Isotope Dilution Measurements Research Articles

A simple and reliable measurement procedure for determination of glycocholic acid in human serum by isotope-dilution liquid chromatography-tandem mass spectrometry

Glycocholic acid (GCA) is an important identified biomarker for various hepatobiliary diseases. However, results obtained from routine clinical measurement significantly varied due to the interference from the extremely similar structure analogues, which cause confusion to clinical diagnosis. In this study, a bracketing calibration-based isotope dilution liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS) method was developed. The established method exhibited good performance criteria through the adoption of isotope dilution, calibrator bracketing, and gravimetric measurements. The satisfactory precision (0.16%–2.01%) and good recoveries were achieved at five spiked levels (98.0–100.9%). The limit of detection (LOD) and limit of quantification (LOQ) were 0.01 ng/mL and 0.05 ng/mL, respectively. No interference, matrix effect, and carryover were observed. The sample preparation process was easier to operate, without further solvent evaporation and time-saving. The optimized method was successfully applied to quantify GCA with a wide range of concentrations in clinical human serum samples and also compared with clinical routine systems.

Application of regression methods to solve general isotope dilution measurement equations

Isotope dilution is among the most accurate quantitation approaches in chemical analysis. This calibration method is often employed using a plurality of mathematical formulations. While most analysts find the calibration curve approach most appealing, there is a lack of rigorous general procedures involving calibration curves in isotope dilution and analysts resort to empirical polynomial calibration functions. In this contribution we discuss the adoption of regression analysis, commonly known as least squares methods, to solve isotope dilution equations of varied complexity. This manuscript introduces general regression-based methods to isotope dilution applicable to all known variants of classical isotope dilution known to date, including the fusion of the isotope dilution and standard additions methods.

A highly‑enriched 244Pu reference material for nuclear safeguards and nuclear forensics measurements.

A highly-enriched 244Pu isotope dilution reference material has been prepared and characterized for metrologically traceable measurements of very small quantities of plutonium. The amount of plutonium in samples associated with nuclear safeguards and nuclear forensic measurements can be significantly less than 1 ng. Accordingly, the ability to quantify the amount and isotopic composition of plutonium from a single mass-spectrometric analysis is particularly desirable. The highly-enriched 244Pu reference material, described here, will minimize the magnitude of spike corrections necessary to obtain accurate information on plutonium isotopic composition from isotope dilution measurements.

Equilibrating stable Cd isotopes with acidic soils

ABSTRACT Cadmium (Cd) in the soil solution is in dynamic equilibrium with the reservoir of bioavailable Cd attached to the solid phase, i.e. the labile pool (CdE). Traditionally, CdE is estimated using the radioisotope 109Cd, which has severely restricted access to estimates of CdE. Using stable isotope dilution and isotope ratio measurement by inductive coupled plasma-quadrupole mass spectroscopy (ICP-QMS) would increase access to estimates of CdE; however, detail remains scant about the optimal conditions for equilibration and measurement. We report optimal conditions for spiking with 110Cd, batch equilibration and ICP-QMS measurement of the ratio of 110Cd to 111Cd using results for six acidic soils with total Cd concentrations of 0.19–6.4 mg Cd kg−1, suspended in three background electrolytes (10 mM CaCl2, 1 M NH4NO3, and 1 M NH4Cl). Our optimised procedure produces robust estimates of CdE. Application of this approach will greatly increase access to estimates of CdE and to the investigation of its role in Cd uptake by plants.

A straightforward separation scheme for the concomitant isolation of Lu, Hf, Sm, and Nd prior to isotope ratio and isotope dilution measurements following sample decomposition by Lithium metaborate fusion or Hydrofluoric acid dissolution

A straightforward separation scheme for the concomitant isolation of Lu, Hf, Sm, and Nd prior to isotope ratio and isotope dilution measurements following sample decomposition by Lithium metaborate fusion or Hydrofluoric acid dissolution

La-Ce isotope measurements by multicollector-ICPMS.

