Comprehensive inter‐laboratory calibration of reference materials for δ18O versus VSMOW using various on‐line high‐temperature conversion techniques

Internationally distributed organic and inorganic oxygen isotopic reference materials have been calibrated by six laboratories carrying out more than 5300 measurements using a variety of high‐temperature conversion techniques (HTC) a in an evaluation sponsored by the International Union of Pure and Applied Chemistry (IUPAC). To aid in the calibration of these reference materials, which span more than 125‰, an artificially enriched reference water (δ¹⁸O of +78.91‰) and two barium sulfates (one depleted and one enriched in ¹⁸O) were prepared and calibrated relative to VSMOW2 b and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study, which yielded:

Nicotine,acetanilide and urea multi‐level 2H‐, 13C‐ and 15N‐abundance reference materials for continuous‐flow isotope ratio mass spectrometry

Accurate determinations of stable isotope ratios require a calibration using at least two reference materials with different isotopic compositions to anchor the isotopic scale and compensate for differences in machine slope. Ideally, the δ values of these reference materials should bracket the isotopic range of samples with unknown δ values. While the practice of analyzing two isotopically distinct reference materials is common for water (VSMOW‐SLAP) and carbonates (NBS 19 and L‐SVEC), the lack of widely available organic reference materials with distinct isotopic composition has hindered the practice when analyzing organic materials by elemental analysis/isotope ratio mass spectrometry (EA‐IRMS). At present only L‐glutamic acids USGS40 and USGS41 satisfy these requirements for δ¹³C and δ¹⁵N, with the limitation that L‐glutamic acid is not suitable for analysis by gas chromatography (GC). We describe the development and quality testing of (i) four nicotine laboratory reference materials for on‐line (i.e. continuous flow) hydrogen reductive gas chromatography‐isotope ratio mass‐spectrometry (GC‐IRMS), (ii) five nicotines for oxidative C, N gas chromatography‐combustion‐isotope ratio mass‐spectrometry (GC‐C‐IRMS, or GC‐IRMS), and (iii) also three acetanilide and three urea reference materials for on‐line oxidative EA‐IRMS for C and N. Isotopic off‐line calibration against international stable isotope measurement standards at Indiana University adhered to the ‘principle of identical treatment’. The new reference materials cover the following isotopic ranges: δ²H_nicotine ?162 to ?45‰, δ¹³C_nicotine ?30.05 to +7.72‰, δ¹⁵N_nicotine ?6.03 to +33.62‰; δ¹⁵N_acetanilide +1.18 to +40.57‰; δ¹³C_urea ?34.13 to +11.71‰, δ¹⁵N_urea +0.26 to +40.61‰ (recommended δ values refer to calibration with NBS 19, L‐SVEC, IAEA‐N‐1, and IAEA‐N‐2). Nicotines fill a gap as the first organic nitrogen stable isotope reference materials for GC‐IRMS that are available with different δ¹⁵N values. Comparative δ¹³C and δ¹⁵N on‐line EA‐IRMS data from 14 volunteering laboratories document the usefulness and reliability of acetanilides and ureas as EA‐IRMS reference materials. Published in 2009 by John Wiley & Sons, Ltd.     

Error propagation in normalization of stable isotope data: a Monte Carlo analysis

A higher analytical precision of a stable isotope ratio mass spectrometer does not automatically guarantee accurate determination of the true isotope composition (δ‐value) of samples, since estimates of true δ‐values are obtained from the normalization of raw isotope data. We performed both Monte Carlo simulations and laboratory experiments to investigate aspects of error propagation during the normalization of carbon stable isotope data. We found that increasing both the number of different reference standards and the number of repetitions of each of these standards reduces the normalization error. A 50% reduction in the normalization error can be achieved over the two‐point normalization by either analyzing two standards four times each, or four standards two times each. If the true δ‐value of a sample is approximately known a priori, the normalization error may then be reduced through a targeted choice of locally optimal standards. However, the difference in improvement is minimal and, therefore, a more practical strategy is to use two or more standards covering the whole stable isotope scale. The selection of different sets of standards by different laboratories or for different batches of samples in the same laboratory may lead to significant differences in the normalized δ‐values of the same samples, leading to inconsistent results. Hence, the same set of standards should always be used for a particular element and a particular stable isotope analytical technique. Copyright © 2010 John Wiley & Sons, Ltd.     

