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Optimization of a field flow pre-concentration system by experimental design for the determination of copper in sea water by flow-injection-atomic absorption spectrometry
Spectrochimica Acta Part B 57 Ž2002. 85᎐93
Optimization of a field flow pre-concentration system by experimental design for the determination of copper in sea water by flow-injection-atomic absorption spectrometry M.C. YebraU , N. Carro, A. Moreno-Cid Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Chemistry, Uni¨ ersity of Santiago de Compostela, 15782-Santiago de Compostela, Spain Received 22 May 2001; accepted 11 September 2001
Abstract A Plackett᎐Burman 2 7=3r32 design for seven factors Žsample pH, sample flow rate, eluent volume, eluent concentration, eluent flow rate, ethanol percentage in the eluent and mini-column diameter. was carried out in order to find the significant variables affecting the field flow pre-concentration system ŽFFPS. and the flow injection elution manifold for copper determination in seawater samples by flame atomic absorption spectrometry. By using the optimized flow systems, seawater samples were collected and pre-concentrated in situ by passing them with a peristaltic pump through a mini-column packed with Amberlite XAD-4 impregnated with the complexing agent 4-Ž2-pyridylazo. resorcinol. Thus, copper is pre-concentrated without the interference of the saline matrix. Once in the laboratory, the mini-columns loaded with copper are incorporated into a flow injection system and eluted with a small volume of a 40% Žvrv. ethanolic solution of 3 mol ly1 hydrochloric acid into the nebulizer-burner system of a flame atomic absorption spectrometer. Analysis of certified reference materials ŽSLEW-3 and NASS-5. showed good agreement with the certified value. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Field sample pre-concentration; Experimental design; Chelating mini-column; Flow-injection; Atomic absorption spectrometry; Copper; Seawater
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Corresponding author. Tel.: q34-9-81-59-10-79; fax: q34-9-81-59-50-12. E-mail address: [email protected] ŽM.C. Yebra..
0584-8547r02r$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 3 5 3 - 6
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M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
1. Introduction Copper is a heavy metal employed extensively in chemical industries and domestic activities. At low levels of concentration, it is considered to be an essential micronutrient for the normal metabolism of some living organisms w1x. However, at higher concentrations, Cu becomes toxic because it can bind to the cell membrane and alters the transport process through the cell wall. Due to Cu pollution by waste effluents released into the marine environment, it is necessary to define control programs to monitor the CuŽII. concentration in marine samples. Shellfish, mainly mussels, accumulate copper from the aquatic environment, resulting in much higher concentrations Ž3000 times. than those present in seawater. Therefore, in order to assess its accumulation, it is essential to determine CuŽII. in effluents from industrial and urban wastes or in seawater from places nearby. Sensitive, reproducible and accurate analytical methods for CuŽII. determination in seawater are required. Atomic absorption spectrometry ŽAAS. is a well-established technique for this purpose. Nevertheless, copper determination in seawater is influenced by several factors, such as the low concentration of metal in the sample, the high saline matrix of seawater and the contamination or losses of analyte during the sampling orrand post-sampling steps Žstorage and pre-treatment .. Therefore, a preliminary pre-concentration andror separation technique is necessary. The use of flow injection ŽFI. sample pre-concentration systems enables trace metal determinations in complex diluted samples using conventional flame atomic absorption spectrometry ŽFAAS.. A number of FI pre-concentration methods have been used for trace copper determination including a mini-column containing a chelating resin or a solid sorbent w2᎐9x. These on-line techniques offer more advantages than manual batch procedures, i.e. better reproducibility, accuracy and efficiency, automated sample management, higher sample throughput and low consumption of sample and reagents w10,11x. Furthermore, on-line mini-column pre-concentration systems using chelating resins are simpler and
less time consuming than other options, allowing ‘in situ’ sample pre-concentration. Mini-column field sampling ŽMFS. techniques have also been developed w12x. In these techniques, seawater is processed in a flow system at the sampling sites. Cu is immobilized on mini-columns, minimizing contamination or losses of analyte during the sampling and storage, and also facilitating sample transport. Later, the mini-columns are returned to the laboratory and directly inserted into an FI-AAS system. Limited literature is available about MFS techniques for copper determination in seawater samples. Nickson et al. w13x proposed the in situ pre-concentration of several trace elements on an iminodiacetate resin and Yebra et al. w14x described the use of a mini-column loaded with PAN w1-Ž2-pyridylazo.-2-naphtholx for the in-situ pre-concentration of copper from seawater samples. A procedure for the determination of copper in seawater using a Field Flow Pre-concentration System ŽFFPS. with mini-columns subsequently incorporated to a FI-FAAS device is described and optimized. Mini-columns containing Amberlite XAD-4 were used as a solid sorbent to prepare a ligand-loaded resin. This substrate was impregnated with 4-Ž2-pyridylazo. resorcinol ŽPAR., a heterocyclic azo compound capable of forming complexes with copper at the pH of seawater Ž7.8᎐8.3. w15x. This avoids the modification of the sample pH, and as a consequence, causes the FFPS to be the simplest possible. Univariate methods are widely employed for FI systems optimization, although it is tedious because a great number of experiments are required to obtain information about the behavior of the system. An experimental design has been used to simultaneously optimize the operational parameters in the FFPS and FI systems. This factorial design provides information about the processes involved. Because of the high number of variables that potentially affect all the system, a Plackett᎐Burman Ž2 7= 3r32. factorial design was applied to a synthetic seawater solution of copper ŽII.. Seven experimental variables were optimized: sample pH, sample flow rate, eluent volume, eluent concentration, eluent flow rate, ethanol percentage in the eluent and mini-col-
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
umn diameter. The developed optimal conditions were applied to the determination of copper in real seawater samples from Galicia ŽSpain..
2. Experimental section 2.1. Instrumentation The FFPS is constituted of a Gilson Minipuls-3 peristaltic pump fitted with polyvinyl chloride ŽPVC. tubes, a Reodyne Žmodel 5301. switching valve, a 0.45- m filter Ž M illipore . and laboratory-made PVC mini-columns packed with 0.05 g of 20 mesh Amberlite XAD-4 saturated with PAR. A portable alternator model 4600-HN ŽHonda. is connected to the peristaltic pump for performing the field pre-concentration. The FI elution system comprised a Gilson Minipuls-3 peristaltic pump, fitted with PVC tubes and a Reodyne Žmodel 5041. four-way valve. Cu detection was carried out on-line by a PerkinElmer 5000 atomic absorption spectrometer provided with a copper hollow-cathode lamp, a deuterium lamp background corrector and an air᎐acetylene flame Ž21.0r2.0 l miny 1 . as atomizer. The peak absorbance was registered at 324.8 nm. The aspiration flow-rate of the nebulizer was set to be the same as the flow-rate of the FI channel. A Perkin-Elmer 50 servograph recorder Ž5 mV range. was connected to the spectrometer output. The heights of the absorbance peaks were measured. Statistical analysis of the experimental designs was carried out by means of the STATGRAPHIC V.4.1 statistical package ŽManugistic, Inc. Rockville, MD, USA.. 2.2. Reagents All chemicals were of analytical-reagent grade. A standard solution of 1000 g mly1 CuŽII. was prepared from CuŽII. nitrate in 0.5 mol ly1 nitric acid ŽBDH Chemicals, Poole, England.. From this solution, other diluted standard solutions were prepared daily. Hydrochloric acid ŽMerck. diluted to 3 mol ly1 in 40% Žvrv. ethanol ŽMerck. was used as eluent. Amberlite XAD-4 Ž20 mesh.
