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The development of an in-situ NMR photoreactor to study environmental photochemistry Liora Bliumkin, Rudraksha Dutta Majumdar, Ronald Soong, Antonio Adamo, Jonathan P.D. Abbatt, Ran Zhao, Eric J Reiner, and Andre J Simpson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00361 • Publication Date (Web): 12 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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Environmental Science & Technology

The Development of an in-situ NMR Photoreactor to Study Environmental Photochemistry

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Liora Bliumkin,†,‡ Rudraksha Dutta Majumdar,† Ronald Soong,† Antonio Adamo,§ Jonathan P.D.

5

Abbatt,‡ Ran Zhao,‡ Eric Reiner,∥ and André J. Simpson*,†,‡

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†

Environmental NMR Centre, Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario, M1C 1A4, Canada

8 9

‡

Department of Chemistry, University of Toronto, Toronto, Ontario, M5S 3H6, Canada

10

§

Teaching and Research in Analytical Chemical and Environmental Science (TRACES),

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Department of Physical and Environmental Sciences, University of Toronto Scarborough,

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Toronto, Ontario, M1C 1A4, Canada

13

∥

Ontario Ministry of the Environment, Toronto, Ontario, M9P 3V6, Canada

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*Corresponding

author.

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[email protected]

Phone

+1

416-287-7547.

Fax

18 19 20 21 22 23

ACS Paragon Plus Environment

+1

416-287-7279.

E-mail:


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ABSTRACT: Photochemistry is a key environmental process directly linked to the fate, source,

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and toxicity of pollutants in the environment. This study explores two approaches for integrating

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light sources with nuclear magnetic resonance (NMR) spectroscopy: sample irradiation using a

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“sunlight simulator” outside the magnet, versus direct irradiation of sample inside the magnet.

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To assess their applicability, the in-situ NMR photoreactors were applied to a series of

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environmental systems: an atmospheric pollutant (paranitrophenol), crude oil extracts, and

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groundwater. The study successfully illustrates that environmentally relevant aqueous

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photochemical processes can be monitored in-situ and in real-time using NMR spectroscopy. A

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range of intermediates and degradation products were identified and matched to the literature.

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Preliminarily measurements of half-lives were also obtained from kinetic curves. The “sunlight

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simulator” was shown to be the most suitable model to explore environmental photolytic

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processes in-situ. Other light sources with more intense UV output hold potential for evaluating

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UV as a remediation alternative in areas such as wastewater treatment plants or oil-spills.

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Finally, the ability to analyze the photolytic fate of trace chemicals at natural abundance in

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groundwater, using a cryogenic probe, demonstrates the viability of NMR spectroscopy as a

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powerful and complementary technique for environmental applications in general.

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TOC ART:

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KEYWORDS. Nuclear Magnetic Resonance Spectroscopy, Environmental Photochemistry,

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Photolysis, Degradation, In-situ, Kinetics

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INTRODUCTION

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Environmental photochemistry involves the transformation of compounds, found on

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Earth’s surface and in the atmosphere, due to the absorption of photons between 290-600 nm.1

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The absorption of photons provides sufficient energy for electrons to be excited from the ground

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state into an excited state (commonly π→π∗ or n→π∗).1 In order for the excited electrons to return

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to ground (stable) state, they must release the excess energy. One way this can be achieved is

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through the initialization of a chemical reaction that requires an input of energy.1 These

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photochemical reactions can then result in mineralization (conversion to CO2 and H2O) or

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generation of new compounds via bond cleavage, isomerization, rearrangement or intermolecular

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reactions, and can take place in both the aqueous (atmospheric aerosols or surface water) and

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solid phases (plant and soil surface).2 Generally, photochemical reactions (photolysis) can be



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direct or indirect. In direct photolysis, a chromophore directly absorbs a photon and becomes

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excited.1 Conversely, indirect photolysis involves a photosensitizer, that upon absorption of

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photons, transfers the energy to initiate a chemical reaction in nearby compounds.1 Not only does

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solar radiation play a pivotal role in the composition and fate of both natural and anthropogenic

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chemicals in the environment, much of the life on Earth also relies on it as its source of energy.3

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Current techniques used in photochemical analysis

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Photochemistry is commonly studied using fluorescence, optical spectroscopy, and mass

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spectrometry (MS) due to the high temporal resolution and sensitivity of the techniques.4

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Chemical properties such as quantum yields can be obtained using optical spectroscopy.

