Joaquim A. Nóbrega

The selection of an analytical method featuring multielemental capability, wide linear range, and ease of operation would be ideal for rare earth element (REE) determination in complex samples. This work targets the determination of Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sc, Sm, Tb, Th, Tm, Y, and Yb in geological samples, fertilizer, and agricultural gypsum samples using inductively coupled plasma–optical emission spectrometry (ICP-OES). Samples were prepared using microwave-assisted acid digestion and the accuracy was evaluated by addition and recovery experiments for spiked samples at three concentration levels. A careful choice of the plasma viewing position and appropriate dilution factors led to accurate and sensitive determinations of REEs. The proposed procedure using external calibration presented good accuracy and sensitivity.   

	Rare earth elements (REEs) comprise the lanthanides series, as well as Sc and Y. Nowadays these elements are essential for high-technology and military applications (1), and some ores can be classified as a rich source of REEs, such as Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sc, Sm, Tb, Th, Tm, Y, and Yb. The most abundant REEs in the Earth's crust are Ce, La, Nd, and Y (31–66 µg/g), while Tm and Lu (0.5–0.8 µg/g) are found in lower contents (1,2).

	The chemical analysis of REEs in geological materials is critical for better exploration of mineral reserves as well as for studies in geology and geochemistry because these elements present unique properties and can be used as geochemical tracers (3). Moreover, the predominant ionic form of some REEs is trivalent, which can replace Ca in phosphate minerals and may reach soil together with phosphorus fertilizers and agricultural gypsum (4,5).

	Among the spectrochemical methods available, instrumental neutron activation analysis (INAA) and inductively coupled plasma–mass spectrometry (ICP-MS) are commonly used for REE determination in mineral samples because of their multielement capability, high sensitivity, and low detection limits. However, these methods rely on expensive instrumentation and are prone to interferences caused by long irradiation times and spectral overlaps, respectively. In addition, X-ray fluorescence (XRF) spectroscopy may also be applied in the direct solid analysis of these samples; however, XRF lacks sensitivity and thus is not suitable for the determination of REEs at low concentrations (6). In this context, inductively coupled plasma–optical emission spectrometry (ICP-OES) has been reported in the literature as a feasible alternative for REE determination because of the multielement capacity, adequate sensitivity, wide linear dynamic range, and ease of operation (6–8).

	Considering these aspects, the goal of this study was to evaluate the performance of ICP-OES for the determination of REEs in geological samples, phosphorus fertilizers, and agricultural gypsum samples. The ICP-OES instrument was evaluated for measurements in both radial view and dual view (axial and radial) modes.

	An ICP-OES system with a dichroic spectral combiner for simultaneous collection of data in radial and axial viewing mode was used to perform all measurements (5100 SVDV, Agilent Technologies). The instrument arrangement involves a vertical argon plasma. The plasma operating conditions and parameters of the sample introduction system are presented in Table I. Samples were prepared using microwave-assisted acid digestion with an Ethos 1 oven (Milestone).

	All glassware was decontaminated by immersion in 10% v/v HNO

 for at least 24 h and rinsed with distilled–deionized water (resistivity ≥ 18.2 MΩ-cm) obtained from a Milli-Q Water system (Millipore). All solutions and analytical blanks were prepared with ultrapure water and nitric acid obtained using a sub-boiling distillation apparatus (Milestone). Concentrated nitric acid (65% m/m) and hydrochloric acid (37% m/m) were used to prepare the aqua regia solution used to digest samples. Analytical calibration solutions in the 0.05–5 mg/L range were prepared by appropriate dilution of a multielement stock solution (Agilent Technologies) of 10 mg/L Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sc, Sm, Tb, Th, Tm, Y, and Yb. Limits of detection (LOD) were obtained by considering background equivalent concentrations (BEC) and relative standard deviations (RSD) for 10 measurements of digested blank solutions.

