Nitrogen Compound Characterization in Fuels by Multidimensional Gas Chromatography

Summary

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.

Abstract

Certain nitrogen-containing compounds can contribute to fuel instability during storage. Hence, detection and characterization of these compounds is crucial. There are significant challenges to overcome when measuring trace compounds in a complex matrix such as fuels. Background interferences and matrix effects can create limitations to routine analytical instrumentation, such as GC-MS. In order to facilitate specific and quantitative measurements of trace nitrogen compounds in fuels, a nitrogen-specific detector is ideal. In this method, a nitrogen chemiluminescence detector (NCD) is used to detect nitrogen compounds in fuels. NCD utilizes a nitrogen-specific reaction that does not involve the hydrocarbon background. Two-dimensional (GCxGC) gas chromatography is a powerful characterization technique as it provides superior separation capabilities to one-dimensional gas chromatography methods. When GCxGC is paired with a NCD, the problematic nitrogen compounds found in fuels can be extensively characterized without background interference. The method presented in this manuscript details the process for measuring different nitrogen-containing compound classes in fuels with little sample preparation. Overall, this GCxGC-NCD method has been shown to be a valuable tool to enhance the understanding of the chemical composition of nitrogen-containing compounds in fuels and their impact on fuel stability. The % RSD for this method is <5% for intraday and <10% for interday analyses; the LOD is 1.7 ppm and the LOQ is 5.5 ppm.

Introduction

Before use, fuels undergo extensive quality assurance and specification testing by refineries to verify that the fuel they are producing will not fail or cause equipment problems once disseminated. These specification tests include flash point verification, freeze point, storage stability, and many more. The storage stability tests are important as they determine if the fuels have a tendency to undergo degradation during storage, resulting in the formation of gums or particulates. There have been incidences in the past when F-76 diesel fuels have failed during storage even though they passed all specification tests¹. These failures resulted in high concentrations of particulate matter in the fuels that could be detrimental to equipment such as fuel pumps. The extensive research investigation that followed this discovery suggested that there is a causal relationship between certain types of nitrogen compounds and the particulate formation^2,3,4,5. However, many of the techniques used to measure nitrogen content are strictly qualitative, require extensive sample preparation, and provide little information on the identity of the suspect nitrogen compounds. The method described herein is a two-dimensional GC (GCxGC) method paired with a nitrogen chemiluminescence detector (NCD) that was developed for the purpose of characterizing and quanitifying trace nitrogen compounds in diesel and jet fuels.

Gas chromatography is used extensively in petroleum analyses and there are over sixty published ASTM petroleum methods associated with the technique. A wide range of detectors are combined with gas chromatography such as mass spectrometry (MS, ASTM D2789⁶, D5769⁷), Fourier-transform infrared spectroscopy (FTIR, D5986⁸), vacuum ultraviolet spectroscopy (VUV, D8071⁹), flame ionization detector (FID, D7423¹⁰), and chemiluminesence detectors (D5504¹¹, D7807¹², D4629-17¹³). All these methods can provide significant compositional information about a fuel product. Since fuels are complex sample matrices, gas chromatography enhances compositional analysis by separating out sample compounds based on boiling point, polarity, and other interactions with the column.

To further this separation ability, two-dimensional gas chromatography (GCxGC) methods can be utilized to provide compositional maps by using sequential columns with orthogonal column chemistries. Separation of compounds occur both by polarity and boiling point, which is a comprehensive means to isolate fuel constituents. Although it is possible to analyze nitrogen-containing compounds with GCxGC-MS, the trace concentration of the nitrogen compounds within the complex sample inhibits identification¹⁴. Liquid-liquid phase extractions have been attempted in order to use GC-MS techniques; however, it was found that the extractions are incomplete and exclude important nitrogen compounds¹⁵. Additionally, others have used solid phase extraction to enhance the nitrogen signal while reducing the potential for the fuel sample matrix interference¹⁶. However, this technique has been found to irreversible retail certain nitrogen species, especially low molecular weight nitrogen-bearing species.

