Headspace-Liquid Phase Microextraction for Attenuated Total

Mar 8, 2010 - A combination of headspace (HS) sampling and liquid phase microextraction (LPME) has been successfully developed to solve sensitivity ...
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Anal. Chem. 2010, 82, 3045–3051

Headspace-Liquid Phase Microextraction for Attenuated Total Reflection Infrared Determination of Volatile Organic Compounds at Trace Levels Ana Gonza´lvez, Salvador Garrigues, Sergio Armenta,* and Miguel de la Guardia Department of Analytical Chemistry, Research Building, University of Valencia, 50 th Dr. Moliner St., E-46100 Burjassot, Valencia, Spain A combination of headspace (HS) sampling and liquid phase microextraction (LPME) has been successfully developed to solve sensitivity problems in attenuated total reflection (ATR) infrared determination of volatile organic compounds (VOCs). The HS sampling facilitates the selective extraction of the target volatile analytes from the sample matrix, while the liquid phase microextraction allows their preconcentration prior to infrared analysis. The direct determination of extracted analytes in the acceptor solvent provides high preconcentration factors of the order of 200 with a reduced consumption of organic solvents and a minimum generation of wastes, being thus the developed methodology a green alternative method. The qualitative and quantitative capability of the proposed approach has been evaluated on the basis of two different examples: (i) screening of benzene, toluene and xylene (BTX) compounds in soil samples and (ii) quantitative determination of toluene in cosmetic nail products. Infrared (IR) spectroscopy is a workhorse technique for qualitative and quantitative analysis. It is a molecular specific detection technique which provides the advantages of direct analysis, spectral selectivity, nondestructive nature and fast monitoring of the whole spectrum, thus, providing a high sampling frequency, for organic and inorganic compounds.1 The two main limitations of the technique are the problems encountered in the analysis of aqueous samples caused by the spectral interference of water, especially when classical transmission measurements are used, and the limited sensitivity of the technique which frequently reduces its scope to the determination of major and minor components. The use of IR cells with optical path length of the order of few µm minimizes the interference of water, but, at the same time, reduces the sensitivity of measurements. On the other hand, the use of attenuated total reflection (ATR) as sampling mode overcomes the limitations of the use of conventional transmission IR mode in handling aqueous solutions, because ATR measurements are based on an evanescent field that penetrates into the aqueous solution for a reduced distance, from a tenth of a µm to several µm. However, it is easy to imagine that the short penetration also limits the sensitivity of the technique. * To whom correspondence should be addressed. Phone: 0034 96 354 31 59. Fax: 0034 96 354 48 38. E-mail: [email protected]. (1) Stuart, B. Infrared Spectroscopy: Fundamentals and Applications; John Wiley & Sons: Chichester, 2004. 10.1021/ac1001838  2010 American Chemical Society Published on Web 03/08/2010

