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Gas-Particle Partitioning of Formaldehyde in Mainstream Cigarette Smoke

Data publikacji: 23 May 2020
Tom & Zeszyt: Tom 29 (2020) - Zeszyt 1 (April 2020)
Zakres stron: 2 - 20
Otrzymano: 11 Nov 2019
Przyjęty: 20 Feb 2020
Informacje o czasopiśmie
License
Format
Czasopismo
eISSN
2719-9509
Pierwsze wydanie
01 Jan 1992
Częstotliwość wydawania
4 razy w roku
Języki
Angielski
INTRODUCTION

Cigarette mainstream smoke (MS) is a dynamic aerosol system consisting of gases, vapours and suspended particulate materials (PM) containing more than 6,000 chemical substances (1, 2). Long-term repeated inhalation of cigarette smoke is a cause of morbidity and mortality among smokers (3, 4), and it is widely held that disease risks associated with smoking are the consequence of repeated lung exposure to cigarette smoke toxicants. More than 150 toxicants have been identified in cigarette smoke (5), and over the past 50 years a significant body of work has attempted to understand the contribution of individual toxicants to the risks of smoking-related diseases (5, 6). Regulatory bodies such as the Food and Drug Administration's Tobacco Product Scientific Advisory Committee (TPSAC) have prioritised over 90 Harmful or Potentially Harmful Constituents in tobacco and smoke (7), while the WHO Study Group on Tobacco Product Regulation (TobReg) identified a priority list of 18 compounds recommended for regulation (8,9,10).

Central to these lists are four toxic, carcinogenic or potentially carcinogenic aliphatic aldehydes, formaldehyde, acetaldehyde, acrolein and crotonaldehyde (Table 1). Formaldehyde is an International Agency for Research on Cancer (IARC) Group 1 human carcinogen (11), and its formation in burning cigarettes has been studied extensively (12,13,14). Formaldehyde is present in cigarette smoke at relatively similar concentrations to acrolein and crotonaldehyde; however, acetaldehyde is present at substantially higher levels (2). In cigarette MS the aldehydes are partitioned to varying degrees between the gas and particulate phases. Acetaldehyde and crotonaldehyde are almost entirely present in the gas-phase (15, 16).

Summary of physical properties of selected carbonyls in cigarette smoke.

Property Compound
Formaldehyde Acetaldehyde Acrolein Crotonaldehyde
Formula HCHO CH3CHO CH2 = CHCHO CH3CH = CHCHO
Boiling point (K) 254 293 326 377
ISO emissions from KY3R4F (mg/cigarette) 21.5 ± 7.8 (37) 540 ± 135 (37) 49 ± 14 (37) 13 ± 5 (37)
Proportion in particulate phase of mainstream smoke 32% (15, 16) 2% (15) 0% (17, 57) 7% (56)
2% (56)
7% (15)
33% (60)
Total retention on cigarette smoke inhalation 95–99% (61) 94–95% (62,19) > 97% (61)
70–97% (19)
97% (61),
98% (61)
99% (63,64)
Mouth retention from cigarette smoke - 30–60% (19) 72% of 97% total retention (19) -
Predicted main deposition regions in the respiratory tract Mouth and oropharyngeal/laryngeal (21) Mouth and oropharyngeal/laryngeal (21) Mouth and oropharyngeal/laryngeal (21) -
Trachea (22) Deposition past generation 8 (22) Deposition past generation 8 (22)
Toxicity IARC Group 1 human carcinogen (11) IARC Group 2B (58, 59) Inflammatory and cytotoxic effects (2, 65) Mutagenic and carcinogenic (55)

The partitioning of acrolein has been subject to some degree of uncertainty, with estimates ranging from 0 to 33% (Table 1). Stabbert et al. (17) attributed this to reactions of acrolein with other smoke constituents or the particulate trapping medium. In contrast, a significant proportion (approximately 32%) of formaldehyde is present in the particulate phase of smoke despite the pure compound being a gas at cigarette smoke temperatures. This has been explained as the result of an equilibrium operating between formaldehyde and its less volatile hydrated form, methanediol, after reaction with MS water. Formaldehyde also reacts with ammonia, particularly in sidestream smoke, to form the particulate phase compound hexamethylenetetramine, although this is a relatively minor factor in MS (15) due to substantially lower ammonia levels. The gas/particle partitioning of formaldehyde has been identified as a constraint on the performance of selective filtration media for reduction of MS carbonyl levels (16, 18). On inhalation, these cigarette smoke aldehydes are retained almost quantitatively in the respiratory tract (Table 1). However, relatively little is known about their sites of deposition during smoking (19). Site of deposition can have an important impact on disease potential, due to localised peak toxicant concentrations in comparison to other tissue sites. For example, preferential deposition of a disease-relevant toxicant in the upper respiratory tract may potentially indicate a higher risk of upper respiratory tract diseases but a lower risk of deep-lung disease, and vice versa. The lack of such information for cigarette smoke formaldehyde is of particular concern, given the established carcinogenicity of the compound in humans. Modelling approaches have been used in an attempt to resolve these uncertainties. With typical occupational breathing patterns Schroeter et al. (20) predicted 85% uptake of atmospheric formaldehyde in the nose, consistent with the nasal cavity and sinuses being the principal site of occupational cancers from formaldehyde exposure. Corley et al. (21) modelled cigarette smoke entry via the oral cavity and predicted maximal formaldehyde surface airway dose in the mouth and oropharynx/larynx regions with decreasing dose by airway generation, thereafter. Similarly, Asgharian et al. (22), disregarded nasal and oral losses, but predicted that formaldehyde would be quickly absorbed by the mucus membranes with a very high uptake in the trachea (airway generation 0), and that no formaldehyde would pass beyond airway generation 8. Similar conclusions were reached when the behaviours of acetaldehyde and acrolein were modelled, but greater penetration past airway generation 8 was predicted for acetaldehyde, partly through its more moderate water solubility.

Distributions of volatile cigarette smoke components between the gaseous and particulate phases (G/P partitioning) can impact deposition sites in the respiratory tract due to potential changes in compound volatility (23). Significant G/P partitioning of a compound in an aerosol could substantially modify its deposition properties from that of the compound in air. Establishing the G/P properties of carbonyls in cigarette smoke is therefore an important contribution to understanding their potential roles in the development of smoking-related diseases. Interestingly, with regards to G/P partitioning in cigarette smoke, the behaviour of formaldehyde seems similar to that of nicotine, with both compounds existing in equilibrated states between less volatile chemically-bound states in the particulate phase (hydrated formaldehyde, and nicotine salts) and more volatile unbound forms (formaldehyde and unprotonated nicotine) that will to greater or lesser degree exist in the gas-phase.

