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TECHNICAL NOTES
Continuous Liquid-Llquid Extraction for Preconcentration with On-Line Monitoring Manuel Agudo, Angel Rfos, and Miguel ValcPrcel' Department of Analytical Chemistry, University of Cbrdoba, Sun Albert0 Magno, sln, E-14004 Cbrdoba, Spain Liquid-liquid extraction is gaining interest as a tool for solving major selectivity and sensitivity problemsin a variety of determinations. The incorporation of liquid-liquid extraction units into continuous-flow systems provides several advantages over time-consuming manual procedures, particularly as regards automatability and sample throughput. Conventional liquid-liquid extraction flow assemblies typically use three basic units: a segmenter, an extraction coil, and a phase Separator.' Correct operation of the segmentor and separator is central to efficiency in the separation process, for which some simplifications have been proposed.2~3Then, iterative reversal flow methodology4 was used for a continuous liquid-liquid extraction by inserting a single plug of organic phase into a carrier stream (aqueous phase) containing the a ~ ~ a l y t The e . ~ gradual enrichment of the organic phase with the solute is continuously monitored, which allows kinetic measurements to be made.6 The new approach proposed in this paper is simple as it avoids the use of the segmenter, extraction coil, and phase separator and enables automatic controlled preconcentration. The Hg(II)/dithizone-carbon tetrachloride system was used to validate this methodology.
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FOUNDATION
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The key to the proposed methodology is the placement of a small volume of organic phase in a conventional cuvette, placed in measuring position in a photometer. The signal is zeroed-in duringthis f i i t step. Afterwards, the aqueous phase containing the analyte is passed through the organic phase and ita gradual enrichment with the analyte is continuously monitored. In this way, a large volume of sample (aqueous stream) can be passed through a small organic volume, to achieve a high preconcentration factor by controlling the overall volume of sample. In addition, the kinetics of the process can be exploited for determining the analyte. Figure 1 illustrates the overall process for the Hg(II)/dithizone carbon tetrachloride system; the position of the input and output tubes allows the sample to flow through the organic phase from bottom to top and the volume of aqueous phase (sample) in the cuvette is kept constant by withdrawal. In order to determine the optimal position of both tubes, the design shown in Figure l c was used as apseudo probe. When the process was finished, the organic plug was drained and a fresh volume introduced for the next experiment (see the manifold description under Experimental Section).
Fburr 1. Arrangement of the Input and output tubes In the photometric
(1) ValcArcel, M.; Luque de Castro, M. D. Non-Chromatographic Continuow Separation Techniques; Royal Society of Chemistry: Cnm-
cuvette for Ilquldllquld extractlon: (a) organic solvent loedlng (o.P., organic phase): (b) sample (a.p., aqueous phase) flowlng through the organic phase. Part c shows both tubes as they were flnally used.
bridge, UK, 1991. (2) SahleetrBm, Y.; Karlberg, B. Anal. Chim. Acta 1986,179, 315. (3) Audunseon, G. Anal. Chem. 1986,68,2714. (4) Rlos, A.; Luque de Castro, M. D.; Valcbcel, M. Anal. Chem. 1988, 60, 1640. (5) CaAete, F.; Rlm, A.; Luque de Castro, M. D.; Valcbcel, M. Anal. Chem. 1988,60,2354. (6) Caftete, F.; Rloe, A.; Luaue de Castro, M. D.: Valcbcel, M. Anal. Chim. Acta 1989,224, 169.