Another way to overcome the water absorption in IR measurements is the generation of a vapor phase from the samples and the determination of the analytes in the gaseous phase, thus reducing drastically matrix effects and increasing the optical path length.2 However, the aforementioned methodology has a reduced sensitivity and application range. To increase the sensitivity of IR procedures, several approaches have been developed in the literature; such as the use of powerful IR sources (quantum cascade,3 CO or CO24 and tunable diode5 lasers) or the combination of preconcentration techniques such as solid phase extraction (SPE)6 or solid phase microextraction (SPME)7 with IR measurements. Recent studies have been reported the use of SPE in combination with transmission measurements for the analysis of carbohydrates in beer8 using agarose polymer beads and caffeine in soft drinks using C18 silica beads.9 On the other hand, ATR sensors generally require the use of a polymer coating the internal reflection element to preconcentrate the analyte. Various polymer films have been investigated as sensing phases. Polyisobutylene has been employed to determine volatile organic compounds (VOCs) in aqueous solutions,10 poly(vinylchloride) for the analysis of chlorinated hydrocarbons and pesticides11 and BTEX,12 silicone for aromatic hydrocarbons determination in aqueous solutions,13 parafilm for the determination of VOCs in water14 and Teflon AF2400, poly(acrylonitrile-co-butadiene) (PAB), polydimethylsiloxane (PDMS), and poly(styrene-co-butadiene) (2) Armenta, S.; Garrigues, S.; de la Guardia, M. TrAC Trends Anal. Chem. 2008, 27, 15–23. (3) Ko ¨lhed, M.; Haberkorn, M.; Pustogov, V.; Mizaikoff, B.; Frank, J.; Karlberg, B.; Lendl, B. Vib. Spectrosc. 2002, 29, 283–289. (4) Mendelson, Y.; Lin, B. C.; Peura, R. A.; Clermont, A. C. Appl. Opt. 1988, 27, 5077–5081. (5) Beckwith, P. H.; Brown, C. E.; Danagher, D. J.; Smith, D. R.; Reid, J. Appl. Opt. 1987, 26, 2643–2649. (6) Daghbouche, Y.; Garrigues, S.; Vidal, M. T.; de la Guardia, M. Anal. Chem. 1997, 69, 1086–1091. (7) Bryant, C. K.; LaPuma, P. T.; Hook, G. L.; Houser, E. J. Anal. Chem. 2007, 79, 2334–2340. (8) Haberkorn, M.; Hinsmann, P.; Lendl, B. Analyst 2002, 127, 109–113. (9) Armenta, S.; Lendl, B. Vib. Spectrosc. 2009, 51, 60–64. (10) Yang, J.; Her, J. W. Anal. Chem. 2000, 72, 878–884. (11) Walsh, J. E.; Maccraith, B. D.; Meaney, M.; Vos, J. G.; Regan, F.; Lancia, A.; Artjushenko, S. Analyst 1996, 121, 789–792. (12) Regan, F.; Walsh, F.; Walsh, J. Int. J. Environ. Anal. Chem. 2003, 83, 621– 631. (13) Albuquerque, J. S.; Pimentel, M. F.; Silva, V. L.; Raimundo Jr, I. M.; Rohwedder, J. J. R.; Pasquini, C. Anal. Chem. 2005, 77, 72–77. (14) Heglund, D. L.; Tilotta, D. C. Environ. Sci. Technol. 1996, 30, 1212–1219.

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Figure 1. Schematic diagram of the manifold of the HS-LPME ATR system.

(PSB)15 have been compared for the determination of VOCs in water. However, the polymer used for internal reflection element coating should have additional properties than analyte enrichment; such as, low MIR absorption in the fingerprint region of the analyte, fast enrichment time, and reversibility, which not always are possible. Moreover, ATR spectroscopy has been also employed in combination with SPE without any external coating substance in the internal reflection element for the quantitative determination of caffeine in soft drinks.16 In summary, SPE and SPME techniques have demonstrated their usefulness in the preconcentration of organic compounds from aqueous solution. However, in most cases, the coating of the internal reflection element is required, thus reducing the versatility of the system and complicating the determinations. As far as we know, there are no precedents on the combination of liquid phase microextraction (LPME) with IR spectroscopy to increase the sensitivity of IR measurements. LPME is a technique which integrates sampling, extraction and preconcentration. In general, there are two modes of LPME sampling: direct-immersed LPME and headspace LPME (HS-LPME). In the direct mode, the organic solvent droplet is immersed into the stirred aqueous sample and the analytes are extracted into the organic droplet based on passive diffusion. In the HS-LPME the droplet is exposed above the interface of the air and the aqueous sample.17 The method is fast, simple, inexpensive and it can be considered as a green analytical procedure because it requires little solvent and produces little waste.18 (15) Flavin, K.; Hughes, H.; Dobbyn, V.; Kirwan, P.; Murphy, K.; Steiners, H.; Mizaikoff, B.; Mcloughlin, P. Int. J. Environ. Anal. Chem. 2006, 86, 401– 415. (16) Alcudia-Leon, M. C.; Lucena, R.; Cardenas, S.; Valcarcel, M. Anal. Chem. 2008, 80, 1146–1151. (17) Theis, L.; Waldack, A. J.; Hansen, S. M.; Jeannot, M. A. Anal. Chem. 2001, 73, 5651–5654. (18) Armenta, S.; Garrigues, S.; de la Guardia, M. TrAC Trends Anal. Chem. 2008, 27, 497–511.