The diffusion denuder method is often used for studying G/P partitioning in flowing aerosols, including MS. A cylindrical denuder is a long tube, the inner surface of which is covered with an active collection layer, termed a ‘sink wall’. When an aerosol is passed through the tube under laminar flow conditions, gas molecules diffuse to the wall, where they are irreversibly removed from the gas-phase, whereas the particulates, which have much lower diffusion coefficients, travel through and exit the tube, generally to be captured on a filter. Principles of operation and applications of the diffusion denuder method have been reviewed by Ali et al. (24) and John et al. (25). Gormley and Kennedy (26) derived an analytical solution for the diffusion of dilute gas molecules in a laminar flow tube of infinite size and perfect sink walls. Lewis, Colbeck and Mariner proposed a semi-empirical model (27, 28) for analysis and interpretation of the nicotine deposition patterns measured in denuders in their smoking experiments. Lipowicz and Piadé (29) proposed a sophisticated theoretical model to describe the wall deposition and G/P distribution of nicotine in MS in diffusion denuders based on partial differential equations of flow, diffusion of gas-phase nicotine and evaporation from particles.

Denuder approaches have also been widely applied to sample and determine carbonyls, for instance from ambient air and automotive exhausts and stack gases (30,31,32,33,34,35). However, to our knowledge, no smoking or smoking-related denuder studies with aldehydes have been reported so far. We have previously developed a diffusion denuder method to study G/P partitioning of nicotine in MS (25), and in this study have adapted this apparatus to examine G/P kinetics of carbonyls in cigarette smoke. The most important prerequisites for quantitative operation of a denuder tube are the flow, which must be laminar, and the collection efficiency of the wall coating, which must be close to 100%. Collection of carbonyls from cigarette smoke is usually achieved through derivatisation with 2,4-dinitrophenylhydrazine (DNPH) forming the corresponding hydrazone (36). This method has been the most frequently applied analytical tool for the determination of aldehydes and ketones (37,38,39), and it was our method of choice.

Our investigations showed that fundamental denuder studies with aldehyde and air gas mixtures were necessary prior to commencing the smoking experiments with cigarettes. Additionally, quantitative denuder results could only be obtained for formaldehyde among the cigarette-smoke carbonyls studied. Therefore, the present study has addressed two main issues: optimisation of the denuder method for measuring formaldehyde, and application of the method in smoking experiments to assess the temperature dependence of formaldehyde G/P partitioning in cigarette MS.

MATERIAL AND METHODS
Preparation of aldehyde-air gas mixtures

Aldehyde-air gas mixtures for denuder studies were prepared in the concentration ranges that were expected to occur in MS, based on published data (37) and our own preliminary smoking experiments.

During preparation of aldehyde-air gas mixtures, a 10-L Pyrex® bulb was used. Acetaldehyde, acrolein and crotonaldehyde were each measured into an evacuated bulb by monitoring pressure.

A few microlitres of formalin were injected in the evacuated bulb and the solution was allowed to evaporate. Finally, the bulb was filled up to 1 bar total pressure with synthetic air at the required relative humidity (RH, 60% or 0%). The water content of the mixture arising from the formalin sample was negligible compared with the RH of the buffer air gas, even in the case of 0% RH formaldehyde-air gas mixtures. The gases mixed for at least 6 h prior to use.

An alternative, ‘diffusion gas’, approach was used for the preparation of formaldehyde-air mixtures, as shown in Figure 1. A stream of synthetic air with a high flow rate and controlled RH enters the apparatus and passes through the upper part of a diffusion vial (‘diffuser’) containing ~35% (wt/v) formalin. The air stream picks up formaldehyde from the formalin solution via diffusion. After leaving the diffuser, the gas flow enters a larger diameter mixing tube, from which a portion is sampled through an inner tube. The sample flow may be varied in the range of Q273 = 200–600 mL min−1 (Q273 designates volumetric flow rates in standard conditions). In test experiments, a constant formaldehyde concentration was achieved using the diffusion gas preparation method, with just a very small increase in the RH of the air flow.

Figure 1

Experimental set-up used for the preparation of flowing formaldehyde–air gas mixtures. In the apparatus the three-way glass valve is connected to a DNPH impinger to determine the concentration of formaldehyde in the gas stream. In the denuder experiments, the impinger is replaced by a DNPH-coated denuder tube.

The denuder apparatus and procedure
The denuder assembly

The experimental apparatus used as the basis for this denuder study has been described previously (25). We modified this apparatus slightly to optimize it for the study of aldehydes (Figure 2). In brief, the heart of the apparatus is the vertically positioned diffusion denuder tube made of thin-walled Pyrex®, with an internal diameter of 0.80 cm and length of 150.0 cm (our standard measurements) and coated internally with DNPH. The denuder tube temperature is regulated to within ± 0.2 K at each of the experimental temperatures of 298–323 K. The lower end of the denuder tube is attached to a Cambridge filter pad (CFP) for the collection of smoke particles leaving the denuder. The exit gases are passed through two impingers, filled with DNPH acetonitrile solution.

Figure 2

Schematic representation of the denuder apparatus. 1) Denuder tube; 2) Cambridge filter pad; 3) synthetic air cylinder; 4) ultrathermostat; 5) membrane pump; 6) Pyrex® hood; 7) retractable electric igniter; 8) water saturator; 9) and 10) impingers, filled with acidic DNPH solution; 11), 12) and 13) mass flow controllers; 14) mixer to mix dry and wet air flows; 15) humidity sensor; 16) control for mass flow controllers; 17) humidity indicator. Based on Figure 1 of (25).

The air flows and the required volumetric flow rate (Q273) are regulated and measured by electronic mass flow controllers, which along with the water-saturator, the gas mixer and the humidity sensor regulate and measure the RH of the sampling air. (The RH values of the sampling air that we report refer to RH values at the laboratory temperature, which was kept constant at 298 ± 2 K.)

Coating of the denuder tube

This was conducted by applying multiple layers of DNPH from acetonitrile solution as detailed in the Supplementary Information.