0003-2700/93/0365-2041$04.00/0
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output
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EXPERIMENTAL SECTION Reagents. All reagents used were of analytical grade, and solutions were prepared in ultrapure water. A dithizone solution was made by dissolving 0.005 g of compound (Riedel) in 100 mL of carbon tetrachloride (Merck). A stock solution containing 1.00g L-1 mercury(I1)prepared from mercury(I1)nitrate (Merck) and 0.1 M nitric acid (Merck) were also used. @ l9Q3 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
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PERISTALTIC PUMP
Blank (Washing) solution (0.1M HNO3)
AU X I L l ARY PUMP S PECTROPHOTOMET ER
Sample or blank
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Water
DISPLACEMENT FLASK
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Scheme of the proposed conflguration used for contlnuous liquid-llquld extraction: IV, injection valve; SV, switchlng valve. A
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B
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a 5 TIME (min) Flguro 4. Regression lines obtained from the recordings for (A) 1.0, (B) 0.1, and (C) 0.01 pg mL-I Hg(I1). I
I
2
4
I
6 TIME (min)
Recordings obtained for 1 pg mL-l Hg(I1) at different flow rates: (A) 2.0, (B) 4.5, (C) 6.0, (D) 7.6, and (E) 9.5 mL min-l.
Flgurr 3.
Apparatus. A Unicam 8625 single-beamspectrophotometer equipped with a Hellma QS cuvette (10-mm light path) and connectedto a Knauer z-t recorder was used. A Gilson Minipuls3, an Ismatecperistalticpump, and three Rheodyne 5041injection valves were also employed. Manifold. The assembly used to implement the methodology is depicted in Figure 2. The loop of the injection valve (IV) was filled with a fixed volume of the organic phase (carbon tetrachloride containing dithizone), for which a displacement flask was used in order to avoid passage of carbontetrachloridethrough the pump tubes. As IV was switched,the organicplug was driven by the blank solution stream (aqueousphase) to the photometric cuvette, where it was kept until each experiment was finished. After the organic plug was placed in the cuvette, the instrument was zeroed, which allowed one to pass a controlled volume of sample as required by switchingon the SVZ valve. At that point, the "selecting valve" SVI was switched on in order to flush the organic plug from the cuvette by having the auxiliary pump operate in the opposite direction. In this way, the system was made ready for a new experiment.
RESULTS AND DISCUSSION The UV-visible spectra of dithizone and Hg(II)-dithizonate, both dissolved in carbon tetrachloride, showed 500 nm to be be best wavelength for monitoring the extraction of mercury(I1) from the sample. A concentration of 93 mM dithizone in the organic phase resulted in the maximum possible difference between the blank and sample signals. The optimal pH for the samples (aqueous phase) waa 1.5, which provided the highest selectivity toward mercury. The ionic strength had no special effect on the signal; however, the repeatability was more favorable at an ionic strength of 0.1 M NaNOs, so the samples were adjusted to this ionic strength. The volume of organic phase used (introduced through the injection valve) had a significant influence. Small volumes resulted in poor precision (a minimum volume of ca. 100 p L was needed). On the other hand, volumes above ca. 300 p L gave rise to dramatically decreased sensitivity. A volume of 210 p L was finally selected. The flow rate also played a major role as it increased the sampling frequency. As the extraction process was quite fast, a high flow rate value could be used, which would allow substantial volumes of sample to be passed over short periods of time. Also, for a given overall amount of mercury(I1) in the samples, the measured signal was found to increase with increasing flow rates, probably as a result of a more efficient contact between
CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
ANALYTICAL
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Table I. Figures of Merit of the Proposed Method for the Determination of Mercury over Various Concentration Ranges sample vol regression det range eampliig detection limit equationb coefficient (ng mL-9 frequency (h-1) (ne mL-9 needed0 (mL) By Measuring the Slopes of Absorbance-Time Recordings 50 S 1.94 X 10% - 0.053 0.9983 1OOo-100 35 30 12 100 S 1.01 X 1 k 2 C- 0.127 0.9958 100-15 20 2 250 S = 1.39 X 1 k 2 C+ 0.074 0.9863 15-4 10 By Measuring the Signal at the End of the Experiment 50 100 250
A 2.07 X 10% A = 1.35 X 10% A = 2.62 X 10%
- 0.022
+ 0.001 + 0.020
0.9999 0.9967 0.9955
1 ~ 1 0 0 100-20 20-4
10 5 2
55 8 3
0 In order to complete the whole experiment. * S, slope of the absorbance-time recordings; A, absorbance, C, concentration of mercury(II), in ng mL-1.
