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GC/MS analysis of trace FAMEs in jet fuel using
Method IP PM DY/09

By James D. McCurry, Ph.D.
Agilent Senior Scientist

Aviation fuels can be contaminated with fatty acid methyl esters (FAMEs) from automotive biodiesel that is transported in the same pipelines. FAMEs are a growing concern because their effects on jet engines are not yet understood. To address the difficult analysis of FAMEs in aviation fuel, the Energy Institute recently developed a new GC/MS method that requires both selective ion monitoring (SIM) and scan modes. The Agilent 5975C Series GC/MSD is ideal for this analysis because it can capture both SIM and scan data in a single run, while meeting all method requirements.

A complex and unwanted contaminant

Multi-product pipelines (MPPs) transport different types of liquid hydrocarbon fuels, including jet fuel. Recently, the increased MPP transport of biodiesel fuel has created a new source of jet fuel contamination, namely FAMEs. While the effects of FAME contamination are still being studied, manufacturers of commercial aircraft engines have placed a 5 mg/kg (ppm) limit on total FAMEs in jet fuel.

Biodiesel fuel is a mixture of 5 to 20 percent by weight FAMEs in petroleum diesel. The chemical structure of FAMEs consists of a nonpolar long-chain hydrocarbon coupled to a polar methyl ester group. FAMEs are made from a variety of renewable resources, principally vegetable oils and animal fats. Due to the varied nature of these oils, many different saturated and unsaturated FAMEs are found in biodiesel. Because it would be difficult to measure every FAME in jet fuel, the Energy Institute has determined that six FAMEs represent 95% of the potential biodiesel sources.[1] These are shown in Table 1.

Figure 1. These total ion chromatograms (TICs) show the improved signal-to-noise ratio obtained with the SIM acquisition. (Enlarge image.)

Figure 2. Calibration curves of each FAME from 0 to 50 mg/kg show linearity that exceeds method requirements. (Enlarge image.)

Figure 3. Calibration curves of each FAME from 0 to 5 mg/kg demonstrate excellent correlation coefficients of better than 0.999. (Enlarge image.)

Figure 4. Comparison of the SIM total ion chromatograms shows that you can clearly detect the FAME spikes above the jet fuel blank. (Enlarge image.)

GC/MS analysis overcomes GC resolution problems

Due to the complexity of jet fuel, a single capillary GC column cannot resolve the FAMEs from the hydrocarbon matrix. To solve this problem, the Energy Institute has developed Method IP PM DY/09 using GC/MS to selectively detect and quantify each FAME in jet fuel.[1] The mass spectrum of each FAME has several unique ions not found in the hydrocarbon matrix, so the method uses SIM to optimize the selectivity and sensitivity. To aid identification, the method also specifies the simultaneous acquisition of a full mass spectrum from m/z 30 to 330 for each FAME. This dual data acquisition technique is known as SIM/SCAN.

To demonstrate this method, we configured an Agilent 5975C Series GC/MSD system with an Agilent 7693A Series Automatic Liquid Sampler and an Agilent J&W HP-INNOWax column (part number 19091N-205). We prepared FAME calibration standards in dodecane, and obtained a commercial sample of jet fuel from a local refiner. This sample did not contain any FAMEs and was used as a blank. We also prepared two matrix spikes using this jet fuel sample, with 5 and 1 mg/kg total FAMEs, respectively. We analyzed the standards, blank, and matrix spikes under identical conditions, as outlined in the method.

Figure 1 shows the SIM/SCAN data obtained for the calibration standard that contained 0.5 mg/kg of each FAME in dodecane. The higher signal-to-noise ratio of the SIM data shows why this data is used to quantify trace levels of FAMEs in jet fuel.

All results meet method requirements

Per the method, we obtained two sets of calibration curves for each FAME. The first set of curves ranges from 0 to 50 mg/kg (Figure 2), and is used to quantify FAMEs in samples that contain greater than 5 mg/kg FAMEs. The second set of curves (Figure 3) ranges from 0 to 5 mg/kg and is used to quantify samples that contain FAMEs below 5 mg/kg. All correlation coefficients must exceed 0.985, and this requirement is met in both sets of plots.

The data from the two matrix spikes and the matrix blank are shown in Figure 4. Compared with the matrix blank, the FAME peaks are easily observed at both concentration levels.

We ran each spiked sample three times to calculate the measurement precision. At the 5 mg/kg total FAME limit, we calculated an average of 5.1 mg/kg with a standard deviation of 0.10 mg/kg. For the more challenging lower concentration spike of 1 mg/kg of total FAME, three runs produced an average of 1.1 mg/kg with a standard deviation of 0.12 mg/kg.

Successful analysis with simultaneous SIM and scan

The Agilent 5975C GC/MSD system is an excellent platform for the measurement of trace FAMEs in jet fuel using Energy Institute Method IP PM DY/09. You can easily set up the system for simultaneous SIM/SCAN data acquisition, to maximize sensitivity and selectivity and to provide full spectra for qualitative analysis. A single analysis is all you need to accomplish both types of acquisition.

Using the calibration procedure described in the method, the 5975C met the linearity requirements for every FAME at both the low and high concentration ranges, and triplicate analyses of matrix spikes demonstrated excellent precision and accuracy. For more details about this method and use of the Agilent 5975C Series GC/MSD to analyze FAMEs in jet fuel, view this recorded e-seminar.

Reference

1. IP PM DY/09 “Determination of fatty acid methyl esters (FAME), derived from biodiesel, in aviation turbine fuel – GC-MS with selective ion monitoring/scan detection method,” the Energy Institute, London, UK.

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