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However, it becomes challenging to elucidate such information from complex samples due to

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spectral overlap, as in the case of polycyclic aromatic hydrocarbons (PAHs) in cosmic water ice.5

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Other studies have demonstrated that UV-Vis spectrometry provides more ambiguous

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information relative to higher resolution techniques such as NMR spectroscopy.6 Fluorescence is

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a highly sensitive alternative, but is restricted to only a small fraction of molecules that fluoresce

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and thus provides only limited information with respect to chemical structure. Mass spectrometry

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(MS) is arguably one of the most efficient and informative techniques and can characterize

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photoproducts in a complex mixture based on their fragmentation patterns.7 High Performance

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Liquid Chromatography (HPLC) and Gas Chromatography (GC) are generally coupled with MS

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to enhance selectivity and reduce spectral overlap.8 Nonetheless, MS may require extensive

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sample preparation that can potentially lead to the introduction of variability and artifacts.9 For

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example, many free radicals (such as nitroxides) can be detected in solution using fluorescence

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while much of this information is lost with MS if the preparation time is too long.10 Furthermore,


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while MS provides critically needed molecular formulae information, identification of exact

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structures may not be possible if novel structures are formed (i.e. library fragmentation not

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available). As such, there is need for complementary techniques, especially those that can

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provide high resolution isomeric information required to solve de-novo molecular structure.

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NMR Spectroscopy as a tool in environmental research

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In recent years, NMR spectroscopy has emerged as an important complementary tool in

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environmental research as it can provide unprecedented information regarding molecular

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structures, mechanisms, and kinetics that are key in the elucidation of photochemical reactions.4

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Moreover, it is a non-selective, versatile, robust, and highly reproducible technique that offers

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efficient and indiscriminate information that can be missed by conventional methods.11,12

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Simpson et al. have previously provided a detailed review of the applicability of NMR

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spectroscopy to environmental research.13 A brief overview of the unique advantages afforded by

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NMR spectroscopy as it pertains to environmental studies is provided in Table S1, with the

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appropriate examples and references.14–31

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The ability of NMR spectroscopy to analyze a sample in its natural state and in a non-

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invasive manner is a very important factor in environmental studies. The technique has been

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shown to be a useful tool to follow the progress of chemical reactions. An excellent example is

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the study of trifluralin degradation by using 19F NMR spectroscopy where samples in NMR tubes

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were placed outside in direct sunlight and then periodically brought in for NMR analysis.32 The

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study identified a range of degradation products and reaction mechanisms. However, while such

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studies are accessible and easy to perform, periodic sampling only provides information on the

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sample at discrete points in time. Determining the correct sampling frequency and timing for



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each sample becomes essential to minimize over- and under-estimations and bias on the complex

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chemistry taking place inside the NMR tube.33 The lack of high temporal resolution is

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specifically problematic if reactive short-lived intermediates or rapid reactions occur. These can

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be avoided using near real-time measurements, as demonstrated with the application of in-situ

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NMR analysis to study reactive organometallic structures with short lifetimes.34 Similarly,

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Henjum et al.33 have shown the importance of in-situ analysis as they compared pollutant

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loadings in streams from near real-time measurements and periodic sampling. It was

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demonstrated that larger calculation errors arise from periodic sampling and that identifying

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pollution sources and sinks are more feasible using near real-time monitoring.33 Thus, in-situ

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monitoring has the capability to improve quantitative analyses, reduce the loss of valuable

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information between sampling points, and follow the fate, source, and toxicity of pollutants in

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the environment for a more comprehensive understanding on environmental processes.35

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In this article, we explore and develop various approaches for performing in-situ

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photochemical NMR spectroscopy to study environmental photochemistry. These include

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comparison of light sources, from relatively cheap xenon arc lamps to more realistic “sunlight

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simulators”, as well as comparing flow systems (light source outside the spectrometer) to optical

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fiber (light directly into the NMR magnet). Three different light setups are tested on a range of

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media including individual compounds, crude oil extracts, and groundwater to test the

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applicability to a wide range of environmental systems. This study demonstrates that

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environmentally relevant photochemical processes in the aqueous phase can be monitored in-situ

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and in real time using NMR spectroscopy. Once constructed, the photochemical NMR systems

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are relatively easy to operate permitting studies with high temporal resolution in an automated

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fashion without user intervention. Considering the highly complementary nature of NMR


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spectroscopy to MS,36 especially in terms of structural elucidation, in-situ photochemical NMR

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will likely play an important role unraveling photochemical processes especially in more

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complex system where MS alone is insufficient.

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EXPERIMENTAL SECTION

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Light Sources and Optical Fiber (for full details see Supporting Information) OceanOptics HPX-2000: 35 W continuous xenon light source (main output: 290-800 nm.

