High-Efficiency Microwave-Assisted Digestion Combined to in Situ

Oct 18, 2013 - A method for heavy and extraheavy crude oil digestion based on microwave-assisted wet digestion (MW-AD) and ultraviolet (UV) radiation ...
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High-Efficiency Microwave-Assisted Digestion Combined to in Situ Ultraviolet Radiation for the Determination of Rare Earth Elements by Ultrasonic Nebulization ICPMS in Crude Oils J. S. F. Pereira,† R. S. Picoloto,‡ L. S. F. Pereira,‡ R. C. L. Guimaraẽ s,§ R. A. Guarnieri,§ and E. M. M. Flores*,‡ †

Instituto de Química, Universidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, Rio Grande do Sul, Brazil Departamento de Química, Universidade Federal de Santa Maria, 97105-900, Santa Maria, Rio Grande do Sul, Brazil § Centro de Pesquisas e Desenvolvimento Leopoldo Américo Miguez de Mello, Petrobras S. A., 21941-945, Rio de Janeiro, Rio de Janeiro, Brazil ‡

ABSTRACT: A method for heavy and extraheavy crude oil digestion based on microwave-assisted wet digestion (MW-AD) and ultraviolet (UV) radiation using diluted HNO3 was applied for the determination of rare earth elements (REE) by inductively coupled plasma mass spectrometry (ICPMS) with an ultrasonic nebulizer (USN). Even using pressurized systems conventional acid digestion is not feasible for efficient crude oil digestion, especially for heavy and extraheavy crude oils that generally present high amounts of asphaltenes and resins. In the proposed system, UV radiation is generated in situ by immersed electrodeless Cd discharge lamps positioned inside quartz vessels. The use of diluted solutions (1− 14.4 mol L−1 HNO3 and 1−4 mol L−1 H2O2) were evaluated for heavy and extraheavy crude oil digestion (API density of 11.1−19.0). With the proposed method the residual carbon content was lower than 13 mg C/100 mg of sample, and it was possible to digest sample masses up to 500 mg using 4 mol L−1 HNO3 and 4 mol L−1 H2O2. Interferences caused by excessive acid concentration and carbon content in digests were minimized allowing limits of quantification for REEs as low as 0.3 ng g−1. Samples were also digested using MW-AD in pressurized systems with concentrated HNO3, but even using 280 °C, 80 bar, and concentrated HNO3, MW-AD method was not suitable for REE determination due to interferences in ICPMS determination. The combination of microwave heating with UV was considered a suitable and effective way to digest crude oil allowing further determination of low concentrations of REE by ICPMS.

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Some of the techniques used for REE determination are neutron activation analysis (NAA) and inductively coupled plasma optical emission spectrometry (ICP OES). Concerning the NAA technique, it allows achieving accurate results but it presents some drawbacks such as a long time for analysis, generation of radioactive wastes, low access for most laboratories, and unsuitability for the determination of all the REE.9 In addition, the limit of detection (LOD) could be not enough for determination of the very low levels of REE in crude oil.10,11 The use of ICP OES for REE determination is normally impaired due to serious spectral interferences caused by emission lines overlap, lower sensitivity, and unsuitable LODs when compared to inductively coupled plasma mass spectrometry (ICPMS).12 Taking into account some of special characteristics, such as sensitivity and multielement capability, ICPMS has been the most recommended technique for REE determination in crude oil.2,4,6,7 For REE determination by ICPMS some interferences are more intense when compared to

ecent studies have pointed out that information related to crude oil origin, maturity, migration, and correlation of oil to source rock, among others parameters, could be properly obtained by the knowledge of concentration of rare earth elements (REE),1,2 and a major part of the works concerning this issue was only published in the last 8 years.2−7 In addition, the determination of REE is important because some of these elements could be indicators of environmental pollution by crude oil refineries.1−3,8 However, despite the relevance of REE determination in crude oil, only a few works have been reported in literature2−8 and most of them refer to light crude oils, lubricating or diesel oils,3,4 or crude oils with relatively low contents of heavy compounds or sulfur or high content of saturated hydrocarbons.5,7 Heavy and extraheavy crude oil are considered as complex matrixes because generally they contain high concentration of asphaltenes and resins that makes difficult achieving effective digestion or suitable solutions to be analyzed by spectrometric techniques. Moreover, REE determination could be considered as an important analytical challenge mainly due to the low levels that these elements are currently found in crude oils. © 2013 American Chemical Society

