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Ensuring Accurate Gulf Oil AnalysesMultiple sample prep solutions are being created to support the oil spill clean up.by Sky Countryman, Manager of Applied Technology, Phenomenex, Torrance, CA
The Gulf Oil Spill will go down as one of the greatest environmental disasters in American history. Revised estimates published by the National Incident Command’s Flow Rate Technical Group (FRTG) put the total at 4.9 million barrels of oil that have been released. While ongoing efforts are in place to help contain the oil and prevent it from further contaminating the coastline, there will undoubtedly be a lasting impact from this spill.
During the clean up process, there is a need for rapid and robust analytical procedures to help understand the impact of the hydrocarbon contamination that is being encountered. The Louisiana Sweet Crude, which constitutes the majority of the oil, comprises a variety of compounds such as aliphatic, aromatic, and alkyl-aromatic hydrocarbons. This article will discuss several different techniques that are currently being used in support of the Gulf Oil Spill clean up.
In order to understand the impact of the Gulf Oil Spill contamination, the oil must first be extracted from the matrix and subjected to various clean up procedures to remove potential interferences. There are various procedures that can be used for this work, including Gel Permeation Chromatography (GPC) and Solid Phase Extraction (SPE).
Oil can be extracted directly from water samples using either liquid-liquid extraction (LLE) or SPE. The advantage of SPE is a reduction in the amount of environmentally unfriendly solvent required to perform the extraction. In addition, recoveries for the lighter Polyaromatic Hydrocarbon (PAH) isomers, thought to be a major component of the Louisiana Sweet Crude, may be improved. The lighter PAH isomers are extremely volatile and can be lost during the concentration steps used in LLE methodologies. Recovery data published in EPA methods such as 505.1 report recoveries of 70.5 percent with greater than 7 percent RSD.
Using the Strata-PAH SPE cartridges, recoveries greater than 85 percent were achieved for even the light PAHs such as Naphthalene (Table 1). A significant benefit derived from these specially designed tubes is the removal of common environmental contaminants such as humic acid, which is formed by decaying plant matter often found in marshes or ground water deposits. Humic acid can interfere with accurate quantitation of PAH isomers using the techniques described later, so their removal is critical.
Table 2: Fractionation of Aliphatic components in a Diesel Fuel Sample Click to enlarge. | Oil leaked into the Gulf of Mexico has contaminated more than just the sea water; it has been found in plants, animals, and on the beach itself. Shellfish are at particular risk of contamination and are the subject of intense study. The analysis of PAHs and other hydrocarbons from shellfish can be challenging due to the lipid content in animal tissue. The National Oceanic and Atmospheric Administration (NOAA) Technical Memorandum NMFS-NWFSC-59 for extraction and clean up of sediments and tissues identifies GPC as a clean-up technique for removing high molecular weight impurities using the EnviroSep-ABC 350- x 21.2-mm column.
Figure 1 shows the calibration solution indicating the window that would be used to collect the more toxic PAHs. The EnviroSep-ABC material was specifically designed to be used for this type of clean-up methodology. The carefully controlled particle size and pore size distribution of this material ensures that it maximizes separation between the target PAH isomers and the high molecular weight impurities.
Table 3: Fractionation of Aromatic components in a Diesel Fuel Sample. Click to enlarge | Traditional approaches to hydrocarbon testing provide a sum total of the material present in the sample but make no distinction between the different types of hydrocarbons. While these methods give a general understanding of the sample, they do not accurately describe its toxic potential. In response, many states have adopted testing methods that characterize a sample based on the specific aromatic and aliphatic compounds present in the sample.
These newer methodologies utilize a silica gel SPE cartridge to fractionate the sample into an aromatic and an aliphatic portion (Tables 2 & 3). The fractions are then analyzed separately by GC-FID or GC-MS and the level of specific compounds that are known to be toxic can be measured. The Strata EPH material was specially designed to overcome common problems with these methods such as tube contamination and tube-to-tube flow consistency. The tube material and flow characteristics are specially controlled to reduce contamination and improve tube-to-tube consistency.
