Article pubs.acs.org/ac
Fast and Precise Method for Pb Isotope Ratio Determination in Complex Matrices using GC-MC-ICPMS: Application to Crude Oil, Kerogen, and Asphaltene Samples Georgia Sanabria-Ortega, Christophe Pécheyran,* Sylvain Bérail, and Olivier F.X. Donard LCABIE, IPREM UMR 5254, CNRS, Université de Pau et des Pays de l’Adour, 64053, Pau cedex 9, France S Supporting Information *
ABSTRACT: A new method to determine Pb isotope ratio without ion-exchangematrix separation is proposed. After acid digestion, Pb was ethylated to Et4Pb, separated from the digested solution (black shale, asphaltene, crude oil and kerogen) by extraction in isooctane, and then injected into a gas chromatograph coupled to a multicollector inductively coupled plasma mass spectrometer. Seven isotopes (202Hg, 203 Tl, 204Pb, 205Tl, 206Pb, 207Pb, 208Pb) were monitored simultaneously with peak duration of 23 s. GC elution was operated under wet plasma conditions where a thallium standard solution was introduced to the mass spectrometer for mass bias correction. The total time of the procedure (sample preparation and analysis, after acid digestion) was reduced by a factor of 15 compared to conventional-continuous sample introduction. Data treatment was carried out using the linear regression slope method. Mass bias was corrected using the double correction method (first thallium normalization followed by classical bracketing). For the 208/206Pb and 207/206Pb ratios, precision (2RSDEXT, n = 21) was 49 and 69 ppm, and the bias between experimental results and reference values was better than 0.0033 and 0.0007 ‰, when injecting 1.2 ng of ethylated Pb SRM NIST 981 solution. Results obtained by this method were validated by comparison with those obtained via conventional-continuous sample introduction. The applicability of this approach was demonstrated with the analysis of black shale, asphaltene, crude oil and kerogen samples.
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elsewhere using a desolvator nebulizer.16 This was attributed to possibility of organic compounds remaining on the membrane of the desolvating nebulizer and producing erratic release of Pb and Tl. An advantage of MC-ICPMS over TIMS is the versatility to couple it to different sample introduction devices,17 such as HPLC,18,19 GC,20−22 laser ablation,23,24 flow-injection,13 cold vapor,25,26 or gold trap.27 Because the isolation is carried out online, using hyphenated techniques like GC or HPLC allows the isotopic analysis of Pb species without previous matrix separation. However, the main challenges of isotope ratio measurements using transient signals (when using GC or HPLC) are (i) the short time for data collection, (ii) the isotope ratio drift during analyte transient passage, and (iii) the complexity of the data treatment. Some authors have tried to explain the drift observed during peak elution.20,28,29 Recently Dzurko29 confirmed the results previously obtained by Krupp and Donard,20 which demonstrated that it is not the changes in instrumental mass bias, chromatographic fractionation, background signal drift, or the analyte concentration that are the source of isotope ratio drift.
b isotopes have been widely used in geology and geochronology for dating minerals and rocks and are of great interest in geological ore prospection.1 In the petroleum industry, Pb isotope ratio determination has a potential for dating hydrocarbon migration in diverse geologic environments2 and to make a correlation between crude oil and derivates as age-sensitive and genetic markers.3 TIMS is one of the reference techniques used for Pb isotope ratio determination together with MC-ICPMS.4,5 However the MC-ICPMS technique has gained popularity in analytical chemistry and geosciences for Pb isotope ratio determination. Because of improved ionization efficiency, the analysis is free from time-dependent instrumental mass fractionation6 (though instrumental mass bias due to interface is higher), and the time of measurement is shorter. Although there are some applications for the precise isotope ratio determination of Pb without matrix separation7−9 (e.g., environmental samples), quantitative isolation of the target element is always required prior to the isotope ratio measurement. Classically, prior to TIMS or MC-ICPMS analysis, Pb is isolated from the matrix by means of ionic exchange resins, such as Sr.Spec resin,1,5,10−12 Pb.Spec resin,13 or AG1-X8 resin,14,15 procedures well studied for geological and environmental samples, like rocks or lichens. However, for crude oil samples, after Pb purification with the resin AG1-X8, the instability of the Tl signal (used as an internal standard) and to a lesser extent the Pb signal have been reported © 2012 American Chemical Society
Received: June 12, 2012 Accepted: July 30, 2012 Published: July 30, 2012 7874
dx.doi.org/10.1021/ac301251b | Anal. Chem. 2012, 84, 7874−7880
Analytical Chemistry
Article
In addition, they support the hypothesis that the drift is produced by the Faraday cup/amplifier designs. Peak−apex, peak area integration or average isotope ratio within the peak are the conventional methods used to calculate the isotope ratio in transient signals.21,22 A new method, the linear regression slope method (LRS), first published by Fietzke23 for Sr isotope ratio measurement by LA-MC-ICPMS in carbonates and then by Epov30 to measure isotope ratios in different species of Hg by GC-MC-ICPMS, improves the accuracy and precision in comparison with the conventional calculation methods. Besides, this method allows the use of the bracketing mass bias correction method, when the standard and the sample have different concentrations, and allows the measurement of isotope ratios for low abundant isotopes. In this work an easy and fast procedure to separate Pb from the matrix is proposed. Pb is ethylated with NaBEt4, the PbEt4 produced is extracted in isooctane, achieving a matrix separation and lead preconcentration, and then the extract is injected into a gas chromatograph coupled to MC-ICPMS where a second Pb separation/isolation is produced. The method was used for the isotopic analysis of crude oil, kerogen and asphalthene samples.