Received: August 22, 2013 Accepted: October 18, 2013 Published: October 18, 2013 11034

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were obtained for Cd, Cu, Fe, and Pb.36 A further study was developed using this method but only devoted to organic matter reduction in water, and no other applications were found in spite of its suitability for other matrixes.37 On the basis of the potentiality of the MW-UV method, in the present work its feasibility is demonstrated for the first time to the digestion of crude oils for subsequent La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y determination by ICPMS. A systematic study of digestion conditions (HNO3 and H2O2 concentration, among others) was performed, and evaluation was carried out taking into account the RCC and final acidity in digests. In order to demonstrate the effectiveness of the proposed method it was applied for digestion of heavy and extraheavy crude oil samples (API density ranging from 11.1 to 19.0). Results obtained with the proposed method by MW-UV were compared with those using MW-AD. As no certified reference material is available for REE in crude oil, the accuracy was evaluated by comparison of results with those obtained by NAA (for La, Ce, Nd, Sm, Eu, Yb) and also by recovery tests using a crude oil emulsion especially produced for this study.

other elements, and in this case, the use of different types of nebulizers and devices, such as an ultrasonic nebulizer (USN), could be recommended.2−4 However, it is well-known that REE determination by ICPMS is prone to drastic spectral interferences caused by polyatomic ions, such as oxides and double-charge ions.13−16 In addition, the normal high acid concentration in solutions after sample digestion could cause interferences in REE determination by inductively coupled plasma techniques.17−19 These drawbacks impair the determination of REE and make it necessary to develop new methods of sample preparation avoiding or minimizing the use of concentrated acids. Regarding crude oil digestion, it is important to consider the complexity of this type of matrix that is hard to bring into solution in comparison with other types, such as biological and botanical samples, mainly due to the presence of some compounds such as asphaltenes and resins that increases its stability.20,21 Microwave radiation combined with concentrated acids has been successfully applied for sample digestion in the last years due to the high efficiency of heat transfer allowing relatively fast and efficient sample digestion.22 Considering the applications described for crude oil digestion, most of them are devoted to the use of microwave-assisted acid digestion (MW-AD) for further metals, metalloids, and nonmetals determination,23,24 and the few works related to subsequent REE determination are based only on conventional heating or dry ashing with the use of concentrated acids.2,3,5,7 In most cases, concentrated sulfuric acid is used in combination with HNO3.2,3 However, the presence of high amounts of residual H2SO4 could change the viscosity of digests causing interferences and increasing blank values. Another drawback related to dry ashing and wet digestion methods using conventional heating is the long time for digestion where for some cases up to 1 week can be necessary for the digestion of only 0.1 g of crude oil.5 These examples show the difficulty of digesting crude oils for REE determination and the respective challenges involved in routine analysis. In this sense, combustion methods have been proposed to digest matrixes with high carbon content and difficult to bring into solution,25−30 such as petrochemical samples.31−34 However, no applications using combustion methods for crude oil digestion and further REE determination were found in literature. It is well-known that ultraviolet (UV) radiation is effective as an aid for digestion of some compounds.35 In this respect, the combination of microwave-assisted acid digestion and ultraviolet radiation, called the microwave-assisted ultraviolet (MWUV) digestion method, could be an alternative to achieve better decomposition efficiency and also minimize the use of concentrated acids. This method was originally developed by Florian and Knapp36 in order to obtain a high-efficiency digestion (low values of residual carbon content, RCC, using diluted acids). In this method, an electrodeless Cd discharge lamp (main emission line of 228 nm) was used for generation of UV radiation. Lamps were directly immersed into quartz vessels conventionally used for MW-AD.36 In the first application, the feasibility of this method was demonstrated for digestion of milk using only a mixture of 50 μL of concentrated HCl (36%), 50 μL of concentrated HNO3 (69.5%), and 1 mL of H2O2 (30%). The microwave program was performed by heating vessels by 30 min at 1000 W and 20 min for cooling. Carbon content was reduced more than 90% even using very low acid volumes, and quantitative recoveries

