SRM 1643c Research Articles

Evaluating inter-study variability in phthalate and trace element analyses within the Children\u2019s Health Exposure Analysis Resource (CHEAR) using multivariate control charts

BackgroundThe Children’s Health Exposure Analysis Resource (CHEAR) program allows researchers to expand their research goals by offering the assessment of environmental exposures in their previously collected biospecimens. Samples are analyzed in one of CHEAR’s network of six laboratory hubs with the ability to assess a wide array of environmental chemicals. The ability to assess inter-study variability is important for researchers who want to combine datasets across studies and laboratories.ObjectiveHerein we establish a process of evaluating inter-study variability for a given analytic method.MethodsCommon quality control (QC) pools at two concentration levels (A and B) in urine were created within CHEAR for insertion into each batch of samples tested at a rate of three samples of each pool per 100 study samples. We assessed these QC pool results for seven phthalates analyzed for five CHEAR studies by three different lab hubs utilizing multivariate control charts to identify out-of-control runs or sets of samples associated with a given QC sample. We then tested the conditions that would lead to an out-of-control run by simulating outliers in an otherwise “in-control” set of 12 trace elements in blood QC samples (NIST SRM 955c).ResultsWhen phthalates were assessed within study, we identified a single out-of-control run for two of the five studies. Combining QC results across lab hubs, all of the runs from these two studies were now in-control, while multiple runs from two other studies were pushed out-of-control. In our simulation study we found that 3–6 analytes with outlier values (5xSD) within a run would push that run out of control in 65–83% of simulations, respectively.SignificanceWe show how acceptable bounds of variability can be established for a given analytic method by evaluating QC materials across studies using multivariate control charts.

Measurement harmonization and traceability for trace element analyses across the Children's Health Exposure Analysis Resource laboratory network

Harmonization and traceability are related metrological principles that are indispensable to assuring measurement comparability across different biomonitoring studies. The Children's Health Exposure Analysis Resource (CHEAR) was established in 2015 with six laboratories providing environmental exposure measurements on biospecimens. To ensure harmonization across studies for trace elements, CHEAR used a multi-faceted approach that included: 1) an initial interlaboratory validation exercise based on the analysis of certified blood and urine reference materials; 2) frequent participation in an established interlaboratory proficiency program for trace elements; and 3) analysis of a common pool of well-characterized biological reference materials with each analytical batch. Method accuracy and precision were established for each laboratory via analysis of NIST SRM 955c Toxic Elements in Caprine Blood, SRM 2668 Toxic Elements in Frozen Human Urine and SRM 3668 Mercury, Perchlorate, and Iodide in Frozen Human Urine. The differences among the six laboratories for As, Cd, Hg, and Mn in urine and Cd, Hg, and Pb in whole blood were judged to be fit-for-purpose. Interlaboratory performance over a 5-year period demonstrated an improvement in performance, such that for 2018–2019, >99% of challenges for urine As, Cd, Hg, and Mn, and 95% for whole blood Cd, Hg, Pb, and Mn, were found to be satisfactory. The CHEAR common reference materials were analyzed by at least 5 laboratories for 22 elements in urine and 13–14 elements in whole blood, thus providing a rich source of data to assess intra- and inter-run performance. The suite of trace elements with assigned values in both blood and urine matrices are more comprehensive than similar reference materials from other sources, and is reflective of the concentrations necessary to support biomonitoring studies. While some areas for future improvement were identified, significant progress was made to improve harmonization of trace element measurements in biological matrices among the CHEAR network labs.

