Atomic Absorption Spectroscopy and Atomic Emission Spectroscopy A. Evaluation of Analytical Parameters in Atomic Absorption Spectroscopy Objective The single feature that contributes most to making atomic absorption unique among energy absorption techniques is the use of a flame to produce the necessary neutral, ground-state atoms. This experiment involves a study of various instrumental and chemical parameters that are important to atomic absorption spectroscopy because they are important to the processes that occur in the flame. Specifically, the parameters to be investigated are: (1) dependence of signal on position in flame; (2) effect of fuel-to-oxidant ratio; (3) effect of the state of the metal; and (4) effect of concentration. Theory Since atomic absorption obviously implies absorption by atoms, the processes by which we can produce atoms are of principal importance. The necessity of a steady signal (uniform production of atoms) and of a system easily suited for use with solutions has limited the methods of atom production almost exclusively to flames. A primary objective is to produce a large number of free atoms in the flame, but the conditions necessary to accomplish this are often different for different systems. Furthermore, the physical environment within a flame varies drastically, causing the atom composition of the flame to vary according to the geometric region. Degradation of solvated molecules to neutral atoms in a flame is a complicated and often poorly understood process. The efficient production of free atoms depends on the fuel-oxidant composition, the temperature, the sample feed rate, the type of burner, and the chemical system itself, including solvent and diverse substances. It is not always possible to sort out the individual effect of each of these parameters, but it is possible to determine empirically optimum analytical conditions. In fact, a study of the effects of these parameters often leads to valuable conclusions about the chemical nature of the system. The mechanism of atom production is complex and apparently is not the same for all solvents and flames. For this reason, the effects of certain parameters are not always the same for all systems. One of the more important factors that must be considered is the possible formation of oxides. Many metal oxides are among the most stable of all known compounds, and consequently, once formed, are very difficult to reduce, even in a high-temperature, fuel-rich flame. For this reason, the reducing nature of the flame (amount of fuel to oxidant) is of principal importance. Furthermore, certain regions in the flame are likely to be more reducing and thus contain a greater predominance of neutral atoms. The free-atom composition of the flame is determined by examining the signal as a function of the position of the optical path in the flame. A plot of such data is referred to as a flame-atom profile.
Experiment #1A: Introduction of PE AAnalyst 200 1. Prepare chromium standards with the following chromium concentrations: 1, 2, 3, 4, and 5 ppm Cr. 2. Prepare an unknown chromium solution that has an approximate concentration between 1 and 5 ppm Cr. 3. Dilute all solutions with distilled water. 4. Using distilled water as the blank, read the absorbance for all solutions using manual and autosampling procedure. Experiment #2A: Dependence of Signal on Position in Flame 1. Using distilled water as the blank, read the absorbance for 10 ppm Cr solution. 2. Lower burner ½ turn and read absorbance for 10 ppm Cr again. 3. Continue to lower burner ½ turn and read absorbance until absorbance equals 0. 4. Set burner height to maximum absorbance position. Experiment #3A: Dependence of Signal on Fuel-to-Oxidant Ratio 1. Set acetylene flow rate to 2.5 L/min and read absorbance of 10 ppm Cr solution. 2. Raise flow rate of acetylene by increments of 0.1 L/min and read absorbance until flow rate equals 3.5 L/min. 3. Set flow rate of acetylene to rate showing maximum absorbance. Experiment #4A: Dependence of Signal on State of Metal With the burner height at the position of maximum absorbance and the flow rate set at the rate that gives the maximum absorbance, record the absorbance for the following 10 ppm Cr solutions: chromium nitrate, sodium chromate, sodium dichromate, chromium acetate, and chromium chloride. Calculations 1. Prepare a flame-atom profile by plotting the burner position (in arbitrary units) vs. signal intensity of (Cr-water). Locate the burner position giving the maximum signal on the graph. 2. Plot the signal vs. fuel/oxidant ratio. 3. Discuss the effect of the state of the metal on results. -2-
