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A spectrometer based on pulsed UV laser-induced breakdown spectroscopy (LIBS) and a highly sensitive intensified charged coupled device camera was developed to determine the carcinogenic substances like fluorine in various brands of cigarettes available commercially. In order to achieve the high sensitivity required for the determination of trace amounts of fluoride in cigarettes and eventually the best limit of detection, the experimental parameters (influence of incident laser energy on LIBS signal intensity and time response of plasma emission) were optimized. In addition, the plasma parameters like electron temperature and electron density were evaluated using Boltzman’s plot for cigarette tobacco for the first time. To the best of our knowledge, LIBS has never been applied to determine the fluorine concentration in cigarettes. Along with the detection of fluorine, other trace metals like Ba, Ca, Ni, Cu, and Na were also detected in cigarettes. For determination of the concentration of fluorine, calibration curve was drawn by preparing standard samples in various fluoride concentrations in tobacco matrix. The concentration of fluorine in different cigarette tobacco samples was 234, 317, 341, and 360 ppm respectively, which is considered to be much higher than the safe permissible limits. The limit of detection of our LIBS spectrometer was 14 ppm for fluorine.
© 2015 Optical Society of America
1. Introduction
Tobacco leaves are the main raw material used in manufacturing of cigarettes. It is a well-established fact that tobacco consists of various toxic elements that can cause health problems during the smoking of the cigarettes. Over 4000 chemicals have been identified as ingredients [1]. Tobacco is known to consist of various compounds among which hydrocarbon, oxygen-containing compounds, nitrogen-containing compounds, heavy metals, nonmetals, ions, and halogen-containing compounds are prominent. It is reported that the approximate composition of halogen-containing compounds is 1.5% in tobacco [1]. Halogens [especially, fluorine (F), chlorine (Cl), and bromine (Br)] can produce very dangerous compounds during the burning of the cigarettes. In the process of burning, 2.4% of these halogens are transferred to smoke and eventually inhaled as organo-halogens, which are also quite harmful to the body [2].
Fluorine is among the top 15 most abundant elements on the Earth’s crust. The main sources of fluorine exposure to human beings are through food, water, and other daily use products. Too much intake of fluoride results in dental and skeletal fluorosis. It is reported that over 25% of the population of Saudi Arabia (SA) suffers from dental fluorosis even though a study revealed that the level of fluoride in drinking water in SA is negligible [3]. Even the lung cancer cases are rising rapidly specially in the smoker population of SA. Due to these facts, we were motivated to investigate the fact that, there must be other sources of fluoride ingestion among the population in SA. It has been proven that the proportion of fluorosis as a result of water fluoridation is now less compared to other sources like cigarettes and diet. Fluoride gets into the body through the gastrointestinal track and is stored right there as a hydrofluoric acid [4]. It causes muscle fiber [5], severely distorts spermatogenesis [6], and disrupts calcium current in neurological systems [7]. Due to limited awareness on the fluoride level in tobacco, it is usually ignored when calculating the total dietary intake of fluoride. The minimal recommended level for daily oral fluoride ingestion was determined to be 0.05 ppm per day [8], as recommended by the nonobservable adverse effect level (NOAEL). Estimation of the lethal fluoride doses are 16–64 ppm in adults and 3–16 ppm in children [8]. Due to these facts, this study is highly desirable to determine the level of fluoride in tobacco cigarettes.
To determine fluorine concentration in various tobacco cigarette brands, the widely used methods are ion selective electrode and ion chromatography [9–11]. These two methods are not straightforward and involve the conversion of a solid sample to a solution, which is in ionic form within the solution. This process of conversion of a solid sample into a solution is very tedious and requires chemicals. Molecular absorption spectrometry is another method of fluorine determination but always requires high homogeneity of the sample to be tested. The principle of this method is based on the molecular absorption of gallium monoflouride (GaF) [12–16] and requires high-resolution continuum sources absorption spectrometry and the limit of detection is not very good.
