Method development for the determination of nickel in petroleum using line-source and high-resolution continuum-source graphite furnace atomic absorption spectrometry

Microchemical Journal 77 (2004) 131 – 140 www.elsevier.com/locate/microc

Method development for the determination of nickel in petroleum using line-source and high-resolution continuum-source graphite furnace atomic absorption spectrometry Maria Goreti R. Vale a, Isabel C.F. Damin a, Aline Klassen a, Ma´rcia M. Silva a, Bernhard Welz b,*, Alessandra F. Silva b, Fa´bio G. Lepri b, Daniel L.G. Borges b, Uwe Heitmann c a

Instituto de Quı´mica, Universidade Federal do Rio Grande do Sul, Av. Bento Goncßalves 9500, 91501-970, Porto Alegre, RS, Brazil b Departamento de Quı´mica, Universidade Federal de Santa Catarina, Campus Trindade, Floriano´polis, SC 88040-900, Brazil c ISAS Institute for Analytical Sciences, Department Berlin, Albert-Einstein-Strasse 9, 12489, Berlin, Germany Accepted 25 February 2004 Available online 12 May 2004

Abstract Several sample preparation methods have been investigated for the direct determination of nickel in crude oil using graphite furnace atomic absorption spectrometry (GF AAS). Xylene was found unsuitable as solvent because of the poor long-term stability of the solutions and the resulting contamination of the equipment. Isobutyl methyl ketone (IBMK) solutions exhibited better stability, but the sensitivity of the organic nickel salt used for the standard solutions showed a high day-to-day variability. An oil-in-water emulsion using Triton X-100 as surfactant gave the best results. Using high-resolution continuum-source (HR-CS) GF AAS, it could be observed that up to 50% of the nickel in crude oil, most likely low molecular weight nickel porphyrins, were lost already at pyrolysis temperatures >400 jC, whereas the rest of the nickel as well as the nickel standard were stable up to 1300 jC. The nickel absorption at a secondary line at 232.138 nm was recorded simultaneously with that at the primary line at 232.003 nm, expanding the dynamic working range by an order of magnitude. The best characteristic mass obtained was m0=27 pg and the limit of detection was around 0.07 Ag g 1 Ni in oil, based on an emulsion of 2 g oil in 10 ml. The accuracy of the procedure was verified by analyzing the certified reference material (CRM) NIST SRM 1634c, Trace Metals in Residual Fuel Oil. D 2004 Elsevier B.V. All rights reserved. Keywords: Nickel in crude oil; Graphite furnace atomic absorption spectrometry; High-resolution continuum-source AAS; Volatile nickel porphyrins; Stability of solutions

1. Introduction Nickel derives largely from the formation of crude oil and its concentration provides, together with that of vanadium, information about the origin of the oil. The ratio between these two elements is actually ‘bore-hole specific’ and is transported proportionally through crude oil refining procedures, even if in markedly reduced concentrations. In addition, nickel is a serious catalyst poison and may cause undesirable side reactions in refinery operations [1]. Inhalable dusts or aerosols of nickel and its compounds that might be generated during combustion of oil are classified * Corresponding author. Tel.: +55-48-9983-1344; fax: +55-48-3319711. E-mail address: [email protected] (B. Welz). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.02.007

as hazardous substances and are carcinogenic. All these properties make it necessary that the content of nickel in crude oil has to be determined routinely, independent of the fact that the content of such elements might have an influence on the price of the crude oil on the market. Nickel typically occurs in crude oil in the concentration range 2– 200 mg kg 1, i.e., in the ‘upper trace range’. A variety of spectrometric techniques have been used for the determination of nickel in crude oil and petroleum products, such as inductively coupled plasma (ICP) optical emission spectrometry [2 –5], ICP mass spectrometry [6,7], X-ray fluorescence spectroscopy [8,9] and even high-performance liquid chromatography with UV detection [10]. However, flame atomic absorption spectrometry (F AAS) is the technique of choice in most publications [11– 16]. The main advantages of that technique are that, firstly, a direct

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determination of the analyte in the hydrocarbon matrix appears to be possible after corresponding dilution with an organic solvent; secondly, the flames used in F AAS, in contrast to an ICP, are quite tolerant to most organic solvents; and thirdly, the concentration of nickel is high enough to allow a reasonable dilution of crude oil to eliminate viscosity effects. Nevertheless, direct determinations are not free of problems since the solvent, the compounds used for calibration and the container material have a substantial influence on the sensitivity, and on the stability of the solutions, respectively. For the analysis of crude oil, it is important to use a solvent mixture that contains a component to completely dissolve asphaltenes and resins, which only have limited solubility in non-polar solvents. Mixtures of 80% xylene or toluene and 20% ethanol or propan-1-ol have been proposed for the determination of nickel in crude oil, heavy fuel oils and bitumen products [13,17]. For a direct determination of nickel in the diluted crude oil sample, complete agreement between the metal compound in the test sample solution and in the calibration solution would be ideal. This is not possible because nickel as well as other trace elements is present in a number of various compounds in crude oil [1] and the composition is a priori not known in advance. A number of reports have been published on differing sensitivities of the various organic compounds of nickel by F AAS [11,18,19]. The most reliable procedure for the determination of nickel in crude oil and petroleum products hence is ashing of the sample and analysis of the residue after taking up in hydrochloric acid [15,20], a procedure, which is quite tedious and requires careful control to avoid analyte losses [21]. Graphite furnace atomic absorption spectrometry (GF AAS) appears to be an alternative since the problem of differing sensitivity for various compounds does not exist; the results from a simple dilution with xylene and ashing were found to be practically identical [22]. In addition, crude oil can be so strongly diluted that the hydrocarbon matrix does not cause any interference [23]. The formation and analysis of an oil-in-water emulsion or microemulsion instead of a dilution of the crude oil sample with an organic solvent has been proposed for F AAS [12] as well as for GF AAS [24 – 26] with the distinct advantage that aqueous standards could be used for calibration. The goal of this work was to develop a routine procedure for the reliable direct determination of nickel in crude oil with a minimum of sample preparation. The work also included an investigation of the stability of sample and calibration solutions in organic solvents and of oil-in-water emulsions. GF AAS has been chosen in order to avoid the problem of different sensitivities for the various organic compounds. Conventional line-source (LS) AAS has been used for method development as well as high-resolution continuum-source (HR-CS) AAS, particularly because of the much higher diagnostic information available with the latter technique, and its superior background correction capability.

