Analysis of Trace Metals in Tobacco and Tobacco Ash using Atomic Emission Spectroscopy

Transcription

1 CHEM 411L Instrumental Analysis Laboratory Revision 1.0 Analysis of Trace Metals in Tobacco and Tobacco Ash using Atomic Emission Spectroscopy In this laboratory exercise, we will analyze cigarette tobacco and the ash from the same tobacco for the trace elements Zinc (Zn), Iron (Fe) and Chromium (Cr). We will carry out this analysis by measuring the emission intensity for each element using Atomic Emission Spectroscopy (AES) and comparing these results with appropriate calibration data. Our sample's trace metals will be excited using an Inductively Coupled Plasma (ICP). We will be following a modification of the procedure of W. Wang and B.J. Finlayson-Pitts as reported in JChemEd [80] The first question to ask is why AES: Plasma, arc and spark emission spectrometry offer several advantages when compared with the flame and electrothermal methods considered [previously]. Among the advantages is their lower susceptibility to chemical interferences, which is a direct result of their higher temperatures. Second, good emission spectra result for most elements under a single set of excitation conditions; consequentl, spectra for dozens of elements can be recorded simultaneously. This property is of particular importance for the multi-element analysis of very small samples. Emission spectra from plasma, arc and spark sources are often highly complex and are frequently made up of hundreds, or even thousands, of lines. This large number of lines, although advantageous when seeking qualitative information, increases the probability of spectral interferences in quantitative analysis. Consequently, emission spectroscopy based on plasmas, arcs and sparks requires higher resolution and more expensive optical equipment than is needed for atomic absorption methods with flame or electrothermal sources. Principles of Instrumental Analysis by Douglas A. Skoog, F. James Holler, Stanley R. Crouch The second question is why use an ICP to excite our sample's elements: The two major advantages of plasma instruments are the high temperatures that they can attain for emission measurements and their capability of examining many elements simultaneously. Most plasma instruments use argon as the plasma source and are powered by a radio-frequency alternating current (AC) that flows through a copper coil that is wound as an inductor around the argon flow. A spark injects electrons into the load coil region of the plasma. These electrons and argon atoms subsequently ionize the argon to create additional electrons which lead to further ionization events. The result is a plasma in a confined region of space that has a very high temperature. An aerosol of the sample is carried into the center of the plasma by a second stream of argon gas. A third stream of argon is passed through the narrow outer tube as a cooling sheath to keep the parts of the instrument that surround the plasma from melting. The flow of argon in these three streams is critical to generate a stable plasma. Analytical Chemistry and Quantitative Analysis by David S. Hage and James D. Carr

2 Back to Wang and Finayson-Pitts; "A sample introduced into the plasma forms atoms and ions, a portion of which are excited into upper electronically excited states. They emit light when they return to the ground state, with the energy or wavelength of the light being characteristic of the particular element. The intensity of this emission can then be used to quantify the element. This method is referred to as.. ICP-AES." Thus, we will: 1. Extract the trace metals in our cigarette tobacco using a "cold" and a "hot" Nitric Acid extraction; for comparison. Filter the resulting solutions and then run an ICP-AES analysis of the solution for Zn, Fe and Cr. 2. Do the same for the ash from the cigarette tobacco. 3. Spike a tobacco sample with Zn and Fe and perform the "cold" extraction and analysis. This will allow us to determine the "Recovery Efficiency" of the method. 4. Perform an ICP-AES analysis of a standard solution containing Zn, Fe, and Cr as well as a series of dilutions to obtain calibration data.

3 Procedure The following procedure is taken from Wang and Finlayson-Pitts. Your laboratory instructor will provide you with appropriate calibration data. Use appropriate caution and techniques when handling the concentrated Nitric Acid. 1. Grind the tobacco into a very fine powder using a mortar and pestle. Slice the paper outer wrap and grind it together with the tobacco. 2. Weigh 0.1g of the tobacco and dissolve it in 2 ml of conc. HNO 3. Stir and let it sit for at least half an hour with occasional stirring. 3. Weigh another 0.1g of the ground tobacco and dissolve it in 2 ml conc. HNO 3 in a high pressure vial (Kontes Microflex 5 ml). Heat up the mixture for 2 minutes (or until all of the tobacco has dissolved) in a boiling water bath. This should be done collectively by all students in a fume hood and under the supervision of your laboratory instructor. Cool the solution down to Room Temperature before you open the cap of the high pressure vial. 4. Dilute each solution with Nanopure water to 30 ml using volumetric flasks. 5. Filter 10 ml of the diluted samples using 0.45 m Acrodisc filters by drawing the solution into a syringe, attaching the filter to end of the syringe, and holding the filter on while forcing the liquid through the filter. (Note: The filter may plug up, making it impossible to push the sample through the filter. In this case, remove the filter and place a new one on the syringe.) 6. Give your prepared samples to your laboratory instructor for the ICP-AES analysis. Spiked Tobacco 1. Make stock solutions of ZnCl 2 and FeCl 3 using spectroscopic grade materials in Nanopure water. They should have concentrations of 1 mg Zn and Fe per ml, respectively. 2. Weigh 0.1g of the ground tobacco. Spike this sample with 4.0 L of the stock ZnCl 2 solution and 9.0 L of the stock FeCl 3 solution. 3. Extract and process as per the "cold" procedure above.

4 Ash 1. Smoke down a cigarette, knocking off the ash as it burns into a clean beaker. 2. Weigh a 0.1g sample of the ash and dissolve in 2 ml of conc. HNO Extract and process as per the "cold" procedure above.

5 Data Analysis 1. Plot signals for each metal as a function of concentration in mg/l. Using a Linear Least Squares regression analysis, determine the slope and intercept, as well their standard deviations, for each metal's calibration line. 2. Determine the method's Detection Limit for each metal. Typically, the Detection Limit is taken as 3 times the Standard Deviation of the Background Signal. Note that the y-intercept of your calibration line represents the Background Signal. 3. Use your calibration data to obtain the concentration of each metal in the tobacco in units of g of metal per gram of tobacco. Compare the results of the "hot" and "cold" extraction methods. 4. Determine the "extraction efficiency" for the Zn and Fe using your calibration data and the data from the spiked tobacco. 5. Determine the concentration of each metal in the ash in units of g of metal per gram of tobacco. Calculate the ash concentration factor; i.e., the ratio of the conctration in the ash to that in the unburned tobacco. You should submit a simple Assay Sheet containing the above findings. Be sure to include all plots requested.

6 Name An ICP-Atomic Emission Spectrophotometric Analysis of Cigarettes Cigarette Brand Instrument Serial Number Calibration Curves for Each of the Following Metals*: Zn slope (m) s m intercept (b) s b Fe slope (m) s m intercept (b) s b Cr slope (m) s m intercept (b) s b Analysis Results Report All Concentrations in Units of g/g sample Metal Tobacco (Cold Extraction) Tobacco (Hot Extraction) Ash (Cold Extraction) Comments level s c level s c level s c Zn Fe Cr 1. Compare the "cold" vs. "hot" extraction techniques. 2. Comment on the "Recovery Efficiency" for the "cold" extraction method.