Geological and Agricultural Samples Preparation 

	Approximately 100 mg of two geological samples (G1 and G2) were accurately weighed directly in the PTFE-PFA digestion vessels, followed by the addition of 9.0 mL of aqua regia. The loosely capped vessels were left overnight for a predigestion step in a laminar flow clean hood. The digestion was further carried out in a closed-vessel microwave cavity oven according to the following three-step heating program: 10-min ramp to 120 °C, 20-min ramp to 220 °C, and a 5-min plateau at 220 °C. After the digests cooled to room temperature, they were transferred to previously decontaminated 50-mL polypropylene flasks and the volume was made up with distilled–deionized water. Seven fertilizer samples (A–G) and two agricultural gypsum samples (Gy1 and Gy2) were provided by Embrapa Pecuária Sudeste. Samples A, B, and C were a mixture of mineral fertilizers with different NPK contents with B, Cu, Mn, and Zn in their compositions. Samples D, E, and F were a mixture containing B, Cu, Mn, and Zn in a 20-fold higher concentration than samples A, B, and C. Sample G was an organomineral fertilizer. The fertilizer samples were submitted to a 2-h predigestion step at room temperature, followed by microwave-assisted digestion. Microwave-assisted digestions were performed by adding 6 mL of HNO

(30% w/w) to approximately 200 mg of sample. The heating program for fertilizer samples was implemented in two steps: a 20-min ramp to 220 °C, followed by a 20-min plateau at 220 °C. After the digested samples cooled down, they were transferred to polypropylene flasks and volumes were made up to 50.0 mL with distilled–deionized water. The accuracy of the proposed procedures was checked by addition and recovery experiments at a single level at 2.5 mg/L for geological samples and two concentration levels (0.1 and 1 mg/L) for fertilizers. Spikes were added before digestion to check the sample preparation step. Both digestion and recovery procedures were performed in triplicate for each sample.

	The determination of REEs in geological samples, fertilizers, and agricultural gypsum was carried out using external calibration. Limits of detection were calculated considering BEC, signal-to-background ratio (SBR), and relative standard deviation (RSD) for 10 measurements of a blank solution, and the values are shown in Table II. Calibration curves with correlation coefficients better than 0.9990 were obtained for all analytes, showing the feasibility of the REE measurements by ICP-OES.

	Eid and colleagues (9) reported LODs ranging from 0.009 to 0.45 mg/mL for eight REEs determined by ICP-OES in phosphate samples. Lichte and colleagues (10) described a procedure for the determination of REEs in geological materials by ICP-MS. The LODs were in the 2–11 ng/g range with precision (RSD) of 2.5%. However, spectral interferences had to be overcome, leading to the formation of oxides and promoting drifts in sensitivity.

	Addition and recovery experiments were performed to check the accuracy of the proposed procedure. Samples of geological materials, fertilizers, and agricultural gypsum were spiked.

	Geological sample G2 was spiked at 2.5 mg/L and the results demonstrated that suitable accuracies were attained in both viewings. Recoveries for all elements, except Ce, ranged from 90.1% to 107% for both viewing modes. The only exception was for Ce (418.659 nm), where recoveries presented a positive error, 110% and 118%, for radial and dual view modes, respectively. Applying a 

-test, no statistical differences at a 95% confidence level were observed between the radial view and dual view modes. Lanthanum, Nd, and Pr were presented at high concentrations in both samples (G1 and G2), thus a fourfold additional dilution was necessary and reading in radial view mode was the best choice.

	The REE concentrations in fertilizers were lower than in the geological samples; therefore spikes were added at lower levels in the fertilizer samples. Addition and recovery experiments (Table III) were applied to three fertilizer samples (B, D, and G) with different compositions. Due to the low REE concentrations in the fertilizers, the dual viewing mode was used because of its higher sensitivity. The procedure presented suitable accuracy, except for Ce, La, Pr, Th, and Y at 0.1 mg/L in spiked sample B, because the concentrations of these REEs in sample B were higher than the spiked concentrations.