2. Experimental 2.1. Instrumentation All measurements with conventional LS AAS were carried out using an AAS 5EA atomic absorption spectrometer (Analytik Jena, Jena, Germany) with deuterium background correction, equipped with a transversely heated graphite tube atomizer. A NARVA hollow cathode lamp for nickel (GLE, Berlin, Germany) was used as the radiation source with a current of 5.0 mA. The main analytical line at 232.0 nm was used for all determinations with a spectral bandwidth of 0.2 nm. The spectrometer was interfaced to an IBM PC/AT-compatible computer. All experiments were carried out using pyrolytically coated graphite tubes with an integrated PIN platform (Analytik Jena Part No. 407A81.025). An MPE 5 furnace autosampler (Analytik Jena) was used for introduction of emulsions and solutions. Argon with a purity of 99.996% (White Martins, Sa˜o Paulo, Brazil) was used as the purge gas with a flow rate of 2.0 l min 1 during all stages, except during atomization, where the flow was stopped. Integrated absorbance (peak area) was used exclusively for signal evaluation and quantification. The optimized graphite furnace temperature program used for all determinations with LS-GF AAS is given in Table 1. All measurements with high-resolution continuumsource (HR-CS) AAS were carried out using a prototype HR-CS atomic absorption spectrometer, built at ISAS Berlin (Berlin, Germany). The prototype is based on a Model AAS 6 Vario (Analytik Jena), from which the entire optical compartment including detector and associated controls had been removed and replaced by a double echelle monochromator (DEMON), similar to the system described by Heitmann et al. [27]. The DEMON consists of a prism pre-monochromator for order separation and an echelle monochromator for simultaneous recording of small sections of the high-resolved spectrum. Both units are in Littrow-mounting with focal lengths of 300 and 400 mm, respectively, resulting in a total spectral resolution of k/ Dkc140,000. Variation of the width of the intermediate slit Table 1 Graphite furnace temperature programs for the determination of nickel in crude oil with direct dilution or emulsification using LS-GF AAS Program stage

Temperature (jC)

Ramp (jC s 1)

Hold time (s)

Gas flow rate (l min 1)

Drying 1 Drying 2 Drying 3 Drying 4a Drying 5a Pyrolysis Auto Zero Atomization Cleaning

90 110 130 180 300 1300 1300 2400 2500

5 5 3 10 10 100 100 1500 100

50 50 50 30 40 20 5 5 4

2.0 2.0 2.0 2.0 2.0 2.0 0 0 2.0

a Drying steps 4 and 5 were used for emulsions only; they were omitted when samples diluted with IBMK were analyzed.

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allows determining the recorded wavelength interval. A xenon short arc lamp XBO 301 (GLE) with a nominal power of 300 W and an electrode distance of V1 mm, operated in a ‘hot-spot’ mode, and emitting intense radiation particularly in the UV region, was used as the continuum radiation source. An S7031-0906 UV-sensitive, thinned back-illuminated CCD array detector with 51258 pixels, size 2424 Am (Hamamatsu Photonics, Herrsching, Germany), operating in full vertical binning mode was used, providing an instrumental bandwidth of 1.6 pm at 200 nm. At 232.003 nm, the main wavelength of Ni, the resolution per pixel was 1.8 pm, and an intermediate slit width of 230 Am was used, making possible the simultaneous evaluation of 200 pixels, corresponding to about F0.2 nm around the analytical wavelength at pixel no. 250. The nickel absorption was measured using the central pixel only and the central pixel F1, i.e., over a spectral interval of about 2 and 6 pm, respectively. In addition, the nickel absorption was also measured at 232.138 nm, a secondary nickel line that was within the selected spectral window, again using the central pixel, and the central pixel F1. The system also includes active wavelength stabilization via spectral lines from an internal Ne lamp. The system was controlled by a Pentium III, 1000 MHz personal computer, running an inhouse developed data acquisition program. The system allows the recording of up to 5000 subsequent scans with a minimum integration time of 10 ms per scan. A very important feature of the software is that all data of a measurement can be stored in the computer, and parameters, such as the time interval for integration, and pixel(s) used for measurement and background correction can be optimized after the measurement. The graphite tube atomizer and the graphite parts were identical to those used with the AAS 5 atomic absorption spectrometer. The graphite furnace temperature program was slightly modified, as shown in Table 2, using only four ‘drying’ stages, a 100 jC higher atomization temperature, and particularly a pyrolysis temperature of only 400 jC, as will be discussed in Section 3. A Unique-Thorton model USC-2850 ultrasonic bath (Thorton, Sa˜o Paulo, Brazil) operated at a frequency of 37F3 kHz, with temperature control up to 80F5 jC, was used for preparation of the emulsions.

