Comparison of direct sampling and emulsion analysis using a filter furnace for the determination of lead in crude oil by graphite furnace atomic absorption spectrometry

Spectrochimica Acta Part B 64 (2009) 530–536

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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b

Comparison of direct sampling and emulsion analysis using a filter furnace for the determination of lead in crude oil by graphite furnace atomic absorption spectrometry☆ Isabel C.F. Damin a, Morgana B. Dessuy a, Tamara S. Castilhos a, Márcia M. Silva a,⁎, Maria Goreti R. Vale a, Bernhard Welz b, Dmitri A. Katskov c a b c

Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre — RS, Brazil Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis — SC, Brazil Tshwane University of Technology (TUT), Faculty of Science, Chemistry Department, Pretoria 0001, South Africa

a r t i c l e

i n f o

Article history: Received 17 November 2008 Accepted 5 March 2009 Available online 12 March 2009 Keywords: Lead Crude oil Filter furnace Direct sampling Graphite furnace atomic absorption spectrometry

a b s t r a c t The determination of trace elements in crude oil is difficult due to the complex nature of the sample and the various different chemical forms in which the metals can occur. The advantage of graphite furnace atomic absorption spectrometry is that only a minimum of sample pretreatment is required. In this work two techniques have been compared to establish a fast and reliable method for lead determination in crude oil. In the first one the crude oil samples were weighed directly onto solid sampling (SS) platforms and introduced into the graphite tube for analysis. In the second one the samples were prepared as oil-in-water emulsions and analyzed in a filter furnace (FF). Twenty μL of a mixture of 0.5 mg L− 1 Pd + 0.3 mg L− 1 Mg+ Triton X-100 has been used as the modifier, and calibration against aqueous solutions has been used for both methods. The sensitivity obtained with the FF was more than a factor of two better than that with SS; however, as a larger sample mass could be introduced in the latter case, so that the limits of detection for both techniques were 0.004 mg kg− 1. Seven crude oil samples were analyzed using the two procedures, and all results were in agreement at a 95% confidence level using a paired Student's t-test. For validation purposes, three crude oil samples have been mineralized using an open-vessel acid digestion, and the results were in agreement with those found with direct sampling and with emulsion sampling using FF according to ANOVA test. Both methods are simple, fast and reliable, being appropriated for routine analysis; however, the direct method using SS technology should be preferred because of its simplicity, speed and commercial availability. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The main constituents of crude oil are hundreds of different complex organic compounds, but it also contains trace metals. Nickel and vanadium are the most abundant origin-specific trace elements and are usually found in the mg kg− 1 range, whereas others, such as lead are present in the μg kg− 1 range [1,2]. Knowledge of trace metal concentration is very important for petroleum technology, i.e., for crude oil cracking processes, fractionation, corrosion and also for environmental purposes because of their toxicity. Lead is a source of destabilization of petroleum products during storage, causing corrosion of boilers that are burning residual oil, and it is causing environmental pollution. An important problem related to lead in petroleum products is poisoning of

☆ This paper was presented at the 10th Rio Symposium on Atomic Spectrometry, held in Salvador-Bahia, Brazil, 7–12 September 2008, and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Fax: +55 51 3308 7304. E-mail address: [email protected] (M.M. Silva). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.03.002

catalysts for crude oil processing since it deactivates noble metals, mainly platinum, which are active components of the catalysts [3]. A variety of analytical techniques and procedures have been proposed for quantification of metallic elements in crude oil and petroleum products [1,4]. Graphite furnace atomic absorption spectrometry (GF AAS) appears to be a good alternative for the determination of trace elements, such as lead, in crude oil, because it is one of the most sensitive techniques with limits of detection in the range from µg L− 1 to ng L− 1 and it is extremely tolerant to complex matrices [5]. The traditional sample preparation strategy for the analysis of crude oil and petroleum products is a complete mineralization; it avoids all problems associated with organic matrices and solvents and makes possible to use aqueous standards for calibration. The conventional ashing and acid dissolution methods, however, are time-consuming and include the risk of losing volatile analytes [4]. Microwave-assisted digestion in closed vessels reduces significantly the risk of losses, but it requires special equipment and has a limited sample throughput. Direct determination of metals in oil samples diluted with organic solvents might be possible and reduces sample preparation time significantly;

