Determination of Copper by Graphite Furnace Atomic Absorption Spectrophotometry A Student Exercise in Instrumental Methods of Analysis Mark A. Williamson1 Northern Arizona University, Flagstaff, AZ 86011 We have found that a laboratory exercise for a class in instrumental methods of analysis involving the determination of copper by graphite furnace atomic absorption spectrophotometry (AAS) is useful as an introduction to this valuable analvtical tool. Students o ~ t i m i z ethe charring and atomization tkmperatures by a plot of absorbance vs. temnerature for each temperature parameter. Suhsequent analysis of an instructor-prepared unknown is completed with little difficulty and is found to he quite accurate. Employment of the percent recovery technique to evaluate the student-optimized instrumental parameters is useful. There are numerous puhlished laboratory exercises that illustrate the quantitative use of flame atomic absorption snectronhotometrv - (.1 3 ) . However.. desnite . the widespread use of h e graphite furnace (or electrothermal atomization, ETA) AAS techniaue in industrial and academic research settings (4-9, theie are no published structured experiments emplovine - - -the method. At Northern Arizona University, our undergraduate chemistry program offers an exercise in flame AAS methods during an introductory (juniorlevel) course in quantitative analy& and a simple exercise in the use of ETA in a senior-level course in instrumental methods of analysis. Atomic absorption spectrophotometry is an analytical technioue that is hiehlv - .s~ecific - for the analvsis of metals metalloids in various matrices (9-11). ~ h method e is based on the selective ahsor~tionof line radiation by atomic species in the vapor phase. when compared to conventional flame excitation methods, ETA offers distinct advantages: high sensitivity, selectivity, accuracy, and speed, coupled with economical (low) sample consumption during analysis. In contrast to flame AAS, ETA absorption signals are a function of the total amount of analyte present in the graphite tube, not the concentration of the solution under study. The ETA technique witnesses a transient atomic vapor in the ontical nath. The vanor often remains in the optical path four times longer than 2 s and this comparatively long residence time is the basis for the enhanced detection limits over flame techniques. A four-stage process is typically involved in the production of the absorption event in ETA: drying, charring, atomizing, and purging. The drying step functions to volatilize the sample solvent, often, hut not necessarily, an aqueous solution. The completion of this step results in the production of a dried precipitate com~osed~of the cations. anions. and oraanic materials originall; found in the bulk solution: ~ f t e n , b r y i n ga 10-pL aliauot for 15 s at 110 OC is sufficient. Newer temperature propammer models for ETA instrumentation offeGariahle ramping times in comparison to the maximum heating rate methods of older varieties. In either case, the sample is made ready for subsequent charring.
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'Present address: Department of Geological Sciences. Virginia Polytechnic Institute. Blacksburg. VA 24061.
Determination of the correct charring temperature by ohserving the effect of charring temperature on analyte signal is vital to the analvsis of a complex sample by ETA-AAS (12). Charring is designed to rembve as much volatile matrix residue as possible before atomization. Absorhances due to chemical interference and background affect the measurement of the analyte signal, giving false concentration data and producing anoisy base-line signal. I t is desirable to use a charring temperature sufficiently high to volatilize as completely as possible any interfering "smoke"-producing materials in the sample, yet low enough to insure no loss of analvte element orior to atomization (13). The presence of certain inorganic salts may require the use of a lower temnerature. For example, while the analysis of zinc in simple aqueous matrices iormally will require charring temperatureson the order of 700 " C ,amatrix high in chloride ion will precipitate zinc(I1) chloride, significantly reducing the optimum charring temperature to 250 OC (mp ZnClz = 283 OC). Use of charring temperatures significantly above the melting point will result in increased volatilization of analyte prior to atomization, leading to reduced measured absorbance and inaccurate results. The atomization step is the analytical event. The residue remaining after charring is heated to a temperature high enough to volatilize the analyte, creating the atomic vapor whose ahsorhance is measured. Establishing the optimum atomization temperature insures that a high enough