Oxygen isotope homogeneity assessment for apatite U-Th-Pb geochronology reference materials

Secondary ion mass spectrometry (SIMS) measurement of oxygen isotopes in apatite has been employed more and more in petrogenetic, metallogenic, and climate change studies. Well-characterised reference materials are needed due to the matrix effect, but they are yet to be well established. In this study, we conducted in-situ oxygen isotopic and chemical analyses on six commonly used apatite reference materials (ie, Emerald, Kovdor, McClure, Mud Tank, Otter Lake, and Slyudyanka) and two in-house apatite references (Qinghu and GEMS 203) to assess their oxygen isotope homogeneity and applicability for microbeam analyses. Our results show that all these apatite references are in general chemically homogeneous. In terms of oxygen isotopes, GEMS 203 (δ¹⁸O = 9.85 ± 0.40‰ [2SD], corrected by Durango 3), Kovdor (δ¹⁸O = 6.55 ± 0.38‰, 2SD), and McClure (δ¹⁸O = 5.94 ± 0.42‰, 2SD) are fairly homogeneous, whereas Emerald (δ¹⁸O = 10.37 ± 0.45‰, 2SD), Mud Tank (δ¹⁸O = 6.35 ± 0.46‰, 2SD), Otter Lake (δ¹⁸O = 9.71 ± 0.47‰, 2SD), Qinghu (δ¹⁸O = 5.44 ± 0.49‰, 2SD), and Slyudyanka (δ¹⁸O = 17.49 ± 0.43‰, 2SD) are less homogenous. This indicates that the former group represents better reference materials for in-situ oxygen isotopic analyses, whilst the latter group can be used as secondary reference material for analytical quality control.     

Making an honest measurement scale out of the oxygen isotope delta-values

The differential measurement of the abundance of oxygen isotopes based on reference materials, such as VSMOW for the case of water, was used because the precision of the absolute mass-spectrometric determination of the abundance fell short of the differences to be measured. Since then these measurements have been much improved, so that a calibration scheme of the oxygen isotope abundance in water, carbonates, silica, phosphates, sulfates, nitrates and organic materials is suggested, based on an accredited primary standard of oxygen in air and using standard fluorination and O(2) to CO(2) conversion techniques.     

Two new organic reference materials for delta13C and delta15N measurements and a new value for the delta13C of NBS 22 oil

Analytical grade L-glutamic acid is chemically stable and has a C/N mole ratio of 5, which is close to that of many of natural biological materials, such as blood and animal tissue. Two L-glutamic acid reference materials with substantially different 13C and 15N abundances have been prepared for use as organic reference materials for C and N isotopic measurements. USGS40 is analytical grade L-glutamic acid and has a delta13C value of -26.24 per thousand relative to VPDB and a delta15N value of -4.52 per thousand relative to N2 in air. USGS41 was prepared by dissolving analytical grade L-glutamic acid with L-glutamic acid enriched in 13C and 15N. USGS41 has a delta13C value of +37.76 per thousand and a delta15N value of +47.57 per thousand. The delta13C and delta15N values of both materials were measured against the international reference materials NBS 19 calcium carbonate (delta13C=+1.95 per thousand ), L-SVEC lithium carbonate (delta13C=-46.48 per thousand ), IAEA-N-1 ammonium sulfate (delta15N=0.43 per thousand ), and USGS32 potassium nitrate (delta15N=180 per thousand ) by on-line combustion continuous-flow and off-line dual-inlet isotope-ratio mass spectrometry. Both USGS40 and USGS41 are isotopically homogeneous; reproducibility of delta13C is better than 0.13 per thousand, and that of delta15N is better than 0.13 per thousand in 100-microg amounts. These two isotopic reference materials can be used for (i) calibrating local laboratory reference materials, and (ii) quantifying drift with time, mass-dependent fractionations, and isotope-ratio-scale contraction in the isotopic analysis of various biological materials. Isotopic results presented in this paper yield a delta13C value for NBS 22 oil of -29.91 per thousand, in contrast to the commonly accepted value of -29.78 per thousand for which off-line blank corrections probably have not been quantified satisfactorily.     