Table 2 Graphite furnace temperature program for the determination of nickel in crude oil with emulsification using HR-CS GF AAS

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2.2. Reagents Analytical grade reagents were used throughout. Nitric acid (Merck, Germany) used in this work was further purified by sub-boiling distillation in a quartz sub-boiling still (Ku¨rner Analysentechnik, Rosenheim, Germany). Distilled, deionized water (DDW) with a specific resistivity of 18 MV cm, from a Milli-Q water purification system (Millipore, Bedford, MA), was used for the preparation of the samples and standards. All containers and glassware were soaked in 3 mol l 1 nitric acid for at least 24 h and rinsed three times with DDW before use. The nickel stock solution, the nickel salt of 2-ethyl-hexanoic acid in oil (980 –1020 mg kg 1, d=0.86 kg l 1) was purchased from Merck. The working standards were prepared by serial dilution of the stock solution in base mineral oil (BMOMS, High Purity Standards, Charleston, SC, USA) and diluted in the same way as the sample, i.e., by dilution with organic solvents isobutyl methyl ketone (IBMK) or xylene (both from Merck). The surfactant used for emulsification was Triton X-100 (Union Carbide). 2.3. Reference material and samples As there is no certified reference material (CRM) for crude oil available, the NIST SRM 1634c, Trace Elements in Residual Fuel Oil (National Institute for Standards and Technology, Gaithersburg, MD, USA) was used in this work for validation of the method. The samples analyzed were acquired from local refineries and were identified as: PCV-105 and TQ-01 are blends of crude oils of different origin; Marlim is a crude oil from Bacia de Campos oil field and GOP a heavy gas oil from the same source. 2.4. Dilution with organic solvent The standards were prepared from a nickel stock solution of 1.0 mg kg 1 in oil, pipetting appropriate volumes to give concentrations in the range of 43– 172 Ag l 1, in 10-ml polyethylene tubes, and diluting directly with the organic solvent (IBMK or xylene). The blank was prepared using the base mineral oil. For samples, between 0.1 and 2.0 g of the CRM or the crude oil samples, depending on their nickel content, were weighed in triplicates in a small beaker, dissolved with the solvent and transferred to 10-ml polyethylene tubes using a Pasteur pipette.

Program stage

Temperature (jC)

Ramp (jC s 1)

Hold time (s)

Gas flow rate (l min 1)

2.5. Emulsification

Drying 1 Drying 2 Drying 3 Drying 4 Pyrolysis Atomization Cleaning

90 150 180 300 400 2500 2600

5 5 10 10 100 1500 100

50 50 50 40 20 5 4

2.0 2.0 2.0 2.0 2.0 0 2.0

The standards were prepared from a nickel stock solution of 1.0 mg kg 1, pipetting the volume to give concentrations in the range of 43 –172 Ag l 1 in 10-ml borosilicate glass tubes, and diluting with 1.0 ml of xylene. The tubes were closed, fitted into a polystyrene support and put into the ultrasonic bath at ambient temperature for 5 min. After, 100

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Al of Triton X-100 was added with a micropipette and the volume was completed with DDW. The tubes were agitated manually during 2 min and then put in the ultrasonic bath at ambient temperature for 5 min and finally agitated manually during 2 min again. The blank was prepared using the base mineral oil. For samples, between 0.1 and 2.0 g of the CRM or the crude oil samples, depending on their nickel content, were weighed in triplicates directly into the tubes and the same procedure described for standards was followed. The homogeneity of the emulsions was maintained by manual agitation with a micropipette just before the sample emulsion was taken up by the autosampler capillary. For both methods, the standard calibration technique was used, introducing 15 Al of standards. In the case of the emulsions, a volume of 5 Al of ethanol (Merck) was introduced into the graphite tube before the sample to avoid foam formation.

Fig. 1. Variation of the integrated absorbance over time for a 2.6-ng Ni standard solution (Ni salt of 2-ethylhexanoic acid) in base mineral oil, and in a crude oil sample (PCV-105), diluted with IBMK, and as oil-in-water emulsion, respectively.

3. Results and discussion 3.1. Stability of solutions and emulsions An important part of a routine procedure is that the sample and calibration solutions are stable at least for a few hours so that the analysis can be carried out using an automatic sampling device once the solutions or emulsions have been prepared. In a first attempt we investigated xylene as a solvent/diluent for samples and standards, but no stability at all could be obtained. The sensitivity for nickel in the standard solutions (Ni salt of 2-ethyl-hexanoic acid) decreased irregularly to about one fourth of its initial value over 400 min, i.e., within a typical working day. It was found that nickel had adsorbed to the autosampler cups (polyethylene), the autosampler capillary (PTFE) and also to the micropipette tips when manual sampling was used. The adsorbed nickel could be dissolved in dilute nitric acid, but thorough cleaning of the autosampler was necessary before it could be used again for the determination of this element. The use of xylene as a solvent was not further investigated after this experience. IBMK was investigated next as a more polar solvent, and the integrated absorbance recorded for a standard solution and for a crude oil sample over a period of 5 h is shown in Fig. 1. The stability observed with this solvent was significantly better, although a poorly reproducible initial drop in sensitivity was observed during the first few minutes, followed by an increase of 25– 30% in integrated absorbance over the 5-h measurement period. This increase in sensitivity was not considered a serious problem for a routine procedure, as it occurred to the same extent for the sample solution and the calibration solution, at least over the first 3 h, so that it would not seriously affect the accuracy of the measurement. It is assumed that this effect was due to a gradual evaporation of the solvent from the autosampler cups caused by the efficient ventilation system in the