Re-Os geochronology for the NIST RM 8505 crude oil: The importance of analytical protocol and uncertainty

The National Institute of Science and Technology (NIST) research material (RM) 8505 is a Venezuelan crude oil certified for its vanadium concentration. It has recently been proposed as a matrix-matched standard for interlaboratory comparison of Re-Os isotopic analyses while serving as a benchmark for petroleum-based Re-Os method development. Here, the NIST RM 8505 oil is characterized using a multi-solvent asphaltene precipitation, permitting interlaboratory Re-Os data comparison and evaluation of techniques used to derive Re-Os isochron ages. We present a detailed discussion of analytical blank sources, critical evaluation of analytical uncertainties, method improvements and data corrections. Additionally, we provide a comprehensive discussion on the effect of data population size and analytical uncertainty on isochron statistics and the robustness of isochron ages. In addition to NIST RM 8505 (2.43 ± 0.15 ng/g Re, 30.0 ± 0.7 pg/g Os), we provide the first Re-Os data for NIST SRM 1634c (1.73 ± 0.01 ng/g Re, 32.5 ± 0.8 pg/g Os) and EnviroMAT SCP Science (39 ± 1 pg/g Re, 1.01 ± 0.02 pg/g Os), two other commercially available hydrocarbon trace metal standards.The application of several asphaltene (ASPH) precipitations using different n-alkane solvents (pentane, heptane and decane) for the NIST RM 8505 significantly improved the spread of data in the 187Re/188Os-187Os/188Os space. Multi-solvent asphaltene precipitations yield an asphaltene-only Model 1 isochron (75 ± 24 Ma) and the maltene-crude oil-asphaltene triplets yield a more precise Model 1 isochron age of 78.5 ± 2.3 Ma with a 187Os/188Osinitial of 0.877 ± 0.021. Our age is considerably older compared to a previously reported Model 1 age of 62.7 ± 5.7 Ma based on C7 ASPH-crude oil-MALT triplets (Liu and Selby, 2017) and considerably younger than the Model 1 age of 98.4 ± 9.5 Ma based on ASPH separates (Liu et al., 2019). The analytical reasons leading to these age differences are discussed in this paper.The geological context for the NIST 8505 oil is unknown, therefore we use trace metal data (Ni, V, Fe, Mo, etc.) to derive plausible constraints for the origin of this oil. The NIST 8505 oil shows trace metal data similar to published data for oils from the Urdaneta district, the La Paz, Mara and Lagunillas oil fields and source rocks of the La Luna Formation in the Maracaibo Basin, Venezuela. In this geological context, our ~79 Ma age for the NIST 8505 oil implies a Late Cretaceous early hydrocarbon generation event from older Upper Cretaceous source rocks and overlaps with the youngest age of potential source rocks in the Maracaibo Basin. However, the age of large-scale oil generation in the Maracaibo Basin is significantly younger, lasting from the Oligocene to the Pliocene.

A rapid and sensitive method for the determination of inorganic chloride in oil samples

A novel isotope dilution method for the analysis of inorganic chloride in fuel oil matrix is presented. The samples were diluted by isopropyl alcohol:toluene, mixed with 37Cl− internal standard and reacted with triethyloxonium tetrafluoroborate at room temperature. This reagent promoted conversion of Cl− into stable ethyl chloride (EtCl) which was selectively detected by headspace GC–MS/MS with no matrix effects. A limit of detection of 0.2 mg kg−1 Cl− was obtained in fuel oil. The method was tested on the NIST SRM 1634c (trace elements in residual fuel oil) and validated through a series of robustness tests. Over 18 days, the variation in the signal response was less than 10% and the RSD for quantitative isotope dilution results was below 3%. Overall, the method is fast, simple and allows robust quantitation of inorganic chloride directly in the non-aqueous media.

Interfacing nanoliter liquid chromatography and inductively coupled plasma mass spectrometry with an in-column high-pressure nebulizer for mercury speciation