Analysis using GC, GC-MS and HPLC
The widespread contamination and high volume of samples being tested creates a need for a rapid and robust testing procedure. Since the capabilities of each lab are different, various approaches will be needed to make informed decisions. The EPA’s sampling plan from May 2010 utilizes EPA Method 8015B for Total Petroleum Hydrocarbon (TPH) and EPA Method 8270 for semi-volatile contaminants including PAHs.
Figure 1: NOAA Calibration Solution on the EnviroSep-ABC Column. Click to enlarge | Total petroleum hydrocarbon analysis is a sum of what are considered the gasoline range organics (GRO) and the diesel or oil range organics (DRO or ORO). The lighter GRO fraction is commonly analyzed using purge and trap or other techniques that introduce only the volatile portion of the sample (Figure 2). The Zebron ZB-1 is able to improve retention for these volatile compounds, allowing for more accurate determination of the light hydrocarbons present in the sample. This is important because the lighter hydrocarbons are the ones that the population in the region is likely to be exposed to through inhalation as these more volatile compounds evaporate.
The heavier DRO or ORO portion of the sample can include hydrocarbons from C10 to greater than C44. Based on the high asphaltene content in the early samples received by Ed Overton’s group at Louisiana State Univ., the gulf oil crude may contain very heavy hydrocarbons that cannot be eluted using traditional polyimide-coated GC columns. In such cases it will require the use of specialized metal GC columns, such as the ZB-1XT SimDist columns, which can be used to characterize hydrocarbon samples that contain species higher than C120. Since the ZB-1XT SimDist column was formulated to work at temperatures up to 450 C, they provide improved lifetime at the lower temperatures used to characterize standard DRO and ORO samples while providing the flexibility to handle high asphaltene samples that may be received from the gulf spill clean up.
Figure 2: Fast Gasoline Range Organic Separation using Zebron ZB-1. Click to enlarge. | PAHs are a particularly toxic class of compounds found in petroleum products that pose a significant health risk to anyone exposed. The EPA gives two primary techniques for the analysis of PAHs in solid materials, GC-MS and HPLC. The primary method proposed by EPA for the clean up effort is Method 8270, which uses GC-MS.
The standard method for PAH analysis utilizes a 30-m GC column and requires greater than 30 minutes to elute all the PAH isomers. Due to the high sample volume being received by the laboratories, it is advantageous to decrease this analysis time so that more samples can be analyzed per day. By optimizing the column dimension and method parameters on the Zebron ZB-5ms, the analysis can be shortened to only 11 minutes, which is more than a 150 percent increase in sample throughput (Figure 4). This simple change would allow a lab to run almost three times as many samples per day.
Figure 3: Characterization of an Oil Sample Using the Zebron ZB-1XT
SimDist Column Click to enlarge. | The analysis of PAHs can also be done using HPLC, following EPA Method 8310 guidelines. The HPLC method allows for improved resolution for certain PAH isomers such as Benzo[b] and Benzo[k]fluoranthene when compared to the GC-MS method. The use of fluorescence detection and large volume injection can also help improve detection limits, allowing for lower level quantitation. Again, standard methodologies require more than 30 minutes per sample, which impacts sample throughput in the laboratories supporting the cleanup efforts.
There have been significant advances in particle technology and HPLC systems in the past several years that allow many older methods to be dramatically improved and analysis times decreased. One such technology is the Kinetex core-shell particle, which provides ultra-high efficiency separations on standard HPLC instruments. Using these columns, the separation of the standard 16 PAH compounds can be done in less than 6 minutes (Figure 5).
Conclusion
Clean-up efforts following the Gulf Oil Spill are ongoing and there is a lot of work to be done. Due to the magnitude of this spill, there will be a lot of different types of samples that need to be tested and there is not one specific technique that can be applied to all. The solutions presented here represent just a few of what is available for the analysis of hydrocarbon samples. Additional solutions are being developed every day for other chemicals, such as dispersants, used in the clean-up process.
For more information about these and other solutions that are available, please visit: www.phenomenex.com
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