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however, direct determination is not free of problems since the solvent and the compounds used for calibration have a substantial influence on the sensitivity [6–8], and the analyte is significantly less stable in organic than in aqueous solution [9]. The formation and analysis of an oil-inwater emulsions or micro-emulsions has been proposed for F AAS [10] and for GF AAS [11–16] with the distinct advantages that aqueous standards could be used for calibration, and that there were no stability problems, particularly in the case of micro-emulsions. Direct analytical techniques are obviously preferred over those that require significant sample preparation, particularly in the case of complex samples, such as crude oil. Direct solid sampling (SS) GF AAS has been shown to be an extremely powerful technique, with high sensitivity, a minimum risk of contamination, high speed and the additional advantage that essentially no reagents are used. It has been shown in recent reviews on this subject that after careful optimization of the experimental conditions this technique is essentially free of interferences and often allows calibration against aqueous standards [17,18]. The only disadvantage, which is common to all SS techniques, is the relatively high uncertainty, which is typically around 5–20% RSD, due to the inhomogeneity of natural samples and the small amount of sample, which is introduced into the atomizer [19,20]. Crude oil, in spite of its often extremely high viscosity, is obviously not a solid sample; however, this does not exclude it from being analyzed using SS techniques. Several papers appeared recently applying SS-GF AAS techniques for direct determination of trace metals in crude oil and petroleum products. Brandão et al. described procedures for the direct GF AAS determination of nickel [21], copper, iron and vanadium [22], investigating the use of modifiers to prevent losses of volatile compounds. Our group proposed the differential determination of volatile and non-volatile compounds of nickel and vanadium in crude oil using SS-GF AAS [23]. Dittert et al. [24] described a procedure for the simultaneous determination of chromium and iron in crude oil without sample preparation using high-resolution continuum source GF AAS. A different approach for the determination of volatile analytes in complex matrices was proposed by Katskov et al. [25], using a transversely heated filter furnace (FF). The FF has several advantages for the analysis of organic solutions, which are summarized in a recent review by Katskov [26]. Anselmi et al. [27] proposed a method for the determination of several trace elements, including lead, in gasoline and diesel fuel using FF AAS. Up until now, the determination of lead in crude oil has not yet been described using SS-GF AAS or FF AAS techniques. The goal of this work has therefore been to compare these two most innovative techniques for trace element determination in view of a sensitive, fast and reliable routine procedure for the determination of lead in crude oil. The SS-GF AAS technique, where the crude oil sample is weighed directly onto a graphite platform without any pretreatment, is obviously the most rigorous approach, and it has the advantage of being commercially available. The use of FF AAS requires some sample preparation, such as emulsification, but has the advantage that an autosampler can be used for easy sample introduction; however, the filter tubes are not yet commercially available.