Flow of aldehyde-air gas mixtures and capture of MS particulates

Denuder experiments were carried out by continuously drawing aldehyde-air gas mixtures or cigarette MS through the denuder tube coated with DNPH, from which particles were collected on a CFP and two impingers in series (Figure 2). The CFP was impregnated with 0.25 g of DNPH, 10 mL of CH3CN and 0.2 mL of 85% H3PO4 solution and the impingers were filled with an acidic solution of DNPH (20 mL of acetonitrile, 0.2 mL of phosphoric acid (85%) and 0.05 g of DNPH). Tip-ventilated Kentucky 3R4F reference cigarettes were used in the smoking experiments. Sample cigarettes were conditioned for 4–8 days at 298 K and 60% RH. Cigarettes were smoked vertically (Figure 2) using a continuous draw process, as described previously (25). Only the first 2 cm of each cigarette was sampled to minimize variations in the MS temperature and humidity when entering the denuder tube, consistent with a continuous sampling time of approximately 120 s.

After sampling, the denuder tube was cut into 5-cm sections. The aldehyde hydrazone derivatives deposited in each 5-cm section, and collected by the Cambridge filter and impingers, were determined by high-performance liquid chromatography (HPLC). Coatings from the 5-cm denuder segments were dissolved in 2 mL of CH3CN and the Cambridge filter was extracted by 20 mL of CH3CN. At least three replicates of denuder runs were made for each experimental condition (e.g., at least three cigarettes were smoked at each given temperature and RH and Q273 value).

HPLC analysis

To measure aldehydes in DNPH hydrazone derivatives, an automated Agilent 1200 HPLC apparatus (Santa Clara, CA, USA) equipped with a Phenomenex LUNA C18 column (Phenomenex, Macclesfield, UK); dimensions: particle size, 5 μm; length, 250 mm; internal diameter, 4.6 mm. The column was kept at 313 K; the flow rate of the mobile phase was 1.5 mL min−1. Detection was achieved by ultraviolet absorption at λ = 360 nm. Calibrations were carried out with a TO11/IP-6A Aldehydes/Ketone-DNPH calibration standard (Sigma-Aldrich, Gillingham, UK) that contained hydrazones of 15 carbonyls in acetonitrile solution.

Two gradient elution methods were used for the analyses. The HPLC protocol described by Restek (40) was used for model aldehydes. Carbonyls in cigarette MS were analysed by a method developed by British American Tobacco (41). Further details of the HPLC analyses and representative chromatograms are presented as Supplementary Information.

Materials

Full details of all materials and reagents are provided in the Supplementary Information.

RESULTS AND DISCUSSION

We report the findings of denuder experiments with air mixtures of four aldehydes (formaldehyde, acetaldehyde, acrolein and crotonaldehyde), optimisation of the denuder method for formaldehyde, and measurements of the G/P partitioning of formaldehyde in cigarette smoke.

Denuder studies with aldehyde-air gas mixtures

Aldehyde-air gas mixtures were prepared either with RH of 60% or 0% (dry synthetic air). Standard denuder tubes, fully coated with DNPH, were used at a denuder temperature of 298 (± 1) K. The sampling flow rate and time were Q273 = 300 (± 6) mL min−1 (5 mL s−1) and 5 min, respectively. Initial concentrations of the aldehydes in the mixtures were 0.01 μg cm−3 of formaldehyde, 0.15 μg cm−3 of acetaldehyde, 0.05 μg cm−3 of acrolein, and 0.02 μg cm−3 of crotonaldehyde. Samples from the denuder experiments were processed and analysed by HPLC.

Measurement of DNPH hydrazine derivatives indicated that depletions of acetaldehyde, acrolein and crotonaldehyde along the denuder tubes are significantly faster with 60% than 0% RH mixtures (Figure 3).

Figure 3

Depletion of acetaldehyde, acrolein, crotonaldehyde and formaldehyde along the distance of denuder tubes obtained with aldehyde–air gas mixtures prepared with 0% and 60% RH synthetic air (temperature T = 298 (± 1) K, flow Q273 = 300 (± 6) mL min−1).

Acetaldehyde showed hardly any depletion with the dry air mixture and even some initial reformation of the DNPH hydrazone seemed to occur. These observations indicate a strong humidity dependence of the heterogeneous DNPH(s) + aldehyde(g) → hydrazone(s) reactions. In support, measurement with dry aldehyde mixtures returned a poor mass balance: all three aldehydes gave ~35% with the 0%-RH mixture versus ~75% obtained with 60% RH. The best denuder performance was found for formaldehyde (Figure 3), although there was a sensitivity of depletion rate to humidity, the mass balance was independent of RH at ~85%.

Optimization of the denuder method for formaldehyde

A prerequisite for the quantitative operation of the diffusion denuder method is the fast reaction of the gaseous component of the aerosol on the surface of the denuder tube (i.e., a perfect sink wall). Ideally, the denuder depletion curves for formaldehyde from the formaldehyde-air mixtures should not depend on volumetric air flow, RH and temperature. Owing to the good results for formaldehyde and to obtain quantitative results for the G/P partitioning of formaldehyde in MS, we investigated whether these operational parameters affect results.

The following parameter variations were applied: Q273 = 200, 300, 400, 500 and 600 mL min−1 at 298 (± 1) K and RH 60%; RH 0, 10, 20 and 60% at Q273 = 300 (± 6) mL min−1 and 298 (± 1) K; and temperature 298, 310 and 323 (± 1) K at Q273 = 300 mL min−1 and RH 60%.

Effect of flow rate, RH and denuder temperature

Denuder depletion data for formaldehyde at different flow rates are presented in Figure 4. The rate of formaldehyde depletion along the denuder was significantly dependent on the flow rate of the formaldehyde-air flow; contact time and denuder efficiency increased with slowing air flow. The difference is substantial between the fastest and slowest air flows, but differences were smaller at slower flows.

Figure 4

Effect of volumetric flow-rate variation on the formaldehyde depletion data in DNPH-coated denuder tubes at 298 K and 60% RH. The curves are two-parameter-fit results to the data using the Gormley-Kennedy equation (26).

The best denuder performance was found at Q273 = 200 mL min−1, but was not substantially different from that with Q273 = 300 mL min−1.

As noted above, denuder depletion slopes were faster at higher RHs (Figure 5), implying that the rate of sink wall reaction between formaldehyde and DNPH coating increases with higher water concentration in the gas stream. Increasing RH at low RH values strongly enhanced efficiency of denuder trapping, although differences between curves became smaller once RH reached 20%. This RH may be regarded as a lower limit for denuder determination of formaldehyde in smoking studies, since the burning process of the cigarette produces combustion water in the ingoing smoke stream.

