Isotope Dilution Calibration Research Articles

Evaluation, comparison and combination of molecularly imprinted polymer solid phase extraction and classical solid phase extraction for the preconcentration of endocrine disrupting chemicals from representative whole water samples

Estrogens are endocrine disrupting chemicals and of high concerns due to demonstrated harmful effects on the environment and low effect levels. For monitoring and risk assessment, several estrogens were included in the "watch list" of the EU Water Framework Directive which sets very low environmental quality standard (EQS) levels for Estrone (E1) and 17β-Estradiol (E2) of 0.4 ng L−1 and for 17α-Ethinylestradiol (EE2) of 0.035 ng L−1 requiring sensitive detection methods, as well as extensive sample preparation. A sensitive, derivatization-free, isotope dilution calibration HPLC-MS/MS method for a panel of 5 selected estrogens (including the 3 estrogens of the EU WFD watchlist), and a procedure for the reproducible preparation of a representative whole water matrix including mineral water, humic acids and solid particulate matter are presented. These are used in a diligent comparison of classical solid phase extraction (SPE) on hydrophilic-lipophilic balanced (HLB) phase to SPE on an estrogen-specific molecularly imprinted polymer phase (MISPE) for ultra-trace levels of the analytes (1–10 ng L−1). Additionally, a two-step procedure combining HLB SPE disks followed by MISPE is evaluated. The tow-step procedure provides superior enrichment, matrix removal and sample throughput while maintaining comparable recovery rates to simple cartridge SPE. Estimated method quantification limits (MQLs) range from 0.109–0.184 ng L−1 and thus meet EQS-levels for E1 and E2, but not EE2. The representative whole water matrix provides a reproducible comparison of sample preparation methods and lays the foundation for a certified reference material for estrogen analysis. The presented method will serve as the basis for an extended validation study to assess its use for estrogen monitoring in the environment.

An isotope dilution liquid chromatography-mass spectrometry method for detection of melamine in milk powder

ABSTRACT Adulteration/unintentional contamination of milk with melamine could have negative health and economic implications especially in the developing countries due to insufficient laboratory support and surveillance. This paper presents an Isotope Dilution Liquid Chromatography-Mass Spectrometry (ID LC-MS) method developed for detection of melamine in powdered milk. The rapid sample preparation involved dissolution of 1g of milk powder in 2.5% formic acid, precipitation of protein with acetonitrile, spiking of samples with melamine (triamine-15N3) at 200 µg L–1 and detection of intrinsic 14N-melamine molecular ratio to the spike. The isotope dilution calibration procedure was free from matrix effects, unlike other methods where the detector sensitivity can fluctuate up to several orders of magnitude. Limit of detection of the method was 13 µg kg–1, and the recovery of melamine at 50, 100, and 250 µg kg–1 was 78.7–126.3%. The method was used to determine melamine levels in 22 milk powder products (local and imported) available in Sri Lanka. Melamine was detected in all the samples (range = 0.33–0.96 mg kg–1). Full cream milk powders (both local and imported) contained melamine in the range of 0.39–0.84 mg kg–1, and various health and pregnancy formulas contained <0.5 mg kg–1 of melamine. Two imported infant formula samples contained the highest levels of melamine (0.96 and 0.94 mg kg–1). Although these melamine levels are below the regulatory limit in Sri Lanka (1 mg kg–1), a monitoring programme would ensure consumer safety.

Automated method to determine pharmaceutical compounds in wastewater using on-line solid-phase extraction coupled to LC-MS/MS.

An automated method was developed using on-line solid-phase extraction (SPE) as a sample preparation step, coupled to liquid chromatography-tandem mass spectrometry (LC-MS/MS), for determination of pharmaceutical compounds in wastewater samples at nanograms per liter to micrograms per liter concentration levels. This method is suitable for use in routine analysis, especially in environmental applications, avoiding cross-contamination and requiring minimal sample handling. Results can be obtained rapidly, with a chromatographic run time of only 24 min (including sample preparation and chromatographic analysis). Using a 50 μL injection volume, the method was validated according to international guidelines, considering parameters included in terms of method detection (MDL) and quantification limit (MQL), linearity, inter-day and intra-day precisions, and matrix effects. Assessment of chromatographic efficiency considered peak resolution and asymmetry, and carryover was evaluated to ensure analytical reliability and the ability to reuse the SPE cartridge. The intra- and inter-day precisions were lower than 10 and 17%, respectively. The MDL values ranged from 1×10-6 to 1 μg L-1, while the MQL values were from 0.001 to 3 μg L-1. Matrix effects were minimized by isotope dilution calibration. Application of the method to 20 wastewater samples showed that caffeine was the most frequently detected compound, with the highest concentration of 715 μg L-1, while other pharmaceutical compounds were detected in fewer samples and at lower concentrations (up to 8.51 μg L-1).