2. Experimental

The experiments with SS-GF AAS have been carried out using SS platforms (Analytik Jena Part No. 407-152.023) and SS tubes without a dosing hole (Analytik Jena Part No. 407-A81.303). An M2P microbalance (Sartorius, Göttingen, Germany) has been used for weighing the crude oil samples directly onto the SS platform, which was introduced into the graphite tube using a pair of pre-adjusted tweezers, which is part of the SSA 5 manual solid sampling accessory (Analytik Jena AG). The sample mass was automatically transmitted to the instrument's computer to calculate the ‘normalized integrated absorbance’ (integrated absorbance calculated for 1.0 mg of sample) after each measurement. This normalized integrated absorbance is commonly used in SS-GF AAS to compare signals, as it is impossible and unnecessary to introduce always exactly the same sample mass in a series of measurements. The aqueous standards and modifier solution were injected manually onto the SS platform using a micropipette. For the measurements with emulsions, experimental FF tubes manufactured at TUT, similar to those described by Katskov [26], were used. The tubes had exactly the same outer dimensions and could be used in the atomizer unit of the AAS 5 EA without any modification. An MPE 5 furnace autosampler (Analytik Jena) was used for sample introduction. A Unique-Thorton model USC-2850 ultrasonic bath (Thorton, São Paulo, Brazil) operated at a frequency of 37 ± 3 kHz, with temperature control up to 80 ± 5 °C, was used for preparing the emulsions. A digester block Model Te-015/1 (TECNAL — Brazil) was used for digesting the samples. Argon with a purity of 99.996% (White Martins, São Paulo, Brazil) was used as purge gas with a flow rate of 2 L min− 1 during all stages, except during atomization, when the flow was stopped. Integrated absorbance (peak area) was used exclusively for signal evaluation and quantification. The standard calibration technique with aqueous standards was used for most of the determinations. Calibration using oil-inwater emulsions was also investigated for comparison under the same experimental conditions. The optimum parameters for the graphite furnace temperature program are given in Table 1 for the two methods.

2.2. Reagents, solutions and samples Analytical grade reagents were used exclusively. Distilled, deionized water with a specific resistivity of 18 MΩ cm from a Milli-Q water purification system (Millipore, Bedford, MA, USA) was used for the preparation of standards, modifier solutions and emulsions. All containers and glassware were soaked in 1.4 mol L− 1 nitric acid for at least 24 h and rinsed three times with water before use. Nitric acid (Merck, Germany) was further purified by sub-boiling distillation in a quartz sub-boiling still (Kürner Analysentechnik, Rosenheim, Germany). The stock standard solution of lead (1000 mg L− 1 in 0.014 mol L− 1 nitric acid) was prepared from Titrisol concentrate (Merck). The working standards were prepared by serial dilution of the stock solution with 0.014 mol L− 1 nitric acid. All standards for FF AAS were prepared in 10% ethanol in order to facilitate sample introduction (lower surface Table 1 Graphite furnace temperature program for the determination of lead in crude oil using SS-GF AAS and FF AAS; chemical modifier: 20 µg Pd + 6 µg Mg + 0.05% Triton X-100 for SS-GF AAS and 10 µg Pd + 3 µg Mg for FF AAS. Stagea

2.1. Instrumentation and operation All measurements have been carried out using a Model AAS 5 EA atomic absorption spectrometer (Analytik Jena AG, Jena, Germany), equipped with deuterium background correction and a transversely heated graphite tube atomizer. The spectrometer was interfaced to an IBM PC/AT compatible computer. A NARVA hollow cathode lamp for lead (GLE, Berlin, Germany) has been used as the radiation source with a lamp current of 6 mA. The analytical wavelength at 217.0 nm has been used in all cases with a spectral band pass of 0.5 nm.

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SS-GF AAS

FF AAS

Temperature/°C Ramp/°C Hold/ Temperature/°C Ramp/°C Hold/ s s− 1 s s− 1 Drying 150 Drying 300 Pyrolysis 1100 Atomization 2000 Cleaning 2400

10 5 100 FPc 1000

50 30 40 6 4

100b 110 1000 2000 2300

15 10 100 FPc 1000

10 15 30 6 4

a Purge gas (argon) flow rate: 2 L min− 1 in all steps, except during atomization, when the gas flow was interrupted. b Injection of sample into the filter furnace. c FP = full power.

