Sulfur Determination in Fuels by ICP-OES and ICP-MS to Meet Increasingly Stricter Legislation Requirements

The increasing number of works on sulfur determination in fuels observed since 2000 is closely related to stricter pieces of legislation regulating the maximum concentrations of this element allowed in diesel fuel. In most countries, ultralow-sulfur diesel should present sulfur concentrations of 15 mg/kg or lower. In this overview, we discuss sulfur determination procedures based on inductively coupled plasma with optical emission spectroscopy (ICP-OES) or mass spectrometry (ICP-MS) detection that were proposed to meet the new regulatory requirements. We critically review the main developments in instrument technology, calibration, and sample preparation that have made it possible to determine low sulfur concentrations in fuels. We discuss some strategies used to minimize spectral interferences related to sulfur determination by ICP-MS, such as collision and reaction cells, high-resolution mass analyzers, and the interference standard method (IFS). Finally, we discuss sample preparation and sample introduction strategies developed to improve ICP-OES performance in sulfur determinations in fuels.

Sulfur is naturally present in petroleum (1), from which several products such as diesel, gasoline, and kerosene are extracted and commercialized as fuels. In diesel, for example, sulfur exists in the form of mercaptans, sulfides, disulfides, and heterocyclic compounds (1,2). The concentration of sulfur varies according to the petroleum source, cracking process, and fuel treatment (2). Environmental problems and health hazards are associated with the presence of this element in fuels. During their combustion, sulfur compounds are burned and form harmful oxides (SO_x) and sulfate particulates (3). This process is the primary anthropogenic source of atmospheric pollution by sulfur compounds, and one of the main sources of particulate matter in urban environments. It has been shown that SO_x contributes to the depletion of the Earth's ozone layer, acid rain, and chronic respiratory diseases (3). At low temperatures, humidity condensation in the engine can result in the formation of sulfurous and sulfuric acids, which causes severe wearing and corrosion of engine parts (4). In addition, sulfur compounds can affect the stability of fuels, lead to sludge formation, and act as catalyst poisons to platinum group elements, which results in lower efficiency of the emission control system in vehicle exhausts (5,6).

Before the 1990s, the sulfur content in diesel fuel was not regulated and it was approximately 3000 mg/kg or even higher (7). As a consequence of environmental concerns, the United States (US) was the first country to control the concentration of this element in diesel. In 1990, the Environmental Protection Agency (EPA) established a maximum allowed concentration of sulfur in on-road diesel as 2000 mg/kg. In 1993, this limit was lowered to 500 mg/kg (8). Nowadays, the total sulfur concentration is also regulated in biodiesel fuel. The time line for the American, European Union (EU), and Brazilian legislations regarding the reduction of sulfur in diesel fuel is shown in Figure 1 (8). It is interesting to note that it took 20 years to drastically reduce sulfur concentrations in diesel in the US. On the other hand, Brazil only began to limit sulfur concentrations in diesel in 2000. These facts are related to the development of desulfurization systems. According to Stanislaus and colleagues (8), the efficient desulfurization of diesel is a complex process, and finding cost-effective ways to produce ultralow-sulfur diesel (ULSD) requires extensive research and investment. Hydrodesulfurization (HDS), a catalytic process that uses extreme conditions of temperature and pressure, is the most commonly used industrial method for removing sulfur from fuels (9). Unfortunately, HDS is not effective at removing heterocyclic sulfur compounds, and it reduces diesel's antifriction (lubricity), which results in early wear of engine parts (8–10). Shifting from normal diesel to ULSD is a technical challenge because factors such as feedstock source and quality, catalysts, process parameters, and reactivities of sulfur compounds significantly affect the degree of desulfurization (8–10).

Figure 1: Time line with the main regulations on sulfur maximum allowed concentration in fuels and milestones in ICP-MS developments.

