested above, its form of delivery to RT tissue is likely to occur almost exclusively through the PDD mechanism. It would therefore seem that the tissues at primary risk for exposure to tobacco-smoke BaP would be those tissues that come into direct contact with tobacco-smoke PM.

III. Acid/Base Chemistry of Nicotine A. Forms of Nicotine Nicotine, cocaine, codeine, morphine, and a large number of other pharmacologically active plant compounds are alkaloids. As such, each of these compounds contains at least one basic nitrogen that can pick up a proton (H+). Nicotine can pick up a total of two protons. The three forms of nicotine are abbreviated herein as Nic, NicH+, and NicH22+ (Figure 5). The species NicH22+ only becomes important at very low pH values and so has usually been assumed to be of negligible importance in tobacco smoke PM. The acid dissociation of protonated nicotine to return the free-base nicotine plus a proton occurs according to

NicH+ ) protonated nicotine (“bound” nicotine) H+ Nic unprotonated nicotine + proton (14) (“free-base” nicotine)

Similarly,

fp )

fg + fp ) 1

The unprotonated form of nicotine is referred to as the “free-base” because the nicotine has literally become free of the proton on NicH+ and free of the restrictions imposed by carrying the associated positive ionic charge. As is discussed below, free-base nicotine can volatilize to the gas phase, but protonated nicotine cannot. Also, free-base nicotine can move into and through tissue lipid

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By definition

Figure 5. Three forms of nicotine.

layers, but protonated nicotine is largely excluded from such layers.

B. Chemical Activities and Activity Coefficients In a liquid solution such as exists in tobacco smoke PM, it is the chemical activity of a species that is the fundamental measure of the degree to which the chemical is available for interaction with other species. The chemical activity may or may not equal the chemical concentration. As an anthropomorphic analogy, consider a group of people in a room with a door. The tendency of the people to exit through the door (their activity) will depend on their general energy level and so can be greater or less than in some other circumstance when the concentration of people in the room is the same, but the average energy level of the people in the room is different. Reaction 14 is taken to be occurring in liquid tobacco smoke PM. In that phase, all of the chemical species in reaction 14 will certainly exist as dissolved species, and will have real and measurable chemical activities that we represent as {NicH+}, {Nic}, and {H+}. These activities are related to the corresponding concentrations [NicH+], [Nic], and [H+] through molal activity coefficients γ:

{NicH+} ) γNicH+[NicH+]

(15)

{Nic} ) γNic[Nic]

(16)

{H+} ) γH+[H+]

(17)

Each γ here is the correction factor (always > 0) that reveals how active the species feels in the solution in question as compared to the “reference” state, which is dilute water. When in the reference state, the species is surrounded exclusively by water molecules, and its γ equals 1.0. In solutions other than dilute water (e.g., in tobacco smoke PM), the value of γ will depend on the nature of the solution and on the nature of the species (e.g., NicH+, Nic, or H+). If a given species feels twice as active as when it is in dilute water, then its γ is 2.0. If it feels half as active, then its γ is 0.50. Since tobacco smoke PM is only about ∼10% water, with the remaining ∼90% being a mixture of many different organic compounds, it is not likely that all three of the activity coefficients γNicH+, γNic, and γH+ will be close to 1.0 in that phase. The equilibrium constant for the acid dissociation in reaction 14 is represented as

Ka )

{Nic}{H+} {NicH+}

(18)

pH t -log{H+}

(19)

pKa t -log Ka

(20)

with pKa ) 7.98 at 20 °C and 7.80 at 37 °C (10). Equation 18 makes it clear that the pH of the solution of interest affects the position of the equilibrium in reaction 14. As a result, measurements of what has been termed the “pH of tobacco smoke” (aka “smoke pH”) have been of considerable interest to the tobacco industry, as is made clear in the documents on cigarette design and nicotine chemistry that became available as a result of litigation in this country (see refs 11 and 12 generally, and 13-16 specifically). It has also been of interest to those who have argued that cigarettes are drug delivery devices, and should be regulated as such (e.g., ref 17). There has, however, been much misunderstanding concerning the definition, meaning, use, and utility of the term “smoke pH”. The confusion regarding “smoke pH” has resulted from the fact that tobacco smoke is composed of both (1) a complex combustion gas mixture and (2) droplets that are largely nonaqueous in composition, containing only ∼10% water. With regard to the gases, although some have felt that the amounts of gases such as CO2 need to be considered when examining the acid/base properties of tobacco smoke, it is the tobacco smoke PM that is the phase of interest relative to pH in the context of reaction 14. The term pH of the tobacco smoke PM phase is therefore to be preferred over both “pH of tobacco smoke” and “smoke pH”. With regard to the largely nonaqueous composition of the droplets making up tobacco smoke PM, although such a phase will have a perfectly well definable pH, measuring pH in such matrixes is very difficult (see below). Rearranging eq 18 yields (see also Table 3)

{NicH+} ) {H+}/Ka {Nic}

(21)

[NicH+] γNicH+ 10-pH ) -pK [Nic] γNic 10 a

(22)

In dilute water, we have that γNic ) 1, and γNicH+ ) 1, so that {Nic} ) [Nic] and {NicH+} ) [NicH+]. Thus, in dilute water (or at least when γNic/γNicH+ ) 1), from eq 22, we have

and

The dashed boxes emphasize that the enclosed equations are limited to the case of dilute water (or at least when γNic/γNicH+ ) 1). The fraction of the total nicotine that is present in the free base form in the liquid phase is represented as Rfb.

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Table 3. Different Ranges in pH and the Corresponding Relations between {Nic} and {NicH+} for Any Type of Solutiona pH

{H+}

any solutionb (by eq 21)

pH < pKa pH ) pKa pH > pKa

{H+} > Ka {H+} ) Ka {H+} < Ka

{Nic} < {NicH+} {Nic} ) {NicH+} {Nic} > {NicH+}

dilute water onlyc (by eq 28) [Nic] < [NicH+] [Nic] ) [NicH+] [Nic] > [NicH+]

NicH+ dominatesd equal amounts of each free base dominates

Rfb < 0.5 Rfb ) 0.5 Rfb > 0.5

a The corresponding ranges in behavior in dilute water for [Nic], [NicH+], and R are also given; at 25 °C, pK ) 7.98; at 37 °C, pK ) fb a a 7.80 (10). b Relative values of Rfb are not specified by means of eq 21 without knowledge of γNic/γNicH+. c Or at least when γNic/γNicH+ ) 1. d Assumes that NicH 2+ is not important. 2

When the diprotonated species can in fact be neglected, then

Rfb )

[Nic] [Nic] + [NicH+]

)

1 (25) 1 + [NicH+]/[Nic]

By eq 22,

γNic 10-pH [NicH+] ) γNicH+ 10-pKa [Nic]

1 γNic 10-pH 1+ γNicH+ 10-pKa

(27)

