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Graphite furnace atomic absorption spectrometric determination of Ni and Pb in diesel and gasoline samples stabilized as microemulsion using conventional and permanent modifiers
Spectrochimica Acta Part B 60 (2005) 615 – 624 www.elsevier.com/locate/sab
Graphite furnace atomic absorption spectrometric determination of Ni and Pb in diesel and gasoline samples stabilized as microemulsion using conventional and permanent modifiersB Mariela N. Matos Reyes, Reinaldo C. CamposT Department of Chemistry, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rua Marqueˆs de Sa˜o Vicente, 225, Rio de Janeiro, RJ 22453-900, Brazil Received 17 September 2004; accepted 2 February 2005 Available online 22 April 2005
Abstract A procedure for the graphite furnace atomic absorption spectrometric determination of Ni and Pb in diesel and gasoline samples was developed. Sample stabilization was necessary because of evident analyte losses that occurred immediately after sampling. Excellent longterm sample stabilization was observed by mixing different organic solvents with propan-1-ol and 50% vol/vol HNO3 at a 3.3:6.5:1 volume ratio. For Pb, efficient thermal stabilization was obtained using aqueous Pd–Mg modifier as well as for Ir as permanent modifier. The drying temperature and ramp rate influenced the sensitivity obtained for Ni, and had to be carefully optimized. Taking this into account, the same sensitivity was attained in all investigated organic media stabilized as microemulsion. Thus, calibration with microemulsions prepared with a single organic solvent was possible, using aqueous or organic stock solutions. Commercial gasoline and diesel samples were directly analyzed after stabilization as microemulsion and by comparative UOP procedures. n-Hexane microemulsions were used for calibration, and good agreement was obtained between the results using the proposed and comparative procedures. Typical coefficients of variation (n = 6) ranged from 1% to 4%, and from 1% to 3% for Ni and Pb, respectively. Detection limits (k = 3) in the original gasoline or diesel samples, derived from 10 blank measurements, were 4.5 and 3.6 Ag l 1 for Ni and Pb, respectively, comfortably below the values found in the analyzed samples. D 2005 Elsevier B.V. All rights reserved. Keywords: GFAAS; Ni and Pb determination; Organic solutions; Petroleum fractions; Microemulsion
1. Introduction Petroleum and its products supply a substantial fraction of the energy requirement of the modern world. They are also used as raw material of a great variety of chemicals and polymers. Petroleum consists predominantly of a mixture of hydrocarbons although organometallic compounds are present as well [1]. Thus metals such as Ag, As, Ba, Ni, Pb, Sn, and V, among others, can be present in petroleum B
This paper was presented at the 8th Rio Symposium on the Atomic Spectrometry, held in Paraty, RJ, Brazil, 1-6 August 2004, and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. T Corresponding author. Fax: +55 21 3114 1309. E-mail address: [email protected] (R.C. Campos). 0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2005.02.020
samples [2,3]. Vanadium and Ni are the most abundant ones, with concentrations around 1000 and 250 Ag g 1, respectively [4,5]. In petroleum products such as gasoline and kerosene obtained from the refining process, metals can have their origin as volatile species formed or released during the distillation process or as contamination during refining or storage. Barium, Ca, and Pb may also be added as catalysts, anticorrosives, or dispersants [6]. It is well known that some metals catalyze oxidative reactions in hydrocarbon mixtures, degrading their thermal stability and impairing their use as fuel. Thus, only low concentrations of metals can be tolerated before the fuel stability degrades to an undesirable extent [7]. Metals in petroleum products may also cause corrosion and catalyst poisoning [1,2] besides their potential environmental impact. Consequently, the accurate determination of trace
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metal concentrations in petroleum products is of fundamental importance for their subsequent use, and different procedures can be found in the literature. Flame atomic absorption spectrometry (FAAS) has been one of the most used techniques, and this preference is related to its selectivity, simplicity of the instrumental procedure, and the low cost of the equipment [8–12]. However, the high organic content of the samples impairs their direct FAAS analysis. This problem might be overcome by dilution with an appropriate organic solvent, although this action deteriorates the limit of detection of the procedure [9]. Alternatively, the sample can be mineralized [13], but these procedures are time-consuming and prone to contamination and analyte loss. Sample emulsification is another alternative [14,15], but in this case, problems with the stability of the emulsion and calibration might arise. Moreover, FAAS usually does not exhibit appropriate sensitivity, and the same concerns are associated to other spectrometric techniques, such as inductively coupled plasma optical emission spectrometry (ICP OES) [16]. Inductively coupled plasma mass spectrometry (ICP-MS) has found increasing application in the analysis of organic solutions, using different sample introduction techniques [17–20]. However, ICP-MS is still considered an expensive technique and its multielemental characteristic is not always required. Graphite furnace atomic absorption spectrometry (GFAAS) offers limits of detection at the order of nanograms per gram or less, at lower expense. Direct sample introduction is in principle possible, permitting faster analysis and reduced risk of contamination or analyte loss. Additionally, considering that the analyte might be present as an unknown organometallic species, an adequate temperature program and a proper modifier could minimize the effect of the chemical nature of the analyte in the GFAAS determination [21–24]. Improved detection limits can be attained by multiple injection procedures [25] or by using a specially designed graphite atomizer that allows the introduction of larger sample volumes [26,27]. Considering the low levels to be determined, the stability of analyte solutions in organic liquids is of special concern, once adsorption losses to the container walls and losses of volatile compounds may occur [28–30]. An efficient stabilization of trace metals in organic solutions could be attained by using microemulsion systems, especially if an acid is initially present in the aqueous phase [28,29,31]. Microemulsions are defined as spherical aggregates of oil (or water) dispersed into water (or oil) and stabilized by an interfacial film of a surfactant (molecules of detergents) [32]. Microemulsions are spontaneously and immediately formed, undefinitively stable, transparent, isotropic, and have low viscosity [33]. Experiments performed with fluorescence spectroscopy have shown that polarity of microemulsions is close to that of the water, following the characteristics of the micelles [32,34]. According to Ruiz et al. [11], the number of carbon atoms of the organic phase molecules is a determining factor for the distribution of the
alcohol (surfactant) between the pseudophases (oil and water) and the interphase. Taking into account the sample stability, the objective of the present study is the development of simple procedures for the determination of Ni and Pb in diesel and gasoline by GFAAS after their stabilization as microemulsions. The influence of the microemulsion composition on the GFAAS response, the use of conventional and permanent modifiers, the adequate calibration procedure, and the proper temperature program for an adequate calibration were investigated.
2. Experimental 2.1. Instrumentation The measurements were carried out with two different instruments: a Perkin Elmer (Norwalk, CT, USA) Model 1100 atomic absorption spectrometer with continuum source (deuterium lamp) background correction, equipped with an HGA-300 graphite furnace (longitudinal heating) and an AS-40 autosampler, was used for the Pb measurements; an Analytik Jena (Jena, Germany) Model Zeenit 60 (transversal heating) atomic absorption spectrometer with Zeeman effect background correction and transversely heated graphite atomizer, equipped with an MPE-52 autosampler, was used for the Ni measurements. Hollow cathode lamps (VWR Scientific, cat no. 58137-452, USA, for Ni; Hamamatsu Photonics KK, cat no. L233-82NQ, Japan, for Pb) were used as a line sources. Pyrolitic graphite-coated tubes (Perkin Elmer B0070699) with totally pyrolitic graphite platforms (Perkin Elmer B0109324) and integrated contact tubes with PIN platforms (Analytik Jena 407-152.023) were used, respectively. The operating parameters and the instrumental settings of both instruments were set according to the respective manufacturers’ recommendations, unless otherwise specified. Sample and modifier injection volumes were always 10 Al. All measurements were based on integrated absorbance and the results are the average of at least three measurements. 2.2. Reagents, solutions, and samples All chemicals were of analytical reagent grade and MilliQ water (Millipore, Bedford, MA, USA) was used throughout. Nitric acid was purified by subboiling distillation (quartz subboiling still; Kuerner Analysentechnik, Rosenheim, Germany). Argon (99.96%; Aga, Rio de Janeiro, Brazil) was used as purge and protective gas. Aqueous Ni and Pb analytical solutions were daily prepared by adequate dilution of 1000 Ag ml 1 solutions prepared from Titrisol concentrates (Merck, Darmstadt, Germany) with 0.2% vol/vol nitric acid. Lead as cyclohexanebutyrate in oil (1000 Ag g 1; Merck, Darmstadt, Germany), as well
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as lead (5000 Ag g 1; Conostan, CONC 0110, Houston, USA) and nickel (1000 Ag g 1; Conostan, CONC 0070, Houston, USA) alkyl aryl sulfonates were used for the preparation of the organometallic salt solutions. The palladium plus magnesium nitrates (Pd–Mg) modifier was prepared from adequate mixing and dilution of palladium (10 g l 1) and magnesium nitrate solutions (both from Merck, Darmstadt, Germany). The Pd and Mg masses deposited onto the platform were 10 and 15 Ag, respectively. For its use as permanent modifier, iridium was deposited onto the platform using a 5000 mg l 1 solution prepared by dissolution of iridium chloride (Fluka Ag, Switzerland) in 10% (vol/vol) nitric acid. The Ir coating procedure was described elsewhere [35]. Propan-1-ol (Vetec, Rio de Janeiro, Brazil) was used for the preparation of the three-component microemulsions. Hydrochloric acid, fuming sulfuric acid, n-hexane, nheptane, n-octane, iso-octane, n-decane, n-hexadecane, acetone, bromoform, and carbon disulfide were all from Vetec. Commercial regular gasoline and diesel samples were obtained from different suppliers at different gas stations of the Rio de Janeiro city. The samples were collected in screw-capped 50 ml polypropylene tubes with conical bottom (Sarstedt, Nqmbrecht, Germany). It is important to mention that Brazilian gasoline contains from 20% v/v to 22% v/v ethanol.