High-temperature elemental analysis and pyrolysis techniques for stable isotope analysis

A universal method for pyrolysis and elemental analysis, suitable for the online determination of deuterium, carbon, nitrogen and oxygen isotopes for organic and inorganic substances, is presented. The samples are pyrolytically decomposed in a high-temperature pyrolysis (HTP) system, at a temperature exceeding 1400 degrees C, in the presence of reactive carbon. The method is suitable for the analysis of stable isotope ratios from hydrogen, carbon, nitrogen and oxygen. The instrumentation and experimental procedure are simple and cost-effective. The reproducibility of the delta values for D/H is better than 3 per thousand, and for (18)O, (13)C (organic) and (15)N (inorganic) it is approximately 0.2 per thousand. The HTP system is suitable for solid and liquid samples and can use an autosampler for the samples. Results are presented for the isotopic composition of international reference materials and selected laboratory reference materials, which demonstrate the precision and accuracy of the method. Possible problems in the measurement of nitrates and their solutions are particularly discussed. The analyses of oxygen isotopes in selected geological samples (carbonates, silicate, biotite) are demonstrated.     

δ 15N measurement of organic and inorganic substances by EA-IRMS: a speciation-dependent procedure

Little attention has been paid so far to the influence of the chemical nature of the substance when measuring δ ¹⁵N by elemental analysis (EA)–isotope ratio mass spectrometry (IRMS). Although the bulk nitrogen isotope analysis of organic material is not to be questioned, literature from different disciplines using IRMS provides hints that the quantitative conversion of nitrate into nitrogen presents difficulties. We observed abnormal series of δ ¹⁵N values of laboratory standards and nitrates. These unexpected results were shown to be related to the tailing of the nitrogen peak of nitrate-containing compounds. A series of experiments were set up to investigate the cause of this phenomenon, using ammonium nitrate (NH₄NO₃) and potassium nitrate (KNO₃) samples, two organic laboratory standards as well as the international secondary reference materials IAEA-N1, IAEA-N2—two ammonium sulphates [(NH₄)₂SO₄]—and IAEA-NO-3, a potassium nitrate. In experiment 1, we used graphite and vanadium pentoxide (V₂O₅) as additives to observe if they could enhance the decomposition (combustion) of nitrates. In experiment 2, we tested another elemental analyser configuration including an additional section of reduced copper in order to see whether or not the tailing could originate from an incomplete reduction process. Finally, we modified several parameters of the method and observed their influence on the peak shape, δ ¹⁵N value and nitrogen content in weight percent of nitrogen of the target substances. We found the best results using mere thermal decomposition in helium, under exclusion of any oxygen. We show that the analytical procedure used for organic samples should not be used for nitrates because of their different chemical nature. We present the best performance given one set of sample introduction parameters for the analysis of nitrates, as well as for the ammonium sulphate IAEA-N1 and IAEA-N2 reference materials. We discuss these results considering the thermochemistry of the substances and the analytical technique itself. The results emphasise the difference in chemical nature of inorganic and organic samples, which necessarily involves distinct thermochemistry when analysed by EA-IRMS. Therefore, they should not be processed using the same analytical procedure. This clearly impacts on the way international secondary reference materials should be used for the calibration of organic laboratory standards.

delta34S measurements on organic materials by continuous flow isotope ratio mass spectrometry

