ANALYSIS OF ZINC IN HAIR USING FLAME ATOMIC ABSORPTION SPECTROSCOPY

Transcription

1 ANALYSIS OF ZINC IN HAIR USING FLAME ATOMIC ABSORPTION SPECTROSCOPY Introduction The purpose of this experiment is to determine the concentration of zinc in a sample of hair. You will use both the calibration curve and the standard addition method. For quality control, you will determine the concentration of zinc in a test solution. Atomic Absorption Spectrochemical methods of analysis for elements are almost all based on the interaction between light and atoms (or atomic ions) in the gas phase. Light can only be absorbed or emitted in discrete energy packets called photons. The energy of a photon will depend on its frequency, ν (or, equivalently, its wavelength, λ): photon energy E = h = hc The energy levels of atoms, molecules and ions are also discrete; in fact, a photon cannot be absorbed unless its energy exactly matches the energy difference between two discrete energy levels, a requirement known as the resonance condition. Thus, for an atom to absorb a photon, resonance condition E atom = h photon If a photon interacts with an atom and satisfies the resonance condition, then its energy may be absorbed by the atom. In doing so, the atom makes a transition from a lower energy state (usually the ground state, which is the lowest energy configuration) to an upper excited energy state. The next figure illustrates this concept. excited state energy level incident photon ground state energy level Where exactly does the photon s energy go? For photons in the ultraviolet and visible region of the spectrum (about nm), the energy is transferred to a valence electron of the atom. Thus, for Page 1

2 atoms and atomic ions, the energy levels depicted in the previous figure would represent the energies of electrons in the atomic orbitals. Let s use an example. The ground state electronic configuration of sodium is [Ne]3s; this configuration occurs when the electrons all have the minimum possible energy. The one valence electron is in the 3s atomic orbital. If an incident photon of wavelength nm interacts with a sodium atom in the ground state, there is a chance that the photon s energy will be absorbed; if so, one of the valence electrons will be promoted (excited) into a 3p atomic orbital. Higher energy photons can also be absorbed by the sodium atom; a photon of nm can excite the 3s electron into the 4p orbital, while a photon at nm can promote the valence electron into the 5p orbital. Beer s Law The previous section described how atoms in the gas phase can absorb photons of a certain frequency/wavelength - i.e., photons that satisfy the resonance condition. This phenomenon can, of course, serve as the basis for quantitative analysis (or we wouldn t be talking about it!). Essentially, the idea is this: a greater number of analyte atoms in the gas phase will absorb more photons. The relationship between the concentration of photons in the gas phase and the number of photons absorbed is described by the Beer-Lambert law, which is usually referred to simply as Beer s law. Beer s law can best be understood by picturing the atoms in the gas phase as a 3-dimensional minefield cloud. Photons, all at the same frequency, enter the cloud, which is mined with photon absorbers. Whenever a photon encounters an absorber, there is a certain chance that the photon will be absorbed ( blown to bits ).What determines the probability that the mine goes off? It is the transition probability, which depends on the nature of the absorber and the quantum states (i.e., the atomic orbitals) involved in the transition. In absorption spectroscopy, it turns out that it is best to deal with the fraction of photons that make it through the sample without being absorbed; this is the transmittance, T, of the sample. T = # of photons that travel through the sample # of photons that entered the sample The transmittance is also the probability that any one photon that enters the sample will make it through the analyte cloud. Its value can be measured, as we will see. Note that transmittance is often given as a percent (as the percent transmittance, %T). There are three important factors that affect the transmittance of light through a sample: 1. The probability that the photon is absorbed when it encounters an absorber (i.e., the sensitivity of the mines, which determines likelihood of the mine blowing up when you encouter it). This probability is the transition probability of the analyte species. We would expect that as this probability increases, the number of photons making it through the analyte cloud decreases, so that the transmittance decreases. 2. The pathlength of the photon through the gas cloud. The farther the photon must travel through the analyte cloud, the less likely that the photon will survive. Thus, as the length of the photon path through the cloud increases, we would expect that fewer photons make it out; the transmittance would decrease. Page 2

