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Energy & Fuels 1998, 12, 1342-1355
Quantitative Extraction of Nitrogen Compounds in Oils: Atomic Balance and Molecular Composition I. Merdrignac,*,†,‡ F. Behar,§ P. Albrecht,† P. Briot,| and M. Vandenbroucke§ Universite´ Louis Pasteur, Laboratoire de Ge´ ochimie Organique, 1 rue Blaise Pascal, 67000 Strasbourg, France, The Ohio State University, Department of Chemistry, 100 West 18th Street, Columbus, Ohio 43210, Institut Franc¸ ais du Pe´ trole, De´ partement de Ge´ ochimie, 1-4 av. de Bois Pre´ au, BP311, 92506 Rueil-Malmaison, France, and Institut Franc¸ ais du Pe´ trole, CEDI Solaize, 69360 Vernaison, France Received May 5, 1998. Revised Manuscript Received August 4, 1998
The aim of this study was to get an atomic mass balance on nitrogen compounds in petroleum fractions such as crude oils, distillation cuts, or rock extracts. Two methods have been developed for the separation of basic compounds: a liquid-solid extraction on a commercial acid-bound silica and a new technique by liquid-liquid extraction. Both methods allowed for a selective and quantitative recovery of the basic compounds. For neutral nitrogen species, a method is proposed based on deprotonation by a strong base and liquid-liquid extraction. The efficiency of the three methods was checked on appropriate standards. These techniques were then applied to a crude oil from the Middle East and its distillation cut 375-550 °C. All the basic and neutral fractions were submitted to elemental analyses in order to get the total nitrogen balance to be compared to the initial amount present in the crude oil. Results show that a large part of nitrogen is contained in the residue of the oil distillation above 550 °C. The recovery of nitrogen as isolated compounds reached 93 wt % in the distillation cut 375-550 °C. However, only 41 wt % of the nitrogen content was recovered as basic and neutral species from the crude oil. This must mean that nitrogen structures in the crude oil are found largely as high molecular weight polyfunctional compounds. The major part of the nitrogen compounds isolated from the petroleum samples was found in the neutral fraction. However, the proportion between basic and neutral nitrogen compounds was higher in the crude oil than in the distillation cut, 375-550 °C, meaning that bases were preferentially found in the high molecular weight fraction of the crude oil.
Introduction Nitrogen compounds are well-known as poisons of acidic and metallic refinery catalysts and cause trouble in petroleum hydrotreatments,1 where they are, in part, responsible for deposit and gum formations.2-4 Furthermore, they show carcinogenic and mutagenic properties. Nitrogen compounds in petroleum products can be classified into two main chemical classes: basic and neutral.5 The predominant family in low molecular weight basic nitrogen structures is the pyridine derivatives, also called azaarenes,6 whereas the low molecular weight neutral nitrogen compounds are mainly pyrrole derivatives.7 The complexity, the lack of commercial standards, the low concentration, and the wide polarity †
Universite´ Louis Pasteur. The Ohio State University. Institut Franc¸ ais du Pe´trole, Rueil-Malmaison, France. | Institut Franc ¸ ais du Pe´trole, Vernaison, France. (1) Mills, G. A.; Boedeker, E. R.; Oblad, A. G. Chem. Charact. Catal. 1950, 72, 1554-60. (2) Dahlin, K. E.; Daniel, S. R.; Worstell, J. H. Fuel 1981, 60, 47780. (3) Worstell, J. H.; Daniel, S. R.; Frauenhoff, G. Fuel 1981, 60, 48587. (4) Batts, B. D.; Fathoni, Z. Energy Fuels 1991, 5, 2-21. (5) Bakel, A. J.; Philp, R. P. Org. Geochem. 1990, 16, 353-67. (6) Mojelsky, T. W.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1986, 3, 25-33.