Figure 5

Effect of the air flow RH on the formaldehyde depletion data in DNPH-coated denuder tubes at 298 (± 1) K and volumetric air flow Q273 = 300 (± 6) mL min−1. The curves are two-parameter-fit results to the data using the Gormley-Kennedy equation (26).

Temperature had little to no effect on depletion of formaldehyde along the denuder tube (Figure 6). The denuder depletion curves seem to indicate a slight negative temperature dependence of the rate of the gas-solid reaction of formaldehyde with DNPH. However, the observation could also be explained by changes in the linear flow rate with temperature. Flow rate is lower at lower temperatures, which produces an increase in wall-gas contact time and a consequent higher wall-capture efficiency.

Figure 6

Effect of the denuder temperature on formaldehyde depletion data in fully DNPH-coated denuder tubes at 60% RH and Q273 = 200 (± 6) mL min−1. The curves are two-parameter fit results to the data using the Gormley-Kennedy equation (26).

Gormley-Kennedy analysis of the formaldehyde denuder data

For our experimental conditions with formaldehyde, the Gormley-Kennedy [GK] equation is mD(z)/mD(z0)=0.819×exp(-7.314h)=0.819×exp-7.314[zπDT/(2QT)] \eqalign{& {m_D}(z)/{m_D}({z_0}) = 0.819 \times \exp (- 7.314\,h) \cr & = 0.819 \times \exp - 7.314[z\,\pi \,{D_T}/(2{Q_T})] \cr} where mD(z) is the formaldehyde mass collected in the jth denuder section per μg; mD(z0) is the formaldehyde mass collected in an arbitrary 0th denuder section (e.g., between 0 and 5 cm) per μg; z is the axial end point of the jth denuder section from the entrance of the denuder tube per cm; DT is the diffusion coefficient of formaldehyde in air at temperature T per cm2 s−1; and QT is the volumetric flow rate of air through the denuder at temperature T per mL s−1. In the case of perfect denuder operation, the depletion of formaldehyde would follow the GK equation, and so a single parameter fit to the experimental data should return the gas-phase diffusion coefficient of formaldehyde as was found previously with nicotine (25). The diffusion coefficient of formaldehyde is well established as D298 (formaldehyde, air) = 0.165 ± 0.020 cm2 s−1 (41, 42).

When the only fitting parameter was the diffusion coefficient of formaldehyde, the GK fitting procedure provided a very poor description of the experimental data (Table 2), giving an estimation of formaldehyde's diffusion coefficient around three to five times smaller than in the literature value. This might be because the formaldehyde mass is underestimated in the first segment of the denuder tube due to saturation effects and/or flow was not fully laminar in the entrance portion of the tube. With the use of two parameters (D298 and the initial formaldehyde mass), formaldehyde decay data are described accurately (Table 2, Figure 4). The estimated D298 values are consistent at flow rates below 400 mL min−1 (Table 2), but lower at faster flow-rates. At the slowest flow the D298 value of 0.184 cm2 s−1 is reasonably close to that in the literature (0.165 cm2 s−1), although the estimated initial formaldehyde masses do not agree with the determined mass balances. The denuder tube, therefore, is most likely not a ‘perfect sink’ even at our optimized operational conditions.

Summary of Gormley-Kennedy analysis of formaldehyde denuder dataa.

Effect of flow rate (temperature = 298 K, RH 60%)

Q273 (mL min−1) 1-parameter fit 2-parameter fit
D298 (cm2 s−1) D298 (cm2 s−1) mD(z0) (μg)
200 0.035 ± 0.016 0.184 499
300 0.049 ± 0.015 0.186 125
400 0.060 ± 0.018 0.188 93
500 0.058 ± 0.014 0.127 33
600 0.064 ± 0.015 0.130 33

Effect of RH (temperature 298 = K, Q273 = 300 mL min−1

RH (%) 1-parameter fit 2-parameter fit
D298 (cm2 s−1) D298 (cm2 s−1) mD(z0) (μg)
0 0.024 ± 0.004 0.042 11.9
10 0.041 ± 0.011 0.112 64.3
20 0.046 ± 0.015 0.162 92.8
60 0.049 ± 0.016 0.192 125.2

Effect of denuder temperature (Q273 = 200 mL min−1, RH 60%)

T (K) 1-parameter fit 2-parameter fit
DT (cm2 s−1) DT (cm2 s−1) mD(z0) (μg)
298 0.035 ± 0.031 0.189 499.2
310 0.035 ± 0.024 0.149 366.6
323 0.032 ± 0.021 0.128 176.5

The errors given are 2σ values and are those returned from the parameter estimations.

Similarly, for the effects of RH, the conclusions are very similar (but in the opposite direction) to those for flow rate (Table 2, Figure 5). For temperature, the single-parameter fits provide very low diffusion coefficients for formaldehyde, while with two-parameters, the estimated D298 values decrease with increasing temperature (Table 2, Figure 6). This effect is more likely due to increasing linear flow rate with increasing temperatures. Nevertheless, the results clearly show that the dependence of the heterogeneous wall reaction on temperature is probably minor, which is advantageous for investigating G/P in cigarette smoke.

Smoking studies

G/P partitioning of formaldehyde in MS from Kentucky 3R4F reference cigarettes was assessed in DNPH-coated denuder tubes at temperatures of 298, 310, and 323 (± 1) K, 60% (± 1%) RH, and volumetric flow rate of Q273 = 300 (± 6) mL min−1. Data were also collected for acetaldehyde, acrolein and crotonaldehyde in the same experiments, but the poor performance of these compounds in the aldehyde-gas mixture experiments had demonstrated that they were not compatible with the current denuder apparatus. Cigarette smoke data for these compounds were also highly scattered in comparison to formaldehyde (data not shown), and depletion patterns were not obvious; consequently, data for these compounds will not be discussed further.

Denuder depletion patterns and overall masses determined for formaldehyde in cigarette MS

Based on the denuder behaviour of MS nicotine (25, 27, 29), double exponential depletion curves were anticipated for formaldehyde if ideal diffusion denuder operation was obtained and initial partitioning between phases was significant. Such a trend was observed with formaldehyde in 3R4F cigarette smoke (Figure 7). Importantly, deposition was much slower than found with formaldehyde vapour under the same conditions, where deposition was complete after formaldehyde traversed 20 cm of the denuder. With cigarette smoke, formaldehyde continued to deposit along the full 1.5 m length of the denuder, and a significant portion was trapped at the CFP and impinger placed after the denuder tube. This observation shows that formaldehyde in cigarette smoke is entrained within the aerosol to a significant degree, resulting in a significantly extended delivery than found with formaldehyde vapour alone.

