Headspace solvent microextraction

Headspace solvent microextraction

Talanta 62 (2004) 265–270 Headspace solvent microextraction A new method applied to the preconcentration of 2-butoxyethanol from aqueous solutions in...

123KB Sizes 4 Downloads 214 Views

Talanta 62 (2004) 265–270

Headspace solvent microextraction A new method applied to the preconcentration of 2-butoxyethanol from aqueous solutions into a single microdrop Yadollah Yamini a , Mohammad Hojjati a , Majid Haji-Hosseini a , Mojtaba Shamsipur b,∗ a

Department of Chemistry, Tarbiat Modarres University, Tehran, Iran b Department of Chemistry, Razi University, Kermanshah, Iran

Received 29 January 2003; received in revised form 2 July 2003; accepted 23 July 2003

Abstract A new procedure and experimental setup for the headspace solvent microextraction of volatile organic materials from aqueous sample solutions is described. The extraction occurs by suspending a 3-␮l drop of the solvent from the tip of a microsyringe to the headspace of a stirred aqueous sample solution for a preset extraction time. The temperature of the microdrop and the bulk of sample solution should be kept constant at optimized values. The sample analyses were carried out by gas chromatography. The procedure was successfully applied to the extraction and determination of 2-butoxyethanol from content of some color samples used for painting the outer coverage of some machines such as coolers, refrigerator, cloths machine, etc. Parameters such as extraction time, nature of extraction solvent, size of microdrop, sample volume, stirring rate, ionic strength and pH of sample solution were studied and optimized, and the method performance was evaluated. © 2003 Elsevier B.V. All rights reserved. Keywords: Headspace solvent microextraction; 2-Butoxyethanol; Gas chromatography

1. Introduction During the past decade, special attention has been paid to sample preparation methods that ensure reduction of the amount of organic solvents, or even their complete elimination, and also keep the number of operations and processes to a minimum [1]. Among these methods are headspace sampling (HSS) [2,3] and solvent microextraction (SME) [4–11], which has been applied to the analysis of a broad type of real samples. In HSS [2], the sample is normally placed in a sealed vial and heated in an oven until the volatile compounds reach equilibrium with the gas phase. An aliquot of the gas phase is finally analyzed by gas chromatography (GC). HSS is not expensive, does not require complicated instrumentation and the use of organic solvents is not necessary. In fact, HSS is the most suitable for the analysis of samples with rather high contents of volatile organic compounds [3]. However, due ∗ Corresponding author. Tel.: +98-831-4223307; fax: +98-831-4228439. E-mail address: [email protected] (M. Shamsipur).

0039-9140/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2003.07.012

to the need for an elevated temperature or the introduction of some inert gas to the headspace, the precision diminishes notably in HSS procedures. SME is based on the traditional LLE technique but involves only a few microliters of organic solvent as extractant [4–11]. Here, a single microdrop (as extraction phase) at the end out of a tip of a GC microsyringe needle [6], is transferred to the injection port of the GC system. Subsequently, another approach termed as dynamic liquid-phase extraction uses a microsyringe as a separatory funnel [8]. In SME, only very immiscible solvents with water such as toluene and n-octane are applicable. Thus, this method is suitable for the extraction of nonpolar and moderately polar components. SME is attractive in terms of sensitivity, precision, analysis time, and relative simplicity. In order to take advantage of the high capabilities of both HSS and SME methods, Jeannot and coworkers [12] introduced a new headspace solvent microextraction system (HSME) using a single microliter-volume drop. In this system, extraction of analytes occurs via suspending a microliter drop of a proper nonaqueous solvent from the tip of a microsyringe that is located in the head space of a stirred

266

Y. Yamini et al. / Talanta 62 (2004) 265–270

aqueous sample solution thermostated at a given temperature for a pre-set extraction time. The drop remained at the tip of the microsyringe throughout the extraction period and then was retracted back into the needle and injected into a GC for the identification and quantification of the extracted analytes. The capability of HSME was studied for the extraction and determination of alcohols [13] and BTEX [14]. In the present study the feasibility of the proposed HSME system was demonstrated by the extraction and determination of 2-butoxyethanol from some color samples used by Absal Company (Tehran, Iran) for painting outer coverage of some of their products including coolers, heaters, refrigerators, washing machines and so on. It is worth mentioning that 2-butoxyethanol is a well-known emulsifier widely used in electroplating industry to improve the quality of deposited paints onto a part or an assembled product. The currently applied method for the determination of the 2-butoxyethanol content of colors is the static headspace gas chromatography [15], which is known to suffer not only from a considerably low precision, but also from a high cost. The decreased precision might be due to some leakage of the volatile sample from the syringe used for its transfer from the headspace into the GC. The use of an

auto-injection device in static headspace GC would also cause the increased cost of the method, in comparison to the procedure proposed in this paper.