range of petroleum products contribute to the difficulty of investigating nitrogen compounds in these complex mixtures. Furthermore, the presence of nitrogen constituents in high-boiling fractions of crude oil require the adaptation of separation methods for polar petroleum fractions.8 The basic requirements of a separation technique are quantitativity, selectivity, and simplicity. Several separation schemes for basic nitrogen compounds in oils have been described in the literature (e.g., Schmitter et al. among others9). They use the basic properties of the azaarenes and are based on liquidsolid extraction. The main differences between the proposed methods concern the solid support properties. Some use adsorbents composed of macroreticulated resins for ion-exchange chromatography,10,11 others use modified supports to obtain a selective complexation with the sample.9,12-14 Unfortunately, these fraction-
‡ §
(7) Frakman, Z.; Ignasiak, D. S.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1987, 3, 131-37. (8) Jewell, D. M.; Weber, J. H.; Bunger, J. W.; Plancher, H.; Latham, D. R. Anal. Chem. 1972, 44, 1391-95. (9) Schmitter, J. M.; Ignatiadis, I.; Arpino, P. J.; Guiochon, G. Anal. Chem. 1983, 55, 1685-88. (10) Snyder, L. R.; Buell, B. E. Anal. Chem. 1968, 40, 1295-302. (11) Green, J. B.; Hoff, R. J.; Woodward P. W.; Stevens, L. L. Fuel 1984, 63, 1290-301. (12) Shue, F. F.; Yen, T. F. Anal. Chem. 1981, 53, 2081-84. (13) Li, M.; Larter, S. R.; Stoddart, D.; Bjoroy M. Anal. Chem. 1992, 64, 1337-44.
10.1021/ef980103r CCC: $15.00 © 1998 American Chemical Society Published on Web 09/10/1998
Extraction of Nitrogen Compounds in Oils
ations have a rather poor selectivity for nitrogen species, and thus, incomplete separation of the specific compound classes is achieved. Moreover, the experimental devices used in these studies are not suitable for recovery of large amounts of fractions and are hardly applicable to viscous crude oils. Finally, purification steps are required, leading to possible contamination and loss of products. The neutral nitrogen compounds have been lessstudied than the azaarenes: their extraction is difficult due to their weak acid-basic character.15 The published methods are also based on liquid-solid chromatography, and the procedures differ by the nature of the solid phase. Some authors have described separation methods carried out by specific complexation on bound adsorbents.16-18 Unfortunately, they lead to partial compound degradations or coelutions. Active centers (SO3H groups) have been used for ion-exchange chromatography, but no accurate separation was performed.19 Other methods with standard adsorbents were used,5,20-24 but several purification steps were needed, leading to complex and long separation schemes with risks of contamination or loss of products. As for basic compounds, there are few quantitative data25,26 and no atomic nitrogen balances. The aim of this work was to get a quantitative mass balance of the nitrogen compounds present in oils according to the following experimental strategy: (1) Development of specific extraction techniques for both basic and neutral compounds. (2) Checking of the selectivity and quantitativity on different standards, a crude oil, and the distillation cut 375-550 °C. (3) Gas chromatographic (GC) calibration to study the detector response toward nitrogen species and to approximate the percentage of GC-amenable nitrogen compounds of any analyzed fraction. (4) Establishment of the nitrogen mass balance by elemental analysis. Samples The standards, purchased from Aldrich, are quinoline, isoquinoline, 2,6- and 2,8-dimethylquinoline, benzo[h]quinoline, indole, carbazole, octadecane, and anthracene. We have also used a standard mixture from our laboratory which is a distillation cut enriched in indole and carbazole homologues. (14) Thomson, J. S.; Green, J. B.; McWilliams, T. B.; Yu, S. K. T. J. High Resolut. Chromatogr. 1994, 17, 415-26. (15) Richter, F. P.; Caesar P. D.; Meisel, S. L.; Offenhauer, R. D. Ind. Eng. Chem. 1952, 44, 2601-05. (16) Jewell, D. M.; Snyder, R. E. J. Chromatogr. 1968, 38, 351-54. (17) Ruckmick, S. C.; Hurtubise, R. J. J. Chromatogr. 1985, 321, 343-52. (18) Grizzle, P. L.; Sablotny, D. M. Anal. Chem. 1986, 58, 238996. (19) Ivanov, S. K.; Al Kane, M. J.; Kaltichin, Z. D. Erdoel Kohle, Erdgas 1992, 45, 401-07. (20) Schiller, J. E.; Mathiason, D. R. Anal. Chem. 1977, 49, 122528. (21) Later, D. W.; Lee, M. L.; Bartle, K. D.; Kong, R. C.; Vassilaros, D. L. Anal. Chem. 1981, 53, 1612-20. (22) Dorbon, M.; Schmitter, J. M.; Garrigues, P.; Ignatiadis, I.; Ewald, M.; Arpino, P. J.; Guiochon, G. Org. Geochem. 1984, 7, 11120. (23) Ignatiadis, I.; Arpino, P. J.; Dorbon, M. Rev. Inst. Fr. Pet. 1986, 41, 551-73. (24) Chang, S. H.; Kuangnan, Q.; Winston, K. R. J. High Resolut. Chromatogr. 1994, 17, 271-76. (25) Nwadinigwe, C. A.; Maduka, M. C. Fuel 1993, 72, 1139-43. (26) Mao, J.; Pacheco, C. R.; Traficante, D. D.; Rosen, W. Fuel 1995, 74, 880-87.