temperarure to volatilize all the analgre is used, thrrehy providing the lowest detection limits. Additionally. the lowest trmneiature that will accom~lishthis ourpose should he used in tube. Purging the order to prolong the life' of the graphite tube a t the atomization temperature for approximately 5 s after the atomization event removes any remaining analyte and prepares the system for the next measurement. In practice, absorbance signal measurement may be accomplished hy measuring peak area or height. In theory, peak area is a hetter choice than height, as the latter is a function of the time taken to atomize the sample and the residence time of the chemical vapor within the analytical volume. On the other hand, peak area depends solely on the residence time (14). However, with conventional ETA instrumentation. sienal integration offers little improvement over the peak heiiht method with respect to senskivity (14), limit of detection, precision, or linear dynamic range of working curves (15,-16). A chart recorderwith a rapid response (0.25 s FSD) will record absorhances satisfactorily and will allow the simpler peak height method to he used. A standard addition methodology may be used to evaluate the accuracy of analyses with graphite furnace AAS. The concentration of an analyzed sample solution is increased by a known amount (in the narts-per-billion range). An absorbance measurement is mide foi this solution and compared to a calculated (expected) concentration. For a 10-pL addition of a 200-pph standard to 500 pL of an analyzed solution, the percent recovery is determined by
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+ 5W(X) = Expected Recovery (pph) 510
(Measured Recovery) Expected Recovery
(100, = aR
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where X is the averaze concentration determined for an unknown solution. TG measured value is compared to a calculated (expected) value, and the percent recovery is determined. his method serves as an eGaluation of theinstrumental parameters chosen for the analysis. A 100%recovery indicates that the optimal conditions for the furnace during the analysis were selected. Experimental A Perkin-Elmer model 306 atomic absorption spectrophotometer equipped with an HGA-2100 temperature programmer and graphite furnace Was used for this exercise, although other makes or models may he substituted. High-density graphite L'vov platforms were mounted in high density graphite tuhes for the sample chamber. Students set all instrument parameters to accommodate copper analysis (any element may he used, however). The drying step for the temperature programmer was 110 *C for 20 s, which was evaluated as sufficientbv the author for the 10-uL iniectians used during the experiment. ~ & n . at n flow ratc of 20-30 m~lmin,was used a: the purgegas, in the normal mode.'l'imrs for charring and atumiration strpr un the programmer were set tu 16 and 4 s, rrsprctively. These timrs were uptimired by the author prmr to acnrnl student work, as the exercise was inwnded t u demonstrate the effect c,f temperature only. The malyrical wavelength was 321.7 nm with a slit width of 0.7 nm. Althmch rt una not necessary with the matrix employed in this exercise, deuterium background correction was used in order to allow the students the opportunity to balance the reference and sample beams of the spectrophotometer. A 1000-ppm'coppersolution was prepared by the students from Cu(NO& and distilled-deionizedwater, with 10%concentrated nitric acid included in the quantitative preparation. This stock was diluted to 10 ppm with standard volumetric pipets and flasks, using distilled deionized water that had also been acidified withnitric acid (10%v/v).In order to illustrate the small quantity of reagent necessary for graphite furnace work, l-mL volumes of 100 and 200 pph copper were prepared from the 10-ppm stock, using Eppendorf ninets. Investigations to determine optimal charring and atomization temperatures were carried out. Using one of the parts-per-billion standards, and an atomization temperature arbitrarily set to a value above 2400 'C, triplicate absorbance measurements were determined when using charring temperatures ranging from 3M1-1700 "C, at 2 0 0 T intervals. Absorbance was measured as peak height on a chart recorder. A plot of peak height vs. temperature was used to determine the charring temperature yielding maximum absorbance during atomization. This experimentally determined charring temperature was used for optimizing the atomization conditions. Using one of the standard copper solutions, ahsorhances were recorded for a series of atomization temperatures from 2200 to 2800
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Figure 1. Peak heigM (absorbance)vs. charring temperature.