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ŽAcros Organics, New Jersey, USA.. A 1 = 10y4 mol ly1 solution of 4-Ž2-pyridylazo. resorcinol ŽPAR. ŽAcros Organics. was prepared in 20% Žvrv. ethanol. The certified reference materials used were SLEW-3 Žestuarine water. and NASS-5 Žseawater. from the National Research Council of Canada. Ultrapure water of 18.3 M ⍀ cm resistivity obtained from a Milli-Q water purification system ŽMillipore. served for dilution and washing, respectively. 2.3. Mini-column preparation Amberlite XAD-4 was kept for 24 h in methanol, then filtered off and air-dried. The mini-columns were manufactured in the laboratory from PVC tubing Ž2.3 mm i.d.. and were packed with 0.05 g of 20 mesh Amberlite XAD-4, and a small amount of co-polymer beads were injected into the mini-column with a syringe. The ends of the tube were fitted with glass wool to keep the beads in the tube. Once packed, PAR was immobilized in dynamics with a flow manifold by passing a solution of PAR Ž1 = 10y4 mol ly1 . through the mini-columns at a flow rate of 0.8 ml miny1 for 30 min. Then, the mini-columns were washed out with ultrapure water until any excess PAR solution was eliminated from XAD-4; later they were kept in a refrigerator until the field sample pre-concentration system is accomplished. In this step, eight mini-columns can be prepared at the same time Žas many as the channels of the peristaltic pump.. Mini-columns packed with Amberlite XAD-4 can be used for several times Žup to approx. 10 operational cycles.. 2.4. Field sample pre-concentration The FFPS used is shown in Fig. 1. The seawater sample Žwithout pH modification. or a standard solution at pH 8 were pumped at 0.5 ml miny1 through the mini-column, which allowed to pass a fixed sample volume. If a real sample was considered, an on-line filtration with a 0.45-m filter was necessary. The Cu was retained on the minicolumn and the sample matrix was sent to waste. After the mini-column was loaded, the resin was rinsed with ultrapure water for 5 min at 10 ml
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M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
Fig. 1. Field flow pre-concentration system. P, peristaltic pump; SV, switching valve; F, filter; MC, mini-column ŽAmberlite XAD-4 loaded with PAR. W, waste.
miny1 and the residual fluid drawn off. The dry mini-columns were disconnected from the FFPS, placed in a portable refrigerator and returned to the laboratory where they were stored in a refrigerator until analysis.
equal to the nebulizer aspiration flow.. Thus, Cu was released directly into the nebulizer of the spectrometer. Calibration and blank columns were prepared in the laboratory in the same manner by pumping through the appropriate solution at pH 8.
2.5. On-line elution᎐FAAS determination of copper The determination of copper concentration was performed by sequentially connecting each minicolumn to the FI manifold shown in Fig. 2. Minicolumns were located immediately before the detector to avoid eluent dispersion. The analysis procedure was based on the injection of 333 l of eluent w3 mol ly1 HCl in 40% Žvrv. ethanolx into a carrier Žultrapure water pumped at a flow rate
3. Results and discussion 3.1. Optimization of adsorption of PAR on Amberlite XAD-4 The optimal conditions of PAR adsorption on Amberlite XAD-4 were obtained by testing several flow rates. Studies of the effect of the flow
Fig. 2. FI manifold for elution-determination of copper. P, peristaltic pump; MC, mini-column; W, waste; IV, injection valve; FAAS, flame atomic absorption spectrometer.
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
rate during the preparation of mini-columns were conducted with the volume-based system with a constant chelating reagent volume Ž5 ml., a PAR concentration of 1 = 10y4 mol ly1 and 0.05 g of Amberlite XAD-4 placed on a mini-column. The effluent of each solution studied and the original solutions of PAR were measured spectrophotometrically at 485 nm. The difference between the absorbance of each tested solution Žbefore and after passing though the resin. gives the quantity of the chelating reagent adsorbed by the resin. Maximum adsorption of PAR on the resin was obtained between 0.4 and 0.8 ml miny1 . Adsorption was decreased at flows ) 1.0 ml miny1 due to short residence times of the chelating reagent in the mini-column. To determine the optimal concentration of PAR, 5 ml of solutions containing concentration of 1 = 10y4 ᎐4 = 10y4 mol l y1 were passed at 0.8 ml miny1 through 0.05 g of Amberlite XAD-4. These experiments showed that 47% of the PAR was adsorbed in the resin with concentration above 1 = 10y4 mol ly1 of this reagent. Lower concentrations attained approximately 100% adsorption. Then, a PAR flow rate of 0.8 ml miny1 and a concentration of 1 = 10y4 mol ly1 PAR were selected for this work. 3.2. Optimization of the FFPS and FI elution process: factorial design The number of variables potentially affecting the overall system is large; seven factors were studied: sample pH, sample flow rate, eluent volume, eluent concentration, eluent flow rate, ethanol percentage in eluent and mini-column
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diameter. If a two-level full factorial design Ž2 7 . is applied, 128 experiments need to be carried out, in addition to the replicates needed for the statistical evaluation of the coefficients for the fitted model and the degree of coincidence of the hyperplane obtained with the true response surface. Plackett᎐Burman designs are used advantageously when the number of factors that could have an influence on the system is large, because they allow us to fractionate the full factorial design, giving numbers of factor combinations that are a multiple of four. In this case, a Plackett᎐Burman Ž2 7= 3r32. type III resolution design has been selected, allowing 4 d.f., which involved 12 non-randomized runs w16x. Table 1 shows the upper and lower levels given to each factor; such values were selected from available data and experience gathered in previous screening experiments. Optimization experiments were performed on standard solutions containing 30 g ly1 of CuŽII., and the sample volume was fixed at 2.5 ml, a sample volume allowing quantitative results taking into account the limit of quantification of the overall procedure. Table 2 lists the design matrix for these experiments and the copper recoveries Žexpressed as percentage. obtained in each of the experiments. The analysis of the results presented in Table 2 is reflected in the standardized main effects Pareto chart w17x shown in Fig. 3. In this chart, bar lengths are proportional to the absolute value of the estimated effects, clarifying the relative importance of effects. In order to test statistical significance, ANOVA Žanalysis of variance. was
Table 1 Studied parameters and factor levels used in the factorial design and the optimum values for the field sample pre-concentration FI-FAAS system Factor
Key
Sample pH Flow rate sample Žml miny1 . Eluent volume Žl. Eluent acid concentration Žmol ly1 . Flow rate eluent Žml miny1 . Ethanol percentage Column diameter Žmm. Sample volume Žml.