3 3. The concentration of absorbers. The concentration of the absorbers is related to the number of mines that are in a unit volume in the sample; higher concentrations means that encounters between photons and absorbers will occur more frequently. We would expect that, as the concentration increases, the transmittance of the sample would decrease. The instruments we will use for spectrochemical analysis actually respond to photon detection rates. The most direct way of thinking about a photon rate is in units of photons/second. However, it is common to quantify the intensity of light as the radiant power in J/s. It is easy to see that the detected radiant power is directly proportional to the rate of detected photons, radiant power = energy (in J) photon $ photons detected second Let s imagine that we shine some light (which satisfies the resonance condition) on a gas that contains a certain concentration of analyte atoms: incident power transmitted power "cloud" of analyte atoms We can measure the incident power, P 0, and the transmitted power, P, through the analyte cloud. Beer s Law that the number of photons that survive the analyte gas cloud decreases exponentially with increasing pathlength, b, and analyte concentration, C, in the cloud: P = P 0 e kbc where k is a constant that depends only on the transition probability between the two atomic energy levels. Now, the ratio P/P 0 is, by definition, the transmittance, T of the system, so we may write T = e kbc Usually, Beer s Law is written in terms of the absorbance, A, where A = logt: Beer s Law A = abc where b is the pathlength of the photon through the cloud, and C is the concentration of analyte atoms in the cloud. The constant a is the absorptivity of the analyte species; it depends only on the transition probability between the two energy levels that match the resonance condition. Beer s Law is the basis of many spectrochemical analytical techniques, such as flame atomic absorption spectroscopy (FAAS) and molecular absorption spectroscopy. Beer s Law will be followed only to the extent that the following assumption is true: each detected photon has an equal probability of being absorbed by an analyte species. What this means in practice is that: Page 3

4 the pathlength of each photon through the gas cloud must be the same (otherwise, photons that travel longer pathlengths will generally have a greater probability of being absorbed); each photon must have the same wavelength (so that the transition probability is the same for each photon); the analyte atoms are evenly distributed throughout the gas cloud (otherwise, photons traveling through particularly dense regions of analyte atoms will be more likely to be absorbed); no stray light is detected. Stray light is light that is registered by the instrument but that has not passed through the gas cloud. In practice, none of these conditions is met fully, and so deviations from Beer s Law will be observed. The extent of the deviation (and the curvature observed in the calibration curve) will depend on how badly the above assumptions are violated. Flame Atomic Absorption Spectroscopy (FAAS) In atomic spectroscopy, light interacts with atoms in the gas phase. Beer s Law relates the absorbance to the concentration of analyte atoms in the gas phase. Of course, we are most interested in the concentration of atoms in the sample solution. We will now look at the instrumentation needed to provide gaseous analyte atoms in a manner that ensures (or tries to) a linear relationship between the measured absorbance and the analyte concentration in the sample. The functional components in an atomic absorption spectrophotometer are illustrated in the next figure. light source atomizer monochromator and detector nebulizer sample solution Here is a very brief description of the function of each of the components of the instrument; see the literature references for more detail. the atomizer and the nebulizer provide the analyte atoms in a gaseous form. The nebulizer converts the solution into very small droplets, which are then transported into the atomizer. In this experiment, we will use an acetylene-air combustion flame as the atomizer. The heat from the combustion of the acetylene fuel provides the thermal energy necessary to convert the droplets of solution into a gaseous state. Page 4

5 the purpose of the light source is to provide photons of the proper wavelength. The most common source in atomic absorption spectroscopy is the hollow cathode lamp. the monochromator screens out any stray light and the detector provides a current that is proportional to the radiant power of the light that makes it through the monochromator. Beer s Law specifies the relationship between the detected absorption and the concentration of analyte atoms in the gas phase. The ability of the nebulizer and atomizer to vaporize the analyte in the sample is called the atomization efficiency. Typically, atomization efficiencies of flame atomizers are quite low; one reason (and there are several) is that the nebulizer is pretty inefficient at converting the sample solution into small liquid droplets, since only about 5% of the solution every reaches the flame. In order to observe a linear relationship between measured absorbance and analyte concentration, it is only necessary that the atomization efficiency is independent of analyte concentration. This is generally the case for low analyte concentrations. It is also important, of course, is that the atomization efficiency remain constant from sample to sample. References Skoog 8A-C, 9A-D Harris 22 Page 5