Figure 7

Formaldehyde depletion in MS from Kentucky 3R4F reference cigarettes; data determined at in DNPH-coated denuder tubes at a temperature of 298 K.

Total masses of formaldehyde determined from denuder smoking experiments are presented in Table 3. Despite a directional decrease in total formaldehyde mass with increasing temperature, the three values are within 13% of the average value, indicating that the combustion processes of the cigarettes were not significantly influenced by the variation of the sampled air temperature in the denuder tube. As noted in the Introduction, to our knowledge, no denuder sampling study has been performed to determine aldehydes, but carbonyls are routinely quantified in cigarette smoke (37, 43,44,45). The values measured in our experiments are lower than those reported elsewhere as very different smoking protocols are used.

Overall formaldehyde masses determined for cigarette MS using the diffusion denuder method.

T (K) Formaldehyde mass (μg)

Denuder, mDL Cambridge pad, mc Impinger, mI Total, mΣ fDL
298 1.00 ± 0.33 0.33 ± 0.37 1.54 ± 1.52 2.86 ± 2.02 35 ± 27%
310 1.03 ± 0.35 0.58 ± 0.15 0.80 ± 0.29 2.42 ± 0.54 43 ± 17%
323 1.43 ± 0.22 0.32 ± 0.004 0.58 ± 0.34 2.34 ± 0.79 61 ± 23%
Average 2.530

Abbreviations: mDL = formaldehyde mass deposited over the whole length of denuder tube; mC = formaldehyde mass determined in Cambridge filter pads; mI = formaldehyde mass determined in impingers; mΣ = total amount of formaldehyde, calculated as mDL + mC + mI; fDL = fraction of formaldehyde deposited over whole length of denuder tube, calculated as (mDL /mΣ) × 100. Data obtained by smoking 2 cm lengths of three 3R4F cigarettes.

In our study, the need for laminar flow meant that we drew continuously on the cigarette, sampling only 2 cm of the 5.7-cm length of the burnt cigarette, whereas the other studies cited used higher flow rate puffing approaches, taken under the ISO or Health Canada Intense puffing regimes (46, 47) to generate smoke, by sampling the entire cigarette.

Temperature dependence of G/P partitioning of form-aldehyde in cigarette MS: application of denuder models

At 298, 310, and 323 K, formaldehyde displayed a relatively high deposition close to the denuder inlet, which declined sharply further along the tube, demonstrating dynamic changes in G/P partitioning (Figure 8). A small but definite increase of denuded formaldehyde mass was seen at all distances with increasing denuder temperature (Figure 8). In accordance, the fraction of formaldehyde deposited over the total length of the denuder tube was roughly 35% at 298 K and around 61% at 323 K (Table 3). At each temperature, deposition of formaldehyde from cigarette smoke occurred over a longer denuder tube distance than found with formaldehyde vapour alone. The formaldehyde depletion data have been analysed by the preferred Lipowicz-Piadé model (29).

Figure 8

Temperature dependence of formaldehyde deposition from cigarette MS in diffusion denuder tubes at a flow rate Q273 = 300 mL min−1.

Theoretical model of Lipowicz and Piadé (2004)

The Lipowicz and Piadé model (29) was adapted to estimate the initial fraction of formaldehyde in the gas-phase and to construct the deposition curves as a function of the denuder distance. The initial fraction may be calculated from the total amount of formaldehyde deposited over the whole length of the denuder tube and the total amount of formaldehyde entering the tube (Table 3). Lipowicz and Piadé (29) proposed an algorithm to construct the nicotine deposition; we have applied this approach to our formaldehyde data. The only input parameter for the Lipowicz and Piadé model is the amount of formaldehyde entering the tube. Our denuder curves agree reasonably well with the theoretical curves (Supplementary Figure S4); the average deviations between experiment and theory are 10, 3 and 11% at T = 298, 310 and 323 K, respectively.

Semi-empirical model of Lewis, Colbeck, and Mariner (1994)

In this model, the depletion of organics from cigarette MS in denuder tubes results from two processes: fast diffusion and deposition of the molecules in the gas-phase, and slower evaporation from aerosol particles with subsequent gaseous diffusion to the walls for deposition. Lewis et al. (27) assumed both processes obey Gormley-Kennedy type equations (26) and fitted the experimental data by a two-exponential function with three fitting parameters. The equation also contains the temperature-dependent gas-phase diffusion coefficient of formaldehyde in air (DT). The room temperature diffusion coefficient of D298 = 0.165 cm2 s−1 was taken from Kincaid et al. (41), from which D310 = 0.175 and D323 = 0.186 cm2 s−1 are estimated (48). The parameters returned from the fitting procedures have been used to estimate the initial formaldehyde fraction in the gas-phase (Table 4).

Comparison of G/P partitioning for formaldehyde and nicotine determined from cigarette MS using the diffusion denuder method.

T (K) Formaldehyde fractiona Nicotine fractionb

Lewis, Colbeck and Mariner model (1994) Lipowicz and Piadé model (2004) Whole length of denuder tube Lewis, Colbeck and Mariner model (1994) Lipowicz and Piadé model (2004) Whole length of denuder tube
298 5.6% 0.49% 35% 1.3% 0.09% 6%
310 7.3% 0.69% 43% 3.0% 0.5% 21%
323 11.6% 1.33% 61% 5.7% 2.5% 53%
Ratio of 323 to 298 K values
2.10 2.71 1.74 4.5 27.8 8.83

a This work.

b Taken from Reference 25.

Supplementary Figure S3 shows that the Lewis model provides a very good description of the denuder depletion of formaldehyde from cigarette MS, but the estimated parameters should be considered empirical without having real physical meaning (29).

G/P partitioning of formaldehyde in cigarette MS: analysis by denuder models and comparison with nicotine

In the current study, cigarette smoke formaldehyde concentrations showed dynamic changes along the denuder distance, similarly to those reported for nicotine (25). At room temperature, the initial gas-phase formaldehyde fraction of smoke obtained by the application of the theoretical model of Lipowicz and Piadé (29) is around 0.5%. This implies that the > 99% formaldehyde deposited in the denuder tubes originates of evaporation from smoke particles.

The corresponding parameter for nicotine is around 0.09%. Therefore, both formaldehyde and nicotine in mainstream cigarette smoke exist predominantly in the particle-phase, and most deposition in the denuder tube is owing to evaporation from particles, subsequent gas-phase diffusion, and wall adsorption. Our results are in line with the previously reported findings that significant portions of formaldehyde (10, 15, 16, 44) and nicotine are found in the particle-phase of cigarette MS (15).