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Fig. 1. Stability of (a) Ni and (b) Pb in gasoline (n) and diesel (E) samples, sampled and stored in 50 ml propylene capped tubes.
reliability of this modification was already confirmed in a previous work [29].
2.3. Comparative procedures Comparative procedures, namely UOP 391-91 [36] for Ni and UOP 848-84 [37] for Pb, were used. In short, they require an open-vessel digestion with an appropriate acid sequence, bclose to drynessQ evaporation, and recovery with diluted HNO3. The whole digestion procedure took about 4 h. The Ni UOP procedure was originally developed for determinations at the micrograms per milliliter concentration level, using FAAS as the final quantification method. Consequently, limits of detection poorer than that demanded in the present study were observed. Therefore the original sample pre-treatment was kept but the final determination was performed by GFAAS. The
3. Results and discussion 3.1. Stability studies To investigate the stability of Ni and Pb in gasoline and diesel samples, the samples were collected in a nearby gas station, brought immediately to the laboratory, and submitted to GFAAS measurements at intervals of at least 20 min. The temperature program used is shown in Table 1. In order to avoid evaporation, the autosampler polyethylene cups were closed tightly in between each the measurements. As Fig. 1 shows, a significant signal drop was observed over
Table 1 Temperature programs for Ni and Pb in the stability studies (no modifier) Step 1 2 3 4 5c 6 7 8 a b c
Nia
Pbb
Temperature (8C)
Ramp (8C/s)
Hold (s)
Ar flow rate
Temperature (8C)
Ramp (s)
Hold (s)
Ar flow rate
80 110 1150 1150 2500 2650
2 2 40 0 2000 1000
5 15 15 6 7 4
Max Max Max Stop Stop Med
90 120 750 20 1750 20 2650 20
1 20 10 1 0 1 1 1
10 10 20 10 5 5 3 5
300 300 300 300 0 300 300 300
Experiments performed with the Zeenit 60 AAS spectrometer. Experiments performed with the P Elmer 1100 AAS spectrometer. Read step.
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Fig. 2. Stability of (a) Ni and (b) Pb in gasoline (n) and diesel (E) samples, sampled and stored in 50 ml propylene capped tubes, stabilized as microemulsions.
time. This drop was attributed to adsorption of the analyte traces on the containers wall, indicating that analyte losses occurred since the very first moment after sampling. Since it took no more than 15 min between sampling and analysis, the process could still be followed in the laboratory. Similar experiments with solutions prepared by direct dilution of the Ni and Pb Conostan standards in different pure organic solvents and mixtures were also performed. They included n-hexane, n-heptane; n-octane; n-decane; n-hexadecane, iso-octane; toluene; n-octane + carbon disulfide (99 + 1, vol/ vol), n-octane + bromoform (99 + 1, vol/vol), and n-octane + acetone (99 + 1, vol/vol). A similar signal drop was observed, in all cases. Sample stability after mixing the diesel and gasoline samples with propan-1-ol and aqueous 50% vol/vol HNO3 was also investigated. The samples were collected in the same type of polypropylene tubes and immediately brought to the laboratory. There, the samples (3.3 ml) were mixed with 6.5 ml of propan-1-ol and 0.1 ml of 50% vol/vol HNO3, spontaneously forming a transparent monophasic long-term stable system (microemulsion). As shown in Fig. 2, excellent stability was then attained. Similar results were also observed for Ni and Pb solutions in the other pure organic solvents and mixtures listed above, even using vessels of different materials demonstrating the wide applicability of the system. 3.2. Investigation of the loss mechanism In order to avoid possible data misinterpretation relative to the actual cause of signal loss with time before
stabilization, several observations that have been made and that support the adsorption hypotheses will be reported here: the kinetics of the signal drop depends on the material of the vessel, which means that the phenomenon depends on the nature of the surface the liquid was in contact with. It was also dependent on the surface/volume ratio of the container: the larger this ratio, using vessels of different radii, the faster was the drop. In an additional experiment, gasoline and diesel samples were collected from a nearby gas station, without any pre-treatment or stabilization, and immediately brought to the laboratory. Twenty-one 50 ml Sarstedt plastic tubes were used. They were divided in seven groups (three tubes per group) and their analysis was performed as follows: aliquots of 10 ml were taken from each tube of each group at different times: group 1, immediately after arriving at the laboratory; group 2, 30 min after arrival; group 3, 60 min; group 4; 90 min; group 5, 120 min; group 6, 24 h; group 7, 48 h after arriving at the laboratory. Immediately after taking the samples, they were submitted to the UOP methods, which lead to a final medium that does not cause any trouble to GFAAS analysis. The same loss pattern as that presented in Fig. 1 was observed, confirming that the loss mechanism is related to a phenomenon external to the equipment. Volatilization losses at room temperature are not a good explanation either. Otherwise, methods such as the UOP that uses much higher temperatures in open vessels would never result in good recoveries. No sedimentation or precipitation was observed either. Thus, adsorption is left as the most probable loss mechanism. 3.3. Influence of the microemulsion composition Different pure organic liquids were used to investigate the influence of the microemulsion composition on the analytical response, and their volumes ranged from 0.9 to 6.9 ml, while propan-1-ol volumes ranged from 8.9 to 2.9 ml. The 50% vol/vol HNO3 solution volume was kept constant at 0.1 ml. Fig. 3 shows the results for Ni as aryl alkyl sulphonate. For n-hexane, no significant change in the analytical signal was observed in the composition range investigated. However, for toluene, a clear signal
Fig. 3. Influence of the microemulsion composition, expressed by the organic liquid/propanol volume ratio on the GFAAS Ni response (Ni as alkyl aryl sulphonate): ( ) n-hexane; ( ) n-octane; ( ) n-decane; (n) isooctane; ( ) toluene; ( ) n-octane + carbon disulfide (99 + 1, vol/vol).