Quantification of tributyltin in seawater using triple isotope dilution gas chromatography–inductively coupled plasma mass spectrometry achieving high accuracy and complying with European Water Framework Directive limits

A triple isotope dilution GC-ICPMS method for the determination of tributyltin (TBT) was developed and validated to meet the European Water Framework Directive (WFD) requirements. The validation procedure involved the evaluation of trueness, precision (repeatability, intermediate precision), limit of detection (LOD) and limit of quantification (LOQ), stability, measurement uncertainty and traceability studies. The method is one of the most sensitive methods published to date with good accuracy, 103% average recovery in the range with %RSDs of 2.8-6.7%. A LOD value of 0.015 ng L−1 for the TBT cation was achieved with a sample volume of 12 mL seawater. TBT was derivatized using 20 µL sodium tetraethylborate solution (0.05% NaBEt4) to make volatile for GC-ICPMS. Measurement uncertainty was in the range of 4.8-13% which was achieved through dissolution of tributyltinchloride (TBTCl) in 1-propanol, a low-volatility solvent combined with the use of a triple isotope dilution (ID) calibration technique. Isotope dilution calibration was performed by adding 117Sn isotopically enriched TBT to the seawater samples. The stability test results showed that TBT concentration was stable for three months in seawater samples after passing through a 0.2 µm filter and stored in amber glass bottles at 4°C. The response surface methodology (RSM) approach was successfully implemented to provide optimal conditions for large volume injection (LVI) to obtain the maximum analytical signal. The key variables selected in the experimental design were evaporation time, evaporation temperature, carrier flow, and injection speed. This method was applied to seawater samples collected from the Bay of Izmit, Kocaeli, Turkey, where TBT pollution has not been measured yet.

Determination of chloride in crude oil using isotope dilution GC–MS: A comparative study

Optimisation and validation of a novel isotope dilution gas chromatography mass spectrometry (GC–MS) method for determination of inorganic chloride (Cl−) in crude oils is presented. The use of stable isotope dilution calibration approach decreases losses and biases and it is considered a higher order measurement approach. Samples were diluted with a 1:1 blend of isopropyl alcohol : toluene with 5% water, mixed with 37Cl− internal standard, water extracted and derivatized with triethyloxonium tetrafluoroborate at room temperature. This reagent converted Cl− into stable ethyl chloride (EtCl) which was detected by headspace GC–MS. A limit of quantitation of 100 µg kg−1 and limit of detection of 30 µg kg−1 were observed in crude oil samples when initial dilution was not required. The performance of this novel method was compared with combustion ion chromatography, ion chromatography with conductivity detection after microwave-induced combustion, and ASTM D3230 method on a set of medium and heavy crude oil samples. Despite the challenges of the measurement, a reasonable level of agreement was observed among methods over the range of 0.38 ± 0.02 to 993 ± 103 mg kg−1 of Cl−. Presented headspace GC–MS method proved to be suitable for analysis of all types of crude oils samples with an excellent reproducibility (RSD < 6%) and high sample throughput (7 min GC–MS run time).

Uranium exposure of the Swiss population based on 24-hour urinary excretion.

