Direct Sampling and Emulsion Analysis for Trace Metal Determination in Crude Oil


due to volatilisation, observed for the ‘light’ oils that contain a volatile fraction (light hydrocarbons), was minimised by keeping the temperature in the laboratory around 20–25°C. The strategy we used to weigh such samples was to record the first weight, not considering the weight loss with time. The error of such procedure was evaluated and was not higher than the errors observed for solid samples, being appropriate for analysis. After a careful optimisation of the heating programme of the GFAAS, it was possible to use aqueous standards for calibration. The use of palladium as chemical modifier allowed the differential determination of volatile and non-volatile compounds in a similar way, as discussed above for emulsion analysis.9


The results were


compared with those obtained by HR-CS GFAAS by emulsion technique; no statistical differences were found for total nickel and vanadium at the 95% confidence level.


Lead was also determined in crude oil by SS-GFAAS,10 using the


same approach discussed for nickel and vanadium. The difference in this case was that only the total lead was determined, using conventional Pd + Mg modifier. The results obtained for several crude oil samples analysed were in agreement with those found with emulsion sampling using FF and acid digestion (discussed above). The results show that the SS-GFAAS technique was very simple, fast and provided accurate results using aqueous standards for calibration.


Final Remarks


Sample pre-treatment is probably the most critical stage of crude oil analysis and consists of submission of the sample to a procedure in which it becomes more appropriate for analysis. Currently there is a trend for methods that avoid intensive sample manipulation, such as emulsion formation, and also allow the use of aqueous standards for calibration instead of the more unstable and expensive organic standards. Besides, it has the advantage that an autosampler can be used for easy sample introduction. The use of SS-GFAAS for analysis of crude oil has the clear advantage of providing the fastest results, as no sample preparation at all is involved, avoiding the use of organic solvents and other reagents, which might be considered a contribution to green chemistry.


1.


Hardaway C, Sneddon J, Beck JN, Determination of metals in crude oil by atomic spectrometry – a review, Anal Lett, 2004;37:2881–99.


2. Welz B, Sperling M, Atomic Absorption Spectrometry, Third edition, New York: Wiley-VCH, 1999.


3.


Korn MGA, dos Santos DSS, Welz B, et al., Atomic spectrometric methods for the determination of metals and metalloids in automotive fuels – a review, Talanta, 2007;73:1–11.


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Sebor G, Lang I, Kolihova D, Weisser O, Effect of the type of organo-metallic iron and copper-compounds on the determination of both metals in petroleum samples by flame atomic-absorption spectroscopy, Analyst, 1982;107:1350–5.


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Duyck C, Miekeley N, da Silveira CLP, et al., The determination of trace elements in crude oil and its heavy fractions by atomic spectrometry, Spectrochimica Acta Part B, 2007;62:939–51.


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Vale MGR, Damin ICF, Klassen A, et al., Method development for the determination of nickel in petroleum using line-source and high-resolution continuum-source graphite furnace atomic absorption spectrometry, Microchem J, 2004;77:131–40.


7.


Márquez N, Ysambertt F, De la Cruz C, Three analytical methods to isolate and characterize vanadium and nickel


9.


Regarding the methods proposed by our group for nickel and vanadium determination, both methods are simple, fast and reliable, being appropriated for routine analysis. Although the direct method using SS technology would be preferred because of its simplicity, speed and commercial availability, emulsion analysis should be preferred for samples with high analyte concentrations because of the dilution possibility. With the use of the HR-CS GFAAS technique it was possible to detect the loss of volatile compounds and to propose methods that allowed the differential determination of volatile and


A variety of analytical techniques and procedures have been proposed for quantification of metallic elements in petroleum products.


non-volatile compounds of nickel and vanadium without any chromatographic separation. For lead determination in crude oil both techniques investigated,10


emulsion in FFAAS or direct SS-GFAAS,


provided accurate results, so that both were suitable for the purpose. The higher sensitivity of FFAAS, however, might be of advantage for the analysis of petroleum derivatives with significant lower lead content, but the filter tubes are not yet commercially available. Another important advantage obtained in all proposed methods was the possibility of using aqueous standards for calibration and the conventional Pd + Mg modifier applied over the sample.


Besides the several elements related in the literature, it might be expected that the application of these techniques could be extended for the determination of other trace elements in crude oil. n


Acknowledgments


The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Nível Superior (CAPES) for financial support and scholarships.


porphyrins from heavy crude oil, Anal Chim Acta, 1999;395:343–9.


8.


Lepri FG, Welz B, Borges DLG, et al., Speciation analysis of volatile and non-volatile vanadium compounds in Brazilian crude oils using high-resolution continuum source graphite furnace atomic absorption spectrometry, Anal Chim Acta, 2006;558:195–200.


Damin ICF, Vale MGR, Silva MM, et al., Palladium as chemical modifier for the stabilization of volatile nickel and vanadium compounds in crude oil using graphite furnace atomic absorption spectrometry, J Anal At Spectrom, 2005;20:1332–6.


10. Damin ICF, Dessuy MB, Castilhos TS, et al., Comparison of direct sampling and emulsion analysis using a filter furnace for the determination of lead in crude oil by graphite furnace atomic absorption spectrometry, Spectrochimica Acta Part B, 2009;64:530–6.


11. Katskov DA, Graphite filter atomizer in atomic absorption spectrometry, Spectrochim Acta Part B, 2007;62:897–917.


12. Vale MGR, Oleszczuk N, Santos WNL, Current status of direct solid sampling for elctrothermal atomic absorption spectrometry—a critical review of the development between 1995 and 2005, Appl Spectrosc Rev, 2006;41:377–400. 13. Welz B, Borges DLG, Vale MGR, Heitmann U, Progress in


HYDROCARBON WORLD – VOLUME 6 ISSUE 1


direct solid sampling analysis using line source and high-resolution continuum source electrothermal atomic absorption spectrometry, Anal Bioanal Chem, 2007;389:2085–95.


14. Kurfürst U, Solid Sample Analysis – Direct and Slurry Sampling Using ET AAS and ETV-ICP, Berlin: Springer, 1998.


15. Brandão GP, Campos RC, Castro EVR, Jesus HC, Direct determination of nickel in petroleum by solid sampling- graphite furnace atomic absorption spectrometry, Anal Bioanal Chem, 2006;386:2249–53.


16. Brandão GP, Campos RC, Castro EVR, Jesus HC, Determination of copper, iron and vanadium in petroleum by direct sampling electrothermal atomic absorption spectrometry, Spectrochim Acta Part B, 2007;62:962–9.


17. Dittert IM, Silva JSA, Araujo RGO, et al., Direct and simultaneous determination of Cr and Fe in crude oil using high-resolution continuum source graphite furnace atomic absorption spectrometry, Spectrochim Acta Part B, 2009;64:537–43.


18. Silva MM, Damin ICF, Vale MGR, Welz B, Feasibility of using solid sampling graphite furnace atomic absorption spectrometry for speciation analysis of volatile and non-volatile compounds of nickel and vanadium in crude oil, Talanta, 2007;71:1877–85.


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