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EXPERIMENTAL SECTION Instrumentation. A microwave oven originally designed for closed vessels wet digestion (Multiwave 3000 microwave sample preparation system, Anton Paar, Graz, Austria) equipped with eight high-pressure quartz vessels was used for the proposed MW-UV method. The volume of vessels was 80 mL, and the maximum values for operational temperature and pressure were 280 °C and 80 bar, respectively. A cadmium lowpressure discharge microwave lamp (part no. 16846, Anton Paar) was used inside each quartz vessel. The emission domain in the UV region is mainly located at 228 nm, and the radiation intensity emitted during the microwave heating program is dependent on the absorbed microwave energy and varies from 1 to 10 W.36 Lamp operation is fully initiated and maintained by the microwave field within the microwave oven cavity. In addition, the system is equipped with a lamp base ring and a lamp spacer made from poly(tetrafluorethylene) (PTFE). These devices are used to avoid damages to the quartz vessels and shocks to UV lamps during microwave heating. The maximum temperature was set at 200 °C in order to avoid excessive heat that could damage PTFE devices. Rare earth elements determination was performed using an inductively coupled plasma mass spectrometer (PerkinElmerSCIEX, model Elan DRC II, Thornhill, Canada) equipped with an ultrasonic nebulizer (model U6000AT+, CETAC Technologies, Omaha, U.S.A.). The USN operates with successive steps of heating (140 °C) and cooling (−5 °C) in a sample flow rate of 2.5 mL min−1. The optimized operational conditions for USN-ICPMS were rf power of 1400 W and plasma, auxiliary, and nebulizer gas flow rates of 15, 1.20, and 1.00 L min−1, respectively. The isotopes used for measurements were 139La+, 140 Ce+, 141Pr+, 146Nd+, 152Sm+, 153Eu+, 160Gd+, 159Tb+, 163Dy+, 165 Ho+, 166Er+, 169Tm+, 172Yb+, 175Lu+, and 89Y+. Other isotopes, such as 143Nd+, 151Eu+, 154Sm+, 155Gd+, 161Dy+, 162Dy+, 167Er+, 173 Yb+, and 174Yb+, were also monitored during analytes measurements in order to evaluate possible interferences. An inductively coupled plasma optical emission spectrometer (Spectro Ciros CCD, Spectro Analytical Instruments, Kleve, Germany) was used for measurements of RCC. A cross-flow nebulizer coupled to a Scott double-pass type nebulization 11035

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yttrium was added as internal standard. For acidity determination, 0.1 mol L−1 KOH was used for titration (this solution was previously standardized using potassium hydrogen phthalate). Proposed Microwave-Assisted UV Digestion Method. Crude oil samples (from 350 to 500 mg) were weighed inside the quartz vessels used for the conventional MW-AD method. Before crude oil weighing, a PTFE device was transferred to quartz vessels in order to maintain the UV lamp in the vertical position. Quartz vessels were charged with a mixture of nitric acid and hydrogen peroxide solutions, and UV lamps were positioned inside vessels. Microwave heating program was based on the heating program performed for the MW-AD method. In this sense, the heating program was performed as follows: (i) 400 W for 10 min (ramp of 10 min), (ii) 900 W for 10 min (ramp of 10 min), and (iii) 0 W for 20 min (cooling step). The maximum temperature and pressure were set at 200 °C and 80 bar, respectively. After digestion and pressure release, resultant solutions were diluted with water to 25 mL. After each run, vessels, UV lamps, and PTFE devices were cleaned using concentrated HNO3 under microwave radiation (1400 W during 10 min). Further, all devices were rinsed with water and dried in a class 100 laminar bench (CSLH-12, Veco, Brazil). Conventional Microwave-Assisted Acid Digestion. For digestion by MW-AD, crude oil samples (up to 500 mg) were directly weighed into the quartz vessels. Further, the quartz vessels were charged with 5 mL of concentrated HNO3 and 1 mL of H2O2. Then, the quartz vessels were closed, capped, fixed in the rotor, and further placed inside the microwave oven cavity. The selected microwave heating program was started for crude oil digestion: (i) 400 W for 10 min (ramp of 10 min), (ii) 1000 W for 10 min (ramp of 10 min), and (iii) 0 W for 20 min (cooling step). The maximum temperature and pressure were set at 280 °C and 80 bar, respectively. After digestion the pressure of each vessel was carefully released and digests were diluted with water to 25 mL. After each run, vessels were cleaned using concentrated HNO3 by microwave radiation at 1400 W during 10 min followed by a rinsing step with water.