To assist the petroleum industry in meeting new low sulfur regulations, in 1992 the National Institute of Standards and Technology (NIST) introduced the first diesel standard reference material (SRM) with a certified value for sulfur: SRM 2724, with 425 ± 4 mg/kg of sulfur (7,11). More recently, NIST began to produce diesel SRMs with sulfur concentrations as low as 9.06 ± 0.25 mg/kg. In this case, sulfur determinations were carried out by wavelength dispersive X-ray fluorescence spectrometry (WDXRF) and isotope dilution inductively coupled plasma–mass spectrometry (ICP-IDMS) (11). In the overview presented here, we focus on the application of inductively coupled plasma with optical emission spectroscopy (ICP-OES) or mass spectrometry (ICP-MS) detection to determine sulfur in fuels in light of the increasingly stricter regulations.

ICP-MS for Sulfur Determination in Fuels

ICP-MS became commercially available in 1983 (12). As the method evolved, it became clear that it would be successful in a plethora of analytical applications. However, some limitations related to polyatomic interfering ions and the difficulty to handle complex sample matrices have always haunted ICP-MS. For sulfur, the mass-to-charge ratios (m/z) of its main isotopes (that is, ³²S⁺ [95.02%] and ³⁴S⁺ [4.21%]) overlap with the ones from polyatomic ions formed in the plasma such as ¹⁶O₂⁺, (¹⁶OH)₂ ⁺, ¹⁶O¹⁸O⁺, and ¹⁴N¹⁸O⁺, which compromise sensitivity, precision, and accuracy (13–15). Several strategies have been proposed to overcome these limitations. For example, Yu and colleagues (15) evaluated the use of electrothermal vaporization coupled to ICP-MS as a strategy to minimize oxygen-based polyatomic interferences caused by solvent introduction into the plasma. Although lower background signals were observed in diesel fuel analysis, accurate sulfur determination using isotope dilution (ID) was only possible with a 5% N₂-Ar plasma. In this case, nitrogen gas acted as an oxygen scavenger, allowing for a limit of detection (LOD) as low as 4 μg/kg.

Figure 1 also presents the milestones in ICP-MS developments to either circumvent or correct for spectral interferences. It is clear that the interest in determining sulfur in fuels by ICP-MS (and the subsequent development of more efficient interference-minimizing strategies) increased after stricter regulations were established by both the US and the EU. Collision and reaction cells, for example, were introduced in the late 1990s. In this case, two main approaches are used to reject unwanted ions: discrimination by kinetic energy and discrimination by mass (16). By applying the former, for example, it was possible to determine sulfur in biodiesel using an octopole reaction system (ORS). According to the authors, a 0.54-mL/min xenon gas flow rate is sufficient to remove O₂-based interferences. Xenon gas interacts with oxygen and the products are discriminated by their kinetic energy, resulting in a LOD of 0.029 mg/kg for kerosene-diluted biodiesel samples (17).

Discrimination by mass using dynamic reaction cells (DRC) is also effective. One strategy in sulfur determinations is to introduce O₂ in a pressurized reaction cell and monitor SO⁺ rather than S⁺. Thus, the analytical signal is recorded in a m/z region with less intense interfering signals (16,20–22). Despite the simplicity of this approach, no applications to sulfur determination in petroleum products have been described in the literature.

In 2009, Heilmann and Heumann described a study to evaluate the determination of sulfur in petroleum products using a quadrupole-based instrument without a collision and reaction cell (23). In this work, isotope dilution with ³⁴S-labeled dibenzothiophene spike was used. The ICP-MS system was coupled to a gas chromatography (GC) injector system capable of promoting thermal vaporization at 340 °C. The sample was injected into a thermal vaporizer, which was directly connected to the plasma torch by a heated transfer line. Helium was used as the carrier gas and a special "sample sandwich" technique using nitrogen gas was used to prevent the introduction of oxygen into the ICP-MS. Relatively low background signals and accurate results were obtained by applying this strategy. One year earlier, the same authors developed a species-specific isotope dilution GC–ICP-MS procedure for the determination of thiophene derivatives in petroleum products (24).