6 FLAME ATOMIC ABSORPTION: PROCEDURE In this experiment, you are to determine the concentration of zinc in a sample of hair by atomic absorption spectroscopy using an air-acetylene flame atomizer. In order to verify the accuracy your analytical technique, a test solution of zinc will also be analyzed. Preliminary Warm up the instrument: turn on the main power supply switches at least thirty minutes before measurements are made (ask the TA or your instructor to show you how to do this). Make sure the zinc hollow cathode lamp is in the proper position. Adjust the lamp current to about 8 ma. We will monitor the absorbance at a wavelength of nm. Note that the monochromator scale zero is off, so a setting of 2164Å actually corresponds to 2139 Å. Sample and Blank Preparation Obtain a sample of hair from the drying oven in the back. After allowing it to cool in a dessicator, measure the mass of your sample. Note that dry hair, when exposed to the air outside of your dessicator, will absorb moisture very quickly. Dissolve your sample by heating it in a 100 ml beaker with approximately 25 ml of concentrated nitric acid (in the hood). Make sure all of the hair is in the solution (use a water bottle to help with this). To avoid excessive foaming, heat the solution gently at first until the hair dissolves; then heat at the boiling point for an additional minutes. Allow the solution to cool before filtering through a Whatman #1 filter. Dilute the filtered solution to the mark in a 100 ml volumetric flask, using deionized water as the solvent. In order to check for possible contamination in the dissolution procedure, you should prepare a blank in the same manner. In other words, boil the same volume of nitric acid (with no hair) for the same amount of time, filter and then dilute to 100 ml. It is easiest, and most accurate, if you simply keep the two beakers sample and blank together at all times. In other words, heat them on the same hot plate for the same amount of time, etc. Standard Solutions Preparation Make up 5 calibration standards between 1 and 20 ppm Zn by diluting the appropriate volume of stock standard solution to 100 ml. Use deionized water as the solvent. Pipet three 25 ml portions of your hair sample solution into three 50 ml beakers; pour the remaining hair sample solution into another 50 ml beaker. Pipet additions of 0.2, 0.4 and 0.6 ml of 1000 ppm Zn solution into the three measured (i.e., 25 ml) volumes of hair sample solution; these are your standard addition solutions. Pour about 30 ml of each of your calibration standards into separate 50 ml beakers, and do the same for your sample prep blank and your test sample solution. At this point you should have ml beakers containing solutions to be measured by flame AA. Page 6

7 Procedure Measurements Before each measurement, you must adjust the scale of the meter using the following two steps: 1. Set the mode-selector switch to Absorption mode. Block the entrance slit to the monochromator and watch the meter. Adjust the Zero control to read 100% absorption (which corresponds to 0% transmission). 2. Now unblock the entrance slit and aspirate blank solution into the flame. Use the Gain control to adjust the meter to 0% absorption (100% transmission). After the second step, block the beam again: it should still read 0% transmission. If so, you are all set to obtain measurements for your calibration curve. The analog meter on the FAAS instrument has separate scales for the faction of light absorbed and for the fraction of light transmitted through the sample. The latter scale (transmittance) is linear, and it is generally easier to read the transmittance and then convert to absorbance later. You will need to obtain at least one measurement on each of the eleven solutions; as well as a measurment of the solvent blank (i.e., deionized water). IMPORTANT: the concentration of zinc in either of your sample solutions might be higher than 25 ppm, which is the concentration of your most concentrated calibration standard. How can you tell if this is the case? What should you do if, indeed, either sample solution is more concentrated than 25 ppm? If you don t know the answers to these questions, ask your instructor before you leave lab! Page 7

8 FLAME ATOMIC ABSORPTION: DATA SHEET Name: Unknown #: Mass of sample: g solvent blank std 1 std 2 std 3 std 4 std 5 hair blank hair sample addition #1 addition #2 addition #3 test sample conc, ppm 0 0 transmittance, % dilution factor zinc conc in hair (95% confidence intervals) zinc conc in test solution (95% confidence interval) Results std addition: calibration curve: DATA TREATMENT You should plot your calibration data to determine whether your calibration curve is linear; if necessary, make use of residual plots to help you with your decision. If the curve is not linear, then you need to fit a nonlinear function (e.g., a second-order polynomial) to your data. In that case, you may need to seek guidance begin with the on-line tutorial in calculating confidence intervals using the calibration curve and standard addition methods.