	For the agricultural gypsum sample, spikes at 0.1 and 1.0 mg/L were performed. The best results were reached after a 10-fold additional dilution and using radial view configuration because of the higher concentrations of REEs in this sample, except for Ce, La, and Nd-the added concentrations for these elements were higher than the concentrations naturally present in the samples. Using the dual view mode, good accuracy was attained for lower level spikes (0.1 mg/L) except for Ce, La, and Nd; however, for spikes at the higher level (1.0 mg/L), recoveries were not satisfactory, because the added concentrations were too high when compared to the real concentrations in the sample. Thus, the careful choice of the viewing position and additional dilution led to proper accuracies and sensitivities for REE determination in both viewing modes. For agricultural gypsum samples, radial viewing mode was better.

Determination of REEs in Geological Samples, Fertilizer, and Agricultural Gypsum Samples

	The developed procedure using dual view mode was applied to the determination of REEs in fertilizers and geological samples (Table IV). Geological sample 1 contained higher concentrations of all the analytes than sample 2, except for Sc. Rare earth element determination by ICP-OES was fast and suitable recoveries were obtained. The addition and recovery experiments showed that spectral interferences were not an issue. Three emission lines for each of the 17 elements were measured in less than 2 min in both viewing modes and required a sample volume of approximately 3 mL, demonstrating a high sample throughput for the procedure. Some concentrated samples were further diluted before the analysis, which was not a problem considering the wide linear range of the calibration curves. In the agricultural gypsum samples, analytes were determined using the radial viewing mode (Table V).

	Lutetium and Tm were not found in any fertilizer sample, and the REEs presented at higher concentration levels were the same as those previously cited by Tyler (2). Agricultural gypsum samples presented concentrations fivefold higher than those found in fertilizer samples.

	Spectral and matrix interferences were reported by Eid and colleagues (9) for REE determination, where suppression in La II 408.6-nm and Nd II 406.1-nm emission intensities were observed because of high Ca concentrations. Jaron and colleagues (11) reported spectral interferences caused by Fe and Ti overlaps and matrix interferences caused by the presence of Ca and Mg during the analysis of geological samples. However, those interferences were not found in the present work. The ICP-OES method was fast and suitable for the determination of rare earth elements in a wide concentration range in geological and agricultural samples.

	The ICP-OES method presented multielement capability and selectivity that was compatible with accurate REE determination in complex materials such as ores and agricultural samples. The high sample throughput and adequate accuracy and precision of the developed procedure indicates that ICP-OES is a suitable alternative for REE determination in geological and fertilizer samples. The feasibility of fast measurements of emission signals in radial and dual viewing modes was demonstrated here.

	The determination of REEs in geological and agricultural materials can provide valuable information about the geochemical formation, plant nutritional status, the need for supplementation, and possible contamination. Moreover, in some cases it is important to have a fast sample profile to check for possible interfering elements on other target analytes when applying different methods-for example, the magnitude of interferences caused by double-charged species of 

	The authors are grateful to grants 2013/26672-5 and 2014/18393-1 of São Paulo Research Foundation (FAPESP), for the scholarships provided to R.C.M. and A.V., and for research grant 2015/14488-0. The authors are also thankful to the Conselho Nacional de Desenvolvimento Científico e TecnolÓgico (CNPq, Grants 443771/2014-6 and 303107/2013-8), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES – Grant 15/2104) for fellowships and financial support. We also would like to thank Agilent Technologies for their support.

