In the Laboratory
Simultaneous Atomic Absorption Spectrometry for Cadmium and Lead Determination in Wastewater
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A Laboratory Exercise
J. Chem. Educ. 2004.81:1174. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/22/18. For personal use only.
Paulo R. M. Correia and Pedro V. Oliveira* Instituto de Química, Universidade de São Paulo, CP 26077, CEP 05513-970, São Paulo, SP, Brasil; *
[email protected] Quantitative and qualitative analytical determinations are carried out by increasingly modern and sophisticated instruments. Instrumental analysis courses attempt to keep pace with this trend. Accordingly, an objective of these courses is to incorporate new equipment in novel experiments designed to familiarize undergraduate students with the actual tools that they will eventually use in analytical practice. Atomic absorption spectrometry (AAS) is one of the most widely used instrumental techniques for elemental analysis owing to its selectivity, low spectral interference, and ease of operation. It is a well-established single-element spectrochemical technique, suitable for trace and ultra-trace element determinations in a large variety of samples. The use of electrothermal atomizers (ETAAS) instead of flame atomizers (FAAS) brings a number of additional advantages such as lower detection limits, reduced sample volumes, and the possibility of in-situ sample thermal pretreatment during the graphite furnace heating program (1). A number of experiments dealing with FAAS and ETAAS can be found in the chemical education literature (2–9). To our knowledge, there has not been any report on simultaneous atomic absorption spectrometry (SIMAAS). Experiments involving SIMAAS could be used to introduce undergraduate students to this new instrument and to generate critical discussion about other multi-element spectrochemical techniques, such as inductively coupled plasma optical emission spectrometry (ICP–OES) and inductively coupled plasma mass spectrometry (ICP–MS). SIMAAS instruments allow the analysis of up to six elements simultaneously, while ICP–OES and ICP–MS can achieve simultaneous or sequential determination for dozens of elements (10). On the other hand, the sensitivity obtained with ICP–OES is usually poorer than that of ETAAS, even when using an axial torch. In most cases, ICP–MS allows the best sensitivity for many elements, but it exhibits isobaric interferences, which can disturb the determination of some elements. In addition, the high instrumentation costs remain a hindrance to the use of ICP–MS in undergraduate labs. Finally, the sample requirement for carrying out the analysis is lower when ETAAS is used: 5 to 50 µL are sufficient, compared to the volume of hundreds of µL necessary for ICP– OES or ICP–MS determinations, for which pneumatic nebulization is generally used (1, 10). In analysis performed by SIMAAS, the time and costs are significantly reduced even when only two elements are simultaneously determined. For instance, a two-element procedure leads to an analytical frequency improvement of 100%. Considering the long duration of the heating program for electrothermal atomizers, typically one to three minutes,
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enhancement of the analytical frequency is significant. Since the number of runs required by SIMAAS is fewer compared to single-element ETAAS, the costs associated with the replacement of the graphite parts, such as the electrodes and the graphite tubes, is lessened. In addition the consumption of high-purity reagents is decreased. The reduction in the generation of wastes is another aspect to be highlighted in view of environmental considerations (11,12). The goal of this laboratory exercise is to verify sources of water contamination in a local river, employing a problem-based learning approach. The simultaneous determination of cadmium and lead by multi-element atomic absorption spectrometry with electrothermal atomization is proposed. Students optimize instrumental parameters to determine the best experimental conditions to carry out the simultaneous analysis. Experiment
Overview This experiment was designed to introduce SIMAAS through the examination of a real-world problem, thus giving undergraduate students the opportunity to make decisions and solve a problem using their experimental data. Although there are three other important pedagogical laboratory approaches, the problem-based approach was chosen owing to its deductive character, and the necessity of active participation by the students in the evaluation of the experimental procedure (13). Students’ motivation is frequently enhanced when they are dealing with a real-world problem, which is an important emotional aspect that supports meaningful learning. Additionally, in a problem-based instruction method, the focus is not only on the analytical topics, but also on the development of other abilities such as team work, communication skills, and critical approach to problems, which are valued by employers and often not emphasized in traditional methods of teaching chemistry (14, 15). The problem of river water contaminated by trace heavy metals is presented as an environmental case. Students receive a request from the local authorities to identify potential cadmium and lead pollution sources among four industries, which discharge pretreated wastewater into the river. Two different samples, collected at each industry, are tested for both elements. The EPA maximum contaminant levels (MCL) can be used as the reference values: 5 µg L᎑1 for cadmium and 15 µg L᎑1 for lead (16, 17). During a guided discussion prior to the laboratory activities, students receive a letter describing the case and plan the experimental procedure with the instructor. The simul-
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In the Laboratory
taneous analysis of cadmium and lead will be carried out by SIMAAS and some theoretical information about atomic absorption spectrometry is discussed. Intentionally, the instructor omits important information about the instrumental conditions, for example, pyrolysis and atomization temperatures. Thus, the students will need to perform a preliminary optimization experiment before the analysis. The analysis of wastewater samples trains the students in trace element determination and the importance of avoiding sample contamination, which is a seminal analytical topic. Additionally, an environmental discussion can emphasize public concern about the water pollution, and a study related to the heavy metal toxicity, even in trace quantities, allows the students to create a new awareness regarding the rational use and preservation of natural resources (18). It is also appropriate to point out principles of green chemistry, which serve as a guide for developing more environmentally benign products and processes (11, 12). The experiment is carried out in two parts: in the first part the instrumental parameters are optimized and in the second part cadmium and lead are simultaneous determined under the optimized conditions. Each part can be carried out in a three- or four-hour laboratory class.