laboratory. In contrast to this, the initial drop in sensitivity, which occurred only for the calibration solutions, and which was poorly reproducible, was considered a problem, as it would cause erroneously high results for the investigated samples. Dilution with IBMK was nevertheless further investigated in the future experiments. The calibration solutions were reasonably stable over a typical working day, but the sensitivity dropped to about 50% after 48 h, i.e., they had to be prepared fresh daily. Our next attempt was to investigate the emulsion/microemulsion technique with nitric acid and Triton X-100 that has been used successfully in earlier work for the analysis of lubricating oils [24] and gasoline [7]. However, a black precipitate formed immediately upon the addition of nitric acid to the crude oil samples so that this preparation method could not be applied for the samples investigated in this work. Most likely, it was the asphaltenes that were precipitated upon addition of the acid. The best performance with respect to stability of the integrated absorbance signal for nickel was obtained for an oil-in-water emulsion with the addition of xylene and Triton X-100, as is also shown in Fig. 1. The sensitivity for both, the standard solution and the crude oil sample did not change over 24 h and the sensitivity loss after 48 h was only of the order of 15%, making this sample preparation most suitable for routine application. The only condition was that the emulsion had to be mixed by manual agitation before introduction into the graphite tube with the autosampler, which means that no unattended operation was possible. 3.2. Program optimization using LS-GF AAS Optimization of the graphite furnace temperature program using conventional LS-GF AAS was carried out in the usual manner by first establishing pyrolysis and atom-

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ization curves for a calibration solution and at least one representative sample. Obviously it would be preferable to use a CRM for that purpose and not a sample with an unknown analyte content; however, there is no crude oil CRM available on the market, and the only available material, SRM 1634c, Trace Metals in Residual Fuel Oil, was not considered to be representative enough for crude oil analysis. It was found necessary, in order to achieve a smooth and complete removal of all the volatile components of crude oil, to apply a rather sophisticated multi-stage ‘drying’ program with several ramp and hold times at gradually increasing temperatures, particularly when the sample was introduced as an emulsion (refer to Table 1). Shorter drying programs included the risk of occasional spattering of the sample upon transition to the pyrolysis temperature, resulting in inferior precision. The pyrolysis curves for both the standard and the crude oil sample in IBMK and as oil-in-water emulsion are shown in Fig. 2. Even without the addition of a modifier, nickel appeared to be stable up to a pyrolysis temperature of at least 1300 jC under all conditions, which is in agreement with literature data [28]. Pyrolysis temperatures lower than 600 jC could not be used due to the excessively high and rapidly changing background absorption signal, caused by the oil matrix. Hence, the slight gradual increase in the integrated absorbance for the crude oil sample at pyrolysis temperatures <1000 jC (Fig. 2) was suspected to be also due to some background absorption phenomena that could not be corrected properly by the deuterium background correction system of this instrument. This effect was not further investigated, as the phenomenon disappeared at higher temperatures, and a pyrolysis temperature of 1300 jC was chosen for all further experiments. The atomization curves (which are not shown here) revealed an optimum atomization temperature of 2400 jC, which is again in

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Fig. 3. Absorbance over time and wavelength for TQ-01 crude oil sample as oil-in-water emulsion, recorded in the vicinity of the nickel line at 232.003 nm using HR-CS GF AAS; pyrolysis temperature: 1300 jC, atomization temperature: 2400 jC.

agreement with literature data for a transversely heated atomizer [28]. The sensitivity obtained for nickel in the calibration solution made up in IBMK was some 30% lower than that for the emulsion, whereas the difference in sensitivity in the crude oil sample, using the two sample preparation procedures, was less than 10%, at least for a pyrolysis temperature of 1300 jC. This means that the results for nickel in oil would be some 25% higher when they were dissolved in IBMK, compared to the emulsion technique. As mentioned earlier, the sensitivity obtained for nickel standards in IBMK was not well reproducible so that varying results were obtained, which are not shown here. The results obtained for nickel in the investigated samples using the emulsion technique are presented in Table 3. The agreement of the value found for the CRM with the certified nickel content was quite satisfactory, so that it might be assumed that the results obtained for the other samples were accurate as well. 3.3. Investigations using HR-CS GF AAS Probably, the most outstanding feature of this technique for method development is its possibility to reveal the spectral environment of the analytical line at high resoluTable 3 Results obtained for nickel in trace elements in residual fuel oil CRM and in four crude oil samples using the emulsion technique and LS-GF AAS

Fig. 2. Pyrolysis curves for a 2.6-ng Ni standard solution (Ni salt of 2ethylhexanoic acid) in base mineral oil, and in a crude oil sample (PCV105), diluted with IBMK, and as oil-in-water emulsion, respectively, using LS-GF AAS.