A B C D E F G
Fixed
2.5
ow Žy.
High Žq.
Optimum
6 0.5 83 0.1 2.5 0 1.1
9 5 333 3 5.5 20 2.3
8 0.5 333 3 3.5 40 2.3
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
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Table 2 Design matrix and response values Plackett᎐Burman design for copper ŽII.
Žrecoveries.
No.
A
B
C
D
E
F
G
Recovery Ž%.
1 2 3 4 5 6 7 8 9 10 11 12
q q y q q q y y y q y y
y q q y q q q y y y q y
q y q q y q q q y y y y
y q y q q y q q q y y y
y y q y q q y q q q y y
y y y q y q q y q q q y
q y y y q y q q y q q y
35.75 16.39 31.87 100.00 49.19 21.24 106.78 100.00 100.00 18.44 36.89 18.44
in
employed. Yates’ order was used to calculate estimated effects. As can be observed, the majority of variables related to the eluent, hydrochloric acid Žvolume, concentration and ethanol percentage., and the sample pH appear to be statistically significant Žat 95% confidence.. All of them, except the sample pH, are positively affected. The negative sign for the sample pH suggests that it inhibits the Cu recovery when its value is increased Žalkaline range.. As expected, the maximum recoveries were obtained at the highest eluent concentration Ž3 mol ly1 hydrochloric acid. because the Cu solubility in the eluent is increased. Higher eluent concen-
trations could dissolve the metallic parts of the spectrometer nebulizer, so further experiments have not been performed. The positive influence of the ethanol percentage on Cu determination may be related to the fact that alcohol favors the desorption of copper from the resin beads. Table 2 shows that the elution of Cu was quantitative at the highest values of the eluent volume. Lower volumes are not enough to achieve a complete copper retrieval, and therefore, 333 l were considered to be a compromise to obtain good signals for CuŽII.. In theory, only factors appearing as statistically significant should be considered further. However, Plackett᎐Burman designs frequently suggest discarding factors that influence the process, but do not exhibit effects large enough to be easily appreciated over the experimental error because of oversimplification of the design and lack of resolution. Although the other factors Žthe sample and eluent flow rates and the column diameter. do not appear as statistically significant in this design Žsee Fig. 3., they present a certain influence on Cu recovery and are affected by negative sign Žthe sample and eluent flow rates. and positive sign Žthe column diameter.. For the tested levels of the sample flow rate Ž0.5᎐5.5 ml miny1 ., these results show that the Cu determination efficiency is inversely proportional to this factor. This may be due to the augmententation of the residence time of copper in mini-column when the flow rate of sample
Fig. 3. Pareto chart of the main effects for the Plackett᎐Burman design, obtained by using copper recoveries. The vertical lines indicate the statistical significance.
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
diminishes, so that 0.5 ml miny1 is considered to be an optimal value to obtain quantitative copper recoveries. The eluent flow rate is considered as a very important factor in metal determination. When changing this flow rate, the aspiration rate on the spectrometer was altered accordingly. In this system, the eluent flow rate hardly presents any influence in the working range. The highest value of eluent flow rate Ž5.5 ml miny1 . provides very short residence times of eluent in the minicolumn and produces problems of overpressure in the narrow PVC tubes. It is necessary to reach equilibrium to obtain a good population of copper into flame. A mini-column with an i.d. of 2.3 mm is considered as the optimal value. Higher dispersion appears when the diameter of the mini-column is increased. According to the results, a new experimental design shifted in the direction of higher values of eluent concentration, eluent volume and ethanol percentage and in the direction of lower values of sample pH would be desirable. In this way, not only could the possible factor interactions be established, but also the systems optimum could be identified because good Cu recoveries were achieved. In accordance with previous considerations Žonly two statistically significant parameters, sample pH and ethanol percentage, and a nonstatistically significant one, eluent flow rate with a very low estimated effect, would be reconsidered., further experiments were performed in order to fine tune these variables without the necessity of applying another factorial design. It is important to consider that the main objective of this work was to optimize the system by reducing the number of experiments and the process time. Two experiments were performed keeping all the other factors at optimum values and varying the values of sample pH between 6 and 8, the results obtained were similar. A pH of 8 is considered as the optimal value because seawater is strongly buffered by the hydrogencarbonate᎐ carbonate᎐carbon dioxide system at approximately this pH. Two additional experiments were conducted to fine-tune the ethanol percentage; the considered values were 20 and 40%; the experiment performed at 40% gave a slightly better recovery.
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Since eluent flow rate hardly affects the Cu recovery and high values of this variable lead to overpressure problems and difficult mini-column handling, this variable has been fixed at a low value. Although high effluent flows cause a Cu population in the flame Žhigh sensitivity., previous experiments in our laboratory suggest that 3.5 ml miny1 is a good compromise value. The last column in Table 2 shows the optimal values of factors affecting the studied system for copper determination. 3.3. Study of sample ¨ olume Large sample volumes are permitted to be pre-concentrated in mini-columns due to their great capacity. In this study, good recoveries Ž98᎐101%. were obtained by passing through mini-columns 2.5 ml of a copper standard solution of 30 g ly1 , 75 ml of a copper standard solution of 1 g ly1 , and 100 ml of a copper standard solution of 0.5 g ly1 . In all instances, eluent volume was 333 l.
4. Analytical figures of merit Cu determinations with different sample volumes Ž2.5, 75 and 100 ml. were carried out by using calibration graphs performed under the optimal experimental conditions. Equations of the calibration curves are as follows: By direct aspiration of copper solution: Absorbance s 1.5 = 10y4 q 5.8 = 10y5 C Ž n s 7. and C s 73.3᎐5000 g ly1 copper. Sample volume 2.5 ml: Absorbance s 3.3 = 10y4 q 4.3 = 10y4 C Ž n s 7. and C s 9.5᎐673.7 g ly1 copper. Sample volume 75 ml: Absorbance s 2.1 = 10y4 q 0.0128C Ž n s 7. and C s 0.3᎐22.6 g ly1 copper. Sample volume 100 ml: Absorbance s 4.7 = 10y4 q 0.0171C Ž n s 7. and C s 0.2᎐16.9 g ly1 copper.