chamber was used throughout, and measurements were performed according to the conditions previously described.38 For ICPMS and ICP OES determinations, argon (99.996%, White Martins−Praxair, São Paulo, Brazil) was used for plasma generation, nebulization, and as auxiliary gas. Determination of acidity in digests was performed using an automatic titrator (model 836, Metrohm, Herisau, Switzerland) equipped with a module of automatic stirring (model 803 Ti Stand, Metrohm) and a combined pH electrode (model 6.0262.100, Metrohm). Neutron activation analysis (Neutron Activation Analysis Laboratory, Comissão Nacional de Energia Nuclear, Instituto de Pesquisas Energéticas e Nucleares, IPEN, São Paulo, Brazil) was performed for accuracy evaluation using neutron flow of 1012 n cm−2 s−1. Before the half-life of each element, γ activity measurements were performed in a hyperpure Ge detector (GX 2020, Canberra) linked to a spectrometer. Samples, Reagents, and Standards. Heavy and extraheavy crude oil samples were used for evaluation of the proposed digestion method by MW-UV for further REE determination, and they were arbitrarily named from “A” to “D”. Crude oil samples presented API density in the range of 11.1−19.0. Water and salt content ranged from 0.1% to 30% and 5 to 100 μg g−1, respectively. Due to high viscosity, samples were heated in an oven at 60 °C (Nova Ética, Brazil) until they started to flow allowing to take samples from the storage vessels. In view of the lack of certified reference materials of crude oil with certified values for REE, accuracy was also evaluated by the use of spikes. Crude oil sample “A” was arbitrarily chosen for the preparation of a spiked sample since REE concentration was below the limits of quantification (LOQ) that were previously determined by USN-ICPMS (after MW-AD). In this case, analytes determination was performed using diluted digests obtained by MW-AD and also using the USN for solvent removal in order to minimize interferences in the determination step. Crude oil emulsion was prepared using 10 g of crude oil sample “A” and about 100 μL of a standard solution (Multi-Element Solution 1, CLMS-1, Spex Certiprep Inc., 10 mg L−1 of all REE). This synthetic emulsion was prepared by aqueous phase incorporation in the oil phase. In this sense, the mixture of aqueous phase and crude oil was homogenized in an oven for 15 min at 80 °C and simultaneously stirred for complete incorporation of standard solution added in the crude oil. Further, the sample was submitted to a mechanical stirring at 2500 rpm during 5 min. Deionized water was further purified using a Milli-Q system (Millipore Corp., Bedford, U.S.A.), and it was used to prepare all the standard solutions and reagents. Multielement stock standard solution containing 10 mg L−1 of all REE (MultiElement Solution 1, CLMS-1, Spex Certiprep Inc., Quebec, Canada) was used to prepare analytical standards by sequential dilution in 5% (v/v) HNO3 (Merck, Darmstadt, Germany) in the range of 10−100 ng L−1. Concentrated HNO3 (65%, Merck) was purified using a sub-boiling system (Milestone, Model Duopur, Bergamo, Italy), and it was used for crude oil digestion methods. Nitric acid was also used to clean the quartz vessels, UV lamps, and PTFE devices after each digestion step. Hydrogen peroxide (ultrapure, purity ≥30%, Merck) was also used for crude oil digestion by MW-UV. Standard solutions used for RCC determination by ICP OES were prepared by sequential dilution of a stock reference solution prepared by citric acid dissolution in water, and