Important regional differences in uranium exposure exist because of varying uranium concentrations in soil, water and food. Comprehensive data on the exposure of the general population to uranium is, however, scarce. Based on the 24-hour urinary excretion, the uranium exposure of the adult Swiss population was assessed in relation to age, sex, place of residence, body mass index (BMI), smoking habit and type of drinking water, as well as risk factors in relation to kidney impairment and indicators of a possible renal dysfunction. Uranium was quantified in 24-hour urine from a nationwide population-based sample (n = 1393). The ratio 238U/233U was measured for isotope dilution calibration with a sector field inductively coupled plasma mass spectrometer (HR-ICP-MS). Overall median and 95th percentile were 15 and 67 ng/24 h, respectively. The place of residence significantly influenced urinary uranium excretion. However, most of the highest urinary uranium excretion levels could not be associated to areas known for their elevated uranium concentrations in the drinking water. Sources other than the local drinking water (e.g., bottled water) might be important, too. Gender as well as albumin excretion also had a significant effect on uranium excretion. The latter was, however, strongly dependent on the presence of diabetes mellitus. No association was found for age, BMI, smoking habit or the other examined kidney related variables. On the basis of uranium exposure, assessed via 24-hour urinary uranium excretion, and current knowledge of the toxicity of naturally occurring uranium, a substantial corresponding health risk for the general adult population is unlikely. However, as long as no specific sensitive biomarker for the biological impact of low-dose chronic uranium exposure has been identified and validated, assessing subtle health impact of such exposure will remain difficult.

&amp;lt;italic&amp;gt;In-situ&amp;lt;/italic&amp;gt; LA-MC-ICPMS U-Pb dating method for low-uranium carbonate minerals

Compared with the reported standards ASH15 and WC-1, AHX-1a is more homogeneous and wider application potential. However, in view of the fact that the original sample size of AHX-1a is small, which makes it not suitable for isotope dilution calibration method before its spatial distribution characteristics of U-Th-Pb are completely delineated. Therefore, AHX-1a is temporarily used only as an in-house standard in our laboratory. Currently, we are continuing to search for and calibrate other potential standards with similar characteristics to AHX-1a, but of a larger sample size. To continuously characterize and identify suitable carbonate reference materials, further calibration using isotope dilution is planned.

Quantitative analysis of progesterone using isotope dilution-matrix-assisted laser desorption ionization-time of flight mass spectrometry as a reference procedure for radioimmunoassay

BackgroundProgesterone is one of the female steroid hormones and plays an important role in the menstrual cycle and during pregnancy. It is especially important in preparing the uterus for the implantation of the blastocyst and maintaining pregnancy. The concentration in human serum is measured to determine the ovarian function retroactively and the cause of abortion in early pregnancy. MethodsA quantification assay based on isotope dilution mass spectrometry to determine the concentration of progesterone in human serum is reported. Incorporated with 13C3-progesterone, serum samples were subjected to progesterone extraction and clean-up by C4 solid-phase-extraction columns and hexane-based liquid/liquid extraction, respectively. The cleaned-up serum samples were then subjected to MALDI-TOF mass spectrometry for the quantification of progesterone. ResultsProgesterone and the internal standard, 13C3-progesterone, were measured in the selected reaction monitoring mode for the transitions m/z 315.4 to 108.9 and m/z 318.4 to 111.9, respectively. We calculated the peak area ratio of progesterone to 13C3-progesterone. The progesterone concentration in human serum was calculated by substituting the peak area ratio into an isotope dilution calibration curve, and then compared with the radioimmunoassay. ConclusionsIn the study, the concentrations of serum progesterone were measured, and the recovered progesterone concentration determined by the assay showed good robustness and consistency in comparison to the conventional radioimmunologic assay. We concluded that the 13C3-progesterone-based quantification assay is a robust method for the measurement of serum progesterone.

A reference isotope dilution headspace GC/MS method for the determination of nitrite and nitrate in meat samples

SummaryA novel method for the determination of nitrite and nitrate in meat products is presented. The samples were ground and extracted in hot water with the presence of and internal standards. The solution was buffered with sodium bicarbonate and reacted with triethyloxonium tetrafluoroborate to convert nitrite and nitrate into EtNO2 and EtONO2. Such derivatives could be detected by headspace GC/MS in positive chemical ionisation mode with 0.05 µg g−1 and 1.0 µg g−1 LOD. The method was used for and quantitation in the 0.5‐300 and 2.5‐300 µg g−1 ranges. The method was applied for the analysis of fifteen meat products. Despite minimal sample preparation, the headspace sampling ensured a clean chromatography for over 135 analyses (throughput ten samples per hour). The proposed method offers selective GC/MS detection combined with high‐precision isotope dilution calibration, it is suitable for metrological applications and can support regulations on meat safety (European Commission, 2011).