The 138La-138Ce decay system (half-life 1.02 × 1011 years) is a potentially highly useful tool to unravel information about the timing of geological processes and about the interaction of geological reservoirs on earth, complementing information from the more popular 147Sm-143Nd and 176Lu-176Hf isotope systems. Previously published analytical protocols were limited to TIMS. Here we present for the first time an analytical protocol that employs MC-ICPMS, with an improved precision and sensitivity. To perform sufficiently accurate La-Ce measurements, an efficient ion-chromatographic procedure is required to separate Ce from the other rare earth elements (REE) and Ba quantitatively. This study presents an improved ion-chromatographic procedure that separates La and Ce from rock samples using a three-step column separation. After REE separation by cation exchange, Ce is separated employing an Ln Spec column and selective oxidation. In the last step, a cation clean-up chemistry is performed to remove all remaining interferences. Our MC-ICPMS measurement protocol includes all stable Ce isotopes (136Ce, 138Ce, 140Ce and 142Ce), by employing a 1010 ohm amplifier for the most abundant isotope 140Ce. An external reproducibility of ±0.25ε-units (2 r.s.d) has been routinely achieved for 138Ce measurements for as little as 150-600 ng Ce, depending on the sample-skimmer cone combinations being used. Because the traditionally used JMC-304 Ce reference material is not commercially available anymore, a new reference material was prepared from AMES laboratory Ce metal (Cologne-AMES). In order to compare the new material with the previously reported isotopic composition of AMES material prepared at Mainz (Mainz-AMES), Cologne-AMES and JMC-304 were measured relative to each other in the same analytical session, demonstrating isotope heterogeneity between the two AMES and different JMC-304 batches used in the literature. To enable sufficiently precise age correction of radiogenic 138Ce and to perform isochron dating, a protocol was developed where La and Ce concentrations are determined by isotope dilution (ID), using an isotope tracer enriched in 138La and 142Ce. The new protocols were applied to determine the variations of Ce isotope compositions and La-Ce concentrations of certified geochemical reference materials (CRMs): BCR-2, BCR-1, BHVO-2, JR-1, JA-2, JB-3, JG-1, JR-1, JB-1b, AGV-1 and one in-house La Palma standard.

Evaluation of an Isotope Dilution HPLC Tandem Mass Spectrometry Candidate Reference Measurement Procedure for Total 17-β Estradiol in Human Serum.

The inaccuracy of 17-β estradiol (E2) measurements affects its use as a biomarker in patient care and research. Clinical and research communities called for accurate and standardized E2 measurements. Reference Measurement Procedures (RMPs), part of the CDC Hormone Standardization Program (HoSt), are essential in addressing this need and ensuring that methods are accurate and comparable across testing systems, laboratories, and over time. A candidate RMP (cRMP) was developed for the measurement of total E2 in serum using liquid chromatography-tandem mass spectrometry (LC-MS/MS) without derivatization. The cRMP meets suggested performance criteria for accuracy and precision through the use of isotope dilution, calibrator bracketing, and gravimetric measurements. The cRMP demonstrated high agreement with certified reference materials (no significant bias to BCR576, 577, and 578) and established RMPs (slope 1.00, 95% CI 1.00-1.01; intercept 0.02, 95% CI -0.01 to 0.06). The cRMP is highly precise with intra-assay, interassay, and total percent CVs of 2.7%, 1.3%, and 2.4%, respectively. A higher specificity was achieved by measuring E2 without derivatization, compared to methods using derivatization agents. The cRMP can serve as a higher-order standard for establishing measurement traceability and provides an accuracy base against which routine methods can be compared in HoSt.

