Graphite Furnace AA, Page 1 DETERMINATION OF METALS IN FOOD SAMPLES BY GRAPHITE FURNACE ATOMIC ABSORPTION SPECTROSCOPY I. BACKGROUND (VERSION 1.0) Atomic absorption spectroscopy (AAS) is a widely used technique for determining a large number of metals. Previously, you gained experience with the most common implementation of AAS in which an aqueous sample containing the metal analyte is atomized by aspirating it into an air-acetylene flame. A line source (hollow cathode lamp) operating in the UV-visible spectral region is then used to cause electronic excitation of the metal atoms, and the absorbance is measured with a conventional UV-visible dispersive spectrometer with photomultiplier detector. In this experiment, an alternative atomization procedure will be investigated. In addition, a wet ashing technique will be used for sample digestion, and the standard addition method of calibration will be explored. These procedures will be used to determine metals in food samples. Graphite Furnace Atomization The disadvantages of flame atomization include: (1) a relatively inefficient sample introduction procedure that results in very little of the sample even being introduced into the flame, (2) the low temperature of the flame that can result in poor atomization of elements with high boiling points, (3) the extremely reactive environment of the flame that can result in loss of analyte through chemical reactions such as oxide formation, (4) the short residence time of the atomized sample in the optical path of the spectrometer, and (5) the relatively large rate of sample consumption. The graphite furnace atomizer addresses each of these limitations. From source To detector Figure 1. Graphite furnace atomizer. The arrows denote the optical path.
Graphite Furnace AA, Page 2 Figure 1 above displays a schematic of the furnace atomizer to be employed in this experiment. The optical path of the spectroscopic measurement is indicated by the arrows. The furnace assembly depicted in the figure replaces the flame atomizer. Optical Path Sample Introduction Port Figure 2. Internal elements of the graphite furnace. The graphite tube contains the sample. Figure 2 displays the internal elements of the graphite furnace. The graphite tube contains the sample and is resistively heated by electrical current supplied through the left and right graphite contacts. A microliter-sized liquid sample is introduced into the graphite tube through the indicated port. In the instrument used in this experiment, an autosampler system will be employed to introduce the sample into the graphite tube. Atomization involves the operation of a sequential temperature program to perform (1) solvent evaporation, (2) sample ashing to decompose organic matter, and (3) intense heating to produce an atomic vapor sample for the absorption measurement. The temperature program is optimized for each element studied. The furnace used in this experiment is capable of reaching 3000 C, although lower temperatures are often used depending on the boiling point of the metal to be measured. Using a higher than optimal temperature is counterproductive as it can lead to ion formation which results in a loss of signal (i.e., ions and atoms have absorption transitions at different wavelengths). Since the sample is placed into the furnace and held there, a purge gas system must be used to flush the sample residue from the graphite tube. In addition, the graphite tube must be in an inert environment during the heating step in order to help prevent compound formation during atomization and to prevent oxidation of the graphite tube itself. This is accomplished with an internal purge gas and external protective gas flow system employing argon. Figure 3 displays a diagram of the gas flow paths. A circulating water system is also used to cool the furnace after each run.
Graphite Furnace AA, Page 3 Optical Path Figure 3. Diagram of internal purge and external protective gas flows using argon. Data collection consists of acquiring the absorbance as a function of time during the furnace operation. Figure 4 displays such a time trace for measuring lead absorbance at 217 nm in several standards and a sample of orange juice. 1 Note that absorbance peaks are obtained during the temperature program. For the standards, single peaks are obtained, while multiple peaks are observed when the orange juice is run. These peaks correspond to the solvent evaporation (drying), ashing, and atomization steps. Molecular species with broad spectral bands are evolved during the drying and ashing steps used with an organic sample. These species may absorb at the characteristic wavelength of the metal and thus must be removed before the atomic sample is generated. The analytical signal is typically taken as the absorbance peak area associated with the atomization phase of the temperature program. Method of Standard Additions The technique of standard additions is a common experimental method in atomic spectroscopy. This technique is a replacement for the conventional "calibration curve" approach to quantitative analysis. It is used primarily in applications in which it is difficult to prepare calibration standards that exactly match the sample matrix of an unknown. In such a case, calibration standards may fail to duplicate key elements of the sample matrix, leading to unpredictable interference effects. The standard addition method is based on the concept of using the unknown sample itself as the sample matrix for the construction of calibration standards. In this approach, a known amount of analyte is added to an aliquot of the unknown and the resulting solution is diluted to a fixed final volume. This process is called "spiking" the unknown. The analyte is added by pipetting a fixed volume of a known "spiking solution". By use of several identical aliquots of the unknown and varying volumes of the spiking solution, a series of spiked unknown samples are prepared. One additional solution is prepared simply by taking the same aliquot of the unknown and diluting to the same final volume used with the other solutions. The absorbance of each solution is then measured.