Energy & Fuels, Vol. 12, No. 6, 1998 1343 Table 1. Geochemical Characteristics of Crude Oil and the Corresponding Distillation Cut 375-550 °C
sample crude oil 375-550 °C a
N(EA)a distillation % recovery ppm of (wt %) ppm crude sat. aro. res. asph. tot 23
1771 1389
1771 319
46 49
33 42
19 9
2 0
100 100
EA: elemental analysis.
It is known that about 90% of the crude oils contain less than 0.2 wt % nitrogen.27 The crude oil chosen here is a typical marine oil from the Middle East27 with a nitrogen content of 1771 ppm. The asphaltene fraction does not exceed 2 wt %, whereas the resins account for 19 wt %. The amounts of saturates and aromatic hydrocarbons are, respectively, 46 and 33 wt %. The distillation cut 375-550 °C of this crude oil (23 wt %) was chosen for its high initial boiling point (375 °C) to minimize the loss of the low molecular weight compounds during the extraction procedures. It has no asphaltene and a low content of resins (9 wt %). Hydrocarbons predominate with 49 wt % for the saturated and 42 wt % for the aromatic hydrocarbons. The nitrogen content amounts to 1389 ppm, which represents only 319 ppm with respect to the whole crude oil (Table 1). The joint study of both crude oil and its distillation cut is necessary for a good crosscheck of the quantitative data. In fact, a standard GC allows the analysis of compounds up to a distillation temperature of approximately 500-550 °C. A distillation cut with an upper limit of 550 °C can, thus, be totally analyzed. Consequently, if the nitrogen distribution of the 375550 °C distillation cut does not correspond to that of the crude oil, the differences may be due to either a possible thermal alteration during vacuum distillation or the presence of nitrogen compounds in the 375- or 550+ °C fractions. Experimental Section Chemicals and Materials. All solvents and thin-layer plates were purchased from Merck (Darmstadt, Germany). Hexane (n-C6), methylene chloride (CH2Cl2), and methanol (CH3OH) were distilled prior to use. Glassware was washed with detergent and then rinsed with distilled water, acetone, and three times with methylene chloride. All glassware, when open, was covered with aluminum foil. Silica was Soxhletextracted with methylene chloride. For thin-layer chromatography (TLC) separation, commercial silica plates of 0.5 mm thickness were eluted with ethyl acetate, kept 24 h under a ventilated hood, and then heated to 120 °C for 3 h before use, whereas reversed-phase RP-18 plates of 0.25 mm thickness were eluted with acetone. Extraction of Basic Nitrogen Compounds. Two methods were compared based on the same principle, e.g. basic compound affinity (Lewis bases) with an acid functionality. The procedures used (Figure 1) were the following: (A) a liquid-solid extraction involving: (1) an adsorption of basic compounds on a commercial sulfonic-acid-bound silica and (2) a purification of the recovered fraction by reversed-phase C18 TLC. (B) A continuous liquid-liquid extraction with strong mineral acid to selectively extract the basic species without emulsion (in opposition with the simple liquid-liquid extraction28). (27) Tissot B.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: Berlin, New York, 1984.
1344 Energy & Fuels, Vol. 12, No. 6, 1998
Merdrignac et al.
Figure 3. Original device for quantitative liquid-liquid extraction of basic nitrogen compounds. (1) Condenser; (2) funnel; (3) glass frit; (4) heater (see explanation in the text). Figure 1. Schematic diagram of the two methods used for extraction of basic nitrogen compounds: (A) liquid-solid and (B) liquid-liquid extraction in continuous flow.