Ultra-trace level speciated isotope dilution measurement of Cr(VI) using ion chromatography tandem mass spectrometry in environmental waters

The reliable analysis of highly toxic hexavalent chromium, Cr(VI), at ultra-trace levels remains challenging, given its easy conversion to non-toxic trivalent chromium. This work demonstrates a novel analytical method to quantify Cr(VI) at low ngL−1 concentration levels in environmental water samples by using speciated isotope dilution (SID) analysis and double-spiking with Cr(III) and Cr(VI) enriched for different isotopes. Ion chromatography tandem mass spectrometry (IC-MS/MS) was used for the analysis of Cr(VI) as HCrO4− → CrO3−. Whereas the classical linear multipoint calibration (MPC) curve approach obtained a method detection limit (MDL) of 7ngL−1 Cr(VI), the modified SID-MS method adapted from U. S. EPA 6800 allowed for the quantification of Cr(VI) with an MDL of 2ngL−1 and provided results corrected for Cr(VI) loss occurred after sample collection. The adapted SID-MS approach proved to yield more accurate and precise results than the MPC method, allowed for compensation of Cr(VI) reduction during sample transportation and storage while eliminating the need for frequent external calibration. The developed method is a complementary tool to routinely used inductively-coupled plasma (ICP) MS and circumvents typically experienced interferences.

A thoroughly validated spreadsheet for calculating isotopic abundances (2H, 17O, 18O) for mixtures of waters with different isotopic compositions.

Oxygen and hydrogen stable isotopes are widely used tracers for studies on naturally occurring and laboratory mixtures of isotopically different waters. Although the mixing calculations are straightforward to perform, there are ample possibilities to make mistakes, especially when dealing with a large number of mixed fluids. To facilitate isotope mixing calculations and to avoid computational mistakes, a flexible tool to carry out these calculations is in demand. We developed, in three independent efforts, spreadsheets to carry out the mixing calculations for a combination of waters with different isotopic compositions using the isotope mass balance equation. We validated our calculations by comparison of the results of the three spreadsheets for a large number of test calculations. For all the cases, we obtained identical results down to the 12(th) to 14(th) significant digit. We present a user-friendly, thoroughly validated spreadsheet for calculating (2) H, (17) O and (18) O stable isotopic abundances and respective isotope delta values for mixtures of waters with arbitrary isotopic compositions. The spreadsheet allows the mixing of up to 10 different waters, of which up to five can be specified using their isotopic abundances and up to five others using their isotope delta values. The spreadsheet is implemented in Microsoft Excel and is freely available from our research groups' websites. The present tool will be applicable in the production and characterization of singly and doubly labeled water (DLW) mother solutions, the analysis of isotope dilution measurements, the deduction of unknown isotope values of constituents for mixtures of natural waters, and many other applications.

Body composition measurement methods and their clinical application

Body composition measurement could significantly facilitate evaluating the content of various body components, getting information about the health status and assessing the diagnosis, treatment, test, and prognosis of several diseases. Human health will be seriously affected by the imbalance of body composition. Body composition measurement methods are generally divided into local measurement methods and general measurement methods. The former ones include skinfold thickness measurement, ultrasonic measurement, computed tomography and nuclear magnetic resonance measurement; while the latter ones include anthropometry, underwater weighing measurement, air displacement measurement, biological resistance measurement, near infrared absorption measurement, dual-energy X-ray absorptiometry measurement, isotope dilution measurement, potassium isotope measurement and three-dimensional body scanning measurement, etc. This article reviews a variety of body composition measurement methods and their clinical application. Key words: Human body composition measurement method; Clinical application; Body fat

Americium and plutonium separation by extraction chromatography for determination by accelerator mass spectrometry