Traceable and accurate quantification of iron in seawater using isotope dilution calibration strategies by triple quadrupole ICP-MS/MS: Characterization measurements of iron in a candidate seawater CRM

Accurate quantification of iron is crucial for the characterization of a candidate seawater certified reference material. Isotope dilution mass spectrometry as a primary method is one of the option for this purpose. In this study, triethylamine (TEA) assisted Mg(OH)2 co-precipitation is developed and combined with isotope dilution mass spectrometry for the characterization of candidate certified reference material, UME CRM 1206. The analytical protocol of co-precipitation was optimized to achieve reproducible yields of ~100% Fe with high salt rejection (residual salinity ~0.003%). Three calibration strategies, single, double and triple isotope dilution mass spectrometry (IDMS) coupled with co-precipitation were investigated in terms of metrological aspects. The determination of Fe was carried out by ICP-MS with tandem configuration (ICP-MS/MS) using NH3/He cell gas which was optimized for the minimum background equivalence concentration to be able to measure highly accurate 56Fe/57Fe ratio with the highest possible sensitivity. The background corrected typical procedural blank levels and limit of detection calculated based on three times the standard deviation of blanks were found as 9 ng/kg (0.16 nM) and 11 ng/kg (0.20 nM) for single IDMS, respectively. Trueness of the developed analytical methods has been demonstrated using CASS-6 and NMIA MX014 with complete and very precise recoveries. SI-traceable values with combined uncertainty (k = 2) were obtained for single, double and triple IDMS as (12.70 ± 0.16) ng/g, (12.728 ± 0.084) ng/g and (12.732 ± 0.062) ng/g, respectively. The relative combined uncertainty for each approach was calculated as 0.64%, 0.33% and 0.24%, respectively.

Cold vapour matrix-independent generation and isotope dilution inductively coupled plasma mass spectrometry for reference measurements of Hg in marine environmental samples.

The analytical procedure for reference measurements of Hg in marine environmental samples based on cold vapour generation (CVG) inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) and isotope dilution calibration strategy (ID) is described in the present study. The procedure was applied on different matrices and varying mass fractions for mercury in oyster, marine sediment and costal seawater samples. The use of CVG for sample introduction resulted in sensitivity enhancement, reduction of blank levels, minimization of matrix interferences and the elimination of memory effects. The entire measurement process was described by mathematical equations, and all factors influencing the final results were systematically investigated. The expanded uncertainty on results was estimated following the ISO/Guide to the Expression of Uncertainty in Measurement guidelines, and all major contributions were identified. Modelling of the entire measurement process and the use of primary method of measurements ID ICP-MS related obtained values for total mercury in different types of marine matrices to the international system of units SI. Obtained results for mercury mass fractions in biota and marine sediments were used as the International Atomic Energy Agency's contribution in the certification campaigns for marine reference materials and were in excellent agreement with the assigned reference values.

The direct and accurate determination of major elements Ca, K, Mg and Na in water by HR-ICPMS

A direct, accurate and precise method is reported for major elements Ca, K, Mg and Na measurements in river and drinking water using a high resolution ICP-MS. Double isotope dilution calibration was used for the determination of Mg whereas the combined standard addition and internal standardization (Sc) was used for Ca, K and Na measurements. The method is validated by analysis fresh water SLRS-5, SLRS-6 and SRM1640a with satisfactory results characterized by high precisions of 0.055% to 0.66% RSD (or 0.29–1.8% relative combined uncertainty) for all four analytes, superior to those reported in earlier studies. As noted, use of internal standard Sc has significantly (3–33 times) improved measurement precisions for Ca, K and Na compared to standard addition calibration alone. The proposed method was applied for the determination of major elements Ca, K, Mg and Na in a candidate drinking water CRM AQUA-1. Values of 1.908 ± 0.007 µg g−1, 8.27 ± 0.14 µg g−1, 0.660 ± 0.010 µg g−1 and 13.76 ± 0.05 µg g−1 (u, k = 1) were obtained for Mg, Ca, K and Na in AQUA-1 drinking water, respectively.