Metal speciation analysis of microsamples in the clinical, biological and forensic fields is important for elucidating the metals’ toxicity, mobility and metabolic behaviors of metals. Such analysis may be achieved by nanoliter high-performance liquid chromatography (nanoHPLC) hyphenated with inductively coupled plasma mass spectrometry (ICP-MS). In this work, an in-column high-pressure nebulizer (ICHPN) was developed to enhance the sensitivity and the separation efficiency of the sheathless nanoHPLC-ICP-MS coupling. The ICHPN consists of two concentric fused-silica capillaries with tapered tips connected via a PEEK tee, where C18 silica particles with a size of 5 μm were packed in the tapered inner capillary based on the keystone effect. Combining a heated single pass spray chamber with a makeup gas, the ICHPN was capable of independently optimizing the nebulization efficiency and the transport efficiency. The ICHPN offered high sensitivities, low detection limits and good precision at nanoflow rates. Compared with commercial nebulizers, the ICHPN fabrication was simple, rapid, reproducible and inexpensive. By implementing the ICHPN, we achieved rapid separation of four mercury species, i.e., Hg2+ and methyl-, ethyl- and phenylmercuric chloride, within 8.0 min with good resolution (2.0–13.9). Detection limits of 0.044-0.13 μg L−1 were obtained with precisions of peak heights and areas ranging from 1.5 to 3.5% for a 50 μg L−1 standard solution. Good agreement between the determined and certified values of mercury species in a certified reference material of caprine blood (SRM 955c) together with good recoveries (93–101%) validated the accuracy of the method.

Sequential extraction procedure for the separation of Ni and V species in crude oil and analysis by ETAAS, GC–MS, and IR

The determination of nickel and vanadium in crude oil has been evaluated in several scientific studies because these elements can contribute to equipment corrosion, the contamination of catalytic processes, and environmental pollution. Many studies have focused on the evaluation of the total concentration of these elements. However, although it is important to know the total elemental concentration to determine the chemical composition of a sample, this does not yield information about the amount of potentially toxic compounds that may be associated with different macromolecules present in the sample. Here, we report the development of a sequential extraction procedure for the separation of nickel and vanadium species. To determine the total concentration of Ni and V, samples, as well as a certified reference material (NIST – SRM 1634C), were acid digested and analyzed by electrothermal atomic absorption spectrometry (ETAAS). The separation of the species was performed by column chromatography with silica gel as the stationary phase, and hexane (fraction 1), toluene (fraction 2), dichloromethane (DCM) + methanol (fraction 3), and methanol (fraction 4) as the eluents. The extracts obtained in each fraction were analyzed by ETAAS, gas chromatography–mass spectrometry (GC–MS), and infrared (IR) measurements. The experimental results showed that the total concentrations of the elements in the crude oil were between 7.4 and 14.3 µg g−1 for Ni and 9.8 to 14.6 µg g−1 for V. The analysis of SRM 1634C showed that 89% of Ni and 103% of V were recovered. The GC–MS analysis showed the following elution order: saturated compounds and aromatics (fraction 1), aromatic and cyclic compounds (fraction 2), aromatic compounds (fraction 3), and polar compounds (fraction 4). The results obtained by ETAAS showed that 100% of the Ni and V contained in crude oil can be separated in three different extracts. However, the IR spectra and GC–MS analysis allowed us to conclude that Ni and V are associated with compounds containing nitrogen and oxygen that were not identified by gas chromatography.

Development of a Common Procedure for the Determination of Methylmercury, Ethylmercury, and Inorganic Mercury in Human Whole Blood, Hair, and Urine by Triple Spike Species-Specific Isotope Dilution Mass Spectrometry.

We report the first common methodology for the simultaneous determination of methylmercury (MeHg), ethylmercury (EtHg), and inorganic mercury (Hg(II)) in human blood hair and urine. With the exception of the initial sample mass (0.15 g for blood, 0.5 g for urine, and 0.1 g for hair), the same sample preparation and gas chromatography-inductively coupled plasma mass spectrometry (GC-ICPMS) measurement conditions are employed for the three matrixes providing experimental values in agreement with the certified values in the analysis of NIST SRM 955c (Caprine Blood) Level 3 and the certified human hairs IAEA 085 and IAEA 086. Also, the method provides quantitative recoveries for the three Hg species in the analysis of fortified human urine samples at 1, 2, and 5 ng Hg g-1. Mercury species concentrations for levels 2 and 4 of SRM 955c are reported here for the first time. A systematic interconversion of EtHg into Hg(II) was obtained for all matrixes reaching values up to 95% in blood, 29% in hair, and 11% in urine. MeHg dealkylation was also observed in a lesser extent in blood and hair analyses, but it was not observed when analyzing urine samples. Hg methylation was not observed in any matrix. The amount of NaBPr4 added for derivatization has been found to be the main factor responsible for Hg species interconversion. This work demonstrates for the first time that experimental conditions optimized for SRM 955c (caprine blood) are not valid for human blood samples as the optimum initial sample amount for a real sample is more than 3 times lower than that for SRM 955c.

