Solvent-Assisted Headspace Sampling Using Solid Phase

Sep 7, 2015 - Despite these improvements, mass transfer into the headspace is a challenge, particularly for less volatile analytes, which influences t...
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Solvent-assisted headspace sampling using solid phase microextraction for the analysis of phenols in water Mosotho J. George, Ljiljana Marjanovic, and D. Bradley G. Williams Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b02539 • Publication Date (Web): 07 Sep 2015 Downloaded from http://pubs.acs.org on September 8, 2015

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Analytical Chemistry

Solvent-assisted headspace sampling using solid phase microextraction for the analysis of phenols in water Mosotho J. George,∫,† Ljiljana Marjanovic∫ and D. Bradley G. Williams*∫,‡ ∫

Department of Chemistry, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa. Department of Chemistry and Chemical Technology, National University of Lesotho, Roma 180, Lesotho. ‡ Ferrier Research Institute, Victoria University of Wellington, 69 Gracefield Rd, Lower Hutt, 5040, New Zealand. †

ABSTRACT: Headspace analysis is used widely and relies on volatilization of analytes into the headspace above the matrix. We detail the dramatic influence that added solvent can have on headspace analysis of phenols, without the requirement for specialized headspace vials. The use of water-immiscible solvents is key and leads to a 1–3 orders of magnitude enhancement in the volatilization of these analytes, and shorter fiber exposure times than are otherwise required.

Headspace analysis is indispensable for the analysis of the volatile contents of various matrices. When the analyte is highly volatile, direct measurements on the headspace contents are possible.1 In this way, the decarboxylation of fatty acids to terminal alkenes is measured with headspace FTIR and also using GC-MS,2 while CO can be similarly measured.3 When the analyte is less volatile or if it is present in small amounts, a pre-concentration technique is required. Some elegant advances have been made recently, including the development of a thin-film solvent based microextraction device for the determination of SO2 in foodstuffs.4 In other work, a headspace solvent microextraction method was employed for the analysis of derivatives of benzene5 and has been applied recently to the analysis of short chain fatty acids.6 The use of microfibers represents a particularly effective technique by which volatile and semi-volatile substances in a matrix or in solution may be pre-concentrated prior to analysis without the need to perform more laborious manipulations to prepare the matrix.7 Solid phase microextraction (SPME) fibers can be used for headspace analysis, in which a microfiber is inserted into the headspace and the matrix is typically heated to volatilize the analytes, which become adsorbed onto the fiber.8 The fiber device is then inserted into the injector port of a GC, where thermal desorption of the analytes occurs. Advances to the microfibers used for this process permit a wide range of materials to become adsorbed to be analyzed, and the technique is now a central feature of analytical chemistry.9 While numerous microfibers with varying properties are now commercially available for specific applications,10 experimental devices continue to show significant promise.11 For example, a silica-based ionogel incorporating ionic liquids has been found to be useful for the analysis of volatile aromatic compounds.12 In addition, a membrane SPME fiber has been developed and was found to be suitable for the determination of volatile organic compounds in water samples.13 Despite these improvements, mass transfer into the headspace is a challenge, particularly for less volatile analytes, which influences the three-phase equilibrium between the

solution, the headspace and the SPME fiber.14 In a steady-state kinetic model for headspace using solvent microextraction, the importance of the equilibrium between these states in determining the uptake kinetics of the analytes into the headspace has been demonstrated.15 While stirring and elevated temperatures improve kinetics and mass transfer, a widely applicable method to improve headspace analysis remains elusive. If such a method were to be devised, it would dramatically improve the sensitivity of headspace-based trace analysis. We herein describe a solvent-assisted approach to enhanced volatilization of waterborne organic compounds, using phenols as model analytes. This technique considerably improves the volatilization of the phenols, which are known environmental pollutants.16 This simple improvement and the benefits it brings are likely to be widely applicable in headspace-based analysis of trace organic compounds. While water-miscible solvents are commonly employed to aid the dissolution of components with low water solubility,17 or for residual solvent analysis of pharmaceutical preparations in accordance with USP Residual Solvents ,18 for example, the present findings confer a level of volatilization of phenols not attainable previously. For the headspace analyses, an aqueous sample (1 mL) containing a mixture of 5 ng.mL–1 each of phenol, 4-chlorophenol, 2,6-dichlorophenol, 5-chloro-2-methylphenol and 2,4,6trichlorophenol in a capped standard 2 mL GC vial was used. To this was added the organic solvent (100 µL of methanol, acetone, diethyl ether or dichloromethane, respectively) and the vial was shaken vigorously before being heated to the required temperature. Headspace sampling was performed with a SPME polydimethylsiloxane microfiber device once thermal equilibrium had been attained. All of the extractions were static. Headspace sampling was conducted for 20 minutes at 50 °C (Figure 1; all analyses were in triplicate; Supporting Information Table S1) and the microfiber was then introduced to the injector port of a GC-MS for analysis (see Supporting Information for experimental details). We were delighted by the results which showed increased efficiency of the headspace