Sample

Certified (Ag g 1)

Found (Ag g 1)

SRM 1634c PCV-105 TQ-01 GOP Marlim

17.5F0.21

17.1F1.1 3.45F0.1 0.69F0.04 2.14F0.03 18.9F0.21

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tion, which makes HR-CS GF AAS an extremely useful diagnostic tool, as shown in Fig. 3, the details of which will be discussed later. Another very important feature are the superior background correction capabilities, which firstly allow to correct for background absorption of at least A=2; secondly, they make possible a correction of even the most rapidly changing background signals, as measurement of atomic and background absorption are truly simultaneous, avoiding all the problems associated with the fast sequential measurement used in conventional LS-atomic absorption spectrometers [29]; thirdly, due to the high resolution of the spectrometer, any molecular absorption with fine structure or atomic absorption due to a matrix element has no influence on the measured signal as long as they are not overlapping directly; fourthly, even in the latter situation, which is the worst case, the problem can be solved by subtracting model spectra using a least squares algorithm [30]. Last not least, although this is not directly related to HR-CS AAS, but rather a feature of the software, the fact that the time frame for signal integration can be selected in steps of about 30 ms, and that this selection can be made after the actual measurement, makes possible to exclude any spectral event that is not directly overlapping with the atomic absorption in time. The first investigations with HR-CS GF AAS were carried out under the conditions optimized with LS-GF AAS, i.e., with a pyrolysis temperature of 1300 jC. The time - and wavelength-resolved absorbance obtained for TQ-01 crude oil using the emulsion technique is shown in Fig. 3, and the wavelength-resolved time-integrated absorbance obtained for PCV-105 crude oil dissolved in IBMK is shown in Fig. 4. In both spectra, there appears a second line at 232.138 nm, which must be another nickel line, as the intensity ratio between this line and the main analytical line at 232.003 nm remained constant for all sample and calibration solutions. This line obviously does not cause any interference in conventional LS AAS, but it contributes

Fig. 4. Integrated absorbance recorded for PCV-105 crude oil sample dissolved in IBMK over a spectral range of 0.35 nm around the nickel line at 232.003 nm using HR-CS GF AAS and a pyrolysis temperature of 1300 jC.

to the non-linearity of the calibration curve if it is not excluded by the exit slit of the monochromator. In HR-CS AAS, however, this line could be used as an alternate analytical line for the determination of high concentrations of nickel, which would be out of the linear range of the main analytical line. And as this line was within the spectral range that was monitored anyway, the integrated absorbance of both lines could be recorded simultaneously, and the decision, which line is used for evaluation, could be made at a later stage. This way it would not be necessary to make another determination in case the nickel content of a sample was out of the calibration range of the primary analytical line. Hence, all determinations were carried out evaluating the integrated absorbance measured with both lines. While the IBMK solutions of standards and samples did not show any other spectral event (Fig. 4), there appeared several additional small equidistant absorption lines in the spectrum for the emulsions (Fig. 3) at the same time as the nickel absorption. These additional lines are obviously part of a molecular absorption spectrum with a rotational fine structure. As they showed up only in the emulsions, but not in the IBMK solutions and, in the samples as well as in the standards, they were probably not due to the oil, but due to a component of the emulsion, most likely Triton X-100; the origin of these molecular absorption lines was not further investigated. However, it should be noticed that these molecular structures could cause some over-correction when deuterium background correction was used, although it is unlikely that this would result in a significant measurement error, as the over-correction should be the same in the samples and the standards. In a next step, we went to the more critical pyrolysis temperature of 600 jC, where the integrated absorbance values for the oil samples were increasing when LS AAS was used (refer to Fig. 2) in order to find out if there was any background absorption phenomenon that could have caused this effect. The time-resolved absorbance signals obtained for TQ-01 crude oil emulsion with HR-CS AAS on the center pixel at 232.003 nm without and with background correction are shown in Fig. 5a. It is obvious that there is already a high and rapidly changing non-specific absorption of Ac1 preceding the analyte signal, which could be corrected without problems with HR-CS AAS, but which might have caused the suspected difficulties for LS AAS with deuterium background correction. However, closer investigation showed that the value for nickel obtained by HR-CS AAS under these conditions increased in a very similar way, as it increased with LS AAS, although the former technique obviously had no background correction problems. A very similar behavior was observed for the IBMK solution of the same crude oil, which is not shown here. In order to shed more light onto this phenomenon we decided to lower the pyrolysis temperature further and to penetrate into the temperature range that had been inacces-

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Fig. 5. Absorbance over time for TQ-01 crude oil sample as oil-in-water emulsion, recorded at the center pixel only at 232.003 nm using HR-CS GF AAS: (A) pyrolysis temperature 600 jC, (B) pyrolysis temperature of 400 jC, black line: background-corrected atomic absorption, gray line: total absorbance without background correction.

sible for LS AAS with deuterium background correction. Fig. 5b shows the situation for the same crude oil sample emulsion using a pyrolysis temperature of only 400 jC. The background absorption under these conditions reached a value of Ac4, i.e., a magnitude that is beyond the capability of any background correction system, particularly as the absorption was changing very rapidly with time. Even with HR-CS AAS, this background absorption caused a significant baseline noise, as there was essentially no more radiation arriving at the detector. Nevertheless it was possible to make a quantitative determination of the nickel content in the analyzed samples because the time interval for integration of the atomic absorption signal could be selected carefully after the determination in a way to avoid any distortion from the background absorption. This was actually even true for a pyrolysis temperature down to 300 jC, where the background absorption reached a value of Ac5. The background signal for the IBMK solution only reached a value of Ac2 when a pyrolysis temperature of 400 jC was used, i.e., it might be within the correction