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In summary, the aim of the present paper has been to develop an appropriate methodology which combines the use of HS sampling, LPME preconcentration and ATR measurement to enhance, in a simple way, the sensitivity and selectivity of IR spectroscopy. The vaporized analytes from a solid or liquid sample are dissolved in the acceptor solvent which is measured by ATRFTIR. The evaluation of the different experimental parameters and the solution of problems encountered in the analysis of samples with relatively high water content will be explained in detail. The capability of the HS-LPME ATR procedure for the qualitative and quantitative analysis of VOCs in different matrices will be evidenced on the basis of two examples: (i) A semiquantitative determination of benzene, toluene and xylene (BTX) in soil samples that can be used as screening methodology and (ii) the quantitative determination of toluene in commercial nail cosmetic products. METHOD DEVELOPMENT The developed procedure involves three steps: (i) analyte vaporization, (ii) transfer, and (iii) analyte dissolution, as it can be seen in Figure 1. The untreated solid or liquid sample is heated inside a 10 mL headspace vial and the analytes vaporized transferred by using a stainless steel line. Few µL of an accepting solvent are used to retain vaporized analytes from the samples. Afterward, the dissolved analytes can be detected and determined by infrared spectroscopy using the ATR sampling mode. The extraction-preconcentration procedure works within three different phases, the original solid phase or liquid phase, the generated gas phase and the acceptor organic liquid phase, and so, analytes were placed in a complex network environment due to the relation with many equilibrium states. As a necessary consequence, many kind of parameters can affect the enrichment factor in the HSLPME. Basically, the amount of analytes present in the accepting solvent should be related to their volatility and their solubility in

the accepting solvent but also depend on the nature of the sample, the sample amount and temperature. During the first step, the temperature and the heating time are the main parameters to be controlled, which affect the vapor pressure of the analytes and control the efficiency of the analytes evaporation. In the second step, the generated vapors are transferred from the sample HS vessel to the acceptor solvent by using a stainless steel tube. Compared to static headspace, dynamic headspace, with open end system, increases the speed of transferring the vaporized analytes to the accepting solution. Finally, the efficiency of the dissolution step is influenced by several factors, such as the nature and physicochemical characteristics of the accepting solvent and the solubility of the analytes in it, the temperature of the vapor and the presence of moisture in the vapor phase. The main difference of the developed procedure in front of conventional HS-LPME is that the extracting solvent remains at room temperature separated from the heated sample. Thus, the effect of vapor pressure of the extracting solvent on the enrichment factor is not as critical as solubility of the analytes on the selected organic solvent. EXPERIMENTAL SECTION Reagents. Benzene, toluene, p-xylene, chloroform, trichloroethylene, tetrachloroethylene, methanol, isopropanol, acetone, methyl ethyl ketone, butyl acetate, and ethyl acetate were used as probe molecules for representing the different volatilities of organic compounds and were obtained from Merck (Darmstadt, Germany). Acetonitrile, also purchased from Merck, was selected as accepting solvent due to its miscibility with the majority of volatile organic compounds. Headspace-Liquid Phase Microextraction Device. A 10 mL headspace vial was placed for 15 min on a dry block heater for tubes Multiplaces JP Selecta (Barcelona, Spain) set at 120 °C controlled temperature. The Multiplaces block heater allows the simultaneous treatment of 15 samples. HS-LPME sampling was performed using 2 mL chromatographic vials equipped with 200 µL inserts filled with 30 µL of acetonitrile, as indicated in Figure 1. A flexible stainless steel capillary of 0.12 mm internal diameter and a total length of 100 mm, commonly employed in liquid chromatography, was used as transfer line. It is important to highlight that one end of the transfer tube was introduced inside of the accepting solution to improve the contact between the VOCs and acetonitrile. Additionally the transfer tube was placed on the block heater to control its temperature and to avoid condensations of the vapor phase generated. Moreover, a needle was introduced in the insert of the chromatographic vial to allow a dynamic headspace sampling. The open end of the system increases the speed of transferring the vaporized analytes to the accepting solution. A Hamilton 100 µL syringe (Bonaduz, Switzerland) was used to fill acetonitrile into the chromatographic vials and to transfer this solution to the ATR sampling device. Infrared Spectroscopy. For ATR spectra acquisition, it was employed a DuraSamplIR II accessory from Smiths Detection Inc. (Warrington, UK) equipped with a nine reflection diamond/ZnSe DuraDisk plate, installed on a Bruker FTIR spectrometer (Bremen, Germany) model Tensor 27 with a KBr beamspliter and a DLaTGS detector. The scanner of the interferometer was operated at an HeNe laser modulation frequency of 10 kHz. Spectra were

