Direct Sampling and Emulsion Analysis for Trace Metal Determination in Crude Oil Emulsion Analysis
The formation and analysis of emulsions or microemulsions instead of a dilution of the crude oil and derivatives with an organic solvent has been proposed for GFAAS, ICP-MS and ICP-OES.1,3,5,6 This technique have been successfully applied for the preparation of fuel samples, due to the homogeneous dispersion and stabilisation of the oil micro-droplets in the aqueous phase, which brings the viscosity close to that of an aqueous solution and reduces the organic load of the system.3
Besides, it allows the use of
aqueous standards for calibration instead of expensive and instable organometallic standards. Better stability for a number of elements in standards and emulsified samples has been reported in all papers.
Our group has carefully investigated the determination of nickel and vanadium in crude oil using the emulsion technique. The oil-in-water emulsion was prepared by weighing up to 2g of crude oil sample, diluting with 1ml of xylene, then adding 100µl of Triton X-100 and completing the volume of 10ml with distilled and deionised water. The analyte signal for both the standard and the crude oil sample remained stable for at least 24 hours, making this sample preparation 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 meant that no unattended operation was possible.6
Using high-resolution continuum source (HR-CS) GFAAS, particularly because of the much higher diagnostic information available with this technique and its superior background correction capability, it was found that up to 50% of the nickel from crude oil samples might be lost during the pyrolysis stage already at temperatures above 400°C, whereas the rest of the nickel was thermally stable up to about 1,400°C.6
Based on literature data7 it has been
assumed that it was the non-polar porphyrin complexes that were lost, whereas the highly polar non-porphyrins were retained in the graphite tube to high temperatures. The same behaviour was also found for vanadium.8
The unsurpassed background
correction capability of HR-CS GFAAS allows pyrolysis temperatures as low as 300°C to be used even with such a complex matrix as crude oil.
Hence, total nickel and vanadium have been determined using a pyrolysis temperature of 400°C, and ‘thermally stable’ compounds using a pyrolysis temperature of 800–1,000°C. The volatile fraction, which is largely nickel and vanadyl porphyrin complexes, was calculated by difference as shown in Figure 1.
For the application of this method in conventional GFAAS, the use of palladium as a modifier has been proposed to stabilise the volatile compounds. A mass of 20µg of palladium, introduced into the graphite tube and thermally pre-treated prior to the injection of the emulsion, efficiently prevented any low-temperature losses of nickel and vanadium from crude oil samples up to pyrolysis temperatures of 1,200 and 1,450°C, respectively.9
In this approach,
the total nickel and vanadium were determined using palladium and the ‘thermally stable’ compounds were determined without a modifier at the same pyrolysis temperature, as shown in Figure 2 for vanadium. The advantage of this procedure was that a kind of
HYDROCARBON WORLD – VOLUME 6 ISSUE 1
Figure 1: Pyrolysis Curves for a 2.6ng Ni Standard Solution and a Crude Oil Sample as Oil-in-water Emulsion, Respectively, Using High-resolution Continuum Source Graphite Furnace Atomic Absorption Spectrometry
2.0 Total Ni 1.6 Volatile Ni 1.2 Stable Ni 0.8 0.4 0.0 200
400 600 800 1,000 1,200 1,400 1,600 1,800 Crude oil sample
Temperature/ºC 2.6ng Ni standard solution
Figure 2: Pyrolysis Curves for Vanadium in Emulsified Crude Oil, Without Modifier and With 20μg Pd as Modifier, Respectively, Obtained by Graphite Furnace Atomic Absorption Spectrometry
0.5 Total V 0.4 Volatile V 0.3 0.2 0.1 0.0 600 900 20µg Pd 1,200 1,500 Temperature/ºC Without modifier
fractionation/speciation was obtained with a relatively simple sample preparation and without any chromatographic pre-separation. It should be stressed that if the presence of volatile compounds is not taken into consideration, even using relatively low pyrolysis temperatures, GFAAS might actually be a source of severe systematic errors.
Our group has also investigated the determination of lead in crude oil using emulsion analysis.10
In this case a transversely heated
filter furnace (FF), shown in Figure 3, was used. There are only a few examples of the use of FFAAS in the literature, and they are summarised in a recent review by Katskov.11
The FF has several
advantages for the analysis of organic solutions, such as: (i) analysis of volatile elements is possible without addition of chemical modifiers; (ii) improved sensitivity and LOD because of the reduced absorption volume and the possibility to inject higher sample volumes; and (iii) background absorption should not play an
55 1,800 2,100 Stable V
Integrated absorbance/s
Integrated absorbance/s
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