Integration of Time and Spatially Resolved In-Situ Temperature and Pressure Measurements With Soft Ionisation Mass Spectrometry Inside Burning Superslim and King-Size Cigarettes

The set-up for temperature, pressure and microprobe chemical sampling has been described by Cui et al. (12). In brief, established spatially and time-resolved methods for mapping gas-phase temperature, pressure distribution, and the concentrations of volatile compounds in a burning cigarette (21, 22, 17, 18, 35) were combined in a single experimental set-up. This consisted of a cigarette holder with the sample cigarette connected to a single-port smoking machine (Borgwaldt A14 Syringe Driver, Borgwaldt, Hamburg, Germany), which was used to generate the puffs. Sampling probes for temperature, pressure or chemical sampling were positioned inside the cigarette via a micro-metre stage with 0.01-mm positioning accuracy in two directions. At defined positions of the cigarette, two-probe sampling was carried out in which synchronized temperature and pressure, or temperature and chemical sampling was effected, while sensor arrays measured temperature and pressure individually at several other positions (Figure 1) (12). Before insertion of the probes, a steel needle was used to pierce holes in the cigarette at an accuracy of 0.1 mm. By manual application of paper glue, a seal was formed around the inserted probes to ensure air-tightness and reduce influence on the flow characteristics of the cigarette. It was attempted to minimize the disturbance of the overall processes by the probes but this could not be completely be excluded. The thermocouple array, measuring the gas-phase temperature, had multiple K-type thermocouples (o.d., 0.254 mm) connected to an analogue-to-digital converter (Model OM-87 DAQ-USB-2401, OMEGA, CT, USA), recording at a frequency of 10 Hz per channel.

Figure 1

The pressure array contained up to six sensors (quartz tubes, o.d., 0.35 mm; i.d., 0.2 mm) connected to a digital pressure transducer (Anhui Institute of Optics and Fine Mechanics, Hefei, China), recording from −2048 to +2048 Pa at a frequency of 10 Hz per sensor (full-range accuracy within 5%) (23, 24). Temperature and pressure data collected at each location were directly processed by MATLAB to produce the respective temperature gradient and axial air flow velocity (23, 24), and interpolated data from multiple locations enabled distribution maps to be generated as a function of time.

Chemical mapping of a cigarette was done as described previously by Adam et al. (2, 3), Hertz et al. (17), Hertz-Schünemann et al. (18), and Cui et al. (12). The vacuum of the mass spectrometer maintained the sampling flow through the chemical microprobe at ~100 μL/s. Vacuum ultraviolet (VUV) 118-nm laser pulses generated from an Nd-YAG laser (Figure 1c) were focused underneath the inlet needle of the capillary-effusive gas inlet system via optical elements. The generated ions were analysed in a reflectron TOF mass spectrometer (Kaesdorf Instrumente für Forschung und Industrie, Munich, Germany) over the mass range 5–500 m/z. Ten mass spectra per second were recorded and normalized to a 1-ppm toluene standard (all spectra recorded in a single experiment were divided by the average peak area of the toluene signal, to exclude sensitivity variations of the MS system from the results). Spectra were then synchronised and averaged over the replicates (time resolution 10 Hz for each concentration map). Compounds were identified by reference to previous MS studies of tobacco smoke by Mitschke et al. (26), Adam et al. (1), Hertz et al. (17), and Hertz-Schünemann et al. (18). Data were recorded and initially evaluated by using a specifically developed software program (Labview; National Instruments NI, Austin, TX, USA).

The burning cigarette was sampled stepwise at insertion points located 20–36 mm from the lit end of the cigarette (see Figure 2) (12). The 28-mm line corresponds to the paper burn line at 0.0 s of a puff. The distribution maps are horizontally centred to 0 mm at this start point. In general, 3–6 replicate measurements were made at each location to obtain an average response. Depending on the target sampling position, the insertion depth was 2.7 mm, 2 mm, 1.25 mm, or 0.5 mm for the superslim cigarettes, and 4.0 mm, 3.0 mm, 2.0 mm and 1.0 mm for the king-size cigarettes.