	(3) K.H. Johannesson, K.J. Stetzenbachand, and V.F. Hodge, 

	(4) A. Bakou, C. Buser, G. Dandulakis, G. Brudvigand, and D.F. Ghanotakis, 

	(5) C.H.R. Saueia, F.M. Le Bourlegat, B.P. Mazzilli, and D.I.T. Fávaro, 

	(6) B. Zawisza, K. Pytlakowska, B. Feist, M. Polowniak, A. Kita, and R. Sitko, 

	(7) M.S. Navarro, H.H.G.J. Ulbrich, S. Andrade, and V.A. Janasi, 

	(8) A.K.G. Silva, J.C. de Lena, R.E.S. Froes, L.M. Costa, and C.C. Nascentes, 

	(9) M.A. Eid, J.A.C. Broekaert, and P. TschÖpel, 

	(10) F.E. Lichte, A.L. Meier, and J.G. Crock, 

	(11) I. Jaron, B. Kudowska, and E. Bulska, 

	(12) B.P. Jackson, A. Liba, and J. Nelson, 

Clarice D. B. Amaral and Raquel C.Machado

 are with the Group for Applied Instrumental Analysis in the Department of Chemistry at Federal University of São Carlos in São Carlos, Brazil and Embrapa Pecuária Sudeste in São Carlos, Brazil. 

Juan A. V. A. Barros, Alex Virgilio, and Joaquim A. Nóbrega

 are also with the Group for Applied Instrumental Analysis in the Department of Chemistry at Federal University of São Carlos. 

 is also with Embrapa Pecuária Sudeste. 

 is with Agilent Technologies in Barueri, Brazil. Direct correspondence to: 

Articles

This method demonstrates that ICP-OES is a suitable alternative to ICP-MS for the determination of rare earth elements in geological and agricultural samples

Determination of Rare Earth Elements in Geological and Agricultural Samples by ICP-OES

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

) 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

 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. 

), 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 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 (

  [4.21%]) overlap with the ones from polyatomic ions formed in the plasma such as 

, 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% N

-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 O

-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 O

 in a pressurized reaction cell and monitor SO

. Thus, the analytical signal is recorded in a 

 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 

S-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 

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 

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

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 O

 absorbs radiation in the low-UV, all modern ICP-OES instruments present a spectrometer evacuated or purged with an inert gas (Ar or N

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 HNO

O 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 H

 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 

-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 O

 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. 

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.  

 (1) H. Shang, H. Zhang, W. Du, and Z. Liu, 

 (2) M.C. Breitkreitz, I.M. Raimundo, Jr., J.J.R. Rohwedder, C. Pasquini, H.A. Dantas Filho, G.E. José, and M.C.U. Araújo, 

 (3) A. Martínez-González, O.-M. Casas-Leuo, J.-R. Acero-Reyes, and E.-F. Castillo-Monroy, 

 (4) M.D. Kass, J.F. Thomas, D. Wilson, S.A. Lewis, and A. Sarles, 

 2005-01-0657 (2005); doi:10.4271/2005-01-0657. 

 (5) V. Meeyoo, D.L. Trimm, and N.W. Cant, 

 (6) M. Kärkkäinen, M. Honkanen, V. Viitanen, T. Kolli, A. Valtanen, M. Huuhtanen, K. Kallinen, M. Vippola, T. Lepistö, J. Lahtinen, and R.L. Keiski, 

 (7) R. Zeisler, K.E. Murphy, D.A. Becker, W.C. Davis, W.R. Kelly, S.E. Long, and J.R. Sieber, 

 (8) A. Stanislaus, A. Marafi, and M.S. Rana, 

(14) C.-F. Yeh, S.-J. Jiang, and T.-S. His, 

(15) L.L. Yu, W.R. Kelly, J.D. Fassett, and R.D. Vocke, 

(18) D. Pröfrock, P. Leonhard, S. Wilbur, and A. Prange, 

(19) P.R.D. Mason, K. Kaspers, and M.J. van Bergen, 

(20) D.R. Bandura, V.I. Baranov, and S.D. Tanner, 

A critical review of the main developments in instrument technology, calibration, and sample preparation that have made it possible to determine low sulfur concentrations in fuels followed by a discussion of strategies to minimize spectral interferences related to sulfur determination by ICP-MS, such as collision–reaction cells, high-resolution mass analyzers, and the interference standard method.

Joaquim A. Nóbrega

Articles

Determination of Rare Earth Elements in Geological and Agricultural Samples by ICP-OES

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