In the Laboratory
Pressure-Assisted Chelating Extraction as a Teaching Tool in Instrumental Analysis
W
Omowunmi A. Sadik,* Adam K. Wanekaya, and Gelfand Yevgeny Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902-6016; *
[email protected] The central focus of a course in instrumental analysis is to understand the fundamental principles behind different chemical instrumentation. In the past, insufficient time was devoted to sampling and sample preparation. In some cases, sampling and sample preparation skills are completely overlooked. This is partly due to the tedious and time-consuming aspects of classical methodologies such as Soxhlet extraction. Depending on parameters such as the class size, laboratory time, and equipment costs, major limitations could be encountered when introducing sample-preparation techniques at undergraduate laboratory levels. In recent times, newer sample-preparation techniques have been developed that meet regulatory requirements, achieve faster assays, and have better automation levels using smaller quantities of organic solvents (1–3). Such progress has resulted in the development of new techniques such as supercritical fluid extraction (SFE), microwave-assisted extraction (MAE) or microwave-assisted digestion (MAD), and accelerated-solvent extraction (ASE) or pressurized-fluid extraction (PFE) (2–4). As these techniques are being explored, optimized, and adopted for modern laboratories, their introduction and implementation for undergraduate laboratories is still rather slow. The objective of undergraduate laboratory curriculum is to develop experiments that are inexpensive to set up, safe to carry out, and can be completed within a reasonable quantity of time. While several excellent laboratory exercises have been designed on sample preparation, metal analysis, and metal digestion, few of these (if any) emphasized digestion protocols using nonhazardous materials or reagents (5–9). In addition, none of these techniques emphasized laboratory experience that allows students to see how wet digestion and instrumental methods can be linked together. This is not at all surprising because most experiments involving the analysis of heavy metals in solid samples are hazardous and time consuming. For example, wet-digestion methods generally require several hours to perform and utilize large quantities of hazardous reagents such as concentrated acid mixtures and strong oxidizing agents. Wet digestion is also prone to atmospheric contamination owing to the use of open systems that result in relatively high blanks. Even at the highest degree of commercially-available purity, and after additional sub-boiling distillation, blank levels of acids remain too high for certain ultra-trace determinations (10, 11). Recently, we reported a novel instrumental–digestion technique using pressure-assisted chelating extraction (PACE; ref 12 ). In PACE, a metal is rapidly digested using pressureassisted thermal equilibration of a selected ligand with this metal. Hence, the use of concentrated acids or strong oxidizing agents is conveniently avoided. We have adapted this procedure for an undergraduate laboratory at SUNY–
www.JCE.DivCHED.org
•
Binghamton. We now report on the use of PACE as an additional teaching tool of sample-preparation techniques in undergraduate laboratories. The major goals of this procedure with respect to undergraduate instruction include (i) exposing students to safe sample-preparation techniques, (ii) correlating wet-chemical methods with modern instrumental analysis, and (iii) comparing the performance of PACE with conventional wet acid-digestion techniques. In a typical exercise, students were first introduced to the basic concepts, followed by sample extraction using a Dionex ASE 200. Finally, the extracts were analyzed using flame atomic absorption spectroscopy (FAAS). Results were correlated with conventional wet acid-digestion techniques using standard reference materials. The ASE 200 is the main instrument used in PACE. It is a commercial instrument manufactured by Dionex Corporation, Sunnyvale, CA. Basic Design and Construction of a Low-Cost PACE Instrumentation The PACE instrument consists of three major components: a pump, a conventional oven, and an inert gas (e.g., nitrogen) supply. These are connected via tubing made of chemically resistive materials such as Teflon. Generally, high pressure metal systems are sufficiently complex that it is uneconomical for the laboratory scientist to undertake their fabrication. A simple and inexpensive design of PACE instrumentation is possible by a careful choice of system components. The requirements for a PACE system include: generation of pressures of up to 2000 psi; temperature control and reproducibility of 0.3% or better; corrosion-resistant components (clean surfaces of refractory metals such as titanium, molybdenum, tantalum, or zirconium will pump nitrogen by chemisorption); seals of fluorocarbon polymers (e.g., Teflon, Kel-F, and Viton-A) with low outgassing rates; and stainless steel valves with Viton-O or polyimide rings that can be baked at 200–300 ⬚C. The majority of these components are commercially available. The ASME (American Society of Mechanical Engineers) has published standards for the construction of metal apparatus under pressure (13). The requirements listed for PACE instrumentation have been abstracted from ASME specifications for type 304 stainless steel. These recommendations should be more than adequate for custom design of a PACE instrument. Many of the commercially available pumping systems are equipped with computer-controlled devices for measuring pressure changes at the pump outlet. A list of manufacturers, accessories, and supplies is listed at the end of the text.