Sulfur (S) isotope ratios of thoroughly dried organic samples were measured by direct thermal decomposition in an elemental analyzer coupled to an isotope ratio mass spectrometer in continuous flow mode (EA-CF-IRMS). For organic samples of up to 13 mg weight and with total S contents of more than 10 microg, the reproducibility of the delta34S(organic) values was +/-0.4 per thousand or better. However, the delta34S values of organic samples measured directly by online EA-CF-IRMS analysis were between 0.3 and 2.9 per thousand higher than those determined on BaSO4 precipitates produced by Parr Bomb oxidation from the same sample material. Our results suggest that structural oxygen in organic samples influences the oxygen isotope ratios of the SO2 produced from organic samples. Consequently, SO2 generated from organic samples appears to have different 18O/16O ratios than SO2 generated from BaSO4 precipitates and inorganic reference materials, resulting in a deviation from the true delta34S values because of 32S16O18O contributions to mass 66. It was shown that both the amount of structural oxygen in the organic sample, and the difference of the oxygen isotope ratios between organic samples and tank O2, influenced the magnitude of the observed deviation from the true delta34S value after direct EA-CF-IRMS analysis of organic samples. Suggestions are made to correct the difference between measured delta34S(organic) and true delta34S values in order to obtain not only reproducible, but also accurate S isotope ratios for organic materials by EA-CF-IRMS.     

Certification of two human hair reference materials issued by the International Atomic Energy Agency

Two new human hair reference materials, with different levels of mercury and methylmercury, have been developed and characterized by the International Atomic Energy Agency, for use in validation of measurements for mercury exposure. The set of materials consists of IAEA-086, with a low level of methylmercury, and IAEA-085, with an elevated methylmercury level. An international intercomparison exercise was carried out, and 68 institutes from 40 countries have contributed data. Based on the evaluation of the results from the intercomparison and analyses by expert laboratories, values of 23.2 and 0.57 mg/kg total mercury are recommended for IAEA-085 and IAEA-086, respectively. Values for methylmercury are recommended at 22.9 mg/kg, MeHg as Hg, for IAEA-085, and at 0.26 mg/kg, MeHg as Hg, for IAEA-086. Recommended and information values are also given for other selected trace elements.     

Methods to reduce interference effects in thermal conversion elemental analyzer/continuous flow isotope ratio mass spectrometry delta18O measurements of nitrogen-containing compounds

On-line determination of the oxygen isotopic composition (delta(18)O value) in organic and inorganic samples is commonly performed using a thermal conversion elemental analyzer (TC-EA) linked to a continuous flow isotope ratio mass spectrometry (IRMS) system. Accurate delta(18)O analysis of N-containing compounds (like nitrates) by TC-EA-IRMS may be complicated because of interference of the N(2) peak on the m/z 30 signal of the CO peak. In this study we evaluated the effectiveness of two methods to overcome this interference which do not require any hardware modifications of standard TC-EA-IRMS systems. These methods were (1) reducing the amount of N(2) introduced into the ion source through He dilution of the N(2) peak and (2) an improved background correction on the CO m/z 30 sample peak integration.Our results show that He dilution is as effective as diverting the N(2) peak in order to eliminate this interference. We conclude that the He-dilution technique is a viable method for the delta(18)O analysis of nitrates and other N-containing samples (which are not routinely measured using He dilution) using TC-EA-IRMS, since it can easily be programmed in the standard software of IRMS systems. With the He-dilution technique delta(18)O values of the nitrate isotope standards USGS34, IAEA-N3 and USGS35 were measured using the shortest possible traceability chain to the VSMOW-SLAP scale, and the results were -28.1 +/- 0.1 per thousand, +25.5 +/- 0.1 per thousand and +57.5 +/- 0.2 per thousand, respectively. An improved background correction was also an effective method, but required manual correction of the raw data.     

