Determination of trace metals in naphtha by graphite furnace atomic absorption spectrometry: Comparison between direct injection and microemulsion pretreatment procedures

Fuel Processing Technology 91 (2010) 1702–1709

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Determination of trace metals in naphtha by graphite furnace atomic absorption spectrometry: Comparison between direct injection and microemulsion pretreatment procedures Marcio V. Reboucas a,⁎, Daniela Domingos a,b, Aline S. O. Santos a, Leilacy Sampaio a a b

Braskem S. A., Unidade de Insumos Basicos Bahia, Rua Eteno 1561, Complexo Industrial de Camacari, Bahia, 42810-000, Brazil Universidade Federal da Bahia, Instituto de Química, Campus Universitário de Ondina, Rua Barão de Geremoabo, Ondina, Salvador, Bahia, 40170-290, Brazil

a r t i c l e

i n f o

Article history: Received 26 December 2009 Received in revised form 7 July 2010 Accepted 7 July 2010 Keywords: Metals Naphtha Petroleum products Microemulsion GFAAS ETAAS

a b s t r a c t Naphtha is a volatile petroleum fraction containing C4–C15 hydrocarbon compounds used as feedstock for petrochemical processes which are seriously affected by trace metals. Simple methods for copper, iron, lead and silicon determination in naphtha using graphite furnace atomic absorption spectrometry (GFAAS) have been developed. Two different approaches are presented: direct injection of the sample and oil-in-water microemulsion formation using a mixture of the sample, propan-1-ol and nitric acid aqueous solution. The calibration curves showed linear response for each concentration range with correlation coefficients ranging from 0.9728 to 0.9998. Precision figures of 1.7–20%, reported as the relative standard deviation, were calculated from at least twenty consecutive measurements of solutions containing the metal in a concentration level below 100 μg L−1. The characteristic masses varied from 8.5 to 44 pg and the limit of detection, defined as the metal concentration that gives a response equivalent to three times the standard deviation of the blank (n = 10), was found to be within the range 0.01–26 μg L−1. A critical analysis is presented by the authors emphasizing the advantages and limitations of both approaches. The proposed procedures have been used for copper, iron, lead and silicon determination in naphtha feeds processed in Braskem S.A. (Camacari, Bahia, Brazil). © 2010 Elsevier B.V. All rights reserved.

1. Introduction Determination of trace elements in crude oil and petroleum products has received considerable attention because of its relevance for petroleum cracking and refining processes. Even trace of these metals can cause serious damages in refineries as they can lead to equipment corrosion, catalytic poisoning or affect the quality of final products. Among the petroleum products, naphtha is one of the main feedstocks for the petrochemical industry, and it is by far the most important raw material for aromatic production. Naphtha is a petroleum fraction containing a complex mixture of C4–C15 hydrocarbon compounds from which important basic petrochemicals such as ethylene, propylene, toluene and p-xylene can be obtained [1]. Catalytic reactors are usually used to produce such basic petrochemicals aiming at removing specific contaminants, converting other compounds into final products or improving the overall unit yield. The presence of some metallic species may cause severe poisoning of the process catalysts. Other problems related to equipment corrosion and release of these metallic contaminants to the atmosphere have been

⁎ Corresponding author. Tel.: +55 71 3413 1637; fax: +55 71 3413 2480. E-mail address: [email protected] (M.V. Reboucas). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.07.007

reported when those feedstocks are cracked or used as an energy source. Copper, iron, lead, silicon, arsenic and mercury are the most important metal contaminants monitored by petrochemical industries due to a high probability of occurrence in petroleum [1] and catalyst poisoning effects. Current literature for determination of metals in naphtha with different approaches is summarized in Table 1 [2–12]. Although copper and iron are two of the most relevant metals to be determined in naphtha, to the best of the author's knowledge, no method for determination of these metals in naphtha is available in literature. Graphite furnace atomic absorption spectrometry (GFAAS) and inductively coupled plasma mass spectrometry (ICP-MS) are the main analytical techniques that have been used for metal determination in naphtha samples. Other techniques such as inductively coupled plasma atomic emission spectrometry (ICP OES) and flame atomic absorption spectrometry (FAAS), widely used for elemental analysis in aqueous samples, are not sensitive enough for direct metal determination down to ppb level in a complex matrix such as naphtha. In such cases timeconsuming preconcentration steps would be required. The main advantage of ICP-MS is its inherent feature of providing simultaneous metal determination with high sensitivity. Comparing with ICP-MS, the main advantages of GFAAS are the cost of acquisition and operation, suitability to handle complex matrices (with application of Zeeman-

Table 1 Summary of literature references on metal analysis in naphtha [2–12]. Analyte

Sample

Sample preparation

Technique

Detection limit

[2]

V, Ni, Zn, As, Cd, Ba, Hg, Pb

Condensates and naphtha

Dilution with xylene

ICP-MS

0.004–0.02 μg kg−1 0.004 μg kg−1 (Pb)

[3]

V, Co, Ni, As, Hg, Pb

Naphtha

Emulsion formation with Triton X-100

ICP-MS

0.05–0.7 μg L−1 0.09 μg L−1 (Pb)

[4]

Si

Naphtha

None

GFAAS

15 μg L−1

mo = 143 pg. No use of chemical modifier.