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Journal of Chemical Education
OC at 100 'C intervals. As with the charring optimization, a plot of peak height vs. temperature was produced. In this instance, however, the minimum temperature providing maximum absorbance with ~ the~ highest ~ reproducibility was determined by the student. This value and the temperature determined far the charring step were used as the instrumental parameters for the generation of a working curve for copper. The concentration of copper in an unknown prepared by the instructor was then determined. A standard curve was produced by the student using a blank of deionized water acidified with reagent-grade nitric acid and a 100and 200-pph series of solutions. While working curves were plotted on graph paper for illustration in student reports, the use of a linear regression routine to evaluate unknown solution concentrations was encouraged. A standard addition experiment was carried out in orderto evaluate the student's success in choosing instrumental parameters for generating the working curve and analyzing the unknown. A standard addition was set up whereby 10 pL of the 200-pph copper standard was added to 500 pL of the unknown. The ahsorhance for this standard addition was measured, and the 90recovery for the method was determined. Results and Discussion On the average, more than one laboratory period was required, with two periods of three hours each the norm. T h e bieeest obstacle the students encountered was in the develo G e n t of the motor skills required to repeat sample injections re~roducihlv.The maioritv of students found the experiment stimula;ing, as the meihod of atomization and the detection limits were sirnificantlv different from the flame AAS work they experienced in an karlier course. The optimization of charring and atomization temperatures to be used for the analysis of copper was carried out successfully by students. Generally, a value of 1500 O C was determined for charring by producing a plot of peak height (absorbance) vs. charring temperature (see Fig. 1).Optimal atomization temperatures varied, but were generally hetween 2550 "C and 2700 OC. Figure 2 displays a plot of absorbance vs. atomization temperature for one student's data, displaying the expected plateau a t the high end of the temperature range. Precision for data points was primarily a function of the student's ability successfully to reproduce sample injection technique and was quite variable. Analysis of instructor-generated unknowns was satisfactory. Student work resulted in an overall 6% error with an excellent 98.4% recoverv. - . sueeestine the abilitv correctlv to determine optimal operating parameters. Beer's law plots had an average correlation coefficient of 0.9992 with the deletion of one anomalous value. The work of one student ~ a 124%recoverv resulted in a reported value of 124 V D with for an instructor-prepared unknown of 125 pph. ~ n o t h e i unknown, prepared a t 100 ppb, was determined to be 97 ppb with a 101% recovery. A small amount of copper was detect-
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Flgure 2. Peak helm (absorbance)us. atomization temperature.
ed by all students in blank solutions. The absorbances due to the presence of copper were observed to be variable between data sets and were considered to be due to improper sample handling rather than contaminated nitric acid or water. To exnose students further to variables affecting the aualityof analysis of ETA-AAS, the exercise may be modified to include ootimization of times for each step of the temperature pro&am. For newer models of prog;ammers on ETA aooaratus. the effect of changing .. - . ramping times may be investigated. The change in instrument s & i t i \ , i t y f& an element as a function of purge gns flow rate may nlsu he of use. ETA-AAS is a valuable and widely used analytical tool in many research and industrial settings. This, combined with the lack of discussion of the method in manv texts on instrumental analysis, increases the value of the exercise outlined here. It e x D o s e s students to a techniaue that extends their abilities with AAS to the realm of trace metal analysis, an increasingly important skill in modern chemistry.
Acknowledgment Thanks to R. W. Zoellner and W. A. Hildebrandt for review and S. Mertz for typing.
Literature Cited 1. Bye,R. J. Chem.Educ. 1987,64,188. 2. Hmkina, L. C.; Reichhardt, P. 8.:Stoltzberg, R.J. J. Chom. E d u r 1981,58,580. 3. Hurlbut,J. A,: Gi1bert.L. K.; Buddington,B.N.:Ren,T. F. J. Chem.Educ. 1971.51, 716 ....
4. Yin,X.;Schlemmer,O.: Welr, B.Anol. C k m . 1987,59,1462. 5. Vdion, C. And. Chem. 1986,58,2602. 6. Crancy, C. L.: Sw-uf K.; Smith, F. W., 111: Weaf C. D. Anol. Chem. 1986.58.658. 7. Chiou,K.Y.; Manuei,O.K.Anoi. Chem. 1984,56,2721. 8. Allain. P.;Mauaa, Y.;Der Khatchadourian, F. A n d Chem. 19&1.56,1196. D.Anol. Chem. 1986,58,616. 9. Hodge,V.;Stailard,M.;Koido,M.;Goidbrg,E. 10. H0enig.M.; VanEhsn,Y.: VanCauter, R. And. Cham. I986,58,777. 11. Subramanian. K.; Meranger, J. C. Anol. Cham. 1985.57,217S8, 12. Beaty, R. D. CancepD,lnsfrumantotionondTechniquesin Atomie AbaorptionSpeeLmphofomelry: Perkin-Elmer: Norwslk. CT, 1937. 13. Anolytieol Methods for Atomie Abaorbfion Spectracopy Using the XCA Gmphife Fumoca; Perkin-Elmer: Norwslk, CT, 1974. 11. Sturge0n.R E. And. Cham. 1978.49,1255A. 15. Stuqem. R. E.; Chakrabarti, C. L.: Maine*. I. S.; B&h, P. C. Anol. Chem. 1975.46. 1240. 16. Sturgeon, R E.: Chakrabarti, C. L.; Bertcls. P. C. A n d Chom. 1975,46,1250.
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