Figure 2. GC-NPD of the basic fraction before the purification step during the liquid-solid extraction of the crude oil. GC conditions: Carlo Erba 4160 GC, DB5 J&W column, 30 m × 0.25 mm × 0.1 µm, 40 °C (1 min), 40-100 °C (10 °C/min), 100300 °C (4 °C/min), 300 °C isothermal. Method A: Liquid-Solid Extraction. Retention on Sulfonic-Acid-Bound Silica. A 30 g amount of sulfonic-acidbound silica (Supelco) was placed in a column and washed several times with CH2Cl2. The samples to be analyzed, e.g. mixture of standards or 3 g of crude oil or distillation cut, were dissolved in a minimum volume of CH2Cl2 (∼1 mL) and deposited on the top of the column. Nonbasic compounds were first collected by elution with CH3OH/CH2Cl2 (15:100, v/v) until the eluted solution was colorless. The basic compounds were then displaced by elution with CH3OH/CH2Cl2/NH4OH (10:10: 4, v/v, ammonia 33% aqueous solution). After addition of 150 mL of distilled water, the mixture was evaporated until total removal of methanol (CH3OH). The basic nitrogen compounds were then extracted with CH2Cl2, and the resulting solution was evaporated under reduced pressure. An aliquot of the basic fraction was analyzed by gas chromatography (GC) with an instrument equipped with a selective nitrogen detector (NPD) in order to test the purity of the bases. For the crude oil and distillation cut, the NPD distribution showed a few poorly resolved peaks followed by an unresolved hump containing high molecular weight nitrogen compounds (Figure 2). The separation of the GC-amenable peaks from the hump, which is necessary for a correct peak (28) Yamamoto, M.; Taguchi, K.; Sasaki, K. Chem. Geol. 1991, 93, 193-206.
identification by GC-MS, was carried out by a further purification step on a RP-C18 thin-layer plate as described below. Purification on Reversed-Phase C18. A 7 mg amount of the basic fraction was deposited at the bottom of a RP-18 plate, which was eluted with acetone. Two fractions were separated: the low molecular weight nitrogen compounds revealed by an intense UV band (254 nm) at the solvent front (fraction 1S) and the compounds of higher molecular weight weakly revealed by UV (254 nm) below the area of the first fraction (fraction 2). These two fractions were recovered by scraping the plate and extracting with acetone, weighing, and submitting to elemental analysis. Nitrogen Balance. The recovery of basic nitrogen is
Nbasic ) Nfraction1S + Nfraction2 The residue of this separation (Nres1), contained in the first eluate (CH3OH/CH2Cl2 eluent; 15:100, v/v), was also submitted to elemental analysis. Method B: Liquid-Liquid Extraction. This method is a liquid-liquid extraction of the basic compounds as ammonium salts with an aqueous acid solution in continuous flow in order to avoid any emulsion. The extraction can be carried out on a crude oil sample; no deasphaltening is required prior to extraction. A few milligrams to 300 g of crude oil can be extracted according to the extractor dimension. Extraction Device. The device set up for this extraction (Figure 3) is comprised of three parts: the heater and flask for the aqueous acid solution (A), the extractor (D) containing a long funnel with a glass frit (porosity 1) at the bottom (C), a Dimroth condenser, fitted on top of the extractor (B). The whole system was kept under argon. The dimensions of the apparatus depend on the quantity of sample to be fractionated, which in turn depends on its ability to be dissolved in the selected organic solvent. The length of the funnel is adapted in order for the distilled solution to counterbalance the pressure of the liquid column on the glass frit in compartment D. Typically, a 400 mm long × 25 mm i.d. apparatus should be able to extract between 10 and 200 g of sample dissolved in 180 mL of CH2Cl2 (D) with 150 mL of hydrochloric acid (6 N) (A). Experimental Procedure. The sample diluted in CH2Cl2 was introduced into the extractor through a funnel. The HClcontaining flask was closed hermetically via one of the two