[5]

Hg

Naphtha

Detergentless microemulsion formation

GFAAS

0.78 μg L−1

Multiple injection technique for direct preconcentration in the graphite tube. Pd solution used as chemical modifier. mo = 990 pg.

[6]

Hg

Naphtha

None

GFAAS AFS-TD

32 μg kg−1 (GFAAS) 3.3 μg kg−1 (AFS-TD)

Pd solution used as chemical modifier. mo = 98 pg.

[7]

Mn

Diesel, gasoline and naphtha

Detergentless microemulsion formation

GFAAS

0.3 μg L−1 (naphtha) 0.5 μg L−1 (gasoline) 0.6 μg L−1 (diesel)

Sample throughput 7–10 h−1. mo = 3.4 pg. No use of chemical modifier.

[8]

Ni, V

Fuel oils and naphtha

Emulsion formation with Triton X-100

GFAAS

2–6 μg kg−1 (naphtha) 100–500 μg kg−1 (oils)

W-Ir permanent chemical modifier. mo = 15-20 pg (Ni) and 80–150 pg (V)

[9]

As, Sb

Naphtha

Emulsion formation with Triton X-100

GFAAS

2.7 μg L−1 (As) 2.5 μg L−1 (Sb)

Ir permanent chemical modifier. mo = 48.9 pg (As) and 67.7 pg (Sb)

[10]

As

Naphtha

None

GFAAS

0.5 μg L−1

mo = 35 pg Pd solution used as chemical modifier. −1

Details

[11]

As

Petroleum condensates and naphtha

None

GFAAS

0.56–1.33 μg L

mo = 63.5–69.3 pg. La solution used as chemical modifier.

[12]

Pb

Petroleum condensates and naphtha

Dilution with xylene

GFAAS

0.8 μg L−1

mo = 35 pg. Pd as permanent chemical modifier.

This work

Cu, Fe, Pb

Naphtha

None

GFAAS

0.64 μg L−1 (Cu) 0.01 μg L−1 (Fe) 0.26 μg L−1 (Pb)

mo = 24.8 pg (Cu), 22.8 pg (Fe) and 24.8 pg (Pb). La solution used as chemical modifier for lead.

This work

Cu, Fe, Pb, Si

Naphtha

Detergentless microemulsion formation

GFAAS

1.58 μg L−1 2.36 μg L−1 2.90 μg L−1 25.6 μg L−1

mo = 15.6 pg (Cu), 8.5 pg (Fe), 29.2 pg (Pb) and 44.0 pg (Si). La solution used as chemical modifier for lead and silicon.

(Cu) (Fe) (Pb) (Si)

M.V. Reboucas et al. / Fuel Processing Technology 91 (2010) 1702–1709

Ref.

ICP-MS: inductively coupled plasma mass spectrometry, GFAAS: graphite furnace atomic absorption spectrometry, AFS-TD: atomic fluorescence spectroscopy with thermal desorption.

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effect background correction) and high sensitivity. Indeed, due to its high cost, the ICP-MS equipment is rarely available in industry laboratories. Besides, regarding the complex nature of the organic naphtha sample, the analysis with ICP-MS requires special attention to minimize the coke formation on the sampler and skimmer cones and avoid the instability or even the complete extinction of the plasma due to organic vapor overloading [13]. Interferences due to polyatomic species, formed with carbon from the organic samples and with oxygen from the auxiliary gas, should also be prevented. In fact, Olsen et al. found the determination of low levels of Pb or Hg in naphtha by ICP-MS very problematic [2]. Some of the proposed procedures employing GFAAS or ICP-MS have suggested an emulsion formation as a first sample pretreatment step. Previous studies showed the advantage of using sample emulsification over acid digestion and sample dilution with an organic solvent as sample preparation procedure for the determination of metals in petroleum product samples [3,8,14]. Most of the published works have proposed the use of a surfactant, e.g. Triton X-100, in the emulsification process. The procedures employed by Kumar et al. [3], Meeravali et al. [8], and Cassela et al. [9] established the use of a magnetic stirrer for 20 min in order to obtain a stable microemulsion between naphtha and Triton X-100. Kumar et al. [3] and Cassela et al. [9] found the microemulsion to be stable for more than 1 h, while Meeravali et al. [8] reported 20 and 50 min as the maximum stability time for Ni and V determination in naphtha, respectively. Santos et al. [5] and Brandão et al. [7] used detergentless microemulsions, by mixing appropriate amounts of propan-1-ol, nitric acid aqueous solution and sample for Hg and Mn determination in petroleum products, including naphtha. The microemulsion was formed after manual shaking of the three components without requiring any further agitation. Brandão et al. [7] confirmed the Mn absorbance signal stability up to 18 days, which is consistent with the 30 day-period observed by Campos et al. [15] in a previous study for trace metal determination in gasoline. In GFAAS methods, the use of chemical modifier may be required to stabilize the more volatile species and prevent analyte losses during drying and pyrolysis steps. The use of permanent chemical modification of the graphite tube, through different coating procedures, have been widely tested and proposed [16]. Although some improvement in method sensitivity is commonly observed, such approach has limited application in a routine quality control laboratory. The permanent impregnation of the graphite tube implies this chemical modification is suitable for all metals and matrices. If such assumption cannot be confirmed, each application would require a different tube with specific pretreatment, demanding extra technician handling and therefore reducing productivity. Besides, a specific quality control routine must be established on the modifier coating to ensure the maintenance of sensitivity of the method within the acceptable range during the whole