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Figure S1a) OceanOptics PX-2: a pulsed xenon lamp, wavelength range from 220-750 nm (Figure

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S1b). Original Hanau Suntest: a xenon burner with daylight filter, specifically designed to mimic the spectrum of sunlight received at the Earth’s surface (Figure S2).

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Chemical Actinometry and Calibration of the Suntest

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The average global shortwave (SW) downward surface radiation (DSR) reported in the

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literature is ~17 mW/cm2, while the average global net absorbed surface shortwave radiation flux

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is ~15 mW/cm2.37 Nuclear magnetic resonance spectroscopy based chemical actinometry using a

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2 mM solution of 2-nitrobenzaldehyde38 in 70% D2O and 30% H2O was used to measure the

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average radiation flux between 290-380 nm reaching the reaction vessel inside the Suntest by

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monitoring the NMR peaks of the protons attached to C(3) and C(6). The radiation flux exposed

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to the reaction vessel inside the Suntest was calculated to be ~8.53 mW/cm2 with an average

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photon flux of 2.46 × 1014 s-1·cm-2·nm-1 which was consistent with that reported by Zhao et al.39

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This is ~2 times lower than the average global SW-DSR and ~1.75 times lower than the net



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absorbed radiation flux reported in the literature and is consistent with the net absorbed

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shortwave radiation in New Orleans, Casablanca, and Beijing in January and Paris and Berlin in

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October.37 No attempts were made to calibrate the HPX-2000 or PX-2 light sources in relation to

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natural sunlight since their spectral output differs greatly from sunlight, and numerous

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disadvantages observed in this work make them less suitable for photochemical studies that aim

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to mimic the natural environment.

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Experimental Summary (see Supporting Information for full details)

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Samples were prepared as described in the Supporting Information. For both the HPX-

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2000 and PX-2 light was transferred via an optical fiber and data were collected on the most

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sensitive 5 -mm cryogenically cooled NMR probe (see Supporting Information for details and

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Figure 1 for schematic). The polyamide coating was removed from a ~ 10 cm section at the end

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of the fiber that entered the NMR tube to increase light transmittance into the sample.40 The

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optical fiber OD was 1 mm, and it was connected to the 4.2 mm ID NMR tube such that it does

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not form a gas tight seal, allowing air exchange. No spinning was performed on the NMR tube.

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Due to the different design of the Suntest (a sunlight chamber, rather than a light source with an

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optical fiber connector), a looped flow system was employed. The easiest integration was via a

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standard, commercial NMR flow probe that permits the sample to be directly flowed through the

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spectrometer. All studies with the Suntest light source were performed using a 1H, 13C, 15N, TXI

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(Triple resonance Inverse) z-gradient 250 µL injection NMR flow probe with the exception of

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the groundwater. For groundwater, while a lot more challenging due to the low concentration, a

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cryogenic probe was employed fitted with a custom designed flow cell as previously described

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by Soong et al.17 All other experimental details are provided in the Supporting Information.


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RESULTS AND DISCUSSION

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Comparison of different light sources

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Riboflavin represents a simple, cheap and well characterized photosensitive compound

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ideal for investigating the basic performance of the in-situ NMR photoreactors before application

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to more environmentally relevant systems later in this paper. Riboflavin is highly sensitive to UV

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and visible light, and forms reactive oxygen species (ROS) upon light exposure.41 Absorption

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maxima have been reported around 224, 268, 373, and 445 nm.42,43 The photodegradation of

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riboflavin has been extensively studied, and is known to form two main photoproducts:

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lumichrome and lumiflavin.41

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HPX-2000, PX-2, and Suntest light systems were selected for comparison. These are

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discussed more in the Supporting Information. Figure 1 provides the schematics, NMR spectra

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and kinetic curves for the degradation of riboflavin using the three different light sources.

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Expanded NMR spectra including detailed assignments and various controls are provided in the

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supporting section (Figures S3 through S8), along with specific assignments of the degradation

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products. The controls included light-off before exposure (to ensure no change prior to analysis),

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light-off after exposure (to ensure changes halt when light is removed), and dark controls (to

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demonstrate that changes are caused by the light). No changes were observed in any of the

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controls, confirming all reactions to be photolytic.

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On light exposure of riboflavin using both PX-2 and HPX-2000, the two singlet (methyl)

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peaks at 2.05 and 2.17 ppm and aromatic peaks at 7.09 and 7.19 ppm decreased in intensity.