Instrumentation A SIMAA-6000 electrothermal atomic absorption spectrometer equipped with a longitudinal Zeeman-effect background correction and standard THGA (Transversal Heating Graphite Atomizer) tube with an integrated pyrolytic coated platform (Perkin-Elmer, Norwalk, CT) is used. The spectrometer is employed in the two-element simultaneous mode, with electrodeless discharge lamps (EDL) as radiation sources. EDLs can be replaced by hollow cathode lamps (HCL). Detailed information about instrumental adjustments is provided in the Supplemental Material.W
Part One Although the optimization of an analytical procedure is an important task, undergraduate students are seldom exposed to this challenge. In the majority of laboratory practices, students are provided with a recipe of the analytical procedure to be carried out, including the best instrument setting. A preliminary experiment is proposed for emphasizing the importance of the optimization step: the students have to discover the most appropriate pyrolysis and atomization temperatures for the heating program considering the requirements for simultaneous determination of cadmium and lead. During this optimization step, the students observe the absorbance signal variations with the pyrolysis or atomization temperature using a multi-element solution containing 5.0 µg兾L of cadmium and 50.0 µg兾L of lead in 0.1% v兾v HNO3. Pyrolysis and atomization temperature curves are obtained by plotting the absorbance signals for cadmium and lead versus the tested temperatures. These graphs allow the selection of the most convenient temperature condition for the simultaneous analysis. The necessity to choose a compromised condition is stressed: the pyrolysis temperature is
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selected based on the most volatile element, whereas the atomization temperature is selected based on the least volatile element (19). These temperature curves are obtained in absence and presence of a solution of 0.5% (w兾v) NH4H2PO4 and 0.03% (w兾v) Mg(NO3)2 in 0.1% (v兾v) of HNO3, which helps the thermal stabilization of both elements. This makes the sample thermal treatment more efficient during the pyrolysis step, and reduces matrix interferences (1, 19). A discussion on the heating program and the importance of chemical modifiers can take place at this stage. This section aims to reiterate the principles of SIMAAS, and to stress the difficulties of establishing compromised conditions for simultaneous analysis, which is further complicated by the restriction of the analytical response range, because a single dilution factor must be selected for the samples. For these reasons, the development of multi-element procedures for the determination of more than two elements is more difficult and less frequently employed than singleelement procedures.
Part Two After establishing an optimized analytical procedure, students carry out the analysis of the water samples, starting with those collected from the potentially contaminating industries. Multi-element standards containing from 0.5 to 5.0 µg L᎑1 of cadmium and from 5.0 to 50.0 µg L᎑1 of lead in 0.1% v兾v HNO3 are used for instrument calibration. The analytical results should be organized to make the identification of contaminating sources easier. In addition to the determinations, students can evaluate the figures of merit for the optimized analytical protocol: the quantification detection limit (QDL) and the characteristic mass (mo ). The QDL can be related as the smallest mass or concentration of an analyte that can be determined quantitatively with a risk of error < 5%. The QDL can be calculated by multiplying the detection limit (DL) by a recommended factor, k = 3 (1), xQDL = kxDL and xDL = kσblank 兾S where xDL represents the concentration of the detection limit (µg L᎑1), σblank the standard deviation of 10 consecutive measurements of blank solution used for instrument calibration, S the calibration curve slope, and xQDL the concentration of the quantification detection limit. The characteristic mass is defined as the analyte quantity responsible for 1% (A = 0.0044) of the radiation absorption, usually expressed in picograms (pg). Its value can also be estimated from the calibration curve slope (m o = 0.0044Vinjection兾S ) (1). This laboratory practice for simultaneous determination of cadmium and lead shows the importance of the development of multi-element analytical procedures to improve analytical frequency, reduce consumption of high-purity reagents and sample, and attenuate costs.