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M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
Experimental enrichment factors of 7.4, 220.7 and 294.8 were obtained for 2.5, 75 and 100 ml of sample volumes, respectively, calculated as the ratio between the slopes of the calibration graphs obtained by each sample volume used and the direct aspiration of Cu solution. The theoretical pre-concentration factors were 7.5, 225.2 and 300.3 Žfor 2.5, 75 and 100 ml, respectively., which confirms the quantitative recovery and the low dispersion of the FI system. Detection limits were defined as three times the S.D. of the peak height for 30 determinations of the blank Žultrapure water. and were 2.8, 0.08 and 0.05 g ly1 for the studied sample volumes of 2.5, 75 and 100 ml, respectively. The quantification limit, calculated as the blank signal plus 10 times its S.D. were 9.4, 0.28 and 0.17 g ly1 for sample volumes, 2.5, 75 and 100 ml, respectively. The pre-concentration factor, detection and quantitation limits could be further improved by increasing sample volume without degradation in the efficiency due to the favorable kinetics of this system. The optimized method was proposed for Cu determination in seawater, if the mean Cu concentration is approximately 0.25 g ly1 , whereas in marine water w18x, a sample volume of 100 ml was considered as the optimal value. The precision of the method obtained for standard solutions containing 0.5 Ž n s 11. and 2 Ž n s 11. g ly1 of copper were 3.8 and 1.1%, respectively, expressed as relative standard deviation ŽR.S.D... Finally, the overall procedure was validated using two certified reference materials ŽSLEW-3, estuarine water and NASS-5, seawater. with a Cu concentration of 1.55" 0.12 g ly1 and 0.297" 0.046 g ly1 , respectively. The copper contents obtained Žmean " S.D., n s 2. were 1.52" 0.03 g ly1 and 0.29" 0.04 g ly1 , which are agree with the certified values.
5. Effect of sample matrix ions A study of interferents was performed by analyzing synthetic seawater solutions Ž100 ml. containing 5 g ly1 of CuŽII. and the most important constituents of seawater w18x. The results are
Table 3 Effect of sample matrix ions on copper recovery Ion
Ratio wionxrwCux
Recovery Ž%.
Al3q Ba2q Ca2q Cr3q Fe3q Kq Mg2q Mn2q Naq Ni2q Pb2q Sr2q Zn2q CO3 2y Cly Fy Iy NO3y SO42y PO43y
2:1 15:1 4 = 105 :1 1:1 2:1 4 = 105 :1 1.3= 106 :1 1:1 6 = 106 :1 1:1 1:1 8 = 103 :1 150:1 3 = 104 :1 2 = 107 :1 1.5= 103 :1 1:60 1.1= 104 :1 2 = 106 :1 60:1
97.0 103.0 97.0 100 95.4 100 95.4 97.0 95.4 103.0 97.0 100 95.4 103.0 97.0 103.0 100 100 97.0 103.0
shown in Table 3. The presence of large amounts of alkali and alkaline earth metals has no influence on the recovery of copper Ž- 5%.. This study shows that copper can be quantitatively determined in seawater samples.
6. Stability of mini-columns and sorption capacity Stability studies were performed at optimal conditions of the whole procedure. Mini-columns with retained CuŽII. were kept for 2, 7, 15 and 30 days at room temperatures and at 4⬚C Žrefrigerator.. Results showed that the mini-columns are stable for 2 days at room temperature and at least 30 days at 4⬚C. The sorption capacity of the Amberlite XAD-4 chelating resin loaded with PAR for the retention of copper was also determined. Increasing amounts of copper were passed through a minicolumn containing 0.05 g of chelating resin. The results demonstrated that the resin has a sorption capacity of 2.2 mol copper per gram of dry resin.
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
7. Conclusions Pre-concentration and elimination of the sample matrix was successfully achieved by the FFPS by using a chelating mini-column packed with Amberlite XAD-4 loaded with PAR. One of the main advantages of the present portable FFPS is that the problems associated with sample transport and storage were avoided. Furthermore, it is the simplest system possible because the seawater samples requires no pH adjustment. The proposed FFPS using Amberlite XAD-4 minicolumns coated with PAR and the FI-AAS determination of copper in seawater samples is simple, sensitive and accurate. Copper was quantitatively recovered from the mini-column with a high precision. The use of factorial design for assessing the main factors that involve all procedures ŽFFPS and FI elution. has a number of advantages over the conventional univariate method, such as fast screening of factors with a relatively small number of experiments and the estimation of the main effects. In addition, factorial design can be used to quickly assess the robustness of the FFPS and FI elution manifolds. In conclusion, the proposed method is excellent as regards simplicity, sensitivity, precision, accuracy, selectivity and mini-column stability.
w4 x
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Acknowledgements The authors are grateful for the financial support provided by the Galicia Government ŽXunta de Galicia. in the framework of Project PGIDT99PX120901A.
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References w1x S.E. Manahan, Environmental Chemistry, CRC Press, Boca Raton, 1994. w2x A.M. Naghmush, M. Trojanowicz, E.W. Olbrych-Sleszynska, Flow injection flame atomic spectrometric determination of copper with preconcentration on ligand loaded Amberlite XAD-2, J. Anal. At. Spectrom. 7 Ž1992. 323᎐328. w3x R.E. Santelli, M. Gallego, M. Valcarcel, Preconcentration and atomic absorption determination of copper traces in waters by online adsorption᎐elution on an activated carbon minicolumn, Talanta 41 Ž1994. 817᎐823.