tube lifetime. Finally, the employment of permanent modifiers usually requires the use of higher atomization and cleaning temperatures, leading to some deterioration in tube lifetime. For further improvement of GFAAS methods, multiple injection technique has also been applied to in situ preconcentration so that lower limits of detection could be reached, dispensing any additional sample pretreatment steps and speeding up the analysis [10]. From what has been mentioned, simple and accurate methods for metal determination in complex naphtha samples are needed and can be developed using the state of the art of some analytical methodologies. Therefore, the present work proposes the use of a widely available analytical technique (GFAAS) for development of the methods based on direct sample analysis and detergentless microemulsion formation. Simple and sensitive methods for trace metal determination were obtained without the use of permanent modifiers and low-stability detergent solutions. A critical comparative analysis between the two approaches has also been presented, providing a more practical rather than academic point of view. 2. Experimental 2.1. Instrumentation A Varian SpectrAA 220Z (Australia) Zeeman atomic absorption spectrophotometer equipped with a GTA 110 model graphite tube atomizer, a PSD 100 model autosampler and Varian hollow cathode lamps (Cu 327.4 nm, Fe 248.3 nm, Pb 283.3 nm, and Si 251.6 nm) were used for all the measurements. The device was set up according to the manufacturer's instructions and run in the single element mode. Pyrolytic graphite-coated tubes with a center-fixed platform (Varian, PN: 63-100026-00) were used. Argon 99.996% and Nitrogen 99.996% (White Martins, São Paulo, Brazil) were employed as a purge and protective gas, as mentioned later on. Further details on operational conditions are presented in Table 2. 2.2. Reagents and solutions All reagents were of analytical grade quality and freshly distilled and deionized water (electrical resistivity of 18.0 MΩ cm−1) was used when necessary. The working standard solutions of each metal were prepared by appropriate stepwise dilution of a 1000 mg L−1 (Cu, Pb, Fe: Merck, Darmstadt, Germany; Si: Conostan Oil Analysis Standards, Ponca City, USA) stock standard solution to the required μg L−1 levels just before use. A 99.99%m/m silicone (polydimethylsiloxane) fluid (PMX-200, Dow Corning Corporation) was also used as standard in the recovery test.

Table 2 Operational conditions for direct injection and microemulsion methods. Operational condition

Sample pretreatment Purge gas Measurement mode Wavelength (nm) Slit Width (nm) Lamp current (mA) Total volume (μL) Sample volume (μL) Modifier volume (μL) Modifier solution (200 mg L− 1 La in 0.1% v/v HNO3) Number of multiple injections Modifier mode Pre last dry step Injection rate

Direct injection methods

Microemulsion methods

Copper

Iron

Lead

Copper

Iron

Lead

Silicon

None Argon Peak area 327.4 0.2 10 40 40 –

None Argon Peak area 248.3 0.2 10 40 40 –

None Argon Peak area 283.3 0.2 10 45 40 5

Microemulsion Argon Peak area 327.4 0.2 10 30 30 –

Microemulsion Argon Peak area 248.3 0.2 10 50 50 –

Microemulsion Argon Peak area 283.3 0.2 10 45 40 5

Microemulsion Nitrogen Peak area 251.6 0.2 10 45 40 5

No 2 – 5 1

No 1 – – 1

Yes 2 Co-inject 5 1

No 1 – – 1

No 1 – – 1

Yes 1 Co-inject – 1

Yes 1 Co-inject – 1

M.V. Reboucas et al. / Fuel Processing Technology 91 (2010) 1702–1709 Table 3 Graphite furnace temperature programs for direct injection methods. Cycle