Figure 2. Effect of the heating time on the IR signals for the analysis by HS-LPME ATR of 0.1 and 1 µL acetone and toluene injected in the headspace vial at 120 °C, using a stainless steel capillary of 0.12 mm internal diameter and a total length of 100 mm as transfer line and 30 µL acetonitrile as acceptor solvent. Values correspond to the mean value of three independent injections.

recorded in the mid infrared region, from 4000 to 400 cm-1, with a spectra resolution of 4 cm-1, a zerofilling value of 2 and a Blackman-Harris 3-term apodization function, averaging 25 scans per spectrum. For instrumental and measurement control, spectra treatment and data manipulation, it was employed the OPUS program (version 6.5) from Bruker. ATR spectra have been corrected using the atmospheric correction function to remove interferences from water vapor and carbon dioxide. To increase the sensitivity of the methodology a Bruker IFS 66/v FTIR spectrometer equipped with a liquid nitrogen refrigerated mercury-cadmium-telluride (MCT) detector was used. The scanner for the interferometer was operated at an HeNe laser modulation frequency of 80 kHz and 250 scans per spectrum were cumulated at 4 cm-1 nominal resolution. RESULTS AND DISCUSSION Experimental Parameters Affecting the HS-LPME Procedure. As it has been aforementioned, temperature is the most important variable in the vaporization step. An increase in the temperature will provide an increase in the vapor pressure of the analytes, which could improve substantially their extraction efficiency from the sample matrix. In this stage, an elevated temperature in the headspace vial was preferred and, thus for samples with a reduced water content, 120 °C could be the best selection. Another important parameter in the analyte volatilization step is the heating time. Time profiles of infrared signals will be related to the volatilization speed of organic compounds, which is directly related to the heating temperature in the vial and the volatility of the considered compounds. To study the effect of the heating time on the ATR signal, two model compounds, acetone and toluene, with different volatilities, were selected. Their amounts were fixed at two different levels, 1 and 0.1 µL inside the headspace vial, and the heating temperature of the vial fixed at 120 °C. Figure 2 shows the effect of the heating time on the IR signals. Peak area values from 1245 to 1200 cm-1 and from 762 to 712 cm-1 were used as analytical response for the analysis of acetone and toluene, respectively. Several conclusions can be extracted from this figure. First of all, it can be seen that the signals of 1 µL Analytical Chemistry, Vol. 82, No. 7, April 1, 2010

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Figure 3. Effect of the diameter of the transfer tubes on the IR signal of 1 µL toluene injection in the headspace vial at 120 °C for 15 min, using 30 µL acetonitrile as acceptor solvent. Values correspond to the mean value of three independent injections.