Extraction of Nitrogen Compounds in Oils
Energy & Fuels, Vol. 12, No. 6, 1998 1345
openings of the extractor. The liquid-liquid extraction was manually initiated by addition of HCl in the funnel until the emergence of two phases was observed. The condenser was then swept with argon, and heating was initiated (Figure 3). It was absolutely necessary to work under inert gas; under these conditions, quinoline was completely recovered from a standard solution, whereas it was totally oxidized in the presence of air. The extraction efficiency was monitored twice a week by recovering the total HCl solution accumulated in flask A. The free bases were regenerated under highly alkaline conditions (potassium hydroxide, 6 N) and extracted with CH2Cl2. The latter was evaporated under reduced pressure to give fraction 1L. The presence of nitrogen in fraction 1L was checked by GC-NPD. The extraction time necessary depends on the initial nitrogen concentration and ranged from about 1 day to several weeks, depending upon the sample under study. The residue of this separation, contained in the extractor, was washed free of residual aqueous acid solution with distilled water. Nitrogen Quantitative Analysis. The recovery of basic nitrogen is
Nbasic ) Nfraction1L The residue (Nres2) was also submitted to elemental analysis. Extraction of Neutral Nitrogen Compounds. The neutral nitrogen compounds were separated from the crude oil in two steps using their slightly acidic character: (1) prefractionation of the oil sample before extraction of nitrogen compounds to eliminate pure hydrocarbon fractions and (2) extraction of neutral nitrogen compounds by sodium methylate in methanol, followed by a liquid-liquid extraction of the isolated compounds after hydrolysis. Prefractionation of the Oil Sample Before Nitrogen Compound Extraction. The low polarity and amphoteric properties of the neutral nitrogen compounds limit the selectivity of the extraction method. This is why it is necessary to prefractionate the crude oil in order to achieve a preconcentration of the neutral species. A 10 g amount of crude oil was loaded onto a 100 g silicagel column (Figure 4). The saturated and aromatic hydrocarbons (fraction a) were obtained by elution with hexane. Once the Rf of the eluted products was below 0.3 (controlled by SiO2 TLC, hexane), the hexane was replaced by CH2Cl2, which allowed an extensive elution of NSO compounds of low polarity (fraction b). Further elution with a mixture of CH3OH/CH2Cl2 (20:100, v/v) enabled the recovery of the most polar compounds, mainly composed of high molecular weight components (fraction c). The three fractions (a, b, and c) were then separately submitted to procedure A (Figure 5) for extraction of neutral nitrogen compounds. The study of fraction a was only carried out to check that it did not contain any nitrogen compounds, which turned out to be the case. Extraction of the Neutral Nitrogen Compounds. In a 100 mL flask, 0.5 g of crude oil fractions (a, b, or c) or standards was diluted in 25 mL of n-C6 and then 50 mL of a 1 M solution of sodium methylate in methanol (not miscible with n-C6) was added. After vigorously stirring under argon for 45 min, the neutral nitrogen compounds were selectively recovered in the methanol solution. After addition of 30 mL of distilled water to the CH3OH solution, the mixture was briefly stirred at room temperature. The solvent was then evaporated under vacuum until complete removal of the methanol. The neutral nitrogen compounds were extracted with CH2Cl2, leading to the first fraction a0, b0, or c0 (Figure 5). This procedure was repeatedly applied to the residual n-C6 solution to lead to fractions ai, bi, or ci and a residual n-C6 solution (fraction 2). Three extractions are usually necessary for complete removal of the neutral nitrogen compounds (i ) n) (controlled by GC-NPD on fractions i).
Figure 4. Schematic diagram of the quantitative extraction of neutral nitrogen compounds.