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Eight new peaks appeared between 6-8.5ppm along with two large signals at 3.61 and 3.77 ppm

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(Figure 1, S3, S4). The solution changed from light to a dark orange, and a dark, orange-brown

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precipitate was formed inside the NMR tube (more visible with HPX-2000, see Figure S3). The

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NMR products were not consistent with products expected to be formed from riboflavin in

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sunlight.41,44 The alkyl region resembled the spectrum of erythritol (Figure 1) and indicates the

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polar branch from riboflavin is cleaved. The aromatic region (Figure S3) exhibited similarities to

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spectra of riboflavin derived polymers previously reported in the literature,45 and constituted a

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very small fraction of the overall spectrum. However, in-depth identification is beyond the scope

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of this paper and was not attempted.

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The results with the Suntest were significantly different. The peaks at 7.09 and 7.19 ppm

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were replaced by five new singlet peaks between 6.7-8.5 ppm. Also, new peaks between 3.4–3.8

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ppm were observed. These are all consistent with the main photoproducts of riboflavin reported

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in the literature for > 254 nm light, namely lumichrome, lumiflavin, erythrose, and 1-deoxy-

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xylulose (Figure 1).41,44 Detailed assignments are provided in Figure S8. The occurrence of

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different photoproducts between the light systems are likely due to the different spectral output.

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While not clear from Figure S1, the manufacturer reports wavelengths down to 185 nm for the

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HPX-2000 and 220 nm for the PX-2. Riboflavin is known to contain several chromophores

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ranging between 200-500 nm, including a chromophore at 224 nm.42,43

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The kinetics also highlighted differences between the light systems. The PX-2 and HPX-

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2000 show unusual sigmoidal curves (Figure 1). However, these are likely result of how the

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optical fiber was placed within the NMR tube. To avoid shimming problems, the end of the

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stripped fiber was placed in the reaction solution but above the detection coil. It appears that the

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light induces photolytic reactions, but the products take time to diffuse into the detection coil


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region. The lack of an immediate and prominent reaction suggests that the light cannot penetrate

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directly into the coil region due to self-absorption from the sample itself. This clearly highlights

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the difficulties in distributing light uniformly through any sample based on an optical fiber,

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which in turn would complicate calculating kinetic parameters such as half-lives.46 To test this

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hypothesis of delayed onset of product detection, the experiment was repeated with the fiber

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completely stripped of polyamide coating and submerged to the bottom of the NMR tube.

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However, this was done using a room temperature probe, since the heating from the fiber may

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adversely affect the cryoprobe detection coil that has cryogenically cooled electronics at very

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close proximity to the sample. It could potentially cause the cryogens to become gaseous and

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crack probe components.

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immediately with the compound completely degrading within 1 hour (Figure S5), thus validating

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our hypothesis that when the fiber is above the coil region, the products take time to diffuse

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downwards for detection.

It was observed that the riboflavin signals started attenuating

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The Suntest system, which is based on a flow design, produces a logical decay profile

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indicating that the reaction starts immediately after light exposure and continues in a two-step

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first-order mechanism,39,44 initial rapid degradation followed by slower degradation. The half-life

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was calculated as ~1.88 h. Based on chemical actinometry (see experimental section), the Suntest

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system produces light that is consistent with that measured in New Orleans, Casablanca, and

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Beijing in January and Paris and Berlin in October which is ~ 1/2 of the global solar average.37

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The average half-life of riboflavin in the environment can be measured by accounting for the

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difference in radiation flux (a factor of two) between the Suntest and the environment. Previous

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kinetic studies suggest a linear relationship between photon flux and the rate of riboflavin

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photodegradation.39,44 Once the reduced light output from the Suntest (~50% compared to



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average global net) is accounted for, the global average half-life of riboflavin is ~0.94 h. This is

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consistent with previous reports of riboflavin photodegradation in milk.47 Considering this along

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with the fact that Suntest produced the expected degradation products,41,44 and is fundamentally

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designed to simulate sunlight, it is clear that this approach is best suited to investigate and

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monitor environmental photochemistry. That said, the drawbacks of such a flow system include

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the requirement for larger volumes of sample, the rigorous cleaning required between samples to

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prevent carry over, the cost of a Suntest simulator, and the need to design a NMR flow cell or

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have access to an NMR flow probe. Arguably, the HPX-2000 and PX-2 may have use if reactions

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in the deep UV range are of interest, for instance, the photoremediation of contaminants.