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RESULTS AND DISCUSSION Performance of the Microwave-Assisted UV Method. Initial studies were performed for the evaluation of safety aspects of MW-UV method (sample “A” was used for this evaluation). In this respect, the mass of crude oil was limited to 350 mg and 10 mL of concentrated HNO3 was used for digestion. Initially, the maximum temperature was set at 250 °C, according to manufacture recommendations. The exhaustion system of microwave oven was maintained at level “2” (126.6 m3 air h−1) during all microwave heating program. In this case, cooling at level “2” during the digestion step allows a higher microwave power necessary to keep pressure and temperature at the respective maximum levels and, consequently, resulting in intensification of UV radiation emitted by microwave UV lamps.39 During microwave heating an increase of pressure was observed in about 6 min of microwave heating, and the maximum pressure exceeded the limit of 0.8 bar s−1, reaching values of about 50 bar and 230 °C (initially, the maximum temperature was set at 250 °C). After cooling, it was observed that the PTFE caps of the quartz vessels were completely damaged probably due to the high temperature reached inside the vessels. By setting the maximum temperature to 200 °C 11036

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these problems were not observed, and this condition was used for further studies. It is well-known that the extent of digestion is dependent on achieved temperature, and especially for some difficult matrixes (such as crude oil), this parameter is important to obtain low RCC values. However, it must be considered that for the proposed MW-UV method, a suitable efficiency of digestion can be achieved even working with a maximum temperature of 200 °C due to the additional effect of in situ UV radiation. Investigation of the Acid Mixture for Crude Oil Digestion by MW-UV. Initially, digestion using only HNO3 was evaluated in the proposed MW-UV method (using about 350 mg of crude oil), and RCC and acidity determinations were performed after each study. The follow HNO3 concentrations were investigated: 1, 3, 5, 7, 10, and 14.4 mol L−1 using a total volume of 10 mL of solution. This volume was necessary to cover the surface of the UV lamp. After the end of the heating program and vessels cooling it was observed that digestion of crude oil (350 mg) was not complete when the HNO3 concentration lower than 5 mol L−1 was used (solid residues remained into solution). In this case, crude oil digestion was apparently suitable using at least 5 mol L−1 HNO3 and sample mass up to 350 mg. Residual carbon content and acidity were determined in digests obtained by MW-UV using only 5 mol L−1 HNO3, and results obtained for RCC for 5, 7, 10, and 14.4 mol L−1 HNO3 were 13.2 ± 0.7, 12.4 ± 0.6, 11.9 ± 1.1, and 11.2 ± 0.6 mg C/ 100 mg of sample, respectively (n = 4). The respective values for final acidity in digests were 1.64 ± 0.12, 3.43 ± 0.28, 5.93 ± 0.43, and 9.49 ± 0.81 mol L−1. As expected, it was observed that the acidity increased with the use of higher HNO3 concentration for crude oil digestion by MW-UV. On the other hand, RCC in digests were similar in the range from 5 to 14.4 mol L−1 HNO3. One reason for this small difference of RCC values can be attributed to the effect of UV radiation that helps to achieve better efficiency of digestion. Additionally, this fact could be indicative that it is possible to use HNO3 concentration as low as 5 mol L−1 for crude oil digestion keeping suitable digestion efficiency. Influence of Hydrogen Peroxide on the MW-UV Method. Studies were performed to evaluate the use of H2O2 in order to improve digestion efficiency40,41 and to reduce the acid concentration used for sample decomposition. In this sense, tests were performed using 5 mol L−1 HNO3 and varying the concentrations of H2O2 (30%, 9.8 mol L−1) from 1 to 4 mol L−1 (values of H2O2 concentration correspond to addition of about 1−4 mL of H2O2). It was observed that with the use of 4 mol L−1 H2O2 (4 mL of concentrated H2O2) the crude oil digestion was not complete probably due to the pressure increase resulting in a reduction of the microwave radiation (pressure sensor caused a reduction from 900 to 400 W of microwave power). Consequently, the temperature of the solution was lower (temperature was reduced to values of about 150 °C instead of 200 °C when 900 W of microwave power was applied). For these tests digestion was not complete (solid residues were observed in digests). Results obtained for RCC and acidity in digests using 5 mol L−1 HNO3 and different concentrations of H2O2 showed that RCC was not changed depending on the volume of H2O2 used, ranging from 15.8 to 16.7 mg C/100 mg of sample. On the other hand, residual acidity increased when high volumes of H2O2 were used. This fact could be explained by the possibility of acid regeneration in the presence of oxygen as previously demonstrated.40,41