137

potential of Zeeman-effect background correction, but it clearly exceeded this limit when a pyrolysis temperature of 300 jC was used. The pyrolysis curves obtained under these conditions are shown in Fig. 6, and they exhibit a very interesting phenomenon, i.e., that a significant part of the nickel in crude oil was apparently lost already at temperatures above 400 jC, whereas the rest of the nickel was thermally stable up to at least 1300 jC. This could only be due to the existence of at least two significantly different nickelorganic compounds or types of compounds in the investigated crude oil samples. Obviously, this phenomenon did not appear in the standard solutions, as they only contained a single nickel compound. A detailed investigation of the nature of these two types of nickel species was considered to be beyond the scope of this work. However, it is well documented in the literature that nickel exists in a variety of different metal organic compounds in crude oil, particularly of non-polar nickel porphyrins and polar non-porphyrins [1]. Ma´rquez et al. [31] have for example demonstrated the existence of volatile, low molecular weight nickel porphyrins and shown that some 30% of the nickel could be volatilized by vacuum sublimation already at temperatures of 90– 120 jC, and more than 50% at temperatures up to 240 jC, respectively. It might well be that future research will show that GF AAS could be used to differentiate between such groups of compounds, such as porphyrins and non-porphyrins, and hence be used for kind of a speciation analysis. Fig. 6, similar to Fig. 2, shows a significant difference in the sensitivity for the nickel-organic standards when the emulsion technique was used in comparison to the IBMK solution, whereas the sensitivity for nickel in the crude oil sample was essentially identical under the two conditions. This suggests that dilution with IBMK might be used for crude oil analysis, but not for the preparation of standard

Fig. 6. Pyrolysis curves for a 2.6-ng Ni standard solution (Ni salt of 2ethylhexanoic acid) in base mineral oil, and in a crude oil sample (PCV105), diluted with IBMK, and as oil-in-water emulsion, respectively, using HR-CS GF AAS and integrated absorbance measured at the center pixel (232.003 nm)F1.

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solution from the Ni-salt of 2-ethyl-hexanoic acid, which exhibited significant day-to-day variation in sensitivity, in spite of its apparent stability over a typical working day (refer to Fig. 1). No results for nickel in the investigated samples are shown here because of this obvious calibration problem. Table 4a shows the results obtained for nickel using the emulsion technique and a pyrolysis temperature of 400 jC, and Table 4b shows the results obtained with a pyrolysis temperature of 1300 jC, respectively. Both tables show the results obtained at the primary analytical line at 232.003 nm and at the alternate line at 232.138 nm, using the center pixel (CP=line core) as well as the center pixel F1 (line core + wings). Firstly, there was no significant difference, neither in the results obtained nor in the precision, when only the CP was used or CPF1, at least not for the primary analytical line. Secondly, as expected, better precision was obtained for low nickel contents using the primary analytical line, but some improvement in precision was observed at the secondary line for the highest nickel concentrations. Thirdly, there was a clear trend to higher results for nickel in samples with high analyte content when they were determined at the secondary line with lower sensitivity. This trend was also confirmed in the IBMK solutions, which are not shown here because of the calibration problems. It has also to be noted that the best results for the CRM, which has high nickel content, have been obtained using the less sensitive secondary line. This demonstrates the advantage of using less sensitive lines for the determination of high analyte contents in order to avoid problems associated with non-linear calibration curves. Most importantly, however, there was a very significant difference in the results obtained for nickel in the crude oil samples, and also in the CRM, when a pyrolysis temperature of 400 jC was used instead of 1300 jC, which is obviously due to the loss of a volatile nickel species at temperatures >400 jC. Hence, the total nickel content in crude oil can only be determined using a pyrolysis temperature not higher than 400 jC, which is not possible with conventional LSGF AAS using deuterium background correction because of the excessive background absorption caused by the oil matrix under these conditions. It should be mentioned that there is a reasonable agreement between the data obtained with LS-GF AAS using a pyrolysis temperature of 1300 jC,

Table 4b Same as Table 4a, but using a pyrolysis temperature of 1300 jC Wavelength