Figure 2

The temperature sensor and puff taken by the smoking machine were synchronized via a reference thermocouple, which triggered the smoking machine to take a puff (35 mL puff volume, 2 s puff duration) when the temperature of the approaching burning coal reached the pre-defined temperature of 400 °C for the superslim cigarettes and 450 °C for the king-size cigarettes. The temperature and chemical measurements were synchronized by assuming the cigarettes had cross-sectional symmetry and by using shared sampling grid locations along the paper burn line.

Both types of cigarettes were Chinese Virginia-style (100% cut leaf, 0% expanded cut tobacco, 0% reconstituted tobacco, 0% cut stem) products. The circumference of the superslim cigarettes was 17 mm, while that of the king-sized was 24 mm. The physical parameters and the mainstream smoke yields of NFDPM, nicotine and carbon monoxide of the cigarettes are summarized in Table 1. The cigarettes were sourced from a single batch and conditioned at 22 °C and 60% relative humidity for 48 h or more before experiments. Both ISO puffing parameters (2-s puff duration, 35-mL puff volume, once every 60 s) and HCI parameters (2-s puff duration, 55-mL puff volume, once every 30 s, filter ventilation sealed) were used, although in this work the measurements were conducted on a single puff. The yields of NFDPM, nicotine and CO in Table 1 have been obtained after 40 repetitions each.

To describe the cigarette burning process in detail, several parameters (T_0.5, V₀, T_max) were defined and measured, together with three sets of burn rate, providing a comprehensive picture of this highly heterogeneous combustion system (21, 22, 13). T_max was the maximium gas-phase temperature in the burning coal, T_0.5 was the average temperature of the hotter 50%-volume of the burning coal, and V₀ was the cumulative coal volume above 200 °C. Cumulative coal volume was given according to a modified Rosin-Rammler [R-R] distribution equation. These distribution parameters can directly provide a combination of coal volume and temperature range information. Table 2 listed these parameters for the two types of cigarette smoked under ISO and HCI puffing conditions. The main observations were: 1) T_0.5 under HCI puffing was greater than that under ISO puffing for the superslim cigarette, but this effect wasn’t observable for the king-size cigarette; 2) a similar trend exists for V₀ when comparing the superslim and the king-size cigarettes; 3) T_max did not differ significantly between the two burning tips; and 4) for both cigarettes, a higher puffing flow rate (HCI vs ISO) induced a faster burn rate, whether it was the average puff burn rate or the maximum puff burn rate. The king-size cigarettes had a marginally slower smouldering rate than the superslim cigarettes.

Figure 2 summarises the gas-phase temperature distributions as a function of puffing time (0 to 2 s) and extended to 1.0 s post puff. Temperature maps recorded at 0.5-s intervals were aligned vertically for the two products from the start of the puff at 0.0 s. The direction of burn was from right to left in the horizontal direction. Visually, for king-size cigarette, the burning coal was larger and temperature maps appeared more rounded, which was mainly attributed to the puffing caused turbulent flow or to boundary effects, resulting from the higher volume available in the king-size cigarettes.

Under ISO conditions, the changes in the coal temperature of the superslim cigarettes in response to the puff flow were similar to those reported previously by Cui et al. (12). In short, the temperature increased noticeably in the first 0.5 s, and then substantially from 0.5 to 1.0 s. After 1.0 s, the temperature of the central internal region of the burning coal remained relatively high. As discussed before by Baker (4) and Cui et al. (12), the delay in temperature increase in the first 0.5 s might be due to the time required for the exothermic energy from tobacco combustion to overcome the initial cooling effect of the incoming ambient air. The temperature differences between the superslim and king-size products were most obvious on the overall shape of the temperature map at 1.0 s, which is consistent with the fact that the gas-phase temperature is responsive to the external puffing flow. In general, the temperature spread was more intense for HCI conditions and stayed higher for longer after the puff; this would be expected due to the higher flow rate, resulting in higher exothermal combustion.

