An on-line coupling of nanofibrous extraction with column-switching high performance liquid chromatography – A case study on the determination of bisphenol A in environmental water samples

Highlights

• On-line SPE-UHPLC method using polyamide 6 nanofibers as an extraction sorbent was developed and validated.

• The possibility to multiple reuse the nanofibrous polymer (700 injections) in the high pressure system was confirmed.

• No mechanical changes in the nanostructure and no spontaneous agglomeration of nanofibers were observed.

• The method was successfully applied as a case study to the analysis of Bisphenol A in river samples.

Abstract

Polyamide 6 nanofiber polymers were used as modern sorbents for on-line solid phase extraction (SPE) coupled with liquid chromatography. The on-line SPE system was tested for the determination of bisphenol A in river water samples. Polyamide nanofibers were prepared using needleless electrospinning, inserted into a mini-column cartridge (5 × 4.6 mm) and coupled with HPLC. The effect of column packing and the amount of polyamide 6 on extraction efficiency was tested and the packing process was optimized. The proposed method was performed using a 50-µL sample injection followed by an on-line nanofibrous extraction procedure. The influence of the washing mobile phase on the retention of bisphenol A during the extraction procedure was evaluated. Ascentis^® Express C18 (10 cm × 4.6 mm) core-shell column was used as an analytical column. Fluorescence detection wavelengths (λ_ex = 225 nm and λ_em = 320 nm) were used for identification and quantification of Bisphenol A in river waters. The linearity was tested in the range from 2 to 500 µg L⁻¹ (using nine calibration points). The limits of detection and quantification were 0.6 and 2 µg L⁻¹, respectively. The developed method was successfully used for the determination of bisphenol A in various samples of river waters in the Czech Republic (The Ohře, Labe, Nisa, Úpa, and Opava Rivers).

Introduction

Finding a reliable sample preparation procedure is still considered to be the bottleneck of the whole analytical method. Current advances in sample preparation have been focused on automation, improving sensitivity and accuracy, low sample and organic solvent consumption, and especially miniaturization. The development of miniaturized sample preparation techniques coupled e.g. with liquid chromatography has shown to be a promising way to achieve these aims. The latest trends in micro-extraction sample clean-up techniques have reduced the volume of organic solvents needed for liquid extraction methods, and have reduced the amount of sorbents in solid phase extraction based methods. Moreover, the development of new extraction materials with improved properties is challenging task in the field of micro-extraction techniques.

Nanofiber polymers, which have demonstrated an excellent potential for extraction purposes, have proven to be new and promising candidates due to their stability, versatility, large surface area due to the small diameter of their fibres (less than 1000 nm), and thus enhanced extraction kinetics and capacity. The most common way to create nanofibers is through electrospinning using an electrostatic field for forming the nanofibers from a solution or melt of polymer [1], [2]. The use of nanofibers and nanomaterials in the micro-extraction context has been recently described [3], [4]. Solid Phase Micro-Extraction (SPME), Micro-Extraction by Packed Sorbent (MEPS), nanofiber disk SPE, and pipette-tip micro-extractions are the most often used off-line nanofibrous extraction approaches before determination by High Performance Liquid Chromatography (HPLC), Gas Chromatography (GC), or other separation techniques with a suitable detection [3], [4]. Nevertheless, the on-line coupling of nanofiber polymers directly with analytical tools has only very rarely been described [3], [4]. Therefore, this almost undescribed area shows good potential to simplify the clean-up step and to speed up the analytical run, most notably in liquid chromatography systems.

The typical on-line SPE-HPLC procedure employs a double position column switching valve to perform the extraction and separation steps directly in the system. The sample is injected directly onto the extraction pre-column in the first step. The target analytes are retained on the pre-column while the rest of the sample with residual interferences is eluted to the waste. Purification of the sample occurs during the washing mobile phase, which flows through the extraction pre-column. In the second step, the analytes are eluted with a mobile phase to the chromatography column, where separation is carried out [2], [5].

An environmental contaminant, bisphenol A (BPA), was chosen as the target analyte for this study. BPA is widely used in the manufacture of plastics and resins. Because of the widespread use of plastics, the presence of residual BPA concentrations in food and environmental samples is widely studied [6], [7], [8], [9], [10], [11], [12]. Some works have used various extraction techniques, for example coacervative extraction [6], liquid-liquid extraction [12], dispersive liquid phase microextraction [7], [12], and solid phase extraction (SPE) [13], [14], [15], [16]. Two works describing the on-line connection of SPE with HPLC for the determination of BPA in samples have been found. In these studies, a C18 cartridge or a methacrylic acid 3-sulfopropyl potassium salt (MASK) modified pre-treatment column were used for the sample extraction [17], [18]. Bisphenol A belongs within the group of endocrine disrupting chemicals. Several studies on BPA have focused on its increased estrogenic activity [10], [11]. The maximal daily intake of BPA was set at 4 µg kg⁻¹ (body weight) per day (EFSA, January 2015). Despite all these facts, no limits of BPA in environmental water have been established in the Czech Republic.

As far as we know, no work describing on-line nanofibrous extraction for the determination of BPA in river samples has been published yet. Therefore, this work was focused on the testing of a nanofibrous extraction with using polyamide 6 nanofibers, which were connected on-line with a UHPLC system to perform an analysis of BPA in rivers. The polyamide nanofibers used in this work were prepared by needleless electrospinning using a nanospider laboratory machine. In addition to their behaviour, changes in their structure, stability in a high back-pressure on-line SPE system, influence of column packing on extraction efficiency, and repeatability of the extraction steps were individual objectives of the presented study.