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tension). The emulsion standards were prepared by pipetting 1.0 mL of base mineral oil (BMOMS, High Purity Standards, Charleston, SC, USA) into 10-mL borosilicate glass volumetric flasks, adding 100 µL of Triton X-100 (Union Carbide), 1.0 mL of xylene (Merck), an appropriate amount of an aqueous standard solution and diluting to volume with water. The composition and mode of preparation of the oil-in-water emulsion has been optimized using a multivariate approach as described previously [28]. The following solutions were used as modifier: a mixture of 500 mg L− 1 Pd + 300 mg L− 1 Mg+ 0.05% Triton X-100 (Pd and Mg as the nitrates from Merck) as conventional modifier added to each sample as described in Section 2.3; iridium stock solution,1000 mg L− 1 Ir in 8% HCl and ruthenium stock solution, 1000 mg L− 1 Ru in 8% HCl (Merck) as permanent modifiers. The crude oil samples analyzed in this work were acquired from refineries of Petrobras; most of them are from the state of Bahia, Brazil. 2.3. Procedure 2.3.1. Direct analysis using SS-GF AAS The less viscous light crude oil samples did not need any sample preparation; however, the most viscous heavy crude oil samples (OB5 and OB7) were heated to 40 °C on a water bath and then mixed vigorously with a glass rod for homogenization; they were allowed to cool before sampling. Aliquots between 0.5 and 3.0 mg of crude oil, depending on the analyte concentration, were weighed directly onto the SS platforms and transferred to the graphite furnace as described in Section 2.1. In case when the Pd + Mg modifier was used, the sample was first weighed onto the SS platform and 20 μL of the modifier solution, corresponding to 10 µg Pd and 6 µg Mg, was added over the sample and submitted to the heating program in Table 1. Each sample was analyzed at least six times. For the measurements with the permanent modifier, the SS platforms were treated by injecting 10 times 40 µL of a 1000 mg L− 1 Ir or Ru standard solution and submitting the platform after each injection to a heating program that has been described previously [29]. 2.3.2. Analysis of oil-in-water emulsions using FF AAS The emulsions were prepared by weighing about 0.5 g of oil sample accurately into 10-mL borosilicate glass volumetric flasks, adding 1.0 mL of xylene and 100 µL of Triton X-100, and completing the volume with water. Three aliquots of each sample were prepared and homogenized in an ultrasonic bath and by manual shaking, as described previously [28]. A volume of 30 µL of sample emulsion was introduced into the FF, corresponding to about 1.5 mg of sample, followed by 10 µL of the Pd + Mg modifier solution, corresponding to 5 µg Pd and 3 µg Mg. The homogeneity of the emulsion was maintained by manual agitation with a Pasteur pipette just before the sample emulsion was taken up by the autosampler capillary. The stability of the analyte in the emulsion was evaluated by measuring the integrated absorbance of lead in one sample (OB3) and one standard (1.0 ng) over a period of 24 h; no change of sensitivity has been observed over this period of time. 2.4. Recovery experiments Recovery experiments with aqueous standards have been carried out using two crude oil samples in order to investigate potential matrix effects. As it is difficult to add and mix the analyte with the sample homogeneously even using an organic standard, in the case of SS-GF AAS the spike recovery tests were carried out injecting 20 μL of the modifier solution onto the platform and drying during 80 s of the temperature program. After cooling, the sample was weighed onto the SS platform and 10 µL of a 50 µg L− 1 Pb aqueous standard was added on top of the sample and the complete temperature program was executed. For recovery tests using FF AAS the sample emulsions were prepared in the same way as described in Section 2.3.2, but with the addition of 10 µL of a 100 µg L− 1 Pb aqueous standard. A spiked