Figure 3 summarises the pressure distribution inside a cigarette as a function of the puffing time, using the same time and dimensional scales for the two burning tips. The data were sampled directly, using a small pressure probe as described in the Experimental section. By combining the gas-phase temperature (Figure 2) and pressure distributions (Figure 3), it is possible to calculate the gas-flow velocity inside the burning zone (not shown), examples of which have been reported previously for a superslim product by Cui et al. (12).

Figure 3

The pressure distribution inside the superslim cigarette smoked under ISO conditions was similar to that reported previously (12). There was a detectable flow at 0.0 s (Figure 3), which might have been induced by the pressure sensor module and/or the combined measurement fluctuation. From 0 to 0.5 s, a relatively uniform increase in pressure occurred in the front (left) part of the burning coal in response to the puff, whereas the back or downstream (right) part of the burning coal experienced a relatively uniform pressure field.

The pressure continued to increase in the front region of the coal from 0.5 s to 1.5 s in response to the puffing air flow and temperature increase (Figure 2) but, unlike the temperature distribution, it dropped off immediately after 1.5 s when the external puff flow rate of the 2-s bell-shaped flow decreased.

The high-pressure region in front of the burning coal is thought to be due to the increase in air viscosity associated with gas-phase temperature, which means that only a small amount of the puffing air is drawn through the tip of the burning coal (4, 5, 6), while the majority of the puffing air flows into the rod from the peripheral band (paper char line), which is located approximately between the high-pressure front region and low-pressure back region of the coal (Figure 3).

The higher pressure differences observed in the superslim cigarette are due to the smaller cross-sectional area of the rod under the equal puffing air flow. The pressure differences in the front (left) region are higher for the HCI puff than for the ISO puff, which is also due to the larger puff volume. Interestingly, the HCI puff does not seem to extend the higher pressure regions significantly relative to the ISO puff for both types of cigarette.

The complex thermophysical conditions experienced by the tobacco inside the two types of cigarette are expected to lead to the characteristic release and formation of smoke constituents (7). Soft ionisation SPI mass spectrometry is suited to screen the dynamic evolution of chemical reactions as reported by Adam et al. (1). Here, we selected two examples of chemical interaction to illustrate the effects of both product design (superslim vs king-size) and puffing conditions (ISO vs HCI) on the smoke formation process. The first system involved benzene and nitric oxide (NO), while the second involved nicotine, indole and ammonia.

3.3.1

Benzene-NO system

NO is an ubiquitous oxide of nitrogenous compounds occurring also in tobacco involved in combustion. Benzene can be formed from multiple sources (e.g., organic acids and terpenes) above 300 °C during tobacco pyrolysis; it is relatively insensitive to tobacco blending (28, 35). Previously, we described an interactive and three-stage NO-formation/destruction mechanism that is coupled to the dynamics of benzene formation as a result of tobacco thermal decomposition and combustion for a king-size 2R4F US-blended cigarette (35). It was interesting to assess whether the proposed NO-benzene mechanism also applies to the Chinese Virginia cigarettes used in the present study. Figures 4 and 5 show the concentration map for NO and benzene as a function of puffing time, respectively. At 0 s, corresponding to the steady-state smouldering of the tobacco, measurable amounts of the two compounds were present in both cigarettes, although the concentrations of NO were higher, consistent with the fact that tobacco is a nitrogen-rich biomass. From 0 to 0.5 s, the benzene concentration initially decreased under both ISO and HCI conditions, before increasing slightly from 1.5 s. This pattern could be explained by a mainly air-dilution-led decrease in benzene prior to its re-formation during the second half of the puff, where the temperature was higher and the incoming dilution air was reduced (35). The interaction between a gradually weakening air dilution and benzene formation extended to 2.0 s, leading to its significant concentration.

