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What Exactly Are You Filtering Out? Chun Kiat Ng‡,§ and Bin Cao*,†,‡ †

School of Civil and Environmental Engineering, Nanyang Technological University, Singapore Singapore Centre on Environmental Life Sciences Engineering, Nanyang Technological University, 639798, Singapore § Interdisciplinary Graduate School, Nanyang Technological University, 639798, Singapore or samples were prepared using a filtration approach, but the detailed information on the filters used in the studies was often missing. We argue that the filtration details are critical because the incorrect choice of filters may result in misleading results. As a proof of concept, we filtered 3 mL of 0.9% sodium chloride (NaCl) solution containing 5 ppm of estrone (E1), estradiol (E2), 17α-ethinyl estradiol (EE2) and testosterone, respectively, through a syringe filter housing a hydrophilic polyethersulfone (PES) membrane with an effective filtration area of 2.8 cm2 and pore size of 0.2 μm (Acrodisc Syringe Filters with Supor Membrane, Pall Corporation). We quantified the concentration of E1, E2, EE2 or testosterone in respective solution prior to and after filtration, and found that a significant amount of hormones (up to 96%) were retained by the hydrophilic PES membrane filter (Figure 1). The hydrophilic ‡

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o quantify organic contaminants in aqueous samples, filtration is often the final step before loading the samples into analytical instruments such as high performance liquid chromatography (HPLC) or gas chromatography (GC), to prevent possible clogging of the separation columns. Many laboratories, perhaps to lessen the logistical and financial burden, adopt a one-size-fit-all approach during the procurement of the filter, as long as deemed critical specifications, such as pore size, are adhered. Such filters are then used in procedures for many different applications. However, other specifications of the filters, for example, membrane materials, may drastically affect the filtration results. For example, polyethersulfone (PES) membrane-based syringe filters are commercially available filters that have been very widely used in environmental and life sciences research because of its low protein binding properties. However, PES membranes have a high adsorption capacity for low-polar organic solvent (e.g., ketone and chlorinated hydrocarbons) and aromatic hydrocarbons.1 Hence, in cases where filters made of PES membranes are used to prepare certain organic contaminant samples, the concentration of the contaminant in the filtrate may be significantly underestimated. In recent years, there have been great interests in environmental hormones studies focusing on fate of hormones as emerging contaminants in natural and engineered ecosystems.2 A simple search in the Google Scholar database resulted in thousands of research papers on estrogens and testosterone studies in the year 2014 alone. In most papers, hormone stocks © 2015 American Chemical Society

Figure 1. Percentage retention of estrone (E1), estradiol (E2), 17αethinyl estradiol (EE2) and testosterone by a syringe filter housing a hydrophilic polyethersulfone membrane with effective filtration area of 2.8 cm2 and pore size of 0.2 μm (Acrodisc Syringe Filters with Supor Membrane, Pall Corporation). Initial concentration and volume were 5 ppm and 3 mL, respectively. All experiments were performed in triplicates.

PES membrane filter has a much higher efficiency to retain EE2 (96%) than E1 (68%), E2 (61%), and testosterone (17%). For each of these hormones, the retention efficiency was found to be higher for hormones at a lower initial concentration. These results clearly demonstrate that filters with PES membranes are not suitable for preparing estrogens samples and also highlight the importance of filtration details during the experimental Received: April 7, 2015 Published: April 23, 2015 5259

DOI: 10.1021/acs.est.5b01704 Environ. Sci. Technol. 2015, 49, 5259−5260

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Environmental Science & Technology execution and the reporting of such information in the papers. We urge the environmental science and engineering community to carefully choose the filters for preparation of stocks and samples and to appreciate the importance of experimental details of filtration (e.g., filter materials etc.) by reporting such critical information.



AUTHOR INFORMATION

Corresponding Author

*Phone: (+65) 6790 5277; fax: (+65) 6791 0676; e-mail: [email protected]. Notes

The authors declare no competing financial interest.



REFERENCES

(1) Røe utvik, T. I.; Durell, G. S.; Johnsen, S. Determining produced water originating polycyclic aromatic hydrocarbons in North Sea waters: Comparison of sampling techniques. Mar. Pollut. Bull. 1999, 38 (11), 977−989. (2) Richardson, S. D.; Ternes, T. A. Water analysis: emerging contaminants and current issues. Anal. Chem. 2005, 77 (12), 3807− 3838.

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DOI: 10.1021/acs.est.5b01704 Environ. Sci. Technol. 2015, 49, 5259−5260

What exactly are you filtering out?

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