Nickel determination in oily sludge leachates using isotope dilution inductively coupled plasma mass spectrometry following photochemical vapor generation: A feasibility study

The feasibility of photochemical vapor generation for the production of volatile Ni species was evaluated and applied to the analysis of acetic acid leachates obtained from oily sludge samples. Sludge samples originating from a petrochemical plant were submitted to extraction using standardized procedures with solutions adjusted to pH 2.88 or 4.93, with acetic acid. The leachate solutions were subsequently treated with a mixture of acetic and formic acid and exposed to UV radiation from a Hg discharge lamp; the irradiated solution was pumped to a gas-liquid separator and the volatile species were transported by a stream of argon for detection using inductively coupled plasma mass spectrometry. Experimental variables, including the nature and concentration of low molecular weight acids, irradiation interval, carrier gas flow rate and the effect of concomitant species on the efficiency of photochemical production of volatile Ni species were systematically evaluated. Oxygenated anions, particularly nitrate, were found to suppress the analytical signal, whereas the effect of cationic species depended on the nature of the cation. Lead was found to induce substantial signal suppression even at ppb levels. Considering the significant influence of concomitant species on the overall efficacy of the photochemical process, isotope dilution calibration was adopted as a strategy to overcome non-spectral interferences. Under optimized conditions, relative standard deviations better than 8.5% were achieved, along with detection limits better than 40 μg L−1. The accuracy was statistically attested upon comparison of the results to those obtained from the analysis of leachates using sample introduction via pneumatic nebulization. The proposed photochemical vapor generation setup was proven efficient to carry out the determination of Ni in the leachates, although the technique is highly sensitive to the presence of concomitants.

Inter-laboratory study for the certification of trace elements in seawater certified reference materials NASS-7 and CASS-6.

Certification of trace metals in seawater certified reference materials (CRMs) NASS-7 and CASS-6 is described. At the National Research Council Canada (NRC), column separation was performed to remove the seawater matrix prior to the determination of Cd, Cr, Cu, Fe, Pb, Mn, Mo, Ni, U, V, and Zn, whereas As was directly measured in 10-fold diluted seawater samples, and B was directly measured in 200-fold diluted seawater samples. High-resolution inductively coupled plasma mass spectrometry (HR-ICPMS) was used for elemental analyses, with double isotope dilution for the accurate determination of B, Cd, Cr, Cu, Fe, Pb, Mo, Ni, U, and Zn in seawater NASS-7 and CASS-6, and standard addition calibration for As, Co, Mn, and V. In addition, all analytes were measured using standard addition calibration with triple quadrupole (QQQ)-ICPMS to provide a second set of data at NRC. Expert laboratories worldwide were invited to contribute data to the certification of trace metals in NASS-7 and CASS-6. Various analytical methods were employed by participants including column separation, co-precipitation, and simple dilution coupled to ICPMS detection or flow injection analysis coupled to chemiluminescence detection, with use of double isotope dilution calibration, matrix matching external calibration, and standard addition calibration. Results presented in this study show that majority of laboratories have demonstrated their measurement capabilities for the accurate determination of trace metals in seawater. As a result of this comparison, certified/reference values and associated uncertainties were assigned for 14 elements in seawater CRMs NASS-7 and CASS-6, suitable for the validation of methods used for seawater analysis.

Determination of total cyanide in soil by isotope dilution GC/MS following pentafluorobenzyl derivatization

The high toxicity of cyanide, along with its widespread industrial use, has fuelled interest in the development of analytical methods for its determination in complex matrices. In this study, we propose a novel approach for the measurement of total cyanide in soil samples based on single-step derivatization with pentafluorobenzyl bromide (F5Bn−Br) followed by quantitation with gas chromatography mass spectrometry in negative chemical ionization mode.The reaction between CN− and F5Bn−Br resulted in the identification of several derivatives such as F5Bn−CN, (F5Bn)(F5Ph)CH−CN, and (F5Bn)2(F5Ph)C−CN. The relative proportion between such compounds was dependent on experimental conditions. When a 100 μL aliquot of an alkaline-aqueous extract was reacted with 700 μL of 1.3% F5Bn−Br in acetone, the tri-alkylated derivative was the most abundant. In such conditions a detection limit of 0.5 ng/g of CN− was attained.Soil samples were initially spiked with an alkaline solution of K13C15N internal standard and suspended in 7.5% aqueous NaOH. Determination of total cyanide was achieved by digestion of the alkaline extract with H3PO4 to produce HCN which was then trapped in 0.1% NaOH in a sealed double vial system, followed by reaction with F5Bn−Br. Isotope dilution calibration was chosen for quantitation, and the validity of the novel method was demonstrated by analysis of soil Certified Reference Materials (CRMs) and by spike recovery tests.