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sampling in the presence of the added solvents. When using the water-immiscible solvents diethyl ether and dichloromethane, the volatilization of the analytes was dramatically higher than with water miscible solvents methanol and acetone. The immiscible solvents probably serve to extract and concentrate the analytes in the organic phase and then release them into the headspace. The effectiveness of diethyl ether was marginally higher than that of dichloromethane, most likely because of its lower density which caused it to float on the surface of the water while dichloromethane sinks to the bottom of the vial. Nonetheless, the sampling temperature of 50 °C was above the boiling point of dichloromethane (approximately 40 °C), which caused the solvent droplets to be small and to circulate upwards towards the surface of the water, buoyed by vapor bubbles, thereby releasing the solvent vapor and the analytes into the headspace. Even without optimization, diethyl ether facilitated volatilization-absorptionanalysis improvements over water alone of 18× for phenol, 3× for 4-chlorophenol, 3× for 2,6-dichlorophenol, 5× for 5chloro-2-methylphenol and 2× for 2,4,6-trichlorophenol, while DCM afforded improvements of 12×, 3×, 3×, 4× and 2×, respectively. With the water-miscible solvents, the enhanced volatilization of the analytes likely relates to co-evaporation of the solvent and analytes which would improve mass transfer between the liquid phase and the headspace. Elevated temperatures were a requirement: when this set of experiments was repeated at ambient temperatures, no significant advantages were noted in the presence of the solvents over the simple aqueous set up. All subsequent experiments were performed at a sampling temperature of 50 °C.

Figure 1. Effect of different solvents on the solvent-assisted headspace analysis of five phenols. Phe = phenol; CP = 4chlorophenol, DCP = 2,6-dichlorophenol; CMP = 5-chloro-2methylphenol and TCP = 2,4,6-trichlorophenol.

Typically, SPME extractions are characterized by relatively long extraction times (longer than 30 minutes19,20) due to lower mass transfer than solvent-based counterparts, such as liquidphase microextraction, where the extraction times are typically less than 30 minutes.21,22 In the present instance, there was a non-uniform response by the analytes to changes in the time allowed for the headspace sampling (Figure 2; Supporting Information Table S2). Most of the analytes benefited from longer exposure periods of the fiber to the headspace, but the benefits were reversed for phenol, possibly due to competitive displacement thereof on the fiber by the less water soluble analytes. This non-uniform response is consistent with the results obtained by Zhao et al.23 using an ionic liquid-based SPME where there was a reduction in extraction efficiency for some compounds with longer extraction times, which is indicative of a dynamic system. This effect has also been noted

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during headspace sampling using a polythiophene fiber,24 while phenols show complex nonlinear extraction–time profiles consistent with the displacement of phenol, with immersion SMPE sampling using a PA fiber.25 What is also evident from our results is that those analytes with the higher boiling points benefit most from the longer exposure times (bp phenol = 189 °C, 4-chlorophenol = 220 °C, 2,6-dichlorophenol = 233 °C, 5-chloro-2-methylphenol = 220 °C, 2,4,6-trichlorophenol = 246 °C). This is consistent with prior results where it was noted that, during SPME sampling, less highly volatile phenols benefit more from elevated temperatures and extended sampling periods than more volatile analogs.26 Overall, optimal efficiency for analysis of the mixture in our case was achieved at around 15 minutes’ exposure of the fiber to the headspace.

Figure 2. Time-dependent profile of solvent-assisted headspace analysis. Phe = phenol; CP = 4-chlorophenol, DCP = 2,6dichlorophenol; CMP = 5-chloro-2-methylphenol and TCP = 2,4,6-trichlorophenol.

The volume of diethyl ether added to the aqueous analyte solution was varied from 10–100 µL and the headspace of the mixtures was analyzed as before in triplicate (Figure 3, 15 minutes extraction; Supporting Information Table S3). There was an increase in efficiency with increasing organic solvent amount, reaching a maximum at 50 µL for most analytes, followed by a drop in efficiency. The increases likely relate to improved extraction efficiencies of the analytes into the solvent. Beyond a given point it is probable that dilution effects in the organic liquid phase outweigh the volatilization effects of the analytes into the headspace. This would become amplified by the headspace becoming saturated by solvent vapor which would force some amount of the solvent to remain in the liquid phase.

Figure 3. Effect of the volume of added solvent on the efficiency of the solvent-assisted headspace analysis. Phe = phenol; CP = 4chlorophenol, DCP = 2,6-dichlorophenol; CMP = 5-chloro-2methylphenol and TCP = 2,4,6-trichlorophenol.