Therefore, Rfb for nicotine in dilute water is easily calculated based on pH (see also Table 3). Because tobacco smoke PM is not dilute water, eq 27 is the correct expression for Rfb for nicotine in that phase, not eq 28. Values of Rfb for nicotine in tobacco smoke PM cannot, however, currently be calculated by eq 27 because of (1) uncertainties regarding the value of the ratio γNic/ γNicH+ in such a phase and (2) considerable difficulties in measuring the pH of tobacco smoke PM (see below). It is very useful to roll up the matter of activity coefficients and pH into one parameter that we rigorously define as the effective pH of the tobacco smoke PM:

γNic {H+} γNicH+

(29)

γNicH+ γNic

(30)

or

pHeff t pH + log Substitution into eq 27 yields

Rfb )

10-pKa + 10-pHeff

-pKa

10

(31)

which can be rearranged to yield

pHeff ) pKa + log

Rfb 1 9 Rfb

any solution (by eq 31) [Nic] < [NicH+] NicH+ dominatesa [Nic] ) [NicH+] equal amounts of each [Nic] > [NicH+] free base dominates

Rfb < 0.5 Rfb ) 0.5 Rfb > 0.5

Assumes that NicH22+ is not important.

(26)

In dilute water (or at least when γNic/γNicH+ ) 1),

pHeff t -log

pHeff pHeff < pKa pHeff ) pKa pHeff > pKa a

In general, then

Rfb )

Table 4. Different Ranges in pHeff and the Corresponding Ranges in Behavior for [Nic] and [NicH+] for Any Type of Solution, Including Tobacco Smoke PM

(32)

In this manner, if a tobacco smoke PM phase is charac-

terized by pHeff ) pKa, then Rfb ) 0.5 just as Rfb ) 0.5 in a dilute aqueous solution when pH ) pKa (see also Table 4). When using eq 32, it is important to note that the accurate estimation of pHeff will become increasingly difficult as Rfb f 1. Inasmuch as eq 31 allows one to take a given value of pHeff and calculate the corresponding value of Rfb, so too can one perform the inverse operation, namely take a value of Rfb for nicotine in some tobacco smoke PM, and backout the corresponding value of pHeff for that PM by means of eq 32. Thus, the calculated PM-phase pH values mentioned in Pankow et al. (5) should be viewed as being pHeff values. Also, Pankow (18) was recently misquoted (by ref 19, page 74) as having presented calculated pH values of 5.7, 6.7, 7.9, and 8.1 for tobacco smoke PM from four unnamed brands of U.S. cigarettes. In actual fact, the values presented were pHeff values, based on measured Rfb values of 0.0045, 0.045, 0.45, and 0.51, respectively.

C. pH of Tobacco Smoke PM Although numerous values of “smoke pH” have been reported in the literature, and many more have been reported within internal tobacco company documents, it is without question true that not a single accurate measurement of the pH of a tobacco smoke PM phase has been made, as calibrated on the pH scale of standard aqueous buffer solutions. In effect, a reported “smoke pH” value cannot be interpreted to mean anything more than the pH reading obtained from tobacco smoke according to the chosen empirical protocol. The following three important observations can be made. (1) Since differences among the protocols that have been used cause differences in measured pH, results that have been obtained with different protocols should absolutely not be compared. (2) Within a highly specific protocol, real relative differences between different smokes might be discernible so that “smoke pH” values of say 5.5 and 6.5 as measured by one of the protocols for smokes A and B might mean that smoke A is somehow more acidic than smoke B; however, the results do not necessarily mean that either tobacco smoke PM phase is acidic in an absolute sense. (3) Contrary to what has occurred (see below), pH results obtained with any of the heretofore used empirical

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Figure 6. (a) Combination pH electrode of the type used to measure “pH of tobacco smoke”. Aqueous filling solutions shown as stipled areas. Sources of the overall electrical signal of the electrode are E1, constant potential of the electrode inside the {H+}-sensing glass bulb; E2, pH-dependent potential across the {H+}-sensing glass bulb; E3, external “liquid junction” potential ELJ that forms at the interface of the test solution with the “salt bridge” leading to the internal reference electrode; and E4, constant potential of the internal reference electrode system, including the internal liquid junction potential. pH measurements are best made when ELJ is the same constant value for the test solution and for the standard aqueous buffer(s) used to calibrate the electrode; changes in electrode response are then due only to changes in E2. (b) Tip of combination electrode showing the conducting film of buffer solution + tobacco smoke PM that forms when tobacco smoke is passed over the electrode according to the method of Sensabaugh and Cundiff (22).

protocols should not be used to infer an understanding of the distribution of nicotine between the free-base and protonated forms within the tobacco smoke PM phase by means of eq 28. Many of the measurements of “smoke pH” that have been made have been obtained by one or the other of two different types of methods. The first type of method involves the addition of what amounts to a large quantity (e.g., 50 mL) of water (preferably high purity, CO2-free water) to a small amount of collected smoke material, then measurement by pH electrode. Different approaches have been used to combine the smoke material with the water, including (20, 21) (a) collecting tobacco smoke PM on a glass fiber filter, then suspending the filter in the water; (b) collecting “whole smoke condensate”, then mixing it with the water; and (c) bubbling tobacco smoke through the water. Although each of these approaches may provide some measure of the overall acid/base properties of the tobacco smoke PM, the large degree of dilution in each means that they cannot provide a measure of the pH of the original tobacco smoke PM, except perhaps by coincidence. For approaches b and c, there is also the issue that acids and/or bases originally in the gas-phase portion of the aerosol can dissolve in the water and affect the final measured pH. The second type of method that has been used to measure “smoke pH” was developed by Sensabaugh and Cundiff (22). It involves no large-scale addition of water. A “combination” pH electrode (Figure 6a) is first calibrated using standard aqueous buffer solutions (e.g., with a pH value in the 6-7 range). A thin film of one of the buffers (often pH 6) is left on the electrode, and tobacco smoke is simply drawn over the electrode. When smoke PM deposits on the electrode, enough of a conducting film