Figure 4

Figure 5

According to the proposed NO formation and destruction mechanism (35), the concentration distribution of NO is controlled by the following three stages, which take place roughly in the direction of the tip to the unburnt cigarette: 1) heterogeneous char oxidation-led formation of NO at the tip (high-temperature region) of the burning coal; 2) NO reburn by a homogeneous reaction with organic species/radicals, leading to significant destruction of NO in the middle region of the coal; and finally 3) homogeneous gas-phase oxidation of nitrogen (reformation via HCN).

In addition Figures 4 and 5 show that the concentrations of benzene and NO continued to increase at the end of the 2-s puff and beyond. A kind of ‘dieseling’ before the cigarette is back in the steady-state smouldering phase could explain this observation. Due to the still present ‘fresh’ air from the outside the formation reaction can go on. Simultaneously, the flow stopped and the formed NO is not diluted, which leads to an increase in concentration. In terms of the effects of the tobacco rod circumference (superslim vs king-size), the dynamic interaction between benzene and NO was largely maintained, except for some variations in intensity. Some of the intensity variations between this and the previous results may be the result of tobacco blend differences (US-blended vs Virginia type of blend in this study) (19).

3.3.2

Nicotine, indole and ammonia system

The second chemical interaction system of nicotine, indole and ammonia further demonstrates the capability of the time-and spatially integrated method to facilitate understanding complex chemistry. This system involves the thermal release of nicotine and its degradation reactions with indole and ammonia. Ammonia represents the end point of non-oxidative degradation processes.

Endogenous nicotine in tobacco leaf mostly exists as salts with carboxylic acids. Numerous pyrolysis studies have shown that nicotine is released by decarboxylation and subsequently transferred into cigarette smoke. The first step in thermal transfer of nicotine from tobacco to the gas phase is known to occur at a temperature around 110−125 °C for nicotine acetate, and 110−210 °C for nicotine malates (31).

Figure 6 shows the concentration maps for nicotine, indole and ammonia as a function of puffing time for both types of cigarettes, obtained under HCI conditions. At 0.0 s, nicotine had the highest concentration and was positioned to the left side of the maps, consistent with its thermal release in the low-temperature region next to the smouldering coal. By contrast, ammonia was mostly located at the tip of the burning coal, while indole was concentrated between the nicotine- and ammonia-rich regions.

Figure 6

With the start of the puff (1.0 s), the incoming air appeared to dilute the concentration of all three compounds, with the increase in indole localized at the centre of the burning zone. Due to higher flows in superslim cigarettes compared to king-size cigarettes, the dilution effect was more distinct. Overall the reactive zone for superslim cigarettes was horizontally wider.

Towards the end of the puff (2.0 s), nicotine remained concentrated at the left (lower-temperature) side, and a significant increase in the ammonia concentration was noted in the superslim cigarette, but not in the king-size cigarette. Thereafter, without incoming air, the concentrations of all three compounds continued to increase in their respective regions. The ‘dieseling’ effect, described for the Benzene-NO system is a likely explanation. In this case the ‘after-run’ effects drove the pyrolysis process.

Even if the chemical distribution maps hypothesizes a potential reaction mechanism, there is no direct evidence to suggest that the evolution of e.g., the three compounds shown in Figure 6 is mechanistically connected; the variations in concentration may be simply the result of their formation processes and spatial chemical environment, as dictated by the temperature and air flow variations. Nevertheless, the presented identification of their main formation regions and their coherence with temperature and air flow presents the opportunity to further investigate their individual formation mechanisms. This can be done by using, for example, analytical pyrolysis set-ups, whereby the heating temperature profile and oxygen levels are clearly different for these three compounds. This enables a truly representative understanding of the formation processes.