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drop occurred as its proportion in the microemulsion increased. In general, the other organic liquids investigated behaved similar to n-hexane. The organic liquid/propan-1ol ratio initially chosen for the stability studies (0.51) lies in a region where it does not weaken the robustness of the method. Thus, this proportion was kept in the further investigations. 3.4. Pyrolysis and atomization temperature optimization and the use of modifiers The rapid loss of the metal traces in the samples hindered the proper optimization of their pyrolysis and atomization temperatures. However, once stabilization studies had just a comparative nature, the temperature programs for these studies were defined based on the temperatures recommended for aqueous medium (Table 1). The stabilization of the Ni and Pb traces in organic media provided by microemulsion formation allowed a more detailed investigation of the temperature program. Table 2 shows the pyrolysis and atomization temperatures obtained from pyrolysis and atomization curves (not shown) in the presence or absence of Pd–Mg and Ir as conventional and permanent modifiers, respectively. Microemulsions prepared with diesel and gasoline samples, as well as those of pure organic solvents, were investigated. Significant enhancement of the pyrolysis temperature was observed in the presence of modifiers, especially for Ir. The analyte forms investigated had no significant influence on these parameters. Once sufficiently high (z 1000 8C) pyrolysis temperatures were observed for Ni without a modifier, modifiers were used for Pb only in further studies. 3.5. Sensitivity studies Once matrix composition may play an important role in the GFAAS response, analytical curves were established using microemulsions prepared with the different organic media, and compared to those established with aqueous standards (0.2% vol/vol HNO3). The general temperature
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programs used were those shown in Table 1, with the optimized pyrolysis and atomization temperatures shown in Table 2. Sensitivity in aqueous medium was significantly larger (ca. 30%) than in all microemulsion-stabilized organic media investigated. This difference was observed for both elements in the different forms available, even in the presence of the modifiers. However, for Pb, the same sensitivity was observed for the different microemulsionstabilized organic media and analyte compounds investigated. This behavior was initially not obtained for Ni (Fig. 4a). However, changing the drying temperatures from 80 and 110 8C to from 90 and 250 8C, respectively, and slowing the pyrolysis ramp from 40 8C s 1 to 20 8C s 1, the same sensitivity was achieved for Ni in all microemulsion systems (Fig. 4b), similar to Pb. 3.6. Peak profiles For Ni, the same general peak profile was observed for the different matrices and analyte forms studied (Fig. 5A). The appearance times were also similar and t max can be considered the same if a F 10% tolerance is accepted. Concerning background, only the typical line splitting background associated with Zeeman effect background correction is observed (i.e., the different matrices do not generate background themselves). For Pb (Fig. 5B), using Pd–Mg as modifier, the peak profiles are also similar in shape. Appearance time and t max differed always less than 0.3 s. Using Ir as permanent modifier, appearance time for analytical solutions was larger than that for the diesel and gasoline samples. However, t max did not differ by more than 0.2 s. Diesel and gasoline samples showed non-specific absorption, but were well within the range of the background correction system. The analysis of the profiles has not offered any plausible explanation for the sensitivity difference observed on comparing aqueous and nonaqueous solutions. Ni losses during the pyrolysis stage are not likely either once, even beginning at very low values, pyrolysis curves have not revealed any loss. Thus, the cause of this difference is still an open question.
Table 2 Optimized pyrolysis and atomization temperatures in organic media stabilized as microemulsion; values valid for all organic media studied Ni
Pb
Analyte forma
Modifier
Pyrolysis temperature (8C)
Atomization temperature (8C)
Analyte formb
Modifier
Pyrolysis temperature (8C)
Atomization temperature (8C)
Niaq Niaq Niaq Niconostan Niconostan Niconostan
No Pd/Mg (NO3)2 Ir, perm No Pd/Mg (NO3)2 Ir, perm
1000 1200 1600 1100 1300 1700
2500 2500 2600 2500 2500 2600
Pbaq Pbaq Pbaq Pbconostan Pbconostan Pbconostan PbCHB PbCHB PbCHB
No Pd/Mg (NO3)2 Ir, perm No Pd/Mg (NO3)2 Ir, perm No Pd/Mg (NO3)2 Ir, perm
700 900 1000 700 1000 1100 800 1000 1100
1800 1800 1900 1750 1800 2000 1700 1800 1900
a b
Conostan=alkyl aryl sulphonate. CHB=ciclohexanebutyrate.
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Fig. 4. Analytical curves of Ni as alkyl aryl sulphonate stabilized in microemulsion using the original (a) and the modified (b) drying and ramp steps: (5) nhexane; (n) n-heptane; (4) n-octane; (E) n-decane; (w) iso-octane; (x) toluene; (+) n-octane + carbon disulfide (99 + 1, vol/vol); () n-octane + bromoform (99 + 1, vol/vol); (o) n-octane + acetone (99 + 1, vol/vol); (.) water.
3.7. Figures of merit No adequate certified reference material is available for the samples investigated in the present study. Thus, accuracy was assessed by the analysis of commercial gasoline and diesel samples in two different ways: their GFAAS analysis as microemulsions, as a proposed procedure, and their analysis by comparative independent procedures. In the proposed procedure, calibration was performed with analytical curves prepared with microemulsions in pure n-hexane, adding increasing microvolume spikes of 10 mg l 1 working solutions of Ni or Pb alkyl aryl sulphonates. Analyte addition was also investigated and, in this case, adequate microvolumes of the working solutions were added to the samples already prepared as microemulsions. In order to avoid any loss during sampling or transport, the samples were immediately stabilized during sampling by collecting 175 ml in PET flasks already containing 5 ml of 50% (vol/vol) HNO3 and 325 ml of propan-1-ol. The stabilized samples were submitted to the proposed and comparative proce-
dures. Table 3 shows the results. For Pb, no significant difference (ANOVA, p = 0.05) was observed between the results obtained by any of the investigated variants of the proposed procedure, and those derived from the comparative procedure (analytical curve calibration). The same output was found by separately comparing the results of each variant of the proposed method with those of the comparative procedure using the paired t test. A similar result was obtained for the results derived from the analyte addition calibration (not shown). For Ni, the confidence interval of the error ranged from 2% to 6%, acceptable for trace analysis. This difference might be due to losses of volatile Ni species during the UOP procedure, which is performed in open vessels. The presence or formation of volatile Ni species and their loss at temperatures like those used in the UOP method is in accordance to what was observed in Section 3.4 concerning the need for slower pyrolysis ramp in the GFAAS temperature program. The limits of detection were calculated (3 s) from 10 consecutive measurements of the analytical curve blank.
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Fig. 5. Absorption pulses of (A) Ni (2 ng) and (B) Pb (1 ng) as (a) aqueous ion in aqueous (0.2% HNO3) medium; (b) alkyl aryl sulphonate in n-hexane microemulsion; (c) in gasoline microemulsion; (d) in diesel microemulsion. Ni measurements were performed with the transversally heated graphite furnace, no modifier. Pb measurements performed with the longitudinally heated graphite furnace using (1) Pd–Mg modifier and (2) Ir as permanent modifier. Correct, AA (—) and background, BG (222222) signals.
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Table 3 Proposed x comparative (UPO 391-91, Ni; UOP 848-84, Pb) procedures results in the determination of Ni (Ag l 1) and Pb (mg l 1) in commercial diesel (D) and gasoline (G) samples (Ag l 1, n = 6, FS.D.) Samples
D1 D2 D3 D4 D5 D6 D7 D8 G1 G2 G3 G4 G5 G6 G7 G8
Ni
Pb
Proposed
UOP
83 F 1 83 F 1 84 F 1 85 F 1 102 F 3 121 F 2 96 F 2 108 F 3 114 F 4 105 F 1 113 F 1 101 F1 132 F 1 158 F 2 115 F 2 123 F 5
78 F 3 80 F 3 84 F 2 83 F 2 97 F 1 116 F 1 90 F 4 102 F 4 100 F 2 105 F 1 111 F 3 99 F 4 129 F 2 152 F 4 110 F 3 119 F 5
Proposed
UOP
No mod
Pd + Mg
Ir, perm
0.51 F 0.02 0.48 F 0.02 0.43 F 0.02 0.52 F 0.02 0.59 F 0.02 0.020 F 0.002 0.030 F 0.002 0.030 F 0.003 0.60 F 0.02 0.020 F 0.003 0.030 F 0.001 0.42 F 0.03 0.36 F 0.01 0.44 F 0.02 0.030 F 0.003 0.030 F 0.002
0.52 F 0.02 0.52 F 0.02 0.41 F 0.02 0.51 F 0.02 0.64 F 0.02 0.030 F 0.001 0.020 F 0.001 0.030 F 0.001 0.63 F 0.04 0.020 F 0.001 0.030 F 0.003 0.45 F 0.003 0.37 F 0.02 0.46 F 0.02 0.040 F 0.003 0.030 F 0.003
0.54 F 0.02 0.51 F 0.02 0.42 F 0.02 0.54 F 0.02 0.53 F 0.03 0.030 F 0.001 0.030 F 0.002 0.030 F 0.002 0.58 F 0.03 0.030 F 0.003 0.020 F 0.002 0.48 F 0.03 0.31 F 0.02 0.48 F 0.02 0.030 F 0.001 0.020 F 0.001
They were 4.5 and 3.6 Ag l 1 for Ni and Pb, respectively in the original samples.