injection of toluene and acetone are approximately 10 times higher than those obtained for 0.1 µL injection after 15 min heating time. Moreover, it can be seen that for 1 µL toluene and acetone injection the infrared signal increases gradually until a constant value is achieved after approximately 10 min, being the IR signal approximately the same for both analytes after 10 min heating time. Comparing data obtained after 2 min heating time, it can be observed that the IR signal of acetone is higher than that of toluene, due to the higher volatility of acetone. However, decreasing the amount of the VOCs injected in the HS vial, the time profile of the IR signal differs considerably, being the IR signal practically constant during all the time range evaluated. Taking into consideration these results, a heating time of 15 min was selected for further experiments. The transfer of the VOCs from the headspace vial to the acceptor solvent can be affected by the nature, length, and diameter of the tube. The transfer tube was placed in contact with the dry block heater in order to maintain an elevated temperature inside it, otherwise, volatile compounds can condensate. Different tubes, from 0.12 to 4 mm ID, made of polypropylene, Teflon and stainless steel were tested, fixing their length at 10 mm; which was the minimum distance to facilitate the handling of the acceptor solvent vial and to maintain this solution at room temperature far enough from the dry block heater. Figure 3 shows the effect of the diameter of the transfer tubes on the FTIR signal obtained from 1 µL toluene injection. A reduction of the internal diameter together with the use of stainless steel increases the analytical response and the repeatability, probably due to the best control of the temperature in the transfer tube which avoids partial condensation of the analytes. The effect of the volume of acceptor solvent on the FTIR signal was also studied. As it was expected, on reducing the volume of acetonitrile, an increase of the FTIR signal can be observed, due to the increase of the preconcentration factor of the analyte. However, there is a minimum volume of acceptor solvent that can be used to avoid its appreciable losses by volatilization inside the chromatographic vial and also to cover completely the surface of the ATR cell. In this sense, the FTIR signal of five independent injections of 1 µL acetone + 1 µL toluene in the headspace vial, using 50 and 30 µL of acetonitrile as acceptor solvent were measured and compared (see Figure 4). The signals obtained using 30 µL of acceptor solvent were approximately 1.8 times higher than those obtained with 50 µL. On the other hand, the 3048

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Figure 4. Effect of the volume of acceptor solvent on the IR signal of 1 µL acetone and 1 µL toluene injection in the headspace vial at 120 °C for 15 min using a stainless steel capillary of 0.12 mm internal diameter and a total length of 100 mm as transfer line. A: acetone absorption bands; T: toluene absorption bands.

Figure 5. IR spectra of probe molecules measured using the HSLPME ATR method. IR bands highlighted with gray color correspond to the analytical signals used for analytical feature calculations. The infrared spectra found for a spiked soil sample and a cosmetic product were included in the top of the figure.

RSD values of both measurements were comparable, approximately 7.5 and 7.0% for toluene and acetone, respectively. In short, taking into consideration the acetonitrile density, the HS-LPME procedure developed provides a preconcentration factor of the order of 200 for a sample amount of 5 g. Qualitative Aspects. The IR spectra of different probe molecules measured using the aforementioned methodology are shown in Figure 5. All the spectra were obtained cumulating 25 scans per spectrum with 4 cm-1 resolution from 1 µL of each pure compound introduced inside the headspace vial. As it can

Table 1. Analytical Features of Merit of the Determination of Different VOCs Using the HS-LPME ATR Procedure calibration line

LOD

VOCs

boiling point (°C)

slope

intercept

R2

nL

µg g-1 a

RSD (%)b

benzene toluene p-xylene chloroform trichloroethylene tetrachloroethylene methanol isopropanol acetone methyl ethyl ketone ethyl acetate butyl acetate

80.1 110.6 138.3 61.2 87.2 121.1 64.7 82.4 56.5 79.6 77.1 126

0.000665 ± 0.000005 0.000414 ± 0.000014 0.000270 ± 0.00003 0.000994 ± 0.000013 0.000352 ± 0.000004 0.000182 ± 0.000010 0.001027 ± 0.000019 0.00062 ± 0.00002 0.001208 ± 0.000007 0.000612 ± 0.000003 0.001979 ± 0.000012 0.00217 ± 0.00008