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M.C. Yebra-Biurrun, A. Bermejo-Barrera, M.P. Bermejo-Barrera, M.C. Barciela-Alonso, Determination of trace metals in natural waters by flame atomic absorption spectrometry following online ion-exchange preconcentration, Anal. Chim. Acta 303 Ž1995. 341᎐345. L.A. Ellis, D.J. Roberts, Determination of copper, cadmium, manganese and lead in saline water with flow injection and atom trapping atomic absorption spectrometry, J. Anal. At. Spectrom. 13 Ž1998. 631᎐634. A. Ali, Y. Ye, G. Xu, Copper determination after FI on-line sorbent preconcentration using 1-nitroso-2naphthol as a complexing reagent, Fresenius’ J. Anal. Chem. 365 Ž1999. 642᎐646. B. Wen, X.Q. Shan, R.X. Liu, Preconcentration of trace elements in sea water with polyŽacrylaminophosphonicdithiocarbamate. chelating fiber for their determination by inductively coupled plasma mass spectrometry, Fresenius’ J. Anal. Chem. 363 Ž1999. 251᎐255. S.L.C. Ferreira, V.A. Lemos, B.C. Moreira, A.C. Spinola-Costa, R.E. Santelli, An on-line continuous flow system for copper enrichment and determination by flame atomic absorption spectroscopy, Anal. Chim. Acta 403 Ž2000. 259᎐264. P.H. Lin, K.S. K. Danadurai, S.D. Huang, Simultaneous determination of cobalt, nickel and copper in seawater with a multi-element electrothermal atomic absorption spectrometer microcolumn preconcentration, J. Anal. At. Spectrom. 16 Ž2001. 409᎐412. Z. Fang, Flow Injection Atomic Absorption Spectrometry, Wiley, Chichester, 1985. J.L. Burguera ŽEd.., Flow Injection Atomic Spectroscopy, Practical Spectroscopy Series, 7, Marcel Dekker, New York, 1989. A. Sanz-Medel ŽEd.., Flow Analysis with Atomic Spectrometric Detectors, Analytica Spectroscopy Library, 9, Elsevier, Amsterdam, 1999. R.A. Nickson, S.J. Hill, P.J. Worsfold, Field preconcentration of trace metals from seawater and brines coupled with laboratory analysis using flow injection and ICP-AES detection, Int. J. Environ. Anal. Chem. 75 Ž1999. 57᎐69. M.C. Yebra, N. Carro, M.F. Enriquez, A. Moreno-Cid, A. Garcıa, ´ Field sample preconcentration of copper in seawater using chelating minicolumns subsequently incorporated on a flow-injection-flame atomic absorption spectrometry system, Analyst, 2001, in press. J. Brown, A. Colling, D. Park, J. Phillips, D. Rothery, J. Wright, Seawater: Its Composition, Properties and Behaviour, Pergamon Press, Oxford, 1992. S.N. Deming, S.L. Morgan, Experimental Design: A Chemometric Approach, Chapter 14, Elsevier, Amsterdam, 1993. STATGRAPHIC V.4.1 Statistical Package, Manugistic, Inc. Rockville, MD, USA. H.J.M. Bowen, Environmental Chemistry of the Elements, Academic Press, London, 1979.
Optimization of a field flow pre-concentration system by experimental design for the determination of copper in sea water by flow-injection-atomic absorption spectrometry M.C. YebraU , N. Carro, A. Moreno-Cid Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Chemistry, Uni¨ ersity of Santiago de Compostela, 15782-Santiago de Compostela, Spain Received 22 May 2001; accepted 11 September 2001
Abstract A Plackett᎐Burman 2 7=3r32 design for seven factors Žsample pH, sample flow rate, eluent volume, eluent concentration, eluent flow rate, ethanol percentage in the eluent and mini-column diameter. was carried out in order to find the significant variables affecting the field flow pre-concentration system ŽFFPS. and the flow injection elution manifold for copper determination in seawater samples by flame atomic absorption spectrometry. By using the optimized flow systems, seawater samples were collected and pre-concentrated in situ by passing them with a peristaltic pump through a mini-column packed with Amberlite XAD-4 impregnated with the complexing agent 4-Ž2-pyridylazo. resorcinol. Thus, copper is pre-concentrated without the interference of the saline matrix. Once in the laboratory, the mini-columns loaded with copper are incorporated into a flow injection system and eluted with a small volume of a 40% Žvrv. ethanolic solution of 3 mol ly1 hydrochloric acid into the nebulizer-burner system of a flame atomic absorption spectrometer. Analysis of certified reference materials ŽSLEW-3 and NASS-5. showed good agreement with the certified value. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Field sample pre-concentration; Experimental design; Chelating mini-column; Flow-injection; Atomic absorption spectrometry; Copper; Seawater
U
Corresponding author. Tel.: q34-9-81-59-10-79; fax: q34-9-81-59-50-12. E-mail address: [email protected] ŽM.C. Yebra..
0584-8547r02r$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 3 5 3 - 6
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1. Introduction Copper is a heavy metal employed extensively in chemical industries and domestic activities. At low levels of concentration, it is considered to be an essential micronutrient for the normal metabolism of some living organisms w1x. However, at higher concentrations, Cu becomes toxic because it can bind to the cell membrane and alters the transport process through the cell wall. Due to Cu pollution by waste effluents released into the marine environment, it is necessary to define control programs to monitor the CuŽII. concentration in marine samples. Shellfish, mainly mussels, accumulate copper from the aquatic environment, resulting in much higher concentrations Ž3000 times. than those present in seawater. Therefore, in order to assess its accumulation, it is essential to determine CuŽII. in effluents from industrial and urban wastes or in seawater from places nearby. Sensitive, reproducible and accurate analytical methods for CuŽII. determination in seawater are required. Atomic absorption spectrometry ŽAAS. is a well-established technique for this purpose. Nevertheless, copper determination in seawater is influenced by several factors, such as the low concentration of metal in the sample, the high saline matrix of seawater and the contamination or losses of analyte during the sampling orrand post-sampling steps Žstorage and pre-treatment .. Therefore, a preliminary pre-concentration andror separation technique is necessary. The use of flow injection ŽFI. sample pre-concentration systems enables trace metal determinations in complex diluted samples using conventional flame atomic absorption spectrometry ŽFAAS.. A number of FI pre-concentration methods have been used for trace copper determination including a mini-column containing a chelating resin or a solid sorbent w2᎐9x. These on-line techniques offer more advantages than manual batch procedures, i.e. better reproducibility, accuracy and efficiency, automated sample management, higher sample throughput and low consumption of sample and reagents w10,11x. Furthermore, on-line mini-column pre-concentration systems using chelating resins are simpler and
less time consuming than other options, allowing ‘in situ’ sample pre-concentration. Mini-column field sampling ŽMFS. techniques have also been developed w12x. In these techniques, seawater is processed in a flow system at the sampling sites. Cu is immobilized on mini-columns, minimizing contamination or losses of analyte during the sampling and storage, and also facilitating sample transport. Later, the mini-columns are returned to the laboratory and directly inserted into an FI-AAS system. Limited literature is available about MFS techniques for copper determination in seawater samples. Nickson et al. w13x proposed the in situ pre-concentration of several trace elements on an iminodiacetate resin and Yebra et al. w14x described the use of a mini-column loaded with PAN w1-Ž2-pyridylazo.-2-naphtholx for the in-situ pre-concentration of copper from seawater samples. A procedure for the determination of copper in seawater using a Field Flow Pre-concentration System ŽFFPS. with mini-columns subsequently incorporated to a FI-FAAS device is described and optimized. Mini-columns containing Amberlite XAD-4 were used as a solid sorbent to prepare a ligand-loaded resin. This substrate was impregnated with 4-Ž2-pyridylazo. resorcinol ŽPAR., a heterocyclic azo compound capable of forming complexes with copper at the pH of seawater Ž7.8᎐8.3. w15x. This avoids the modification of the sample pH, and as a consequence, causes the FFPS to be the simplest possible. Univariate methods are widely employed for FI systems optimization, although it is tedious because a great number of experiments are required to obtain information about the behavior of the system. An experimental design has been used to simultaneously optimize the operational parameters in the FFPS and FI systems. This factorial design provides information about the processes involved. Because of the high number of variables that potentially affect all the system, a Plackett᎐Burman Ž2 7= 3r32. factorial design was applied to a synthetic seawater solution of copper ŽII.. Seven experimental variables were optimized: sample pH, sample flow rate, eluent volume, eluent concentration, eluent flow rate, ethanol percentage in the eluent and mini-col-
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
umn diameter. The developed optimal conditions were applied to the determination of copper in real seawater samples from Galicia ŽSpain..