Pankow

exists between the {H+}-sensing bulb and the liquid junction to the interior reference electrode (Figure 6b) that a pH reading can be obtained. Many of the measurements of “smoke pH” made during the last 30 years have been carried out using variations of this method. These include the determinations made by (1) Sensabaugh and Cundiff (22) and Brunnemann and Hoffmann (23); (2) commercial laboratories on behalf of various clients (e.g., refs 24 and 25); and (3) tobacco company scientists for purposes of product development (e.g., refs 13-16), and to determine the extent to which brand-dependent “smoke pH” and “calculated” free-base nicotine deliveries can be correlated with cigarette sales (e.g., refs 14 and 26). The “smoke pH” values that have been obtained by the general method of Sensabaugh and Cundiff (22) for cigarette smoke have frequently been in the range 5.06.5. There are two main reasons why the electrode method of Sensabaugh and Cundiff (22) cannot provide accurate measures of the true pH of tobacco smoke PM as calibrated against standard aqueous buffer solutions, except by chance. First, when making pH measurements with a combination electrode, it is well-known that (2730) (a) an electrical potential ELJ forms at the external side of the liquid junction to the internal reference electrode and (b) the value of that potential when the electrode is in a largely nonaqueous organic liquid such as tobacco smoke PM will not be the same as when it is in an aqueous buffer (ELJ,buffer). Second, the liquid film that covers the electrode during the measurement will not, in any event, be the same as the smoke PM, but rather a combination of constituents from the collected smoke PM, the initial buffer solution, and the gases from the smoke that dissolve in the new mix. With regard to liquid junction potentials, let ELJ,smoke be the value of ELJ during exposure of the electrode to a given smoke. Since the value of ELJ for a given pH measurement is part of the overall potential measured by the electrode, any significant difference between ELJ,smoke and ELJ,buffer will invalidate the calibration. We note here that liquid junction effects in partially aqueous solutions can bias pH measurements significantly (27). We also note that the simple usage by different researchers of combination pH electrodes of different designs may well complicate the making of “within protocol” comparisons of the results from the various researchers. In such a case, one cause could be electrode-to-electrode differences in the difference between ELJ,smoke and ELJ,buffer. It is apparent that neither the first nor the second type of “tobacco smoke pH” measurement method can be expected to provide an accurate measure of the pH of tobacco smoke PM, as calibrated against standard aqueous buffers. Moreover, even if any such values were accurate, it is far from clear that they could be applied in eq 28 to calculate Rfb for tobacco smoke PM because of the requirement for that equation that γNic/γNicH+ ) 1. These huge problems and questions have been ignored in past studies of tobacco smoke. For example, Morie (31) used eq 28 directly, writing in 1972 that “The pH from the smoke of domestic blend cigarettes ranges from 5.2 to 6.2 (average 5.6). It is obvious from this that the percentage of unprotonated nicotine in the TPM of smoke from these cigarettes is very low (0.22-2.17%).” Related comments found in the internal tobacco company documents that became available as a result of litigation in this country during the 1990s include “In

Perspective

essence, a cigarette is a system for delivery of nicotine to the smoker in attractive, useful form. At ”normal” smoke pH, at or below about 6.0, essentially all of the smoke nicotine is chemically combined with acidic substances, hence is nonvolatile and relatively slowly absorbed by the smoker. As the smoke pH increases above about 6.0, an increasing proportion of the total smoke nicotine occurs in “free” form, which is volatile, rapidly absorbed by the smoker, and believed to be instantly perceived as nicotine “kick.” [Teague, C. E. (1974) R. J. Reynolds Tobacco Company (14)]. and “The “pH” of cigarette smoke is approximately 6.0 (1). At a “pH” of 6.0, virtually all of the nicotine is protonated. In order for a substantial amount [of] nicotine to exist in the unprotonated state, the smoke “pH” would have to be around 8.0 (1). If it could be demonstrated that the “pH” of the smoke from cigarettes on the American market is 7.0 or less, this would constitute proof that the cigarettes are delivering protonated nicotine.” [Anonymous, Philip Morris Tobacco Company, June 24, 1994 (13)]. We conclude that “typical” measured values of cigarette “smoke pH” have been naively treated as if they were pHeff values for the PM in cigarette smoke. The result has been that tobacco smoke PM from cigarettes has been inferred to typically contain mostly protonated nicotine, with relatively little free-base nicotine. This unjustified conclusion has been continuously repeated over many years [as in the above quotes and again recently by Lauterbach (19)] and so, regrettably, has come to be viewed as the common “wisdom” for cigarette smoke.

IV. Acid/Base Dependence of the Volatility of Nicotine A. General One does not need to measure the pH of tobacco smoke PM to estimate how the nicotine is apportioned between the free-base and protonated forms in such PM. Because the volatility of liquid-phase nicotine depends directly on Rfb, measuring Kp for nicotine from tobacco smoke PM can provide a direct measure of Rfb. The neutral, free-base form of nicotine can exist as a solid or liquid and in the gas phase. Protonated nicotine is not capable of existing in the gas phase to any significant extent. Rather, it will usually be found together with oppositely charged “counterions” in a neutral solid salt, or dissolved in a liquid that is again neutral due to a positive/negative charge balance. Protonated nicotine is limited from entering the gas phase because (1) in solids and liquids it is attracted to counterions; (2) in liquids, it can be attracted to liquid “solvent” molecules; and (3) in the gas phase, there is no way for it to find a “solvated” environment.

B. Mainstream Tobacco Smoke Particulate Material (PM) When a tobacco product is heated and burned, a great many compounds are released to the hot gas phase in and near the combustion zone. The gaseous free-base nicotine that is released can originate directly from any free-base nicotine that may be present in the tobacco product and from solid salts in the product in which the nicotine is combined with organic acids. One example of such a salt can be represented as (NicH+)(HA-)(s) where HA- is the first conjugate base of the diprotic organic

Chem. Res. Toxicol., Vol. 14, No. 11, 2001 1473

Figure 7. NicH+ ) H+ + Nic reaction within the tobacco smoke particle phase, along with the coupled gas/particle distribution reaction for free-base nicotine (Nic).

acid H2A (e.g., malic acid). During thermal release from this type of nicotine salt, the anions in the salt take back protons from NicH+ cations according to

(NicH+)(HA-)(s) heat Nic + H2A f

(33)

yielding free-base nicotine (Nic) that can then volatilize. This process has been studied by Seeman et al. (32) for several nicotine salts. As noted above, condensation occurs when the hot, complex, gaseous mixture cools as it is drawn away from the combustion zone and toward the smoker. It is the less volatile of the compounds in the gas mixture that condense to form the majority of the mainstream tobacco smoke PM; the volatility (p°L) of nicotine is sufficiently low that most of the initially gaseous nicotine condenses. The nicotine that enters the PM-phase may remain in the free-base form, or it may become protonated by the action of acids that also condense. At any given instant in the PM phase, it is only the free-base nicotine as set by Rfb that can volatilize to the gas phase (Figure 7).

