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