4. Conclusion Ni and Pb losses occur already during gasoline and diesel sampling, turning immediate sample stabilization mandatory. Adsorption appears to be the most probable loss mechanism. Excellent long-term sample stabilization was obtained by mixing these samples with propan-1-ol and 50% vol/vol HNO3 in an appropriate volume ratio, forming microemulsions. Their immediate and spontaneous formation and long-term physical stability turns the procedure in an almost direct determination, in contrast to time-consuming conventional mineralization procedures. No previous stirring, sonication, or intermittent homogenization is necessary in contrast to systems using surfactants such as Triton X-100. Its applicability to a wide range of pure organic solvents and mixtures justifies similar investigations with other petroleum fractions. The propan-1-ol/organic liquid ratio influenced the analytical response to some extent and must be defined. The conventional aqueous Pd + Mg as well as the Ir permanent modifier showed good performance for Pb in the microemulsion-stabilized media. However, their use was not indispensable once similar Pb results were found without any modifier at low background absorption levels. The drying temperature and ramp rate must be carefully optimized for Ni determination. This may be due to the presence or formation of volatile Ni compounds. The presence of very volatile Ni compounds was also recently reported by Vale et al. in the Ni determination in crude oil samples [38]. Aqueous calibration was not advisable. However, external calibration with microemulsions prepared with a single organic solvent was possible, using
0.49 F 0.02 0.54 F 0.02 0.35 F 0.02 0.46 F 0.01 0.61 F 0.01 bLD bLD bLD 0.59 F 0.04 bLD bLD 0.39 F 0,02 0.38 F 0.02 0.46 F 0.02 bLD bLD
aqueous or organometallic stock solutions. The results found by the proposed procedure were in close agreement with those obtained with comparative UOP procedure in the determination of Ni and Pb in commercial diesel and gasoline samples. Observed Ni and Pb contents in diesel samples ranged from 83 to 121 Ag l 1 and from 20 to 640 Ag l 1, respectively. In gasoline, these ranges were 101–158 Ag l 1 and 20–600 Ag l 1. The overall sample dilution factor associated with the microemulsion preparation is just 1 + 2, permitting detection limits well below the levels found in the investigated samples. The use of larger injection volumes or in situ pre-concentration [25] might improve these figures, if necessary. Alternatively, filter atomizers [27] could also be used. The present work poses concerns on data reported previously for Ni and Pb concentrations in gasoline and diesel at the nanograms per milliter level, for which the question of sample stability was not addressed. However, instrumental developments related to this determination are not necessarily under suspicion since sample stability refers to stages prior to the instrumental determination. It is also intriguing why problems related to the stability of these samples are not commented in the literature more frequently. First, they show up only at the nanograms per milliliter (or ng g 1) range. This range is in contrast to the micrograms per milliter range associated with many of the initial efforts for metal determination in oil derivatives, which aimed at determinations of Pb and other additive metals in fuels and lubricants. At the micrograms per milliter range, these losses are negligible. Moreover, if the samples are left in natural form in their original vessels for a long enough time, adsorption equilibrium between the solution and the container wall will be reached. Then, if samples in this situation are directly taken to the instrument (as frequently occurs in FAAS and ICP OES techniques, for
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instance), no change will be noticed in relation to the time. However, the liquid carried to the equipment is not representative of the original sample anymore. Only if these samples were transferred to another vessel (such as an autosampler cup of a GFAAS instrument) and consequently brought in contact with a new surface would the signal drop be noticed. Furthermore, it is still possible to observe reasonable repetitive results in replicate GFAAS readings if the reading period is not too long, as can be deduced from Fig. 1. In such case, losses may also pass unnoticed. Another point to be considered is that gasoline, diesel, and related samples vary widely in relation to their composition, and only little is known about the metal speciation in these samples. Thus, it is also a point to investigate if these losses occur equally in samples coming from different sources.
References [1] V. Sychra, I. Lang, G. Sebor, Analysis of petroleum and petroleum products by atomic absorption spectroscopy and related techniques, Prog. Anal. At. Spectrosc. (1981) 342 – 402. [2] W.K. Robbins, H.H. Walker, Analysis of petroleum for trace metals— determination of trace quantities of cadmium in petroleum by atomic absorption spectrometry, Anal. Chem. 47 (1975) 1269 – 1275. [3] I. Lang, G. Sebor, V. Sychra, D. Kolihova´, O. Weisser, The determination of metals in petroleum samples by atomic absorption spectrometry, Anal. Chim. Acta 84 (1976) 299 – 305. [4] M. Bettenelli, P. Tittarelli, Evaluation and validation of instrumental procedures for the determination of vanadium in fuel oils, J. Anal. At. Spectrom. 9 (1994) 805 – 812. [5] J.G. Reynolds, Nickel in petroleum refining, Pet. Sci. Technol. 19 (2001) 979 – 1007. [6] E. Cardarelli, M. Cifani, M. Mecozzi, G. Sechi, Analytical application of emulsions: determination of lead in gasoline by atomic absorption spectrophotometry, Talanta 33 (1986) 279 – 280. [7] I.M. Gonc¸alves, M. Murillo, A.M. Gonza´les, Determination of metals in used lubricating oils by AAS using emulsified samples, Talanta 47 (1998) 1033 – 1042. [8] O. Platteau, M. Carrillo, Determination of metallic elements in crude oil–water emulsions by flame AAS, Fuel 5 (1995) 761 – 767. [9] J.R. Baena, M. Gallego, M. Valca´rcel, Speciation of inorganic lead trialkyl-lead compounds by flame atomic absorption spectrometry following continuous selective pre concentration from aqueous solutions, Spectrochim. Acta, Part B 54 (1999) 1869 – 1879. [10] J.M. Guidroz, J. Sneddon, Fate of vanadium determined by nitrous oxide–acetylene flame atomic absorption spectrometry in unburned and burned Venezuelan crude oil, Microchem. J. 73 (2002) 363 – 366. [11] A.I. Ruiz, A. Canais, V. Hernandes, Effect of surfactants in flame atomic absorption spectrometry with pneumatic nebulization: influence of hydrophobic chain length, J. Anal. At. Spectrom. 8 (1993) 109 – 113. [12] J.L. Fabee, M.L. Ruschak, Determination of nickel, vanadium and sulfur in crude and heavy crude fractions by inductively coupled argon plasma, atomic emission spectrometry and flame atomic absorption spectrometry, Anal. Chem. 57 (1985) 1853 – 1863. [13] T. Wondimu, W. Goessler, K.J. Irgolic, Microwave digestion of residual fuel oil for the determination of three elements by inductively coupled plasma-mass spectrometry, J. Anal. Chem. 367 (2000) 35 – 42. [14] A. Salvador, M. de la Guardia, V. Berenger, Determination of the total iron content of used lubricant oils by atomic absorption with use of emulsions, Talanta 30 (1983) 986 – 988.