2. Experimental 2.1. Chemicals Methanol, 1-propanol, 1-butanol, 1-pentanol, methyleneglycol acetate, cyclohexane, cyclododecance, 2-butoxyethanol (Merck, Darmstadt, Germany), benzylalcohol, toluene (Fluka, Busch, Switzerland), and dodecane (Aldrich, Milwakee, WI) were all of reagent grade and used as received. Sodium chloride, calcium chloride, and sodium sulfate (Merck) were of the highest purity available and used without any further purification except for vacuum drying. Color samples were obtained from Absal Company. 2.2. Apparatus The HSME apparatus designed is shown diagrammatically in Fig. 1. The apparatus was equipped with a 10-␮l

Fig. 1. Schematic diagram of the HSME apparatus.

Y. Yamini et al. / Talanta 62 (2004) 265–270

microsyringe with a utility stop (Hamilton, Reno, NV), a 7-ml extraction vial, and a magnetic stirrer bar of 8 mm length × 1.5 mm diameter (VWR Scientific Products, West Chester, PA). Two separate re-circulation cell compartments connected to two corresponding water baths were employed to control the temperature of the vial’s solution and the microsyringe needle, respectively. It should be noted that the diameter of the internal part of the re-circulating cell, made of stainless steel, was closely matched with the needle, so that the equilibrium temperature of needle can be reached rapidly. All analyses were carried out on a Hewlett-Packard (Avondale, PA) Model 5890 series II gas chromatograph equipped with a flame ionization detector. High purity helium (99.99%, Sabalan, Tehran) was used as the carrier gas at a constant flow of 4.0 ml min−1 . The injection port was held at 250 ◦ C and used in the split mode with a split flow of 20 ml min−1 . Separations were carried out on a 20 m × 0.52 mm fused-silica capillary column with a 0.5-␮m DB-1 coating (J&W Scientific, Folsom, CA). Oven temperature programming was used to facilitate separation with an initial over temperature of 80 ◦ C ramping at a rate of 50 ◦ C min−1 to a temperature of 100 ◦ C and then at a rate of 20 ◦ C min−1 to a final temperature of 150 ◦ C. 2.3. Procedure The 10-␮l microsyringe containing the appropriate solvent was clamped above the vial containing sample solution. The microsyringe was then lowered and its needle passed through the vial septum until the tip of the needle was 5–15 mm above the surface of the stirred sample solution (depending on the sample volume). Then, the syringe plunger was depressed so that the solvent drop was suspended from the needle’s tip. The drop was exposed to the head space of the stirring sample solution for pre-set extraction time. After extraction, the plunger was withdrawn and the microdrop was retracted into the microsyringe. The syringe was then removed from the top of the sample vial a 1 ␮l of 1000 ␮g ml−1 methyleneglycol acetate in water (as an internal standard) was drown up into the microsyringe and, finally, the mixture was injected into the gas chromatograph for analysis.

3. Results and discussion 3.1. Method development Method development was examined on 2-butoxyethanol from a univariate approach. The parameters influencing the HSME procedure including the extraction time, the temperature of microdrop and bulk analyte solution, the nature of solvent used as extractant, the volume of microdrop and analyte solution, the stirring speed, the ionic strength and pH of analyte solution were optimized independently. All quan-

267

Fig. 2. Extraction time profile for determination of optimum sampling time: 2-butoxyethanol concentration, 1000 ␮g ml−1 ; sample volume, 5 ml; water drop size, 2 ␮l; sample temperature, 25 ◦ C; microsyringe needle temperature, 5 ◦ C; ionic strength, 1 M of Na2 SO4 ; stirring rate, 400 rpm.