Multi-element analysis of crude oils using ICP-QQQ-MS

Determination of trace element contents in crude oils constitutes a powerful geochemical tool, complementing organic geochemical analyses and allowing a deeper insight into the processes of oil generation, migration and maturation. Inductively coupled plasma mass spectrometry (ICP-MS) has been widely applied to trace element determination in petroleum samples due to its low detection limits and fast multi-element capability. However, quantification of several elements by quadrupole-based ICP-MS is hampered by low sensitivity due to high ionisation energy, severe polyatomic interferences and/or low abundance. In the present contribution an ICP tandem mass spectrometer (ICP-MS/MS), or triple quadrupole ICP-MS (ICP-QQQ-MS) has been used to overcome these limitations. Prior to analysis, samples were subjected to high pressure acid digestion, allowing external calibration by means of aqueous standards. 25 elements, including P, S, the first-row transition metals, As, Se, Re, Pb and U, could be quantified in one measurement. Polyatomic interferences on the lighter elements could be removed by reaction of the target ions with O2 or NH3 in the collision/reaction cell of the ICP-MS/MS. The accuracy of the analytical method was assessed by the analysis of the standard reference materials NIST SRM 1634c (trace elements in fuel oil) and 1084a (wear-metals in lubricating oil). The measurement procedure was applied to the analysis of crude oil samples from the Alpine Foreland Basin of Austria.

Measurement of mercury species in human blood using triple spike isotope dilution with SPME-GC-ICP-DRC-MS.

The measurement of different mercury compounds in human blood can provide valuable information about the type of mercury exposure. To this end, our laboratory developed a biomonitoring method for the quantification of inorganic (iHg), methyl (MeHg), and ethyl (EtHg) mercury in whole blood using a triple-spike isotope dilution (TSID) quantification method employing capillary gas chromatography (GC) and inductively coupled dynamic reaction cell mass spectrometry (ICP-DRC-MS). We used a robotic CombiPAL(®) sample handling station featuring twin fiber-based solid-phase microextraction (SPME) injector heads. The use of two SPME fibers significantly reduces sample analysis cycle times making this method very suitable for high sample throughput, which is a requirement for large public health biomonitoring studies. Our sample preparation procedure involved solubilization of blood samples with tetramethylammonium hydroxide (TMAH) followed by the derivatization with sodium tetra(n-propyl)borate (NaBPr(4)) to promote volatility of mercury species. We thoroughly investigated mercury species stability in the blood matrix during the course of sample treatment and analysis. The method accuracy for quantifying iHg, MeHg, and EtHg was validated using NIST standard reference materials (SRM 955c level 3) and the Centre de Toxicologie du Québec (CTQ) proficiency testing (PT) samples. The limit of detection (LOD) for iHg, MeHg, and EtHg in human blood was determined to be 0.27, 0.12, and 0.16 μg/L, respectively.

Certification of Total Arsenic in Blood and Urine Standard Reference Materials by Radiochemical Neutron Activation Analysis and Inductively Coupled Plasma - Mass Spectrometry.