A simple method was developed to separate Pu and Am using single column extraction chromatography employing N,N,N′,N′-tetra-n-octyldiglycolamide (DGA) resin. Isotope dilution measurements of Am and Pu were performed using accelerator mass spectrometry (AMS) and alpha spectrometry. For maximum adsorption Pu was stabilized in the tetra valent oxidation state in 8M HNO3 with 0.05M NaNO2 before loading the sample onto the resin. Am(III) was adsorbed also onto the resin from concentrated HNO3, and desorbed with 0.1M HCl while keeping the Pu adsorbed. The on-column reduction of Pu(IV) to Pu(III) with 0.02M TiCl3 facilitated the complete desorption of Pu. Interferences (e.g. Ca2+, Fe3+) were washed off from the resin bed with excess HNO3. Using NdF3, micro-precipitates of the separated isotopes were prepared for analysis by both AMS and alpha spectrometry. The recovery was 97.7±5.3% and 95.5±4.6% for 241Am and 242Pu respectively in reagents without a matrix. The recoveries of the same isotopes were 99.1±6.0 and 96.8±5.3% respectively in garden soil. The robustness of the method was validated using certified reference materials (IAEA 384 and IAEA 385). The measurements agree with the certified values over a range of about 1–100Bqkg−1. The single column separation of Pu and Am saves reagents, separation time, and cost.

Correction of spike contribution for strontium isotopic measurement by thermal ionization mass spectrometry: a test for spike-standard mixed solutions

The Rb-Sr isotope system has long been used for radiometric dating and petrogenetic investigation. The concentrations of Rb and Sr could be precisely measured by isotope dilution thermal ionization mass spectrometry combined with chemical purification of these elements. For the simultaneous measurement of Sr isotopic composition and isotope dilution, the contribution from the added spike should be carefully corrected. Reliable 87Sr/86Sr ratios of the spike-standard mixed solutions were obtained using the new mass bias factor calculated on the basis of measured Sr isotopic ratios. This correction yielded reasonable 87Sr/86Sr ratios for overspiked standard solutions whose spike fractions reaching 25 wt.%. The correction procedure described in this study shows that the simultaneous measurement of Sr isotopic composition and isotope dilution is available for the case that the spike proportion in the sample-spike mixture is significantly high (84Sr/86Sr > 3.7). The principle of this correction protocol can also be applied to other isotope systems such as Sm-Nd and Lu-Hf pairs.

Measurement of non-separated U/Pu samples: optimization of TIMS procedures for safeguards purposes at Rokkasho on-site laboratory

The on-site laboratory (OSL) at Rokkasho Reprocessing Plant (RRP) is jointly operated by the Japanese authority Nuclear Material Control Centre and the International Atomic Energy Agency (IAEA) and provides, together with the Nuclear Material Laboratory (NML) at Seibersdorf, analytical services to the IAEA’s inspectorate. OSL deals with a variety of samples typical to a reprocessing plant including pure product solutions of uranium and plutonium but also mixed U/Pu solutions originating from various stages of the chemical process. For a significant proportion of the samples, the requirement on measurement accuracy and precision from the Inspectorate makes the use of thermal ionization mass spectrometry (TIMS) indispensible. Until recently, all samples intended for TIMS had to undergo time-consuming U/Pu separation before isotope dilution measurement. The need for rapid reporting of analytical results for certain safeguards samples evoked the idea of performing TIMS measurements without prior U/Pu separation for mixed U/Pu products as they are obtained from the PUREX process at RRP. For this purpose, a systematic study was initiated to probe the figure of merits and limitations of conducting TIMS analyses on mixed U/Pu samples and, in particular, whether the accuracy and precision of the main ratios of interest, n(235U)/n(238U) and n(240Pu)/n(239Pu), are influenced by the presence of larger amounts of the other element. A series of synthetic mixtures with U/Pu ratios ranging from 1:10 up to 100:1 were prepared and measured in both laboratories—OSL and NML—using ThermoFisher TRITON multi-collector TIMS instruments. For the n(235U)/n(238U) ratio, interference due to 238Pu was observed which can be significant depending on the U/Pu ratio and the 238Pu abundance. However, for the n(240Pu)/n(239Pu) ratio, which is of premier importance for safeguarding RRP, no significant interference arising from the concomitant U was detected independently of enrichment. Even in samples with an excess of U (U/Pu ratio of 100:1), compliance with International Target Values (ITV2010) was demonstrated for n(240Pu)/n(239Pu) results with a relative difference to certified not exceeding 0.01 %.