both phases, but in fact because a higher amount of analyte passes through the organic phase (although the total time of contact between the phases decreases with increased flow rate). The maximum possible flow rate was thus chosen, viz. 9.4 mL min-1. In this way, high sample (aqueous-phase)/ organic-phase ratios were achieved in reasonable times that resulted in very favorable concentration factors of Hg(I1) in the organic phase. Figure 3 shows the absorbance-time signals obtained for an aqueous sample containing 1pg mL-1 Hg(I1) that was passed through the organic plug at different flow rate values. The size of the aqueous droplets rising through the organic phase has influence on extraction efficiency. The smaller the droplets the larger their surface area/volume ratio and thus the extraction. As the droplet size is governed by surface tension (and it is influenced by a wide variety of experimental variables), samples and standardsshould be carefully matched in order to avoid errors. Determination of Mercury(I1). The proposed method is applicable over a wide range of Hg(I1) concentration, as the volume of sample to be passed through the organic phase retained at the detection point can be varied over a wide range, thus enabling simultaneous preconcentration determination. Figure 4 shows the regression lines obtained from the recordings for 1.0, 0.1, and 0.01 r g mL-l monitored for 6 min. As can be seen, kinetic information inherent in the dynamic process involved is the most useful for this methodology. In fact, the slope (the rate of enrichment of the organic phase with the analyte), and the signal obtained at a fixed time (the largest signals were obtained at the final time), are suitable parameters for determination purposes. The results provided by applying the proposed method at three different mercury levels are given in Table I. As can be seen, the determination of mercury(I1) based on measurements of absorbance-time slopes on the recordings offer the most advantageous features from an analytical point of view, especially as regards the sampling frequency,since the measurements of slopes does not entail waiting for the experiment to complete. The precision of the method (expressed as relative standard deviation) at 60 and 6 ng mL-l was i6.4% and i8.5%, respectively (n = 8). Various synthetic samples of mercury(I1) [prepared by adding different amounts of mercury(I1)nitrate in a tap water matrix] were analyzed by using the proposed method. The results obtained from the slopes and fiial signal measurements are listed in Table 11.
Table 11. Analysis of Synthetic Samples of Mercury(I1) by Using the Proposed Method amt found (ng mL-l) amt found (ng mL-9 amt rial amt final added slope absorbance added slope absorbance (ngmL-1) method method ( n g d - 1 ) method method 60 50 40 30
80 100 70 250 500 750
57.3 49.2 39.2 31.6 82.5 98.3 69.6 247.7 501.2 749.3
56.0 51.8 41.5 31.4 81.6 101.0 70.9 249.2 500.7 749.6
8.0 6.0 5.0 4.0 10.0 20.0 35.0 15.0 25.0 30.0
7.8 5.8 4.8 4.1 9.1 19.2 34.3 14.5 24.6 30.6
7.8 6.1 5.3 3.8 9.2 20.6 35.2 15.3 25.4 30.5
CONCLUSIONS A simple,reliable automated methodology for liquid-liquid extraction in continuous flow systems were developed for the determination of mercury. The proposed methodology could be very useful for analyzing samples containing analytes at low concentrations since the preconcentration process takes place just at the detection point, where an additional reaction between the analyte (in the aqueous phase) and a chelating reagent dissolved in the organic plug can also be implemented. In this way, preconcentration, reaction, and detection are integrated in the same system (a conventionalcuvette). This multipurpose methodology can be extended to other specific applications,involving photometricor fluorometric detection. Also, the manifold can be altered to accommodate high analconcentrations in the samples. If needed, an injection valve can be used to insert very small volumes of samples (aqueous phase) relative to those used in this work, which was aimed at solving the sensitivity problems usually associated with trace and ultratrace analysis. As a result of small amounts of organic solvents needed by this methodology, the toxicity hazards of the methods thus developed will decrease.
ACKNOWLEDGMENT The Spanish CICyT is acknowledged for financial support awarded (GrantPB90/0925). M.A.isalsogratefultothe Junta de Andalucla for financial support received in the form of a fellowship.
RECEIVED for review May 24, 1993. Accepted July 8, 1993.