Vol. 81 No. 8 August 2004
•
Journal of Chemical Education
1177
In the Laboratory
Experimental
Hazards The principal hazards are associated with wet acid-digestion. Proper attention and precaution should be exercised when handling concentrated acids. Acids are corrosive in any form and in high concentrations destroy body tissue and cause severe burns on contact with the skin. The addition of concentrated HNO3 and the digestion on a hot plate should be conducted in a fume hood. EDTA may be irritating to the eyes, skin, and upper respiratory system. Lead(III) nitrate may cause irritation to the skin, eyes, and respiratory tract. It may also affect the central nervous system, kidneys, blood, and reproductive system. Gloves, goggles or safety glasses, and laboratory coats should be used whenever handling chemical reagents. The use of liquids at elevated temperatures and high pressures in the PACE procedure may present potential safety concerns for the students. All PACE procedures should be carried out using stainless steel instruments from the manufacturer. Extraction cells were cooled before removal from the oven. Alternatively, insulated gloves or tongs were used for these removals. Reagents The following reagents are available for the students: • Concentrated HNO3 • 1000-ppm lead stock solution, prepared by dissolving 0.4000 g of Pb(NO3)2 in 250 mL of 0.1% aqueous HNO3. • 0.02 M EDTA stock solution, made by dissolving 7.4448 g of disodium salt of EDTA in deionized water and then bringing the solution to 1.0 L • Diatomaceous earth (celite 545) • 30% H2O2
Lead Standard Solutions The students prepared a 50-mL solution of aqueous 100ppm Pb by diluting a 1000-ppm Pb stock solution (provided) with 0.1% aqueous HNO3. Using micropipets or appropriate volumetric flasks, five standard solutions (25 mL each of 1.0, 2.0, 3.0, 4.0, and 5.0-ppm Pb) were similarly prepared from the 100-ppm Pb solution by diluting with 0.1% aqueous HNO3. The Pb(NO3)2 for making the standard solution was obtained from J. T. Chemical Co. (Phillipsburg, NJ). Concentrated HNO3 was obtained from Fisher Scientific (Fair Lawn, NJ). Buffalo River Sediment (SRM 8704) was obtained from the Standard Reference Materials Program through the National Institute of Standards and Technology (Gaithersburg, MD). All glassware was soaked overnight in 1 M HNO3, rinsed with deionized water, and then dried in the oven before use. Disodium ethylenediamine tetraacetate, 30% H2O2, and diatomaceous earth were obtained from Fisher Scientific (Fair Lawn, NJ). Dionex Corporation’s ASE 200 (Sunnyvale, CA) was used for extraction while an AAnalyst 300 atomic absorption spectrometer (from Perkin Elmer Corporation, Norwalk, CT) was employed for Pb analysis using acetylene–air, 217.0 nm, and a slit width of 0.7 nm. 1178
Journal of Chemical Education
•
Wet Acid-Digestion Procedure Approximately 1.00 g triplicate samples of the sediment were carefully weighed by difference into three 100-mL beakers. Another 100-mL beaker was used for a reagent blank. Concentrated HNO3, 10 mL, was added to each beaker. All the beakers were covered with watch glasses. The mixtures were heated on a hot plate to a mild reflux for 60 min and the beakers were checked periodically. If the volumes were too low, then the beaker sides were washed with a few mL of deionized water and if bumping occurred, the heat was turned down slightly. At the end of 60 min, 1 mL of 30% H2O2 was added to each beaker (caution: vigorous degassing occurred sometimes) and the reaction mixtures heated for a few more minutes until no more bubbles were present. The beakers were removed from the hot plate and given ample time to cool. After cooling, the digested samples were each filtered separately into three 50-mL volumetric flasks and the filters were washed with copious quantities of deionized water. The reagent blank was transferred to a fourth 50-mL volumetric flask. Finally, the flasks were filled to the mark with deionized water. The samples were then analyzed for Pb with FAAS. During the digestion, students were asked to proceed with the preparation of the six standard Pb solutions. The absorbance of each of the six standard solutions was determined and a calibration plot was constructed. The plot was used to determine the concentration of Pb as mg of Pb per L of the sample solutions, which was then converted to mg of Pb per kg of sediment as follows: mPb