C. “Impact” The term “impact” occurs repeatedly in the internal tobacco company documents on cigarette design and nicotine chemistry that have become available (11, 12). In these documents, “impact” (1) consistently refers to a sensation in the back of the throat that smokers report feeling during the inhalation of some types of tobacco smoke; (2) is considered to be related to both the total smoke nicotine, and in particular to the fraction of the PM-phase nicotine that is in the free-base form. Synonyms that have been used for “impact” include “sensory stimulation” and “kick”. A relevant quote that provides some additional synonyms and explanation is “The sensory attribute most associated with nicotine is described within the company as “Impact”. It is described by consumers as Throat Catch, Throat Hit, Throat Grip, etc. Our definition of Impact is the sudden, sharp but short-lived sensation (typically less than one second in duration) which is noticed immediately [when] the smoke makes contact with the back of the throat. The higher the nicotine delivery per puff of a product the higher the Impact felt by the inhaling smoker.” [D. E. Creighton, (1987) Brown and Williamson Tobacco Corp. (15)]. The available documents indicate that “impact” has been viewed by the industry as an important characteristic of any successful commercial cigarette brand. They are also consistent in stating that a smoke with too much “impact”

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is “harsh”: “If nicotine satisfaction were the only criteria [sic], increasing the pH would be the obvious step. This, however, introduces a smoke with high physiological impact and a harsh bite, which would seem to offset the advantages gained from increased nicotine.” [L. Chen, (1976) Lorillard Tobacco Company (16)]. If “impact” in the pharynx is indeed caused by nicotine in tobacco smoke, then there are three possibilities regarding its underlying mechanism(s): (1) deposition of just gaseous nicotine; (2) deposition of both gaseous and PM-phase nicotine; or (3) deposition of just PM-phase nicotine. In studies during the 1980s to 1990s at the R. J. Reynolds Tobacco Company, Ingebrethsen and co-workers (33-39) considered how smoke nicotine could deposit in the pharynx and cause “impact” (see especially ref 37, Bates no. 508297980). Referring to what we have here named the EGD mechanism, they wrote “We have hypothesized that a mechanism, possibly the predominant mechanism, of sensory stimulation by nicotine during smoking is via what we have termed evaporative deposition or evaporative mass transfer. Evaporative deposition is the process by which nicotine evaporates from smoke particles during inhalation and is adsorbed to respiratory tract (RT) surfaces.” [B. J. Ingebrethsen et al., (1991) R. J. Reynolds Tobacco Company (38)]. Ingebrethsen and Lyman (37) listed four observations as being supportive of their EGD hypothesis. With italics used for all directly quoted material, these observations (which are also consistent with a role for DGD in “impact”) are (1) “particle deposition in the pharyngeal region is low...”; (2) “[in smoke inhalation experiments with human subjects,] more volatile smoke components, including nicotine, deposited at higher efficiencies than less volatile components...”; (3) “aged smoke was judged to be smoother...” (comment: smoke allowed to coagulate to a larger mean particle size exhibited less impact despite the larger efficiency with which such particles would be expected to deposit in the pharynx); (4) “Throat Impact or Harshness... B ) D < A ...”: cigarette prototype A, 0.53 mg of nicotine delivered/cigarette (control); cigarette prototype D, 0.58 mg of nicotine delivered/cigarette, with 4.5% added malic acid (hydroxybutanedioic acid) in the tobacco blend; cigarette prototype B, 0.74 mg of nicotine delivered/cigarette, with 5.6% added levulinic acid (4-oxopentanoic acid) in the tobacco blend (comment: amending the tobacco blends with organic acids in molecular acid form reduced the ‘impact” of the resulting smoke for comparable delivered nicotine levels, as would be expected if the pHeff of the smoke PM was thereby reduced.)

D. Kp and Kp,fb for Nicotine When the overall Kp partitioning of nicotine is considered (eq 1), then cp (ng/µg) represents the total (protonated + free-base) nicotine that is in the PM-phase; cg (ng/m3) represents just gas-phase free-base nicotine (only the free-base nicotine is volatile). As discussed by Pankow et al. (3, 5) when the partitioning of just the free-base form of nicotine is considered, we have

Kp,fb )

cp,fb cg

(34)

Figure 8. Equilibrium distribution of nicotine between the protonated (NicH+) and free-base (Nic) forms in tobacco smoke particles at two different pHeff values. (a) At a lower pHeff, Rfb is low, and so Kp is relatively large. (b) At a higher pHeff, Rfb approaches 1, and so Kp decreases and approaches Kp,fb. At the higher pHeff, more of the total nicotine is in the gas phase. The particle-to-gas-phase ratio of just the free-base nicotine is the same at both pHeff values because of the constancy of Kp,fb.

where cp,fb (ng/µg) represents just the free-base nicotine that is in the PM-phase, with

cp,fb ) Rfbcp

(35)

Rfbcp cg

(36)

Therefore

Kp,fb ) and

Kp )

cp Kp,fb ) cg Rfb

(37)

The composition of tobacco smoke PM is subject to some variation (e.g., concentration of water, natures and concentrations of the other condensed compounds, etc.). As a result, at a given temperature, unlike the case of nicotine partitioning to a well-defined phase such as dilute water, the value of Kp,fb for partitioning of nicotine to tobacco smoke PM at a given temperature will not take on the same exact value for all different types of such PM. As noted by Pankow et al. (5), however, the potential certainly exists for different tobacco smoke PMs to exhibit very similar values of Kp,fb. In eq 37, since we always have that Rfb < 1, it is always true that Kp > Kp,fb. However, as bases are added (and/ or acids removed) and the pHeff of the PM phase increases so that Rfb in the PM phase approaches unity, then Kp will decrease and approach Kp,fb for that PM. This exact behavior was observed in the laboratory by Pankow et al. (5) when different cigarette smoke PMs collected on filters were exposed to increasing gas-phase concentrations of ammonia, yielding 10-4.94 m3/µg as the average measure of Kp,fb at 20 °C for the cigarette smoke PMs examined [Kp,fb ) 10-5.54 at 37 °C (5b)]. The interplay of Rfb and Kp is depicted schematically in Figure 8. The equation that describes the physical chemical equilibrium of a given neutral compound (e.g., free-base

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Table 5. Parameters in Kp According to Eq 38 R T p°L MWPM ζ

ideal gas constant ) 8.2 × 10-5 m3 atm/mol K temperature (K) vapor pressure (Torr) of the compound of interest as a liquid (temperature dependent)a average molecular weight of the particle phase mole fraction activity coefficient of the compound of interest in the particle phaseb

a May need to be adjusted for the Kelvin effect when the particles are very small (0) that reveals how active the compound of interest feels when it is surrounded by the various chemical species making up the mixture of interest (in this case tobacco smoke PM) as compared to when the compound is present in its own pure phase and surrounded by other like molecules, in which case ζ ) 1.0. Of the variables in eq 38, p°L plays the largest role in determining the value of Kp that a particular compound will exhibit for given aerosol conditions. It is very useful to investigate what eq 38 would predict for the partitioning of free-base nicotine to tobacco smoke PM. Since that equation applies to neutral compounds, any such prediction will be for Kp,fb. At 20 °C, p°L for freebase nicotine is reported to be 10-1.61 Torr (9). The mole fraction scale activity coefficient ζ of free-base nicotine can probably be assumed to be close to 1.0 in tobacco smoke PM. Taking a tobacco smoke PM composition range of 78-86% tar, 7% nicotine, and the balance (15 to 7%) water, with a range of assumed MWtar values of 100-250 g/mol, one obtains a range for MWPM of 60129 g/mol. Using these values in eq 38 with T = 293 K yields a range for Kp,fb of 10-4.91 to 10-5.24 m3/µg, which closely brackets the value of Kp,fb of 10-4.94 measured in the laboratory at 20 °C by Pankow et al. (5).