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Photooxidation and phototransformation of p-nitrophenol, an atmospheric pollutant

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Solar radiation is known to be the driving force behind several atmospheric processes and

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responsible for the generation of atmospheric radicals which are considered “cleansers” of the

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atmosphere.48–50 Nitrophenols are introduced into the atmosphere via biomass burning emissions,

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as well as the atmospheric oxidation of aromatic pollutants.51 Nitrophenols are phytotoxic;52 and

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therefore, their photochemistry and decay products are of great interest to the atmospheric

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chemistry community. In particular, p-nitrophenol is sufficiently water-soluble and consequently

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subject to aqueous-phase photooxidation and phototransformation in cloud and fog waters.53

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Hence, the phototransformation of p-nitrophenol was monitored here to demonstrate the

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phototransformation process of a simple environmentally relevant compound using in-situ NMR

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photoreactors.

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In-situ and real time information from NMR spectroscopy enabled the identification of

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p-nitrophenol degradation products, as shown in Figure 2, along with the kinetic curve for the


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phototransformation using the Suntest as a light source. The various control spectra collected

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under the light-off conditions (see Supporting Information, Figure S9) confirmed the spectral

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changes as photolytic.

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The signals from the parent compound and photoproducts decrease over the course of

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light exposure, suggesting either phototransformation to CO2 or release of small volatile organic

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products (Figure 2). As stated in the literature, the major photoproducts of p-nitrophenol are

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hydroquinone and 4-nitrocatechol (4-nitrobenzene-1,2-diol).54 These primary intermediates

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reacted further with hydroxyl radicals leading to ring-opening products and formation of

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oxygenated aliphatic compounds such as 2-butenedioic acid (fumaric acid).54,55 Additionally,

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benzoquinone was formed from hydroquinone while terephthalic acid, and 5-nitrobenzene-1,2,3-

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triol were detected based on their chemical shifts (Figure 2).56 The ability to monitor a reaction’s

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progress with high temporal resolution using in-situ NMR spectroscopy and NMR’s highly

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complementary nature to MS should prove useful in the elucidation of reaction mechanisms in

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general. The half-life of p-nitrophenol was determined to be 2.47 h (Figure 2). Unlike riboflavin,

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estimating the half-life of p-nitrophenol in the environment has proven to be more challenging

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with a wide range of half-lives reported since the photooxidative degradation of p-nitrophenol is

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highly dependent on ·OH and substrate concentrations as well as the light source used.54

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In contrast, only slight phototransformation was observed with HPX-2000 as the light

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source (Figure S10). This is most likely related to the spectral output of the two sources. The

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Suntest produces 90 µW/cm2/nm (converted from ~0.9 W/m2/nm, Figure S2) which is ~9 times

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more intense than the ~10 µW/cm2/nm for the HPX-2000. Furthermore, loss in the optical fibers

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and low surface area exposure would render an external optical fiber solution, such as the HPX-

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2000, less appealing for most environmental applications. A schematic summarizing the



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photodegradation behavior of riboflavin and p-nitrophenol under the different light sources is

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shown in Fig S11.

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Oil spills: the fate of Water Soluble Fraction of crude oil upon exposure light

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To demonstrate the application of photochemical NMR spectroscopy to a more complex

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environmental mixture, the water soluble fraction (WSF) of crude oil was studied. Crude oil is a

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complex heterogeneous mixture containing > 30,000 different hydrocarbon molecules57

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(saturates, aromatics, resins and asphaltenes)58 some of which contain aromatic rings and

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heteroatoms, as observed by Fourier transform ion cyclotron resonance mass spectrometry

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(FTICR-MS).57,59–61 Oil spills are a significant environmental problem where toxic chemicals are

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released into the environment.62–64 Certain gasoline components are resistant to biodegradation,

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but are photoliable.65 Here, the fate of water soluble oil components upon light exposure using

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the HPX-2000 and the Suntest light sources was investigated.

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Sodium dodecyl sulfate (SDS) was added to simulate the use of surfactants which are

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often used to help disrupt large oils spills and disperse oil components into the aqueous phase.66

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H2O2, an oxidant, was also added as an essential catalyst in the photodegradation of the WSF.

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Photooxidation in the presence of H2O2 has been proven to be an important remediation

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technique involving the generation of reactive hydroxyl radicals that are capable of degrading a

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wide range of organic pollutants.67 An SDS control experiment containing H2O2 confirmed that

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SDS is not photolabile (Figure S12). Other controls, including light-off prior and following

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photoirradiation and dark controls containing H2O2, confirmed all reactions to be photolytic


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(Figures S13, S14, S18). With both models, new peaks were observed between 0.7-1.5 ppm,

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suggesting the formation of aliphatic compounds (Figure 3B, pink),68,69 but to a greater extent

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with Suntest. Furthermore, new photoproducts between 3.5-3.8 ppm hint at hydroxylation of

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crude oil precursors (Figure 3B orange).61,68,69 This trend is in agreement with previous