The evaporation-diffusion-deposition process for formaldehyde is more rapid than that for nicotine, as shown by the measured total denuded formaldehyde fraction at room temperature, which is significantly higher than that determined for nicotine (roughly 35% versus 6%). Three explanations can be advanced to interpret this observation.

First, the evaporation rate of formaldehyde from cigarette MS particles is likely to exceed that of nicotine due to the significantly higher vapour pressure of formaldehyde (>105 Pa) in comparison to nicotine (5.7 Pa) at room temperature.

Second, the chemical processes holding both species in cigarette smoke particles [Equation 1] are also likely to favour formaldehyde release over nicotine release. Formaldehyde in the presence of water is in equilibrium with methanediol, its hydrates and oligomers, whereas nicotine is in equilibrium with acidic species generated by burning tobacco. Due to its volatility, water will rapidly evaporate from cigarette smoke particles under our experimental conditions, driving the methanediol-formaldehyde equilibrium to generate higher levels of free formaldehyde in the smoke particles. In contrast, organic acids have relatively low volatilities compared to water.

Third, the measured diffusion coefficient for formaldehyde is 2–3 times larger than that for nicotine.

wallHCHO(g)HCHO(p)CH2(OH)2(p)CH2(OH)2(g)HCHO(g)wallwallC10H14N2(g)C10H14N2(p)+HXC10H15N2(p)++X(p)- \matrix{{{\rm{wall}} \leftarrow {\rm{HCH}}{{\rm{O}}_{({\rm{g}})}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftarrow\over {\smash{\rightarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {\rm{HCH}}{{\rm{O}}_{({\rm{p}})}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftarrow\over {\smash{\rightarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {\rm{C}}{{\rm{H}}_2}{{({\rm{OH}})}_{2({\rm{p}})}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftarrow\over {\smash{\rightarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}}} \cr {{\rm{C}}{{\rm{H}}_2}{{({\rm{OH}})}_{2({\rm{g}})}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftarrow\over {\smash{\rightarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {\rm{HCH}}{{\rm{O}}_{({\rm{g}})}} \to {\rm{wall}}} \cr {{\rm{wall}} \leftarrow {{\rm{C}}_{10}}{{\rm{H}}_{14}}{{\rm{N}}_{2({\rm{g}})}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftarrow\over {\smash{\rightarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {{\rm{C}}_{10}}{{\rm{H}}_{14}}{{\rm{N}}_{2({\rm{p}})}} + {\rm{HX}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftarrow\over {\smash{\rightarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {{\rm{C}}_{10}}{{\rm{H}}_{15}}{\rm{N}}_{2({\rm{p}})}^ + + {\rm{X}}_{({\rm{p}})}^ -} \cr}

The impact of temperature on G/P partitioning of the two species can be explained by use of the Lipowicz and Piadé (29) model, which predicts that the initial gas-phase formaldehyde fractions will increase considerably with increasing denuder temperature (Table 4). At the temperatures 298 K and 323 K, the model returns fractions of around 0.5% and 1.3%, respectively; that is, they are increased by roughly a factor of ~3. These observations are consistent with the physical state of formaldehyde at ambient temperature, namely a gas, which is dissolved in the MS particle-phase. Assuming equilibrium, the gas-to-particle concentration ratio is proportional to the Henry's law constant of formaldehyde in the aerosol phase. Using pure water as a proxy for the smoke aerosol, the temperature dependence of the G/P partitioning of formaldehyde in cigarette MS could be estimated by the temperature dependence of the Henry's law constant of formaldehyde in water (49). Using the data of Allou et al. (50) leads to an estimated ratio of 2.9, in close agreement with the temperature dependence of the initial gas-phase fraction of formaldehyde from the Lipowicz and Piadé model (29).

The importance of water in the G/P partitioning of cigarette smoke formaldehyde was observed by Pang and Lewis (44). They determined carbonyl emissions from a number of cigarettes at normal tobacco moisture and in dried form where much of the tobacco water content had been removed through prolonged oven heating. Pang and Lewis found that the gas-phase formaldehyde emissions increased, but the particulate-phase formaldehyde emissions decreased on moving from normal to dry tobacco cigarettes. The G/P formaldehyde ratio was therefore higher (2 times) with dry tobacco cigarettes. Although smoke water content was not measured in Pang and Lewis's study, in general, smoke water content is made up from two sources, tobacco water and combustion water. By reducing tobacco water content from around 13% to near zero, the water content of the smoke can be anticipated to be reduced markedly in moving from normal tobacco moisture cigarettes to dry tobacco cigarettes. Pang and Lewis's observations are therefore consistent with our explanation of formaldehyde/water interactions influencing G/P ratios of formaldehyde in cigarettes smoke. Pang and Lewis (44) also noted that similar gas/particle partitioning ratios of carbonyls were found among all cigarettes and cigars, which means that the findings of the present study are more generically applicable to cigarettes beyond the reference product studied in this work.

G/P partitioning of nicotine in cigarette MS shows a much stronger temperature dependence than formaldehyde. For instance, the initial gas-phase nicotine fraction estimated by the Lipowicz and Piadé model (29) is around 30 times higher at 323 K than that at 298 K (Table 4). Nicotine is a semi-volatile organic and its denuder behaviour in cigarette MS can be rationalised by the theory of absorptive partitioning developed by Pankow and co-workers (51,52,53). Within this theoretical framework, the strong increase of the gas-phase fraction of nicotine with increasing denuder temperature is explained by the high positive temperature dependence of both nicotine release from salts in the particle [Equation 1] and the vapor pressure of nicotine (25, 54).

Implications for human smoking dosimetry

When considering the implications of these findings for formaldehyde exposure to the respiratory tract of smokers, caution must be exercised. The flowrates used in this study are considerably slower than those occurring on the inhalation step of cigarette puffing. However, what is clear is that cigarette smoke retains formaldehyde through chemical interactions within the particulate phase, leading to relatively slow formaldehyde release. Consequently, on the basis that chemical interactions of formaldehyde with particulate phase compounds in cigarette smoke are reversible (as indicated by our experimental denuder data), cigarette smoke will continue to generate formaldehyde further into the respiratory tract than would occur with formaldehyde vapour alone. It may therefore present a greater genotoxic risk to the deeper areas of the respiratory tract beyond airway generation 8, than predicted by formaldehyde vapour deposition. Other small aldehydes in cigarette smoke are significantly less retained in the particulate phase of smoke than formaldehyde and can be expected to deposit earlier in the respiratory tract than formaldehyde. Acetaldehyde has been reported to offer mouth-retention of up to 60% (19), and up to 72% of acrolein has been reported to be retained in-mouth; therefore, lower levels of mouth retention and greater levels of deeper respiratory tract retention can be expected with formaldehyde than other small aldehydes.