V. Denuder Studies of Gas/Particle Distributions of Nicotine in Tobacco Smoke A. fg, fg,e, TPM, and Kp Figure 9 contains curves for log fg,e vs log TPM for selected values of Kp. The lines were calculated using eq 9. The coordinates of each point defined by (log fg,e, log TPM) values pertain to a specific Kp, as is made apparent by rearrangement of eq 9 according to

Kp )

1 - fg,e fg,eTPM

(39)

Therefore, (a) knowledge of fg,e and TPM allows calculation of Kp for the smoke PM of interest; and (b) knowledge

Figure 9. Curves of log fg,e vs log TPM for various values of Kp, including Kp,fb for nicotine at 20 °C (∼10-4.94) and at 37 °C (∼10-5.54).

of Kp and Kp,fb allows calculation of Rfb (by eq 37) and pHeff (by eq 32) for the smoke PM of interest. Only a small number of fg values have been reported in the literature for mainstream cigarette smoke. In the work of Lewis et al. (40, 41), gas- and particle-phase nicotine concentrations were individually sought using a denuder/filter/sorbent sampler. Fresh mainstream smoke was drawn through an oxalic acid-coated “denuder” tube followed first by a glass fiber filter, then by a tube of XAD sorbent. As smoke passes through this type of denuder, gaseous (free-base) nicotine diffuses to the walls of the tube and is trapped there with high efficiency because it becomes protonated by the acid coating. Tobacco smoke particles, on the other hand, diffuse more slowly than do gas molecules and so can pass through a properly designed and applied denuder largely unretained, for collection by the filter. Particle-phase organic compounds that evaporate from the filter are collected using the “sorbent” portion of the sampler. The filter- and sorbentcollected material are then assumed to have been associated with the particle phase as the aerosol entered the sampler. It is often assumed that little or no PM-phase material is collected on the denuder walls. This may not always be a good assumption. When it is not, the measured value of fg will be made artificially high by that collection. Corrections for such collection can be made by means of analyses of the amounts of a denuder-collected, nonvolatile, PM-phase tracer (e.g., solanesol or dotriacontane for tobacco smoke PM). As in the human inhalation of tobacco smoke, the occurrence of DGD in a denuder creates a driving force for evaporation from the particles. PM-phase nicotine can consequently deposit in an acid-coated denuder without actual particle deposition: it can evaporate and deposit by EGD. For the nicotine collected in their denuder tubes, Lewis et al. (40, 41) sought to explicitly distinguish between the fraction that was initially in the gas phase and deposited in the denuder by DGD (referred to herein as fg,init) and the fraction that was initially in the particle phase and deposited from the gas phase by EGD. This distinction was sought by cutting up the denuder tube

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Table 6. Data of Lewis et al. (41) Obtained at Room Temperature (presumably ∼20 °C) for Nicotine fg,init Values Together with Values of the Group (1 - fg,init)/(fg,initTPM) Calculated Based on Those Dataa % of total nicotine entering denuder total smoke nicotine (mg) cigarettesc

unmodified 1.8 ( 0.2 “80%” (1:5) ventilated cigarettese 0.7 ( 0.1

(a) initially in gas-phase (fg,init × 100)

(b) evaporated to as phase in denuder

(c) collected by denuder (fdenuder × 100)

fg,init

TPMb (µg/m3)

(1 - fg,init)/(fgTPM)

0.7 ( 0.4

4(3

4.7

10-2.15

107.08

10-4.94 d

0.5 ( 0.3

21 ( 5

21.5

10-2.30

105.94

10-3.64

At equilibrium, (1 - fg,init)/(fg,initTPM) ) (1 - fg,e)/(fg,eTPM) ) Kp. Cigarette smoke was drawn into samplers utilizing denuder tubes coated with oxalic acid. b Calculated assuming a geometric standard deviation of 1.3 for the tobacco smoke particle size distributions, and the following values assumed by Lewis et al. (41): (1) mass median diameter, 0.30 µm for the smoke from the unmodified cigarettes; 0.39 µm for the smoke from the ventilated cigarettes; and (2) number of particles/cm3, 1.9 × 109 for the smoke from the unmodified cigarettes; 3.8 × 107 for the smoke from the ventilated cigarettes. c Rothmans King Size Filter (international) cigarettes (42). d This value differs somewhat from the 10-5.08 value previously calculated by Pankow et al. (5) using the approximation that the mass median diameter for the smoke aerosol was similar to the diameter of average mass for the aerosol. For the number calculated for this table, we do not make that approximation (see footnote a). e Rothmans King Size Filter (international) cigarettes (42), ventilated 1:5 with perforation holes. a

and examining how the deposited nicotine was distributed down the length of the tube. Data of Lewis et al. (41) are summarized in Table 6 along with values of the group (1-fg,init)/(fg,initTPM) derived from that data. The fg,init and TPM values are given as powers of 10 to facilitate comparison with the log-log plot in Figure 9. The value of (1 - fg,init)/(fg,initTPM) computed for the unmodified cigarettes is identical to the value of Kp,fb reported by Pankow et al. (5) for partitioning to tobacco smoke PM at 20 °C. If this value of (1 - fg,init)/(fg,initTPM) for the unmodified cigarettes in fact pertained to a state of equilibrium, then by eq 37, this result would suggest that Rfb ≈ 1 in that smoke PM. Such a result would certainly not be not consistent with the conventional view that cigarette smoke PM is typically acidic with Rfb , 1. However, such a result would be consistent with the fact that, as is discussed above, Pankow (18) recently presented calculated tobacco smoke Rfb values of 0.45, and 0.51 for two unnamed brands of U.S. cigarettes. The value of (1 - fg,init)/(fg,initTPM) computed for the ventilated cigarettes studied by Lewis et al. (41) is 20 times larger than the 10-4.94 value for Kp,fb reported by Pankow et al. (5) for tobacco smoke PM at 20 °C. If the 10-4.94 value of (1 - fg,init)/(fg,initTPM) computed for the smoke PM from the unmodified cigarettes does equal the nicotine Kp for that smoke, then the smoke from the ventilated cigarettes was either characterized by (a) a different Kp and/or (b) an absence of equilibrium. The nicotine in the smoke from the ventilated cigarettes may well have been further from equilibrium than in the smoke from the unmodified cigarettes. Indeed, it is always true that a sudden introduction of dilution air will disrupt all gas/particle equilibria within an aerosol. For example, for a compound for which Kp does not change with dilution, a new initial state will be created in which (a) more of each of the smoke compounds is in the particle phase than will be the case at equilibrium; so that (b) (1 - fg,init)/(fg,initTPM) > Kp.