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[15] S.J. Kumar, S. Gangadharan, Determination of trace elements in naphtha by inductively coupled plasma mass spectrometry using water-in-oil emulsions, J. Anal. At. Spectrom. 14 (1999) 967 – 971. [16] R.J. Brown, Determination of trace metals in petroleum and petroleum products using an inductively coupled plasma optical emission spectrometer, Spectrochim. Acta, Part B 38 (1983) 283 – 289. [17] T.D. Saint Pierre, L.F. Dias, S.M. Maia, A.J. Curtius, Determination of Cd, Cu, Fe, Pb in gasoline as emulsion by electrothermal vaporization ICP-MS with analyte addition and isotope dilution calibration techniques, Spectrochim. Acta, Part B 59 (2004) 551 – 558. [18] C. Duyck, N. Miekeley, C.L.P. da Silveira, P. Szatmari, Trace element determination in crude oil and its fractions by inductively coupled plasma mass spectrometry using ultrasonic nebulization of toluene solutions, Spectrochim. Acta, Part B 57 (2002) 1979 – 1990. [19] K. Kahen, A. Strubinger, J.R. Chirinos, A. Montaser, Direct injection high efficiency nebulizer-inductively coupled plasma mass spectrometry for analysis of petroleum samples, Spectrochim. Acta, Part B 58 (2003) 397 – 413. [20] N.H. Bings, Direct determination of metals in lubricating oil by laser ablation coupled to inductively plasma time of flight mass spectrometry, J. Anal. At. Spectrom. 17 (2002) 759 – 767. [21] Z. Kowalewska, E. Bulska, A. Hulanicki, Organic palladium and palladium–magnesium chemical modifiers in direct determination of lead in fractions from distillation of crude oil by electrothermal atomic absorption analysis, Spectrochim. Acta, Part B 54 (1999) 835 – 843. [22] R.Q. Aucelio, A. Curtius, Evaluation of electrothermal atomic absorption spectrometry for trace determination of Sb, As and Se in gasoline and kerosene using microemulsion sample introduction for chemical modification, J. Anal. At. Spectrom. 17 (2002) 242 – 247. [23] N.N. Meeravali, S.J. Kumar, The utility of a W–Ir permanent modifier for the determination of Ni and V in emulsified fuel oil and naphta by transverse heated electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 16 (2001) 527 – 532. [24] N.S. Thomaidis, E.A. Piperaky, Comparison of chemical modifiers for the determination of vanadium in water and oil samples by ETAAS, Analyst 121 (1996) 111 – 117. [25] M.V. Rebouc¸as, S.L.C. Ferreira, B.D.B. Neto, Arsenic determination in naphta by electrothermal atomic absorption spectrometry after preconcentration using multiple injections, J. Anal. At. Spectrom. 18 (2003) 1267 – 1273. [26] A. Anselmi, P. Tittarelli, D.M. Katskov, Determination of trace elements in automotive fuels by filter furnace atomic absorption spectrometry, Spectrochim. Acta, Part B 57 (2002) 403 – 411. [27] C.M. Mbileni, P. Ngobeni, D. Katskov, N. Panichev, Determination of lead and cadmium in organic solutions by electrothermal atomic absorption spectrometry with a transverse heated filter atomizer, J. Anal. At. Spectrom. 17 (2002) 236 – 241. [28] I. Silva, R.C. Campos, A.J. Curtius, S.M. Sella, Determination of lead and copper in kerosene by electrothermal atomic absorption spectrometry: stabilization of metals in organic media by a threecomponent solution, J. Anal. At. Spectrom. 8 (1993) 749 – 754. [29] R.C. Campos, H.R. dos Santos, P. Grinberg, Determination of copper, iron, lead and nickel in gasoline by electrothermal atomic absorption spectrometry using three component solutions, Spectrochim. Acta, Part B 57 (2002) 15 – 28. [30] R.J. Cassela, B.A.R.S. Barbosa, R.E. Satelli, A.T. Rangel, Direct determination of arsenic and antimony in naphta by electrothermal atomic absorption spectrometry with microemulsion sample introduction and iridium permanent modifier, Anal. Bioanal. Chem. 379 (2004) 66 – 71. [31] R.Q. Auce´lio, A. Doyle, B.S. Pizzorno, M.L. Trista˜o, R.C. Campos, Electrothermal atomic absorption spectrometric method for the determination of vanadium in diesel and asphaltene prepared as detergentless microemulsions, Microchem. J. 78 (2004) 21 – 26. [32] A. Sanz Medel, M. Fernandez de la Campa, E. Blanco Gonzalez, M. Fernandez-Sanchez, Organic surfactant assemblies in analytical atomic spectrometry, Spectrochim. Acta, Part B 54 (1999) 251 – 287.
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[33] B.A. Kelser, D. Varle, R.E. Barden, S.L. Halt, Detergentless water/oil microemulsions composed of hexane and 2-propanol. Nuclear magnetic resonance studies, effect of added NaCl, J. Phys. Chem. 83 (1979) 1276 – 1280. [34] T. Pitaksuteepong, N.M. Davies, I.G. Tucker, T. Rodes, Factors influencing the entrapment of hydrophilic compounds in nanocapsules prepared by polymerization of water in oil microemulsions, Eur. J. Pharmacol. Biopharm. 53 (2002) 335 – 342. [35] P. Grinberg, R.C. Campos, Iridium as permanent modifier in the determination of lead in whole blood and urine by electrothermal atomic absorption spectrometry, Spectrochim. Acta, Part B 56 (2001) 1831 – 1843.
[36] UOP Method 391-91, Trace Metals in Petroleum Products, AAS, 1991. [37] UOP Method 848-84, Nickel, Vanadium, Iron, Lead, Copper, and Sodium in Gas Oils, AAS, 1984. [38] M.G.R. Vale, B. Welz, M.M. Silva, F.G. Lepri, I.C.F. Damin, W.N.L. dos Santos, U. Heitmann, Stabilization of volatile nickel and vanadium compounds in crude oil using palladium modifier and high resolution continuum source graphite furnace atomic absorption spectrometry, VIII Rio Symposium on Atomic Spectrometry, Paraty, Rio de Janeiro Brazil, August, 1–6, Book of Abstracts, 2004, p. 21.
Graphite furnace atomic absorption spectrometric determination of Ni and Pb in diesel and gasoline samples stabilized as microemulsion using conventional and permanent modifiersB Mariela N. Matos Reyes, Reinaldo C. CamposT Department of Chemistry, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rua Marqueˆs de Sa˜o Vicente, 225, Rio de Janeiro, RJ 22453-900, Brazil Received 17 September 2004; accepted 2 February 2005 Available online 22 April 2005
Abstract A procedure for the graphite furnace atomic absorption spectrometric determination of Ni and Pb in diesel and gasoline samples was developed. Sample stabilization was necessary because of evident analyte losses that occurred immediately after sampling. Excellent longterm sample stabilization was observed by mixing different organic solvents with propan-1-ol and 50% vol/vol HNO3 at a 3.3:6.5:1 volume ratio. For Pb, efficient thermal stabilization was obtained using aqueous Pd–Mg modifier as well as for Ir as permanent modifier. The drying temperature and ramp rate influenced the sensitivity obtained for Ni, and had to be carefully optimized. Taking this into account, the same sensitivity was attained in all investigated organic media stabilized as microemulsion. Thus, calibration with microemulsions prepared with a single organic solvent was possible, using aqueous or organic stock solutions. Commercial gasoline and diesel samples were directly analyzed after stabilization as microemulsion and by comparative UOP procedures. n-Hexane microemulsions were used for calibration, and good agreement was obtained between the results using the proposed and comparative procedures. Typical coefficients of variation (n = 6) ranged from 1% to 4%, and from 1% to 3% for Ni and Pb, respectively. Detection limits (k = 3) in the original gasoline or diesel samples, derived from 10 blank measurements, were 4.5 and 3.6 Ag l 1 for Ni and Pb, respectively, comfortably below the values found in the analyzed samples. D 2005 Elsevier B.V. All rights reserved. Keywords: GFAAS; Ni and Pb determination; Organic solutions; Petroleum fractions; Microemulsion
1. Introduction Petroleum and its products supply a substantial fraction of the energy requirement of the modern world. They are also used as raw material of a great variety of chemicals and polymers. Petroleum consists predominantly of a mixture of hydrocarbons although organometallic compounds are present as well [1]. Thus metals such as Ag, As, Ba, Ni, Pb, Sn, and V, among others, can be present in petroleum B
This paper was presented at the 8th Rio Symposium on the Atomic Spectrometry, held in Paraty, RJ, Brazil, 1-6 August 2004, and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. T Corresponding author. Fax: +55 21 3114 1309. E-mail address: [email protected] (R.C. Campos). 0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2005.02.020
samples [2,3]. Vanadium and Ni are the most abundant ones, with concentrations around 1000 and 250 Ag g 1, respectively [4,5]. In petroleum products such as gasoline and kerosene obtained from the refining process, metals can have their origin as volatile species formed or released during the distillation process or as contamination during refining or storage. Barium, Ca, and Pb may also be added as catalysts, anticorrosives, or dispersants [6]. It is well known that some metals catalyze oxidative reactions in hydrocarbon mixtures, degrading their thermal stability and impairing their use as fuel. Thus, only low concentrations of metals can be tolerated before the fuel stability degrades to an undesirable extent [7]. Metals in petroleum products may also cause corrosion and catalyst poisoning [1,2] besides their potential environmental impact. Consequently, the accurate determination of trace