Online preconcentration ICP‐MS analysis of rare earth elements in seawater

The rare earth elements (REEs) with their systematically varying properties are powerful tracers of continental inputs, particle scavenging intensity and the oxidation state of seawater. However, their generally low (∼pmol/kg) concentrations in seawater and fractionation potential during chemical treatment makes them difficult to measure. Here we report a technique using an automated preconcentration system, which efficiently separates seawater matrix elements and elutes the preconcentrated sample directly into the spray chamber of an ICP‐MS instrument. The commercially available “seaFAST” system (Elemental Scientific Inc.) makes use of a resin with ethylenediaminetriacetic acid and iminodiacetic acid functional groups to preconcentrate REEs and other metals while anions and alkali and alkaline earth cations are washed out. Repeated measurements of seawater from 2000 m water depth in the Southern Ocean allows the external precision (2σ) of the technique to be estimated at <23% for all REEs and <15% for most. Comparison of Nd concentrations with isotope dilution measurements for 69 samples demonstrates that the two techniques generally agree within 15%. Accuracy was found to be good for all REEs by using a five point standard addition analysis of one sample and comparing measurements of mine water reference materials diluted with a NaCl matrix with recommended values in the literature. This makes the online preconcentration ICP‐MS technique advantageous for the minimal sample preparation required and the relatively small sample volume consumed (7 mL) thus enabling large data sets for the REEs in seawater to be rapidly acquired.

Precise measurement of 228Ra/226Ra ratios and Ra concentrations in seawater samples by multi-collector ICP mass spectrometry

We have developed a method for the precise measurement of 228Ra/226Ra ratios and 228Ra and 226Ra concentrations in seawater by multi-collector ICP mass spectrometry (MC-ICP-MS). The measurement of Ra isotopes has typically relied on decay counting techniques with large volume seawater samples. Adapting pre-concentration approaches from such previous studies, our new protocol pumps 50 to 150 L of seawater through Mn-fibre cartridges to pre-concentrate Ra. The Mn-fibres are ashed and then leached with 6 N HCl to remove Ra which is then precipitated with SrSO4. The SrSO4 is converted to SrCO3 for dissolution, before purification of Ra using two ion-exchange columns—one with AG50-X8 resin and the other with Sr-spec resin. The resulting purified Ra aliquot from the Mn-fibre is used to assess the 228Ra/226Ra ratio of seawater. A separate 120 to 500 mL sample of the same seawater is also collected and used to assess 226Ra concentrations by MC-ICP-MS, by applying isotope dilution with a 228Ra spike. Seawater 228Ra concentrations are calculated by multiplying 228Ra/226Ra ratios from the Mn-fibre by 226Ra concentrations from the isotope dilution measurement. Measurements of 228Ra/226Ra ratios on an in-house Ra standard demonstrate an internal precision of 1.7% (2 S.E.) and an external repeatability of 2% (2 S.D.). Similar external repeatability is demonstrated for four 100 litre seawater samples from Southampton Docks. The accuracy of the technique, relative to commonly used decay-counting approaches, has been demonstrated for seawater 228Ra and 226Ra concentrations using samples collected as part of the GEOTRACES intercalibration effort. Using comparable volumes of seawater to those for existing analytical techniques, MC-ICP-MS provides a lower detection limit (∼0.1 fg 228Ra), higher precision (2%) and a shorter counting time (20 minutes) for measurement of 228Ra and 226Ra concentrations (although it requires a longer pretreatment time for Ra purification). This technique will allow study of 228Ra in the ocean interior where concentrations are low and will extend the powerful use of this isotope to understand mixing and trace element fluxes in the oceans.