B. fdenuder: The Total Denuder-Collected Nicotine Fraction As discussed above, in terms of the behavior of aerosol nicotine, an acid-coated denuder is analogous to the upper RT. Acid-coated denuders could therefore provide general indications regarding the types of amounts of tobacco smoke nicotine that could be available for deposition in the RT by a combination of DGD and EGD,

although a detailed comparison of aerosol nicotine behavior in denuders and in the RT would need to consider a number of factors, including residence time (multiple seconds for the denuders that have been used for smoke nicotine vs less than a second for smoke passing through the pharynx) and any dilution of the aerosol that could promote nicotine evaporation (little to none for mainstream smoke drawn directly into a denuder tube vs some for smoke drawn into the pharynx). As utilized in Table 6, fdenuder × 100% represents the percent of the total smoke nicotine that is collected by a denuder. In the measurements of Lewis et al. (41), fdenuder × 100% was measured to be 21.5 and 4.7% for the ventilated and unmodified cigarettes, respectively. The large percentage of nicotine that was measured as having volatilized from the ventilated cigarette smoke PM may have been the result of an increased driving force for evaporation (and therefore EGD in the denuder) as brought about by the dilution of that smoke. It is moreover interesting to note for the ventilated cigarettes that even though the total nicotine in the smoke was lower, the total mass amount of denuder-collected nicotine was found to be greater (ventilated cigarettes, 0.215 × 0.7 mg ) 0.15 mg; unmodified cigarettes, 0.047 × 1.8 mg ) 0.085 mg.) An examination of the available tobacco company documents reveals that measurements with denuders were made at Philip Morris USA both prior to (43-49) and after (50-56) the appearance of the 1994 and 1995 papers of Lewis et al. (40, 41). An internal document of that company from 1991 states (45) “The experiments described in this memo were made to see if vapor phase nicotine can be differentiated from nicotine in the particulate phase. Since any scheme that involves separation of the phases by simple removal of the particulate phase may artificially perturb the system due to absorption and/ or desorption of nicotine from particulate to vapor phase, it was decided to provide separation by removal of the vapor phase selectively from that of the particulate phase.” [D. C. Watson, Philip Morris USA, October 3, 1991 (45)]. The work discussed in that memo utilized denuder/ impaction trap samplers with several configurations of acid-coated denuder tubes. The impaction trap was intended to collect the particle-phase nicotine that passed through the denuders. This initial phase of Philip Morris research did not attempt to distinguish between DGD and EGD in the denuder tubes (even though the stated reason for focusing on denuder tech-

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Table 7. Total Smoke Nicotine and Denuder-Collected Fractions of Total Smoke Nicotine (fdenuder × 100%) Measured at Room Temperature (presumably ∼20 °C) by Philip Morris USA (45)a experiment

cigarette type

total smoke nicotine (mg)

fdenuder × 100%

fdenuder × 100% (adjusted)b

5 6 8 9 12 13 14 15 16

Marlboro Marlboro with KOH added to cigarette filter 100% burley tobacco 100% bright tobacco Marlboro Marlboro Marlboro Marlboro after treatment with 0.5% NH3 Marlboro after treatment with 5.0% NH3

1.1 0.9 1.9 2.2 1.5 1.4 1.5 1.4 1.8

1.9 2.2 3.1 1.8 2.7 3.3 4.0 4.0 6.0

1.2 1.5 2.2 1.3 1.9 2.3 2.8 2.8 4.2

a Smoke from each cigarette was drawn into a denuder/impaction trap sampler with a denuder tube coated with p-toluenesulfonic acid. Adjusted assuming that 70% of the measured value of fdenuder was due to deposition from the gas phase, as based on Philip Morris data on dotriacontane deposition in the denuders.

b

Table 8. Nicotine fg,init Values Together with Corresponding Values of fg,init/fg,init,ref from Philip Morris (55, 56)a

cigarette type

no. of experiments (N)

% dry weight of additive in filler (found)

“filler pH”

% of total nicotine entering denuder initially in gas phase (fg,init × 100%) (as reported for best fit)

“reference” NH3 amended DAP amended KOH amended

18 7 4 5

0 0.55% (as NH3) 1.13% (as NH3) 2.0% (as KOH)

5.30 6.15 6.40 7.20

1.60 1.83 2.03 2.47

a

standard deviation for fg,init (as reported for best fit)

average same day (paired) fg,init/fg,init,ref

0.54 0.41 0.23 0.47

1.45 1.41 1.39

All data were apparently obtained at room temperature (presumably ∼20 °C).

nology could have led to the conclusion that both DGD and EGD could be expected to occur in a denuder). It is therefore not possible to determine a posteriori, for that work, the portion of each fdenuder value that was deposited by DGD (fg,init) vs the portion deposited by EGD. Table 7 summarizes the results (45) obtained for Marlboro cigarettes, modified Marlboro cigarettes, cigarettes prepared from just “burley” tobacco, and cigarettes prepared from just “bright” tobacco. The fdenuder values are presented both as tabulated in the Philip Morris memo (45) and after multiplication by a factor of 0.7 to adjust for the stated possibility that ∼30% of the fdenuder values may have been due to deposition of particles containing nicotine. Although the degree of experimental detail provided in the memo is not sufficient to thoroughly evaluate the reliability of that Philip Morris data, and replicate measurements were not carried out for some of the cigarette types, the following features of the data do stand out: (1) the measured fdenuder values are roughly similar to those reported by Lewis et al. (41); (2) treating the Marlboro cigarette blend material with 5.0% ammonia increased the total smoke nicotine relative to the smoke from untreated Marlboro cigarettes; (3) treating the Marlboro cigarette blend material with 5.0% ammonia increased fdenuder, suggesting a larger smoke-PM Rfb and a lower Kp relative to the smoke PM from the untreated Marlboros; and (4) the 100% burley tobacco cigarette yielded a larger fdenuder value as compared to that from the 100% bright tobacco cigarette, suggesting a larger smoke-PM Rfb and a lower Kp for the burley smoke relative to the bright smoke [we note here that burley smoke has generally been considered to be more alkaline than bright smoke (22)]. As noted, Philip Morris scientists conducted additional experimentation with denuders for the purpose of measuring the relative amounts of gas and PM-phase nicotine

in smoke from both control (“reference”) and additiveamended cigarettes (50-56). In one particularly detailed study (55, 56), a total of 36 denuder experiments were conducted in which the approach of Lewis et al. (41) was followed so as to the distinguish the fg,init values for the smokes from the corresponding fdenuder values. In addition, the effects of particle deposition in the denuders were apparently also investigated by means of solanesol analyses. The percent dry weight values at which the additives were present in the tobacco “fillers” used to make the test cigarettes are given in Table 8 along with the results of the measurements. The data are consistent with the results in Table 7. Since experiments with the “reference” cigarettes were conducted on the same days as with the amended cigarettes, the stated purpose of the computations of the ratio of fg,init to that for the same day reference cigarette (fg,init,ref) was to improve the reliability of the statistical comparisons.

