Elsevier

Microchemical Journal

Volume 82, Issue 2, April 2006, Pages 137-141
Microchemical Journal

Determination of Mo, Zn, Cd, Ti, Ni, V, Fe, Mn, Cr and Co in crude oil using inductively coupled plasma optical emission spectrometry and sample introduction as detergentless microemulsions

https://doi.org/10.1016/j.microc.2006.01.005Get rights and content

Abstract

A procedure to prepare crude oil samples as detergentless microemulsions was optimized and applied for the determination of Mo, Zn, Cd, Si, Ti, Ni, V, Fe, Mn, Cr and Co by ICP OES. Propan-1-ol was used as a co-solvent allowing the formation of a homogeneous and stable system containing crude oil and water. The optimum composition of the microemulsion was crude oil / propan-1-ol / water / concentrated nitric acid, 6 / 70 / 20 / 4 w/w/w/w. This simple sample preparation procedure together with an efficient sample introduction (using a Meinhard K3 nebulizer and a twister cyclonic spray chamber) allowed a fast quantification of the analytes using calibration curves prepared with analyte inorganic standards. In this case, Sc was used as internal standard for correction of signal fluctuations and matrix effects. Oxygen was used in the nebulizer gas flow in order to minimize carbon building up and background. Limits of detection in the ng g 1 range were achieved for all elements. The methodology was tested through the analysis of one standard reference material (SRM NIST 1634c, Residual Fuel Oil) with recoveries between 97.9% and 103.8%. The method was also applied to two crude oil samples and the results were in good agreement with those obtained using the acid decomposition procedure. The precision (n = 3) obtained was below 5% and the results indicated that the method is well suited for oil samples containing low concentrations of trace elements.

Introduction

Crude oil represents a complex mixture containing both organic and inorganic chemical species, being trace metals one group the inorganic components present in this type of matrix. Information on trace element concentrations in crude oil is getting increasingly important for the geochemical characterization of source rocks and basins and also to allow corrective actions during crude oil processing [1]. Trace metals have been found in different proportions in different crudes and consequently in their derivatives. Frequently Ni and V are found in largest concentrations contributing to environmental pollution. Because of their mutagenic and carcinogenic potential Ni and V emissions have been strictly controlled in several countries [2]. In addition, V is a catalyst poison and causes corrosion in furnaces and boilers during oil processing. The knowledge of the concentration ratio between V and Ni in crude oil provides powerful geological information allowing oil–oil and oil–rock correlation and evaluation of the palaeoenvironmental conditions of sedimentation [3]. Other metals, such as Fe, Cu and Zn, may also be present in significant amounts. Chemical species of these metals can be partially transferred to fractions (fuels for instance), decreasing their quality and performance.

The American Society for Testing Materials (ASTM) [4] and the Institute of Petroleum (IP) indicate standard procedures to prepare crude oil derivatives previously to spectrometric trace metal determinations [5], [6]. Oil samples can be examined directly or after ashing of the sample. The direct determination is usually avoided due to the incompatibility of the sample with the instrumental apparatus employed for the analysis. In addition, the complexity of the matrix may affect the accuracy of the measurements. Procedures that require sample ashing are time consuming and the complete recovery of the analytes is often affected by vapor-phase loss due to the high volatility of some of the analyte chemical species. Open vessel acid decomposition, microwave acid decomposition in closed vessels and extraction methods involve intricate steps and are time consuming [7]. Alternatively, these water non-miscible organic samples can be prepared as emulsions or microemulsions. In this case, the hydrophobic phase is evenly dispersed in the aqueous phase as microdroplets stabilized in micelles or vesicles created by the addition of a detergent [8]. Detergentless microemulsions can also be used although it is a less common approach. In this case, a co-solvent allows the formation of a homogeneous and long-term stable three-component solution. The use of emulsions or microemulsions is a simple approach for sample preparation, making possible the use of simple inorganic standards for calibration when the emulsion or microemulsion is acidified with a mineral acid [8], [9], [10], [11]. Therefore, the traditional use of the expensive and unstable organometallic standards is avoided.

The determination of metals in crude oils and fractions can be achieved routinely using a number of spectrometric techniques such as flame (FAAS) [2], [12], [13], [14], [15] and electrothermal (ETAAS) [3], [7], [15], [16], [17], [18], [19] atomic absorption spectrometry, X-ray fluorescence spectrometry (XRF) [20], [21], neutron activation analysis (NAA) [22], inductively coupled plasma mass spectrometry (ICP-MS) [1], [23], [24], [25] and inductively coupled plasma optical emission spectrometry (ICP OES) [26], [27]. ICP OES is a mature and robust multielement analytical technique suitable to trace determination of many elements, in special the refractory ones. The cost of instrumentation also makes it an attractive technique when compared to ICP-MS, NAA and XRF. Because of the high viscosity, the analysis of crude oil using ICP OES generally requires some degree of sample preparation previous to the aspiration into the plasma. The direct dilution of sample in organic solvent may cause serious technical problems related to the overloading of the plasma with solvent vapors making it unstable by changing its physical characteristics and energy, and, consequently, affecting the overall performance of the technique [23], [28], [29]. Therefore, other strategies of sample introduction into the plasma are required.

In the present work, an ICP OES method was developed for trace-level determination of ten analytes (Mo, Zn, Cd, Ti, Ni, V, Fe, Mn, Cr and Co) in crude oil prepared as detergentless microemulsions. The acidification of these microemulsions allowed the use of inorganic standards for calibration (analytical curve with Sc as internal standard). A study was made to evaluate the stability of the analytes in detergentless microemulsion indicating a long term stability of signal. The right choice of the nebulizer system (Meinhard nebulizer with a twister cyclonic spray chamber) and optimized instrumental conditions guaranteed maximum analyte signal, accuracy and precision when analyzing one standard reference material (SRM 1634c) and two crude oil samples.

Section snippets

Instrumentation

A Perkin Elmer model Plasma P1000 inductively coupled plasma optical emission spectrometer (radial viewing) from Perkin-Elmer, Norwalk, CT, USA was employed for this study. This instrument is equipped with an Ebert type monochromator with a holographic grating of 2400 grooves mm 1 and a Rf generator of 27.12 MHz. A Meinhard TR-30-K3 concentric nebulizer (36 psi, Glass expansion, Hawthorn, AUS) was used together with a glass twister cyclonic spray chamber (Glass Expansion, Hawthorn, AUS) of 0.75

Optimization of detergentless microemulsions

Sample preparation is a crucial step for the establishment and application of a spectroanalytical method for the analysis of crude oils. The sample preparation mainly aims to reduce viscosity of sample in order to obtain a form compatible with the introduction system and atomizer. However, it is also desirable that preparation of sample to be simple and led to a uniformity of analyte chemical species in the sample, which minimizes potential interferences associated with the complex composition

Conclusions

Microemulsion sample introduction was found to be a very effective technique for ICP OES analysis of trace metals in crude oil. The major advantages of the microemulsion approach are: allowing access to the high sensitivity of ICP OES for petroleum analysis without requiring acid decomposition and allowing the use of stable inorganic standards. Microemulsion tolerates higher amounts of crude oil, minimizing the dilution factor when compared to more traditional sample preparation procedures such

Acknowledgements

R.M. Souza and A.L.S. Meliande thank CNPq for a scholarship grant. The skill and dedication of our laboratory technician M. Dupin are also acknowledged.

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