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Enhanced extraction efficiency with an increase of the ionic strength of a solution is a widely-utilized phenomenon, and is one which we have used to great effect when developing the bubble-in-drop single-drop microextraction technique for the analysis of pesticides in various matrices.27-29 To investigate this phenomenon in the present instance, different NaCl solutions of the phenols were subjected to headspace extraction for 15 minutes following addition of diethyl ether (50 µL; Figure 4; Supporting Information Table S4). Notable improvements in extraction efficiency were secured with added NaCl from 0 to 5% NaCl (m/v). At and above 10% NaCl (m/v), the results become erratic with unacceptably high RSD values (RSD > 20%), but nevertheless demonstrated quite stunning enhancements to the solvent-assisted volatilization-absorption-analysis outcomes. Volatilization-absorption-analysis improvements attainable using water with added diethyl ether when adding 5% NaCl were 17× for phenol, 6× for 4-chlorophenol, 5× for 2,6-dichlorophenol, 5× for 5-chloro-2-methylphenol and 3× for 2,4,6-trichlorophenol. These values are generated by comparison against an identical set-up with added solvent but without the addition of NaCl.

Figure 5. Repeatability of solvent-assisted headspace analysis of phenols. Phe = phenol; CP = 4-chlorophenol, DCP = 2,6dichlorophenol; CMP = 5-chloro-2-methylphenol and TCP = 2,4,6-trichlorophenol. The final bar for each analyte represents the mean with its standard deviation.

the various sampling methods highlight immediately the improvements obtainable with an added water-immiscible solvent. Using the 5 ng.mL–1 working solutions, we obtained S/N ratios of 210, 270, 2450, 750 and 1940 for phenol, 4chlorophenol, 2,6-dichlorophenol, 5-chloro-2-methylphenol and 2,4,6-trichlorophenol, respectively, which afford a preliminary indication that low LOD and LOQ values will be obtained in the course of our follow up studies. Table 1. Ratio of measured values of various phenols sampled by different techniques, using conditions optimized for solvent-assisted headspace sampling Sampling methods SA-HS / HSa SA-HS / DI

a

Phe

CP

DCP

CMP

TCP

1220

42

25

40

9

16

31

6

6

3

a

Figure 4. Improved solvent-assisted headspace analysis due to increased ionic strength. Phe = phenol; CP = 4-chlorophenol, DCP = 2,6-dichlorophenol; CMP = 5-chloro-2-methylphenol and TCP = 2,4,6-trichlorophenol.

The vials could be sampled repetitively under optimum conditions without loss of precision (Figure 5; Supporting Information Table S5). The water/solvent mixture (50 µL diethyl ether, 5% NaCl, 15 minutes sampling time) was sampled n = 8 without further addition of solvent, with %RSD of 6– 12%. The results (Figure 5) show that the extraction is repeatable, suggesting a non-exhaustive adsorption of the analytes into the headspace and onto the fiber. Cumulative enhancements due to the presence of solvent spiked at the optimum level, NaCl at the optimum concentration and sampling for 15 minutes were 1–3 orders of magnitude over a solvent-free setup (1220× for phenol, 42× for 4-chlorophenol, 25× for 2,6dichlorophenol, 40× for 5-chloro-2-methylphenol and 9× for 2,4,6-trichlorophenol). We have investigated the use of a polyacrylate fiber for this type of headspace sampling but its ability to absorb and concentrate the analytes was far inferior to the PDMS fiber (by a factor of between 2 and 10). The benefits of the new solvent-assisted headspace sampling method becomes immediately evident when comparing data against direct immersion of the PDMS fiber into the aqueous solution of phenols at 5 ng.mL–1 and also headspace sampling without the addition of solvent (Table 1). All data were generated using the same fiber, with the sampling technique set up according to the optimized protocol. The ratios of

SA-HS is solvent-assisted headspace sampling; HS is headspace sampling; DI is direct immersion sampling.

In summary, the first solvent-assisted headspace analysis is demonstrated in this preliminary study. Significant preconcentration and analysis benefits are achieved by this simple manipulation of standard headspace sampling protocols. It is likely that this method will be widely applicable to the analysis of a range of substances that benefit from the dual effect of extraction into an organic solvent and volatilization from the analyte-enriched solvent phase into the headspace. The use of GC vials as opposed to larger headspace vials also reduces the amount of analyte needed, which can be advantageous when working with small samples. During these studies it was noted that the microfibers suffered some swelling, in a few cases leading to damage to the fiber when retracting it into the barrel of the SPME device for analysis. This effect has also been noted in the total volatilization-SPME method.30 Various fruitless efforts were made to reduce this phenomenon. Overcoming this surmountable hurdle will lead to widespread acceptance and adoption of this technique in (automated) trace analysis of organic substances. For example, a polydimethylsiloxane/divinylbenzene fiber has been found to be resistant to chloroform while polyethylene glycol fibers are resistant to dichloromethane.30 We are presently extending the study to address these issues and to assess the scope of this new technique to produce a validated method.