The nitrogen chemiluminescence detector (NCD) is a nitrogen-specific detector and has been successfully used for fuel analyses^17,18,19. It utilizes a combustion reaction of nitrogen-containing compounds, the formation of nitric oxide (NO), and a reaction with ozone (see Equations 1 & 2)²⁰. This is accomplished in a quartz reaction tube that contains a platinum catalyst and is heated to 900 °C in the presence of oxygen gas.

The photons emitted from this reaction are measured with a photomultiplier tube. This detector has a linear and equimolar response to all nitrogen-containing compounds because all nitrogen-containing compounds are converted to NO. It is also not prone to matrix effects because other compounds in the sample are converted to non-chemiluminescence species (CO₂ and H₂O) during the conversion step of the reaction (Equation 1). Thus, it is an ideal method for measuring nitrogen compounds in a complex matrix such as fuels.

The equimolar response of this detector is important for nitrogen compound quantitation in fuels because the complex nature of fuels does not allow for calibration of each nitrogen analyte. The selectivity of this detector facilitates the detection of trace nitrogen compounds even with a complex hydrocarbon background.

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Protocol

CAUTION: Please consult relevant safety data sheets (SDS) of all compounds before use. Appropriate safety practices are recommended. All work should be performed while wearing personal protective equipment such as gloves, safety glasses, lab coat, long pants, and closed-toed shoes. All standard and sample preparations should be done in a ventilated hood.

1. Preparation of standards

1. Prepare a 5,000 mg/kg (ppm) solution of carbazole (calibration standard, minimum of 98% purity) by placing 0.050 g in a vial and bring the total mass of each solution to 10.000 g with isopropyl alcohol. Cap the vial immediately to prevent loss of isopropyl alcohol. This is the calibration stock solution.
2. Prepare a carbazole solution with 100 ppm nitrogen content by diluting 1.194 mL of stock solution to 5 mL with isopropyl alcohol. This is designated as "100 ppm nitrogen carbazole" and is used to create the calibration standards.
   NOTE: The concentrations of the calibration standards indicate the concentration of nitrogen in the standard, not the carbazole concentration
3. Prepare the following calibration standards by serial dilution:
   20 ppm nitrogen carbazole
   10 ppm nitrogen carbazole
   5 ppm nitrogen carbazole
   1 ppm nitrogen carbazole
   0.5 ppm nitrogen carbazole
   0.025 ppm nitrogen carbazole
4. Put 1 mL of the calibration standards into separate GC vials (6 vials total).
5. Prepare individual 10 ppm solutions of each standard compounds listed in Table 1 in isopropyl alcohol. Place 1 mL of each standard solution in separate GC vials (10 vials total).
   NOTE: The standard compounds listed in Table 1 will be used to classify the unknown nitrogen compounds as 'light nitrogen compounds', 'basic nitrogen compounds', or 'non-basic nitrogen compounds'.

Standard Compound	Elution Time Classification Group
Pyridine	Group 1 – light nitrogen compounds
Trimethylamine	Group 1 – light nitrogen compounds
Methylaniline	Group 1 – light nitrogen compounds
Quinoline	Group 2 – basic nitrogen compounds
Diethylaniline	Group 2 – basic nitrogen compounds
Methylquinoline	Group 2 – basic nitrogen compounds
Indole	Group 2 – basic nitrogen compounds
Dimethylindole	Group 2 – basic nitrogen compounds
Ethylcarbazole	Group 3 – Non-basic nitrogen compounds
Carbazole	Group 3 – Non-basic nitrogen compounds

Table 1: Nitrogen standards and their elution classification groups.

2. Sample preparation

1. For diesel fuels: In a GC vial, add 250 µL of fuel sample and 750 µL of isopropyl alcohol.
2. For jet fuels: In a GC vial, add 750 µL of fuel sample and 250 µL of isopropyl alcohol.
   NOTE: If the total nitrogen concentration of either diesel or jet fuel falls below the calibration curve (0.025 ppm nitrogen) when diluted as instructed above, do not dilute. If the nitrogen concentration in a specific nitrogen group in either diesel or jet fuel falls above the calibration curve (20 ppm nitrogen), dilute sample further.

