Ricardo E.
Rivas
,
Ignacio
López-García
and
Manuel
Hernández-Córdoba
*
Department of Analytical Chemistry, Faculty of Chemistry, University of Murcia, 30071, Murcia, Spain. E-mail: hcordoba@um.es; Fax: +34 868 884148
First published on 8th January 2010
A procedure for the determination of ultratraces of lead and cadmium by liquid-phase microextraction based on solidification of a floating organic droplet (LPME-SFO) separation and electrothermal atomic absorption spectrometry (ETAAS) has been developed. For this purpose, 50 μL of pre-heated (50 °C) undecanoic acid (UA) are added to 25 mL of aqueous sample solution at pH 5. The mixture, maintained at 50 °C, is stirred at 1000 rpm for 10 min and then the vial is immersed in an ice-bath, which results in the solidification of the UA drop that is easily separated. Injection into the atomizer is carried out after a gentle heating. By impregnating the atomizer with tungstate as a permanent chemical modifier, the detection limits were 10 and 0.5 ng L−1 with enrichment factors of 380 and 420, for lead and cadmium, respectively. The relative standard deviation was in the 2.8–3.2% range (n = 5, 25–400 ng L−1 Pb(II) and 1–15 ng L−1 Cd(II)). The proposed method has been applied to the determination of lead and cadmium in bottled, tap and sea water samples, the reliability of the results being verified by means of recovery tests and by using ICP-MS.
In recent years there has been a growing interest in the development of miniaturized preconcentration methods based on liquid–liquid or solid phase extraction,4,5 an approach that allows high preconcentration factors to be obtained, thus rendering the ETAAS determination feasible even at such low levels. There is a large diversity of closely related miniaturized techniques for the purpose of preconcentration, dispersive liquid–liquid microextraction (DLLME)6,7 being one of the most significant for inorganic ultratrace analysis. One of these techniques is that termed liquid phase microextraction by solidification of a floating organic droplet (LPME-SFO).8 Here, a few microlitres of an organic solvent with a low melting point are used for extraction. The aqueous sample solution is stirred and maintained at such a temperature that the organic droplet remains melted in order so that the extraction occurs. Once the process is finished and the liquid cooled, the solidified extractant floats on the aqueous sample and can be easily separated by means of a spatula. This technique has been successfully applied using undecanol and dodecanol to concentrate substances like trihalomethanes,9 organochlorine10 and organophosphorus pesticides,11 polycyclic aromatic hydrocarbons,8 pyrazoline derivatives12 and some metals like lead,13 cobalt and nickel.14
Recently, a modification to this technique has been proposed, in which the extractant is mixed with a suitable amount of a dispersing agent, like in DLLME. Thus, the analyte extraction process has been reported to be very rapid, the emulsion being broken by centrifugation. This combination of DLLME with LPME-SFO has been named as dispersive liquid–liquid microextraction with solidification of a floating organic droplet (DLLME-SFO).15,16
As has been pointed out, the solvent to be used for LPME-SFO should have a low volatility, as well as a low melting point14 and, up to date 1-undecanol (m.p. 11–14 °C) and 2-dodecanol (m.p. 22–26 °C) have been recommended.13,14 This manuscript reports our studies for lead and cadmium preconcentration by using undecanoic acid (UA), that acts both as the complexing and extracting agent. The physico-chemical properties of UA are similar to those of the reagents cited, but its use can be advantageous since no additional complexing agent is required. On the contrary, it is important to bear in mind that UA is a weak acid (pK close to 5) so that its solubility in water increases in alkaline medium hindering its use for the purpose here considered. Since the UA melting point is low (28–31 °C) it is suitable for extraction using the solidified organic drop approach. LPME-SFO proved to be more effective than DLLME-SFO for this purpose. After the preconcentration stage, final measurement was carried out by ETAAS and, since UA has a relatively low boiling point, its easy elimination during the heating cycle allowed low background values to be obtained. In this way, the optimized procedure makes available the measurement of very low levels of these elements to most of the laboratories worldwide.
Temperature programme for conditioning the impregnated atomizer | |||
---|---|---|---|
Step | Temperature, °C | Ramp, °C s−1 | Hold, s |
1 | 120 | 1 | 120 |
2 | 200 | 5 | 120 |
3 | 1200 | 5 | 30 |
4 | 2400 | 1 | 6 |
To decrease the risk of lead and cadmium contamination, only plastic (polypropylene) vessels were used for preparing and storing the solutions. Pipette tips were also of polypropylene. All plasticware was nitric acid-washed and rinsed with ultrapure water.