Drying

Pyrolysis

Atomization

Cleaning / cooling Cycle time (min)

Copper

Iron

T (°C)

Time (s)

50 90 115 122 40 180 1000 1000 1000 2300 2300 2300 2600 40

2.0 5.0 15.0 25.0 6.0 15.0 5.0 40.0 2.0 0.6 2.0 1.3 1.0 21.1

3.23

T (°C)

Lead Time (s)

T (°C)

Time (s)

50 90 115 122 180

2.0 5.0 15.0 25.0 15.0

850 850 850 2500 2500 2500 40

5.0 20.0 2.0 0.9 2.0 0.5 21.3

50 90 115 122 40 180 500 500 500 2000 2000 2000 50

2.0 5.0 15.0 25.0 6.0 20.0 5.0 20.0 2.0 4.8 2.0 1.8 19.0

2.93

Purge gas flow (L min−1) 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 0 0 0 3.0 3.0 3.0

2.13

Lanthanum oxide (La2O3) supplied by Merck (Darmstadt, Germany) was used to prepare the 200 mg L−1 La modifier solution in 0.1% v/v HNO3. The HNO3 solution was of ultrapure grade, prepared by diluting concentrated suprapure nitric acid, supplied by Merck (Darmstadt, Germany). Propan-1-ol (Merck, Darmstadt, Germany) was used as co-solvent and Braskem internal product known as mixed xylenes was used as the organic blank sample. The typical mixed xylene composition is 50%m/m ethyl benzene, 25%m/m m-xylene, 13%m/m p-xylene, 8%m/m o-xylene and 4%m/m other aromatic compounds. 99.9%m/m ethanol (Merck, Darmstadt, Germany) was used for apparatus cleaning. 2.3. Samples All naphtha samples were collected into clean bottles and stored in a freezer at 4 °C prior to analysis. The sampling apparatus was cleaned by nitric acid, deionised water and ethanol washings. The samples were collected from the plant's raw material storage tank or directly from carrier ship tanks. Typically, the collected samples presented distillation curves ranging from 30 to 200 °C and specific gravity values around 0.70. 2.4. General analytical procedure The analysis was performed by both methods in a quite similar approach, although different sample preparation steps were applied.

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In the direct injection method, the pure sample was placed in the autosampler vial right before the analysis, without any pretreatment. In the microemulsion method, an aliquot of 3.0 mL of sample was placed in 10 mL volumetric flasks and 1.0 mL of propan-1-ol and 600 μL of 0.1% v/ v HNO3 were added. Then, the volumes were made up with propan-1-ol and the mixtures were vigorously manually shaken. One-phase transparent and stable microemulsion was obtained. Blank solutions were prepared with a xylene mixture replacing the sample. All analytical solutions used organometallic standards and were prepared in the same way as described above in the microemulsion methods and by solubilization with the xylene solvent in the direct injection methods. In the case of direct injection method, the standard must be prepared and placed in the vial right before analysis, which must be performed within 40 min to avoid errors due to analyte decay, as it will be discussed later in this paper. The naphtha or microemulsioned naphtha sample was injected (30–50 μL) at a rate of ca. 2.3 μL s−1 into the graphite tube. When multiple injections technique was applied, analyte preconcentration took place before pyrolysis and atomization steps. The sample aliquots were introduced in the tube and the temperature was raised, according to the appropriate temperature program presented in Tables 3 and 4. Whenever the modifier was used, an aliquot of the solution was co-injected with the sample. After the temperature program was completed the total amount of metal was determined from the integrated absorbance signal. It is worthy to point out that the calculations using peak heights exhibited worse data precision, in spite of a slightly better sensitivity. 3. Results and discussion 3.1. Methods development and optimization The procedure of preparation of the microemulsion was optimized in a previous work of our group and therefore it was maintained in this study [14]. The microemulsion was prepared by mixing 3.0 mL of naphtha, 1.0 mL of propan-1-ol and 600 μL of 0.1% v/v HNO3 and the volume completed with propan-1-ol in 10 mL volumetric flasks. Such solution was found to be transparent, homogeneous and stable for a period of at least 6 h, as discussed in the following paragraph. This time period is longer than the time required to complete the entire analytical procedure. Studies on absorbance signal stability of the metal standards were performed and are presented in Fig. 1. The metal standards were prepared in xylene solvent and as microemulsion solutions, except for silicon which was only prepared as a microemulsion. The absorbance signal of the metal standard in xylene was followed until a drop of at least 50% of the initial signal was observed. For comparison, the metal

Table 4 Graphite furnace temperature programs for microemulsions methods. Cycle

Copper

Iron

Lead

Silicon

T (°C)

Time (s)

T (°C)

Time (s)

T (°C)

Time (s)

T (°C)

Time (s)

