Direct Sampling and Emulsion Analysis for Trace Metal Determination in Crude Oil
Figure 3: Transversely Heated Furnace Filter Consisting of Filter (Carbon Fibre) Inserted into the Conventional Graphite Tube
The results obtained in our work10 show that the FFAAS provided
twofold better sensitivity, with LOD in the range of ng/g and better relative standard deviation (RSD) than the conventional tube. As there is no crude oil or similar reference material available with a certified value for lead, an open-vessel digestion with sulphuric acid, nitric acid and hydrogen peroxide at 200°C for about 24 hours was adopted for validation purpose. The results obtained did not differ statically from the values obtained by FFAAS; this shows that the proposed method provided accurate results although it is very simple and uses aqueous standards for calibration.
Direct Sampling
Direct analytical techniques are obviously preferred over those that require significant sample preparation, particularly in the case of complex samples such as crude oil. Direct solid sampling (SS) GFAAS has been shown to be an extremely powerful technique that, after careful optimisation of the conditions, is essentially free of interferences and often allows calibration against aqueous standards.12,13
The use of A = conventional graphite tube; B = carbon fibre filter.
Figure 4: Accessories for Direct Solid Sampling Electrothermal Atomic Absorption Spectometry
A
SS-GFAAS has been increasing in recent years mainly because rugged and reliable accessories for direct SS-GFAAS became available in the late 1990s.12
autosampler for SS-GFAAS.
In the petroleum industry there is increasing interest not only in total trace element content but also in speciation analysis.
B
The main advantages of this technique are: (i) the extremely high sensitivity, as no dilution at all is involved; (ii) the minimum risk of contamination, as only a minimum of sample handling is involved; (iii) results are available within a very short period of time; and (iv) essentially no reagents are used. The only disadvantage, which is common to all SS techniques, is the relatively high uncertainty, which is typically around 5–20% RSD, due to the heterogeneity of natural samples and the small amount of sample, typically around 1mg, that is introduced into the atomiser.14
Crude oil, in spite of its often extremely high viscosity, is obviously not a solid sample; however, this does not exclude it from being analysed using SS techniques. Several papers appeared recently applying SS-GFAAS techniques for direct determination of metals in crude oil and petroleum products. Brandão et al. described procedures for the direct GFAAS determination of nickel,15,16 iron and vanadium,16
copper, losses of volatile compounds. Dittert et al.17
investigating the use of modifiers to prevent described a procedure
for the simultaneous determination of chromium and iron in crude oil without sample preparation using HR-CS GFAAS.
A: Manual system SSA 5, consisting of a pre-adjusted pair of tweezers for reproducible insertion of the solid sampling (SS) platform; B: SS autosampler SSA 62 for automatic transfer of up to 62 SS platforms to a microbalance and into the graphite furnace (courtesy of Analytik Jena AG, Jena, Germany).
important role in the vaporisation of hydrocarbon blends due to the filtering action of the furnace. However, this technique is not commercially available.
56
Our group proposed the differential determination of volatile and non-volatile compounds of nickel and vanadium in crude oil using SS-GFAAS.18
Figure 4 shows a picture of a manual and a fully automatic
In this method, the crude oil samples were weighed directly onto solid sampling platforms using a microbalance and introduced into a transversely heated solid sampling graphite tube using a manual solid sampler. The small drop in weight with time
HYDROCARBON WORLD – VOLUME 6 ISSUE 1
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