2. Experimental section 2.1. Instrumentation The FFPS is constituted of a Gilson Minipuls-3 peristaltic pump fitted with polyvinyl chloride ŽPVC. tubes, a Reodyne Žmodel 5301. switching valve, a 0.45- m filter Ž M illipore . and laboratory-made PVC mini-columns packed with 0.05 g of 20 mesh Amberlite XAD-4 saturated with PAR. A portable alternator model 4600-HN ŽHonda. is connected to the peristaltic pump for performing the field pre-concentration. The FI elution system comprised a Gilson Minipuls-3 peristaltic pump, fitted with PVC tubes and a Reodyne Žmodel 5041. four-way valve. Cu detection was carried out on-line by a PerkinElmer 5000 atomic absorption spectrometer provided with a copper hollow-cathode lamp, a deuterium lamp background corrector and an air᎐acetylene flame Ž21.0r2.0 l miny 1 . as atomizer. The peak absorbance was registered at 324.8 nm. The aspiration flow-rate of the nebulizer was set to be the same as the flow-rate of the FI channel. A Perkin-Elmer 50 servograph recorder Ž5 mV range. was connected to the spectrometer output. The heights of the absorbance peaks were measured. Statistical analysis of the experimental designs was carried out by means of the STATGRAPHIC V.4.1 statistical package ŽManugistic, Inc. Rockville, MD, USA.. 2.2. Reagents All chemicals were of analytical-reagent grade. A standard solution of 1000 g mly1 CuŽII. was prepared from CuŽII. nitrate in 0.5 mol ly1 nitric acid ŽBDH Chemicals, Poole, England.. From this solution, other diluted standard solutions were prepared daily. Hydrochloric acid ŽMerck. diluted to 3 mol ly1 in 40% Žvrv. ethanol ŽMerck. was used as eluent. Amberlite XAD-4 Ž20 mesh.
87
ŽAcros Organics, New Jersey, USA.. A 1 = 10y4 mol ly1 solution of 4-Ž2-pyridylazo. resorcinol ŽPAR. ŽAcros Organics. was prepared in 20% Žvrv. ethanol. The certified reference materials used were SLEW-3 Žestuarine water. and NASS-5 Žseawater. from the National Research Council of Canada. Ultrapure water of 18.3 M ⍀ cm resistivity obtained from a Milli-Q water purification system ŽMillipore. served for dilution and washing, respectively. 2.3. Mini-column preparation Amberlite XAD-4 was kept for 24 h in methanol, then filtered off and air-dried. The mini-columns were manufactured in the laboratory from PVC tubing Ž2.3 mm i.d.. and were packed with 0.05 g of 20 mesh Amberlite XAD-4, and a small amount of co-polymer beads were injected into the mini-column with a syringe. The ends of the tube were fitted with glass wool to keep the beads in the tube. Once packed, PAR was immobilized in dynamics with a flow manifold by passing a solution of PAR Ž1 = 10y4 mol ly1 . through the mini-columns at a flow rate of 0.8 ml miny1 for 30 min. Then, the mini-columns were washed out with ultrapure water until any excess PAR solution was eliminated from XAD-4; later they were kept in a refrigerator until the field sample pre-concentration system is accomplished. In this step, eight mini-columns can be prepared at the same time Žas many as the channels of the peristaltic pump.. Mini-columns packed with Amberlite XAD-4 can be used for several times Žup to approx. 10 operational cycles.. 2.4. Field sample pre-concentration The FFPS used is shown in Fig. 1. The seawater sample Žwithout pH modification. or a standard solution at pH 8 were pumped at 0.5 ml miny1 through the mini-column, which allowed to pass a fixed sample volume. If a real sample was considered, an on-line filtration with a 0.45-m filter was necessary. The Cu was retained on the minicolumn and the sample matrix was sent to waste. After the mini-column was loaded, the resin was rinsed with ultrapure water for 5 min at 10 ml
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Fig. 1. Field flow pre-concentration system. P, peristaltic pump; SV, switching valve; F, filter; MC, mini-column ŽAmberlite XAD-4 loaded with PAR. W, waste.
miny1 and the residual fluid drawn off. The dry mini-columns were disconnected from the FFPS, placed in a portable refrigerator and returned to the laboratory where they were stored in a refrigerator until analysis.
equal to the nebulizer aspiration flow.. Thus, Cu was released directly into the nebulizer of the spectrometer. Calibration and blank columns were prepared in the laboratory in the same manner by pumping through the appropriate solution at pH 8.
2.5. On-line elution᎐FAAS determination of copper The determination of copper concentration was performed by sequentially connecting each minicolumn to the FI manifold shown in Fig. 2. Minicolumns were located immediately before the detector to avoid eluent dispersion. The analysis procedure was based on the injection of 333 l of eluent w3 mol ly1 HCl in 40% Žvrv. ethanolx into a carrier Žultrapure water pumped at a flow rate
3. Results and discussion 3.1. Optimization of adsorption of PAR on Amberlite XAD-4 The optimal conditions of PAR adsorption on Amberlite XAD-4 were obtained by testing several flow rates. Studies of the effect of the flow
Fig. 2. FI manifold for elution-determination of copper. P, peristaltic pump; MC, mini-column; W, waste; IV, injection valve; FAAS, flame atomic absorption spectrometer.