More recently, an ICP-MS-MS instrument became commercially available. It presents an octopole-based collision–reaction cell located between two quadrupole analyzers. The first quadrupole prevents all off-mass ions from entering the reaction cell, which allows more control and more efficient collision and reaction interactions inside the octopole. The second quadrupole is then set to one particular m/z of interest. Balcaen and colleagues (25) evaluated the new ICP-MS-MS instrumentation combined with the isotope dilution method to determine low levels of sulfur in ethanol-diluted biodiesel samples (25). Oxygen gas was used in the collision–reaction cell and LODs of 5, 4, and 7 mg/kg were obtained by monitoring ³²S¹⁶O⁺, ³³S¹⁶O⁺, and ³⁴S¹⁶O⁺, respectively.

In 2011, Donati and colleagues (26) described a strategy to minimize polyatomic interferences that is less dependent on instrumental modifications. The interference standard method (IFS) is based on the idea that Ar species naturally present in the plasma experience the same signal fluctuations as polyatomic interfering ions. By using the analytical-to-IFS signal ratio and the external calibration method, it is then possible to minimize the contributions of the interfering ions to the overall analytical signal, which results in improved accuracy. The IFS method allowed the accurate determination of sulfur in biodiesel microemulsions using a quadrupole-based instrument (27). In another work using the same instrumentation, this strategy was used to accurately determine sulfur in diesel and biodiesel samples by monitoring the oxygen-based species SO⁺ and SOH⁺ formed in cool plasma conditions (28).

The limitations associated with quadrupole-based instruments and the difficulties caused by polyatomic interfering ions also prompted the development of high-resolution mass analyzers. The double-focusing magnetic-sector technology represents a 30-fold improvement in resolution power when compared to quadrupole analyzers (29). Even though these inductively coupled plasma–high resolution mass spectrometry (ICP-HRMS) instruments were first commercialized in the late 1980s, the first study on S determination in diesel fuel using this technology was only published in 2001 (30). Interestingly, this work came out just after the EU decided to reduce the maximum sulfur concentrations allowed in fuels from 500 to 350 mg/kg in 2000 (8). Isotope dilution associated with different strategies of sample preparation or sample introduction have been used to improve precision and accuracy in ICP-HRMS determinations. Adequate results were obtained with microwave-assisted digestion (31,32), laser ablation (LA) (33,34), and microemulsion preparation followed by sample introduction with a direct injection high-efficiency nebulizer (DIHEN) (31). Despite their efficiency, high-resolution instruments come at a higher cost, which has prevented a wider application in routine analysis.

Table I summarizes the analytical procedures using ICP-MS and ICP-OES to determine sulfur in fuels. It is interesting to observe the correlation between instrumental improvements and the increasing demand because of environmental and health concerns as well as the impact of those concerns on the number of publications over the years.

Table I: Procedures and sample preparation strategies used for total sulfur determination in fuels

ICP-OES for Sulfur Determination in Fuels

The first report using ICP-OES in elemental analysis was published in 1964 and the first commercial instrument was released in 1974 (35,12). Today, ICP-OES is a well-established multielement method that may be an interesting alternative to ICP-MS in routine fuel analysis because of its lower cost and robustness. Kirkbright and colleagues were the first to investigate the sulfur main analytical lines in the ultraviolet (UV) region (180.73, 182.04, and 182.63 nm) (36). Because O₂ absorbs radiation in the low-UV, all modern ICP-OES instruments present a spectrometer evacuated or purged with an inert gas (Ar or N₂).

Before the 1990s, few papers described sulfur determination in petroleum products. In 1985, Fabec and Ruschak discussed the difficulties of introducing xylene-diluted solutions of petroleum crude oil and its products into an ICP-OES system (37). The authors emphasized the importance of sample introduction system and torch maintenance to avoid memory effects and remove carbon deposits in the system. They also used a base oil in the calibration solutions to minimize viscosity differences. The first studies to assess the sulfur composition in petroleum products used size-exclusion liquid chromatography coupled to ICP-OES to determine the molecular size distribution of sulfur compounds (38,39).

