A novel approach of rare earth elements (REE) determination in crude oil is suggested. Special application of countercurrent chromatography (CCC) is used as a sample pretreatment tool. An oil sample is continuously pumped through the rotating coil column (RCC) as a mobile phase, while an aqueous phase (nitric acid solution) is retained as a stationary phase. Two phases are kept well mixed and agitated, but there is no emulsion at the interface under the chosen conditions. Special features of CCC give an opportunity to vary the volume of oil samples to be analyzed from 10 mL to 1 L or more. Trace metals are preconcentrated into 10 mL of stationary phase (acidic solutions) pumped out of the column so that analysis can be easily determined with inductively coupled plasma mass spectrometry (ICP-MS) without additional sample preparation procedures. Optimal concentration of nitric acid in the stationary phase for preconcentration of REE from oil by CCC has been investigated. The combination of CCC with ICP-MS gives the possibility to develop a rapid, reliable, and accurate method of trace metal including rare earth elements (REE) determination in crude oils and oil products. Such method could be an alternative for unexpanded and expensive neutron-activation analysis (NAA).
Microelemental and REE contents of oil are very important for estimating oil’s age and for developing theories of oil’s origin. It is known that information about hydrocarbons genesis could be obtained using element ratios. Such ratios also could be determinative as the georeconnaissance data. From this point of view, REE content/ratios are of the prime interest. REE ratios could be used as the reference points for different oils (from different deposits/fields) like for most geological samples as rocks, minerals, and ores [
In the meantime, REE analysis of oils is still a very complicated analytical task. There are no methods of preconcentration of metals from oil to aqueous phases, and REE contents are usually at ppt level and lower. Also there are no standard methods (ASTM D, IP, EN, or UOP) or standard/reference samples for REE analysis of oils. It should be mentioned that articles and studies dedicated to REE content in oils could be found really seldom. Thus, ICP-MS determination of REE in oils was published [
Suggesting hybrid method (CCC and ICP-MS) enables lessening detection limit (DL) and detecting most of metals in oil including REE.
Abilities of the liquid stationary phase retention during pumping of oil samples through column enable isolation and preconcentration of the inorganic impurities into acidic aqueous solutions. In this case, CCC can be used as a tool for the sample pretreatment and preparation. Preliminary investigations on retention features of oil/oil products—aqueous phase systems in CCC, were published [
The main characteristic of sample preparation methods is recovery values. It is preferable to compare recoveries of inorganic impurities from oil by extraction method and other instrumentation techniques. If that instrumentation method enables direct oil analysis, then comparison will be more reliable. Thus, NAA was used in this work. Aforesaid issue of standard/reference samples is a real problem for extraction/leaching methods as well. Therefore, relative contents of REE were mainly used.
Thus, regardless of the geological and tectonic structure of the source regions, oil composition, depth of deposit occurrence, and lithology of the enclosing rocks, the distribution curves of REE are usually highly differentiated. It has been shown also that total quantity of lanthanides (ΣLnN) has a very wide range from 0.5 to 5.8 mg/t (3.0 mg/t on average) in West Siberian and Kazakhstani oils. As an example, fluids of cooling magma pockets of increased alkalinity are the major source of lanthanides in oil. They are involved in oil enrichment with light lanthanides and formation of the positive Eu anomaly [
Main objective of this work was to estimate CCC-ICP-MS approach to REE determination of oils from different sources and to compare with the results obtained by NAA.
High-purity deionized water (resistivity 18.2 mΩ) obtained from a Milli-Q water purification system (Millipore, Canada) and ultrapure nitric acid HNO3 (Merck, Germany) for aqueous solution preparing and decomposition of oil samples and as a stationary phase has been used.
In the quantitative elemental analysis by ICP-MC, we prepared standard solutions by diluting multielement agilent standard A (Al, As, Ba, Be, Bi, B, Cd, Ca, Ce, Cs, Cr, Co, Cu, Dy, Er, Eu, Gd, Ga, Ho, In, K, Fe, La, Li, Lu, Mg, Mn, Na, Nd, Ni, P, Pb, Pr, Rb, Re, Sm, Sc, Se, Sr, Tb, Th, Tl, Tm, U, Yb, Y, V, and Zn 10
Calibration coefficients of determination for all elements were 0.9995–0.9999. Recovery tests were performed using “Trace metals in drinking water” solution (with 2–250
Experiments on trace and rare earth elements preconcentration were performed with light Kazakhstan (Tengizskaya) oil with density
Figure
Scheme of planetary centrifuge.
RCC experiments were performed with “Spring-3 M” J-type single-layer synchronous horizontal planetary centrifuge (made in the Institute for Analytical Instrumentation, St. Petersburg, Russia) with the total column volume of 19 mL at constant
The column was filled with aqueous HNO3 solution while stationary. Then the planetary centrifuge is set to rotate at constant speed, and the mobile phase (oil or diesel) was pumped through the column from the top of the column to the bottom or “Head to Tail” [
Working parameters of planetary centrifuge.
Mobile-phase volume | Stationary-phase volume | Centrifuge rotation speed | Beta ratio (spool radius/rotor radius) | Flow rate of mobile phase | Column tubing bore | Temperature |
19–110 | 10 | 750 | 0.65 | 0.5; 1.2 | 0.8 | 20 |
Toluene, iso-octane, and acetone of chemically pure grade were used for column rinsing. After every CCC experiment, 70–100 of each solvent were pumped through the stationary column.