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observations on photo-degraded and weathered oil by Islam et al. using FT-ICR MS.60

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Two-dimensional (2D) Distortionless Enhancement by Polarization Transfer -

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Heteronuclear Single Quantum Coherence (DEPT-HSQC) Spectroscopy provides additional

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spectral dispersion and 1H-13C connectivity over one bond (1JCH). The DEPT component encodes

326

CH3/CH and CH2 with opposite phases based on the difference in evolution behaviour of these

327

units during the experiment. In simple terms, DEPT-HSQC provide a high dispersion map of the

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H-C units in the mixture with the CH2 units phased negative (coloured green, Figure S15) and

329

the CH/CH3 units phased positive (coloured blue, Figure S15). Region 6 in the DEPT-HSQC

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data (Figure S15) supports the production of a small quantity of hydroxylated products.

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Additional naphthenic acid moieties were observed for the Suntest system (~2.7 ppm, Figure 3B,

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green),70 but no corresponding change was detected for the HPX-2000. A notable reduction in

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aromaticity between 6.5-8.0 ppm following light exposure (Figures 3C&D) was also observed,

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as previously noted by other groups.60,71,72 The aromaticity decreased by ~13% with HPX-2000

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and by ~35% with the Suntest model (Figure 3). Based solely on the spectral output of the lamps,

336

the Suntest is ~9 times more intense than HPX-2000 between 300-800 nm and likely explains the

337

increased breakdown down of more aromatic structures (Figures 3C&D). It is proposed that

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photooxidation of aromatic compounds was initiated by ring oxidation followed by ring-opening

339

reactions that yielded a range of oxygenated compounds,60 mainly aldehydes and acids, and

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unsaturated products (Figures 3B&D).57,61,71–73 The clearest indicators are the new signals



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between 5.0-6.0 ppm (HC=C) region following irradiation with HPX-2000 further, which are

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consistent with the formation of double bonds (Figure 3D, green and HSQC region 5, Figure

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S15).

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Other protons in the same 1H-1H spin system can be isolated using selective TOCSY. In

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this experiment the double bond signals are selectively excited and then a homonuclear spinlock

346

transfers the magnetization down the chain to other protons within the same 1H-1H spin system.

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The result is a sub-spectrum of the structural motif that contains the double bonds (Figure S16).

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The units are consistent with linear aliphatic constructs and are most likely the result from

349

aromatic ring opening reactions. Interestingly, these products are more intense with the HPX-

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2000. One argument is that the deeper UV offered by this lamp leads for enhanced degradation

351

of the aromatics. However, the overall aromatic region decreases more with the Suntest (Figure

352

3D). Double bonds are seen to form with the Suntest system (Figure 3D) but they do not appear

353

to accumulate. These unsaturated products are known to be photolabile, and hence, can further

354

react via oxidative cleavage of double bonds to form saturated aldehydes, ketones and

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acids.60,71,74 Figure S15 (HSQC) also supports the production of a small quantity of hydroxylated

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products. Hydroxylated products associated with long chain aliphatics are further confirmed by

357

2D 1H-1H TOCSY (Figure S17). The singlet peak at ~8.3 ppm in Fig. 3C (purple, likely formic

358

acid or alkyl formate) was shifted to a slightly lower frequency (~8.2 ppm, Fig. 3D), and the

359

singlet at ~9.6 ppm in Fig. 3D (brown, an aldehyde) was formed following photoirradiation, with

360

both the Suntest and HPX-2000. However, signals corresponding to -CH2-/-CH- signals adjacent

361

to carboxylic groups at ~2.5 ppm (red) following irradiation were larger with the Suntest model

362

(Figure 3B). The kinetic profile of this region shows that carboxylic groups accumulated over

363

time with the Suntest while it remained relatively the same with HPX-2000 (Figure S19). This


Page 17 of 36


364

provides further evidence that ring-opening products continue to degrade further with the Suntest

365

model due to greater light intensity, forming acidic end products. These are further confirmed by

366

1

H-1H TOCSY (see Figure S17).

367

In summary, NMR is a useful tool to help explain the overall changes occurring during

368

photochemical process, for example, the formation of new structural categories (double bonds)

369

and the degradation of aromatics. With additional assignments from a list of specific compounds

370

of interest (for example benzene, xylene, ethylbenzene, xylene or BTEX) it should be possible to

371

combine both non-targeted and targeted analysis and extract a wealth of process information in a

372

relatively short amount of time and in a non-invasive fashion.