emulsion volume of 30 µL was injected into the FF followed by 10 µL Pd + Mg modifier and submitted to the heating program. 2.5. Sample digestion The digestion of the crude oil samples, that has been carried out in order to validate the proposed procedures, was based on a procedure described by Amorim et al. [30]. A sample mass of about 0.25 g has been weighed into an open Teflon vessel, 2.5 mL of concentrated sulfuric acid were added and heated to 200 ± 10 °C in a digester block for 60 min. Then, 2.5 mL of concentrated nitric acid was added drop by drop to avoid foam formation. After keeping the mixture overnight at room temperature, it was heated again to 200 ± 10 °C for 2 h, followed by the addition of another 1.0 mL of concentrated nitric acid and 2 more hours in the digester block, always taking care that the mixture does not get dry. After cooling, 2.5 mL of 30% (v/v) hydrogen peroxide was added to complete the digestion. The content, approximately 3.0 mL, was left to cool to room temperature, was transferred quantitatively to a volumetric flask and diluted to a final volume of 10 mL with water for the subsequent analysis. All digestions have been carried out in duplicate, and the determinations have been carried out using conventional graphite tubes with PIN platform (Analytik Jena, Part No. 407-A81.025). 3. Results and discussion 3.1. Modifiers and temperature program 3.1.1. Direct analysis using SS-GF AAS Before the pyrolysis stage of the heating program could be investigated, it has been found necessary to optimize the “drying stage”. While this stage is straight-forward when aqueous solutions are used, and it might even be omitted in the analysis of solid samples, it requires special attention in the case of oil samples. In order to achieve a smooth and complete removal of all the volatile components from up to 3 mg of crude oil, it has been necessary to apply two “drying” steps with relatively slow ramp rates and long hold times, as shown in Table 1. It should be mentioned that the temperatures for SS-GF AAS are usually somewhat higher than those for platform atomization due to the greater mass of the SS platform and the resulting slower heat transfer. Among the modifiers that have been proposed for the determination of lead in petroleum products are Pd [3,31], Pd + Mg [3,14,15] and tetrabutylammonium dihydrogenphosphate [15] as conventional chemical modifiers added in solution and, Ir [15] and Ir-methyltrioctylammonium chloride (MTOACl) [3] as permanent modifiers. As the injection of a modifier solution over each sample on the SS platform prior to its introduction into graphite furnace obviously complicates the analytical procedure, and the use of 400 µg of iridium and 400 µg of ruthenium, respectively, as permanent modifiers has been investigated first. However, the sensitivity was lower for the aqueous standard and for crude oil sample and the stabilizing power of the permanent modifiers was also not satisfactory, resulting in analyte losses at relatively low temperatures. It is suspected that nickel and vanadium, which are always present in crude oil at relatively high concentration, cause kind of poisoning of the permanent modifier, as has been observed in an earlier work [23], making it incapable of stabilizing lead. For this reason permanent modifiers have not been further considered in this work. The conventional Pd + Mg modifier has therefore been investigated, and the addition of Triton X-100 (0.05% v/v) was adopted, which was found essential in earlier work [23] for spreading the modifier over the entire platform surface, avoiding analyte losses when the SS technique was used. A preliminary study of the volume of the modifier solution using a pyrolysis temperature of 900 °C was carried out from 10 to 30 µL, i.e., from 5–15 µg Pd and 3–9 µg Mg. The best signal was obtained with 20 µL of modifier solution, containing 10 µg Pd and 6 µg Mg. The modifier has been added in two different