3. Instrument setup

1. Instrument configuration
   1. Auto-sampler: Ensure that the autosampler tray and tower are installed with a splitless inlet and wash vials in place.
   2. Nitrogen Chemiluminescence Detector: Ensure that the nitrogen chemiluminescence detector is installed with the appropriate gas lines (i.e., helium and hydrogen). A hydrogen generator can be utilized instead of a tank, if available.
   3. Duel Loop Thermal Modulator: Ensure that the duel loop thermal modulator is installed and aligned properly so that the column loop will be centered between the cold and hot jet flows during modulation.
2. Column installation
   1. Ensure that the instrument is in maintenance mode (i.e., all burners and gas flows are turned off).
   2. Insert the 30 m primary column into the GC oven and connect to the splitless inlet.
   3. Measure and cut 2.75 m of the secondary column. Put a mark on the secondary column at 0.375 m and 1.375 m using a white-out pen.
   4. Place the secondary column into the Zoex modulator column holder and use the marks as guides for creating a 1 m loop within the holder for modulation.
   5. Connect the shorter end of the secondary column to the primary column using a micro-union. Check for a successful connection by turning on the gas flow and inserting the open end of the column into a vial of methanol. A successful connection is confirmed by the presence of bubbles.
   6. Place the column holder into the modulator and adjust the loops as necessary so that the loops line up properly with the cold and hot jets, as pictured in Figure 1.
   7. Insert the other end of the column into the NCD burner. Then turn on all burners and gas flows to ensure there are no leaks.
   8. Turn the oven on at the maximum temperature limit for a minimum of 2 h in order to bake-out the columns. Once completed, verify that there are no new leaks. Then, cool the oven.

Figure 1: Schematic representation of the GCxGC-NCD instrumentation. This figure has been reprinted from Deese et al. Please click here to view a larger version of this figure.

3. Method parameters
   1. Using the computer software, set the instrument to the parameters listed in Table 2.
   2. Set the initial oven temperature to 60 °C with a ramp rate of 5 °C/min to 160 °C, and then change the ramp rate to 4 °C until 300 °C. The total run time is 55 minutes per sample.
   3. Set the hot jet temperature to be 100 °C higher than the oven temperature at any point in time. Thus: set the initial hot jet temperature to 160 °C with a ramp rate of 5 °C/min to 260 °C, and then change the ramp rate to 4 °C until 400 °C.
   4. Set the ancillary liquid nitrogen Dewar connected to the GC to stay between 20% and 30% full during the run.

Instrument Parameters
NCD	Nitrogen Base Temperature	280 °C
	Nitrogen Burner Temperature	900 °C
	Hydrogen flow rate	4 mL/min
	Oxidizer flow rate (O2)	8 mL/min
	Data collection rate	100 Hz
Inlet	Inlet Temperature	300 °C
	Inlet liner	Splitless
	Purge flow to split vent	15 mL/min
	Septum Purge Flow	3 mL/min
	Carrier gas	He
	Carrier gas flow rate	1.6 mL/min
	Syringe size	10 µL
	Injection volume	1 µL
Modulator	Modulation time	6000 ms
	Hot pulse duration	375 ms
Columns	Flow	1.6 mL/min
	Flow type	Constant flow

Table 2: Instrument parameters.

4. Instrument calibration

1. Place the GC sample vials containing the prepared carbazole standards and load the previously configured method into the GC software.
2. Create a sequence that aliquots the blank (isopropyl alcohol) at the beginning followed by the prepared carbazole standards by increasing concentration.
3. Verify that the liquid nitrogen Dewar is between 20-30% full and all instrument parameters are in "ready" mode. Start the sequence.
4. Once the calibration standard set analysis is completed, use the GCImage software to load each chromatogram, background correct, and detect each carbazole peak or blob.
   NOTE: In GCImage, the detected peaks within the chromatogram are termed as "blobs" by the software.
5. In a spreadsheet program, plot the response (blob volume) against the nitrogen concentration (ppm) of each calibration standard to create a calibration curve (see Figure 2). The trend line of the curve should have R² ≥ 0.99.