Sample preparation based on oxidative decomposition is usually the most effective alternative to prevent matrix effects associated with organic solvents in ICP-OES. Because of the formation of sulfur volatile species during sample digestion, the most successful procedures are based on closed vessels. In this case, microwave-assisted digestion is one of the best alternatives because of its localized fast heating, decomposition efficiency at high pressures and temperatures, and less proneness to contamination and analyte losses (40). For ICP-OES determinations of sulfur in petroleum, diesel fuel, biodiesel, and oil sludge waste samples, a mixture of 4 mL HNO₃, 2 mL H₂O₂, and 2 mL H₂O can be used in closed-vessel microwave-assisted digestions (residual carbon content from 3% to 9% m/m) (41). Another simple and fast sample preparation alternative is combustion. Murillo and colleagues (42) evaluated the performance of an oxygen combustion bomb at decomposing crude oils and related materials before sulfur determination by ICP-OES. In this case, aqueous standards and regular sample introduction devices can be used, and no internal standardization is required. Combustion was also used by Geng and colleagues (43) for coal sample preparation. An oxygen flask and an absorbing solution of H₂O₂ were used in ICP-OES sulfur determinations. According to the authors, the main advantages of this strategy are its simplicity, ease of operation, and low analyte losses.

First described in 2004 (44), microwave-induced combustion (MIC) has been successfully used to decompose difficult samples such as petroleum coke (45), extra-heavy crude oil (46), and crude oil distillation residues (47) before sulfur determination by ICP-OES. Considering its efficiency at decomposing high carbon content samples and its high-pressure closed-vessel nature, MIC may be the most suitable sample preparation method for sulfur determination in petroleum products.

Because ICP-OES is so robust, simpler sample preparation procedures can be used in sulfur determinations. For example, a simple dilution of biodiesel in ethanol or n-propanol was described by Chaves and colleagues (48). A cooled (-5 °C) spray chamber was used to reduce the amount of organic solvent reaching the plasma and no oxygen gas introduction was necessary. Background emission in the low-UV region was minimized using an efficient background correction system. Emulsification of crude oil (49), gasoline, kerosene, and diesel fuel (49) may be a fast alternative to total matrix decomposition without the difficulties observed with the direct introduction of organic solvent–diluted samples. The main drawback of this approach is that O₂ gas must be introduced into the plasma to minimize background emission and carbon deposition on the instrument parts (50). Microemulsion preparation is an interesting alternative to other direct sample introduction methods. It also presents some advantages when compared to emulsion preparation, such as instant formation and thermodynamic stability with no phase separation (51). Biodiesel microemulsion sample preparation combined with the summation of multiple emission lines was successfully used to improve precision and sensitivity in sulfur determinations by ICP-OES (52).

From the works cited here, one can conclude that, in general, the main parameters effecting accuracy and sensitivity in sulfur determinations by ICP-OES are related to sample preparation and the sample introduction system used.

Final Considerations

During the past three decades, inductively coupled plasma methods experienced significant improvements with different instrumental approaches. As discussed by Sánches and colleagues (53) in a recently published review, ICP-OES and ICP-MS have become important tools in elemental analysis of petroleum products. On the other hand, trace sulfur determination in these samples still remains a challenge. The overview presented here clearly shows the correlation between instrumental improvements and stricter regulations. Sometimes it is difficult to point out which one comes first, but instrument innovation, environment and health impact studies, and pieces of legislation restricting maximum levels of chemicals in different products are closely connected. As discussed in this overview, improvements in sample preparation and instrumentation have contributed to minimizing matrix and spectral interferences, allowing for the determination of sulfur at increasingly lower concentrations by both ICP-OES and ICP-MS.

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Joaquim A. Nóbrega is with the Group of Applied Instrumental Analysis in the Department of Chemistry at Federal University of São Carlos in São Carlos, Brazil. His experience is mainly related to spectrochemical analysis, particularly in AAS, ICP-OES and ICP-MS, and microwave-assisted sample preparation. Direct correspondence to: dja@terra.com.br

Joaquim A. Nóbrega

George L. Donati is with the Department of Chemistry at Wake Forest University in Winston-Salem, North Carolina.

George L. Donati

Renata S. Amais is with the Group of Applied Instrumental Analysis in the Department of Chemistry at Federal University of São Carlos in São Carlos, Brazil.

Renata S. Amais