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Aldehyde and Ketone Photoproducts from Solar Irradiated Crude Oil-Seawater Systems Determined by ESI-MS/MS Xian Cao, and Matthew A Tarr Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01991 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017
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Aldehyde and Ketone Photoproducts from Solar
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Irradiated Crude Oil-Seawater Systems Determined
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by ESI-MS/MS
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Xian Cao, Matthew A. Tarr*
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Department of Chemistry, University of New Orleans, 2000 Lakeshore Drive, New Orleans, LA
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70148, United States.
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*Corresponding Author: Tel: (504) 280 6323. Fax: (504) 280 6860. Email:
[email protected] 8
Current Address: Department of Chemistry, University of New Orleans, 2000 Lakeshore Drive,
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New Orleans, LA 70148, United States.
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TOC/Abstract Art
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ABSTRACT
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Aldehyde and ketone photoproducts were observed in the aqueous phase under oil exposed to
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simulated sunlight by using 2,4-dinitrophenylhydrazine (DNPH) derivatization and electrospray
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ionization-tandem mass spectrometry (ESI-MS/MS). Oil samples were spread over seawater in a
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jacketed beaker held at 27.0 °C and exposed to simulated sunlight. The aq. phase was collected
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after irradiation and derivatized with DNPH, which selectively reacts with aldehydes and
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ketones. The derivatized hydrazones (aldehyde/ketone-DNPH derivatives) were washed and
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enriched with a solid phase extraction cartridge prior to analysis by ESI-MS/MS in negative ion
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mode. Over 80 aldehyde and ketone photoproducts were observed from scan range 200-1000
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amu in the aq. phase after irradiation but were absent in dark controls. Based on the MS/MS
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fragmentation of the aldehyde/ketone-DNPH derivatives, most of the aldehyde and ketone
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photoproduct mass spectra observed from the aq. phase were consistent with dicarbonyls,
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hydroxycarbonyls, and oxo-carboxylic acids. The formation of the photoproducts can be
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attributed to photoinduced oxidation of oil. The approach in this study allows easy identification
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of molar mass and other structural features of aldehyde and ketone photoproducts without
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interference from the many tens of thousands of parent compounds in the oil. These results will
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provide insight into the impact of photochemistry on the fate of oil in environmental systems and
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will have implications for oil spill response decisions.
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INTRODUCTION
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Oil is an extensively utilized energy resource and precursor for many materials, such as plastics,
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synthetic rubber, solvents, dyes, waxes, and lubricants. While it provides benefits to society, it
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can also be released into the environment during the production and transport, by land or by sea,
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and lead to damage to life and surrounding environments.1,2 Since spilled oil is often exposed to
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sunlight, it is necessary to understand the natural solar driven degradation processes of oil. When
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oil is spilled in water, the weathering consists of physical, biological, and photochemical
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processes.3,4 Physical and biological degradation have been extensively studied.5,6,7,8,9,10 In
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contrast, photochemical processes have received much less attention and are only poorly
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understood. However, they have been recognized as significant weathering processes in some
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regions such as tropical waters, which receive a high solar flux.4 The photochemical processes
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also depend on the latitude and season since the oils absorb strongly in the ultraviolet and visible
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regions of the solar spectrum.4 Furthermore, synergies have been observed between biological
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and photochemical degradation in the dissolved organic matter in which photochemistry oxidizes
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recalcitrant organic components into products that are biologically metabolized.11,12 The
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susceptibility of crude oil to biodegradation was reportedly increased by photooxidation.13
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To date, most studies on photochemical degradation have focused primarily on the observation
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of chemical composition changes such as polar species converted from aromatic compounds,
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formation of a significant number of surface-active compounds, and generation of oxygenated
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species in water-soluble fractions of the oil. 14 Several studies have reported the photoproducts of
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crude oil under irradiation conditions. Hansen et al. identified the primary photoproducts as
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aliphatic and aromatic acids and to a lesser extent alcohols and phenols in the surface oil film by
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using infrared absorption.15 Boukir et al. investigated photooxidation of resins and showed that
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the principal effect of photooxidation is an increase in carbonyl group content, particularly
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formation of carboxylic groups.16 Ehrhardt et al. found a number of aromatic alcohols, aldehydes
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and ketones in a fossil fuel-water system using authentic reference materials and comparing mass
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spectra and relative chromatographic retention indices.17 Other researchers also observed that the
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solubility of oil increased due to the formation of polar oxygenated species including ketones,
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alcohols, hydroperoxides, sulfoxides, phenols and carboxylic acids.18,19,20,21 Patton et al.