CONCLUSIONS

A diffusion denuder apparatus has been used to examine the gas-particle partitioning of toxic carbonyls in cigarette smoke. Initial experiments using mixtures of formaldehyde, acetaldehyde, acrolein and crotonaldehyde in air showed that the denuder system only offered acceptable performance with formaldehyde. Reactions of the other three carbonyls at the denuder surface was too slow under the experimental conditions to allow effective denuder operation. Experiments with cigarette smoke therefore focused only on formaldehyde.

Formaldehyde was partially entrained within cigarette smoke, such that formaldehyde was delivered much further along the denuding path than found with formaldehyde vapour alone with > 99% of the formaldehyde deposited in the denuder originating in the particulate phase of cigarette smoke. These observations are consistent with the operation of an equilibrium within cigarette smoke between formaldehyde and its less volatile hydrate, methanediol. Formaldehyde deposition from cigarette smoke was much higher than found with nicotine under the same conditions.

Gas/particulate partitioning of formaldehyde, which initially is predominantly in the particle-phase, has significant implications for regional dosimetry by lung-generation, owing to the potential for deeper airway penetration of particulate formaldehyde as compared with formaldehyde vapour alone. The aqueous-based retention of formaldehyde within the particle-phase may also explain constraints observed previously in removal of formaldehyde by selective filtration techniques.

Figure 1

Experimental set-up used for the preparation of flowing formaldehyde–air gas mixtures. In the apparatus the three-way glass valve is connected to a DNPH impinger to determine the concentration of formaldehyde in the gas stream. In the denuder experiments, the impinger is replaced by a DNPH-coated denuder tube.
Experimental set-up used for the preparation of flowing formaldehyde–air gas mixtures. In the apparatus the three-way glass valve is connected to a DNPH impinger to determine the concentration of formaldehyde in the gas stream. In the denuder experiments, the impinger is replaced by a DNPH-coated denuder tube.

Figure 2

Schematic representation of the denuder apparatus. 1) Denuder tube; 2) Cambridge filter pad; 3) synthetic air cylinder; 4) ultrathermostat; 5) membrane pump; 6) Pyrex® hood; 7) retractable electric igniter; 8) water saturator; 9) and 10) impingers, filled with acidic DNPH solution; 11), 12) and 13) mass flow controllers; 14) mixer to mix dry and wet air flows; 15) humidity sensor; 16) control for mass flow controllers; 17) humidity indicator. Based on Figure 1 of (25).
Schematic representation of the denuder apparatus. 1) Denuder tube; 2) Cambridge filter pad; 3) synthetic air cylinder; 4) ultrathermostat; 5) membrane pump; 6) Pyrex® hood; 7) retractable electric igniter; 8) water saturator; 9) and 10) impingers, filled with acidic DNPH solution; 11), 12) and 13) mass flow controllers; 14) mixer to mix dry and wet air flows; 15) humidity sensor; 16) control for mass flow controllers; 17) humidity indicator. Based on Figure 1 of (25).

Figure 3

Depletion of acetaldehyde, acrolein, crotonaldehyde and formaldehyde along the distance of denuder tubes obtained with aldehyde–air gas mixtures prepared with 0% and 60% RH synthetic air (temperature T = 298 (± 1) K, flow Q273 = 300 (± 6) mL min−1).
Depletion of acetaldehyde, acrolein, crotonaldehyde and formaldehyde along the distance of denuder tubes obtained with aldehyde–air gas mixtures prepared with 0% and 60% RH synthetic air (temperature T = 298 (± 1) K, flow Q273 = 300 (± 6) mL min−1).

Figure 4

Effect of volumetric flow-rate variation on the formaldehyde depletion data in DNPH-coated denuder tubes at 298 K and 60% RH. The curves are two-parameter-fit results to the data using the Gormley-Kennedy equation (26).
Effect of volumetric flow-rate variation on the formaldehyde depletion data in DNPH-coated denuder tubes at 298 K and 60% RH. The curves are two-parameter-fit results to the data using the Gormley-Kennedy equation (26).

Figure 5

Effect of the air flow RH on the formaldehyde depletion data in DNPH-coated denuder tubes at 298 (± 1) K and volumetric air flow Q273= 300 (± 6) mL min−1. The curves are two-parameter-fit results to the data using the Gormley-Kennedy equation (26).
Effect of the air flow RH on the formaldehyde depletion data in DNPH-coated denuder tubes at 298 (± 1) K and volumetric air flow Q273= 300 (± 6) mL min−1. The curves are two-parameter-fit results to the data using the Gormley-Kennedy equation (26).

Figure 6

Effect of the denuder temperature on formaldehyde depletion data in fully DNPH-coated denuder tubes at 60% RH and Q273= 200 (± 6) mL min−1. The curves are two-parameter fit results to the data using the Gormley-Kennedy equation (26).
Effect of the denuder temperature on formaldehyde depletion data in fully DNPH-coated denuder tubes at 60% RH and Q273= 200 (± 6) mL min−1. The curves are two-parameter fit results to the data using the Gormley-Kennedy equation (26).

Figure 7

Formaldehyde depletion in MS from Kentucky 3R4F reference cigarettes; data determined at in DNPH-coated denuder tubes at a temperature of 298 K.
Formaldehyde depletion in MS from Kentucky 3R4F reference cigarettes; data determined at in DNPH-coated denuder tubes at a temperature of 298 K.

Figure 8

Temperature dependence of formaldehyde deposition from cigarette MS in diffusion denuder tubes at a flow rate Q273= 300 mL min−1.
Temperature dependence of formaldehyde deposition from cigarette MS in diffusion denuder tubes at a flow rate Q273= 300 mL min−1.

Supplementary Figure S1

Representative HPLC chromatogram. The HPLC peaks are the 2,4-DNPH derivatives of the following carbonyls: #1) formaldehyde; #2) acetaldehyde; #3) acetone; #4) acrolein, #5) propionaldehyde; #6) crotonaldehyde, #7) butyraldehyde; #8) benzaldehyde; #9) isovaleraldehyde.
Representative HPLC chromatogram. The HPLC peaks are the 2,4-DNPH derivatives of the following carbonyls: #1) formaldehyde; #2) acetaldehyde; #3) acetone; #4) acrolein, #5) propionaldehyde; #6) crotonaldehyde, #7) butyraldehyde; #8) benzaldehyde; #9) isovaleraldehyde.