2.0 5.0 15.0 10.0 10.0 5.0 10.0 2.0 0.7 2.0 1.0 3.0

40 90 115 122 200 800 800 800 2400 2400 2400 2700

2.0 5.0 10.0 10.0 5.0 5.0 10.0 2.0 0.9 2.0 1.0 2.0

40 90 115 122 200 500 500 500 2000 2000 2000 2400

2.0 5.0 10.0 10.0 5.0 5.0 10.0 2.0 0.7 2.0 1.0 2.0

40 90 115 122 180 500 500 500 2850 2850

2.0 8.0 12.0 12.0 5.0 5.0 5.0 1.0 1.0 2.5

Cleaning /cooling

40 90 115 122 200 900 900 900 2300 2300 2300 2600

2850

3.0

Cycle time (min)

1.10

Drying

Pyrolysis

Atomization

0.92

0.91

0.96

Purge gas flow (L.min−1) 3.0 3.0 3.0 3.0 3.0 3.0 3.0 0 0 0 3.0 3.0

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Fig. 1. Absorbance signals of 50 μg L−1 metal standards prepared in xylene solvent and as a microemulsion (shown as the average of duplicates).

standard in microemulsion was then monitored in a similar period of time as the respective standard prepared in xylene. The maximum RSD defined by Horwitz (16.7%) for the 50 μg L−1 concentration level used for all standards was established as a criterion for evaluation of signal stability [17]. From Fig. 1, we can observe a fast and significative drop in the analytical signal for all metal standards in xylene solvent. A signal drop beyond the Horwitz limits was observed after approximately 40, 50 and 60 min for iron, lead and copper, respectively. After only 2 h the iron and lead signals were less than 50% of the original signal, while a similar drop for the copper standard was observed after 5 h. On the other hand, a good signal stability performance was observed for all metal standards prepared as microemulsion solutions. Iron and lead showed the best performance. Some signal variability was noticed on silicon, but probably more related to the poorer precision of such determination. A slight decrease on copper signal was also observed after 6 h, but it could be related to the graphite tube deterioration since more than 250 firings were achieved at this point. Therefore, whenever the direct injection method was used, immediate analysis after preparation of the metal standard in xylene was performed. It is worth emphasizing the signal drop observed for metal standards in xylene does not apply to the samples, since the metals present in the samples are stabilized as naturally-occurring organometallic species. The GFAAS temperature programs were developed and optimized from methods previously used in routine analyses and based on the large experience of this research team in petroleum products analyses. Key parameters such as pyrolysis and atomization temperatures were optimized for each analyte, following the univariate and classical approach. Firstly, the atomization temperature was kept high enough to guarantee a significative absorbance signal, based on manufacturer's manual and literature. The pyrolysis temperature was then randomly and increasingly varied reaching temperatures until no significative analyte signal was observed. The minimum pyrolysis temperature tested was 300 °C to ensure a suitable sample ashing. The atomization temperature was then randomly varied while setting the pyrolysis temperature at the optimum condition, i.e. as high as possible to improve sample matrix removal without analyte loss. The optimized pyrolysis and atomization temperatures for each metal and method are shown in Tables 3 and 4. Besides pyrolysis and atomization temperatures, the drying step proved to be a quite critical step in the temperature program, with noticeable influence on data repeatability. Naphtha composition is quite complex and comprises a mixture of both light and heavy compounds, which demands a very careful drying program optimization