373

In this study, 1H NMR data of the crude oil WSF using HPX-2000 as the light source was

374

acquired with a cryogenically cooled probe while the 1H NMR data using the Suntest system was

375

acquired with a room temperature flow injection probe, as the latter is much easier to integrate

376

into a flow system. However, as can be seen from Figure 3C, the sensitivity of the cryoprobe

377

(HPX-2000, right) is ~2 times that of the flow probe (left). While it is possible to integrate flow

378

into cryoprobe systems it involves developing custom flow cells as previously reported by Soong

379

et al.17 To demonstrate a flow-application taking advantage of additional sensitivity of the

380

cryoprobe, the next section deals with the flow analysis of a low concentration environmental

381

sample, at natural abundance.

382 383

Monitoring photochemical changes of groundwater at natural abundance

384

The final example demonstrates the application of cryogenically cooled NMR probe for

385

analysis of complex environmental samples. For centuries, groundwater has been used as a main

386

source of drinking water. Today, it is still a favorable source of drinking and agricultural water.75



Page 18 of 36

387

There has been a growing interest in evaluating the application of sunlight in water treatment as a

388

result of groundwater contamination originating from leeching of industrial chemical waste

389

discharge and pesticides through the soil.76 Aquatic organic matter ranges in complexity and

390

heterogeneity. At one extreme over 80% of the total organic carbon in Antarctic glacial ice can

391

be assigned to simple biomolecules77 while at the other extreme 100,000’s of structures have

392

been demonstrated in river, lake and ocean samples by both NMR and FT-ICR.28,78–80 These

393

humic substances can be divided into 4 main classes: material derived from linear terpenoids

394

(MDLT), carboxyl-rich alicyclic molecules (CRAM), carbohydrates, and aromatics.81 In this

395

manuscript, groundwater was analyzed at its natural state and in a non-invasive manner, a key

396

factor in environmental studies, using a sunlight simulator. Dissolved organic carbon (DOC)

397

concentration in groundwater in North America can be as low as 1-2 ppm depending on the

398

season.82 The total organic carbon (TOC) of the groundwater discussed in this paper was 1.96

399

ppm. Considering the total volume of the NMR is ~300 µL, there is only ~588 ng of total organic

400

matter in the coil with many species in the low ng range. The ground water here, is consistent

401

with previous natural abundance NMR of ground water83 (see supporting Figure 2 in reference

402

81) that demonstrates strong biological inputs. However, it has a very low concentration in

403

comparison and is at the limits of NMR detection. As such it is possible that a more

404

characteristic broad profile for more heterogeneous “humic/fulvic” mixtures that can be observed

405

by natural abundance NMR (see Figure 4 ref. 83) is present but below detection limits in this

406

particular sample. As such, this analysis which uses a relatively low number of scans (4096) will

407

be biased towards the more homogeneous components with any heterogeneous “broad profile”

408

potentially lost in the baseline. Future studies could use more concentrated ground water, more

409

scans, or higher magnetic fields to achieve more comprehensive detection.


Page 19 of 36


410

Control experiments confirmed all reactions to be photolytic (Figure S20). The chemical

411

fingerprints of groundwater prior to light exposure were relatively easy to identify as it consisted

412

of mostly biological molecules that are well represented in bio-reference NMR databases (Figure

413

4). The molecular composition of groundwater was shown to contain many similarities to the

414

spectral composition of DOM in glacial ice. It was found to consist of: acetic acid, alanine,

415

glycerol, glycine, lactic acid, pyruvic acid, and short chain organic acids (SCA) (Figure 4).84 The

416

degradation products were more challenging to identify using the NMR database alone (Figure

417

4B). Acetone is a likely mineralization intermediate of an oxygenated precursor while formic

418

acid, at ~8.1 ppm, is a general breakdown product found in many DOM samples.85 Other

419

possible photodegradation products of DOM reported in literature are: methylglyoxal, glyoxal,

420

acetaldehyde, formaldehyde, pyruvate, glyoxylate,85 malonate,86 succinate, oxalate, acetate,

421

formate87 and ammonium,88 but the signal to noise ratio is too low here to make unambiguous

422

assignments. Figure S21 provides a kinetic profile of the photodegradation of lactic acid and

423

dual photogeneration and consumption of acetone over the course of light exposure,

424

demonstrating the viability of in-situ NMR analysis in understanding photochemical processes of

425

environmental samples taken directly from the environment and analyzed at natural abundance.