Supplementary Figure S2

HPLC chromatograms of samples from smoking denuder experiments.A)HPLC of a denuder sample obtained with 3R4F cigarette at 298 K and 60% RH.B)addition of acrolein + DNPH reaction mixture to the denuder sample.C)addition of crotonaldehyde + DNPH reaction mixture to A + B.
Other DNPH peaks (RT/min): CH2O: ~8.3, CH3CHO(Z): ~11.03, CH3CHO(E): ~11.33, acetone:~14.5.
HPLC chromatograms of samples from smoking denuder experiments.A)HPLC of a denuder sample obtained with 3R4F cigarette at 298 K and 60% RH.B)addition of acrolein + DNPH reaction mixture to the denuder sample.C)addition of crotonaldehyde + DNPH reaction mixture to A + B. Other DNPH peaks (RT/min): CH2O: ~8.3, CH3CHO(Z): ~11.03, CH3CHO(E): ~11.33, acetone:~14.5.

Supplementary Figure S3

Application of the semi-empirical denuder model of Lewis, Colbeck, and Mariner (1994) to model the depletion of formaldehyde from cigarette MS at a flow rate Q273 = 300 mL min−1, and 60% RH with different temperatures (T). Data are non-linear least-squares means.
Application of the semi-empirical denuder model of Lewis, Colbeck, and Mariner (1994) to model the depletion of formaldehyde from cigarette MS at a flow rate Q273 = 300 mL min−1, and 60% RH with different temperatures (T). Data are non-linear least-squares means.

Supplementary Figure S4

Application of the theoretical denuder model of Lipowicz and Piadé (2004) to describe the depletion of formaldehyde from cigarette MS in diffusion denuder tubes. Kentucky 3R4F cigarette MS was assessed at different temperatures (T) at flow rate Q273 = 300 mL min−1 and RH 60%.
Application of the theoretical denuder model of Lipowicz and Piadé (2004) to describe the depletion of formaldehyde from cigarette MS in diffusion denuder tubes. Kentucky 3R4F cigarette MS was assessed at different temperatures (T) at flow rate Q273 = 300 mL min−1 and RH 60%.

Overall formaldehyde masses determined for cigarette MS using the diffusion denuder method.

T (K) Formaldehyde mass (μg)

Denuder, mDL Cambridge pad, mc Impinger, mI Total, mΣ fDL
298 1.00 ± 0.33 0.33 ± 0.37 1.54 ± 1.52 2.86 ± 2.02 35 ± 27%
310 1.03 ± 0.35 0.58 ± 0.15 0.80 ± 0.29 2.42 ± 0.54 43 ± 17%
323 1.43 ± 0.22 0.32 ± 0.004 0.58 ± 0.34 2.34 ± 0.79 61 ± 23%
Average 2.530

Summary of physical properties of selected carbonyls in cigarette smoke.

Property Compound
Formaldehyde Acetaldehyde Acrolein Crotonaldehyde
Formula HCHO CH3CHO CH2 = CHCHO CH3CH = CHCHO
Boiling point (K) 254 293 326 377
ISO emissions from KY3R4F (mg/cigarette) 21.5 ± 7.8 (37) 540 ± 135 (37) 49 ± 14 (37) 13 ± 5 (37)
Proportion in particulate phase of mainstream smoke 32% (15, 16) 2% (15) 0% (17, 57) 7% (56)
2% (56)
7% (15)
33% (60)
Total retention on cigarette smoke inhalation 95–99% (61) 94–95% (62,19) > 97% (61)
70–97% (19)
97% (61),
98% (61)
99% (63,64)
Mouth retention from cigarette smoke - 30–60% (19) 72% of 97% total retention (19) -
Predicted main deposition regions in the respiratory tract Mouth and oropharyngeal/laryngeal (21) Mouth and oropharyngeal/laryngeal (21) Mouth and oropharyngeal/laryngeal (21) -
Trachea (22) Deposition past generation 8 (22) Deposition past generation 8 (22)
Toxicity IARC Group 1 human carcinogen (11) IARC Group 2B (58, 59) Inflammatory and cytotoxic effects (2, 65) Mutagenic and carcinogenic (55)

Summary of Gormley-Kennedy analysis of formaldehyde denuder dataa.

Effect of flow rate (temperature = 298 K, RH 60%)

Q273 (mL min−1) 1-parameter fit 2-parameter fit
D298 (cm2 s−1) D298 (cm2 s−1) mD(z0) (μg)
200 0.035 ± 0.016 0.184 499
300 0.049 ± 0.015 0.186 125
400 0.060 ± 0.018 0.188 93
500 0.058 ± 0.014 0.127 33
600 0.064 ± 0.015 0.130 33

Effect of RH (temperature 298 = K, Q273 = 300 mL min−1

RH (%) 1-parameter fit 2-parameter fit
D298 (cm2 s−1) D298 (cm2 s−1) mD(z0) (μg)
0 0.024 ± 0.004 0.042 11.9
10 0.041 ± 0.011 0.112 64.3
20 0.046 ± 0.015 0.162 92.8
60 0.049 ± 0.016 0.192 125.2

Effect of denuder temperature (Q273 = 200 mL min−1, RH 60%)

T (K) 1-parameter fit 2-parameter fit
DT (cm2 s−1) DT (cm2 s−1) mD(z0) (μg)
298 0.035 ± 0.031 0.189 499.2
310 0.035 ± 0.024 0.149 366.6
323 0.032 ± 0.021 0.128 176.5

Comparison of G/P partitioning for formaldehyde and nicotine determined from cigarette MS using the diffusion denuder method.

T (K) Formaldehyde fractiona Nicotine fractionb

Lewis, Colbeck and Mariner model (1994) Lipowicz and Piadé model (2004) Whole length of denuder tube Lewis, Colbeck and Mariner model (1994) Lipowicz and Piadé model (2004) Whole length of denuder tube
298 5.6% 0.49% 35% 1.3% 0.09% 6%
310 7.3% 0.69% 43% 3.0% 0.5% 21%
323 11.6% 1.33% 61% 5.7% 2.5% 53%
Ratio of 323 to 298 K values
2.10 2.71 1.74 4.5 27.8 8.83

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