tifications made in this study were based on the relative peak area of analyte to the internal standard (methyleneglycol acetate), from the average of three replicate measurements. 3.2. Extraction time A series of 5.0-ml water samples spiked with 1000 ␮g ml−1 of 2-butoxyethanol were prepared and the variation of the analytical signal was studied as a function of extraction time (Fig. 2). In general, the amount of analyte transferred into the microdrop is expected to increase with increasing its exposure time to the headspace of the stirred sample solution. However, the HSME is not an exhaustive extraction method and the analyte is partitioned between the bulk aqueous phase, the headspace, and the microdrop. Thus, the amount of analyte transferred into the microdrop reaches its maximum when this equilibrium is established. Fig. 2 shows that the equilibrium is reached after about 20 min. Similar results were reported for the head space solid-phase microexraction of BTEX from aqueous samples [3]. In the SPME of such low volatility analytes as tributyltinchloride [19], PAHs [20] and organophosphorous pesticides [21], with equilibrium times greater than 30 min, usually an extraction times 20–30 min have been selected in order to extract enough analytes for the analysis. A relatively similar extraction time for the static headspace extraction of PAHs is also reported in the literature [22]. It is worth noting that, for the quantitative analysis, it is not necessary for the analytes to reach the equilibrium. But instead, to only allow a sufficient mass transfer into the microdrop in an exact reproducible extraction time is adequate [11]. Thus, in the present work a 15-min extraction time was adopted for further studies. 3.3. Sample solution and microdrop temperature The temperature at which the sample solution is equilibrated strongly affects the amount of analyte partitioned into the headspace of the solution. Fig. 3A shows that the amount

268

Y. Yamini et al. / Talanta 62 (2004) 265–270

Fig. 4. Effect of nature of microdrop solvent on extraction efficiency: 2-butoxyethanol concentration, 1000 ␮g ml−1 ; sample volume, 1 ml; water drop size, 3 ␮l; microsyringe needle temperature, 4 ◦ C; sample temperature, 50 ◦ C; ionic strength, 1 M of Na2 SO4 ; extraction time, 15 min; stirring rate, 400 rpm: (1) propanol, (2) butanol, (3) pentanol, (4) benzyl alcohol, (5) methylen glycol, (6) toluene, (7) dodecane, (8) cyclohexane, (9) cyclododecane, (10) water.

Fig. 3. Effect of sample solution (A) and microsyringe needle (B) temperature on extraction efficiency: 2-butoxyethanol concentration, 1000 ␮g ml−1 ; sample volume, 5 ml; water drop size, 2 ␮l; microsyringe needle temperature, 4 ◦ C (in A); sample temperature, 50 ◦ C (in B); ionic strength, 1 M of Na2 SO4 ; extraction time, 15 min; stirring rate, 400 rpm.

of analyte delivered into the microdrop, and consequently the sensitivity of the method, is increased with increasing temperature of the stirred sample solution. In order to avoid over-pressurization of the sample vial (and the consequent problems and damages) in the HSME method, it is recommended not to work at very high temperatures. Thus, the sample solution temperature was held at 50 ◦ C for further studies. The over-pressurization phenomenon is also a critical problem in SPME as well as in classical static headspace methods. Fig. 3B shows the influence of the microsyringe needle temperature (at a range of 2.5–25.0 ◦ C) on the extraction efficiency of 2-butoxyethanol. As is obvious, the lower the needle temperature, the higher the extraction efficiency. Thus, further extractions were performed at a temperature of 4.0 ◦ C. It should be noted that, over an extraction period of 15-min, the needle temperature is close to the temperature of cooling bath, but the temperature of microdrop is not expected to be exactly the same as the temperature of the cooling bath.

benzyl alcohol and, at a lower extent, water revealed the highest extraction efficiencies in the series. Thus, in the light of these results, benzyl alcohol was chosen as the extraction solvent for the remainder of the study. This was despite the fact that, due to the safely considerations and the green chemistry involved, water can also be employed as a suitable extraction solvent. It should be noted that because of the exothermic behavior of the dissolution of the head space vapors in microdrop solvent, the use of a water microdrops are also expected to result in a considerable pre-concentration of the analyte, under the experimental conditions employed. It is noteworthy that, since the use of water as solvent may damage the stationary phase of the GC columns, it would be safer to use LC instead of GC for sample analysis. 3.5. Microdrop and sample volume Fig. 5A shows the influence of the microdrop volume on the extraction efficiency of 2-butoxyethanol. As it was

3.4. Nature of microdrop solvent In order to determine which solvent would be optimal for the extraction of 2-butoxyethanol from water samples, several solvents were examined. Each extraction solvent was tested using aqueous solutions containing 5 ml of 1000 ␮g ml−1 of 2-buloxyethanol and the results are shown in Fig. 4. It is seen that, among 10 different solvents tested,

Fig. 5. Effect of microdrop volume (A) and sample volume (B) on efficiency: 2-butoxyethanol concentration, 1000 ␮g ml−1 ; sample volume, 5 ml (in A); water drop size, 3 ␮l (in B); microsyringe needle temperature, 4 ◦ C; sample temperature, 50 ◦ C; ionic strength, 1 M of Na2 SO4 ; extraction time, 15 min; stirring rate, 400 rpm.