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demonstrated that weathering of oil from the Ixtoc I spill caused an increase in the polar fraction
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from an initial 8% to 20-30% in the weathered samples.22 Aeppli et al. observed that oxygen
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content increased in the hydrocarbon residues of Macondo spill oil (MC252 oil), surface slicks,
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sand patties, and rock scrapings.23 Ray et al. reported that oxygen containing molecules were
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much more abundant in irradiated MC252 oil water-soluble fractions compared to dark
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samples.24
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These previous studies showed that a wide array of photoproducts was generated during natural
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photochemistry of crude oil subjected to solar irradiation. One group of these photoproducts is
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made up of aldehyde and ketone compounds, including oxo-carboxylic acids. Most of the prior
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studies have used Fourier transform infrared spectroscopy (FTIR), chromatographic analysis, and
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mass spectrometry (MS) to monitor the photoproducts. However, the structures of aldehyde and
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ketone compounds have not been well elucidated. This limitation is primarily due to the absence
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of methods suitable to determine the photoproducts in crude oil. Therefore, more methods and
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experimental results are desired to better understand the structures of aldehyde and ketone
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photoproducts from crude oil subjected to solar irradiation.
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The most widely used method for measuring aldehyde and ketone compounds is based on
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derivatization with 2,4-dinitrophenylhydrazine (DNPH) and detection by high-performance
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liquid chromatography (HPLC),25,26 gas chromatography-mass spectrometry (GC-MS),25,27 and
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tandem mass spectrometry (MS/MS).28 Numerous studies have used DNPH derivatization
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methods to identify aldehyde and ketone compounds in disinfected water,28 commercial liquid
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soaps, automobile exhaust,29 cigarette mainstream smoke,30 ambient air,31,32 and aerosols.33 A
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variety of aldehydes and ketones were identified including formaldehyde, acetaldehyde, acetone,
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acrolein, propionaldehyde, methyl ethyl ketone, butyraldehyde, crotonaldehyde, glyoxal,
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methylglyoxal, and glycolaldehyde. A series of polar aldehydes and ketones, including hydroxy-
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aldehydes and hydroxy-ketones, dialdehydes, keto-aldehydes, aldo-acids, and keto-acids in
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drinking water were also derivatized and analyzed using DNPH methods.34 Nevertheless, there
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have not been any investigations that use DNPH derivatization to identify aldehyde and ketone
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photoproducts in the aq. phase in contact with crude oil subjected to solar irradiation.
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Herein, we report an investigation using DNPH derivatization to identify aldehyde and ketone
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photoproducts in the aq. phase in contact with oil subjected to solar irradiation. The study was
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based on the hypothesis that aldehyde and ketone photoproducts dissolve into the aq. phase after
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their formation. This is the first study that the uses DNPH reagent to determine aldehyde and
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ketone photoproducts in crude oil systems irradiated with simulated sunlight. It is also the first
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report to study the aldehyde and ketone photoproducts from crude oil by using MS/MS methods
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based on the fragmentation of the aldehyde/ketone-DNPH derivatives.
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EXPRIMENTAL SECTION
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General Information. Pure water was obtained by purification of distilled water with a
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Barnstead Nanopure UV water treatment system (NP water). Natural seawater (pH = 7.9) was
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collected from the open Gulf of Mexico by the Louisiana Universities Marine Consortium
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(prefiltered and UV sterilized), filtered through a 0.45 µm membrane filter (Gelman Science 90
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mm Supor-450), and stored at 4 °C in the dark. Acetonitrile (ACN) used for sample preparation
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and MS/MS analysis was obtained from Fisher (HPLC grade). Three oil samples were studied in
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this investigation: Deepwater Horizon (DWH) crude oil (MC252) and surrogate oil (A0067T)
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from BP, and SRM 2717a oil (3% sulfur in residual fuel oil) from the National Institute of
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Standards and Technology (NIST). 2,4-Dinitrophenylhydrazine was obtained from Sigma
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Aldrich. Hexanal, glyoxal, 4-hydroxy-3-hexanone, 9-anthracencecarboxaldehyde, pyruvic acid,
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and levulinic acid were purchased from VWR. All chemicals were of high purity and used as
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received.
