Isotope dilution analysis in column-coupling isotachophoresis

The use of isotope dilution technique to eliminate problems associated with demands for a high load capacity in column-coupling capillary isotachophoresis was studied. Determinations of phosphate present in a model mixture and in white wine served for the evaluation of this approach to quantitative analysis in isotachophoresis. A high selectivity of the analysis with acceptable accuracy and precision of the determination also in complex ionic mixtures in a short time are obvious advantages of this approach.     

Isotope dilution analysis as a definitive tool for the speciation of organotin compounds

Different spike solutions available for the determination of butyltin compounds by isotope dilution analysis are described and applied for the determination of butyltin compounds in PACS-2 certified reference material. Additionally, those spike solutions were evaluated during the course of an interlaboratory exercise organised by the National Research Council of Canada and the Laboratory of the Government Chemist (UK) in order to quantify tributyltin in a pilot sediment. The aim of this project was to evaluate the capabilities of isotope dilution mass spectrometry to reduce the uncertainty in the certification of Reference Materials for the speciation of organotin compounds. All participants were supplied with a 17Sn-enriched TBT solution from the Laboratory of the Government Chemist (UK). In our case, we performed the analysis of the pilot sediment also using a 119Sn enriched spike (mixed mono-, di- and tributyltin) and a 118Sn-119Sn double spike. The use of these additional spike solutions not only allowed the determination of monobutyltin and dibutyltin in the pilot sediment but also the evaluation and correction of possible extraction-derived rearrangement reactions. An excellent agreement amongst our results and between the participants was obtained with a precision of 8.4% RSD at a level of ca. 80 ng TBT g(-1) (as Sn).     

Isotope dilution analysis mass spectrometry for the routine measurement of butyltin compounds in marine environmental and biological samples

The highly qualified primary method of species-specific isotope dilution analysis has been employed in this work to evaluate for the first time the levels of butyltin compounds in the estuary of the river Eo (Northwest Spain) where there is, since many years, a high oyster farming activity. A spike solution containing mono-, di- and tributyltin enriched in ¹¹⁹Sn allowed the simultaneous determination of the three compounds in different marine environmental and biological samples collected in this area (seawater samples, sediments and biological tissues of four different marine species). The results obtained in this work showed toxic TBT levels for many marine species in 45% of the seawater samples analyzed whereas significant organotin concentrations were found to be obtained only in one of the sediments analyzed. On the other hand, TBT levels ranging from 20 to almost 200 ng g^− 1 (dry weight) were obtained in the different biological tissues analyzed demonstrating the bioaccumulation of organotin compounds in certain marine species.     

Isotope dilution analysis of selenite and selenate in natural water using microwave-induced nitrogen plasma mass spectrometry.

Isotope dilution analysis of the sub-microg l(-1) levels of selenite and selenate in natural water samples by microwave-induced nitrogen plasma mass spectrometry (MIP-MS) was performed. An appropriate amount of a spike solution containing 78Se-selenite and 78Se-selenate was added to the natural water sample to be analyzed. Both analytes in the water were then concentrated simultaneously by passing the sample through a column that was filled with an anionic exchange resin. After the concentration process, all of the selenite and some of the selenate on the resin were eluted by 0.03 M nitric acid. The residual selenate was eluted by 0.13 M nitric acid. The eluted sample solutions were injected into MIP-MS, and isotope dilution analyses were carried out. Selenite and selenate concentrations as low as 0.01 microg l(-1) in the natural water sample were successfully determined by the proposed method.     

Isotope exchange of iodine in iodide-iodate melts studied by radio paper chromatography

The isotopic exchange reaction of iodine in fused NaI–NaIO₃ and KI–KIO₃ systems, which does not proceed until the mixture is melted, is found to be almost instantaneous in homogeneous melts. The equilibrium is attained within a period of less than 5 min at a temperature 5°C above the melting temperature of each composition for both systems. No noticeable exchange is observed even after heating the samples for 30 min at a temperature 20°C below the corresponding melting point. A single-step bimolecular association-dissociation mechanism is proposed for the exchange.     