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ways: (i) depositing the modifier solution onto the platform and drying it before the sample has been weighed onto the platform, and (ii) injecting the modifier solution over the sample on the platform. In addition the sample mass has been varied between 0.5 and 3.0 mg using the crude oil sample OB 12 to see if this parameter had any influence. The correlation curves between sample mass and integrated absorbance were Aint = 0.0346 + 0.0481 m (r = 0.9865) for procedure (i) and Aint = 0.0352 + 0.0495 m (r = 0.9785) for procedure (ii). Both modes presented a very similar behavior and a linear response for a sample mass up to 3 mg. Based on this result the modifier has been injected on top of the sample in all further experiments because this procedure is simpler and faster. Pyrolysis curves have been established for an aqueous standard containing 0.5 ng Pb and for the crude oil sample OB 12 with the addition of 20 µL of the Pd + Mg modifier + 0.05% Triton X-100 as shown in Fig. 1. Due to excessively high background absorption measurement of analyte signals for the crude oil sample was only possible when pyrolysis temperatures of 900 °C or higher were used. The maximum pyrolysis temperature that could be used for the oil sample was 1100 °C, whereas a temperature of 1300 °C could be used for the aqueous standard. As mentioned before the temperatures found with the SS-GF AAS technique are usually somewhat higher than those obtained with platform atomization. A pyrolysis temperature of 1100 °C and an atomization temperature of 2000 °C were chosen for the determination of lead in all further experiments with SS-GF AAS. 3.1.2. Emulsion analysis using FF AAS Based on the above experience, only the conventional Pd + Mg modifier has been investigated for the determination of lead in crude oil using FF AAS. The modifier volume and the necessity of using Triton have also been investigated with FF AAS. The absorbance signal for lead in the OB12 emulsion did not change when the modifier volume was increased from 10 µL to 30 µL; the addition of Triton X-100 to the modifier also did not show any effect; hence, 10 µL of modifier solution, corresponding to 5 µg Pd + 3 µg Mg without the addition of Triton was used for emulsion analysis. The pyrolysis curves shown in Fig. 2 for the crude oil sample and the aqueous standard are similar to those obtained with SS-GF AAS, except that the maximum pyrolysis temperatures were slightly lower. The major difference was the about two times higher sensitivity obtained for lead in the aqueous standard and in the crude oil sample. It has been kind of surprising that even in the FF the background signal for the crude oil sample was very high, so that measurements were only possible with pyrolysis temperatures above 800 °C. The most likely explanation is that the emulsion could penetrate into and through the porous graphite of the inner tube,

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Fig. 2. Pyrolysis curves for lead using FF AAS; the data for crude oil sample OB12 are in integrated absorbance normalized for 1.5 mg of sample. Atomization temperature: 2000 °C; chemical modifier: Pd + Mg.

hence impeding its filter function; the penetration of organic solvents into porous graphite is a well-known phenomenon. The best pyrolysis temperature has been 1000 °C for both, aqueous standard and crude oil. A pyrolysis temperature of 1000 °C and an atomization temperature of 2000 °C have therefore been chosen for the determination of lead in all further experiments using FF AAS. 3.2. Atomization signals Peak shapes and background absorption have also been considered when choosing the proper furnace conditions using SS-GF AAS and FF AAS; some typical atomization and background signals are shown in Fig. 3. The similarity of the atomization signals for lead obtained for an aqueous standard and the crude oil sample by SS-GF AAS is obvious, although the signal from the aqueous standard has an earlier appearance time. The background signal is negligible, demonstrating the efficient removal of the crude oil matrix prior to the atomization stage. The signals for lead in the standard and the crude oil emulsion are also similar for FF AAS with the same trend of a later appearance for the crude oil sample. The relatively high and broad background (in comparison to SS-GF AAS) that is observed for the crude oil sample supports the assumption that the crude oil matrix is penetrating into the porous graphite of the inner tube and released only slowly in the atomization stage. The broad, although lower, background for the standard further supports this explanation, as the standard has been made up with base oil, which apparently also penetrates, but causes less background absorption, as it does not contain any asphaltenes or resins. 3.3. Figures of merit

Fig. 1. Pyrolysis curves for lead using SS-GF AAS; the data for crude oil sample OB12 are in integrated absorbance normalized for 1.5 mg of sample. Atomization temperature: 2000 °C; chemical modifier: Pd + Mg + Triton X-100.

The figures of merit obtained for SS-GF AAS and FF AAS are shown in Table 2. Calibration curves were established using a blank and five calibration solutions in the concentration range of 25–150 µg L− 1 Pb (0.25–1.5 ng Pb) for SS-GF AAS, and from 5–50 µg L− 1 Pb (0.05–0.5 ng Pb) for FF AAS. The calibration curves for SS-GF AAS and FF AAS were established using aqueous standards and emulsified aqueous standards. The sensitivity was very similar for aqueous and emulsified standards in each atomizer, as can be seen in Table 2, which suggests that aqueous standards could be used for calibration. The sensitivity, expressed as the slope of the calibration curve, is more than 2.5 times higher for FF AAS, compared to SS-GF AAS. The characteristic mass values obtained for SS-GF AAS are in good agreement with those reported in the literature [5]. The most obvious effect is the significant increase in sensitivity with FF AAS, which is due to the reduced inner diameter of the tube and the resulting higher density of the atom cloud.