B. Trace Analysis of Metals by Flame Emission Objective This part of the experiment is designed to acquaint the student with the techniques of atomic emission for the analysis of metals. The use of a multipurpose emission-absorption instrument is illustrated for the determination of Na+ by emission. Theory The energy available in a flame can be used to excite the elemental components of an unknown sample, and the emission of radiation can be used to both qualitatively and quantitatively identify the unknown sample. The irreproducible nature of a flame makes quantitative determination virtually 100% empirical, but if conditions are accurately controlled, the emission intensity can be a quantitative indication of the amount of material present. The amount of material can be measured in reference to a working curve established with standard solutions. Some of the difficulties involved in the variation of flame conditions can be circumvented by the inclusion of a known concentration of a similarly emitting element and relating concentration of the unknown to the ratios of the emission intensities of the unknown and the standard. The techniques of atomic emission and absorption are complementary rather than competitive. Many metals, notably the alkali and alkaline earths, are easily excited in flames and consequently can be determined at much lower concentrations by flame emission. Other metals, such as the transition metals, are easily atomized in flames but not so readily excited and thus have much lower detection limits in absorption. For some elements (Mg) the two techniques may have identical detection limits. This is illustrated below: Detection Limits in Flame Emission and Absorption Spectroscopy Element Emission (ppm) Absorption (ppm) Al 0.01 0.5 As 1.0 0.04 Ba 0.01 1.0 Cd 0.3 0.0004 Cr 0.003 0.001 Cu 0.01 0.0005 Fe 0.03 0.003 Mg 0.003 0.003 K 0.00001 0.005 Na 0.00001 0.005 Zn 3.0 0.0002 The instrument used in this experiment is designed for both flame emission and absorption studies. It utilizes an integrated aspirator burner with electric ignition, the burner being fed with air and -3-
acetylene, and the sample being directly aspirated into the flame. For emission studies, the grating monochromator is employed between the flame and the detector. The wavelength of radiation allowed to pass onto the detector is chosen by selecting the element to be studied which automatically adjusts the monochromator system. Five adjustments must be made to set the monochromator at peak emission. Signal strength adjustment is accomplished by adjusting the flame position vertically with respect to the AA optical path, by controlling the fuel and oxidizer flow rates to the burner, and by electronic adjustments on the control panel. For absorption studies, the hollow cathode is selected for the element under analysis, and is operated at about 50% of its current rating. Monochromation of radiation is effected in a manner similar to the emission procedure, except that here the monochromator is simply used to filter out the transition radiation between electronic states other than the principal one desired, since the emission lines from the hollow cathode are essentially monochromatic. Calibration curves are prepared in flame emission and atomic absorption from solutions of the sample in known concentrations. In the assay of sodium, for example, one could make a calibration curve using several solutions of sodium, from say, 1-100 ppm. The concentrations of these solutions could vary over the complete range of 1-100 ppm. The sample would be atomized and the emission (or absorption) measured. The upper limit is reached when an increase in sodium concentration causes only a slight increase in emission (or absorbance); the lower limit is determined by the signal to noise ratio for the particular element analyzed. In quantitative analysis, the unknown sample is atomized and the absorbance or emission measured under exactly the same conditions as those used for preparation of the calibration plot. This is particularly important since many diverse ions interfere with the emission or absorbance of the element assay by effecting an increase or decrease in the emission of absorption. In this experiment the emission mode is used for assay of sodium ion. Experiment #1B: Atomic Emission 1. Transfer 0.5, 1, 3, 5, 7, and 10 ml of 100 ppm Na solution into 25 ml volumetric flasks. 2. Dilute to mark with distilled water. 3. Transfer 10 ml of tap water and 10 ml of distilled water (dip fingers in distilled water for 30 s) to 2 separate 25 ml volumetric flasks. 4. Dilute to mark with distilled water. 5. Record Na emission of all solutions. Calculations 1. Plot the emission intensity vs. concentration of sodium ion from the direct-intensity data. -4-
2. Perform a regression analysis over the linear portion of the graph. 3. Calculate the sodium-ion concentration in the unknown solutions in parts per million by the direct-intensity method. Questions 1. Propose a detailed sample preparation method for testing total sodium concentration in whole blood by Atomic emission. 2. A premix burner does not introduce all the material into the flame, and the larger droplets are drained to waste. How is air prevented from backing up into the burner and possibly causing an explosion? What other safety features are incorporated into the burner assembly. 3. In atomic absorption spectroscopy, why is the monochromator located after the sample compartment (the flame) rather than before as in the case of a UV-visible absorption spectrophotometer? 4. What is beam modulation and why is it used in atomic absorption? 5. Why are atomic absorption lines so sharp compared to the absorption spectrum of a molecule dissolved in solution? Procedure for Operation of PE AAnalyst 200 Amanda Wroble 10/21/03 1. Turn the AA hood fan on and make sure that the door to the fume hood on the far wall of the lab is lowered to the yellow mark. The power switch for the hood fan is located on the back of the bench supporting the AA. 2. Open the compartment on the right side of the front of the PE AAnalyst 200 and turn the power switch on. 3. Open the main valve to the acetylene tank and the small acetylene valve. Turn on the air pressure. It is located directly above the PE AAnalyst 200. 