On-line species-unspecific isotope dilution analysis in the picomolar range reveals the time- and species-depending mercury uptake in human astrocytes

In order to reveal the time-depending mercury species uptake by human astrocytes, a novel approach for total mercury analysis is presented, which uses an accelerated sample introduction system combined on-line with an inductively coupled plasma mass spectrometer equipped with a collision/reaction cell. Human astrocyte samples were incubated with inorganic mercury (HgCl₂), methylmercury chloride (MeHgCl), and thimerosal. After 1-h incubation with Hg²⁺, cellular concentrations of 3 μM were obtained, whereas for organic species, concentrations of 14–18 μM could be found. After 24 h, a cellular accumulation factor of 0.3 was observed for the cells incubated with Hg²⁺, whereas the organic species both showed values of about 5. Due to the obtained steady-state signals, reliable results with relative standard deviations of well below 5 % and limits of detection in the concentration range of 1 ng L^?1 were obtained using external calibration and species-unspecific isotope dilution analysis approaches. The results were further validated using atomic fluorescence spectrometry.

Isotope dilution determination of polycyclic aromatic hydrocarbons in olive pomace oil by gas chromatography-mass spectrometry

A gas chromatographic (GC) method with mass spectrometry detection (MS) for the determination of eight polycyclic aromatic hydrocarbons (PAHs) in olive pomace oil has been developed. The oil was diluted with n-pentane and extracted by liquid-liquid partition with dimethyl sulphoxide (DMSO). After water addition and back-extraction with cyclohexane, a thin-layer chromatography on silica gel was performed as a further purification step. The PAHs spot was scraped off from the plate and the final extract was concentrated and analysed by GC-MS in full scan mode. The eight PAHs under investigation were determined in the presence of the corresponding labelled compounds added as internal standards to the sample at the beginning of the analytical process. The identified PAHs were then quantified by the isotope dilution methodology assuring the compensation of the concentration of each analyte for any variation in the sample preparation. The method precision was satisfactory with relative standard deviation (R.S.D.) values in the range 3.6-12.7% for all PAHs. The average recovery rates ranged from 69.0 to 97.5%. Accuracy was also calculated for benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene and benzo[ghi]perylene by analysing a certified reference material (CRM 458, coconut oil) with adequate results. All response curves exhibited a linear fit from 0.1 to 10 microg ml(-1) and the determination coefficients R2 were better than 0.9942. The limits of detection (0.1-0.4 microg kg(-1)) were acceptable when compared with the maximum permitted limit of 2 microg kg(-1) for each of the eight considered PAHs and 5 microg kg(-1) for the sum of the eight PAHs established by the Italian legislation. Measurement uncertainty was finally calculated identifying and quantifying the uncertainty components of the analytical process. The relative expanded uncertainties (Uc), expressed as percent values were in the range 8.5-11.4% thus appropriate for residues quantification in the range of concentrations considered in the present study.     

Isotope dilution GC-MS routine method for the determination of butyltin compounds in water

A standard GC-MS instrument with electron impact ionisation has been used to develop a fast, simple and reliable method for the simultaneous determination of monobutyltin (MBT), dibutyltin (DBT) and tributyltin (TBT) in water samples. Isotope dilution analysis (IDA) is used for the determination of species, taking advantage of a commercially available spike solution containing a mixture of MBT, DBT and TBT enriched in ¹¹⁹Sn. Method detection limits for 100-mL samples were between 0.18 and 0.25 ng L⁻¹ for the three butyltin compounds with typical RSD between 2 and 4% at levels between 100 and 10 ng L⁻¹, respectively. Recovery of tin species in spiked samples (natural water, wastewater and seawater) was quantitative. The stability of butyltin compounds in collected seawater samples was also studied. The addition of a 1% (v/v) glacial acetic acid preserved tin species in the samples for at least 5 days at room temperature. The IDA method was finally implemented in a routine testing laboratory and it was subsequently accredited by the Spanish National Accreditation Body according to the requirements of UNE-EN ISO/IEC 17025.     