C. “Impact” (Reprise) Because it is known that nicotine is highly water soluble, and is largely protonated at the pH values characterizing the physiological fluids of the RT, it is already known that deposition of gaseous (free-base) nicotine is very efficient and occurs rapidly in the mouth and pharynx (57). Therefore, a remaining important step in ascertaining whether a causal link exists between gaseous nicotine and “impact” would be a comparison of (a) the types of amounts of gaseous nicotine initially present in fresh mainstream cigarette smokes, or producible by evaporation while such smokes are in the pharynx with (b) the amounts of gaseous nicotine that can elicit an “impact” sensation in smokers. Regarding point a, we have the above reported data on the fraction fg of nicotine that is initially in the gas phase of mainstream cigarette smoke, as well as the above reported data on the fraction fdenuder of total nicotine that can deposit in denuders. As has been discussed,

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Pankow

these amounts are about 1-5% of total smoke nicotine values of ∼1-2 mg/cigarette, or the equivalent of ∼0.0010.010 mg/puff. Regarding point b, then, nicotine inhalers that deliver ∼0.010 mg of gaseous nicotine per ∼35 mL inhalation (58, 59) have been observed by researchers interested in the sensory aspects of smoking to exhibit “impact” (60). In fact, such inhalers have been observed to cause throat irritation (61), which brings to mind the observation that tobacco smokes with high levels of “impact” are “harsh” (16). We conclude that all of the available evidence is consistent with gaseous free-base nicotine as the cause of “impact”.

VI. Deposition of a Tobacco Smoke Compound in the Respiratory Tract A. Overall Deposition Fraction F The general equations developed in this section apply to any tobacco smoke compound of interest; the equations in this section that are expressed in specific terms for nicotine have exact analogues for any other smoke compound. We start by letting F be the total fraction of an inhaled smoke compound that deposits in the RT. If fDGD, fEGD, fPDE, and fPDD are the fractions that are deposited by the DGD, EGD, PDE, and PDD mechanisms, then

F ) fDGD + fEGD + fPDE + fPDD < 1

(40)

F < 1 in eq 40 because overall deposition in the RT will never be 100% efficient. F values for nicotine as large as 0.9, have however, been reported for inhaled cigarette smoke (62). As noted above, a single particle that ultimately deposits in the RT can contribute to fEGD, fPDE, and to fPDD: before the particle deposits, nicotine can volatilize and deposit by EGD; after the particle deposits, the remaining nicotine can deposit by PDE and PDD.

B. fDGD Because gaseous nicotine deposits very efficiently in the RT, essentially all of the initially gas-phase nicotine will deposit by the DGD mechanism, and we have

fDGD ≈ fg

(41)

For the mainstream cigarette smokes that have been studied, the available ambient-temperature denuder studies suggest that fg < ∼0.03 and fp ≈ 1, so that most mainstream cigarette smoke nicotine is in the particle phase. (This is as predicted by eqs 9 and 11 for equilibrium partitioning, including for cases when Kp for nicotine is minimized at Kp,fb.) Under these conditions,

fDGD ≈ fg , 1

(42)

This means that when Fnicotine g ∼0.3 so that fDGD can be neglected in eq 40, then

Fnicotine ≈ fEGD + fPDE + fPDD

(43)

For all three of the mechanisms represented in eq 43, the nicotine starts out in the particle phase. Thus, the available information indicates that in cases when Fnicotine g ∼0.3, a large percentage of the nicotine that did deposit must have been in the particle phase when it was initially inhaled.

known for nicotine when cigarette smoke from major brands is inhaled according to a typical smoker’s inhalation pattern. At the least, it would be useful to seek to distinguish within eq 43 between the nicotine deposition that occurs without particle deposition (fEGD) and the nicotine deposition that occurs with particle deposition (fPDE + fPDD). In this context, let FTr represent the efficiency by which a nonvolatile tracer in the tobacco smoke PM (e.g. solanesol) is deposited in the RT [TPM would not be an adequate conservative tracer in this context because of the contributions of nicotine, water, and other volatile compounds to the value of TPM; also, tar ()TPM-nicotine-water) would not be an adequate tracer unless it could be shown that significant EGD of volatile components in tar did not occur.] The value of FTr is unaffected by processes which, by themselves alone, do not bring about the deposition of particles. Examples of such processes include changes in mean size while in the RT because of (a) coagulation of particles; (b) losses of compounds by EGD (or simply evaporation without deposition); and (c) exchange of water with that relatively high RH environment. By the time the particles involved in nicotine PDE and PDD deposit the nonvolatile tracer that they carry, they will already have become partially depleted in nicotine by EGD (i.e., nicotine is not a nonvolatile tracer). Therefore,

FTr > fPDE + fPDD

(44)

D. fEGD Subtracting eq 44 from eq 43 leads to the conclusion that when fDGD is indeed small relative to Fnicotine, then

Fnicotine - FTr < fEGD

(45)

The quantity Fnicotine - FTr is thus a lower bound on fEGD. Measured values for FTr for inhaled tobacco smoke are not generally available. One approximation to this quantity might be Fparticles, the efficiency with which the total initial number of smoke particles deposits in the respiratory tract. For inert particles in the 0.2-0.5 µm size range, experimental respiratory deposition efficiencies have typically been reported to be less than ∼0.3 (63). If Fparticles ≈ 0.3 was valid for tobacco smoke and Fparticles ≈ FTr for such smoke, then the above cited value of Fnicotine ≈ 0.9 together with eq 45 would indicate that fEGD > ∼0.6 for some inhaled cigarette smokes. This is a large number. Indeed, in order for fEGD ) 0.6, at least 60% of the total PM-phase nicotine would have to have existed in the free-base form sometime during the period between inhalation and exhalation so that it could volatilize and deposit by EGD. Whether or not fEGD can indeed be as large as 0.6 when smokers inhale smoke from some commercial cigarettes is not known, partly because of the uncertainties in the values of FTr and Fparticles for cigarette smoke PM. For example, Davies (64) has noted that fresh mainstream cigarette smoke is subject to rapid coagulation and therefore growth in mean particle size, so that 0.3-0.4 µm may be an underestimate of the relevant particle size for inhaled cigarette smoke.