A simple and rapid method to the aforementioned methods tested in this study is laser-induced breakdown spectroscopy (LIBS). This method uses the solid sample directly in ambient air at atmospheric pressure, with minimal sample preparation of just making the solid sample into a pallet. LIBS is a very fast developing technique for quantitative and qualitative analysis of all elements present in the solid sample. This technique involves a short laser pulse evaporating a small amount of material (usually in micrograms), thereby creating a plasma plume. This plasma plume consists of free atoms and ions in different excitation states. As the plasma cools down, the excited elements de-excite and emit radiations, these emitted radiations are then recorded with a high-resolution spectrometer to investigate elemental composition with the help of a spectroscopy technique. Such spectral lines are basically utilized [17–19]. Plasma is generally characterized by different parameters, namely electron density, plasma temperature, and degree of ionization. Generally speaking, there are two types of ionized plasma, the weakly and the highly ionized plasma. The weakly ionized plasma is the one in which the ratio of the electron to other species is less than 10%, while in highly ionized plasma the atoms are usually tripped of many of their electrons resulting in very high electron to atoms/ions ratios. LIBS plasma is characterized under the weakly ionized plasma region [20]. To the best of our knowledge LIBS has never been applied to determine the fluoride level in tobacco cigarettes and therefore the goal of this study is to optimize the experimental parameters with the aim of improving the limit of detection, the signal-to-noise ratio, and linearity of the calibration curve for detection of fluoride in tobacco cigarettes. Laser fluence and the various experimental parameters of LIBS setup were optimized using the fluorine fingerprint line 690.2 nm in order produce optically thin plasma in local thermodynamic equilibrium (LTE). This parametric optimization was essential to achieve the best limit of detection to detect the trace amount of fluorine in tobacco cigarette samples.
2. Experimental
A. Sample Preparation
Four different brands of commercially available cigarettes that are common among smokers in SA were purchased from a local market. The selection of these brands was based on price tag ranging from lower price to higher price. The tobacco was separated from each brand of the cigarettes by removing the filter, tipping paper, and the rolling paper. This tobacco was then hydraulically pressed into pallets with maximum rigidity to withstand the high laser power thrust and to reduce the error for accurate LIBS signal analysis. The tobacco under this study were pressed into round pallet shape by applying a pressure of 5.2 bars for 10 min while placing the tobacco in a special die. The cylindrical die was 2 cm in diameter and of 2 mm thickness. Figure 1 depicts the pictorial view of the palletization process of tobacco cigarettes. There was no need to use any type of binder because the tobacco bonded well on its own. The pallets were preserved in a clean petri dish and stored in a desiccator to avoid any absorption of moisture and other contaminants.
Fig. 1. Pictorial view of the palletization of tobacco cigarettes (a) as prepared cigarettes, (b) separated tobacco grain from a cigarette, and (c) palletized tobacco cigarettes for LIBS analysis.
B. LIBS System
The detailed description and schematic of the experimental setup applied in this work is published in detail in our previous publications [21–24]. In this work, a laser pulse having a pulse duration of 8 ns and wavelength of 266 nm generated from fourth harmonic of a pulsed Nd:YAG laser and a 20 Hz pulse repetition rate was collimated and focused on a cigarette sample surface using a UV convex lens of focal length 30 mm. The laser fluence applied was high enough to create the required plasma plume needed at the surface of the cigarette sample. The crater formed on the surface of the sample as a result of focused laser beam was minimized by placing the sample on a X-Y translator, which was kept moving during the analysis. The laser energy was measured using a calibrated energy meter (Ophir Model 300) for the study of LIBS signal intensity dependence on the laser energy. An optical fiber supported with a miniature lens was placed at an angle of 45° to collect signals from the plasma spark created on the surface of the tobacco cigarette. The high resolution 500 mm spectrograph (Andor SR 500i-A) with grating groove density of 1200 lines/mm was used to collect the generated LIBS signal from the fiber optics. The vertical output port through a built-in delay generator coupled with spectrometer was connected with a time-gated intensified charged coupled device (ICCD, Andor iStar) camera, this delay generator synchronized with the -switch sync out of the exciting Nd:YAG laser. The spectrograph was integrated with a computer processing unit, and with the help of software built in spectrograph reads the data from the chip and prints out the spectra on the PC monitor.