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
rate during the preparation of mini-columns were conducted with the volume-based system with a constant chelating reagent volume Ž5 ml., a PAR concentration of 1 = 10y4 mol ly1 and 0.05 g of Amberlite XAD-4 placed on a mini-column. The effluent of each solution studied and the original solutions of PAR were measured spectrophotometrically at 485 nm. The difference between the absorbance of each tested solution Žbefore and after passing though the resin. gives the quantity of the chelating reagent adsorbed by the resin. Maximum adsorption of PAR on the resin was obtained between 0.4 and 0.8 ml miny1 . Adsorption was decreased at flows ) 1.0 ml miny1 due to short residence times of the chelating reagent in the mini-column. To determine the optimal concentration of PAR, 5 ml of solutions containing concentration of 1 = 10y4 ᎐4 = 10y4 mol l y1 were passed at 0.8 ml miny1 through 0.05 g of Amberlite XAD-4. These experiments showed that 47% of the PAR was adsorbed in the resin with concentration above 1 = 10y4 mol ly1 of this reagent. Lower concentrations attained approximately 100% adsorption. Then, a PAR flow rate of 0.8 ml miny1 and a concentration of 1 = 10y4 mol ly1 PAR were selected for this work. 3.2. Optimization of the FFPS and FI elution process: factorial design The number of variables potentially affecting the overall system is large; seven factors were studied: sample pH, sample flow rate, eluent volume, eluent concentration, eluent flow rate, ethanol percentage in eluent and mini-column
89
diameter. If a two-level full factorial design Ž2 7 . is applied, 128 experiments need to be carried out, in addition to the replicates needed for the statistical evaluation of the coefficients for the fitted model and the degree of coincidence of the hyperplane obtained with the true response surface. Plackett᎐Burman designs are used advantageously when the number of factors that could have an influence on the system is large, because they allow us to fractionate the full factorial design, giving numbers of factor combinations that are a multiple of four. In this case, a Plackett᎐Burman Ž2 7= 3r32. type III resolution design has been selected, allowing 4 d.f., which involved 12 non-randomized runs w16x. Table 1 shows the upper and lower levels given to each factor; such values were selected from available data and experience gathered in previous screening experiments. Optimization experiments were performed on standard solutions containing 30 g ly1 of CuŽII., and the sample volume was fixed at 2.5 ml, a sample volume allowing quantitative results taking into account the limit of quantification of the overall procedure. Table 2 lists the design matrix for these experiments and the copper recoveries Žexpressed as percentage. obtained in each of the experiments. The analysis of the results presented in Table 2 is reflected in the standardized main effects Pareto chart w17x shown in Fig. 3. In this chart, bar lengths are proportional to the absolute value of the estimated effects, clarifying the relative importance of effects. In order to test statistical significance, ANOVA Žanalysis of variance. was
Table 1 Studied parameters and factor levels used in the factorial design and the optimum values for the field sample pre-concentration FI-FAAS system Factor
Key
Sample pH Flow rate sample Žml miny1 . Eluent volume Žl. Eluent acid concentration Žmol ly1 . Flow rate eluent Žml miny1 . Ethanol percentage Column diameter Žmm. Sample volume Žml.
A B C D E F G
Fixed
2.5
ow Žy.
High Žq.
Optimum
6 0.5 83 0.1 2.5 0 1.1
9 5 333 3 5.5 20 2.3
8 0.5 333 3 3.5 40 2.3
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
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Table 2 Design matrix and response values Plackett᎐Burman design for copper ŽII.
Žrecoveries.
No.
A
B
C
D
E
F
G
Recovery Ž%.
1 2 3 4 5 6 7 8 9 10 11 12
q q y q q q y y y q y y
y q q y q q q y y y q y
q y q q y q q q y y y y
y q y q q y q q q y y y
y y q y q q y q q q y y
y y y q y q q y q q q y
q y y y q y q q y q q y
35.75 16.39 31.87 100.00 49.19 21.24 106.78 100.00 100.00 18.44 36.89 18.44
in
employed. Yates’ order was used to calculate estimated effects. As can be observed, the majority of variables related to the eluent, hydrochloric acid Žvolume, concentration and ethanol percentage., and the sample pH appear to be statistically significant Žat 95% confidence.. All of them, except the sample pH, are positively affected. The negative sign for the sample pH suggests that it inhibits the Cu recovery when its value is increased Žalkaline range.. As expected, the maximum recoveries were obtained at the highest eluent concentration Ž3 mol ly1 hydrochloric acid. because the Cu solubility in the eluent is increased. Higher eluent concen-
trations could dissolve the metallic parts of the spectrometer nebulizer, so further experiments have not been performed. The positive influence of the ethanol percentage on Cu determination may be related to the fact that alcohol favors the desorption of copper from the resin beads. Table 2 shows that the elution of Cu was quantitative at the highest values of the eluent volume. Lower volumes are not enough to achieve a complete copper retrieval, and therefore, 333 l were considered to be a compromise to obtain good signals for CuŽII.. In theory, only factors appearing as statistically significant should be considered further. However, Plackett᎐Burman designs frequently suggest discarding factors that influence the process, but do not exhibit effects large enough to be easily appreciated over the experimental error because of oversimplification of the design and lack of resolution. Although the other factors Žthe sample and eluent flow rates and the column diameter. do not appear as statistically significant in this design Žsee Fig. 3., they present a certain influence on Cu recovery and are affected by negative sign Žthe sample and eluent flow rates. and positive sign Žthe column diameter.. For the tested levels of the sample flow rate Ž0.5᎐5.5 ml miny1 ., these results show that the Cu determination efficiency is inversely proportional to this factor. This may be due to the augmententation of the residence time of copper in mini-column when the flow rate of sample
Fig. 3. Pareto chart of the main effects for the Plackett᎐Burman design, obtained by using copper recoveries. The vertical lines indicate the statistical significance.
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
diminishes, so that 0.5 ml miny1 is considered to be an optimal value to obtain quantitative copper recoveries. The eluent flow rate is considered as a very important factor in metal determination. When changing this flow rate, the aspiration rate on the spectrometer was altered accordingly. In this system, the eluent flow rate hardly presents any influence in the working range. The highest value of eluent flow rate Ž5.5 ml miny1 . provides very short residence times of eluent in the minicolumn and produces problems of overpressure in the narrow PVC tubes. It is necessary to reach equilibrium to obtain a good population of copper into flame. A mini-column with an i.d. of 2.3 mm is considered as the optimal value. Higher dispersion appears when the diameter of the mini-column is increased. According to the results, a new experimental design shifted in the direction of higher values of eluent concentration, eluent volume and ethanol percentage and in the direction of lower values of sample pH would be desirable. In this way, not only could the possible factor interactions be established, but also the systems optimum could be identified because good Cu recoveries were achieved. In accordance with previous considerations Žonly two statistically significant parameters, sample pH and ethanol percentage, and a nonstatistically significant one, eluent flow rate with a very low estimated effect, would be reconsidered., further experiments were performed in order to fine tune these variables without the necessity of applying another factorial design. It is important to consider that the main objective of this work was to optimize the system by reducing the number of experiments and the process time. Two experiments were performed keeping all the other factors at optimum values and varying the values of sample pH between 6 and 8, the results obtained were similar. A pH of 8 is considered as the optimal value because seawater is strongly buffered by the hydrogencarbonate᎐ carbonate᎐carbon dioxide system at approximately this pH. Two additional experiments were conducted to fine-tune the ethanol percentage; the considered values were 20 and 40%; the experiment performed at 40% gave a slightly better recovery.
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Since eluent flow rate hardly affects the Cu recovery and high values of this variable lead to overpressure problems and difficult mini-column handling, this variable has been fixed at a low value. Although high effluent flows cause a Cu population in the flame Žhigh sensitivity., previous experiments in our laboratory suggest that 3.5 ml miny1 is a good compromise value. The last column in Table 2 shows the optimal values of factors affecting the studied system for copper determination. 3.3. Study of sample ¨ olume Large sample volumes are permitted to be pre-concentrated in mini-columns due to their great capacity. In this study, good recoveries Ž98᎐101%. were obtained by passing through mini-columns 2.5 ml of a copper standard solution of 30 g ly1 , 75 ml of a copper standard solution of 1 g ly1 , and 100 ml of a copper standard solution of 0.5 g ly1 . In all instances, eluent volume was 333 l.