Two time-point assessment of bile acid kinetics in humans using stable isotopes

The stable isotope dilution measurement of bile acid pool sizes and turnover rates in humans has involved the collection of nine blood samples over four days. This precludes widespread application to larger population studies. This study describes a two time-point approach for blood sampling without loss of statistical power. Isotopic decay curves of cholic acid, chenodeoxycholic acid and deoxycholic acid acquired in three recent human studies were analysed. The optimal combination of two time-points was determined. Time-points around 12 and 72 h after administration allowed for the most accurate description of the decay curves and prediction of kinetic parameters. Analyses of 39 statistical comparisons of kinetic parameters based upon the two time-points and all time-points approaches exhibited only one slightly discrepant result. In conclusion, for group comparison of bile acid kinetics in humans, a two time-point blood collection approach at time-points near 12 and 72 h provides statistically reliable data.

Effects of Non-protein Energy Intake on Whole Body Protein Synthesis, Nitrogen Retention and Glucose Turnover in Goats

The responses of whole body protein and glucose kinetics and of nitrogen (N) metabolism to non-protein energy intake (NPEI) were determined using an isotope dilution approach and measurement of N balance in three adult male goats. The diets containing 1.0, 1.5 and 2.0 times ME maintenance requirement, with fixed intake of CP (1.5 times maintenance) and percentage of hay (33%), were fed twice daily for each 21 d experimental period. After an adaptation period of 11 d, N balance was determined over 3 d. On day 17, whole body protein synthesis (WBPS) and glucose irreversible loss rate (ILR) were determined during the absorptive state by a primed-continuous infusion of []phenylalanine, []tyrosine, []tyrosine and []glucose, with simultaneous measurements of plasma concentrations of metabolites and insulin. Ruminal characteristics were also measured at 6 h after feeding over 3 d. Nitrogen retention tended to increase (p) that of tyrosine. Whole body protein synthesis increased (p). These results suggest that increasing NPEI may enhance WBPS and glucose turnover at the absorptive state and improve the efficiency of digestible N retention in goats, with possibly decreased ammonia and increased amino acid absorption. In addition, simultaneous increases in WBPS and glucose ILR suggest stimulatory effect of glucose availability on WBPS, especially when sufficient amino acid is supplied.

Age calibration of the Fish Canyon sanidine 40Ar/ 39Ar dating standard using primary K–Ar standards

The 40Ar/ 39Ar dating technique requires the use of neutron fluence monitors (standards). Precise calibrations of these standards are crucial to decrease the uncertainties associated with 40Ar/ 39Ar dates. Optimal calibration of 40Ar/ 39Ar standards should be based on K/Ar standards having independent isotope dilution measurements of 40K and 40Ar*, based on independent isotope tracers (spikes) because this offers the possibility to eliminate random interlaboratory errors. In this study, we calibrate the widely used Fish Canyon sanidine (FCs) standard based on four primary K/Ar standards (GA-1550, Hb3gr, NL-25, and GHC-305) on which K and Ar* concentrations have been determined in different labs with independently calibrated tracers. We obtained a mean age of 28.03 ± 0.08 Ma (1 σ; neglecting uncertainties of the 40K decay constants) for FCs, based on the decay constant recommended by Steiger and Jäger [Steiger R.H., Jäger. E. 1977. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362.]. This age corresponds to a mean 40Ar*/ 40K value of (1.6407 ± 0.0047) × 10 −3. We also discuss several criteria that prevent the use of previous calibrations of FCs based on other primary standards (LP-6, SB-3 and MMhb-1). The age of FCs obtained in this study is based on the 40K decay constants of Steiger and Jäger (1977) but we anticipate the imminent need for revision of the value and precision of the 40K decay constants (representing the main source of uncertainties in 40Ar/ 39Ar dating). The 40Ar*/ 40K result of FCs obtained in this study allows therefore a rapid calibration of the age of FCs with uncertainties at the 0.29% level but perhaps more importantly this value is independent of any particular value of the 40K decay constants and may be used in the future in conjunction with revised decay constants.