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Fig. 3. Absorbance signals for lead using (a, b) SS-GF AAS and (c, d) FF AAS. Temperature program as in Table 1; chemical modifier: Pd + Mg.

The limits of detection (LOD) and quantification (LOQ) are defined as the analyte concentration corresponding to an integrated absorbance signal equal to three times and ten times the standard deviation of the blank, respectively, divided by the slope of the calibration curve. The values have been calculated for the maximum sample mass investigated in each atomizer, i.e., 3 mg for SS-GF AAS and 1.5 mg for FF AAS. For SS-GF AAS the blank measurements were carried out according to the ‘zero mass response’ technique [19], introducing repeatedly a solid sampling platform, containing only the modifier, followed by a regular atomization cycle. The blank solutions of the calibration curve were used with the addition of the modifier to establish LOD and LOQ for FF AAS. The results obtained for SS-GF AAS and FF AAS are essentially identical, which means that the greater sample mass that could be applied in SS-GF AAS has been compensating for the higher sensitivity of FF AAS. Both techniques might actually allow using a higher sample mass or volume of emulsion to further improve LOD and LOQ; however, this

Table 2 Figures of merit for the determination of lead using oil-in-water emulsion and aqueous standards for SS-GF AAS and FF AAS; the Pd + Mg modifier was used for all techniques. Parameter

SS-GF AAS

FF AAS

Linear regression aqueous standard Linear regression emulsion Correlation coefficient aqueous standard Correlation coefficient emulsion Characteristic mass, pg LOD (n = 10) mg kg− 1 LOQ (n = 10) mg kg− 1

Aint = 0.0024 + 0.471 m

Aint = 0.0162 + 1.190 m

Aint = 0.0304 + 0.517 m 0.9988

Aint = 0.0063 + 1.146 m 0.9993

0.9985 8.7 0.004a 0.014a

0.9986 3.5 0.004b 0.014b

a LOD and LOQ are based on the ‘zero mass response’ technique and calculated for 3 mg sample mass. b LOD and LOQ are calculated for 1.5 mg of sample in the emulsion.

option has not been investigated, as the sensitivity was more than adequate for the determination of lead in crude oil samples. The precision for SS-GF AAS expressed as relative standard deviation (RSD) of 6 measurements of each sample varied between 7% and 25%. These relatively high values are probably due to the low concentration of the analyte in the sample that makes the homogeneity play an important hole for the precision. The highest imprecision has been obtained for the most viscous heavy crude oil sample. Considering the complexity of the crude oil matrix and the direct sampling, that avoids errors of manipulation, the RSD found in this work has been considered acceptable. These values are higher than the values reported in direct crude oil analysis for Ni and V [23], elements that are present in much higher concentrations, but similar to values reported for Fe, Cu and Cr [22]. For FF AAS the RSD of 3 measurements of each emulsified sample varied between 1% and 4%, showing that the manual agitation has been efficient to maintain the homogeneity of the emulsion during the sampling stage. Nevertheless, the reproducibility of the preparation of the emulsions, expressed as the RSD of the three aliquots prepared of the same sample, varied between 5% and 20%, values that are similar to those obtained with SS-GF AAS. 3.4. Analytical results The results obtained for the determination of lead in seven crude oil samples using the two techniques are summarized in Table 3. The viscous heavy crude oil samples (OB5 and OB7) were homogenized before analysis as described in Section 2.3.1; the less viscous light crude oil samples did not need any pretreatment. Using a paired Student's t-test there was no significant difference between the results on a 95% confidence level (t = 1.098). Thus, reliable results were obtained with both methods. The concentration of lead in the seven samples was between about 0.05 and 0.3 mg kg− 1. The agreement between the results confirms that aqueous calibration standards can be used

