Compartmental analysis of metals in waterpipe smoking technique

Four samples of waterpipe tobacco, which are commercially available on the Jordanian market, were selected for use in this study. The samples were purchased from the Jordanian local market in July 2012 and represent the most popular brands and flavours. The brands used were either made locally in Jordan or imported from the United Arab Emirates [16]. The samples were labelled using code numbers detailing the flavour and colour (Table 1).

The selected samples were analysed in a two-step process. The first step involved determining the total metal concentration using an acid digestion. For the second step, a shisha (waterpipe) smoking machine was used to analyse the concentration of metals in the four compartments. The amount of metal in the smoke stream was calculated as the difference between the total metal in the waterpipe tobacco and the sum of metal in the water vessel and the ash residue. The values reported as the concentration of metal in smoke are used as a measure of the metal that is not filtered by the water and has a smaller particle size than that of the filter but is still available for contact with the user, as it is not trapped by the remaining residue. The 01APPRD tobacco sample was used as an independent quality control sample due to the unavailability of suitable reference material with a comparable matrix. The metals analysed were Bi, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, V, and U. All chemical analysis was performed in the Royal Scientific Society Laboratories in Amman, Jordan.

Smoking was reproduced by connecting a Borgwaldt Shisha Smoker machine (model number 80220100) to a standard waterpipe (Borgwaldt KC) using a plastic hose (Figure 1). Waterpipe tobacco was placed into the waterpipe head and covered with perforated aluminium foil. A commonly locally used charcoal (Royal, China) was lit by means of cooking gas and placed above the foil in order to start the smoking process. The charcoal was placed on top of the foil to avoid direct contact with the tobacco such that the mixture was heated to a high temperature but not burned. The temperature under the aluminium foil and in the middle of the waterpipe tobacco head was found to be 250 and 130°C, respectively. The temperatures of the downstream smoke (after the waterpipe head and before the water vessel) and the upstream smoke (after the water vessel and before the mouth tip) were found to be 72 and 32°C, respectively. The smoke drawn through the plastic hose was then bubbled through water, thus cooling the smoke temperature by more than half. This is in accordance with a number of other studies supporting the role of the water vessel as a cooling stage for the smoke generated [4,10,18]. The water vessel temperature was increased from the ambient temperature (22°C) up to 24°C during the smoking period. Whatman filter paper with a 90 mm diameter and a pore size of 0.45 μm was placed at the end of the plastic hose to filter the emerging smoke, which allowed the fraction of smoke with particle size of 0.45 μm and above to be determined. Each smoking session completely consumed 22 g of waterpipe tobacco. The duration of each inhale was 2.6 s with a volume of 530 ml at a rate of 3 inhales (puffs) per minute (at intervals of 17 s). These parameters were selected according to the draft of the ISO/TC 126/SC standard. Ten consecutive smoking sessions were used for each sample. This equated to a total tobacco weight of 220 g analysed for each sample; the water volume in the vessel was 750 ml and was not replaced during the ten sessions. This was done in order to ensure detectable metal concentrations and a more representative sample. The pH of the water vessel before smoking was found to be 6.0 and after ten successive smoking sessions was 3.6 (Figure 2). After sampling, the water vessel samples were acidified using 1.0 ml of analytical grade concentrated nitric acid and the pH value was measured as 1.8. Five filters were replaced for each smoking session to sustain the least possible suction resistance and that was done by maintaining a pressure drop of less than 1500 Pa between the head of waterpipe and the suction device. The filter replacement was done during the 17 s interval of puff frequency.

Fresh waterpipe tobacco samples, filter paper samples, and ash residue from the smoking machine were digested using a nitric acid/hydrogen peroxide solution. Ultra-pure grade 65% nitric acid and 30% pure grade hydrogen peroxide (Scharlau Chemie S.A, Spain) was used. Prior to digestion both reagents were further purified through sub-boiling distillation in a Teflon still (MLS GmbH, Germany). For each acid digestion analysis an acid washed 100 ml tall form quartz digestion beaker covered with watch glass was used and by adding various volumes (25–30 ml) of HNO₃ and 3–5 ml of H₂O₂ (added in five steps). The duration of the digestion was the time required to obtain a clear solution during continuous heating at about 80°C. For the acid digestion analysis, 10.0 g of homogenised waterpipe tobacco was used. Ash residue analysis used the residue remaining after 22.0 g of fresh tobacco was used during a smoking session. Finally, the amount of metal remaining on the filters used in the smoking machine was analysed by digestion. Measurement accuracy and quality control were maintained for the digestion procedure through digestion of certified and reference standard material in the form of tealeaves (Certified reference no. 23, National Institute for Environmental Studies (NIES)) and wheat flour (Certified reference no. 1567a, National Bureau of Standards, (NIST)) for Cu, Fe, Mg, Mn, Mo, and Ni. Whereas for Cr, Pb, V, U, and Bi, an in-house standardized was formulated by spiking the matrix of Ma’assel sample no. 01APPRD with three different concentrations of aqueous standard multi-element solution, following standard addition methodology. It was concluded that the digestion was accurate with deviation levels lower than 7% of the relative standard deviation (RSD) from the assigned figures (Table 2).

All acidified water samples from the smoking machine and acid digestion samples were analysed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Elan DRCe 9000 (Perkin-Elmer-Sciex, USA). The metals investigated were Bi, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, V, and U. The operational conditions followed the 1994 United States Environmental Protection Agency (EPA) method entitled Laboratory Methods for ICP-MS Analysis of Trace Metals in Precipitation. This method covers all the necessary validation parameters for ICP-MS analysis, such as the operating conditions, instrument multi-element calibration standards, sensitivity checks, internal standard checks, and reagent blank.

The lower limits of detection were calculated as three times the standard deviation of the blank and were found to be 0.1, 225, 13, 436, 620, 17, 6, 43, 12, 16, and 2 ppb of Bi, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, V, and U, respectively. An independent quality control sample was used to certify all measurements in addition to the aforementioned standard reference material, the multi-element solution used originated from CertiPUR Merck No. OC528489. The percentage relative standard error for all measured metal concentrations was found to be 5.25%; this value represents the error on all metal concentrations reported unless otherwise specified.

The reagents used to carry perform the analyses include; ultra-pure grade Nitric Acid (Scharlau Chemie S.A, Spain), pure grade Hydrogen Peroxide (Scharlau Chemie S.A, Spain), multi-element solution (CertiPUR Merck No. OC528489), Tealeaves (Certified reference no. 23, National Institute for Environmental Studies (NIES)), and Wheat flour (Certified reference no. 1567a, National Bureau of Standards (NIST)).