Investigation of preparation techniques for δ2H analysis of keratin materials and a proposed analytical protocol

Accurate hydrogen isotopic measurements of keratin materials have been a challenge due to exchangeable hydrogen in the sample matrix and the paucity of appropriate isotopic reference materials for calibration. We found that the most reproducible δ(2)H(VSMOW-SLAP) and mole fraction of exchangeable hydrogen, x(H)(ex), of keratin materials were measured with equilibration at ambient temperature using two desiccators and two different equilibration waters with two sets of the keratin materials for 6 days. Following equilibration, drying the keratin materials in a vacuum oven for 4 days at 60 °C was most critical. The δ(2)H analysis protocol also includes interspersing isotopic reference waters in silver tubes among samples in the carousel of a thermal conversion elemental analyzer (TC/EA) reduction unit. Using this analytical protocol, δ(2)H(VSMOW-SLAP) values of the non-exchangeable fractions of USGS42 and USGS43 human-hair isotopic reference materials were determined to be -78.5 ± 2.3 ‰ and -50.3 ± 2.8 ‰, respectively. The measured x(H)(ex) values of keratin materials analyzed with steam equilibration and N(2) drying were substantially higher than those previously published, and dry N(2) purging was unable to remove absorbed moisture completely, even with overnight purging. The δ(2)H values of keratin materials measured with steam equilibration were about 10 ‰ lower than values determined with equilibration in desiccators at ambient temperatures when on-line evacuation was used to dry samples. With steam equilibrations the x(H)(ex) of commercial keratin powder was as high as 28%. Using human-hair isotopic reference materials to calibrate other keratin materials, such as hoof or horn, can introduce bias in δ(2)H measurements because the amount of absorbed water and the x(H)(ex) values may differ from those of unknown samples. Correct δ(2)H(VSMOW-SLAP) values of the non-exchangeable fractions of unknown human-hair samples can be determined with atmospheric moisture equilibration by normalizing with USGS42 and USGS43 human-hair reference materials when all materials have the same powder size.     

The future demand for geological reference materials

Geological RMs (G-1 and W-1) were introduced in 1951 for the purpose of validating the accuracy of silicate rock analysis by dc arc spectrography. Since then the introduction of an array of other spectrographic methods has greatly enhanced research into geological processes. The range of elements that could be determined was expanded, and the detection limits for measurement was lowered repeatedly through the years. The development and use of reference materials was critically important in supporting this rapid expansion of geological research. Essentially, all RMs are of importance to the geosciences community since G-1 and W-1 have been prepared and distributed by national geological institutions, first by the USGS or the CRPG, rather than by national metrology institutions. These geological institutions are not yet certifying their RMs according to ISO Guides. The International Association of Geoanalysts (IAG) is seeking to meet this higher metrological requirement. Since the inception of the IAG certification program in 2003, five powdered silicate rock materials have been issued to meet the demand with respect to calibration, method validation, traceability, etc. for whole rock major and trace element analysis. The introduction of microanalytical techniques nearly decades ago and the more recent advent of MC-ICP-MS have become new driving forces in geochemical research. The first opened the possibility of performing in situ elemental composition studies at the ??m scale. The second led to the discovery of small isotope composition variations of mass- and non-mass-dependent processes in ??non-traditional?? stable isotopes (e.g., Fe, Cu, Zn, Mo, W, and Hg) through cosmo- and geochemical processes. Coupling the two techniques expands in situ analysis to isotopic studies. These developments have created great demand for (certified) RMs for both isotope ratio and microanalytical measurements for the geochemical community that is not yet being met. Homogeneity at a ??m scale and unmatched matrices of the natural minerals or synthetic doped glasses hamper the progress in certification of RMs for the microanalytical measurement community. A challenge for the production of isotope RMs is to prepare an RM solution with an isotopic composition similar to the natural systems under investigation. Refined cadmium and nickel metals, for example, have fractionated isotopic compositions far above the range observed in natural systems of interest. Yet, the calibration RM cannot fulfill its purpose when the uncertainty of its isotopic composition exceeds that of the unknowns being measured against it. In this regard, the IAG has recently certified a calibration solution for the determination of Os isotopic ratios. It is also working through member organizations, USGS, and MPI for Geochemistry (Mainz) to develop appropriate microanalytical standards. In addition to these current and future challenges, establishing metrological traceability of geological reference materials in the absence of starting points developed by national metrology institutions is a major issue that needs attention in all future certifications.     