I.C.F. Damin et al. / Spectrochimica Acta Part B 64 (2009) 530–536 Table 3 Analytical results obtained for lead in crude oil samples using SS-GF AAS and FF AAS, and Pd + Mg as the modifier. Sample OB3 OB5 OB7 OB10 OB11 OB12 OBA a

Concentration of Pb/mg kg− 1 (mean ± SD) SS-GF AAS (n = 6)

FF AAS (n = 3)a

0.332 ± 0.048 0.048 ± 0.012 0.272 ± 0.025 0.063 ± 0.007 0.059 ± 0.011 0.076 ± 0.008 0.261 ± 0.022

0.281 ± 0.015 0.048 ± 0.006 0.258 ± 0.025 0.081 ± 0.017 0.073 ± 0.011 0.082 ± 0.018 0.263 ± 0.018

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Acknowledgments The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Projeto Universal — Processo 478271/ 2007-7) for financial support. I.C.F.D., M.B.D., M.G.R.V. and B.W have scholarships from CNPq. T.S.C. has a scholarship from FAPERGS. The authors are also grateful to Analytik Jena AG for the donation of an atomic absorption spectrometer. The authors would like to thank Alessandro Menegatti, Roger Rampazzo and Glaucia S. Costa for their careful experimental work. We also thank Petrobras for providing the crude oil samples.

SD was calculated for the concentration values of three aliquots of each sample.

References for the determination of lead in crude oil samples with the proposed techniques. 3.5. Validation As there is no crude oil or similar reference material available with a certified value for lead, it became necessary to use an independent technique for validation. As sample preparation is the most critical part in crude oil analysis, and as it is well documented that different organic compounds of the same analyte might exhibit different sensitivity, the most rigorous approach, obviously, is a complete mineralization of the crude oil. An open-vessel digestion with sulfuric acid, nitric acid and hydrogen peroxide at 200 °C for about 24 h has been adopted for that purpose (refer to Section 2.5). The results obtained for lead in three crude oil samples, OB3, OB10 and OB11 were 0.339 ± 0.006 mg kg− 1, 0.081 ± 0.017 mg kg− 1 and 0.069 ± 0.014 mg kg− 1 Pb, respectively. These values did not differ significantly from the values obtained by SS-GF AAS and FF AAS, shown in Table 3, according to the Analysis of Variance (ANOVA) test at the 95% confidence interval (F = 0.012 and P = 0.988). This shows that the proposed methods are providing accurate results although they are very simple and use aqueous standards for calibration. Recovery experiments have also been carried out, adding an aqueous lead standard to two samples, OB11 and OB12, as described in Section 2.4; the experiment resulted in recoveries of 91 ± 6 and 92 ± 9% for SS-GF AAS and 97 ± 1 and 102 ± 2% for FF AAS, respectively. These results show that the compound in which lead is present does not have any significant influence on the sensitivity in GF AAS, particularly as in the case of SS-GF AAS the aqueous standard was added over the sample without any mixing. 4. Conclusion The results show that the two techniques investigated in this work provided accurate results for the determination of lead in crude oil using aqueous standards for calibration and a conventional Pd+ Mg modifier. FF AAS provided the better sensitivity, but SS-GF AAS allowed to use a higher sample mass, resulting in the same LOD. FF AAS also provided results with significantly better RSD than SS-GF AAS; however, when the emulsion preparation was included in the consideration, the uncertainty of the method was again very similar, so that both techniques were suitable for the purpose. The higher sensitivity of FF AAS, however, might be of advantage for the analysis of petroleum derivates with significant lower lead content. For the analysis of crude oil SS-GF AAS has the clear advantage of providing the fastest results, as no sample preparation at all is involved, besides the homogenization of the more viscous samples, avoiding the use of organic solvents and other reagents, which might be considered a contribution to the green chemistry. It might be expected that this technique could be extended to the determination of other trace elements in crude oil.

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