-0.003 ± 0.002 0.005 ± 0.006 -0.0013 ± 0.0013 0.009 ± 0.011 -0.004 ± 0.003 0.010 ± 0.010 0.004 ± 0.007 0.007 ± 0.006 0.000 ± 0.012 0.003 ± 0.014 0.007 ± 0.006 -0.004 ± 0.007

0.9996 0.999 0.9999 0.9999 0.9998 0.996 0.9999 0.998 0.9998 0.9992 0.999 0.998

12 54 46 23 60 97 26 36 31 84 44 58

2 9 8 7 18 31 4 6 5 13 8 10

6.2 6.9 8.4 4.8 5.3 8.5 4.8 3.7 6.5 5.5 3.6 7.7

a

LOD expressed for a sample mass of 5 g. b RSD obtained from 6 independent analysis of a 0.5 µL injection of pure standard solution.

be seen a wide variety of VOCs, including aromatic hydrocarbons, chlorinated hydrocarbons, alcohols, ketones and esters, were tested. The comparison of the obtained IR spectra with those reported previously in the spectral libraries19 provides a perfect matching. So, it can be seen that IR spectroscopy is a molecular specific detection technique which provides fingerprint details of the analytes with similar molecular structures for their unambiguous characterization. Additionally, the spectra obtained from the treatment of a soil sample spiked with 0.5 µL of a solution containing benzene, toluene, and xylene (1:1:1 v/v) and that of a cosmetic nail product can be seen in the top of Figure 5. Clearly, the presence of the spiked compounds and that of toluene, ethyl acetate and butyl acetate can be identified. The spectral simplicity is an attribute of the HS-LPME which removes the spectral contribution of the sample matrix, thus providing the fingerprint of the VOCs present. It simplifies the unambiguous identification of VOCs in relatively complex samples where the direct ATR analysis is hindered by nonvolatile compound interferences. Quantitative Aspects. Table 1 shows the analytical figures of merit of the ATR determination of several VOCs analyzed by the developed HS-LPME procedure. The analytical response used for the analysis of each VOC, corresponding to the area of the IR band with the maximum intensity, is highlighted in gray in Figure 5. The linearity of the methodology was explored through the examination of the correlation coefficients of the calibration curves obtained by injecting different volumes of pure solvents in the HS vial from 0.1 to 10 µL. The values of the correlation coefficients were close to the unity, indicating a linear correspondence between the FTIR signal and the analyte volume. The precision of the methodology, expressed as the relative standard deviation (RSD) was established from six independent measurements of a 0.5 µL injection of pure solvents in the headspace vial. As it can be seen in the Table 1, those values varied between 3.6 and 8.5%, for ethyl acetate and tetrachloroethylene, respectively. The absolute limits of detection (LOD), calculated as three times the standard deviation of the IR signal obtained from the injection of a 0.5 µL injection of pure solvent divided by the slope (19) Stein, S. E. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg MD, 2008; http://webbook.nist. gov.

Table 2. Calibration line and Limit of Detection of the Determination of Different VOCs Using the HS-LPME ATR Procedure and the MCT Detector calibration line VOCs benzene toluene p-xylene chloroform trichloroethylene tetrachloroethylene methanol isopropanol acetone methyl ethyl ketone ethyl acetate butyl acetate a

LOD

slope

intercept

R

µg g-1 a

0.0005431 ± 0.0000009 0.0002620 ± 0.0000018 0.0001190 ± 0.0000008 0.001067 ± 0.000010 0.000263 ± 0.000004 0.000389 ± 0.000005 0.000604 ± 0.000005 0.0001284 ± 0.0000004 0.000961 ± 0.000006 0.001280 ± 0.000013