3. Results and Discussion
A. Investigation of Local Thermodynamic Equilibrium Condition of LIBS Plasma
Spectral line intensities in the recorded LIBS spectra were used to detect and quantify the elements present in the test sample with the conditions that the laser-induced plasma is optically thin and in the LTE. The elemental composition of the optically thin plasma is the same as that of the sample. In an optical system like the plasma generated by pulsed laser ablation, the LTE condition holds if the electron-ion and electron-atoms collision processes are very fast and prevails the radiative process. The plasma generated as a result of laser ablation is complicated and can be understood by the following physical laws: the plasma particle obeys Maxwellian velocity distributions, population in the energy level follows the Boltzmann’s statistics, ionization process and can be described by Saha’s equation, and radiation density obeys Plank’s law. Along the boundary of the plasma where electron density is low and movement of the boundary region is rapid, LTE is not a good assumption [20–22]. All that is needed for LTE is for equilibration to occur in small regions of space, although it varies from region to region [20]. However, moving slightly deeper into the plasma volume, the conditions could change more slowly and collisions occur more rapidly, and in this case LTE is valid. In order to ascertain if LTE is reached, the electron density must sufficiently be high enough for collision to dominate the population of the levels. This criterion was originally formulated by McWhirter and is now called McWhirter criterion [17,18]. One of the forms of this criterion is
where is the electron density in , is plasma temperature in Kelvin (K), and in eV. Here, is the energy of first level above the ground state. In order to ascertain if the plasma is in LTE condition, its plasma temperature and electron density were calculated using Boltzmann’s plot method and stark broadening, respectively, where ln is the natural logarithm, is the integrated signal intensity of the spectral line occurring between upper level and the lower level of the species in the ionization stages in an optically thin plasma, is the Boltzmann constant, is the plasma temperature, is the transition probability, is the transition line wavelength, is the degeneracy of the upper level , is the partition function of the species in ionization stage , is the characteristic length of the plasma, and all other symbols carry their usual meaning [24]. This equation is a straight line with a slope of . Hence if one plots the quantity on the left-hand side of Eq. (2) against (of the upper-state emission), then a straight line is expected to be obtained if there is a Boltzmann distribution. Some of the crucial factors in obtaining a good Boltzmann plot are the accurate line intensities, accurate transition probabilities, and well-spaced upper levels [15,25]. In order to have a Boltzmann plot, spectrally isolated characteristic atomic transition lines of neutral barium (Ba I) in wavelength ranges of 300–400 nm and 580–680 nm were recorded for sample #1, using optimized conditions. The different wavelengths identified and selected of (Ba I) were 307.158, 350.110, and 652.731 nm, as depicted in Fig. 2. All these characteristic transition lines of barium were then used to estimate the plasma temperature, where plasma temperature was estimated from the slope of Fig. 3. Table 1 shows the statistical weight, transitional probabilities, and upper energy level that were used for plasma temperature estimation, which were all obtained from the National Institute of Standard and Technology (NIST) database [26].Fig. 2. Selected isolated atomic transition line of barium (Ba I) for plasma temperature estimation.
Table 1. Selected Wavelength for Characteristics Atomic Transition Lines of Neutral Barium (Ba I) and Other Parameters Used for Boltzmann’s Plot
A spectral line in an optical spectrum is characterized with a nonzero linewidth and its central line usually shifts from its nominal central wavelength. This broadening and shift could be as a result of Doppler, instrumental, natural, and stark broadening [27,28]. Since our experiment was carried out under atmospheric pressure, stark broadening is the only dominant mechanism, other aforementioned mechanisms are negligible under these conditions [29]. Stark broadening basically occurs in spectral lines due to collisions between ions and electrons. Stark broadening can also introduce a shift of energy levels, which results in a shift of wavelength positions of the spectral lines [30].
The line profile for Stark broadening is described by a Lorentzian function with full width at half-maximum (FWHM) , and electron density is related by the expression [27,28]
where is the electron impact factor, is the electron density, is the ion broadening parameters, and is the number of particles in Debye sphere [27–31]. The first term on the right-hand side of Eq. (3) represents the broadening due to electron contribution and the second term is due to ions contribution. For nonhydrogenic ions, stark broadening is mainly due to electron impact, since the perturbation by ions is negligible compared to that of electrons. Therefore, the equation reduces to the form [28]In order to calculate the electron density of our plasma, the stark broadening profile of a singly ionized atomic transition line of barium (Ba I) at 307.158 nm was used because it is isolated and free from interference from other spectral lines. Figure 4 depicts a fitted Lorentzian curve for an isolated line of barium (Ba I) at 307.158 nm used for the electron density estimation.