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metal concentrations in petroleum products is of fundamental importance for their subsequent use, and different procedures can be found in the literature. Flame atomic absorption spectrometry (FAAS) has been one of the most used techniques, and this preference is related to its selectivity, simplicity of the instrumental procedure, and the low cost of the equipment [8–12]. However, the high organic content of the samples impairs their direct FAAS analysis. This problem might be overcome by dilution with an appropriate organic solvent, although this action deteriorates the limit of detection of the procedure [9]. Alternatively, the sample can be mineralized [13], but these procedures are time-consuming and prone to contamination and analyte loss. Sample emulsification is another alternative [14,15], but in this case, problems with the stability of the emulsion and calibration might arise. Moreover, FAAS usually does not exhibit appropriate sensitivity, and the same concerns are associated to other spectrometric techniques, such as inductively coupled plasma optical emission spectrometry (ICP OES) [16]. Inductively coupled plasma mass spectrometry (ICP-MS) has found increasing application in the analysis of organic solutions, using different sample introduction techniques [17–20]. However, ICP-MS is still considered an expensive technique and its multielemental characteristic is not always required. Graphite furnace atomic absorption spectrometry (GFAAS) offers limits of detection at the order of nanograms per gram or less, at lower expense. Direct sample introduction is in principle possible, permitting faster analysis and reduced risk of contamination or analyte loss. Additionally, considering that the analyte might be present as an unknown organometallic species, an adequate temperature program and a proper modifier could minimize the effect of the chemical nature of the analyte in the GFAAS determination [21–24]. Improved detection limits can be attained by multiple injection procedures [25] or by using a specially designed graphite atomizer that allows the introduction of larger sample volumes [26,27]. Considering the low levels to be determined, the stability of analyte solutions in organic liquids is of special concern, once adsorption losses to the container walls and losses of volatile compounds may occur [28–30]. An efficient stabilization of trace metals in organic solutions could be attained by using microemulsion systems, especially if an acid is initially present in the aqueous phase [28,29,31]. Microemulsions are defined as spherical aggregates of oil (or water) dispersed into water (or oil) and stabilized by an interfacial film of a surfactant (molecules of detergents) [32]. Microemulsions are spontaneously and immediately formed, undefinitively stable, transparent, isotropic, and have low viscosity [33]. Experiments performed with fluorescence spectroscopy have shown that polarity of microemulsions is close to that of the water, following the characteristics of the micelles [32,34]. According to Ruiz et al. [11], the number of carbon atoms of the organic phase molecules is a determining factor for the distribution of the
alcohol (surfactant) between the pseudophases (oil and water) and the interphase. Taking into account the sample stability, the objective of the present study is the development of simple procedures for the determination of Ni and Pb in diesel and gasoline by GFAAS after their stabilization as microemulsions. The influence of the microemulsion composition on the GFAAS response, the use of conventional and permanent modifiers, the adequate calibration procedure, and the proper temperature program for an adequate calibration were investigated.
2. Experimental 2.1. Instrumentation The measurements were carried out with two different instruments: a Perkin Elmer (Norwalk, CT, USA) Model 1100 atomic absorption spectrometer with continuum source (deuterium lamp) background correction, equipped with an HGA-300 graphite furnace (longitudinal heating) and an AS-40 autosampler, was used for the Pb measurements; an Analytik Jena (Jena, Germany) Model Zeenit 60 (transversal heating) atomic absorption spectrometer with Zeeman effect background correction and transversely heated graphite atomizer, equipped with an MPE-52 autosampler, was used for the Ni measurements. Hollow cathode lamps (VWR Scientific, cat no. 58137-452, USA, for Ni; Hamamatsu Photonics KK, cat no. L233-82NQ, Japan, for Pb) were used as a line sources. Pyrolitic graphite-coated tubes (Perkin Elmer B0070699) with totally pyrolitic graphite platforms (Perkin Elmer B0109324) and integrated contact tubes with PIN platforms (Analytik Jena 407-152.023) were used, respectively. The operating parameters and the instrumental settings of both instruments were set according to the respective manufacturers’ recommendations, unless otherwise specified. Sample and modifier injection volumes were always 10 Al. All measurements were based on integrated absorbance and the results are the average of at least three measurements. 2.2. Reagents, solutions, and samples All chemicals were of analytical reagent grade and MilliQ water (Millipore, Bedford, MA, USA) was used throughout. Nitric acid was purified by subboiling distillation (quartz subboiling still; Kuerner Analysentechnik, Rosenheim, Germany). Argon (99.96%; Aga, Rio de Janeiro, Brazil) was used as purge and protective gas. Aqueous Ni and Pb analytical solutions were daily prepared by adequate dilution of 1000 Ag ml 1 solutions prepared from Titrisol concentrates (Merck, Darmstadt, Germany) with 0.2% vol/vol nitric acid. Lead as cyclohexanebutyrate in oil (1000 Ag g 1; Merck, Darmstadt, Germany), as well
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as lead (5000 Ag g 1; Conostan, CONC 0110, Houston, USA) and nickel (1000 Ag g 1; Conostan, CONC 0070, Houston, USA) alkyl aryl sulfonates were used for the preparation of the organometallic salt solutions. The palladium plus magnesium nitrates (Pd–Mg) modifier was prepared from adequate mixing and dilution of palladium (10 g l 1) and magnesium nitrate solutions (both from Merck, Darmstadt, Germany). The Pd and Mg masses deposited onto the platform were 10 and 15 Ag, respectively. For its use as permanent modifier, iridium was deposited onto the platform using a 5000 mg l 1 solution prepared by dissolution of iridium chloride (Fluka Ag, Switzerland) in 10% (vol/vol) nitric acid. The Ir coating procedure was described elsewhere [35]. Propan-1-ol (Vetec, Rio de Janeiro, Brazil) was used for the preparation of the three-component microemulsions. Hydrochloric acid, fuming sulfuric acid, n-hexane, nheptane, n-octane, iso-octane, n-decane, n-hexadecane, acetone, bromoform, and carbon disulfide were all from Vetec. Commercial regular gasoline and diesel samples were obtained from different suppliers at different gas stations of the Rio de Janeiro city. The samples were collected in screw-capped 50 ml polypropylene tubes with conical bottom (Sarstedt, Nqmbrecht, Germany). It is important to mention that Brazilian gasoline contains from 20% v/v to 22% v/v ethanol.
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Fig. 1. Stability of (a) Ni and (b) Pb in gasoline (n) and diesel (E) samples, sampled and stored in 50 ml propylene capped tubes.
reliability of this modification was already confirmed in a previous work [29].
2.3. Comparative procedures Comparative procedures, namely UOP 391-91 [36] for Ni and UOP 848-84 [37] for Pb, were used. In short, they require an open-vessel digestion with an appropriate acid sequence, bclose to drynessQ evaporation, and recovery with diluted HNO3. The whole digestion procedure took about 4 h. The Ni UOP procedure was originally developed for determinations at the micrograms per milliliter concentration level, using FAAS as the final quantification method. Consequently, limits of detection poorer than that demanded in the present study were observed. Therefore the original sample pre-treatment was kept but the final determination was performed by GFAAS. The
3. Results and discussion 3.1. Stability studies To investigate the stability of Ni and Pb in gasoline and diesel samples, the samples were collected in a nearby gas station, brought immediately to the laboratory, and submitted to GFAAS measurements at intervals of at least 20 min. The temperature program used is shown in Table 1. In order to avoid evaporation, the autosampler polyethylene cups were closed tightly in between each the measurements. As Fig. 1 shows, a significant signal drop was observed over
Table 1 Temperature programs for Ni and Pb in the stability studies (no modifier) Step 1 2 3 4 5c 6 7 8 a b c
Nia
Pbb
Temperature (8C)
Ramp (8C/s)
Hold (s)
Ar flow rate
Temperature (8C)
Ramp (s)
Hold (s)
Ar flow rate
80 110 1150 1150 2500 2650
2 2 40 0 2000 1000
5 15 15 6 7 4
Max Max Max Stop Stop Med
90 120 750 20 1750 20 2650 20
1 20 10 1 0 1 1 1
10 10 20 10 5 5 3 5
300 300 300 300 0 300 300 300
Experiments performed with the Zeenit 60 AAS spectrometer. Experiments performed with the P Elmer 1100 AAS spectrometer. Read step.
