Chemical Analysis of Water-accommodated Fractions of Crude Oil Spills Using TIMS-FT-ICR MS

Summary

The low-energy water-accommodated fraction (LEWAF) of crude oil is a challenging system to analyze, because over time, this complex mixture undergoes chemical transformations. This protocol illustrates methods for the preparation of the LEWAF sample and for performing photo-irradiation and chemical analysis by trapped ion mobility spectrometry–FT-ICR MS.

Abstract

Multiple chemical processes control how crude oil is incorporated into seawater and also the chemical reactions that occur overtime. Studying this system requires the careful preparation of the sample in order to accurately replicate the natural formation of the water-accommodated fraction that occurs in nature. Low-energy water-accommodated fractions (LEWAF) are carefully prepared by mixing crude oil and water at a set ratio. Aspirator bottles are then irradiated, and at set time points, the water is sampled and extracted using standard techniques. A second challenge is the representative characterization of the sample, which must take into consideration the chemical changes that occur over time. A targeted analysis of the aromatic fraction of the LEWAF can be performed using an atmospheric-pressure laser ionization source coupled to a custom-built trapped ion mobility spectrometry–Fourier transform–ion cyclotron resonance mass spectrometer (TIMS-FT-ICR MS). The TIMS-FT-ICR MS analysis provides high-resolution ion mobility and ultrahigh-resolution MS analysis, which further allow the identification of isomeric components by their collision cross-sections (CCS) and chemical formula. Results show that as the oil-water mixture is exposed to light, there is significant photo-solubilization of the surface oil into the water. Over time, the chemical transformation of the solubilized molecules takes place, with a decrease in the number of identifications of nitrogen- and sulfur-bearing species in favor of those with a greater oxygen content than were typically observed in the base oil.

Introduction

There are numerous sources of environmental exposure to crude oil, both from natural causes and from anthropogenic exposure. Upon release to the environment, particularly in the ocean, the crude oil can undergo partitioning, with the formation of an oil slick on the surface, a loss of volatile components to the atmosphere, and sedimentation. However, low-energy mixing of the poorly soluble oil and the water does occur, and this mixture, which is not classically solubilized, forms what is referred to as the low-energy water-accommodated fraction (LEWAF). The solubilization of the oil components in the water is typically enhanced during exposure of the oil-water interfa....

Protocol

1. Preparation of the Low-energy Water-accommodated Fractions (LEWAF)

1. Clean 2-L aspirator bottles by rinsing the bottles with methylene chloride in order to remove any potential contaminants.
2. Fill bottles with 50 mL of methylene chloride, close them, and agitate for 30 s. Drain them into the appropriate waste container. Repeat for a total of three washes.
3. Use one aspirator bottle for the irradiation exposure and the other bottle as a control sample (perform duplicate sets of each if possible).
4. Prepare artificial seawater by mixing 35 g of sea salt mixture per 1 L of water used for a final salinity of 33 parts-per-thousand....

Results

LEWAF analysis by TIMS-FT-ICR MS results in a two-dimensional spectrum based on m/z and TIMS trapping voltage. Each of the samples, taken at different time points, can therefore be characterized based on the changing chemical composition, as observed by the distribution of chemical formulas and the isomeric contribution identified by the IMS (see Figure 1). Typically, the m/z information can be utilized to assign elemental formulas to the analyzed peaks........

Discussion

Critical Steps within the Protocol

The chemical complexity of LEWAFs requires accurate preparation in order for the laboratory experiments to accurately reflect what occurs naturally. A valid assessment of the data hinges on three criteria: minimizing the introduction of artifacts throughout sample handling (e.g., preparation of the LEWAF, sampling, extractions, and preparation of the sample for analysis), validating the experimental protocol (i.e., using dark controls for the p.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Reagents
methylene chloride, Optima	Fisher Scientific	DS-151-4	Used as received
methanol, Optima LC/MS	Fisher Scientific	A456-4	Used as received
toluene, Optima	Fisher Scientific	T291-4	Used as received
Na2SO4, Granular, Anhydrous	Fisher Scientific	S415-212	Combusted at 450 °C/Stored at 105 °C
Crude oil	Various sources
Instant Ocean®	Aquarium Systems	SS1200	33 ppt salinity with 0.45 μm pore filtration
Name	Company	Catalog Number	Comments
Equipment
Suntext XLS+	Atlas Chicalo Ill, USA		1,500 W xeon arc lamp, light intensity of 765 W/m2
Atmospheric Pressure Laser Ionization	Bruker Daltonics Inc, MA		Note a 266 nm laser is used
TIMS-FT-ICR MS Instrument	Bruker Daltonics Inc, MA		The set up we had consisted of a 7 T magnet with an infinity cell
Name	Company	Catalog Number	Comments
Software
DataAnalysis 4.2	Bruker Daltonics Inc, MA
Python 2.7			Requires Numpy, Scipy, Pandas, glob, oct2py, and os
Octave 4.0