Fig. 4. Stark broadening profile for characteristics atomic transition lines of neutral barium (Ba I) to estimate the electron density.
The FWHM of 0.47376 was estimated from the fitted Lorentzian curve and electron impact parameter “” from the reference database [32]. The electron density was estimated to be . Since the critical value of plasma electron density is , then our electron density is much greater than the critical value, which implies that our plasma in this study is indeed in LTE. Therefore, we can conclude that there was not much self-absorption and our emission is fully radiated from the plasma and also the plasma is transparent to the laser beam. Consequently our spectra are optimized and our plasma can be described using thermodynamic parameters.
B. Optimization of LIBS Signal Intensity for Detection of Fluoride in Tobacco
In a typical LIBS study, it is highly recommended to detect the optimum time delay between the incident laser beam and the opening of the shutter of the camera for detection of any element of interest [33–36], like in our case for the fluoride (F I) in a cigarette using the characteristic fingerprint wavelength of 690.2 nm for F I. The time delay between the laser excitation and spectrum acquisition determines the LIBS signal intensity level of the atomic specie to be detected. Usually due to high plasma temperature after laser excitation, there are many kinds of excited ionic, atomic, and molecular species present in the plasma plume, giving rise to an unstructured broad continuum in the LIBS spectrum. In an attempt to check this broadening, the spectrum acquisition time was delayed to a certain value ranging from few hundreds of nanoseconds to few thousands of nanoseconds, as depicted in Fig. 5. In LIBS analysis, both neutral and singly ionized atomic species are of interest and when the time delay is below 200 ns, most of the atomic transitions are from the singly ionized atoms, and for time delay above 200 ns, the transitions from neutral atoms are recorded [37]. A typical plot of LIBS signal intensity (at a fingerprint wavelength of 690.2 nm) dependence on time delay for persistent line of F I is depicted in Fig. 5. As clear from Fig. 5, 720 ns is the optimum delay time for maximum LIBS signal intensity.
Fig. 5. LIBS signal intensity dependence on time delay for fluorine line (F I 690.2 nm) in tobacco cigarettes.
Moreover, in order to determine the optimum laser energy for the LIBS signal intensity of F I (at same fingerprint wavelength of 690.2 nm) in tobacco, with all other parameters such as the LTE, time delay and laser beam diameter were kept constant in accordance with the previous findings. The laser energy was varied from 15 to 18.46 mJ at different intervals, as depicted in Fig. 6, with the corresponding LIBS signal for F I (690.2 nm) having been recorded and is depicted in Fig. 6. It can be noticed from Fig. 6 that the LIBS signal intensity shows a linear dependence on the laser energy with the intensity increasing with increase in laser energy from 15 to 18.46 mJ per pulse. At higher laser energies () the increment factor of the LIBS signal intensity reduces steadily as saturation sets in, with the optimum laser energy found at 18.46 mJ per pulse. However, at 17.54 mJ per pulse of incident laser energy was found to generate appreciable LIBS signal intensity and precision for detection of F I (690.2 nm) in our cigarette samples.
Fig. 6. LIBS signal intensity dependence on laser energy for fluorine line (F I 690.2 nm) in tobacco cigarette.
C. Detection of Fluoride in Tobacco Samples
After optimizing all the main experimental parameters, our LIBS system was set for fluoride detection in the various brands of cigarettes. Time delay between the laser excitation and spectrum acquisition was kept at 720 ns, the laser energy was maintained at 17.54 mJ per pulse, the distance between the focal volume of the plasma plume and the optical fiber was set as 10 mm, and the angle between the test sample and the optical fiber was fixed at 45°. Under these experimental conditions, typical LIBS spectra of the tobacco samples were recorded in the wavelength range of 660–760 nm and are depicted in Fig. 7. A persistent spectral line of fluorine (at fingerprint wavelength of 690.2 nm) was identified in all our samples within the specified wavelength range of the spectrum. The identification of recorded spectral lines was conducted, using the standard database published by NIST [26]. Figure 7 here indicates all the identified spectral lines in all different brands of tobacco samples, the identified F I lines are enclosed in the box. Table 1 shows other elements identified within this wavelength range, which are bromine, barium, sodium, copper, carbon, nickel, and calcium. The maker F line at 690.2 nm is due to atomic transition from (3p) (3p) 3s electronic state and the line is usually of moderate intensity. There are other transition lines of neutral F I within our wavelength range, which are very weak peaks due to self-absorption.