A newly developed procedure for determination of arsenic by radiochemical neutron activation analysis (RNAA) was used to measure arsenic at four levels in SRM 955c Toxic Elements in Caprine Blood and at two levels in SRM 2668 Toxic Elements in Frozen Human Urine for the purpose of providing mass concentration values for certification. Samples were freeze-dried prior to analysis followed by neutron irradiation for 3 h at a fluence rate of 1×1014cm-2s-1. After sample dissolution in perchloric and nitric acids, arsenic was separated from the matrix by extraction into zinc diethyldithiocarbamate in chloroform, and 76As quantified by gamma-ray spectroscopy. Differences in chemical yield and counting geometry between samples and standards were monitored by measuring the count rate of a 77As tracer added before sample dissolution. RNAA results were combined with inductively coupled plasma - mass spectrometry (ICP-MS) values from NIST and collaborating laboratories to provide certified values of (10.81 ± 0.54) μg/kg and (213.1 ± 0.73) μg/kg for SRM 2668 Levels I and II, and certified values of (21.66 ± 0.73) μg/kg, (52.7 ± 1.1) μg/kg, and (78.8 ± 4.9) μg/kg for SRM 955c Levels 2, 3, and 4 respectively. Because of discrepancies between values obtained by different methods for SRM 955c Level 1, an information value of < 5 μg/kg was assigned for this material.

Development of a Styrene Based Molecularly Imprinted Polymer and Its Molecular Recognition Properties of Vanadyl Tetraphenylporphyrin in Organic Media

A highly selective molecularly imprinted polymer (MIP) was synthesized and employed as an adsorbent material for the removal of vanadyl tetraphenylporphyrin (VTPP) from organic media. The maximum adsorption capacity of MIP (0.600 mg/g) toward VTPP was higher than that of the nonimprinted polymer (NIP; 0.281 mg/g). The accuracy of the method was validated by analyzing the certified reference material (CRM) NIST SRM 1634c, Trace Metals in Residual Fuel Oil, and the concentration of VTPP in the CRM was found to be 28.49 mg/kg, which was significantly comparable with the certified value of 28.19 mg/kg.

Molecularly imprinted electrospun nanofibers for adsorption of nickel-5,10,15,20-tetraphenylporphine (NTPP) in organic media

An electrospun nanofiber containing a molecularly imprinted polymer (MIP) was fabricated by electrospinning and employed as a sorbent for the removal of nickel-5,10,15,20-tetraphenylporphine (NTPP) from organic media. Three different PET/PEI-based molecularly imprinted nanofibers (MIN) were prepared in order to evaluate the impact of the template on the affinity of recognition for NTPP. ICP-OES analysis showed that the NTPP adsorption capacity increased as the molar ratio of NTPP to styrene was increased from 1:1 to 3:1. The optimal ratio of template to functional monomer which yielded the best specific affinity and the highest recovery (99.9 %) was 3:1. The degree of adsorption of NTPP attained with the MIN was much higher than that achieved with the non-imprinted nanofiber (NIN). When the fiber masses were 10, 20, and 30 mg, 40.4, 60.2, and 99.9 %, respectively, of the NTPP was bound by the MIN without overloading the active sites. The corresponding values for the NIN were 12, 15, and 18 %, respectively. Furthermore, the MIN showed remarkable stability in relation to reusability, as it could be reused nine times without any loss in performance. The accuracy of the method was validated by analyzing the certified reference material (CRM) NIST SRM 1634c (Trace Metals in Residual Fuel Oil), and the concentration of NTPP in the CRM was found to be 17.64 mg/kg, which was within the error associated with the certified value of 17.54 mg/kg. The MIN was therefore found to be a selective sorbent that is well suited for handling trace metals in organic media.

Iron telluride nanorods-based system for the detection of total mercury in blood

We have developed a simple, colorimetric iron telluride (FeTe) nanorods (NRs) based system for the detection of mercury, mainly based on the cation exchange reaction between FeTe NRs and Hg2+. FeTe NRs (length, 105±21nm) react with Hg2+ to form HgTe NRs (length, 112±26nm) and consequently release Fe2+ ions that catalyzes the oxidation between a peroxidase substrate 2,2′-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) diammonium salt (ABTS) and H2O2. The concentration of Fe2+ and thereby Hg2+ can be determined by measuring the absorbance of the ABTS oxidized product at 418nm. This approach allows the detection of Hg2+, with a limit of detection of 1.31nM at a signal-to-noise ratio 3 and a linear range 5–100nM (R2=0.99). The low-cost, simple, sensitive, and reproducible assay has been validated for the detection of Hg2+ in a blood sample (SRM 955c), with the result being in good agreement with that provided by National Institute of Standards and Technology.