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Fig. 1 Effect of some experimental parameters. Graph A: Effect of pH on the signal obtained using the recommended conditions of solutions containing 200 and 5 ng L−1 lead and cadmium (curves a and b), respectively; Graph B: Volumetric ratio of aqueous to organic phase for lead (curve a) and cadmium (curve b), respectively; the volume of the organic phase was fixed at 50 μL; Graph C: Effect of the extraction time for both first and second extraction (curves a and b, respectively) of a solution containing 200 ng L−1 lead. |
An important aspect to be considered is the efficiency of preconcentration. To this purpose, the effect of varying the volume of the aqueous phase maintaining a 50 μL UA constant volume was studied. There was a linear behavior up to 50 mL which corresponded to an aqueous to organic volume ratio of 1000. However, this high volume ratio is not recommended since it requires more contact time between the two phases to reach an equilibrium, and so a ratio of 500 which corresponds to 25 mL of aqueous phase and 50 μL of organic phase was selected (Fig. 1B). In these conditions, an apparent efficiency of preconcentration of 380 and 420 times for lead and cadmium, respectively, was obtained indicating a high extraction percentage. These values were calculated by dividing the calibration slope obtained from aqueous solutions submitted to the microextraction process by the calibration slope obtained for aqueous solutions that were not extracted. In the latter case, the heating programme shown in Table 1 was slightly modified by using 130 °C as the drying temperature. No significant differences in sensitivity were noted when the measurements were carried out in the aqueous solutions or UA extracts.
The volume of UA used as the extractant was also studied in the 15 to 50 μL range. Very small volumes are not advisable because a part of UA dissolves in the aqueous solution during the process. In the conditions recommended in Experimental, it was found that when 50, 40 and 30 μL UA were used, the amounts of organic phase recovered after the cooling stage (18–20 °C) were approximately 40, 30 and 20 μL, respectively, that is, about 10 μL UA passed to the aqueous phase. In order to have a suitable volume of organic phase to be transferred to the atomizer in a safe way, the recommendation is to use 50 μL UA.
The effect of the extraction time was studied for both analytes and the results for lead are shown in Fig. 1C. As can be seen, the signal obtained (curve a) was practically constant and reached a maximum after 10 min. Similar results were found for cadmium. On the other hand, to verify the high percentage of extraction, each solution was extracted twice, the analytical signal being also obtained after the second extraction. The results for lead shown in Fig. 1C (curve b) prove that although the extraction was not complete, the percentage of analyte extracted was very high. The values calculated, that taking into account the way in which they were obtained have to be considered as indicative, were 92 and 94% for lead and cadmium, respectively.
The relevance of using a high stirring rate to fragment the UA drop into droplets, thus speeding up the extraction process should be emphasized. Using a 29 mm outer diameter vial containing 25 ml of aqueous solution, the height of the liquid column is about 50 mm. When the solution is stirred (1000 rpm) by means of a 10 × 3 mm stir bar, a vortex that extends down to the base of the vial is obtained. In this way, the 50 μL UA drop is easily fragmented into small droplets that easily regroup when the stirring speed is reduced to 300 rpm. On the other hand, the extraction temperature has to be high enough to allow UA to be in the liquid form. However, high temperatures lead to excessive UA solubility in the aqueous phase, while the global process slows down because more time is needed for cooling the solution after extraction. The recommended value is 50 °C.
The ionic strength was varied by incorporating potassium nitrate in the 0.01–1 mol L−1 range, and no effect was observed. The effect of potential interferences was also examined (Table 2). In these experiments, solutions of 200 and 5 ng L−1 Pb(II) and Cd(II), respectively, containing the added interfering ions were treated according to the recommended procedure under the optimal conditions. The common cations and anions present in natural water possess no adverse effects (maximum amounts tested 5 g L−1). The presence of more than 10 mg L−1 of Ni2+, Fe3+, Al3+ and Cr3+ led to a decrease of 10% in the analytical signal. This is not a serious drawback since the sensitivity is so high that the sample may be diluted, if necessary.
