Chemistry
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Nitrogen Compound Characterization in Fuels by Multidimensional Gas Chromatography
Chapters
Summary May 15th, 2020
Here, we present a method utilizing two-dimensional gas chromatography and nitrogen chemiluminescence detection (GCxGC-NCD) to extensively characterize the different classes of nitrogen-containing compounds in diesel and jet fuels.
Transcript
This is a method for the detailed characterization of nitrogen species in jet and diesel fuel. It allows a user to generate compound class-level information about the distribution of nitrogen within a fuel, which can provide greater insight into fuel behavior and performance. This method uses cost-efficient, commercially available instrumentation, allows for quantitation from a single standard, and requires limited sample preparation.
A secondary GC column must be looped within the modulator. And if it's not aligned perfectly, the chromatography would be useless, so just take your time to get it aligned. Before beginning an experiment, perform the calibration standards as indicated, and add one milliliter of each standard into individual gas chromatography vials.
For column installation, first place a 30-meter primary column into the gas chromatography oven, and connect the column to the splitless inlet. Cut 2.75 meters of the secondary column, and use a Wite-Out pen to place a mark on the column at 0.375 and 1.375 meters. Place the secondary column into the Zoex modulator column holder, and use the marks as guides for creating a one-meter loop within the holder.
Use a micro-union to connect the shorter end of the secondary column to the primary column. To check for successful connection, turn on the gas flow, and insert the open end of the column into a vial of methanol. A successful connection can be confirmed by the presence of bubbles.
Place the column holder into the modulator, and adjust the loops as necessary so that they line up properly with the cold and hot jets. Then insert the other end of the column into the nitrogen chemiluminescence detection burner, and turn on all of the burners and gas flows to ensure there are no leaks. In the computer software, set the instrument parameters as indicated in the table.
Set the initial oven temperature to 60 degrees Celsius with a ramp rate of five degrees Celsius per minute to 160 degrees Celsius. When the oven reaches the target temperature, change the ramp rate to four degrees Celsius per minute until the oven reaches 300 degrees Celsius, with a total run time of 55 minutes per sample. Set the hot jet temperature to be 100 degrees Celsius higher than the oven temperature at any point in time.
Then set the ancillary liquid nitrogen Dewar connected to the gas chromatographer to stay between 20 and 30%full during the run. To calibrate the gas chromatographer, place the gas chromatography sample vials containing the prepared carbazole standards into the instrument, and load the previously configured method into the gas chromatography software. Create a sequence that aliquots the blank at the beginning of the analysis, followed by the prepared carbazole standards by increasing concentration.
Once the calibration standard set analysis is completed, use the Gas Chromatography Image software to load each chromatogram, to correct the background, and to detect each carbazole peak or blob. In a spreadsheet program, plot the blob volume response against the nitrogen concentration of each calibration standard to create a calibration curve. The trend line of the curve should have an R squared of greater than or equal to 0.99.
To analyze the samples, place the gas chromatography sample vials into the autosampler tray, and load the previously configured method. Create a sequence that has a blank at the beginning and after every five samples to limit any buildup of fuel within the columns. Then verify that enough liquid nitrogen is available in the Dewar for the modulator and that all of the instrument parameters are in ready mode before starting the sequence.
To analyze the sample data, open the chromatogram of interest in the Gas Chromatography Image software for data analysis, and perform a background correction. To detect blobs, set the filter parameters to a minimum area of 25, a minimum volume of zero, and a minimum peak of 25. Use the Gas Chromatography Image template function to create or load a template to the appropriate group nitrogen compound classes based on the elution times of the known standards.
When all of the compounds have been grouped, export the blob set table into a spreadsheet program, and use the sum, the volume of all of the blobs and peaks within each compound class group, and the calibration equation to calculate the concentration in parts per million for the nitrogen compounds in each group. Carbazole elutes at approximately 33 minutes from the primary column and at two seconds from the secondary column. There is not any tailing, and the standard response is outside of the noise.
Here, a two-dimensional gas chromatography and nitrogen chemiluminescence detection chromatogram with a carbazole standard and the resulting blob table are shown. As observed, there are two detected blobs that are not within the carbazole elution time that are excluded from the blob table. In these figures, a typical chromatogram obtained using this method on a diesel fuel sample and a jet fuel sample are shown.
Typically, jet fuel has fewer nitrogen compounds at lower concentrations than diesel fuel, which can be clearly observed when comparing the two chromatograms. In contrast, in this failed sample measurement, the modulation time was incorrect for the oven temperature, resulting in wrap-around in the column. In this failed sample measurement, a streaking effect of the blobs occurred due to the compounds being retained on the column for too long and destroying any compound separation.
Standards can be utilized to determine the groups associated with each nitrogen compound class, for example, as illustrated in this figure. A similar GC-by-GC sulfur chemiluminescence detection method can also be performed in order to provide even more information about the heteroatomic content in the fuel. This method allows fuel chemists to explore the impact of specific nitrogen compounds on fuel stability and performance beyond a typical total nitrogen measurement.
Both the fuel samples and the solvents are flammable, and the instrument gets extremely hot. Be sure to wear personal protective gear, and do not touch the instrument unless it is cool.
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