Figure 5. Experimental procedure for quantitative extraction of neutral nitrogen compounds (procedure A). The ai, bi, or ci fractions were gathered to obtain fraction 1 (a1 ) ∑ai, b1 ) ∑bi, or c1 ) ∑ci, respectively). Qualitative Analysis of the Crude Oil. The neutral nitrogen compounds were examined by gathering equal amounts with respect to every fraction b1 and c1 in CH2Cl2, and this was labeled total fraction (Figure 4). The nitrogen content of this last fraction was checked by elemental analysis. Analyses by GC-NPD and GC-FID indicated that this fraction also contained non-N-containing compounds, which were extracted or partially soluble in CH3OH. These non-N-containing compounds were mainly eliminated by a SiO2 TLC purification
1346 Energy & Fuels, Vol. 12, No. 6, 1998 step with n-C6 as the eluent, where the nitrogen neutral species, which stayed at the bottom of the plate, were recovered by scraping the plate and extracted with CH2Cl2/MeOH (20: 100, v/v). The GC-NPD and GC-FID traces of these totally purified fractions were then correct for the qualitative analysis. Nitrogen Quantitative Analysis. The total recovery of the neutral nitrogen compounds can be summarized as follows:
Nneutral ) Na(1+2) + Nb(1+2) + Nc(1+2) which, according to qualitative analysis, reduces to
Nneutral ) Nb1 + Nc1 ) Ntotal neutral fraction In the same way, Nresidue () Nres3) ) Nb2 + Nc2. Quantitative GC Analysis. Before interpreting and comparing the distributions of the nitrogen compounds, it was necessary to study the FID response in order to quantify the amount of non-GC-amenable nitrogen products. The signal integration, calibrated on external standards and comprised of peaks and an unresolved hump in the case of the oil or distillation cut, was compared to the injected weight to calculate the percentage of GC-amenable products. The gas chromatograph was a Carlo Erba instrument equipped with a flame ionization detector (FID) for the quantitative study and a nitrogen-phosphorus detector (NPD) for selective detection of nitrogen-containing compounds. The test and the description of the NPD detector have already been published.13,30,31 An on-column injector and a 30 m DB-5 (J&W) fused-silica capillary column with a 0.25 µm film thickness were used with the following chromatography conditions: 40-100 °C (10 °C/min), 100-300 °C (4 °C/min), 300 °C isothermal for 30 min. An HP 3350 laboratory automation system (LAS) integrator was used for peak area measurements. The acquisition of data was carried out in real time with an HP 1000 micro computer, type A600. The calibration measurements were carried out with a Varian 8200CX auto sampler. The FID response was studied according to the N/C atomic ratio. Three nitrogen standards were chosen: quinoline, 2,6dimethylquinoline, and benzo[h]quinoline diluted at various concentrations in the range 50-400 mg/L. The injections were carried out with an automatic on-column injector in order to optimize reproducibility. Calibration curves, drawn for each standard, and correlation coefficients were satisfactory, the maximum variation between the three standards being 4%. Consequently, the average of the three individual calibration curves was taken for the quantification of any nitrogen compound extracted either from crude oil or distillation cut. The GC-FID was regularly recalibrated and this average calibration curve was also regularly recalculated. However, caution should be exercised concerning the determination of the GC-amenable percentage of nitrogen in an injected fraction. Indeed, (1) GC-amenable compounds are quantified with an FID detector, which is a measurement of the number of carbon atoms; (2) as we assumed that the involved nitrogen compounds contained only one nitrogen atom, the N/C atomic ratio decreases with increasing molecular weight compounds. As a consequence, the FID signal at the beginning of the GC trace corresponds to a higher nitrogen amount than the FID signal with the same intensity at the end of the trace. Since C/(N + C) < 1 at the beginning of the GC trace, the nitrogen amount is underestimated. So, the determination of the GC-amenable nitrogen amount of a fraction can only be an estimation of the minimal value. (29) Behar, F. The`se Doct. Etat, Universite´ Louis Pasteur, Strasbourg, 1982. (30) Kolb, B.; Auer, M.; Pospisil, P. J. Chromatogr. 1977, 134, 6571. (31) Albert, D. K. Anal. Chem. 1978, 50, 1822-29.
Merdrignac et al. The molecular identification was performed on a GC-MS Finnigan MAT INCOS 50 using chromatographic conditions similar to those described above. The mass spectrometer was operated in electron-impact mode with an electron energy of -70 eV. Elemental Analysis. The nitrogen compounds in a natural sample were separated into several fractions, and the corresponding atomic nitrogen balance was determined as follows:
Ntotal ) Nbasic + Nneutral + Nothers where Nbasic contains the basic nitrogen compounds extracted by the liquid-solid (fraction 1S + fraction 2) or the liquidliquid method (fraction 1L), Nneutral is comprised of the extracted neutral nitrogen compounds, and Nothers is composed of all the N compounds different from the isolated basic and neutral ones, including compounds irreversibly adsorbed on liquid-chromatography columns. The nitrogen content of the initial sample, basic, neutral, and residual fractions was measured by elemental analysis (ATX, laboratoires Wolff, Clichy and IFP, Solaize) and compared to the initial content in the sample, Ninitial. The atomic nitrogen balance is then calculated from the liquid-liquid extraction data, where all fractions are recovered, in contrast to other extractions using liquid chromatography columns, because of irreversible adsorption of nitrogen compounds on the solid phase:
Ω)
Nbasic + Nresidue2 Ninitial
The elemental analysis was carried out on high nitrogen content fractions (>500 ppm) with a detection limit of 500 ppm. For low concentrations (