Determination of arsenic and cadmium in crude oil by direct sampling graphite furnace atomic absorption spectrometry

In this work, a direct sampling graphite furnace atomic absorption spectrometry method has been developed for the determination of arsenic and cadmium in crude oil samples. The samples were weighed directly on the solid sampling platforms and introduced into the graphite tube for analysis. The chemical modifier used for both analytes was a mixture of 0.1% Pd+0.06% Mg+0.06% Triton X-100. Pyrolysis and atomization curves were obtained for both analytes using standards and samples. Calibration curves with aqueous standards could be used for both analytes. The limits of detection obtained were 5.1μgkg−1 for arsenic and 0.2μgkg−1 for cadmium, calculated for the maximum amount of sample that can be analyzed (8mg and 10mg) for arsenic and cadmium, respectively. Relative standard deviations lower than 20% were obtained. For validation purposes, a calibration curve was constructed with the SRM 1634c and aqueous standards for arsenic and the results obtained for several crude oil samples were in agreement according to paired t-test. The result obtained for the determination of arsenic in the SRM against aqueous standards was also in agreement with the certificate value. As there is no crude oil or similar reference material available with a certified value for cadmium, a digestion in an open vessel under reflux using a “cold finger” was adopted for validation purposes. The use of paired t-test showed that the results obtained by direct sampling and digestion were in agreement at a 95% confidence level. Recovery tests were carried out with inorganic and organic standards and the results were between 88% and 109%. The proposed method is simple, fast and reliable, being appropriated for routine analysis.

A comparison of clinical laboratory data for assigning a consensus value for manganese in a caprine blood reference material.

Biomonitoring for manganese (Mn) exposure is important due to its potential to cause adverse health effects. In this study, we investigate how different sample preparation methods (simple dilution, digestion, volumetric, gravimetric), calibration protocols (aqueous, blood-based, standard additions), and instrumental techniques affect Mn method bias and analytical imprecision. The techniques used included graphite furnace atomic absorption spectrometry (GFAAS), dynamic reaction cell inductively coupled plasma mass spectrometry (DRC-ICP-MS), and sector field (SF-) ICP-MS. We analyzed NIST SRM 1643e Trace Elements in Water and SRM 1598a Inorganic Constituents in Animal Serum (both certified for Mn), and SRM 955c Toxic Metals in Caprine Blood - Level 1 (not certified for Mn). Various matrix effects in ICP-MS produced inaccurate results for SRM 1643e and discrepant results for SRM 955c. In the absence of a certified value for Mn in SRM 955c, we assigned a "consensus" value by combining data from the New York State Department of Health (NYS), the Centers for Disease Control and Prevention (CDC) and the Centre de toxicologie du Québec (CTQ). With this interlaboratory approach, we established an "all-lab" consensus value of 16.3 ± 0.8 μg L-1 based on data from DRC-ICP-MS with simple dilution sample preparation and blood-based calibration. We also assigned an "all-method" consensus value of 16.3 ± 0.9 μg L-1 based on GFAAS and SF-ICP-MS data from the NYS lab and the DRC-ICP-MS all-lab consensus value. Although the expanded uncertainty (U) calculated for the consensus values may not fully account for all sources of uncertainty, it does show the relative variation that might be expected from one study to the next for the determination of Mn in blood.