232.003 nm

232.138 nm

Sample

CP

CPF1

CP

CPF1

SRM 1634c PCV-105 TQ-01 GOP Marlim

14.9F0.58 2.79F0.08 0.64F0.01 2.13F0.13 17.1F1.12

14.7F0.54 2.72F0.08 0.62F0.01 2.12F0.12 17.1F0.79

14.4F0.56 2.88F0.07 0.70F0.04 2.13F0.01 18.1F1.20

15.7F0.44 2.88F0.11 0.71F0.04 2.33F0.23 18.5F0.85

and those of Table 4b, using HR-CS GF AAS and the same pyrolysis temperature. The ratio between ‘volatile’ and ‘stable’ nickel in the various samples investigated in this study is shown in Table 5. It is interesting to note that close to 50% of the nickel in the blended crude oil samples PCV-105 and TQ-01 are volatile, which is in good agreement with the results of Ma´rquez et al. [31]. It is also interesting to note that the percentage of volatile Ni in Marlim crude oil is transferred without alteration to the heavy gas oil fraction GOP from the same origin. Last not least, Table 5 shows that the CRM ‘Trace Metals in Residual Fuel Oil’ is behaving significantly different and cannot be considered a reliable reference material for method validation, unless the entire temperature range down to at least 400 jC can be investigated during method development. 3.4. Figures of merit Only the results for the oil-in-water emulsion are given here, as the sensitivity for the oil standard dissolved in IBMK was varying too much from day to day to report reliable data. The reason for this variation, which was observed for the standard solution only, but not for the sample solutions, could not be found out, but might be due to the specific nickel-organic compound used in this work. The linear regression equation of the calibration function, the correlation coefficient and the characteristic mass obtained with pyrolysis temperatures of 400 and 1300 jC, at 232.0030 and 232.1373 nm, and with the center pixel only and center pixel F1, respectively, are shown in Table 6. In contrast to the samples investigated here, there is no significant difference in sensitivity for the calibration solutions using a pyrolysis temperature of 400 or 1300 jC. The sensitivity for nickel increases roughly by a factor of two

Table 4a Results for the determination of nickel in crude oil (all values in Ag g 1) using the emulsion technique and a pyrolysis temperature of 400 jC with HR-CS GF AAS; the certified value for SRM 1436c is 17.5F0.21 Ag g 1; for details, see text

Table 5 Total nickel content and distribution between ‘volatile’ and ‘stable’ nickel in the investigated samples of crude oil and in the CRM

Wavelength

232.003 nm

Sample

Sample

CP

CPF1

CP

CPF1

Ni content (Ag g 1)

Volatile Ni (%)

Stable Ni (%)

SRM 1634c PCV-105 TQ-01 GOP Marlim

17.0F0.02 4.99F0.05 1.32F0.01 3.22F0.05 23.9F1.40

16.8F0.06 4.91F0.09 1.30F0.01 3.16F0.07 23.6F1.40

17.4F0.31 5.24F0.04 1.24F0.04 2.99F0.25 27.1F0.34

17.4F0.36 5.29F0.17 1.22F0.07 3.08F0.54 27.2F0.16

SRM 1634c PCV-102 TQ-01 GOP Marlim

17.4 5.26 1.31 3.19 27.2

14 47 49 33 33

86 53 51 67 67

232.138 nm

M.G.R. Vale et al. / Microchemical Journal 77 (2004) 131–140 Table 6 Linear regression equation, correlation coefficient (R) and characteristic mass (m0) for the determination of nickel under different experimental conditions, using oil-in-water emulsion and HR-CS GF AAS Wavelength (nm)

Pixel

232.003

CP CPF1 CP CPF1 CP CPF1 CP CPF1

232.138 232.003 232.138

Tpyr (jC) 400

1300

Linear regression equation

R

m0 (pg)

A=0.01193+0.07495 m A=0.02945+0.15723 m A=0.00261+0.01402 m A=0.01280+0.02874 m A=0.00012+0.07201 m A=0.01032+0.15217 m A= 0.0019+0.01497 m A= 0.0021+0.03112 m

0.9979 0.9983 0.9917 0.9892 0.9981 0.9977 0.9944 0.9961

56 27 291 136 60 28 292 139

139

losses. Another effect that has rarely been mentioned in the literature, and which has been found to be one of the major problems for routine analysis, is the poor stability of nickelorganic standard solutions. An oil-in-water emulsion was the only sample preparation method that allowed to obtain reliable results over a typical working day. Lastly, the sensitivity obtained for the organic nickel salt used for calibration purposes, as well as for the investigated samples was about a factor of two inferior to that for aqueous standards. Although this effect did not cause any interference, it obviously calls for further investigation.

Acknowledgements when the center pixel F1 is used for evaluation, compared to the center pixel only, and the sensitivity at the secondary line at 232.138 nm is about a factor of five lower than that at the primary analytical line. This means that the working range is increased by another order of magnitude using all the possibilities, and all this information is obtained simultaneously in one single measurement. The best characteristic mass of m0=27 pg, obtained at the primary analytical line using the center pixel F1, is roughly a factor of two inferior to published values for this element [28], but it is better than the value of m0=33 pg, obtained for the same oil-in-water emulsions using conventional LS-GF AAS with the same atomizer. The reason for this discrepancy is unknown, but it must be due to the organic nickel compounds or the oil matrix, as values around m0=13 pg were obtained for aqueous nickel standards. It is unlikely that this phenomenon is due to analyte losses, unless they occur at temperatures below 300 jC, and are very reproducible even for different nickel compounds, as the value found for SRM 1634c was within the 95% confidence interval of the certificate. This effect cannot be called an interference either, because it appears to the same extent for the samples and the calibration solutions [32]. The limit of detection, expressed as the analyte signal that corresponds to three times the standard deviation of a blank (n=10) was determined as 0.22 ng, corresponding to a nickel concentration of 0.07 Ag g 1 in oil, based on a concentration of 2 g of oil in 10 ml of emulsion.