4. After the PE AAnalyst 200 computer has completed the setup procedures, select Flame AA. Manual Sampling Procedure 5. Click on Tools and then Select Method. Choose the method named Chromium demo. 6. Once Chromium demo has been selected, the Lamp tab should be opened. The wavelength, slit width, and identity of the lamp used in the method are shown. Click on Setup Instrument to turn the lamp on. It will take about 1 minute for the lamp to turn on. 7. Using a small white piece of paper, verify that the burner head is not blocking the optical beam. (The burner should be unlit at this time!) If the burner head blocks the beam, adjust the burner head height. Ask your TA how to make the adjustment. 8. Click on the Flame tab and ignite the flame by selecting the on/off switch. It is important that the sampling tube is placed in a solution whenever the flame is on. 9. While distilled water is being aspirated, click on Autozero. The absorbance of distilled water should now read 0. 10. Place the sampling tube into a chromium solution. 11. Optimize the absorbance of the chromium solution by adjusting the burner height and the flow of the solution into the nebulizer. Ask your TA to assist you as you optimize the -5-
burner height. The nebulizer uptake can be adjusted by turning the red ring into which the sampling tube flows. a. Turn the ring counterclockwise until the sampling tube is blowing bubbles in the solution. b. Turn the ring clockwise until there are no bubbles. c. Turn the ring ¼ of a turn clockwise. 12. Select the Analyze tab. 13. With distilled water aspirating, select Analyze Blank. Once an absorbance reading has been taken, the PE AAnalyst 200 will automatically autozero. 14. Place the sampling tube into the 1 ppm Cr solution and click Analyze Standards. Once the reading has been taken for the 1 ppm Cr solution, place the tube into the 2 ppm Cr solution and click Analyze Standards. Continue for all standards. In between readings, sampling tube should be placed in distilled water briefly. 15. Click on Calibration to display calibration curve. The calibration curve can be printed by clicking on Print. 16. Place tube in unknown Cr solution and click Analyze Sample. The PE AAnalyst 200 automatically calculates the concentration of the unknown based on the calibration curve. 17. The method is set up to print once there is a full page of data. If nothing has printed after step 16, click Printer on/off, and your data should print. Autosampling Procedure 5. Click on Tools and then Select Method. Choose the method named Chromium demo 2. 6. Once Chromium demo 2 has been selected, the Lamp tab should be opened. The wavelength, slit width, and identity of the lamp used in the method are shown. Click on Setup Instrument to turn the lamp on. It will take about 1 minute for the lamp to turn on. 7. Using a small white piece of paper, verify that the burner head is not blocking the optical beam. (The burner should be unlit at this time!) If the burner head blocks the beam, adjust the burner head height. Ask your TA how to make the adjustment. 8. Click on the Flame tab and ignite the flame by selecting the on/off switch. It is important that the sampling tube is placed in a solution whenever the flame is on. The sampling tube on the autosampling apparatus should be in the distilled water wash station. 9. While distilled water is being aspirated, click on Autozero. The absorbance of distilled water should now read 0. 10.Place the autosampling tube into a chromium solution. 11. Optimize the absorbance of the chromium solution by adjusting the burner height and the flow of the solution into the nebulizer. Ask your TA to assist you as you optimize the burner height. The nebulizer uptake can be adjusted by turning the red ring into which the sampling tube flows. a. Turn the ring counterclockwise until the sampling tube is blowing bubbles in the solution. b. Turn the ring clockwise until there are no bubbles. c. Turn the ring ¼ of a turn clockwise. 12. Select the Analyze tab. 13. Place blank solution in position 1 of the tray, standards in positions 2-9, and unknown samples in positions 10-44. -6-
14. Select Analyze Blank. The autosampling device will go to position 1 in the tray, which should be a blank. Once an absorbance reading has been taken, the PE AAnalyst 200 will automatically autozero. 15. Select Analyze Standards. The autosampler will place the sampling tube in 1 ppm Cr solution in position 2 and take an absorbance reading. Once the reading has been taken for the 1 ppm Cr solution, click Analyze Standards again. Continue for all standards. Click on Calibration to display calibration curve. The calibration curve can be printed by clicking on Calibration Curve and Print. 16. Click Analyze Sample. The autosampler will place the sampling tube in position 10. The PE AAnalyst 200 automatically calculates the concentration of the unknown based on the calibration curve. 17. The method is set up to print once there is a full page of data. If nothing has printed after step 16, click Printer on/off, and your data should print. -7-