Y. Yamini et al. / Talanta 62 (2004) 265–270

expected, an increase in the volume of the microdrop (up to 3 ␮l) resulted in a sharp enhancement in the extraction efficiency of the system. However, at larger volumes (i.e. >3 ␮l), the microdrop revealed a great tendency to fall down from the tip of the microsyringe. Thus, a 3-␮l drop size was chosen as the optimized volume. It should be noted that, after an exposure time of 15 min, the volume of a 3-␮l microdrop of benzylalcohol at 4 ◦ C found to increase by 4–7%, presumably due to condensation of some water vapors on the droplet, while, under the same experimental conditions, the change in volume of a 3-␮l microdrop of water was negligible. Since all previous extractions had been done on 5-ml aqueous samples, the next stage of method development investigated the effect of sample volume on extraction efficiency (Fig. 5B). As seen, extractions from samples having volumes of 0.5, 1, 2, 5, and 6 ml were performed and sample volume of 1 ml was found to give the optimum extraction. 3.6. Ionic strength and pH of test solution Fig. 6A shows the influence of salt addition (Na2 SO4 ) on the extraction efficiency of 2-butoxyethanol. As is immediately obvious, the extraction efficiency increases with increasing salt concentration in the test solution. It should be noted that the suitability of the head space techniques for the extraction of organic compounds from aqueous solutions depends on the transfer of analyte from aqueous solution into the gaseous phase [16]. Since, in this work, 2-butoxyethanol possesses a relatively low volatility and high water solubility, its mass transfer from the aqueous phase to the gaseous state is presumably the rate-determining step in the HSME process. In this case, the increased ionic strength of the sample solution is expected to decrease the water solubil-

Fig. 6. Effect of ionic strength (A) and pH of sample solution (B) on extraction efficiency: 2-butoxyethanol concentration, 1000 ␮g ml−1 ; sample volume, 1 ml; water drop size, 3 ␮l; microsyringe needle temperature, 4 ◦ C; sample temperature, 50 ◦ C; ionic strength, 1 M of Na2 SO4 (in B); extraction time, 15 min; stirring rate, 400 rpm.

269

ity of 2-butoxyethanol and, consequently, to enhance the amount of the analyte in the head space. Thus, the presence of sodium sulfate at a 1 M concentration was recommended for the HSME analysis of 2-butoxyethanol. The pH of test solution is known to play a key role in the headspace extraction of analytes [10,17]. Usually, the analytes differ with regard to the pH at which they exist in an ionic form, while the form of analyte in the sample matrix is expected to affect significantly its extraction efficiency. In this study, the pH of the original aqueous solution was varied from 0 to 12 and the HSME procedure was followed to examine how the pH influences the extraction efficiency (Fig. 6B). The results shown in Fig. 6B revealed that the extraction efficiency of 2-butoxyethanol increases significantly with increasing pH of the sample solution up to a pH of 8, and then remains more of less constant. This is due to the possible protonation of the alcohol at lower pH values. However, for the sake of convenience and simplicity all extractions carried out at the natural pH of the original sample solutions (i.e. about pH 6). 3.7. Stirring speed Since the HSME method is based on an equilibrium distribution process, the maximum amount of analytes will be extracted at the equilibrium conditions. Sample stirring reduces the time required to reach the equilibrium and reduces extraction time by enhancing the diffusion of the analytes towards the microdrop, especially for higher molecular mass analytes [5,11]. Thus, in this work, the aqueous sample solution of the same 2-butoxyethanol contents was extracted at various stirring rates and the results are shown in Fig. 7. As it is seen, for a 15-min extraction time, the increase in stirring rate from zero to 400 rpm improved significantly the analytical signal. At higher stirring rates, the amount of analyte extracted continued to increases although with a very smaller slope.

Fig. 7. Effect of the stirring speed on extraction efficiency: 2-butoxyethanol concentration, 1000 ␮g ml−1 ; sample volume, 1 ml; water drop size, 3 ␮l; microsyringe needle temperature, 4 ◦ C; sample temperature, 50 ◦ C; ionic strength, 1 M of Na2 SO4 (in B); extraction time, 15 min.