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Irradiation Procedures. An Atlas CPS+ solar simulator was used to irradiate oil samples at an
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intensity equivalent to approximately 1.3 times that of solar noon (AM 1.5). The simulator
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maximum output (750 W/cm2, 300-800 nm only) was equivalent to 1.26 times the intensity of
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full sunlight (1000 W/cm2 full spectrum AM 1.5). Approximately 200 µL of each oil sample
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were spread over 50 mL of natural seawater in a 100 mL jacketed beaker (80 mm outer diameter,
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47 mm inner diameter, 73 mm internal height). The oil samples were irradiated in the chamber
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of the solar simulator by simulated sunlight with water circulating through the jacket at 27.0 °C.
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For the viscous SRM 2717a oil, the sample was added to the water by first dispersing the oil into
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a mixture of 1.0 mL pentane and 50 µL toluene, swirling for 3 min, and pouring the mixture over
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the seawater in the jacketed beaker. Samples were kept in a fume hood for 5 min to allow
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evaporation of the added solvent. A quartz lid was placed over the top of jacketed beaker to
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reduce evaporation (see Supporting Information Figure S1b). The beaker was subsequently
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transferred to the chamber of the solar simulator. The seawater-oil system was slowly stirred (60
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rpm) with a magnetic stir bar during irradiation. The stirring did not disrupt the oil film on the
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surface of the water. All samples were exposed continuously for 24 h (equivalent to 6 days of
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exposure to average Northern Gulf of Mexico sunlight). Based on solar irradiance data from the
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National Solar Radiation Database at latitude 28.85, longitude -88.35 (near the DWH spill site),
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the average daily amount of direct and diffuse solar radiation received on a horizontal surface at
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this location in 2009 was 4972 W h/m2.35 Our solar simulator produced about 1260 W/m2.
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Dividing this irradiance by the average daily irradiation yields a value of 3.95 h, representing the
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amount of time in our solar simulator equivalent to an average day of irradiation at this location.
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To provide a control sample, each exposure included a dark control (wrapped in aluminum foil to
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prevent exposure to light) without solar irradiation. In addition, a blank irradiated control without
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oil was carried out under the same conditions as irradiated samples. To prevent the oil slick or
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droplets from entering into collected water samples, a small plastic tube was immersed into the
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seawater (close to the bottom of the beaker) and held in place with tape on the outside wall of the
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beaker before adding oil over the seawater. In this manner, the open tip of the tube was not
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exposed to oil at any time. After each exposure, the water samples (40 mL) were collected by
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using a syringe to withdraw the sample through the small tube. Collected samples were kept at 4
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°C for further use.
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Derivatization Reaction. The derivatization reagent solution was prepared according to the
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literature.36 Briefly, 20 mg of DNPH was dissolved in 16 mL of a solution containing HCl (12 M
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aq.), NP water, and ACN in the ratio 2:5:1 (v/v/v). The reagent solution was stored in the dark in
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a glass vial with a Teflon® seal at 4 °C for later use. Aldehyde and ketone stock solutions (10
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mM) were prepared in 50% ACN in water (v/v) and stored at 4 °C. Prior to derivatization, all
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stock solutions were diluted to 100 µM in 50/50 ACN/water. The irradiated water samples and
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the dark controls were derivatized by directly adding 150 µL of DNPH reagent solution to a 15-
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mL sample in a Teflon® capped glass vial. The reaction was allowed to proceed for at least 12 h
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at room temperature. Standard solutions of aldehydes and ketones were derivatized with the
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same procedure as for the irradiated samples.
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Solid Phase Extraction. All aq. samples and the dark controls were extracted with Bond Elut
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C18 cartridges (500 mg adsorbent, Agilent Technologies) after derivatization. Prior to use, the
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cartridges were preconditioned with 20 mL of ACN and 10 mL of NP water, sequentially. For
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sample extraction, 15 mL of aq. sample were passed through the preconditioned cartridges at a
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flow rate of ~15 mL/min. After that, the cartridges were washed with 50 mL of NP water to
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remove salts from the seawater. Excess DNPH reagent was washed off the cartridges with 20 mL
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of 17% ACN in water. Subsequently, the DNPH-derivatized compounds and other adsorbed
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compounds were eluted with 3 mL of ACN into Teflon® capped glass vials. Prior to analysis, the
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extracts were evaporated to dryness with a gentle stream of ultra high purity nitrogen and then
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redissolved in 1.5 mL of 50% ACN in water, resulting in an overall 10-fold enrichment. The
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samples were stored at 4 °C in the dark for later MS/MS analysis.