mg L
( 0.050 L ) msediment g
1000
g kg
= f
mg Pb kg sediment
PACE Procedure The schematic and procedures for PACE are shown in Figure 1. The system consists of two trays. The top tray has 24 extraction cells, and the vial (bottom) tray contains the 24 vials, which correspond to each cell. The maximum temperature and pressure are 200 ⬚C and 3000 psi, respectively. Approximately 1.00 g of the sediment samples (in triplicate) were individually and thoroughly mixed with 0.500 g of diatomaceous earth and transferred to 33-mL extraction cells. A typical extraction sequence begins by heating the oven to a programmed set point. When the set temperature is reached, for example, 150 ⬚C, the chelating solution is pumped into the cells. When the cells are full, the static valves close stopping the flow. Then the cells are heated for a set time to ensure that the samples attain thermal equilibrium with the sequestrant. During this step, the static valves open periodically to maintain the set pressures. After the first cycle, the extracts are purged into the collection vials, fresh solvent (∼60% of the extraction cell volume) is pumped through the cells and the cycle is repeated. At the end of the extraction, the cells are purged with nitrogen for 90 s to ensure complete transfer of the complexed chelating solution from the cells into the collection vials. The combined extracts are transferred to 50-mL volumetric flasks and diluted to volume with Nanopure water. A blank containing only 0.5 g of the diato-
Vol. 81 No. 8 August 2004
•
www.JCE.DivCHED.org
In the Laboratory
maceous earth in the cell is then treated similarly. The following experimental conditions were used for the extraction process: Temperature: 150 ⬚C Pressure: 2000 psi Static cycles: 3 Static time: 5 min Flush % volume: 15 Purge time: 90 s
The samples were then analyzed for Pb using the FAAS. During the extraction the students made six standard Pb solutions from the same 1000-ppm Pb stock solution from the previous lab. The Pb analysis and calculations were carried out as explained for acid digestion. Results and Discussion The laboratory experiment described above is suitable for undergraduate quantitative analysis and instrumental analysis courses. Lead was chosen as a model analyte because it is the most common among serious childhood illnesses. An estimated 3–4 million American children under the age of six years have lead poisoning in their body system. Lead poisoning has been linked to various complications such as memory impairment, shortened attention span, stunted
growth, and reduced intellectual ability (14). The procedure described above was first introduced into a sophomore-level quantitative analysis class during the spring of 2000 and subsequently used as a two three-hour laboratory for the instrumental analysis course at SUNY–Binghamton. An NSF-sponsored external survey of this activity was found to be favorable and this continues to generate new projects using FAAS, digestions, and sample preparation in environmental, biological, and food samples. Students were first introduced to the principles of PACE during lectures and comparison was made with other extraction techniques such as liquid–liquid, microwave, supercritical fluid, and Soxhlet extraction. Modern extraction methods were compared including MAD, ASE, and supercritical extraction methods. Students learned that PACE provided an ideal platform through which metal ions could be quickly extracted using pressure-assisted thermal equilibration of the chelate with the metal. Aqueous EDTA, 0.02 M, was used to statically extract the sample under moderate temperatures (up to 200 ⬚C) and pressures (up to 3000 psi). The applied temperature and pressure helped to increase the speed and the yield of the process. At elevated temperatures, the viscosity of the chelating solution decreased, thus leading to improved penetration of the solution into the sample matrix, and hence an increase in the dissolution process. Elevated temperatures also helped the formation of the complex between the chelate and the metal by shifting the equilibrium to the right according to: + − Mn + Y 4
metal
Figure 1. Schematic diagram of the Dionex ASE 200 system; one set of cells and vials are shown.