Table 1. Instrumental Operating Conditions Used for the GC-MC-ICPMS Coupling Nu MC-ICPMS (GC coupling) plasma gas flow rate (L min−1) auxiliary flow rate (L min−1) RF power (W)
13
spray chamber
cinnabar
0.8
low
nebulizer
microconcentric, selfaspiration, 200 μL min−1 19
instrument resolution accelerating voltage (V) measurement time per sample (s) TRA integration time (s) interface cones
1300
nebulizer pressure (psi) 0.4 Ar makeup gas flow rate (L min−1) isotopes (monitored on Faraday cups): 208Pb, 202 Hg focus GC injector injection volume (μL)
■
EXPERIMENTAL SECTION Instrumentation. A Nu Plasma HR MC-ICPMS (Nu instrument, Wrexham, U.K.) was used for isotope ratio measurement. This instrument was coupled with a gas chromatographic system (Focus GC, Thermo Scientific, Milan, Italy) by means of a commercially produced doubleinlet torch, allowing the simultaneous introduction of a 200 ng mL−1 Tl solution via a 200 μL min−1 self-aspirating microconcentric nebulizer coupled to a cinnabar spray chamber. Three microliters of the preadjusted solutions containing 400 ng mL−1 PbEt4 (i.e., 1,2 ng as Pb) were injected into the GC-MC-ICPMS. Though the limit of detection for a given Pb isotope was estimated to be 65 fg, this concentration was selected as it produced a signal intensity of ∼18 V on mass 208Pb (at the top of the peak), which is close to the maximum intensity signal recordable on the Faraday cup (20 V), this maximizes counting statistics. Operating conditions are summarized in Table 1. The PbEt4 transient signal was measured in time-resolved analysis mode (TRA). To validate this method, total Pb was measured in two different ways. The first method called here “wet plasma”, consists in nebulization of a 200 ± 10 ng mL−1 Pb solution using a micro concentric nebulizer (Micromist 200 μL min−1, Glass Expansion) combined with a mini-cyclonic spray chamber (Cinnabar, Glass Expansion). Under these conditions a maximum intensity signal of 5.65 V on mass 208 was measured. The second method, called here “dry plasma”, was carried out using a DSN desolvating nebulizer system from Nu Instruments (Wrexham, U.K.). By producing a dry aerosol, this system significantly improved the signal sensitivity of the ICPMS in such a way that solutions of 20 ± 1 ng mL−1 of Pb produced a signal intensity of 6.20 V on mass 208. Acquisition conditions for dry and wet plasma configurations were: integration time 10 s, number of cycles 20, number of blocks 3. Sequence of analysis and time of measurement, by the three methods, are described in Table 2. Correction for isobaric interference on mass 204 was unnecessary, since Hg species do not elute at the same time as PbEt4.22 In all our samples, we never found any evidence of mercury species on our chromatograms (monitored using
injector temperature (°C)
column
He carrier gas flow rate (mL min−1)
906 0.5
Ni wet plasma cones (type A) 207 Pb, 206Pb, 205Tl, 204Pb, 203Tl,
split/splitless 3
250
MXT-1, 30 m, 0.53 mm i.d., (1 μm 100% dimethyl polysiloxane)
25
6000
GC temperature program initial temperature (°C) initial time (min) ramp 1 (°C/min)
60
final temperature 1 (°C) hold time (min)
95
ramp 2 (°C/min) final temperature 2 (°C) hold time (min)
60
2 60
5
250 1
202