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Fig. 2. Stability of (a) Ni and (b) Pb in gasoline (n) and diesel (E) samples, sampled and stored in 50 ml propylene capped tubes, stabilized as microemulsions.
time. This drop was attributed to adsorption of the analyte traces on the containers wall, indicating that analyte losses occurred since the very first moment after sampling. Since it took no more than 15 min between sampling and analysis, the process could still be followed in the laboratory. Similar experiments with solutions prepared by direct dilution of the Ni and Pb Conostan standards in different pure organic solvents and mixtures were also performed. They included n-hexane, n-heptane; n-octane; n-decane; n-hexadecane, iso-octane; toluene; n-octane + carbon disulfide (99 + 1, vol/ vol), n-octane + bromoform (99 + 1, vol/vol), and n-octane + acetone (99 + 1, vol/vol). A similar signal drop was observed, in all cases. Sample stability after mixing the diesel and gasoline samples with propan-1-ol and aqueous 50% vol/vol HNO3 was also investigated. The samples were collected in the same type of polypropylene tubes and immediately brought to the laboratory. There, the samples (3.3 ml) were mixed with 6.5 ml of propan-1-ol and 0.1 ml of 50% vol/vol HNO3, spontaneously forming a transparent monophasic long-term stable system (microemulsion). As shown in Fig. 2, excellent stability was then attained. Similar results were also observed for Ni and Pb solutions in the other pure organic solvents and mixtures listed above, even using vessels of different materials demonstrating the wide applicability of the system. 3.2. Investigation of the loss mechanism In order to avoid possible data misinterpretation relative to the actual cause of signal loss with time before
stabilization, several observations that have been made and that support the adsorption hypotheses will be reported here: the kinetics of the signal drop depends on the material of the vessel, which means that the phenomenon depends on the nature of the surface the liquid was in contact with. It was also dependent on the surface/volume ratio of the container: the larger this ratio, using vessels of different radii, the faster was the drop. In an additional experiment, gasoline and diesel samples were collected from a nearby gas station, without any pre-treatment or stabilization, and immediately brought to the laboratory. Twenty-one 50 ml Sarstedt plastic tubes were used. They were divided in seven groups (three tubes per group) and their analysis was performed as follows: aliquots of 10 ml were taken from each tube of each group at different times: group 1, immediately after arriving at the laboratory; group 2, 30 min after arrival; group 3, 60 min; group 4; 90 min; group 5, 120 min; group 6, 24 h; group 7, 48 h after arriving at the laboratory. Immediately after taking the samples, they were submitted to the UOP methods, which lead to a final medium that does not cause any trouble to GFAAS analysis. The same loss pattern as that presented in Fig. 1 was observed, confirming that the loss mechanism is related to a phenomenon external to the equipment. Volatilization losses at room temperature are not a good explanation either. Otherwise, methods such as the UOP that uses much higher temperatures in open vessels would never result in good recoveries. No sedimentation or precipitation was observed either. Thus, adsorption is left as the most probable loss mechanism. 3.3. Influence of the microemulsion composition Different pure organic liquids were used to investigate the influence of the microemulsion composition on the analytical response, and their volumes ranged from 0.9 to 6.9 ml, while propan-1-ol volumes ranged from 8.9 to 2.9 ml. The 50% vol/vol HNO3 solution volume was kept constant at 0.1 ml. Fig. 3 shows the results for Ni as aryl alkyl sulphonate. For n-hexane, no significant change in the analytical signal was observed in the composition range investigated. However, for toluene, a clear signal
Fig. 3. Influence of the microemulsion composition, expressed by the organic liquid/propanol volume ratio on the GFAAS Ni response (Ni as alkyl aryl sulphonate): ( ) n-hexane; ( ) n-octane; ( ) n-decane; (n) isooctane; ( ) toluene; ( ) n-octane + carbon disulfide (99 + 1, vol/vol).
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drop occurred as its proportion in the microemulsion increased. In general, the other organic liquids investigated behaved similar to n-hexane. The organic liquid/propan-1ol ratio initially chosen for the stability studies (0.51) lies in a region where it does not weaken the robustness of the method. Thus, this proportion was kept in the further investigations. 3.4. Pyrolysis and atomization temperature optimization and the use of modifiers The rapid loss of the metal traces in the samples hindered the proper optimization of their pyrolysis and atomization temperatures. However, once stabilization studies had just a comparative nature, the temperature programs for these studies were defined based on the temperatures recommended for aqueous medium (Table 1). The stabilization of the Ni and Pb traces in organic media provided by microemulsion formation allowed a more detailed investigation of the temperature program. Table 2 shows the pyrolysis and atomization temperatures obtained from pyrolysis and atomization curves (not shown) in the presence or absence of Pd–Mg and Ir as conventional and permanent modifiers, respectively. Microemulsions prepared with diesel and gasoline samples, as well as those of pure organic solvents, were investigated. Significant enhancement of the pyrolysis temperature was observed in the presence of modifiers, especially for Ir. The analyte forms investigated had no significant influence on these parameters. Once sufficiently high (z 1000 8C) pyrolysis temperatures were observed for Ni without a modifier, modifiers were used for Pb only in further studies. 3.5. Sensitivity studies Once matrix composition may play an important role in the GFAAS response, analytical curves were established using microemulsions prepared with the different organic media, and compared to those established with aqueous standards (0.2% vol/vol HNO3). The general temperature
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programs used were those shown in Table 1, with the optimized pyrolysis and atomization temperatures shown in Table 2. Sensitivity in aqueous medium was significantly larger (ca. 30%) than in all microemulsion-stabilized organic media investigated. This difference was observed for both elements in the different forms available, even in the presence of the modifiers. However, for Pb, the same sensitivity was observed for the different microemulsionstabilized organic media and analyte compounds investigated. This behavior was initially not obtained for Ni (Fig. 4a). However, changing the drying temperatures from 80 and 110 8C to from 90 and 250 8C, respectively, and slowing the pyrolysis ramp from 40 8C s 1 to 20 8C s 1, the same sensitivity was achieved for Ni in all microemulsion systems (Fig. 4b), similar to Pb. 3.6. Peak profiles For Ni, the same general peak profile was observed for the different matrices and analyte forms studied (Fig. 5A). The appearance times were also similar and t max can be considered the same if a F 10% tolerance is accepted. Concerning background, only the typical line splitting background associated with Zeeman effect background correction is observed (i.e., the different matrices do not generate background themselves). For Pb (Fig. 5B), using Pd–Mg as modifier, the peak profiles are also similar in shape. Appearance time and t max differed always less than 0.3 s. Using Ir as permanent modifier, appearance time for analytical solutions was larger than that for the diesel and gasoline samples. However, t max did not differ by more than 0.2 s. Diesel and gasoline samples showed non-specific absorption, but were well within the range of the background correction system. The analysis of the profiles has not offered any plausible explanation for the sensitivity difference observed on comparing aqueous and nonaqueous solutions. Ni losses during the pyrolysis stage are not likely either once, even beginning at very low values, pyrolysis curves have not revealed any loss. Thus, the cause of this difference is still an open question.
Table 2 Optimized pyrolysis and atomization temperatures in organic media stabilized as microemulsion; values valid for all organic media studied Ni
Pb
Analyte forma
Modifier
Pyrolysis temperature (8C)
Atomization temperature (8C)
Analyte formb
Modifier
Pyrolysis temperature (8C)
Atomization temperature (8C)
Niaq Niaq Niaq Niconostan Niconostan Niconostan
No Pd/Mg (NO3)2 Ir, perm No Pd/Mg (NO3)2 Ir, perm
1000 1200 1600 1100 1300 1700
2500 2500 2600 2500 2500 2600
Pbaq Pbaq Pbaq Pbconostan Pbconostan Pbconostan PbCHB PbCHB PbCHB
No Pd/Mg (NO3)2 Ir, perm No Pd/Mg (NO3)2 Ir, perm No Pd/Mg (NO3)2 Ir, perm
700 900 1000 700 1000 1100 800 1000 1100
1800 1800 1900 1750 1800 2000 1700 1800 1900
a b
Conostan=alkyl aryl sulphonate. CHB=ciclohexanebutyrate.
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Fig. 4. Analytical curves of Ni as alkyl aryl sulphonate stabilized in microemulsion using the original (a) and the modified (b) drying and ramp steps: (5) nhexane; (n) n-heptane; (4) n-octane; (E) n-decane; (w) iso-octane; (x) toluene; (+) n-octane + carbon disulfide (99 + 1, vol/vol); () n-octane + bromoform (99 + 1, vol/vol); (o) n-octane + acetone (99 + 1, vol/vol); (.) water.