270

Y. Yamini et al. / Talanta 62 (2004) 265–270

Thus, the extraction equilibrium may be approximately established at a stirring rate of 400 rpm, after 15 min. 3.8. Evaluation of method performance Calibration curves were calculated using six spiking levels of 2-butoxyethanol in the concentration range of 1–100 ␮g ml−1 (1 ␮l of 50 ␮g ml−1 methyleneglycol acetate as internal reference), 100–1000 ␮g ml−1 (1 ␮l of 500 ␮g ml−1 methyleneglycol as internal reference), and 1000–25,000 ␮g ml−1 (1 ␮l of 25,000 ␮g ml−1 methyleneglycol as internal reference). For each spiking level three replicate analyses were performed. The regression equations and the correlation coefficients (in parenthesis) were Ar = 0.68C + 2.52 (r = 0.992), Ar = 0.1055C − 4.65 (r = 0.993), and Ar = 0.090058C − 0.74 (r = 0.956), respectively, where Ar is relative area and C is concentration (␮g ml−1 ). The proposed HSME method revealed good reproducibility with R.S.D. values in the range of 2.0–15.0%. The limit of detection (LOD) was determined according to published guidelines by comparing the signal-to-noise (S/N) ratio of the lowest detectable concentration to a S/N of 3 [18]. The LOD thus determined was 0.5 ␮g ml−1 . In order to test the applicability of the proposed HSME method to real samples, two color samples containing 0.48 and 0.23% 2-butoxyethanol were obtained from Absal Company and tested by the recommended procedure. The 2-butoxyethanol content of the two samples were determined as 0.50% (R.S.D. = 2%) and 0.22% (R.S.D. = 6%), respectively.

4. Conclusions A new HSME using very low organic solvent (1–3 ␮l) and sample solution volumes (1–4 ml) has been described.

There is no need of dedicated and expensive apparatus for the proposed method. Although the precision and accuracy are still in need, the extreme simplicity and cost-effectiveness of HSME makes it quite attractive when compared to the SPME and such labor-intensive methods as LLE or SPE. The method was successfully applied to the extraction and determination of 2-butoxyethanol from color samples.

References [1] J. Namiesnik, W.J. Wardenck, High Resol. Chromatogr. 23 (2000) 297. [2] M.F. Mehran, N. Golkar, W.J. Cooper, J. Chromatogr. Sci. 34 (1996) 122. [3] J.C.F. Menendez, M.L.F. Sanchez, J.E.S. Uria, E.F. Martinez, A. Sanz-Medel, Anal. Chim. Acta 415 (2000) 9. [4] H. Liu, P.K. Dasgupta, Anal. Chem. 68 (1996) 1817. [5] M.A. Jeannot, F.F. Cantwell, Anal. Chem. 68 (1996) 2236. [6] M.A. Jeannot, F.F. Cantwell, Anal. Chem. 69 (1997) 235. [7] M.A. Jeannot, F.F. Cantwell, Anal. Chem. 69 (1997) 2935. [8] Y. He, H.K. Lee, Anal. Chem. 69 (1997) 4634. [9] L.S. de Jager, A.R.J. Andrews, Chromatographia 50 (1999) 733. [10] L.S. de Jager, A.R.J. Andrews, J. Chromatogr. A 911 (2001) 97. [11] E. Psillakis, N. Kalogerakis, J. Chromatogr. A 907 (2001) 211. [12] A.L. Theis, A.J. Waldack, S.M. Hansen, M.A. Jeannot, Anal. Chem. 73 (2001) 5651. [13] A. Tankeviciute, R. Kazlauskas, V. Vickackaite, Analyst 126 (2001) 1674. [14] A. Przyjazny, J.M. Kekoss, J. Chromathogr. A 977 (2002) 143. [15] Absal Company Catalogue, Absal, Tehran, Iran, 1998. [16] T. Gorecki, J. Pawliszyn, Analyst 122 (1997) 1079. [17] B.H. Hwang, M.R. Lee, J. Chromatogr. A 898 (2000) 245. [18] L.H. Keith, W. Grummett, J. Deegan, R.A. Libby, J.K. Taylor, G. Wntler, Anal. Chem. 55 (1983) 2210. [19] L. Ji-Yan, J. Gui-bin, Z. Qun-Fang, Y. Ke-Wu, J. Sep. Sci. 24 (2001) 459. [20] D.J. Djozan, Y. Assadi, Microchim. J. 63 (1999) 276. [21] D.A. Lambropoulou, T.A. Albanis, J. Chromatogr. A 922 (2001) 243. [22] J.C. Florez Menendez, M.L. Fernandez Sanchez, J.E. Sanchez Uria, E. Fernandez Martinez, Anal. Chim. Acta 415 (2000) 9.