Characteristic | Pb(II) | Cd(II) |
---|---|---|
a Calculated on the basis of 3sy/x. b Compared with direct injection of aqueous solutions. | ||
Linear dynamic range (ng L−1) | 25–400 | 1–15 |
Detection limit a (ng L−1) | 10 | 0.5 |
Calibration function (5 standards, n = 3) | A int = 0.034 + 0.0011·CPb | A int = 0.025 + 0.041·CCd |
Correlation coefficient | 0.9985 | 0.9978 |
Relative standard deviation (n = 5) (%) | 2.8 | 3.2 |
Sampling frequency (samples h−1) | 3 | 3 |
Sample volume (mL) | 25 | 25 |
Preconcentration factor b | 380 | 420 |
Extraction method | Analyte | Detection technique | Preconcentration factor | Extraction time (min) | Sample volume (mL) | Linear range (μg L−1) | Ref. |
---|---|---|---|---|---|---|---|
a Dispersive liquid–liquid microextraction. b Sequential injection dispersive liquid–liquid microextraction. c Single drop microextraction. d Continuous flow single drop microextraction. e Ionic liquid single drop microextraction. f Ionic liquid ultrasound assisted dispersive liquid–liquid microextraction. g Liquid-phase microextraction. h Cloud point extraction. | |||||||
DLLME a | Pb | FAAS | 450 | 0 | 25 | 1–70 | 19 |
SI-DLLME b | Pb | FAAS | 265 | 2 | 12 | 2.3–260 | 20 |
SDME c | Pb | ETAAS | 16 | 20 | 1 | 0–40 | 21 |
CF-SDME d | Pb | ETAAS | 45 | 15 | 7.5 | 0–60 | 22 |
IL-SDME e | Pb | ETV-ICP-MS | 60 | 10 | 1.5 | 0.05–40 | 23 |
IL-SDME | Pb | ETAAS | 76 | 7 | 1.75 | 0.025–0.8 | 24 |
IL-USA-DLLME f | Cd | ETAAS | 67 | 2 | 10 | 0.02–0.15 | 25 |
LPME g | Cd | ETAAS | 390 | 15 | 2 | 0.01–1 | 26 |
LPME-SFO | Cd | FI-FAAS | 640 | 15 | 160 | 0.08–30 | 27 |
SDME | Cd | ETAAS | 65 | 10 | 5 | 0.01–1 | 28 |
SDME | Pb,Cd | ETC-ICP-MS | 190,140 | 15 | 0.2 | 0.01–50 | 29 |
HF-LPME | Pb,Cd | ICP-MS | 73,29 | 15 | 2.5 | 0.02–30 | 30 |
CPE h | Pb,Cd | FI-FAAS | ∼18 | 5 | 15 | 25–2000, 2.5–500 | 31 |
DLLME | Pb,Cd | ETAAS | 115 | 0 | 5 | 0.03–1, 0.01–0.3 | 18 |
LPME-SFO | Pb,Cd | ETAAS | 380,420 | 10 | 25 | 0.025–0.4, 0.001–0.015 | This work |
The proposed procedure was applied to the determination of cadmium and lead in twelve samples, namely seven bottled mineral waters of different brands, three tap waters and two sea waters obtained near of the shore but faraway of a small harbour. In all cases, recovery tests were carried out to check the reliability of the measurements (Table 5). The results for the mineral waters, that are not shown for shortening, provided values in the 16–180 and 2.5–9 ng L−1 ranges for lead and cadmium, respectively. Despite the very low DLs of the procedure, no cadmium nor lead could be detected in two of these samples. The differences found between the samples labeled as tap water 1 and 2 are noteworthy. The first sample was obtained in a 25-years old lab, while the second correspond to a building provided with a modern water supply system. The reliability of the procedure was checked, in addition, by comparing the results with those obtained by means of ICP-MS (Table 5), and the application of a common comparison test (Wilcoxon test) did not reveal significant differences at the 95% confidence level.
Sample | Added (ng L−1) | Found a (ng L−1) | ICP-MS valuesb (ng L−1) | |||
---|---|---|---|---|---|---|
Pb(II) | Cd(II) | Pb(II) | Cd(II) | Pb(II) | Cd(II) | |
a Mean of five determinations ± standard deviation, values in brackets indicate relative recovery (%). b Mean of three determinations ± standard deviation. | ||||||
Tap water 1 | 0 | 0 | 320 ± 4 | 64 ± 0.8 | 307 ± 9 | 56 ± 2 |
200 | 5 | 513 ± 5 | 68.9 ± 0.9 | |||
(96.5) | (98) | |||||
Tap water 2 | 0 | 0 | 85 ± 2 | 16.2 ± 0.7 | 91 ± 5 | 19 ± 3 |
200 | 5 | 286 ± 3 | 21.1 ± 0.6 | |||
(100.5) | (98) | |||||
Tap water treated with a domestic inverse osmosis system | 0 | 0 | 52 ± 2 | 3.1 ± 0.5 | 50 ± 6 | 5 ± 1 |
200 | 5 | 251 ± 3 | 8.2 ± 0.6 | |||
(99.5) | (102) | |||||
Sea water 1 | 0 | 0 | 67 ± 2 | 13.5 ± 0.6 | 60 ± 5 | 11 ± 2 |
200 | 5 | 261 ± 3 | 18.4 ± 0.6 | |||
(97) | (98) | |||||
Sea water 2 | 0 | 0 | 158 ± 5 | 27 ± 0.9 | 167 ± 8 | 32 ± 3 |
200 | 5 | 357 ± 6 | 32.1 ± 0.8 | |||
(99.5) | (102) |
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