426

Additional information such as diffusion, connectivity information, dynamics, and

427

conformation, could not be obtained as the trace amounts of organic material in groundwater

428

prevent 2D NMR analysis at natural abundance. This stated it should be possible to concentrate

429

(freeze dry, speed vacuum) the sample and run 2D NMR at high concentration to elucidate

430

structures. Proton NMR can then be used to follow these assigned molecules at natural

431

abundance. Interestingly, the flow system employed using the Suntest could theoretically permit

432

a small flow to split to MS. The direct combination of NMR and MS is proving very powerful in



433

metabolic research, where the co-variance between signals over time in the two instruments can

434

be used to statistically correlate peaks in NMR and MS.15 Such applications in environmental

435

research could be very powerful and should directly relate molecular formulae (MS) and

436

isomeric information (NMR) to provide an unrivalled combination in terms of identifying new

437

species. The looped photochemical NMR reactor described here paves the way to make such

438

future studies possible.

Page 20 of 36

439 440

The study has successfully demonstrated that photolytic reactions in the aqueous phase

441

can be explored in-situ and in real time using NMR spectroscopy. It provides unambiguous

442

information on kinetics and structural identification of intermediates and degradation products

443

which can be further used to elucidate reaction mechanisms. It was determined that the Suntest

444

light source in combination with a loop flow system is the most suitable model to explore

445

environmental photolytic processes using in-situ NMR spectroscopy. Its application to a range of

446

environmental systems have illustrated that NMR is a powerful complementary tool that can be

447

used to study simple chemical transformations down to groundwater at natural abundance. The

448

isomeric information provided by NMR spectroscopy is extremely complementary to MS and

449

has an important role in unraveling photochemical processes in complex environmental systems.

450 451

ASSOCIATED CONTENT

452

Supporting Information

453

Additional 1H NMR spectra for riboflavin, p-nitrophenol, WSF of crude oil, and groundwater are

454

provided, including a detailed experimental section. This material is available free of charge via

455

the Internet at http://pubs.acs.org.


Page 21 of 36


456 457

AUTHOR INFORMATION

458

Corresponding Author

459

* Corresponding author phone: 416-208-4798; email: [email protected]

460

Notes

461

The authors declare no competing financial interest.

462 463

ACKNOWLEDGMENTS

464

A.J.S. thanks NSERC (Strategic and Discovery Programs), the Canada Foundation for

465

Innovation (CFI), the Ministry of Research and Innovation (MRI), and the Krembil Foundation

466

for providing funding. A.J.S. also thanks the Government of Ontario for an Early Researcher

467

Award.

468 469

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Figure 1. Schematics, 1H NMR data (500 MHz), preliminary assignments, and kinetic information for the photodegradation of the reference sample, 34.52 mM riboflavin solution (in 70% D2O and 30% H2O; pH 11.43) at 296 K, using three different light sources. A: lumiflavin, B: lumichrome, C: 1-deoxy-xylulose, D: erythrose. For detailed assignments and control experiments, see Supporting Information. The blue highlights indicate structure units and NMR signals arising from the riboflavin aliphatic “sidechain” and its derivatives.



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Figure 2. 1H spectra (500 MHz, at 296 K) of p-nitrophenol solution (7.74 mM p-nitrophenol with 38 mM H2O2 in 70% D2O and 30% H2O solution; pH 5.40) and its photoproducts at three different time points during the light exposure inside the Suntest. The percentages represent the relative distribution of the observable photodegradation products, calculated by integrating the areas under the peaks corresponding to the individual products, and then normalizing to the number of protons in each peak such that the percentages between different the products are directly comparable. The products in bottom panel represent ~75% of the NMR signal with ~25% of the parent compound remaining.


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Figure 3. 1H spectra (500 MHz, at 296 K) showing the phototransformation of the WSF of crude oil (extracted from a mixture of 4 mL crude oil : 1 mL of 17.51 mM SDS dissolved in 70% D2O and 30% H2O) containing 68 mM H2O2 with Suntest (left) and HPX-2000 (right) light sources. A (aliphatic) and C (aromatic) are prior to light exposure and B (aliphatic) and D (aromatic) are



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following light exposure. 1: CH/CH2 adjacent to –OH or carboxylic groups; 2: naphthenic acid CH2 groups; 3: CH/CH2 adjacent to carboxylic groups; 4: CH3 groups attached to aromatic rings; 5: aliphatic CH2; 6: aliphatic CH3. The vertically truncated aliphatic signals are from SDS.

Figure 4. 1H spectrum (500 MHz, at 296 K) of A: groundwater sample (TOC: 1.96 ppm; pH = 7.91; conductivity: 1448 µS/cm) after 6 hours in the dark (0-4.5 ppm region), B: after the sample was exposed to light for 1 d 12 h inside the Suntest solar simulator.


Development of an in Situ NMR Photoreactor To ... - ACS Publications

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