to avoid both insufficient solvent evaporation and analyte losses. In addition, whenever the multiple injection approach was used, a cool down step (to 40 °C) was included to ensure the next aliquot would be delivered to the tube in a lower temperature, improving precision and atomization efficiency. Finally, for direct injection methods an additional and long cleaning step was included to ensure complete removal of sample and analyte residues and therefore minimize any memory effect. In order to achieve the minimum acceptable limits of detection, the technique of in situ preconcentration by multiple injections was applied. In direct injection mode, two successive injections were used for copper and lead. Single injections were used for all microemulsion methods, as presented in Table 2. The use of a chemical modifier was required for lead determination in both operation modes, due to the highly volatile nature of the analyte. Without a modifier, the lead determination would require to be performed at a much lower pyrolysis temperature, not sufficient for complete matrix removal. For silicon determination, the use of a chemical modifier was also recommended as mentioned later in this section. Lanthanum was chosen as the chemical modifier for this application since it is already in use in our routine for arsenic determination [10,11], and therefore, the same modifier solution could be applied for the other metals leading to better laboratory working flow. It is worthy to mention that the microemulsion method for silicon was subjected to specific adjustments. Recent results from interlaboratory studies conducted by and with some laboratories in Brazil showed some degree of divergence on silicon results. Besides, the possibility of naphtha contamination with some volatile silicon species demanded special care. Therefore, further improvements in silicon determination were required and implemented during the development of the microemulsion method. Firstly, argon was replaced by nitrogen as purge gas since it led to some signal enhancement, according to the experience of Amaro et al. [4]. The use of a chemical modifier was also introduced to reduce the risk of analyte loss, if different and more volatile silicon species than that used in the method development appeared in the sample. Finally, the pyrolysis temperature was reduced from the optimum temperature previously established down to 500 °C. A direct injection method for silicon had also been developed and validated together with the other metals. Due to the nature of the species present in the samples at the time the work was performed, the pyrolysis temperature was kept at 1200 °C and there was no need of a chemical modifier. However, as more volatile silicon species started occurring in naphtha samples, probably related to changes in the crude oil extraction and refining processes, this method did not prove to be robust enough to provide accurate results. Therefore, since the method described by Amaro and Ferreira [4] did not include a similar optimization as performed in the microemulsion method, it may not be able to provide accurate determination when volatile silicon species are present in naphtha samples. 3.2. Major figures of the proposed methods The analytical performance of each proposed method was evaluated following standard validation procedures. Results are shown in Tables 5, 6 and 7. The lowest detectable concentration of copper, iron and lead was between 1 and 3 μg L−1 in microemulsion mode while concentration levels below 0.6 μg L−1 were found in direct injection mode. Due to blank contamination and poorer detection the silicon detection limit for microemulsion method was much higher (26 μg L−1). Low characteristic concentrations were obtained in most cases, confirming the method's high sensitivity. Therefore, the limits of detection are thought to be mainly influenced by the precision in the measurement of the blank signal. The linearity over the typical concentration range of 0.5 to 100 μg L−1 was demonstrated. The analytical curves were obtained from at least five

M.V. Reboucas et al. / Fuel Processing Technology 91 (2010) 1702–1709

1707

Table 5 Analytical figures of merit of the direct injection methods. Figure of merit

Copper

Iron

Lead

Characteristic concentration or sensitivitya (μg L−1) Limit of detectionb (μg L−1) Limit of quantificationc (μg L−1) Characteristic mass (pg) Working range (μg L−1) Analytical curve equation Correlation coefficient (r) Concentration level for precision study (μg L− 1) Relative standard deviation—RSD (%) Maximum acceptable Horwitz RSD (%)

0.31 0.64 1.28 24.8 1.5–80 Y = 0.0144 X − 0.0042 0.9996 16.8 2.4 16.7

0.57 0.01 0.05 22.8 0.6–100 Y = 0.0077 X + 0.0031 0.9728 17.0 16.5 16.7

0.31 0.26 0.52 24.8 0.5–80 Y = 0.014 X + 0.0025 0.9995 4.5 5.7 23.7

a b c

Calculated from the analytical curve as the minimum detectable concentration for an integrated absorbance of 0.0044. 3sblank criterion based on 10 replicates of a sample at concentration level near blank. 6sblank criterion based on 10 replicates of a sample at concentration level near blank.

standards covering this range. The limits of detection and working ranges were suitable to the process requirements all metals, as demonstrated by sample typical results and process requirements presented in Table 8. Precision was evaluated from 20–25 replicates of a spiked naphtha sample for copper, lead and silicon since the naphtha samples available at the time of the precision study were quite near or below the limit of detection. For iron precision study a naphtha sample was used instead. The observed Cochran coefficient and the ANOVA F-test values were both below the critical values at the 95% confidence level. The data sets were then assumed to be homogeneous, and the overall relative standard deviation (RSD) was calculated. The obtained results were

found to be below the maximum acceptable RSD defined by the Horwitz criterion at each concentration level [17]. Accuracy was assessed from spiked naphtha samples. As shown in Table 7, good analyte recovery (86–114%) was obtained. Sample spiking was used for accuracy assessment since a standard reference material is not available in a suitable matrix such as naphtha, xylene, toluene or light oil at the working concentration range. Additionally, there is no standard method (e.g. ASTM) available for comparison for copper, iron, lead and silicon determination in naphtha at this concentration level. The silicon recovery tests (at 30 and 60 μg L−1) were performed with a silicone standard, since it is the most probable

Table 6 Analytical figures of merit of the microemulsion methods. Figure of merit

Copper

Iron

Lead

Silicon

Characteristic concentration or sensitivitya (μg L− 1) Limit of detectionb,c (μg L− 1) Limit of quantificationd,c (μg L− 1) Characteristic mass (pg) Working range (μg L− 1) Analytical curve equation Correlation coefficient (r) Concentration level for precision study (μg L− 1) Relative standard deviation (%) Maximum acceptable Horwitz RSD (%)

0.52 1.58 3.15 15.6 1.6–100 Y = 0.0085 X − 0.0108 0.9988 22.2 1.7 16.7

0.17 2.36 4.72 8.5 2.4–40 Y = 0.0265 X + 0.0192 0.9970 8.6 20.4 21.3

0.73 2.90 5.80 29.2 2.9–150 Y = 0.0060 X − 0.0025 0.9998 12.7 3.0 16.7

1.10 25.6 51.2 44.0 26–200 Y = 0.0042 X + 0.0589 0.9949 86.0 14.7 15.1

a b c d

calculated from the analytical curve as the minimum detectable concentration for an integrated absorbance of 0.0044. 3sblank criterion based on 10 replicates of a sample at concentration level near blank. calculated considering the dilution factor (3.33). 6sblank criterion based on 10 replicates of a sample at concentration level near blank.