Graphite Furnace AA, Page 4 Time (s) Figure 4. Data acquisition trace of absorbance vs. time for graphite furnace operation. 1 Absorbance is then plotted vs. amount of analyte added. Least-squares analysis can be performed on these data, and the slope (m) and intercept (b) determined. By extrapolating the least-squares line back to the x-axis, the amount of analyte can be determined that was present in the fixed aliquot of the unknown. This amount of analyte gave rise to an absorbance that was effectively added to the responses corresponding to the spiked amounts of analyte. An illustration of this plot is presented in Figure 5. Computationally, we are solving for the amount of analyte (x) denoted by a response of y bkg, where y bkg denotes the absorbance signal produced when no analyte is present. If there is no background (blank) signal or if it has been subtracted automatically during the data acquisition, y bkg = 0. This is the case depicted in Figure 5. Thus, if the linear response is characterized by y = mx + b and we are solving for the intersection of this line with y = y bkg, then x = (y bkg -b) /m. This gives the amount of analyte in the specified aliquot of the unknown (in units of the x-axis). The x-axis can be plotted in weight of analyte added or volume of spiking standard used. In either case, one can convert the computed x to the amount of analyte in the aliquot of the unknown used in the preparation of the standard addition solutions. II. REAGENTS Reagent-grade deionized water (verify with the instructor that the correct water is being used) Atomic absorption reference standards, 1000 ppm (provided)
Graphite Furnace AA, Page 5 Figure 5. Example standard addition plot. Units of the x-axis can either be weight of metal added or volume of spiking solution added. III. PROCEDURE A. Wet ashing of unknown. Note: begin this step immediately upon entering the laboratory. You will be given a food sample. Prepare a representative sample by mixing, blending, or grinding as appropriate. Accurately weigh approximately 5 g of the prepared sample and transfer to a 500 ml Kjeldahl flask. Do the following in the fume hood. Point the neck of the flask away from you. Add approximately 5 ml of concentrated HNO 3 and cautiously heat with a Bunsen burner until the first vigorous reaction subsides. The flask will now contain a black residue. Add 2.0 ml of concentrated H 2 SO 4 and continue heating, maintaining oxidizing conditions by adding concentrated HNO 3 in small (~1 ml) increments until the solution is a clear yellow/orange and no solid residue remains. During this stage of the procedure, brown nitrogen dioxide gas (NO 2 ) will be released after you add HNO 3. When the NO 2 production begins to subside, add the next increment of HNO 3. Also, rotate the flask by the neck periodically to aid in the digestion of all material. A total of 25 to 30 ml of HNO 3 will be required, and this part of the procedure will take 1.5 to 2 hours. Once the solution clears, continue heating until dense white fumes of H 2 SO 4 are evolved and all HNO 3 has been removed (i.e., no further evidence of NO 2 production exists).
Graphite Furnace AA, Page 6 Cool the solution and dilute with approximately 20 ml of reagent-grade water. Transfer this solution to a 100 ml volumetric flask and rinse appropriately to ensure quantitative transfer of the sample. Dilute to volume with reagent-grade water. B. Solution Preparation. The instructor will provide information regarding the working range of the metal being studied and an approximate concentration range for the unknown. For your solution calculations, assume the unknown concentration is at the lower limit of the range provided to you. Assuming the use of 250 ml volumetric flasks, compute the amount of your unknown solution that must be added in order to reach the lower limit of the working range of the metal being studied. This will likely require a serial dilution of your unknown solution in order to obtain a workable delivery volume. Deliver this volume into five 250 ml volumetric flasks. Compute the amount of metal that must be added to the assumed amount of unknown placed in the 250 ml volumetric flask in order to reach 80% of the upper limit of the working range of the metal. Divide this range into four increments. These increments represent the amount of analyte that must be spiked into the standard addition solutions. For example, assume your calculations indicate that 20 µg of analyte must be added in order to reach 80% of the upper limit of the working range. Division into four increments produces values of 5, 10, 15, and 20 µg. These amounts must be delivered in turn to produce four spiked solutions. Using the provided 1000 ppm reference standard of the analyte metal, prepare a spiking solution that allows the specified amounts of metal to be delivered to four of the 250 ml flasks containing the aliquot of the unknown. Dilute all five flasks to volume with reagent-grade water. Note that one flask contains only the unknown aliquot (i.e., it is not spiked with additional analyte. C. Measurement procedure. The instructor will provide information regarding the setup and operation of the graphite furnace atomic absorption instrument. 1. Using reagent-grade water as the blank, make three replicate background measurements and use the average of these absorbance areas as your estimate of y bkg. 2. Measure each standard addition sample in triplicate. After every 5 runs, operate the furnace with no sample in order to help remove any buildup of residual material. 3. Save the integrated absorbance values in an Excel-compatible file for use in subsequent calculations. IV. CALCULATIONS 1. In your calculation section, provide a detailed presentation of your solution calculations that produced the standard addition samples. 2. Tabulate the mean, standard deviation, and 95% confidence interval for your absorbance data. 3. Prepare a standard addition plot with error bars of absorbance vs. amount of analyte
Graphite Furnace AA, Page 7 added. 4. Use least-squares procedures to determine the slope and intercept of the standard addition plot. Report the value of r 2 and the standard error of estimate. 5. Use your slope, intercept, background measurement, and sample preparation scheme to compute the amount of analyte in the original food sample. Express this as ppm metal. 6. In your calculation section, provide a discussion that compares the advantages and disadvantages of flame vs. graphite furnace atomizers for atomic absorption measurements. V. REFERENCES 1. Skoog, D. A.; Holler, J. F.; Nieman, T. A. Principles of Instrumental Analysis, Fifth Ed.; Saunders, Philadelphia, 1998; p. 212.