Routine hydrogen isotope measurement of cellulose nitrate by high-temperature pyrolysis--reference materials and precision

The determination of isotope ratios of non-exchangeable hydrogen in tree-ring cellulose is commonly based on the nitration of wood cellulose followed by online high-temperature pyrolysis and isotope ratio mass spectrometry measurement of cellulose nitrate samples. The application of this method requires a proper calibration using appropriate reference materials whose delta(2)H values have been reliably normalized to the V-SMOW/SLAP scale. In our study, we achieve this normalization by a direct alternating measurement of reference waters (V-SMOW and SLAP) and three cellulose nitrates chosen as reference materials. For that purpose, both water and solid organic samples are introduced into the pyrolysis reactor by silver capsule injection. The analytical precision of the water measurement using the capsule method is +/-1.5 per thousand. The hydrogen isotopic composition of three cellulose nitrate standards measured ranges from -106.7 to -53.9 per thousand. The standard deviation of the calculated means from five measurement periods of those standards is better than 1 per thousand. Twenty-four different measurements of the hydrogen isotope composition of cellulose nitrate were evaluated in order to assess the precision of the described method. We obtained an analytical precision of +/-3.0 per thousand as representative for the 95% confidence interval applicable for routine measurements of cellulose nitrate samples. Evidence was found for significant differences in the behavior of cellulose nitrate and PE foil during the pyrolitic conversion that emphasizes the need for a proper calibration of the routine measurements. This calibration can only be successful if the reference materials used have a very similar chemical composition and undergo the same preparation procedure as the samples.     

Use of IAEA reference materials (IAEA-373 and 375) as low-level99Tc references

The concentration of⁹⁹Tc in IAEA reference materials has been determined with an ICP-MS as 0.86±0.07 Bq·kg⁻¹ dry for IAEA-373 (grass) and 0.25±0.02 Bq·kg⁻¹ dry for IAEA-375 (soil). These being sufficiently higher than the detection limits of typical measurement methods, both materials can be used as reference materials for determining low-level⁹⁹Tc in environmental samples.     

Discrepancies between isotope ratio infrared spectroscopy and isotope ratio mass spectrometry for the stable isotope analysis of plant and soil waters

The use of isotope ratio infrared spectroscopy (IRIS) for the stable hydrogen and oxygen isotope analysis of water is increasing. While IRIS has many advantages over traditional isotope ratio mass spectrometry (IRMS), it may also be prone to errors that do not impact upon IRMS analyses. Of particular concern is the potential for contaminants in the water sample to interfere with the spectroscopy, thus leading to erroneous stable isotope data. Water extracted from plant and soil samples may often contain organic contaminants. The extent to which contaminants may interfere with IRIS and thus impact upon data quality is presently unknown. We tested the performance of IRIS relative to IRMS for water extracted from 11 plant species and one organic soil horizon. IRIS deviated considerably from IRMS for over half of the samples tested, with deviations as large as 46‰ (δ²H) and 15.4‰ (δ¹⁸O) being measured. This effect was reduced somewhat by using activated charcoal to remove organics from the water; however, deviations as large as 35‰ (δ²H) and 11.8‰ (δ¹⁸O) were still measured for these cleaned samples. Interestingly, the use of activated charcoal to clean water samples had less effect than previously thought for IRMS analyses. Our data show that extreme caution is required when using IRIS to analyse water samples that may contain organic contaminants. We suggest that the development of new cleaning techniques for removing organic contaminants together with instrument‐based software to flag potentially problematic samples are necessary to ensure accurate plant and soil water analyses using IRIS. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of 24 elements in four algae reference materials by neutron activation analysis and inductively coupled plasma mass spectrometry

