Preparation of a reference material for crude oil trace elements: Study of homogeneity and stability

Highlights

• A candidate crude oil CRM was prepared.

• ICP OES, ICP-MS and AAS were used for the homogeneity and stability study.

• Homogeneity and stability study based on the determination of Ba, Ca, Mg, Na, Ni, Sr and V were carried out.

• Statistical analysis showed that candidate CRM met the requirements for the stability condition.

Abstract

Trace elements can be found naturally in crude oil or can be added during the production process, transport, or storage. Due to the complexity of the determination of trace elements in crude oil, which is mainly accomplished using multi-element techniques, many reports have been published describing the optimization of instrumental parameters, the development of procedures for sample preparation, and new sample introduction devices. The goal of such research is to improve the sensitivity, precision, and accuracy of the measurements. In all the cases mentioned above, the use of reference materials (RMs) and certified reference materials (CRMs) is recommended to verify the accuracy of the new methods proposed. This paper describes the development of a natural-matrix reference material for quantification of vanadium, nickel, magnesium, sodium, calcium, strontium, and barium in crude oil. 3 kg of crude oil was processed and distributed into 115 units, each containing 20 g. The homogeneity and stability of the prepared reference material were investigated using atomic absorption spectrometry (AAS), inductively coupled plasma optical emission spectrometry (ICP OES), and inductively coupled plasma mass spectrometry (ICP-MS) techniques. The results were evaluated using standard univariate analysis of variance (ANOVA), regression analysis, and principal component analysis (PCA). Statistical evaluation of the data showed good homogeneity and stability of the candidate reference material under the conditions studied.

Introduction

Trace elements are naturally found in oil, or are added during the recovery, transport, or storage processes [1,2]. The presence and quantity of some of these elements provide meaningful information about the origin, migration, and even maturation of the petroleum, and their concentrations may vary depending on the source rocks [3]. The presence of these elements in crude oil has been investigated since the 1950s [4,5] to the present day [6], [7], [8], [9]. These elements are routinely determined in petroleum refineries due to their significance in the quality and consequently the market value of the product [10].

Although there are some standardized methods for the determination of trace elements in crude oil using atomic spectrometry techniques, some of them use large amounts of samples and reagents or involve several steps, increasing the sample preparation time. Thus, there are many opportunities for improvement and the development of new methods. These developments have been driven by the principles of modern analytical chemistry, most specifically when it comes to green chemistry [11], [12], [13], [14], [15]. In all the studies cited above, it was necessary to use certified reference materials (CRMs) or reference materials (RMs) to verify the accuracy of the proposed new method. In 2012, Mello et al. [11] had already identified the lack of specific materials for crude oil and had been described the need for develop new CRMs for metals, nonmetals and for speciation analysis. However, it is often not possible to obtain or acquire an RM or CRM containing the required elements in a specific matrix, and in such cases, a similar material is often substituted. However, the use of materials with similar matrices may cause inadequate results due to differences in their behavior during sample preparation or measurements [11].

Most commercially available RMs and CRMs for trace elements in liquid fuels matrices involve gasoline, diesel, fuel oil, or lubricating oil matrix, although it is also possible to find standards with other oil matrices. Most are certified for the analysis of physical-chemical properties and for some trace elements, like sulfur, due to the economic and technological importance of this trace element. In the literature, reference materials from NIST (National Institute of Standards and Technology) such as SRM 1085b, 1084a, and 1634c are usually used to verify the accuracy of new methods for trace elements in crude oil [5,11,[16], [17], [18]]. Although these materials are commercially available, the acquisition cost, especially when it needs to be imported from another country, is still a difficulty for many research centers.

According to the COMAR database, a search for the term "Petroleum" returned about 94 registered on the database of CRMs. Of these, only 2 CRMs are related to trace elements in oil. Despite these CRMs produced in 1997 are available on the COMAR database, not found the product available for marketing in the producer's website (National Institute of Metrology, China), which justifies the use of reference materials related arrays in literature [19].

Reports in the literature describe the production of reference materials for trace elements in matrices such as gasoline and coal ash in research laboratories. An important advantage of the production of reference materials by research laboratories is their lack of commercial interest in the material produced, which is usually distributed to other laboratories. For example, Linsinger et al. [20] and Tanaka et al. [21] produced reference material with certified sulfur contents in the gasoline matrix and for the main constituent elements (Si, Al, Ca, Mg, Fe, K, Na, P, Sr, Ti, C, and S) and trace elements (As, B, Be, Cd, Co, Cr, Cu, F, Hg, Mn, Ni, Pb, Se, V, and Zn) in the ash matrix. Shehata et al. [22] have developed an oil reference material for the trace elements sulfur, iron, nickel, vanadium, and magnesium in a crude oil matrix.

The difficulty of obtaining reference materials in different matrices stems from the fact that the production of a CRM according to ISO 17,034 [23] is an expensive and time-consuming process that involves multiple steps and requires experience and special resources. Additionally, CRM manufacturers must consider the potential market demand before investing their resources and time in this type of development [24]. Two important steps in verifying the quality of a reference material during its production are homogeneity and stability studies. Homogeneity testing is an important requirement to ensure that each RM unit produced has the same properties. Stability testing is important to determine the storage conditions required for the material, as well as the conditions needed during the transport of the material from the producer to the laboratory [24,25].

Homogeneity and stability studies consume much of the producer's time and are crucial for the continuity of MRC development. There are several works dedicated to these studies, since they are the result of a successful preparation and that can be replicated in the next batches produced [26], [27], [28], [29]. Depending on the type of material, feasibility studies of the reference material are often required with the production of a test lot [24].

Therefore, the aim of this work was to prepare a reference material using a natural Brazilian crude oil matrix and study its homogeneity and stability with the goal of preserving its physicochemical properties and composition during the required period of stability. The trace elements studied were vanadium and nickel (most commonly found in crude oil) and the additional elements barium, calcium, magnesium, sodium and strontium (with chloride counterions), which are present in saline oils. The proposed material may fill a gap in reference materials for trace elements in crude oil from the pre-salt layer, the main characteristic of which is a significant amount of chloride counterions compared to other elements such as vanadium and nickel. The reference material was produced according to the international requirements and recommendations available in ISO 17,034 and the ISO Guides 31, 33 and 35. The steps for production involved: the selection of oil samples with the desired characteristics; the preparation and packaging of a homogeneous batch of the prepared material; and homogeneity and stability assessment, in which the elements of interest were determined using atomic spectrometric techniques.