3.7. Figures of merit No adequate certified reference material is available for the samples investigated in the present study. Thus, accuracy was assessed by the analysis of commercial gasoline and diesel samples in two different ways: their GFAAS analysis as microemulsions, as a proposed procedure, and their analysis by comparative independent procedures. In the proposed procedure, calibration was performed with analytical curves prepared with microemulsions in pure n-hexane, adding increasing microvolume spikes of 10 mg l 1 working solutions of Ni or Pb alkyl aryl sulphonates. Analyte addition was also investigated and, in this case, adequate microvolumes of the working solutions were added to the samples already prepared as microemulsions. In order to avoid any loss during sampling or transport, the samples were immediately stabilized during sampling by collecting 175 ml in PET flasks already containing 5 ml of 50% (vol/vol) HNO3 and 325 ml of propan-1-ol. The stabilized samples were submitted to the proposed and comparative proce-
dures. Table 3 shows the results. For Pb, no significant difference (ANOVA, p = 0.05) was observed between the results obtained by any of the investigated variants of the proposed procedure, and those derived from the comparative procedure (analytical curve calibration). The same output was found by separately comparing the results of each variant of the proposed method with those of the comparative procedure using the paired t test. A similar result was obtained for the results derived from the analyte addition calibration (not shown). For Ni, the confidence interval of the error ranged from 2% to 6%, acceptable for trace analysis. This difference might be due to losses of volatile Ni species during the UOP procedure, which is performed in open vessels. The presence or formation of volatile Ni species and their loss at temperatures like those used in the UOP method is in accordance to what was observed in Section 3.4 concerning the need for slower pyrolysis ramp in the GFAAS temperature program. The limits of detection were calculated (3 s) from 10 consecutive measurements of the analytical curve blank.
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Fig. 5. Absorption pulses of (A) Ni (2 ng) and (B) Pb (1 ng) as (a) aqueous ion in aqueous (0.2% HNO3) medium; (b) alkyl aryl sulphonate in n-hexane microemulsion; (c) in gasoline microemulsion; (d) in diesel microemulsion. Ni measurements were performed with the transversally heated graphite furnace, no modifier. Pb measurements performed with the longitudinally heated graphite furnace using (1) Pd–Mg modifier and (2) Ir as permanent modifier. Correct, AA (—) and background, BG (222222) signals.
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Table 3 Proposed x comparative (UPO 391-91, Ni; UOP 848-84, Pb) procedures results in the determination of Ni (Ag l 1) and Pb (mg l 1) in commercial diesel (D) and gasoline (G) samples (Ag l 1, n = 6, FS.D.) Samples
D1 D2 D3 D4 D5 D6 D7 D8 G1 G2 G3 G4 G5 G6 G7 G8
Ni
Pb
Proposed
UOP
83 F 1 83 F 1 84 F 1 85 F 1 102 F 3 121 F 2 96 F 2 108 F 3 114 F 4 105 F 1 113 F 1 101 F1 132 F 1 158 F 2 115 F 2 123 F 5
78 F 3 80 F 3 84 F 2 83 F 2 97 F 1 116 F 1 90 F 4 102 F 4 100 F 2 105 F 1 111 F 3 99 F 4 129 F 2 152 F 4 110 F 3 119 F 5
Proposed
UOP
No mod
Pd + Mg
Ir, perm
0.51 F 0.02 0.48 F 0.02 0.43 F 0.02 0.52 F 0.02 0.59 F 0.02 0.020 F 0.002 0.030 F 0.002 0.030 F 0.003 0.60 F 0.02 0.020 F 0.003 0.030 F 0.001 0.42 F 0.03 0.36 F 0.01 0.44 F 0.02 0.030 F 0.003 0.030 F 0.002
0.52 F 0.02 0.52 F 0.02 0.41 F 0.02 0.51 F 0.02 0.64 F 0.02 0.030 F 0.001 0.020 F 0.001 0.030 F 0.001 0.63 F 0.04 0.020 F 0.001 0.030 F 0.003 0.45 F 0.003 0.37 F 0.02 0.46 F 0.02 0.040 F 0.003 0.030 F 0.003
0.54 F 0.02 0.51 F 0.02 0.42 F 0.02 0.54 F 0.02 0.53 F 0.03 0.030 F 0.001 0.030 F 0.002 0.030 F 0.002 0.58 F 0.03 0.030 F 0.003 0.020 F 0.002 0.48 F 0.03 0.31 F 0.02 0.48 F 0.02 0.030 F 0.001 0.020 F 0.001
They were 4.5 and 3.6 Ag l 1 for Ni and Pb, respectively in the original samples.
4. Conclusion Ni and Pb losses occur already during gasoline and diesel sampling, turning immediate sample stabilization mandatory. Adsorption appears to be the most probable loss mechanism. Excellent long-term sample stabilization was obtained by mixing these samples with propan-1-ol and 50% vol/vol HNO3 in an appropriate volume ratio, forming microemulsions. Their immediate and spontaneous formation and long-term physical stability turns the procedure in an almost direct determination, in contrast to time-consuming conventional mineralization procedures. No previous stirring, sonication, or intermittent homogenization is necessary in contrast to systems using surfactants such as Triton X-100. Its applicability to a wide range of pure organic solvents and mixtures justifies similar investigations with other petroleum fractions. The propan-1-ol/organic liquid ratio influenced the analytical response to some extent and must be defined. The conventional aqueous Pd + Mg as well as the Ir permanent modifier showed good performance for Pb in the microemulsion-stabilized media. However, their use was not indispensable once similar Pb results were found without any modifier at low background absorption levels. The drying temperature and ramp rate must be carefully optimized for Ni determination. This may be due to the presence or formation of volatile Ni compounds. The presence of very volatile Ni compounds was also recently reported by Vale et al. in the Ni determination in crude oil samples [38]. Aqueous calibration was not advisable. However, external calibration with microemulsions prepared with a single organic solvent was possible, using
0.49 F 0.02 0.54 F 0.02 0.35 F 0.02 0.46 F 0.01 0.61 F 0.01 bLD bLD bLD 0.59 F 0.04 bLD bLD 0.39 F 0,02 0.38 F 0.02 0.46 F 0.02 bLD bLD
aqueous or organometallic stock solutions. The results found by the proposed procedure were in close agreement with those obtained with comparative UOP procedure in the determination of Ni and Pb in commercial diesel and gasoline samples. Observed Ni and Pb contents in diesel samples ranged from 83 to 121 Ag l 1 and from 20 to 640 Ag l 1, respectively. In gasoline, these ranges were 101–158 Ag l 1 and 20–600 Ag l 1. The overall sample dilution factor associated with the microemulsion preparation is just 1 + 2, permitting detection limits well below the levels found in the investigated samples. The use of larger injection volumes or in situ pre-concentration [25] might improve these figures, if necessary. Alternatively, filter atomizers [27] could also be used. The present work poses concerns on data reported previously for Ni and Pb concentrations in gasoline and diesel at the nanograms per milliter level, for which the question of sample stability was not addressed. However, instrumental developments related to this determination are not necessarily under suspicion since sample stability refers to stages prior to the instrumental determination. It is also intriguing why problems related to the stability of these samples are not commented in the literature more frequently. First, they show up only at the nanograms per milliliter (or ng g 1) range. This range is in contrast to the micrograms per milliter range associated with many of the initial efforts for metal determination in oil derivatives, which aimed at determinations of Pb and other additive metals in fuels and lubricants. At the micrograms per milliter range, these losses are negligible. Moreover, if the samples are left in natural form in their original vessels for a long enough time, adsorption equilibrium between the solution and the container wall will be reached. Then, if samples in this situation are directly taken to the instrument (as frequently occurs in FAAS and ICP OES techniques, for
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instance), no change will be noticed in relation to the time. However, the liquid carried to the equipment is not representative of the original sample anymore. Only if these samples were transferred to another vessel (such as an autosampler cup of a GFAAS instrument) and consequently brought in contact with a new surface would the signal drop be noticed. Furthermore, it is still possible to observe reasonable repetitive results in replicate GFAAS readings if the reading period is not too long, as can be deduced from Fig. 1. In such case, losses may also pass unnoticed. Another point to be considered is that gasoline, diesel, and related samples vary widely in relation to their composition, and only little is known about the metal speciation in these samples. Thus, it is also a point to investigate if these losses occur equally in samples coming from different sources.
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