Measurements of methylmercury, ethylmercury, and inorganic mercury species in a whole blood standard reference material: SRM 955c—Toxic Elements in Caprine Blood

Trace elemental speciation analysis is assuming more critical importance in nutritional and toxicological assessments, and despite the necessity to quantitatively differentiate elemental species in environmental and clinical samples, there are a limited number of reference materials suitable for this purpose. A whole blood certified reference material, SRM 955c Toxic Elements in Caprine Blood was developed for the determination of toxic metals in whole blood including mercury. The certification of methylmercury, ethylmercury, and inorganic mercury was carried out by triple spike speciated isotope dilution inductively coupled plasma mass spectrometry (TS-SID-GC/ICP-MS) and represents the first whole blood material certified for three mercury species. The certified values (±expanded uncertainty) of methylmercury, ethylmercury, inorganic mercury and total mercury in SRM 955c are 4.5 ± 1.0 µg L−1, 5.1 ± 0.5 µg L−1, 9.0 ± 1.3 µg L−1, and 17.8 ± 1.6 µg L−1 respectively.

Microwave assisted mixed-micelle cloud point extraction of Au and Tl from environmental samples without using a chelating agent prior to ICP-MS determination

A rapid microwave assisted mixed-micelle cloud point extraction procedure is developed for the pre-concentration and separation of ultra trace levels of Au and Tl simultaneously from environmental samples prior to their determination by ICP-MS. The method is based on the formation of extractable complexes of anionic species AuCl4− and TlCl32− with solubilizing cationic and hydrophobic sites of mixed-micelles, in the presence of HCl. The mixed-micelles consisting of cetyltrimethylammonium bromide (CTAB) and Triton X-114 are used for pre-concentration and extraction of formed extractable complexes of Au and Tl from a bulk aqueous phase into a small surfactant-rich phase, avoiding the addition of an external chelating agent. The phase separation is induced by microwave irradiation. The potential oxide interferences of Ta and Hf are eliminated effectively up to 2 and 5 μg mL−1 levels, respectively, enabling interference free determination of mono isotopic 197Au. The main parameters affecting the cloud point extraction process are evaluated and optimized. Under the optimized conditions, the pre-concentration factors and limits of detection are 125 and 110, and 0.12 and 0.02 pg mL−1, respectively, for Au and Tl. The recoveries of analytes are in the range of 97–103% at 10–40 pg mL−1 for Au and Tl with relative standard deviations of 0.5–2.2%. The accuracy of the procedure is validated by analyzing certified reference materials such as NIST SRM 1643c and 1643d (Trace elements in water), 2709 (San Joaquin soil) and 2711 (Montana soil), and NRCC NASS-5 (Open ocean seawater), as well as being applied to tap and seawater samples.

Feasibility of using solid sampling graphite furnace atomic absorption spectrometry for speciation analysis of volatile and non-volatile compounds of nickel and vanadium in crude oil

A method for the direct determination of volatile and non-volatile nickel and vanadium compounds in crude oil without previous treatment using direct solid sampling graphite furnace atomic absorption spectrometry is proposed. The crude oil samples were weighed directly onto solid sampling platforms using a microbalance and introduced into a transversely heated solid sampling graphite tube. In previous work of our group losses of volatile nickel and vanadium compounds have been detected, whereas other nickel and vanadium compounds were thermally stable up to 1300 and 1600 °C, respectively. In order to avoid this problem different chemical modifiers (conventional and permanent) have been investigated. With 400 μg of iridium as permanent modifier, the signal started to drop already after two atomization cycles, possibly because of an interaction of nickel (which is a catalyst poison) with iridium. Twenty micrograms of palladium applied in each determination was found to be optimum for both elements. The palladium was deposited on the platform and submitted to a drying step at 150 °C for 75 s. After that the sample was added onto the platform and submitted to the furnace program. The influence of sample mass on the linearity of the response and on potential measurement errors was also investigated using four samples with different nickel content. For the sample with the lowest nickel concentration the relationship between mass and integrated absorbance was found to be non-linear when a high sample mass was introduced. It was suspected that the modifier had not covered the entire platform surface, which resulted in analyte losses. This problem could be avoided by using 40 μL of 0.5 g L −1 Pd with 0.05% Triton X-100. Calibration curves were established with and without modifier, with aqueous standards, oil-in-water emulsions and the certified reference material NIST SRM 1634c (trace metals in residual fuel oil). The sensitivity for aqueous standards and emulsions was close to that for SRM 1634c, making possible the use of aqueous standards for calibration. The limits of detection and quantification obtained for nickel and vanadium under this condition were found to be 0.02 and 0.06 μg g −1, respectively, for both elements, based on 10 mg of sample. Nickel and vanadium were determined in the samples with (total Ni and V) and without the use of Pd (thermally stable compounds), and the concentration of volatile compounds was calculated by difference. The results were compared with those obtained by high-resolution continuum source graphite furnace atomic absorption spectrometry by emulsion technique; no significant differences were found for total Ni and V at the 95% confidence level according to a Student's t-test.