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Students’ Report
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After carrying out the procedure optimization (part one) and the simultaneous determination of cadmium and lead in wastewaters (part two), the students prepare a final report containing the following items:
The authorities’ request letter, the instructions for the students, including questions for the prelab and the report, and the instructor notes with the detailed comments are available in this issue of JCE Online.
1. The pyrolysis and atomization temperature curves for cadmium and lead, in absence and presence of the chemical modifier 2. Some considerations on the use of the pyrolysis and atomization temperature curves for the optimization of the heating program 3. Some comments about the thermal behavior of cadmium and lead in the presence of the chemical modifier 4. The analytical results and figures of merit for the simultaneous analytical procedure, such as quantification detection limits, characteristic masses, and analytical frequency 5. A letter addressed to the authorities, in response to their request for the identification of potential contamination sources
Hazards Cadmium and lead are highly toxic and proper safety precautions should be adopted. These include the use of gloves, safety glasses, and lab coats; sample handling in fume hoods whenever possible; and ventilation of instrument exhaust. All solutions should be stored in decontaminated and clearly labeled laboratory flasks. Disposal of waste, even in small quantities, should follow environmental regulations. Conclusion In our pilot study, students encountered no difficulties in carrying out the experiments, after guided discussion with the instructor. The adoption of a problem-based approach enhanced the students’ motivation and kept them involved in the experiment. The final reports indicated that the students assimilated the principles of SIMAAS, the role of the chemical modifier, the procedures to perform trace element analysis, and the advantages of simultaneous techniques.
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Supplemental Material
Acknowledgments The authors thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for financial support (Processo 01/07048-1) and for P. R. M. Correia’s fellowship (Processo 01/02590-2). The authors are grateful to Andrea Maurizio Cavicchioli for the English revision. Literature Cited 1. Welz, B.; Sperling, M. Atomic Absorption Spectrometry, 3rd ed.; Wiley-VCH: Weinheim, Germany, 1999; pp 149–153, 170–181. 2. Kieber, R. J.; Jones, S. B. J. Chem. Educ. 1994, 71, A218. 3. Quigley, M. N. J. Chem. Educ. 1994, 71, 800. 4. Quigley, M. N.; Reid, W. S. J. Chem. Educ. 1995, 72, 440. 5. Quigley, M. N.; Vernon, F. A. J. Chem. Educ. 1996, 73, 980. 6. Rocha, F. R. P.; Nóbrega, J. A. J. Chem. Educ. 1996, 73, 982. 7. Stolzberg, R. J. J. Chem. Educ. 1997, 74, 216. 8. Gilles de Pelichy, L. D.; Adam, C.; Smith, E. T. J. Chem. Educ. 1997, 74, 1192. 9. Tarr, M. A. J. Chem. Educ. 2001, 78, 61. 10. Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of Instrumental Analysis, 5th ed.; Harcourt Brace College Publishers: San Francisco, CA, 1998; pp 231–244, 251–271. 11. Kirchhoff, M. M. J. Chem. Educ. 2001, 78, 1577. 12. Hjeresen, D. L.; Schutt, D. L.; Boese, J. M. J. Chem. Educ. 2000, 77, 1543. 13. Domin, D. S. J. Chem. Educ. 1999, 76, 543. 14. Wilson, G. S.; Anderson, M. R.; Lunte, C. E. Anal. Chem. 1999, 71, 677A. 15. Wenzel, T. J. Anal. Chem. 1999, 71, 693A. 16. U.S. EPA Consumer Fact sheet on Cadmium. http://www.epa.gov/ OGWDW/dwh/c-ioc/cadmium.html (accessed Apr 2004). 17. U.S. EPA Consumer Fact sheet on Lead. http://www.epa.gov/ OGWDW/dwh/c-ioc/lead.html (accessed Apr 2004). 18. Manahan, S. E. Environmental Chemistry, 6th ed.; Lewis Publishers: Boca Raton, FL, 1994; pp 179–185, 675–677. 19. Correia, P. R. M.; Oliveira, E.; Oliveira, P. V. Anal. Chim. Acta 2000, 405, 205.
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