Table 7 Metals concentration obtained in spiked naphtha samples. Metal

Copper

Iron

Lead

Silicon

a b c

Sample

Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha

Direct injection method

1 2 3 1 2 3 1 2 3 1 2 3

Microemulsion method

Added (μg L− 1)

Founda (μg L− 1)

Recovery (%)

Added (μg L− 1)

Founda (μg L− 1)

Recovery (%)

10 15 – 10 15 – 10 20 – – – –

10.23 ± 0.46 15.01 ± 0.37 – 11.4 ± 1.7 14.55 ± 2.1 – 11.21 ± 0.93 17.6 ± 1.4 – – – –

102 100 – 114 97 – 112 88 – – – –

10 20 30 10 16 20 10 20 30 28b 30c 60c

10.12 ± 0.25 21.17 ± 0.61 31.9 ± 1.1 9.4 ± 2.6 15.3 ± 1.1 17.3 ± 1.5 10.21 ± 0.64 20.2 ± 1.1 29.13 ± 0.91 27.0 ± 4.2 33.4 ± 2.8 64.9 ± 3.7

101 106 107 94 96 86 102 101 97 96 111 108

Confidence interval estimated from one standard deviation of three replicates. Conostan standard. Silicone standard.

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Table 8 Metal determination in naphtha samples. Sample or item

Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha Naphtha

1 2 3 4 5 6 7 8 9 10

Typical rangeb Specification or maximum recommended concentration a b c

Copper (μg L− 1)a

Iron (μg L− 1)a

Lead (μg L− 1)a

Silicon (μg L− 1)a

Direct injection

Microemulsion

Direct injection

Microemulsion

Direct injection

Microemulsion

Microemulsion

bLOD 11.4 ± 1.2 29.71 ± 0.95 5.77 ± 0.45 11.73 ± 0.88 ND ND ND ND ND

b LOD 9.71 ± 0.72 31.5 ± 2.3 6.45 ± 0.31 15.73 ± 0.72 ND ND ND ND ND

12.3 ± 3.7 10.5 ± 4.1 bLOD 47.3 ± 1.2 21.91 ± 0.99 ND ND ND ND ND

15.4 ± 4.6 8.1 ± 2.0 b LOD 40.7 ± 2.3 18.8 ± 1.2 ND ND ND ND ND

1.32 ± 0.25 3.50 ± 0.18 8.45 ± 0.57 5.31 ± 0.43 3.10 ± 0.51 ND ND ND ND ND

b LOD 4.11 ± 0.37c 7.71 ± 0.48 5.97 ± 0.38 2.45 ± 0.27 ND ND ND ND ND

ND ND ND ND ND b LOD 267 ± 31 116 ± 11 2290 ± 91 93.4 ± 9.2

2–20 10

b 2–10 20

5–60 300

5–300 100

Confidence interval estimated from the one standard deviation of three replicates. Obtained from a 5-month period and ca. 50 samples. Concentration between limit of detection and quantification.

source of silicon contamination of naphtha (once silicone is used as an anti-foaming additive in petroleum exploration). 3.3. Methods comparison Both approaches, using direct injection and microemulsion formation, proved to be valid and fit for the analytical purpose. However, singular differences and features have been observed. The procedures based on direct injection are less prone to contamination since no reagent is added, except the blank solvent used for standards preparation and the chemical modifier solution used for lead analysis. Besides, lower limits of detection could be achieved since no sample dilution is required. On the other hand, the main advantages of using microemulsion procedures rely on the higher sample throughput and sample/standards stability. Since a much lower organic load is used, the microemulsion GFAAS program is at least 58% faster than the respective direct injection program. As a result of using shorter furnace programs we can also observe an improvement in the graphite tube lifetime, leading to reduction of the overall analysis cost. As already pointed out by Campos et al. [15], the analyte stabilization by microemulsion formation is remarkable. Consequently, both standards and samples can be prepared and simultaneously placed in the autosampler and left for analysis. Such procedure cannot be applied in the direct injection mode because of the evaporative losses of the samples and the instability of metal standards in xylene. If the volatile naphtha sample is placed in the autosampler from the very beginning of the analysis run, a significative amount of the sample is evaporated from the vial, as already pointed out elsewhere [8,9]. The evaporative losses can be minimized by dilution with xylene, but this approach leads to deterioration of the limits of detection and quantification. Besides, as already discussed from the results shown in Fig. 1, accurate results from direct injection methods can only be obtained if the standard is analyzed immediately after preparation, due to a significant drop in the absorbance signal after 40–60 min. In microemulsion methods, the same standard solution can be used for at least 6 h without any significative impact in sensitivity and accuracy. As one would expect, since no dilution step is required in the direct injection approach, lower limits of detection and quantification were obtained compared to the microemulsion methods. On the other hand, lower characteristic masses, i.e. higher sensitivity, for copper and iron were achieved. Such apparent contradiction may be better understood through the evaluation of other parameters such as characteristic concentration and sample volume. Although the characteristic concentrations were in the same order of magnitude in both modes, the total sample volumes used in direct injection mode were much higher, except