To compare results of CCC elements isolation from oil with other techniques, autoclave decomposition of oil samples was used. Module “MKP-04” with autoclave NPVF “ANKON-AT-2” (Russia) was used for decomposition in a closed system. Standard conditions of autoclave decomposition are provided in Table
Conditions and working parameters of autoclaves decomposition of oil sample.
Sample mass | Acid volume | Time and temperature | ||
1 | 2 | 3 | ||
0.5 | 10 | 1 h—160°C | 1 h—180°C | 2 h—200°C |
ICP-MS measures most of the elements with very low (ppb or even ppt) DLs. Minimization of large interferences was provided with a simple matrix of analyzed samples (diluted HNO3 solutions), maximal compliance with the standards matrix, and optimization of the instrument parameters.
Performance of the ICP-MS instrument strongly depends on the operating conditions. Tuning solution containing Li, Y, Ce, Tl, Co (10
Measurement parameters such as carrier gas flow, torch position, speed of peristaltic pump, and dwell time were optimized with the aim of high sensitivity and low isobaric interferences. ICP-MS analysis was performed following the operating program and parameters shown in Table
Instrumental parameters for determination of metals by ICP-MS.
Forwarded power | 1200 W |
Analog stage voltage | −1800 V |
Pulse stage voltage | 1100 V |
Argon consumption | |
Plasma generating flow | 15 L/min |
Auxiliary flow | 0.8 L/min |
Carrier flow | 1.4 L/min |
Sample consumption | 0.5 mL/min |
Peak resolution | ~0.6 amu |
Detector operating modes | Pulse count and analog |
Speed of peristaltic pump | 24 rpm |
Spray chamber | Cooling (4°C) quartz |
Mass range | 27–238 amu |
Sweeps/reading | 40 |
Replicates | 3 |
Dwell time | 50 ms |
Scan mode | Peak hopping |
Abundance sensitivity | |
Low mass | <5·10−7 |
High mass | <1·10−7 |
All ICP-MS data (final concentrations in analyzed solutions, calibrations, tunes, statistics, etc.) were obtained from ICP-MS Top Agilent program pack.
Final REE concentrations (
Measurements were conducted at Central Laboratory of Substance Analysis of Vernadsky Institute of Geochemistry and Analytical Chemistry RAS. Analyzed oil samples of 10–50 mg were placed in a bag of aluminum foil 10 × 10 mm in size together with certified reference materials (ST-1A, effusive basalt; SGD-1A, intrusive basic rock; AN-G, intrusive basic rock). The bag is placed in an aluminum case and irradiated with a neutron flux of 1.2 × 1013 neutron cm−2 s−1 for 15 h. The samples are cooled for 6 days to reduce activity of radionuclides with
Application of CCC for REE recovery was tested on Tengizskaya oil. Table
Measured REE concentration in Tengizskaya oil by ICP-MS after CCC preconcentration from different sample volumes of Tengizskaya oil.
Element | Concentration, | ||
Y | |||
La | |||
Ce | |||
Pr | |||
Nd | <0.001 | <0.001 | <0.001 |
Sm | |||
Eu | |||
Gd | |||
Tb | |||
Dy | |||
Ho | |||
Er | |||
Yb | |||
Lu | <0.0005 | <0.0005 | <0.0005 |
It should be noted that concentrations of REE in oils are at very low level that cause difficulties of their direct detection.
There are comparative data of REE determination in 3 different oils: Tengizskaya, West Siberian, and composite in Table
Measured REE concentration in Tengizskaya, West Siberian, and composite oils by ICP-MS after CCC preconcentration.
Element | Concentration in oils, | ||
Tengizskaya | West Siberian | Composite | |
Y | |||
La | |||
Ce | |||
Pr | |||
Nd | <0.001 | ||
Sm | |||
Eu | |||
Gd | <0.0002 | ||
Tb | |||
Dy | |||
Ho | |||
Er | |||
Yb | |||
Lu | <0.0005 | <0.0005 | <0.0005 |
Comparative REE ratios of Tengizskaya, West Siberian, and composite oils obtained by ICP-MS after CCC preconcentration.
Oil | Elements ratios | |||
La/Ce | Nd/Sm | Eu/Gd | Dy/Ho | |
Tengizskaya | — | |||
West Siberian | — | |||
composite |
Relative REE distributions in Tengizskaya, West Siberian, and composite oil samples.
REE contents and rations of oils from 3 source regions are different (Figure
Therefore, NAA was used in this work as an instrumentation method that enables direct oil analysis (REE detection in the native forms). Only comparison with NAA could be applied as correctness criteria for combined CCC-ICP-MS method. Comparative REE ratios obtained via NNA and CCC-ICP-MS are presented in Table
Comparative REE ratios of Tengizskaya oil obtained by ICP-MS after CCC preconcentration and by NAA.
Method | Elements ratios | |||
La/Ce | Nd/Sm | Eu/Gd | Dy/Ho | |
NAA | ||||
ICP-MS | — |
It has been shown that REE contents and rations of oils from 3 source regions are different. Concentrations of REE are very low with ΣLnN ~0.2 mg/t in 3 oil samples studied. Composite oil has ΣLnN ~0.1 mg/t as diluted one with light fraction. Such ratios are usually used as geochemical certificates for oils. Comparative REE ratios obtained via NNA and suggested combined CCC-ICP-MS method are in close agreement. Correct REE ratios could be used for geochemical studies dedicated to oil genesis research.
The authors are grateful to Russian Foundation for Basic Research (RFBR) Project no. 09-03-00757.