ASSOCIATED CONTENT Supporting Information

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Analytical data and experimental set-up details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Notes

25. Simões, N. G.; Cardoso, V. V.; Ferreira, E.; Benoliel, M. J.; Almeida, C. M. M.; Chemosphere 2007, 68, 501–510. 26. Portillo, M.; Prohibas, N.; Salvadó, V.; Simonet, B. M. J. Chromatogr. A 2006, 1103, 29–34. 27. George, M. J.; Marjanovic, L.; Williams, D. B. G. Talanta, 2015, 144, 445–450. 28. Williams, D. B. G.; George, M. J.; Marjanovic, L. J. Agric. Food Chem. 2014, 62, 7676–7681. 29. Williams, D. B. G., George, M. J., Meyer, R., Marjanovic. L. Anal. Chem. 2011, 83, 6713–6716. 30. Rainey, C. L.; Bors, D. E.; Goodpaster, J. V. Anal. Chem. 2014, 86, 11319–11325.

The authors declare no competing financial interests.

ACKNOWLEDGMENT We gratefully acknowledge the University of Johannesburg, Victoria University of Wellington, the NRF, and THRIP for funding of this project. M.J.G. thanks the National University of Lesotho for study leave.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

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Gad, M.; Zaazaa, H.; Amer, S.; Korany, M. RSC Adv. 2015, 5, 17150–17159. Grant, J. L.; Hsieh, C. H.; Makris, T. M. J. Am. Chem. Soc. 2015, 137, 4940–4943. Medina-Ramos, J.; Pupillo, R. C.; Keane, T. P.; di Meglio, J. L.; Rosenthal, J. J. Am. Chem. Soc. 2015, 137, 5021–5027. Deng, Z.; Chen, X.; Wang, Y.; Fang, E.; Zhang, Z.; Chen, X. Anal. Chem. 2015, 87, 633–640. Theis, A. L.; Waldack, A. J.; Hansen, S. M.; Jeannot, M. A. Anal. Chem. 2001, 73, 5651–5654. Chen, Y.; Li, Y.; Xiong, Y.; Fang, C.; Wang, X. J. Chromatogr. A. 2014, 1325, 49–55. Hengel, M. J.; Shibamoto, T. J. Agric. Food Chem. 2000, 48, 5825–5825. Vas, G.; Vékey, K. J. Mass Spectrom. 2004, 39, 233–254. Kumar, A.; Gaurav, Malik A. K.; Tewary D. K.; Singh, B. Anal. Chim. Acta. 2008, 610, 1–14. Cacho, J. I.; Campillo, N.; Viñas, P.; Hernández-Córdoba, M. J. Chromatogr. A. 2015, 1399, 18–24. Spietelun, A.; Kloskowski, A.; Chrzanowski, W.; Namieśnik, J. Chem. Rev. 2013, 113, 1667–1685. Pena-Pereira, F.; Marcinkowski, Ł.; Kloskowski, A.; Namieśnik, J. Anal. Chem. 2014, 86, 11640–11648. Spietelun, A.; Marcinkowski, Ł.; Kloskowski, A.; Namieśnik, J. Analyst. 2013, 138, 5099–5106. Kalua, C. M.; Boss, P. K. J. Chromatogr. A. 2008, 1192, 25– 35. Schnobrich, C. R.; Jeannot, M.A. J. Chromatogr. A 2008, 1215, 30–36. Michałowicz, J.; Duda, W. Pol. J. Environ. Stud. 2007, 16, 347–362. (a) Ho, T. D.; Joshi, M. D.; Silver, M. A.; Anderson, J. L. J. Chromatogr. A 2012, 1240, 29–44. (b) Shen, G.; Lee, H. K. Anal. Chem. 2003, 75, 98–103. http://www.usp.org/sites/default/files/usp_pdf/EN/USPNF/ge neralChapter467Current.pdf (accessed 18 June 2015) Simões, N. G.; Cardoso, V. V.; Ferreira, E.; Benoliel, M. J.; Almeida, C. M. M. Chemosphere. 2007, 68, 501–510. Campillo, N.; Peñalver, R.; Hernández-Córdoba, M. J. Chromatogr. A. 2006, 1125, 31–37. Huang, S.-P.; Huang, S.-D. J. Chromatogr. A 2007, 1176, 19– 25. Lambropoulou, D. A; Albanis, T. A. J. Biochem. Biophys. Methods. 2007, 70, 195–228. Zhao, F.; Lu, S.; Du, W.; Zeng, B. Microchim. Acta. 2009, 165, 29–33. Ma, Y.; Zhao, F.; Zeng, B. Talanta 2013, 104, 27–31.

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