C. FTr

E. Components of fEGD: fEGD,dep-particles and fEGD,exh-particles

The significance of each of the three nicotine deposition terms in eq 43 is of great interest, but not presently

Given the likely importance of fEGD for nicotine, it would be of considerable interest to have better estimates

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Chem. Res. Toxicol., Vol. 14, No. 11, 2001 1479

of this parameter for representative cigarette smoke and inhalation circumstances. The fact that particles can contribute to EGD and then still deposit nicotine by PDE and PDD, however, presents challenges in the measurement of fEGD. fEGD has two components, fEGD,dep-particles and fEGD,exh-particles. The former arises from those particles that lose nicotine by evaporation and then deposit, and the latter is due to the particles that lose nicotine and are then exhaled:

fEGD ) fEGD,dep-particles + fEGD,exh-particles

(46)

For both components, it is assumed that 100% of the nicotine that evaporates then deposits from the gas phase. As an alternative to seeking estimates of fEGD for nicotine, it will be easier experimentally to determine unambiguous estimates of fEGD,exh-particles as those particles lose nicotine only by evaporation. Let Tri and Tre represent the inhaled and exhaled mass amounts of some nonvolatile tracer compound in the tobacco smoke PM phase. The nonvolatility of the tracer prevents it from depositing by EGD to any significant extent. Solanesol is a good candidate compound for such a tracer because it has a very low vapor pressure p°L (and therefore a very large value of Kp) and is also relatively abundant in tobacco smoke PM. The aggregate mix that is tar would not be a good candidate inasmuch as significant portions of some of the compounds comprising tar may deposit by EGD. On either a per smoked cigarette or per puff basis, let

Tri × (N/Tr)i ) total mass of inhaled nicotine

(47)

Tre × (N/Tr)e ) mass of exhaled nicotine

(48)

Tre × (N/Tr)i ) mass of nicotine that would have been exhaled from the particles that did not deposit had EGD not occurred from those particles (49) where (N/Tr)i and (N/Tr)e are the mass/mass ratios of nicotine/tracer in the inhaled and exhaled PM, respectively. Assuming that most of the nicotine is in the PM phase, we now write

fEGD,exh-particles × total nicotine ≈ fEGD,exh-particles × Tri × (N/Tr)i (50) ≈ Tre × (N/Tr)i - Tre × (N/Tr)e (51) so that

fEGD,exh - particles ≈

(

)

(52)

Since Tre/Tri is the fraction of the tracer that is exhaled, then 1 - Tre/Tri is the fraction that is deposited, i.e., FTr so that

Tre ) 1 - FTr Tri

(53)

and

(

)

(N/Tr)e (N/Tr)i

fEGD,exh-particles ≈ (1 - FTr) (exhaled particles have lost all nicotine by off-gassing) (55)

F. The Case When EGD Operates Similarly on Exhaled and Deposited Particles If the amounts of PM-phase nicotine that deposit by EGD are about the same for particles that are exhaled as for those that are deposited, then we can write the proportion

fEGD,exh-particles fEGD,dep-particles fEGD ≈ ≈ 1 - FTr FTr 1

(56)

or more specifically for our purposes,

fEGD ≈

fEGD,exh-particles 1 - FTr

(57)

(

(58)

By eqs 54 and 57,

fEGD ≈ 1 -

)

(N/Tr)e (N/Tr)i

When the off-gassing of nicotine goes to completion before any particles deposit,

fEGD ≈ 1 (all particles lose all nicotine by off-gassing) (59) and EGD becomes the only important deposition mechanism. The EGD process will occur most rapidly when the smoke particles are freshly inhaled, i.e., when the driving force for volatilization is the greatest. These kinetics will be acting in the same manner for the particles that are ultimately deposited as well as for those particles that are ultimately exhaled. Thus, these kinetics will tend to act in a direction so as to make the per particle EGD for the deposited particles more rather than less similar to that for the exhaled particles, thus acting to favor the validity of the approximations in eqs 56-59.

VII. Conclusions

Tre (N/Tr)e 1Tri (N/Tr)i

fEGD,exh-particles ≈ (1 - FTr) 1 -

The ratio (N/Tr)e/(N/Tr)i is the tracer-normalized fraction of nicotine that remains in the exhaled particles. When this fraction is unity, no evaporation took place from the exhaled particles and fEGD,exh-particles ) 0. When (N/Tr)e/(N/Tr)i equals zero, the off-gassing of nicotine has gone to completion before the particles were exhaled and

(54)

Of the four mechanisms by which an organic compound in tobacco smoke such as nicotine can deposit in the respiratory tract (RT), three (DGD, EGD, and PDE) involve the volatilization of the compound from the tobacco smoke particulate material (PM) phase. The volatility of the compound from the PM phase is therefore of critical importance in determining the relative roles of the four mechanisms for a given smoke. This volatility is set by the compound’s gas/particle partitioning constant Kp, which in turn is determined in part by the compound’s vapor pressure p°L. In the case of an alkaloid such as nicotine that can exist in protonated as well as free-base forms, the fraction Rfb of the compound that is in the neutral free-base form in the PM phase also plays a critical role.

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The value of Rfb for nicotine in tobacco smoke PM is set by pHeff, the effective pH of the PM phase. Historically, a primary method for measuring “smoke pH” has involved the direct exposure of a pH electrode to tobacco smoke. Unfortunately, this method cannot yield direct insight into pHeff values. First, the method does not measure the true pH of the tobacco smoke PM phase. Second, the resulting values say nothing about the effects of PM-phase activity coefficients of protonated and freebase nicotine on the nicotine species distribution. Thus, although “acidic” values have typically been measured by the direct electrode method of Sensabaugh and Cundiff (22) for cigarette “tobacco smoke pH”, such values can neither be viewed as reliable nor utilized as if they are pHeff values to indicate the relative distribution of PMphase nicotine among the free-base and protonated forms. The dependence of the volatility of nicotine from tobacco smoke PM on Rfb means that measuring the gas/ particle distribution of nicotine under equilibrium conditions in a tobacco smoke by a denuder sampler (or by another method) can yield information about the nicotine Kp for that smoke. Knowledge of Kp,fb, the partitioning constant for nicotine in the free-base form, then allows calculation of Rfb through the relation Kp ) Kp,fb/Rfb (eq 37). The available data suggest that the smoke PM from some commercial cigarettes can have Rfb g 0.4, i.e., 40% or more of the nicotine in the free-base form. This conclusion is consistent with (1) the gas-sampling denuder results obtained by Philip Morris (45, 55, 56) in which significant tobacco smoke nicotine was observed to deposit in acid-coated denuder tubes, with more depositing when the cigarette tobacco blend was treated with ammonia; (2) the view that sensory “impact” is caused by the deposition of gaseous nicotine in the pharynx; (3) the observed throat irritation caused by nicotine inhalers; and (4) the high overall respiratory tract deposition efficiencies as large as 0.9 and greater that have been reported for some cigarette smokes. All of the above pieces of information combine to create a picture of nicotine in cigarette smoke that contradicts the traditional view that cigarette smoke PM is typically acidic, with little free-base nicotine typically present in the smoke PM phase. Given the fundamental role that pHeff plays in determining the behavior of nicotine in tobacco smoke and the considerable attention that the tobacco industry has given to the concept of “smoke pH” in corporate research and market share studies, government agencies interested in establishing a framework for the testing and monitoring of nicotine delivery may wish to consider requiring the measurement and publication of the PM-phase Rfb values of the cigars and cigarettes marketed in their jurisdictions. Such information could be obtained by means of eq 37 together with measurements of the volatility of tobacco smoke nicotine through measurements of Kp and Kp,fb.

VIII. Acknowledgment This work was funded by Grant R01-DA10906-02 from the National Institute on Drug Abuse (NIDA) within the National Institutes of Health (NIH). The author sincerely appreciates the efforts of Drs. Rao Rapaka and Paul Hillery of NIDA/NIH to facilitate this work.

Pankow

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