for iron, leading to higher characteristic masses. The application of simpler and faster temperature programs in microemulsion methods, due to less organic sample content and total sample volume (no multiple injections), may also lead to less analyte losses and therefore better characteristic masses figures. The application of multiple injection technique was required to achieve low limits of detection but was also applied to ensure suitable measurement repeatability. In some cases, single injection would lead to lower absorbance signals and poorer precision, since the direct injection of pure naphtha samples would require a more elaborated and long temperature program even when a single aliquot was delivered into the graphite tube. The use of multiple injections was not introduced in the microemulsion mode so that the advantages of using faster methods were retained. However, it is known this is a straightforward approach that can be applied to improve the sensitivity and achieve lower limits of detection, if required. Finally, further improvements in the limits of detection of the microemulsion methods can be achieved using lower dilution factors, i.e. using higher sample volumes in the microemulsion formation. The slopes of the silicon standard addition curves were not significantly different when sample volumes of 3 mL (analytical curve equation: Y=0.0048x+0.291) or 5 mL (analytical curve equation: Y=0.0045x+ 0.113) were used. Similar behavior was observed for copper, iron and lead. 3.4. Analytical application Ten naphtha samples taken from different lots were analyzed for copper, iron, lead and silicon under the recommended analytical settings described in Tables 2–4. The results are shown in Table 8. The samples results were above the limits of detection, except for one determination of each metal. One could suggest the direct injection method would be more suitable for lead determination, since quite low levels of this contaminant are usually observed in naphtha samples. However, the limit of detection of the microemulsion method for lead is almost ten times below the required process specification, ensuring its fitness for this analytical purpose. Finally, the concentration level of silicon in some samples was quite high and therefore a proper dilution with blank xylene solution was used for these samples before microemulsion formation. 4. Conclusions Two sets of methods for trace metal determination in naphtha samples with and without sample microemulsion formation were developed using graphite furnace atomic absorption spectrometry. Successful techniques of in situ preconcentration with multiple

M.V. Reboucas et al. / Fuel Processing Technology 91 (2010) 1702–1709

injections and chemical modification by solution co-injection were used whenever necessary. Both modes are simple and do not require the sample to be subjected to any drastic or time-consuming pretreatment, such as concentrated acids heating or microwave digestion. The detergentless microemulsion methods showed singular advantages over the direct injection methods and therefore have been used for routine analyses in Bahia Basic Petrochemicals Unit Braskem Laboratory (Camacari, Bahia, Brazil). The direct injection methods were kept as backup methods, when lower detection limits were required or in case of suspect of reagents contamination. The analytical methods were developed using the more widely available GFAAS analytical technique, instead of ICP-MS. In addition, the chemical modifier was applied as a solution, instead of the commonly used and routine-troublesome permanent modifier approach. Finally, the use of detergentless microemulsion provided the long-stability standard solution required for accurate and routine-friendly methodology. Acknowledgements The authors are grateful to the Brazilian research funding agency CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), Braskem S. A. for supplying technical resources required for this work and Ann Marie Moreira for the final manuscript revision. References [1] S.C. Pandey, D.K. Ralli, A.K. Saxena, W.K. Alamkhan, Physicochemical characterization and applications of naphtha, J. Sci. Ind. Res. India 63 (2004) 276–282. [2] S.D. Olsen, S. Westerlund, R.G. Visser, Analysis of metals in condensates and naphtha by inductively coupled plasma mass spectrometry, Analyst 122 (1997) 1229–1234. [3] S.J. Kumar, S. Gangdharran, 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. [4] J.A.A. Amaro, S.L.C. Ferreira, Application of factorial design and Doehlert matrix in the optimisation of instrumental parameters for direct determination of silicon in naphtha using graphite furnace atomic absorption spectrometry, J. Anal. At. Spectrom. 19 (2004) 1–5.

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