Iodine determination in milk by isotope dilution analysis

Iodine is an essential trace element in the human diet. It is utilized in the synthesis of L-monoiodothyrosine, which is a precursor of the thyroid hormones, thyroxin and triiodothyronin. In this study, a method that can determine iodine concentrations in milk has been developed. The method is based on substoichiometric isotope dilution analysis (IDA). Iodine concentrations were measured in milk, which has been collected from three big cities and environment municipalities of Aegean Region. The iodine concentration ranges were within 436.8 and 896.7 μg/kg in milk samples. Mean iodine concentration was not greatly changed from season to season, between 1998 and 2002. This revised version was published online in August 2006 with corrections to the Cover Date.     

Automated headspace-solid-phase micro extraction-retention time locked-isotope dilution gas chromatography-mass spectrometry for the analysis of organotin compounds in water and sediment samples

An automated method for the simultaneous determination of six important organotin compounds namely monobutyltin (MBT), dibutyltin (DBT), tributyltin (TBT), monophenyltin (MPhT), diphenyltin (DPhT) and triphenyltin (TPhT) in water and sediment samples is described. The method is based on derivatization with sodium tetraethylborate followed by automated headspace-solid-phase micro extraction (SPME) combined with GC-MS under retention time locked (RTL) conditions. Home-synthesized deuterated organotin analogues were used as internal standards. Two high abundant fragment ions corresponding to the main tin isotopes Sn118 and Sn120 were chosen; one for quantification and one as qualifier ion. The method was validated and excellent figures of merit were obtained. Limits of quantification (LOQs) are from 1.3 to 15 ng l(-1) (ppt) for water samples and from 1.0 to 6.3 microg kg(-1) (ppb) for sediment samples. Accuracy for sediment samples was tested on spiked real-life sediment samples and on a reference PACS-2 marine harbor sediment. The developed method was used in a case-study at the harbor of Antwerp where sediment samples in different areas were taken and subsequently screened for TBT contamination. Concentrations ranged from 15 microg kg(-1) in the port of Antwerp up to 43 mg kg(-1) near a ship repair unit.     

Isotope dilution high-resolution gas chromatography/high-resolution mass spectrometry method for analysis of selected acidic herbicides in surface water

In this work, an isotope dilution method for determination of selected acidic herbicides by high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS) was developed for surface water samples. Average percent recoveries of native analytes were observed to be between 70.8 and 93.5% and average recoveries of labeled quantification standards [(13)C(6)]2,4-D and [(13)C(6)]2,4,5-T were 85.5 and 101%, respectively. Using this method, detection limits of 0.05 ng/L for dicamba, MCPA, MCPP, and triclopyr, and 0.5 ng/L for 2,4-D were routinely achieved. The method was applied to measuring the concentration of these analytes in surface water samples collected from five sampling locations in the Lower Fraser Valley region of British Columbia, Canada. All of the herbicides monitored were detected at varying levels in the surface water samples collected. The highest concentrations detected for each analyte were 345 ng/L for 2,4-D, 317 ng/L for MCPA, 271 ng/L for MCPP, 15.7 ng/L for dicamba, and 2.18 ng/L for triclopyr. Average detection frequencies of the herbicides were 95% for MCPA, 80% for MCPP, 70% for dicamba, 65% for 2,4-D, and 46% for triclopyr. Seasonal variations of herbicide levels are also discussed.     