Palladium as chemical modifier for the stabilization of volatile nickel and vanadium compounds in crude oil using graphite furnace atomic absorption spectrometry

It has been observed in previous work using high-resolution continuum-source atomic absorption spectrometry that up to 50% of the nickel and vanadium in crude oil, most likely non-polar nickel and vanadyl porphyrin complexes, was already lost at pyrolysis temperatures >400 °C, whereas the rest of the analyte was stable up to at least 1200 °C. In this work we were investigating the use of a chemical modifier in order to avoid these low-temperature losses and also to make possible a differential determination of the volatile analyte fractions. Oil-in-water emulsions, using Triton X-100 as surfactant, were used for easy sample handling. A mass of 20 μg of Pd, introduced into the graphite tube and thermally pretreated prior to the injection of the emulsion, was found to efficiently prevent any low-temperature losses of nickel and vanadium from crude oil samples up to pyrolysis temperatures of 1200 °C and 1450 °C, respectively. The best characteristic mass obtained was m0 = 16 pg for Ni and m0 = 33 pg for V. The limits of detection and quantification were 0.02 μg g−1 and 0.065 μg g−1, respectively, for Ni, and 0.06 μg g−1 and 0.17 μg g−1, respectively, for V in oil, based on an emulsion of 2 g of oil in 10 mL. The precision expressed as relative standard deviation (n = 9) for a crude oil containing about 5 μg g−1 Ni and 3 μg g−1 V was 2.0% and 3.1% for Ni and V, respectively. The accuracy of the procedure was verified by analyzing the certified reference material NIST SRM 1634c, trace metals in fuel oil, and the research material RM 8505, vanadium in crude oil.

Method development for the determination of nickel in petroleum using line-source and high-resolution continuum-source graphite furnace atomic absorption spectrometry

Several sample preparation methods have been investigated for the direct determination of nickel in crude oil using graphite furnace atomic absorption spectrometry (GF AAS). Xylene was found unsuitable as solvent because of the poor long-term stability of the solutions and the resulting contamination of the equipment. Isobutyl methyl ketone (IBMK) solutions exhibited better stability, but the sensitivity of the organic nickel salt used for the standard solutions showed a high day-to-day variability. An oil-in-water emulsion using Triton X-100 as surfactant gave the best results. Using high-resolution continuum-source (HR-CS) GF AAS, it could be observed that up to 50% of the nickel in crude oil, most likely low molecular weight nickel porphyrins, were lost already at pyrolysis temperatures >400 °C, whereas the rest of the nickel as well as the nickel standard were stable up to 1300 °C. The nickel absorption at a secondary line at 232.138 nm was recorded simultaneously with that at the primary line at 232.003 nm, expanding the dynamic working range by an order of magnitude. The best characteristic mass obtained was m 0=27 pg and the limit of detection was around 0.07 μg g −1 Ni in oil, based on an emulsion of 2 g oil in 10 ml. The accuracy of the procedure was verified by analyzing the certified reference material (CRM) NIST SRM 1634c, Trace Metals in Residual Fuel Oil.