www.JCE.DivCHED.org
•
(EDTA)
M Yn
−4
EDTA-metal complex
PACE operates through a programmed sequence of temperature, static times, and pressure and thermal equilibration. The use of a weak acid like aqueous EDTA for extraction ensures that the cell material is not destroyed during the extraction process. The extract is automatically filtered as it is expelled from the cell. The procedure results in a clean and almost complete solubilization of the metal while leaving the sample intact. The laboratory exercise in sample preparation was implemented using both PACE and wet acid-digestion techniques. The lab was carried out in one week during two three-hour sessions. A summary of the results is shown in Table 1. The certified value for Pb in the Buffalo River Sentiment is 150 mg兾kg. Student results are 143 mg兾kg (95.3% recovery) as the overall mean with a standard deviation of ± 14 mg兾kg using the PACE technique and 166 mg兾kg (111% recovery) with a standard deviation of ± 61 mg兾kg using the wet aciddigestion technique. Relative standard deviations (RSDs) were 10% and 37% respectively for the two procedures. It is evident that using PACE the recovery is more accurate and more precise than the recovery with the wet acid-digestion technique. The reason for the lower accuracy and precision in the case of wet acid-digestion technique is that the procedure is more operator-dependent compared to the PACE procedure. The automation process seems to improve the accuracy and precision of the PACE technique. An elevated pressure increases the recovery of lead as the pressure enables the chelating solution to remain in the liquid state even above
Vol. 81 No. 8 August 2004
•
Journal of Chemical Education
1179
In the Laboratory
its boiling point. It also enhances the penetration of the chelating solution into the sample matrix. The comparisons between other important experimental factors between PACE and acid digestion are shown in Table 2. PACE procedure takes only 25 min per extraction while a single acid digestion takes up to 100 min. The use of concentrated acids (HNO3) and strong oxidizing agents (H2O2) introduces hazards into the wet acid-digestion procedure. In this regard, the PACE procedure is attractive as it uses only a dilute solution (0.02 M) of a weak acid (EDTA). Since PACE is automated, it means that operator skill and experience are not crucial as required by the wet acid-digestion method. This procedure occurs in enclosed vessels and the risk of airborne contamination is minimized.
Table 1. Summary of Wet Acid-Digestion and PACE Techniques Using Standard Reference Materialsa
Group
Evaluation We conducted an external evaluation of the laboratory revision using three stages of longitudinal assessment prior to commencement, during, and after the project. Questionnaires were handed out to the students to evaluate whether the laboratory procedure enhanced their conceptual development and learning. Overall, positive feedback was received from the students: greater than 90% of the students polled were very enthusiastic about the concept and the laboratory exercises. Some students felt the lab provided them a better understanding of the class materials because it helped relate what was learned in class to real-life situations. Through this project, we have introduced students to modern samplepreparation techniques using PACE. It was found that PACE provided a novel and powerful approach for wet digestion. These advances have significantly improved undergraduate curriculum at Binghamton University. Conclusions A novel digestion technique for undergraduate laboratory has been described. This new metal-digestion technique operates through a programmed sequence of temperature, static times, and pressure and thermal equilibration. The technique uses less concentrated reagents and also requires significantly less time for metal dissolution. The use of EDTA as the extracting medium instead of concentrated acids is attractive, thus resulting in a clean and less hazardous metal dissolution. The percentage of Pb recoveries from the sample was 95% with an RSD of 10% with PACE versus 111% and an RSD of 37% with the wet acid-digestion technique. Better recoveries with a higher precision were obtained using PACE, which means that it is possible to obtain accurate and precise results from the analysis of heavy metals in solid samples safely without the use of concentrated acids and strong oxidizing agents. Acknowledgments The authors acknowledge the financial support of the National Science Foundation through the Course Curriculum Laboratory Improvement grant #9952730. We also thank Ronak Talati for participating in this work as summer intern at Binghamton University.
1180
Journal of Chemical Education
•
Wet Acid-Digestion Recovery
PACE Recovery
Mean / mg kg᎑1
Percentb
Mean / mg kg᎑1
Percent
1
134
89.3
136
90.7
2
172
114
167
3
225
150
127
84.7
4
134
89.3
131
87.3
5
136
90.6
155
103
6
134
89.3
147
98
7
130
86.7
132
88
8
107
71.3
153
102
9
148
98.7
129
86 106
10 c
Mean
312
208
159
166
111
143
111
95.3
a
Buffalo River Sediment sample from the Standard Reference Materials Program was used. b
Percent recovery is [(recovery/NIST certified value) x 100].
c
Wet Acid Digestion recovery is 166 ± 61 mg/kg, PACE recovery is 143 ± 14 mg/kg, and NIST certified value is150 ± 17 mg/kg.