Isotope dilution gas chromatography-mass spectrometry for the study of eicosanoid metabolism in human blood platelets

Stable isotope dilution gas chromatography-mass spectrometry provides one of the most important techniques for the quantitative measurement of eicosanoids. This technique was applied to the quantitation of hydroxyeicosatetraenoic acids, hydroxyheptadecatrienoic acid, thromboxane B2 and prostaglandin F2 alpha formed during platelet aggregation after stimulation of gel-filtered platelets with thrombin (0.25 U/ml) or collagen (2 micrograms/ml). Similar amounts of hydroxyheptadecatrienoic acid and thromboxane B2 were found after platelet activation. The ratio of formation of 12-hydroxyeicosatetraenoic acid to thromboxane B2 varied from donor to donor. Only small amounts of prostaglandin F2 alpha (up to 200 pg per 2.0.10(8) platelets) and basic values of 15-hydroxyeicosatetraenoic acid (up to 100 pg per 2.0.10(8) platelets) were measured using gas chromatography with negative ion chemical ionization mass spectrometry. In addition, different stable isotope dilutions were prepared and are discussed in detail.     

Isotope dilution SPME GC/MS for the determination of methylmercury in tuna fish samples

The development of a rapid, precise and accurate analytical method for the determination of methylmercury in tuna fish samples is described. The method is based on the use of isotope dilution GC/MS with electron impact ionization, a widespread technique in routine testing laboratories. A certified spike containing (202)Hg-enriched methylmercury was used for the isotope dilution of the samples. After extraction of the methylmercury from the sample, methylmercury was propylated using sodium tetrapropyl borate in SPME vials and the analytes were sampled from the headspace for 15 min. For isotope measurements, the molecular ion (MePrHg(+)) was used in the SIM mode. Five molecular ions were monitored, corresponding to the (198)Hg, (199)Hg, (200)Hg, (201)Hg and (202)Hg isotopes. The detection at masses corresponding to (198)Hg was used to correct for m + 1 contributions of (13)C from the organic groups attached to the mercury atom on the (199)Hg, (200)Hg, (201)Hg and (202)Hg masses with simple mathematical equations, and the concentration of methylmercury was calculated on the basis of the corrected (200)Hg/(202)Hg isotope ratio. The (202)Hg-enriched methylmercury spike was applied, with satisfactory results, to the determination of methylmercury in the certified reference material BCR 464. The method was successfully applied to the determination of methylmercury in tuna fish samples, and the obtained results were included in the CCQM-P39 interlaboratory exercise, organized by the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium) with excellent agreement between our results and the average obtained by the other participants.     

Extension of the dynamic range of flame atomic absorption spectrometry using flow injection analysis with variable-volume dilution chambers

A simple and inexpensive procedure is proposed for the extension of the dynamic range of flame atomic absorption spectrometry measurements using on-line dilution. The proposed methodology is based on the use of a manifold with two coupled dilution chambers and a zone injection system. The samples are prediluted in a closed system which includes a variable-volume mixing chamber (10–120 ml) and two injection valves. The samples are injected through one of these valves, and the other is employed to take 100 μl of prediluted samples which are then passed through a new dilution chamber (volume 1–10 ml) and aspirated by the nebulizer of the instrument. A third injection valve mounted in the last part of the manifold is used for the direct injection of diluted standard solutions. Various dilution factors are obtained, ranging from 2 to 130 000 times, thus extending the analytical range of copper determination to more than 100 000 mg l⁻¹.     

Determination of arsenic in germanium by the combination of isotope dilution and activation analysis

The determination of arsenic by activation analysis in germanium may be disturbed by arsenic isotopes formed in the irradiation of the germanium matrix itself. The isotopic dilution technique will eliminate such interferences, but it requires weighing of very small masses of arsenic, after appropriate chemical separation. This weighing procedure can be avoided if the mass of the recovered arsenic is determined by activation analysis. In order to apply such a procedure, a known amount of arsenic labelled with⁷⁶As, whose specific activity is also known, is added to the germanium sample to be analysed. The sample is processed, the arsenic separated, and the chemical yield established. The recovered mass of arsenic is then determined by activation analysis applying the classical isotopic dilution analysis formula. The accuracy precision and linearity of the method are discussed.