Four algae reference materials, IAEA-391, 392, 393 and IAEA-140, prepared by the International Atomic Energy Agency for intercomparison under different preparation conditions were analyzed for 24 elements. Conventional neutron activation analysis (NAA) was used to determine Al, Ca, Cl, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Sb, V and Zn and epithermal NAA using BN and Cd as shielding material for I, Br, As, Ni, Mo and Cd. Inductively coupled plasma mass spectrometry (ICP-MS) as a complementary and comparative method was applied to determine Pb, Sn, Ni, Sr, Rb, As, Co, Cr, Cu, Mn, V, Mo and Zn. Two analytical quality control standard reference materials IAEA-V-10 (hay powder) and IAEA-331 (spinach) were analyzed simultaneously with real samples. The results agree quite well with each other and with the certified values.     

Improved online δ18O measurements of nitrogen- and sulfur-bearing organic materials and a proposed analytical protocol

It is well known that N(2) in the ion source of a mass spectrometer interferes with the CO background during the δ(18)O measurement of carbon monoxide. A similar problem arises with the high-temperature conversion (HTC) analysis of nitrogenous O-bearing samples (e.g. nitrates and keratins) to CO for δ(18)O measurement, where the sample introduces a significant N(2) peak before the CO peak, making determination of accurate oxygen isotope ratios difficult. Although using a gas chromatography (GC) column longer than that commonly provided by manufacturers (0.6 m) can improve the efficiency of separation of CO and N(2) and using a valve to divert nitrogen and prevent it from entering the ion source of a mass spectrometer improved measurement results, biased δ(18)O values could still be obtained. A careful evaluation of the performance of the GC separation column was carried out. With optimal GC columns, the δ(18)O reproducibility of human hair keratins and other keratin materials was better than ± 0.15 ‰ (n=5; for the internal analytical reproducibility), and better than ± 0.10 ‰ (n=4; for the external analytical reproducibility).     

Particle size determination of some IAEA and NIST environmental and biological reference materials

Particle size distributions in fifteen International Atomic Energy Agency (IAEA) and sixteen National Institute of Standards and Technology (NIST) reference materials (RMs) were measured with the basic aim to investigate the potential of these materials to be used as reference or quality control materials in analyses where a small sample mass is required (< 100 mg). Most of the investigated materials are commercially available environmental or biological natural matrix RMs with certified values for trace elements, radionuclides, or organometallic compounds. The laser diffraction technique was used in all measurements. From the point of particle size distribution, materials IAEA-390 (Algae, a set of three materials), IAEA-396m (Urban Dust, 3x air jet milled), NIST-SRM 1515 (Apple Leaves), NIST-SRM 1547 (Peach Leaves), NIST-SRM 1566a (Oyster Tissue), NIST-SRM 1570a (Spinach), NIST-SRM 1573a (Tomato Leaves), and NIST-SRM 1648 (Urban Particulate) can be considered appropriate for small sample mass analysis. However, additional analytical tests are needed to confirm the appropriate homogeneous distribution of chemical composition, the level of heterogeneity for individual elements, at this sample mass level.     

Critical evaluation of laboratory performance from IAEA-452 (fish scallop) inter-comparison exercise results

The International Atomic Energy Agency Marine Environmental Laboratory (IAEA-MEL) conducted an intercomparison exercise for the “Trace elements and methyl mercury in fish scallop: IAEA-452” in 2009. The Neutron Activation Analysis (NAA) laboratory at PINSTECH, Pakistan took part in this exercise. Comparison with the IAEA results showed that of the 18 elements reported to the IAEA 7 had unacceptable z-scores while the data for two elements was questionable (2< |z-score| <3). Upon investigation it was discovered that the large number of elements having unacceptable z-scores was due to human error and the greater difficulty in analyzing biological samples which have low amounts of trace elements. Therefore selection of values corresponding to matrix matched reference materials (RMs), such as IAEA-436 and IAEA-407, and to non-interfering peaks the results obtained became comparable to the IAEA results with only the result for As having |z-score|> 3.