Hg). Isotope ratios were calculated by the linear regression slope method23 (LRS). The slope of the simultaneously measured intensities represents their isotope ratio. The uncertainty of the slope represents the internal precision of the single measurement of isotope ratio. Data reduction was done using the LINEST function of Microsoft Excel23 which calculates the statistics for a line by using the “least-squares” method. Mass bias correction was carried out for both transient and continuous signals, by means of a double mass bias correction method recently reported by Sanabria-Ortega et al.,16 to improve accuracy and precision. It is based on an internal mass bias correction using Tl normalization followed by a second external mass bias correction using the standard bracketing method. To assess the Pb recovery after ethylation during the optimization step, tetraethyl-lead was quantified by coupling an inductively coupled plasma mass spectrometer X-SERIES 2 (Thermo Scientific, USA) to a gas chromatography system (CGC, Focus series, Thermo Scientific, USA). Instrumental operating conditions where the same as used for butyltin species.31 Sample Preparation Procedure. All sample preparation and measurements were carried out in an ISO 7 clean room. 7875
dx.doi.org/10.1021/ac301251b | Anal. Chem. 2012, 84, 7874−7880
Analytical Chemistry
Article
Table 2. Sample Analysis Sequence and Acquisition Time for the “Dry”, “Wet”, and GC Methodsa analysis sequence for a whole bracketing under “dry” and “wet” plasma conditions: wash + blank + NIST981 + wash + blank + sample + wash + blank + NIST981
wet dry
wash
blank uptake
3.3 10−120b
2 3
GC
blank measurement
NIST981 (or sample) uptake
NIST981 (or sample measurement)
total time for a whole bracketing
sample preparation time (hours)c
0.3 2 10 53 0.3 3 10 79−189b analysis sequence for a whole bracketing under GC conditions: NIST981 + sample + NIST981
minimum 18 minimum 18
NIST981
sample
NIST981
total time for a whole bracketing
sample preparation time (min)c
15.1
15.1
15.1
46
30
a
Time is expressed in minutes (min). bWashing time usually required to recover blank level after measuring a crude oil sample. cAfter mineralization and acid evaporation.
Figure 1. (a) Typical chromatogram showing the sequence analysis of a standard (NIST 981) and a sample. (b) Elution peaks of different isotopes of PbEt4. (c) Signal response of m/z = 208 vs m/z = 204, where the slope of the straight line gives the isotopic ratio 208Pb/204Pb = 37.70391.
then redissolved in 2.5 mL of 2% HNO3. In the case of lowconcentration samples (e.g., crude oils), the residue was redissolved directly in 5 mL of the buffer solution. For asphaltene analysis, the same procedure was followed, but only ∼0.1 g of sample was digested. In the case of kerogen, 0.1 g of sample was quantitatively weighed in Teflon vials, then 6 mL of HNO3, 2 mL of HF and 2 mL of H2O2 were added and samples digested in a microwave digestion system (Ethos Touch Control, Milestone). A ramp of 20 min to increase the temperature from ambient to 200 °C was used, followed by a dwell time of 20 min. For PbEt4 derivatization, an aliquot of the Pb standard solution (SRM 981 at 8 μg mL−1) or the sample was put in a 15 mL polypropylene centrifuge tube, then 5 mL of the buffer solution were added, the pH was adjusted to 4.9 with ammonium solution or HCl. Then, 2 mL of isooctane were added, followed by 0.5 mL of 1% NaBEt4 (freshly prepared in Milli-Q water). The solution was manually shaken for 5 min, and the isooctane supernatant was transferred to 2 mL GC vials
Details of standards, reagents and materials used in this work are given in the Supporting Information. One medium crude oil (CO1), 2 asphaltenes from two medium crude oils A1 and A2 (separated according to the procedure used by Dreyfus32), and 3 kerogen samples named K1, K2 and K3 (separated from 3 black siliceous shale samples) were analyzed. Approximately 0.5 g of crude oil was weighed (precision 0.01 mg) in quartz vessels, then 0.5 mL of H2O2 and 6 mL of HNO3 were added. Samples were mineralized in a High Pressure Asher (Anton Paar GmbH, Graz, Austria); a 1 h ramp to increase the temperature from ambient to 320 °C was used, followed by a dwell time of 30 min. Due to the low Pb concentration, in some samples (