Fig. 7. Typical LIBS spectra for F I line in tobacco cigarettes (sample 1–4) within the 660–760 nm wavelength range. The identified F I line is indicated as enclosed in the box.
D. Calibration Curve for the Quantitative and Qualitative Estimation of Fluorine Concentration
As discussed in previous sections, we ensured the presence of optically thin plasma under our experimental conditions, thus the system can be well suited to high degree of precision. The calibrated samples were prepared by adding known concentrations of fluoride to the sample matrix (sample #1) and subsequently we used fluorine line 690.20 nm as the marker line for drawing the calibration curve. The added concentrations of sodium fluoride were (NaF) 15, 30, 60, 122, 231, 348, 420, and 537 ppm. The respective LIBS signal intensity spectra for each respective fluoride concentration added to the sample #1 matrix are depicted in Fig. 8, which shows a linearity in the growth of the LIBS signal intensity versus the F I concentration. The results presented in Fig. 8 were used to plot the linear calibration curve, as depicted in Fig. 9 for the detection of fluoride in various cigarette samples. The calibration curve was used to quantify the fluoride concentration present in the test samples #1, #2, #3, and #4, which were found to be 233.9, 340.9, 316.6, and 360.1 ppm, respectively. The concentrations of fluoride detected in this study are considered to be much higher than the minimum permissible level set by food and drug regulatory agencies [38]. In order to ascertain the degree of sensitivity of our LIBS system, one has to estimate the limit of detection (LOD) using the calibration data [39]. The LOD is the minimum amount of concentration of an analyte on a sample that can be reliably detected by a system [40] and can be estimated by using the equation
where SD is the standard deviation of calibration data and S is the slope of calibration curve [37]. Using Eq. (5), the calculated LOD of our LIBS system is 14.4 ppm, as shown in Table 2.Table 2. Concentration of Fluorine Detected in Various Tobacco Cigarette Samples Using our LIBS Setup
Fig. 8. Superimposed LIBS spectra of standard samples having 122, 231, 348, 420, and 537 ppm fluoride concentrations for plotting the calibration curve.
In this study, we were able to ascertain the level of exposure and ingestion of fluoride concentration in different brands of tobacco cigarettes, which was between 224 and 360 μg for each gram of tobacco cigarette intake. The level of intake depends on the habit of different persons, and generally since tobacco is not swallowed, only some fraction of the aforementioned amount is ingested during its consumption, which ultimately becomes available in the body through absorption. The experience gained from this work can be used for the elemental analysis of leaves, plants, and vegetables.
4. Conclusion
A UV-pulsed LIBS system equipped with a highly sensitive gated ICCD camera was developed to detect the toxic species like fluorine in cigarettes. The maximum concentration of fluorine present in various brands of cigarettes was 234, 371, 341, and 360 ppm, respectively. The concentration detected with our setup for the toxic fluorine was higher than the safe permissible limits set by the Saudi Royal commission responsible for environmental standards in the Kingdom of Saudi Arabia. In addition, the sensitive lines for other elements like Ba, Na, Ca, Ni, and Cu were detected and identified in the recorded LIBS spectra of tobacco cigarettes. For the quantitative analysis, the calibration curves were drawn to determine the trace concentration of fluorine in cigarettes. The experience gained through this work can be useful for the development of a portable system for on-line analysis of toxic chemicals present in cigarettes, especially during the manufacturing process of cigarettes. The limits of detection of fluoride were also estimated by using standard samples containing the fluoride in known concentrations in the cigarette tobacco and drawing a calibration curve.
FUNDING INFORMATION
Deanship of Scientific Research (DSR), King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia (RG1311-1, RG141-PHYS-23).
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