Further studies were performed in order to reduce the amount of HNO3 used for crude oil digestion combined with addition of H2O2. Therefore, different mixtures of HNO3 and H2O2 were evaluated for digestion of 350 mg of crude oil, as follows (the total volume of solution was kept at 10 mL, and the concentration of HNO3 informed is the final concentration in total volume of solution): (i) 4.5 mol L−1 HNO3 and 1 mol L−1 H2O2, (ii) 4 mol L−1 HNO3 and 2 mol L−1 H2O2, (iii) 3.5 mol L−1 HNO3 and 3 mol L−1 H2O2, (iv) 3 mol L−1 HNO3 and 4 mol L−1 H2O2. Crude oil digestion was considered as suitable (RCC < 15%) by using all the mixtures of HNO3 and H2O2 (RCC ranged from about 8.5 to 12.1 mg C/100 mg of sample). In order to use higher sample mass in the digestion step by MW-UV for further determination of REE in low concentrations, the proposed method was investigated for digestion of 400 and 500 mg of crude oil, using similar solutions tested before. With the use of 3.0 mol L−1 HNO3 and 4 mol L−1 H2O2 the digestion of 400 and 500 mg of crude oil was not complete (the presence of solid residues was observed in final solutions), and the same behavior was observed when 3.5 mol L−1 HNO3 and 3 mol L−1 H2O2 were used. In this sense, the concentration of H2O2 was kept at 4 mol L−1 and HNO3 concentration (final concentration in solution after addition of H2O2) was increased to 4 mol L−1. In this case, no solid residues were observed in digests and RCC was lower than 10 mg C/100 mg of sample that is suitable for further analysis by USN-ICPMS. Therefore, relatively higher sample mass of crude oil could be digested by MW-UV using diluted acid (final acidity in digests was lower than 3 mol L−1). This is an important aspect considering heavy and extraheavy crude oil samples since for this type of matrix traditional methods for sample digestion are not efficient due to the content of asphaltenes and resins.42 In addition, with the use of diluted acids it is possible to reduce residues generation and to minimize interferences in the REE determination by ICPMS due to the relatively low acid content in digests. It is important to mention that the same microwave heating program and digestion solution used (4 mol L−1 HNO3 and 4 mol L−1 H2O2) was applied without the insertion of the UV lamp inside the quartz vessels (500 mg of sample). As expected, digestion was not complete, showing the applicability of microwave UV lamps for digestion of difficult matrixes even using diluted acid solutions. Additional tests were performed using the previously optimized conditions (4 mol L−1 HNO3 and 4 mol L−1 H2O2 for digestion of 500 mg of crude oil) using crude oil sample “A” with a known concentration of REE (after a preparation of a synthetic crude oil emulsion with analytes addition). In this case, after digestion by MW-UV, analytes were determined by USN-ICPMS, and recoveries in the range of 97−102% were obtained for all investigated elements. It is important to mention that digests obtained by MW-AD were diluted previous to the analysis by ICPMS and USN was also used to minimize interferences in the determination step by solvent removal. Pressurized Microwave-Assisted Acid Digestion. Crude oil digestion by MW-AD was performed using 500 mg of sample and concentrated HNO3 (5 mL) and H2O2 (1 mL). During the microwave heating program, it was observed that, in general, a rapid increase in the pressure occurred in the second step of digestion (irradiation of 1000 W), and consequently microwave power was about 500 W until the end of the microwave program. After cooling, it was observed that digests obtained by MW-AD presented a yellowish aspect and RCC 11037

