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Screening and quantitation of micro-pollutants from sewage water in the process of bank filtration using UHPLC-HRAM MS

Authors: Patricia van Baar,(1) Florian Wode,(1) Uwe Dünnbier,(1) Maciej Bromirski,(2) Olaf Scheibner,(2)
(1)Berliner Wasserbetriebe, Berlin, Germany  (2)Thermo Fisher Scienti­c, Bremen, Germany

Screening for and quantitation of micropollutants in ground water was carried out along a transect at the lake Tegeler See in the city of Berlin, Germany. The analyses were done by Solid-Phase Extraction on-line coupled to ultrahigh performance liquid chromatography-high resolution accurate mass spectrometry (SPE-UHPLC–HRAM MS). Several contaminants could be detected and quantified; their behavior under different redox conditions was studied.

Key words: ground water analysis, pollutant residues, UHPLC–HRAM MS, Solid-Phase Extraction, SPE–LC–MS

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1.  Introduction
In the city of Berlin, Germany, drinking water is completely derived from ground water containing large portions of river bank filtrated water. In this study, the barrier function of bank filtration for micro-pollutants was investigated along a transect at the lake Tegeler See in the city of Berlin. The transect consisted of multiple ground water sampling sites between the lake and a water supply well. The derived samples were analyzed by UHPLC–HRAM MS, conducting a screening study for contaminants not yet detected in the urban water cycle of Berlin. The lake contains up to 30% of efflux water from a municipal waste water treatment plant and contaminants penetrating the ground water were put into focus in this study.1

2. Purpose
To show how bank filtration can help to generate a closed water cycle in areas of limited water availability.

3. Method
Samples from different ground water probing sites and a ground water well were taken and injected onto the online-SPE system directly. An aliquot of 1 mL of sample was injected onto the online-SPE system. For pre-concentration, a C18 column, 2.1 × 20 mm with 12 μm particle size was used. For compound separation, a C18 column, 2.1 × 50 mm with 1.8 μm particle size was used. A gradient formed from water and methanol, both spiked with 0.1% formic acid, was ramped up from 2% B to 95% B in 6.7 minutes. Total chromatographic cycle time including online reconcentration was 15 minutes. Mass spectrometric analysis was run on a hybrid quadrupole-Orbitrap mass spectrometer. For screening purposes, the full scan and variable Data Independent Acquisition (vDIA) modes were adopted. The vDIA scan mode creates all fragments possible in a data independent approach, and all fragments are present all the time. For quantitative analysis, full scan mode was used.

4. Results and Discussion
The first step in the investigation was to carry out a non-target analysis of the samples drawn from the ground water probing sites and the ground water well. Starting with a set of samples, suspect screening was carried out, leading to 260 suspects, of which 94 could be confirmed by using confirmation criteria like isotopic pattern match and matching of known fragments. For these 94 confirmed suspects, reference standards were purchased and run in a dilution series, used for identification and quantitation at the same time. By this approach, 58 components could be identified and quantified at the same time.
An important part of the investigation was to determine the degradation behavior of the identified compounds and the evaluation of the ability of the bank filtration process to eliminate these compounds. According to the finding of three regions with differing redox potential, three groups of components could be identified that were predominantly degraded in one of the three regions. The first group of compounds was identified as the one facing aerobical degradation, where the compounds show fast decrease of concentration over the course of the transect and mostly are not detectable in the sample from the ground water well. The second group was identified as the one where the compounds undergo mostly anaerobical degradation and show slower decrease of concentration over the course of the transect, but still show low concentrations in the sample from the ground water well used for drinking water production. The third group identified as the critical one, where the components barely show any degradation, so the concentrations found in the sample from the ground water well were still comparable to the ones in the surface water. A total number of 31 components still was detected in the ground water well, with 11 components exceeding a concentration of 0.1 μg/L.
In addition to the suspect screening and quantitative analysis, an unknown screening with unbiased peak detection was carried out to screen for additional compounds of interest that had not come to attention yet. The first approach was to look into the 20 most intense compounds after summing up all signal intensities from different samples for all detected compounds. Eight of these could be easily identified as suspect components already found in the earlier screening. Additional compounds could be identified which had not been found before.
To find out more about the fate of the compounds detected in the process of bank filtration, a search for degradation products was carried out. Compounds already identified during the screening process were used as parent compounds and an exhaustive search for degradation products was carried out. In a fully automated process, all possible transformation steps are applied to the given compounds in up to three steps. As a result, degradation products were detected. The software is able to perform several confirmation steps for the suspected transformation products. The first is an isotopic pattern match of the signals found in comparison with the theoretical pattern derived from the elemental composition of the supposed degradation products. The second is a process called Fragment Ion Search (FISh). Since the data was acquired using the vDIA scan mode, which creates all fragments possible in a data independent approach, all fragments are present all the time.
Due to the vDIA scan mode, where all precursors are present in a given mass range, many more fragments are present than only the ones of the suspected compound. Since the fragment data is acquired with high resolution and high mass accuracy, the FISh process can easily pick the fragments belonging to the suspected transformation product and annotate them, resulting in an unambiguous confirmation of the suspected compound.

5. Conclusion
Bank filtration is an important way to reuse surface water that is influenced by effluents from municipal and industrial waste water treatment plants. A wide range of contaminants found in the according surface water is degraded aerobically or anaerobically, although a number of contaminants turned out to be persistent, which still makes it necessary to carry out additional steps of water purification for drinking water generation. The screening and quantitation process, using LC-HRAM MS is a versatile method to quickly assess the quality of the surface water going into the reuse process as well as the raw water used for drinking water generation. It minimizes the analytical effort needed and combines every day routine analyses with extended risk assessment in an easy to handle process.

6. Reference

  1. Steffen Grünheid, Gary Amy, Martin Jekel. Removal of bulk dissolved organic carbon (DOC) and trace organic compounds by bank filtration and artificial recharge. Water Research, Volume 39, Issue 14, September 2005, Pages 3219–3228.