4. Conclusions The major finding of this work was that as much as 50% of the nickel, most likely volatile, low molecular weight nickel porphyrins, are lost from crude oil samples, and maybe also from other petroleum products, already at pyrolysis temperatures >400 jC. This has not been described in the literature up to now, most likely because of the excessive background absorption that is observed under these conditions. Although this effect could probably be used for kind of a speciation analysis, it also calls for further investigation about ways to avoid these low-temperature

The authors are grateful to Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq), to Coordenacß a˜o de Aperfeicß oamento de Pessoal de Nı´vel Superior (CAPES) and to Fundacßa˜o de Amparo a Pesquisa do Rio Grande do Sul (FAPERGS). B.W. has a research scholarship from CAPES; I.C.F.D. has a scholarship from CAPES; A.F.S., F.G.L. and D.L.G.B. have scholarships from CNPq; and A.K. has a scholarship from FAPERGS. The authors are also grateful to CNPq-Programa de Cieˆncias e Tecnologia do Petro´leo (CTPetro Process 479333/01-7) to the Senatsverwaltung fu¨r Wissenschaft, Forschung und kultur des Landes Berlin, and the Bundesministerium fu¨r Bildung und Forschung for financial support, and to Analytik Jena for financial support and the loan of an atomic absorption spectrometer.

References [1] R.H. Fish, J.J. Komlenic, B.K. Wines, Anal. Chem. 56 (1984) 2452. [2] R.J. Brown, Spectrochim. Acta, Part B: Atom. Spectrosc. 38 (1985) 283. [3] J.L. Fabec, M.L. Ruschak, Anal. Chem. 57 (1985) 1853. [4] B. Pahlavanpour, E.K. Johnson, in: G.B. Crump (Ed.), Petroanalysis ’87—Developments in Analytical Chemistry in the Petroleum Industry, Wiley, London, 1988, pp. 189 – 209. [5] D.W. Hausler, R. Carlson, Spectrochim. Acta Rev. 14 (1991) 124. [6] A.A. Van Heuzen, in: A.R. Date, A.L. Gray (Eds.), Application of Inductively Coupled Plasma Mass Spectrometry, Blackie, London, 1989, pp. 169 – 188. [7] T.D. Saint’Pierre, L.F. Dias, D. Pozebon, R.Q. Auce´lio, A.J. Curtius, B. Welz, Spectrochim. Acta, Part B: Atom. Spectrosc. 57 (2002) 1991. [8] K. Iwasaki, K. Tanaka, Anal. Chim. Acta 136 (1982) 293. [9] E.R. Denoyer, L.A. Siegel, Anal. Chim. Acta 192 (1987) 361. [10] M.Y. Khuhawar, S.N. Lanjwani, Talanta 43 (1996) 767. [11] I. Lang, G. Sˇebor, V. Sychra, D. Kolihova, O. Weisser, Anal. Chim. Acta 84 (1976) 229. [12] M. De la Guardia, M.J. Lizondo, At. Spectr. 4 (1983) 208. [13] S.H. Ka¨gler, Neue Mineralo¨lanalyse, I. Spektroskopie, Hu¨thig, Heidelberg, 1987. [14] M. Bettinelli, P. Tittarelli, J. Anal. At. Spectrom. 9 (1994) 805. [15] O. Platteau, M. Carrillo, Fuel 74 (1995) 761. [16] J.L. Hammond, Y.I. Lee, C.O. Noble, J.N. Beck, C.E. Proffitt, J. Sneddon, Talanta 47 (1998) 261.

140

M.G.R. Vale et al. / Microchemical Journal 77 (2004) 131–140

[17] S.H. Ka¨gler, Erdo¨l, Kohle, Erdgas, Petrochem. 39 (1986) 269. [18] I. Lang, G. Sˇebor, O. Weisser, V. Sychra, Anal. Chim. Acta 88 (1977) 313. [19] G. Sˇebor, I. Lang, D. Kolihova, O. Weisser, Analyst 107 (1982) 1350. [20] S.H. Ka¨gler, Erdo¨l, Erdgas, Kohle, Petrochem. 104 (1988) 131. [21] A.P. Udoh, S.A. Thomas, E.J. Ekanem, Talanta 39 (1992) 1591. [22] P. Bermejo-Barrera, C. Pita-Calvo, F. Bermejo-Martı´nez, Anal. Lett. 24 (1991) 447. [23] M.C. Gonza´lez, A.R. Rodrı´guez, V. Gonza´lez, Microchem. J. 35 (1987) 94. [24] R.Q. Auce´lio, A.J. Curtius, B. Welz, J. Anal. At. Spectrom. 15 (2000) 1389. [25] R.Q. Auce´lio, A.J. Curtius, J. Anal. At. Spectrom. 17 (2002) 242.

[26] J.L. Burguera, R.M. Avila-Go´mez, M. Burguera, R.A. Salager, J.L. Salager, C.L. Bracho, M. Burguera-Pascu, C. Burguera-Pascu, R. Brunetto, M. Gallignani, Y.P. Pen˜a, Talanta 61 (2003) 353. [27] U. Heitmann, M. Schu¨tz, H. Becker-Ross, S. Florek, Spectrochim. Acta, Part B: Atom. Spectrosc. 51 (1996) 1095. [28] B. Welz, M. Sperling, Atomic Absorption Spectrometry, 3rd ed., Wiley-VCH, Weinheim, New York, 1999, pp. 531 – 533. [29] J.A. Holcombe, J.M. Harnly, Anal. Chem. 58 (1986) 2606. [30] H. Becker-Ross, S. Florek, U. Heitmann, J. Anal. At. Spectrom. 15 (2000) 137. [31] N. Ma´rquez, F. Ysambertt, C. De la Cruz, Anal. Chim. Acta 395 (1999) 343. [32] Pure Appl. Chem. 45 (1976) 105.