0.0029 ± 0.0019 0.002 ± 0.004 0.0025 ± 0.0015 0.00 ± 0.03 0.003 ± 0.015 0.004 ± 0.018 0.003 ± 0.009 0.0008 ± 0.0008 0.002 ± 0.011 0.00 ± 0.02

0.9999 0.9999 0.9999 0.9998 0.9992 0.9995 0.9998 0.9999 0.9998 0.9997

1.1 1.9 4.2 0.5 2.3 2.2 2.6 1.2 0.8 1.8

0.00 ± 0.05 0.9993 0.007 ± 0.006 0.9998

1.1 1.8

0.00198 ± 0.00002 0.000792 ± 0.000003

2

LOD expressed for a sample mass of 5 g.

of the calibration line, were lower than 100 nL, which corresponds to the low ppm range, for a sample mass of 5 g. In order to improve the detection limits of the methodology, the DLaTGS detector was replaced by a MCT one, the scanner velocity was increased to 80 kHz HeNe laser modulation frequency and the number of cumulated scans per spectrum was fixed at 250, providing a significant reduction of the spectral noise without sacrificing sampling frequency. As it can be seen in Table 2, the LODs of the procedure have been reduced by a factor ranging from 2 to 14, depending on the analyte. Those values varied from 500 to 4200 ng g-1 obtained for chloroform and p-xylene, respectively. Application of the HS-LPME ATR to the Analysis of Real Samples. Semiquantitative Analysis: BTX in Contaminated Soils. One of the most straightforward applications of this new technique is the analysis of VOCs in soil samples. In environmental site characterization, the collection of many samples is usually required, making these studies very expensive and time-consuming. For this reason, new screening techniques to do an efficient monitoring are essential.20 HS sampling has been used successfully in qualitative screening and quantitative analysis of soils and waters for VOCs in combination with GC.21,22 (20) The Harzadous Materials Training and Research Institute (HMTRI), Site Characterization: Sampling and Analysis (Preserving the Legacy); John Wiley and Sons Inc.: Hoboken, NJ, 2002.

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Table 3. Recovery Data Found for the Analysis of BTX Compounds in Spiked Soil Samples by HS-LPME ATR Procedure amount added (nL-µg g-1)

compound

amount found (nL-µg g-1)

recovery data (%)

50-8.8 8.7 8.6 100-17.5 17.3 17.2 500-87.6 86.7 86.1

benzene toluene xylene benzene toluene xylene benzene toluene xylene

56-9.8 -a 65-11.4 104-18.2 85-14.9 125-21.9 517-90.6 571-100.1 538-94.3

113 -a 131 104 85 125 103 115 107

a

Figure 6. Effect of the amount of Na2SO4 on the IR signal for the analysis of BTX compounds in soil samples.

However, this methodology is time-consuming and the use of fast methods for sample screening could reduce considerably the number of samples to be analyzed by HS-GC, improving substantially the productivity of the laboratory. BTX concentration in uncontaminated areas would be very low and elevated concentrations of BTX in soils are most likely to result from localized spills of petroleum products from historical use and management practices or related to industrial usage. As an example, the environmental agency of the UK reported in 2004 the Soil Guideline Value for Toluene depending on the land-use.23 Those values vary from 3 to 14 mg kg-1 in residential use with plant uptake and from 150 to 680 mg kg-1 in commercial/ industrial uses. Values provided by the environmental agency of the UK for benzene in soils depending on their use vary from 0.03 mg kg-1 in residential use to 95 mg kg-1 in commercial uses.24 Taking into consideration the LOD obtained for BTX using the HS-LPME ATR developed procedure, it seems an appropriate methodology for at least a semiquantitative screening of BTX in soil samples. The main problem of the BTX determination in soil samples using the HS-LPME ATR method is the moisture of the soil. Usually, soil samples contain a certain amount of water which can strongly interferes BTX determination. The vaporized water is rapidly dissolved in the acetonitrile acceptor solvent distorting the IR spectra. Thus, it is absolutely necessary to reduce the amount of water vapor present in the headspace to obtain a good quality IR spectrum of the VOCs. To do it, different amounts of anhydrous Na2SO4 were mixed with 5 g of a soil sample and introduced inside the HS vial. As it can be seen in Figure 6, the interference of the moisture of the soil can be completely reduced by increasing the amount of the drying agent. Soil samples were sieved to obtain a fine powder, with particle sizes lower than 250 µm. Before being used as spiked sample, an (21) Voice, T. C.; Kolb, B. Environ. Sci. Technol. 1993, 27, 709–713. (22) Esteve-Turrillas, F. A.; Armenta, S.; Garrigues, S.; Pastor, A.; de la Guardia, M. Anal. Chim. Acta 2007, 587, 89–96. (23) UK Environmental agency, Soil Guideline Values for toluene in soil, Science Report SC050021/toluene SGV, 2009. http://www.environment-agency.gov. uk/static/documents/Research/SCHO0309BPQJ-e-e.pdf (accessed December 15, 2010). (24) UK Environmental agency, Soil Guideline Values for benzene in soil, Science Report SC050021/ benzene SGV, 2009. http://www.environment-agency. gov.uk/static/documents/Research/SCHO0309BPQI-e-e.pdf (accessed December 15, 2010).

