Chromatography Focus Analysis of Samples from the Gulf of Mexico Oil Spill by GCxGC-TOFMS “
Two petroleum samples from the Gulf of Mexico Oil Spill were obtained: a Well Head Source Oil sample and a sample of Red Mousse. The Well Head Source Oil sample was collected by a Remotely Operated Vehicle (ROV) at the initial leaks in the damaged riser pipe. The Red Mousse sample was collected on the surface in the offshore marine environment. It had not been exposed to the surf zone. The samples provided had already been diluted in dichloromethane and had internal standards added prior to shipment to the lab for GCxGC- TOFMS analysis.
This analysis took advantage of a new feature allowing the analyst to vary the modulation period and hot pulse duration during an acquisition. Leco would like to take this opportunity to thank Professor Ed Overton for providing the samples analysed in this project.
Instruments and Methods
In this study, measurements were made with a Leco Pegasus 4D GCxGC-TOFMS system. It was equipped with a Gerstel MPS2 rail-type auto sampler. Data collection and work-up were performed using ChromaTOF version 4.30.
For this study, the primary analytical column was a 30.0m x
0.25mm ID x 0.25µm df Rtx-5SilMS. The secondary column was a 1.50 m x 0.10mm ID x 0.10µm df BPX-50. The temperature program started at 45°C with a 3 minute
hold, and then ramped at 5°C/min to 340°C with a final hold of 15 minutes. The column offset was +5°C with a +20°C modulator offset. Variable modulation period and hot pulse durations were utilised. From time 0 seconds to 1280 seconds, a 3 second modulation period with a 500 ms hot pulse was utilised. From time 1280 seconds to 2044 seconds, a 4 seconds modulation period with a 600 ms hot pulse was utilised. From time 2044 seconds to 2999 seconds, a 5 seconds modulation period with a 800 ms hot pulse was utilised. From time 2999 seconds to the end of the analysis, a 5 seconds modulation period with a 1000 ms hot pulse was utilised.
Acquired data was saved for a range from 45 to 400 m/z at 200 spectra/seconds. Helium was used as the carrier gas at a corrected constant flow of 1.0mL/min. A split/splitless inlet, operated in split mode, was used for sample introduction. The inlet was maintained isothermally
The ability to identify the chemical make-up of these incredibly complex samples, including the source oil as well as the changes in composition resulting from weathering and additional chemicals used in response to the spill will be an important tool utilised by those responding to this event
at 280°C. An injection size of 1.0µL was used for each analysis. The Well Head Source Oil was analysed with a split ratio of 200:1. The Red Mousse sample was analysed with a split ratio of 50:1.
Results
Both of the samples were data processed with a s/n ratio threshold of greater than or equal to 100:1 necessary to be identified as a peak. The peak finding algorithm of the data processing method was not applied in the regions where the solvent tail and column bleed are located. A Contour Plot of the ‘Well Head Source Oil’ sample is shown in Figure 1. The Contour Plot with peak markers is shown in Figure 2. When data processed, 2670 peaks were identified in the Well Head Source Oil sample. Four deuterated internal standards were added to this sample, naphthalene-d8, acenaphthene-d10, chrysene-d12 and perylene-d12. The peaks identified in this analysis range
from benzene, with a 1st dimension retention time (tr) of 368 seconds and a 2nd dimension tr of 1.530 seconds, to dotetracontane (n-C42), which has a 1st dimension tr of 4559 seconds and a 2nd dimension tr of 3.315 seconds. [The most retained analyte in the 2nd dimension is 2-methyl
chrysene, which has a 1st dimension tr of 3114 seconds and a 2nd dimension tr of 3.525 seconds.] [The most retained compound in the 2nd dimension is the internal standard perylene-d12, which has a 1st dimension tr of
3369 seconds and a 2nd dimension tr of 4.260 seconds]. The four internal standards are highlighted in Figure 3.
Figure 3: A contour plot highlighting the retention plane locations of the four internal standards, naphthalene d-8, acenaphthene-d10, chrysene-d12 and perylene-d12.
Figure 1: A contour plot of the Well Head Source Oil sample. The use of three distinct modulation periods is indicated by the ‘notched’ appearance in the upper left corner of the contour plot.
Figure 4: A surface plot of the Well Head Source Oil sample showing peak shape of the early eluting compounds.
Author Details: Leco Corporation
Life Science & Chemical Analysis Centre St. Joseph, Michigan USA
info@leco.com
Figure 2: A contour plot of the Well Head Source Oil sample showing peak markers for the 2670 peaks with a s/n ratio of ≥ 100. The 4 peaks indicated by white peak markers are the deuterated internal standards.
The use of three distinct modulation periods is indicated by the ‘notched’ appearance in the upper left corner of the Contour Plot. In the past, a modulation period time of at least as long as that of the analyte with the longest 2nd dimension retention time was required. The duration of the hot pulse was set to provide the best overall 2nd dimension peak shape across the entire chromatogram. Decreasing the modulation period in regions of the analysis that do not contain analytes that are highly retained on the 2nd
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