4. Analytical figures of merit Cu determinations with different sample volumes Ž2.5, 75 and 100 ml. were carried out by using calibration graphs performed under the optimal experimental conditions. Equations of the calibration curves are as follows: By direct aspiration of copper solution: Absorbance s 1.5 = 10y4 q 5.8 = 10y5 C Ž n s 7. and C s 73.3᎐5000 g ly1 copper. Sample volume 2.5 ml: Absorbance s 3.3 = 10y4 q 4.3 = 10y4 C Ž n s 7. and C s 9.5᎐673.7 g ly1 copper. Sample volume 75 ml: Absorbance s 2.1 = 10y4 q 0.0128C Ž n s 7. and C s 0.3᎐22.6 g ly1 copper. Sample volume 100 ml: Absorbance s 4.7 = 10y4 q 0.0171C Ž n s 7. and C s 0.2᎐16.9 g ly1 copper.
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M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
Experimental enrichment factors of 7.4, 220.7 and 294.8 were obtained for 2.5, 75 and 100 ml of sample volumes, respectively, calculated as the ratio between the slopes of the calibration graphs obtained by each sample volume used and the direct aspiration of Cu solution. The theoretical pre-concentration factors were 7.5, 225.2 and 300.3 Žfor 2.5, 75 and 100 ml, respectively., which confirms the quantitative recovery and the low dispersion of the FI system. Detection limits were defined as three times the S.D. of the peak height for 30 determinations of the blank Žultrapure water. and were 2.8, 0.08 and 0.05 g ly1 for the studied sample volumes of 2.5, 75 and 100 ml, respectively. The quantification limit, calculated as the blank signal plus 10 times its S.D. were 9.4, 0.28 and 0.17 g ly1 for sample volumes, 2.5, 75 and 100 ml, respectively. The pre-concentration factor, detection and quantitation limits could be further improved by increasing sample volume without degradation in the efficiency due to the favorable kinetics of this system. The optimized method was proposed for Cu determination in seawater, if the mean Cu concentration is approximately 0.25 g ly1 , whereas in marine water w18x, a sample volume of 100 ml was considered as the optimal value. The precision of the method obtained for standard solutions containing 0.5 Ž n s 11. and 2 Ž n s 11. g ly1 of copper were 3.8 and 1.1%, respectively, expressed as relative standard deviation ŽR.S.D... Finally, the overall procedure was validated using two certified reference materials ŽSLEW-3, estuarine water and NASS-5, seawater. with a Cu concentration of 1.55" 0.12 g ly1 and 0.297" 0.046 g ly1 , respectively. The copper contents obtained Žmean " S.D., n s 2. were 1.52" 0.03 g ly1 and 0.29" 0.04 g ly1 , which are agree with the certified values.
5. Effect of sample matrix ions A study of interferents was performed by analyzing synthetic seawater solutions Ž100 ml. containing 5 g ly1 of CuŽII. and the most important constituents of seawater w18x. The results are
Table 3 Effect of sample matrix ions on copper recovery Ion
Ratio wionxrwCux
Recovery Ž%.
Al3q Ba2q Ca2q Cr3q Fe3q Kq Mg2q Mn2q Naq Ni2q Pb2q Sr2q Zn2q CO3 2y Cly Fy Iy NO3y SO42y PO43y
2:1 15:1 4 = 105 :1 1:1 2:1 4 = 105 :1 1.3= 106 :1 1:1 6 = 106 :1 1:1 1:1 8 = 103 :1 150:1 3 = 104 :1 2 = 107 :1 1.5= 103 :1 1:60 1.1= 104 :1 2 = 106 :1 60:1
97.0 103.0 97.0 100 95.4 100 95.4 97.0 95.4 103.0 97.0 100 95.4 103.0 97.0 103.0 100 100 97.0 103.0
shown in Table 3. The presence of large amounts of alkali and alkaline earth metals has no influence on the recovery of copper Ž- 5%.. This study shows that copper can be quantitatively determined in seawater samples.
6. Stability of mini-columns and sorption capacity Stability studies were performed at optimal conditions of the whole procedure. Mini-columns with retained CuŽII. were kept for 2, 7, 15 and 30 days at room temperatures and at 4⬚C Žrefrigerator.. Results showed that the mini-columns are stable for 2 days at room temperature and at least 30 days at 4⬚C. The sorption capacity of the Amberlite XAD-4 chelating resin loaded with PAR for the retention of copper was also determined. Increasing amounts of copper were passed through a minicolumn containing 0.05 g of chelating resin. The results demonstrated that the resin has a sorption capacity of 2.2 mol copper per gram of dry resin.
M.C. Yebra et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 57 (2002) 85᎐93
7. Conclusions Pre-concentration and elimination of the sample matrix was successfully achieved by the FFPS by using a chelating mini-column packed with Amberlite XAD-4 loaded with PAR. One of the main advantages of the present portable FFPS is that the problems associated with sample transport and storage were avoided. Furthermore, it is the simplest system possible because the seawater samples requires no pH adjustment. The proposed FFPS using Amberlite XAD-4 minicolumns coated with PAR and the FI-AAS determination of copper in seawater samples is simple, sensitive and accurate. Copper was quantitatively recovered from the mini-column with a high precision. The use of factorial design for assessing the main factors that involve all procedures ŽFFPS and FI elution. has a number of advantages over the conventional univariate method, such as fast screening of factors with a relatively small number of experiments and the estimation of the main effects. In addition, factorial design can be used to quickly assess the robustness of the FFPS and FI elution manifolds. In conclusion, the proposed method is excellent as regards simplicity, sensitivity, precision, accuracy, selectivity and mini-column stability.
w4 x
w5x
w6x
w7x
w8x
w9x
w10x w11x
w12x
Acknowledgements The authors are grateful for the financial support provided by the Galicia Government ŽXunta de Galicia. in the framework of Project PGIDT99PX120901A.
w13x
w14x
References w1x S.E. Manahan, Environmental Chemistry, CRC Press, Boca Raton, 1994. w2x A.M. Naghmush, M. Trojanowicz, E.W. Olbrych-Sleszynska, Flow injection flame atomic spectrometric determination of copper with preconcentration on ligand loaded Amberlite XAD-2, J. Anal. At. Spectrom. 7 Ž1992. 323᎐328. w3x R.E. Santelli, M. Gallego, M. Valcarcel, Preconcentration and atomic absorption determination of copper traces in waters by online adsorption᎐elution on an activated carbon minicolumn, Talanta 41 Ž1994. 817᎐823.
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