Methods for the microsampling and high-precision analysis of strontium and rubidium isotopes at single crystal scale for petrological and geochronological applications

Micromilling of single crystals to yield microgram-sized solid samples for subsequent isotopic analysis can yield important petrogenetic information from crystals (especially feldspars) in magmatic rocks. Exceptional detail can be gained because the textural context of the sample area can be fully evaluated before milling. Such information is otherwise unknown or lost when performing analysis at the bulk rock scale. Here we present a comprehensive methodology for the precise analysis of ng-levels of Rb and Sr purified from microgram-sized solid samples extracted from minerals in thin sections. Physical sampling techniques are based around a computer numerical control (CNC) milling machine (Micromill™), newly designed specifically for the sampling of complex accretionary and growth structures in crystalline materials. Chemical procedures for the isolation of Sr and Rb for TIMS and MC-ICPMS analysis, respectively, are presented, and the performance of these analytical techniques is assessed in the context of the small sample sizes produced by micromilling. Despite being time consuming, mechanical sampling, conventional dissolution and chemical separation followed by analysis by TIMS remain the best methods for both highly accurate and precise determinations of Sr isotopic compositions in most geological materials over a wide range of Sr concentrations, Rb/Sr ratios and matrix types. Using these techniques, it is possible to achieve long-term 2 S.D. external precisions of 50 ppm for load sizes as small as 3 ng Sr. We demonstrate the validity of these techniques using two examples. The first is from a < 50 ka single feldspar crystal from Parinacota volcano (Chile) to show that 87Sr/ 86Sr isotope ranges as small as 0.00006 (2 × 2 S.D. external) can be resolved under conditions where radiogenic-Sr ingrowth can be neglected. The second is from the 28.4 Ma Fish Canyon Tuff (Colorado) to demonstrate the utility of isotope dilution measurements of Rb and Sr contents to calculate 87Rb/ 86Sr and thus perform age corrections to the 87Sr/ 86Sr ratios in order to establish 87Sr/ 86Sr i variations between single crystals or zones. We demonstrate that biotite crystals in the Fish Canyon Tuff show Sr-isotopic variations well beyond analytical errors and hence that the crystals involved were not in isotopic equilibrium and cannot be used to establish an isochron age. On the other hand, the precision of our isotope dilution measurements may be used to obtain Rb–Sr geochronological information at the single crystal scale, provided that homogeneity in 87Sr/ 86Sr at the time of crystallisation can be demonstrated.

Suitability of tandem-in-time mass spectrometry for polybrominated diphenylether measurement in fish and shellfish samples: Comparison with high resolution mass spectrometry

The first part of the present study focused on the development of an alternative automated sample preparation method for the measurement of selected polybrominated diphenyl ethers (PBDEs) in fish and shellfish. A previously developed automated method has been further optimized and simplified to decrease blank levels and cost. Sorbent and solvent quantities have been significantly reduced without altering extract quality. The second part of the study consisted of comparing tandem-in-time quadrupole ion storage mass spectrometry (QISTMS/MS) to high resolution mass spectrometry (HRMS) for the isotope dilution (ID) measurement step after gas chromatography (GC) separation. Both mass spectrometric methods performed similarly in terms of accuracy but better precision was observed for HRMS. Although better sensitivity can be attained with the high resolution sector instrument, method limits of quantification (mLOQs) were very similar for both approaches as they were dependent on the procedural blanks levels. The mLOQ values ranged between 0.04 and 3.56 ng/g fat, depending on the congener. They allowed the unambiguous identification and quantification of all target analytes, except for BDE-183, in most considered fish extracts. An analytical procedure based on rapid automated sample preparation and QISTMS/MS appeared to be suitable for the measurement of PBDEs in fish and shellfish speciment under quality assurance/quality control (QA/QC) criteria.