David Glöckler successfully defended his PhD Thesis-Congratulations!

"Tailor-made Sorbents to Enhance Sensitivity in Stable Isotope Analysis of Aquatic Micropollutants"

Abstract
The ubiquitous occurrence of organic micropollutants (MPs), such as pesticides and
pharmaceuticals, in aquatic systems raises questions regarding their environmental behavior
and fate. Compound-specific isotope analysis (CSIA), which measures changes in stable
isotope ratios of individual compounds at natural abundance (e.g., 13 C/ 12 C, 2 H/ 1 H, 15 N/ 14 N,
18 O/ 16 O), offers unique means to detect, characterize, and quantify their degradation in
environmental systems, especially when concentration-based data alone would not be
conclusive. However, applying CSIA to MPs in field settings faces challenges of limited
sensitivity and chromatographic resolution of gas and liquid chromatography coupled with
isotope ratio mass spectrometry (GC- and LC-IRMS). Given the low contaminant
concentrations in natural waters, extraction of large sample volumes (i.e., tens to hundreds
of liters) becomes inevitable to enrich sufficient analyte mass for accurate isotope analysis.
Although solid-phase extraction (SPE) methods are available for this task, conventional SPE
sorbents typically lack the selectivity to exclusively extract the target analytes, resulting in
concomitant enrichment of dissolved organic matter (DOM). In consequence, an unresolved
complex mixture (UCM) of unknown isotopic composition interferes in GC- or LC-IRMS
measurements and compromises the reliable isotope analysis of target compounds. Hence,
the overarching goal of this dissertation was to extent the scope of CSIA to MPs at low
concentrations by advancing sample preparation methodologies.
The major part of the thesis specifically aimed at (i) increasing the selectivity of SPE by
employing cyclodextrin polymers (CDPs), which have shown promise as selective sorbents
in water purification technologies, and (ii) gaining detailed knowledge on the control of
sorbent selectivity toward both target analytes and interfering DOM. In the first research
chapter, the efficacy of CDPs with different cavity sizes (α-, β-, γ-CDP) was systematically
investigated for the selective extraction of a range of MPs in the presence of DOM.
Applicability to carbon isotope analysis was assessed for selected pesticides, and the results
were compared with commercially available sorbents. The presented CDP-based SPE
method proved effective in obtaining environmental extracts that meet the stringent criteria
of CSIA, notably by significantly reducing the UCM in GC-IRMS chromatograms after SPE
with β-CDP. The sensitivity of carbon isotope analysis was enhanced by a factor of 7.5
compared to conventional SPE-CSIA. The new CDP-based SPE-CSIA method was
successfully applied to surface water matrices without inducing isotopic fractionation. The
superior selectivity was further confirmed by up to six-fold lower carbon-normalized
CDOM/Canalyte ratios in β-CDP extracts as derived from dissolved organic carbon (DOC) and
liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. In addition, a
weak competition between DOM and analyte on the three CDPs was proven by Gibbs free
energies of adsorption obtained through column chromatography experiments.
The second research chapter employed nontargeted Fourier transform ion cyclotron
resonance mass spectrometry (FTICR MS) analysis to comprehensively explore the
selectivity of β-CDP at the molecular level. The ultrahigh-resolution MS data elucidated the
molecular chemodiversity of DOM extracted from surface and groundwater, highlighting
the reduced matrix complexity in β-CDP extracts compared to conventional sorbents. The
selectivity of β-CDP was shown to be characterized by discrimination against highly
oxygenated and unsaturated compounds, which are associated with classes of lignin-like,
tannin-like, and carboxylic-rich alicyclic molecules (CRAMs). In contrast, conventional
sorbents exhibited a more universal extraction behavior across a wide range of DOM
compositions. The provided molecular-level insights can effectively be used as an a priori
assessment for extraction procedures and cleanup strategies for environmental samples.
The third research chapter aimed to elucidate the molecular drivers underlying the
observed selectivity of CDPs. The study employed a multifaceted approach: (i) synthesizing
CDPs with different cavity sizes (α-, β-, γ-CDP), (ii) evaluating their extraction efficiencies
for selected MPs in competition with different DOM size fractions (<1, 1-3, 3-10, >10 kDa)
to test for size-selectivity, and (iii) performing FTICR MS analysis on CDP-extracted DOM
compounds (<1 kDa) of different origin (surface water and Suwannee River Fulvic and
Humic Acid) to probe for molecular properties governing their selective sorption. First, no
evidence of size-selectivity was observed across the different CDP cavity sizes (i), or
through the two independent approaches (ii) and (iii). Second, a dominant impact of sorbate
oxygenation and polarity on extraction of DOM and MPs was found, with relatively oxygen-
poor/nonpolar compounds exhibiting preferential retention on all three CDPs. Third, the
results indicated the exclusion of anionic matrix, such as carboxylic acids, but revealed
preferential sorption of cationic DOM. Consequently, the selectivity was attributed to a
synergistic effect of nonpolar and additional electrostatic interactions with the negatively
charged polymer cross-linker. The improved understanding of CDPs’ sorption behavior can
facilitate the refinement of sorbent design, enhancing both efficiency and selectivity for
environmental and analytical applications.
The second part of the thesis addressed the challenging task of processing large sample
volumes required for micropollutant CSIA. To tackle this issue, the fourth study evaluated
the feasibility of graphene-modified polymer monoliths for high-throughput extraction of
MPs from water. The aim was to harness the synergistic effects of the high porosity of
monolithic adsorption filtration (MAF) polymers and the unique sorption properties of
graphene, a carbon-based nanomaterial, to enable efficient SPE at high flow rates. The study
successfully immobilized graphene oxide (GO) nanosheets onto the MAF surface, and the
chemical reduction of GO enhanced its sorption affinity and capacity for selected pesticides
by approximately half an order of magnitude. However, the low sorption kinetics and the
insufficiently increased surface area resulted in a poor extraction performance of the
modified polymer in a proof-of-concept SPE experiment. The presented findings provide a
knowledge base for future method development and optimization.
Overall, the present work underscores the critical importance of meticulously selecting
SPE sorbent materials, of conducting comprehensive sorbent selectivity assessment toward
target analyte and matrix constituents, and of performing rigorous method evaluation and
validation to ensure accurate and sensitive micropollutant CSIA. Specifically, β-CDP is
presented as a favorable selective model sorbent for discriminating against environmental
DOM in matrix-susceptible analytical applications. The developed CDP-based SPE-CSIA
procedure, in combination with established sample cleanup techniques, offers new prospects
for CSIA at environmentally relevant concentrations ranging from tens to hundreds of
nanograms per liter. The extension of this method to a wide range of organic MPs for
investigating their environmental behavior and fate holds significant promise for
environmental protection, enabling a better understanding of pollutant sources and
transformation pathways, thus facilitating the informed development of effective mitigation
strategies to safeguard water resources and ecosystem health.