Figure 2: Example GCxGC-NCD carbazole calibration curve. Please click here to view a larger version of this figure.

5. Sample analysis

1. Place the GC sample vials in the autosampler tray and load the previously configured method.
2. Create a sequence that has a blank (isopropyl alcohol) at the beginning and then every 5 subsequent samples to limit any build-up of fuel within the columns.
3. Verify that enough liquid nitrogen is available in the modulator's Dewar and that all instrument parameters are in "ready" mode. Then, start the sequence.

6. Data analysis

1. Open the chromatogram in the GCImage software for data analysis and perform a background correction
2. Detect blobs using the following filter parameters:
   Minimum area = 25
   Minimum volume = 0
   Minimum peak = 25
   NOTE: These parameters are subject to change based on instrument response or sample matrix.
3. Use the GCImage template function to create or load a template to group nitrogen compound classes based on the elution times of the known standards (see Table 1).
   NOTE: Further explanation of template usage can be found in the representative results and Figure 8.
4. Once the compounds have been grouped, export the "blob set table" into a spreadsheet program. Use the sum the volume of all blobs/peaks within each compound class group, and the calibration equation determined in Section 4.4 to calculate the concentration in ppm for the nitrogen compounds in each group.
5. If desired, use the following density calculations to correct for differences in the injection volume of sample vs. standards for quantitation:
   
   NOTE: *percent difference between the ng N injected in the sample matrix vs. the standard matrix
6. Sum all nitrogen content in each compound class to obtain the total nitrogen content of the sample, if desired. If the total nitrogen content is determined to be above 150 ppm nitrogen or a compound class bin is outside of the calibration range, dilute the sample further for analysis. Compare these results with total nitrogen content as determined by ASTM D4629¹³ for quantification verification.

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Results

The nitrogen-containing compound, carbazole, was used in this method as the calibration standard. Carbazole elutes at approximately 33 min from the primary column and at 2 s from the secondary column. These elution times will vary slightly depending on the exact column length and instrumentation. In order to obtain a proper calibration curve and, subsequently, good quantitation of nitrogen compounds within a sample, the calibration peaks should not be overloaded nor have any nitrogen cont...

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Discussion

The purpose of this method is to provide detailed information on the nitrogen content of diesel and jet fuels without extensive sample preparation such as liquid extractions. This is achieved by pairing a two dimensional GC system (GCxGC) with a nitrogen-specific detector (nitrogen chemiluminescence detector, NCD). The GCxGC provides significant separation of the compounds relative to traditional one-dimensional GC. The NCD provides trace nitrogen compound detection without any background interferences. Other nitrogen-sp...

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Materials

Name	Company	Catalog Number	Comments
10 µL syringe	Agilent		gold series
180 µm x 0.18 µm Secondary Column	Restek	Rxi-1MS	nonpolar phase column, crossbond dimethyl polysiloxane
250 µm x 0.25 µm Primary Column	Restek	Rxi-17SilMS	midpolarity phase column
Autosampler tray and tower	Agilent	7963A
Carbazole	Sigma	C5132	98%
Diethylaniline	Aldrich	185898	≥ 99%
Dimethylindole	Aldrich	D166006	97%
Duel Loop Thermal Modulator	Zoex Corporation	ZX-1
Ethylcarbazole	Aldrich	E16600	97%
Gas chromatograph	Agilent	7890B
GC vials	Restek	21142
GCImage Software, Version 2.6	Zoex Corporation
Indole	Aldrich	13408	≥ 99%
Isopropyl Alcohol	Fisher Scientific	A461-500	Purity 99.9%
Methylaniline	Aldrich	236233	≥ 99%
Methylquinoline	Aldrich	382493	99%
Nitrogen Chemiluminescence Detector	Agilent	8255
Pyridine	Sigma-Aldrich	270970	anhydrous, 99.8%
Quinoline	Aldrich	241571	98%
Trimethylamine	Sigma-Aldrich	243205	anhydrous, ≥ 99%