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Below the LOD of the method.

IR spectrum of the sample to be spiked was obtained by HS-LPME ATR to ensure the absence of BTX in the soil, being observed that no absorption bands were present in the spectral region of interest. After that, BTX was added to the soil at different concentration levels, 5 g of sample portions were weighted inside the HS vials together with 5 g of anhydrous Na2SO4 and the vials were placed on a dry block set at 120 °C during 15 min. Using the set up of Figure 1, the VOCs were volatilized, transferred to the acceptor 30 µL of acetonitrile and measured by ATR-FTIR spectroscopy. The recovery data found for the considered compounds, at concentration levels from 8 to 88 µg g-1, in spiked soil samples varied from 85 to 131%, as it can be seen in Table 3, being thus acceptable the use of HS-LPME ATR procedure for a fast screening of BTX compounds in industrial or contaminated soil samples. Quantitative Application: Toluene in Cosmetic Products. The capability of the HS-LPME ATR for the quantitative analysis of VOCs has been evaluated through the determination of toluene in nail products. Toluene is a compound classified as a CMR substance of category 3 under Annex I to Directive 67/548/EEC of 27 June 1967 and its use in cosmetic products is restricted in Europe.25 The Scientific Committee on Consumer Products considers that toluene is safe from the general toxicological point of view when present up to 25% in nail products. On the other hand, the maximum level of toluene in nail products allowed by the U.S. Food and Drug Administration is 50%.26 So, there is a need for fast, precise and accurate analytical methodologies for the determination of toluene in commercial cosmetic products. To demonstrate the quantitative capability of the new HSLPME ATR method, several commercially available nail products made in Colombia (3), Venezuela (4), Brazil (1), Switzerland (1), and Spain (6) were analyzed and data found were compared with a reference method based on HS-GC-MS (see Table 4). The reference values for the samples used in this study were determined as follows: some mg of nail products were accurately weighted inside a 10 mL standard glass vial, which was capped and heated in a static headspace autosampler Thermo Finnigan model HS 2000 (Waltham, MA) at 120 °C for 15 min with shaking. (25) Official Journal of the European Union, COMMISSION DIRECTIVE 2009/ 6/EC, amending Council Directive 76/768/EEC, concerning cosmetic products, 2009. (26) U.S. Department of Health & Human Services, U.S. Food and Drug Administration, Nail Care Products, 2006.

Table 4. Results Obtained for Toluene Determination in Nail Products by the Developed HS-LPME ATR Procedure and the Reference HS-GC-MS Methoda sample

origin

HS-LPME ATRb

HS-GC-MS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Spain Spain Spain Spain Spain Colombia Brazil Venezuela Colombia Spain Switzerland Spain Spain Spain Colombia Venezuela Venezuela