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(1.73−3.05 mol L−1) instead of MW-AD (6.18−10.6 mol L−1) that were not suitable for further analysis by USN-ICPMS without previous dilution. Determination of Rare Earth Elements in Crude Oil after MW-UV and MW-AD Methods. After crude oil digestion, REE determination was performed by USNICPMS, and results obtained after by MW-UV and MW-AD are shown in Table 1. In general, most of all REE were quantified in crude oil samples by USN-ICPMS after MW-UV. For sample “A”, all REE concentrations were lower than LOQs (10σ, n = 10) obtained by USN-ICPMS after MW-UV and also MW-AD. It was observed that LOQs obtained for MW-AD were higher in comparison with those obtained by MW-UV in view of the high values of residual carbon content and acidity obtained for MWAD. For REE determination in digests obtained by MW-AD it was necessary to perform a dilution step previous to analysis due to high residual acid concentration and RCC in digests to avoid interferences during ICPMS determination. In general, using solutions obtained by MW-AD without previous dilution, a reduction of about 10−30% of analytes signal intensity was observed and an additional dilution at least 2 times was necessary. Therefore, the LOQs obtained for MW-AD were about 2 times higher than those obtained for MW-UV. Additionally, it could be observed that, in general, results obtained by MW-AD were slightly lower than those obtained by MW-UV. Concerning the REE determination in digests obtained by MW-UV, no interferences were observed due to acid or RCC content in final solutions. In this case, 4 mol L−1 HNO3 and 4 mol L−1 H2O2 mixture was used for sample digestion, making it possible to obtain better LOQs and to perform REE determination in lower concentration. Since no previous dilution of final solutions is necessary to avoid interferences, some REE could be determined in crude oil samples only when MW-UV was used for sample digestion. As an example, Tm and Lu were quantified in sample “B”, Tb and Tm in sample “C”, and La, Dy, and Tm in sample “D” only when MW-UV was used. Due to the lack of certified reference materials with reference or even recommended values for REE in crude oil, accuracy was

values for crude oil samples ranged from 19 to 26 mg C/100 mg of sample. These results shown the efficiency of the proposed MW-UV method because using diluted acids and maximum temperature of 200 °C the RCC were significantly lower (RCC lower than 10 mg C/100 mg of sample) than those obtained using hard conditions (concentrated acids, 80 bar, 280 °C). Therefore, the use of UV lamps combined with microwave radiation could be considered as a promising alternative in order to obtain complete crude oil decomposition. A comparison of RCC and acidity values is shown in Figure 1 for all crude oil samples investigated after digestion by

Figure 1. Comparison of RCC and final acidity for digests obtained after different crude oil sample (“A” to “D”) decompositions by the proposed MW-UV method and MW-AD (n = 4). RCC obtained by MW-UV (open bars) and by MW-AD (solid bars) methods. The lines represent the final acidity in digests (mol L−1) after MW-UV (○) and after MW-AD (■), respectively.

MW-UV and MW-AD. It is important to mention that MW-AD was performed using concentrated acid instead of diluted acid used for MW-UV method. According to Figure 1, results obtained for RCC after MWUV were lower in comparison with those obtained by MW-AD showing the better efficiency of the proposed method. In this case, the MW-UV method was performed using 4 mol L−1 HNO3 instead of concentrated HNO3 that was necessary to obtain complete sample decomposition. In addition, results for final acidity in digests were significant lower by using MW-UV

Table 1. Results (ng g−1) Obtained for REE in Crude Oil Samples by USN-ICPMS after MW-UV and MW-ADa crude oil sample B element La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y a

MW-UV 12.7 32.1 4.85 28.7 8.58 1.90 10.5 1.66 10.1 1.94 4.82 0.54 2.84 0.41 50.9

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.95 0.9 0.20 2.1 0.74 0.12 0.6 0.11 0.5 0.11 0.45 0.03 0.22 0.03 2.8

sample C MW-AD 11.7 30.9 4.98 26.4 8.33 1.77 9.45 1.48 9.22 1.76 4.39