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HPLC Conditions. HPLC experiments were carried out on an Agilent 1200 Series liquid
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chromatograph (Agilent Technologies, Santa Clara, CA, USA) system equipped with a GraceTM
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EconosphereTM C18 column (L: 150 mm, ID: 4.6 mm, particle size: 5 µm) and diode array
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absorbance detector. Gradient elution was performed using NP water (A) and ACN (B) with the
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following gradient: isocratic at 20% B for 5 min, 20% B to 100% B in 25 min, and then isocratic
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at 100% B for 5 min. The flow rate was 1.0 mL/min. The column was kept at ambient
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temperature, and the injection volume was 200 µL.
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MS/MS Analysis. Tandem mass spectrometry measurement was performed using a Micromass
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Quattro MicroTM API tandem quadrupole MS/MS system with electrospray ionization (Waters
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Corp., Beverly, MA, USA). MassLynx® software (version 4.0) was used for data acquisition. The
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samples were directly introduced into the MS source with a syringe pump at a flow rate of 5
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µL/min. The MS source and desolvation temperatures were maintained at 220 °C and 450 °C,
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respectively. The desolvation and cone gas flows were set at 100 L/h and 50 L/h, respectively.
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The MS capillary voltage was 3.0 kV. The cone voltage was 20 V. The MS/MS experiments
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were carried out utilizing collision-induced dissociation (CID) with argon as the collision gas
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(1.85 × 10-3 mbar). The collision energy was set at 15 eV for all measurements. The scan time
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and run duration time were 2 sec and 2 min, respectively.
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RESULTS AND DISCUSSION
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Solid Phase Extraction. The main aim of this work was to characterize the aldehyde and ketone
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photoproducts from crude oil-water systems subjected to solar irradiation. All the experiments
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were carried out with natural seawater since it is more relevant to the marine environment. The
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DNPH derivatization was performed in acidic solution with excess DNPH reagent in order to
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promote the reaction efficiency. Unfortunately, the unreacted DNPH reagent was found to
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interfere with the subsequent MS/MS analysis. Therefore, C18 extraction cartridges were used to
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remove unreacted DNPH with ACN in water. It was found that 17% ACN in water was
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appropriate for washing the samples.36 A previous study indicated that under these conditions,
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99% of the reagent was removed with 100% recovery of formaldehyde, acetaldehyde, and
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glyoxal derivatives.36 Here hexanal was used to successfully confirm the previously published
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data with DNPH in excess at a molar ratio of approximately 200. Prior to the HPLC analysis, the
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derivatized hexanal sample was loaded onto the C18 cartridges, washed with 17% ACN in water,
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and then collected with pure ACN. The HPLC result demonstrated that 98.6% of the unreacted
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DNPH reagent was washed away compared with the unwashed sample. At the same time, only
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1.4% of the DNPH derivatized analyte was washed off as shown in Figures 1a and 1b.
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Washed and unwashed irradiated oil samples were also analyzed using tandem mass
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spectrometry. Two representative mass peaks of oil photoproducts ([M-H]-: m/z 295 and m/z
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417) were monitored during the measurement as indicated in Figures 1c, 1d, 1e, and 1f. Based on
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the MS/MS spectra, there was a high background generated and no obvious mass peak was
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observed without washing with 17% ACN. In contrast, there were clearly observed typical
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fragments of DNPH derivatives in the washed samples. Crude oil is an extremely complex,
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natural mixture. The MS/MS results indicate that C18 cartridge washing with 17% ACN in water
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reduced the complexity of the background caused by crude oil compounds. These results show
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that C18 cartridge washing with 17% ACN is suitable for determining aq. aldehyde and ketone
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photoproducts in a complex, crude oil containing aq. matrix.
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The results of this study do not include photoproducts of volatile oil compounds or volatile
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photoproducts due to limitations of the method. The volatiles in crude oil primarily include
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alkanes with