Table 2. Comparison of Conventional Wet AcidDigestion with Instrumental PACE Digestion Property
PACE
Acid Digestion
Time per sample digestion/min
25
100
Recovery (%)
95
111
RSD (%)
10
037
Reagents
0.02 M EDTA
Conc HNO3, H2O2
Airborne contamination
unlikely
likely
Safety
safe
not very safe
W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Abu-Samra, A. Anal. Chem. 1975, 47, 1475. 2. Kingston, H. M.; Jassie, L. B. Introduction to Microwave Sample Preparation. Theory and Practice; American Chemical Society: Washington, DC, 1988.
Vol. 81 No. 8 August 2004
•
www.JCE.DivCHED.org
In the Laboratory 3. Richter, B. E.; Jones B. A.; Ezzell, J. L.; Porter, N. L.; Avdalovic, N.; Pohl, C. Anal. Chem. 1996, 68, 1033. 4. Phelps, C. L.; Smart, N.; Wai, C. M. J. Chem. Educ. 1996, 73, 1163. 5. Goltz, D. M.; Hall, T.; Grant, A.; Segstro, A. J. Chem. Educ. 2000, 77, 1486. 6. Dunn, J.; Phillips, D.; van Bronswiik, W. J. Chem. Educ. 1997, 74, 1188. 7. (a) Cox P. J. Chem. Educ. 1977, 54, 717. (b) Beller, N.; Hilleary, C. J. Chem. Educ. 1976, 53, 498. 8. (a) Thompson, L. J. Chem. Educ. 1974, 51, 60. (b) GuistoNorkus, R.; Gounili, G.; Wisniecki, P.; Hubball, J. A.; Smith, S. R.; Stuart, J. D. J. Chem. Educ. 1996, 73, 1176. (c) Sundback, K. A. J. Chem. Educ. 1996, 73, 669. (d) Kegley, S. E.; Hansen, K. J.; Cunnigham, K. L. J. Chem. Educ. 1996, 73, 558. 9. (a) Markow, P. G. J. Chem. Educ. 1996, 73, 178. (b) WeinsteinLloyd, J.; Lee, J. H. J. Chem. Educ. 1995, 72, 1053. (c) Welch, L. E.; Mossman, D. M. J. Chem. Educ. 1994, 71, 521. (d) Kegley, S. E.: Stacy, A. M. J. Chem. Educ. 1993, 70, 151. (e) Coleman, M. F. M. J. Chem. Educ. 1985, 62, 261. (f ) Hindy, K. T. Int. J. Environ. Educ. Information 1992, 11, 111. 10. Begerow, J.; Turfeld, M.; Duneman, L. J. Anal. At. Spectrom. 1996, 11, 913. 11. Begerow, J.; Turfeld, M.; Duneman, L. J. Anal. At. Spectrom. 1997, 12, 1095. 12. Wanekaya, A. K.; Myung, S.; Sadik, O. A. Analyst 2002, 127, 1272–1276.
www.JCE.DivCHED.org
•
13. ASME Boiler and Pressure Vessel Code, Section VIII, Division I, Appendix V, 1974. http://www.asme.org/bpvc/ (accessed May 2004). (b) O’Hanlon J. A User’s Guide to vacuum Technology; Wiley: New York, 1980. 14. Human Lead Exposure; Needleman, H. L., Ed.; CRC Press: Boca Raton, FL., 1991; Chapters 11, 14, 15.
Manufacturers and Suppliers 1. Wallace & Tiernan, 25 Main Street, Belleville, NJ 07109. 2. Combination Pump Valve Company, 851 Preston St., Philadelphia, PA 19104. 3. Beckman Instruments, Inc., 2500 Harbor Blvd, Fullerton, CA 92634. 4. Alcatec Vacuum Products, 7 Pond Street, Hanover, MA 02339. 5. Hewlett-Packard, 2850 Centerville Rd, Wilmington, DE 19808. 6. Dionex Corp, POB 3603, 1228 Titan Way, Sunnyvale, CA 94088-3603. http://www.dionex.com/app/tree.taf?asset_id=10988 (accessed May 2004).
Supplies and Accessories 1. Teflon 2. Kel-F 3. Viton A
Vol. 81 No. 8 August 2004
•
Journal of Chemical Education
1181
