Analytical Instrumentation
GC-VUV Method Provides a Diesel Tank Corrosion Screening Tool
VUV Analytics, Inc. recently developed a GC-VUV and static headspace method to monitor for low-level ethanol and methanol contamination of ultra-low sulfur diesel (ULSD) underground storage tanks. Ethanol is a common “oxygenate” added to automobile gasoline in the United States to enable more complete combustion and reduce air pollution. Typically, ethanol is not used in diesel fuel but may find its way into diesel storage tanks through cross-contamination with ethanol-containing gasoline (e.g., E10, which contains 10% ethanol). According to a recent EPA report, ethanol may support microbiologically-influenced corrosion (MIC) of diesel storage and delivery systems when low molecular weight organic acids, such as acetic acid, are formed during ethanol’s microbial degradation by certain bacteria1
contribute to MIC via formation of formic acid when diesel contains biodiesel. Corrosion has been reported to cause failures in valves, filters, seals, and other equipment which ultimately leads to underground storage tank leaks and groundwater pollution.
Detectability of polar alcohols at low levels on a gas chromatograph (GC) can be problematic due to its tendency to produce broad, tailing peaks on GC columns or be lost completely due to adsorption. Static headspace offers an elegant solution to the GC sample introduction problem by employing a non-volatile mineral oil diluent that allows diesel sample volumes of several hundred microliters. 200 µL of diesel is placed in 2 mL mineral oil in a 20-mL headspace vial, heat/shake equilibrated for 10 min in a Gerstel MPS2 autosampler, followed by injection of the headspace into a GC-VUV system to collect absorbance spectra for methanol and ethanol. Hydrophobic mineral oil diluent suppresses volatilisation of the diesel hydrocarbons that would otherwise overload the GC column, while simultaneously promoting ethanol transfer to the headspace for sampling and analysis.
GC-VUV absorbance data is three dimensional (time, absorbance, wavelength) and specific to compound chemical structure. Compounds that indirectly contribute to corrosion such as water and alcohols can be positively identified by VUV spectroscopy in complex matrices like ultra-low sulfur diesel (ULSD). In addition, ethanol and methanol spectra are easily differentiated. Gasoline hydrocarbon compounds of interest that co-elute during the GC run can be spectrally deconvolved for accurate quantitative analysis.
An example of the GC-VUV and static headspace method for ethanol and methanol analysis is shown in Figure 1. Methanol and ethanol diluted in DMSO to concentrations of 5, 10, 20, 50, 100, 200, 500, and 1000 ppm were spiked into 200 µL diesel. An Agilent 6890 GC was used with its injection port set to 250°C. A helium carrier gas flow rate of 4 mL / min was used throughout the experiment. The oven profile for the residual solvent analysis began at 35°C (held for 1 min), followed by an increase to 275°C at a rate of 30°C/min. A split ratio of 2.5 was utilized to help maximise sensitivity. A total GC runtime of 12 minutes was programmed for each sample analyzed. A VGA-100 GC detector was used in this experiment with the transfer line & flow cell temperatures set to 275°C. The makeup gas pressure used was approximately 0.36 psi. An acquisition range of 120 to 240 nm was selected with an acquisition speed of 4.5 spectra/sec. A GERSTEL Multi-Purpose Sampler (MPS) was used with the syringe temperature held constant at 90°C, an incubation temperature set to 80°C, and an incubation time set to 10 min. An injection volume of 250 µL was selected to ensure good peak shape while the agitator was set to 250 rpm with an injection speed of 200 µL/sec. Figure 1 demonstrates excellent dynamic quantitative range for methanol and ethanol between 5 – 1000 ppm in diesel. Inset shows an overlay of peaks corresponding to water, methanol, and ethanol at concentrations ranging from 5 – 100 ppm concentration spikes. The ability to simultaneously detect water opens the possibility of a more rigorous approach to water analysis in fuel stream samples.
References
(1) Investigation of Corrosion-Influencing Factors in Underground Storage Tanks with Diesel Service, U.S. Environmental Protection Agency Office of Underground Storage Tanks, EPA 510-R-16-001, July 2016.
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. Methanol, a component of biodiesel production, is also suspected to
Figure 1: GC-VUV and static headspace GC-VUV method for methanol and ethanol detection in Diesel. A chromatographic overlay of 5, 10, 20, 50, 100, 200, 500, 1000 ppm methanol and ethanol spikes in 200 µL diesel is shown. An overlay of peaks corresponding to water, methanol, and ethanol at concentrations ranging from 5 – 100 ppm concentration spikes can be seen in the chromatogram zoom inset.
VUV Absorption Spectroscopy.
GC detection in a whole new light.
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Universal, yet selective detector with very sensitive spectral response
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