Quantification of Polybrominated and Polychlorinated Biphenyls in Human Matrices by Isotope-Dilution Gas Chromatography-Tandem Mass Spectrometry.

We have developed a highly sensitive and selective analytical method capable of quantifying a total of 15 polybrominated and polychlorinated biphenyls (11 PBBs and 4 PCBs) in human serum. Samples were subjected to liquid-liquid extraction followed by solid-phase extraction prior to measurement using gas chromatography-tandem mass spectrometry in multiple reaction monitoringmode. Quantification was performed using isotope-dilution calibration covering a concentration range of 0.005-12.5 ng/mL. Limits of detection for all target compounds were in the lowrange (0.7-6.5 pg/mL). The method was validated using in-house pooled human serum fortified at two concentrations (0.5 ng/mL and 1.0 ng/mL), whole semen fortified at one concentration (0.25 ng/mL), and NIST Standard Reference Material (SRM) 1958, which includes five of the target compounds. Method accuracies for all target compounds ranged from 84 to 119% with relative standard deviations (RSDs) of <19%. The measured values for the five target compounds present in the SRM agreed with the certified reference values (89-119% accuracy with RSDs <9%). As this method was developed to support ongoing epidemiologic investigations, we evaluated its suitability by analyzing subsets of serum and whole semen samples from the Michigan PBB Registry cohort. PBB-153, PCB-118, PCB-138, PCB-153 and PCB-180 were detected in all serum samples analyzed, with PBB-77 and PBB-101 detected less frequently in serum. PBB-153, PCB-118, PCB-138, PCB-153 and PCB-180 were detected in at least one whole semen sample.

Isotope-labelling derivatisation: a broadly applicable approach to quantitation of algal toxins by isotope dilution LC-MS/MS

Derivatisation with a labelled reagent allows for isotope dilution calibration of algal toxins for which no labelled standard is available.

On-line UV photochemical generation of volatile copper species and its analytical application

Volatile Cu species were on-line generated by photochemical reduction in a formic acid medium utilizing a unique flow-through photochemical reactor. Digests of biological samples in 18% formic were introduced to the photochemical vapor generation (PVG) reactor under conditions wherein copper-containing species were initially retained on the reactor wall following a purge of the line with air and desorbed in the presence of a plug of 60% formic acid to yield volatile species which were transported to a gas–liquid separator and introduced to multi-collector inductively coupled plasma spectrometry (MC-ICPMS). Sensitivity of the proposed method could be further improved if a sample solution is repeatedly introduced using several cycles comprising loading and drying steps. However, the unstable volatile Cu species generated by PVG together with the early release of Cu during drying step resulted in a poor measurement precision of near 30% (n=4, RSD) and a non-linear correlation between the signal intensity and the Cu2+ concentration. Despite the above shortcomings, the accurate and precise determination of Cu concentrations in biological samples was achieved by applying isotope dilution (ID) calibration. A method detection limit of 8ngg−1 normalized to a nominal test mass of 0.25g of sample was obtained based on ID quantification. Method validation was demonstrated by analysis of three biological certified reference materials of TORT-3, SRM 1566b and DOLT-5 with satisfying results. Concentrations of 490.8±0.6, 71.88±0.06 and 34.95±0.04μgg−1 (1SD, n=4) with precisions of 0.12, 0.08 and 0.13% RSD for Cu were obtained in TORT-3, SRM 1566b and DOLT-5, respectively, in good agreement with certified values.

Calibration graphs in isotope dilution mass spectrometry

Isotope-based quantitation is routinely employed in chemical measurements. Whereas most analysts seek for methods with linear theoretical response functions, a unique feature that distinguishes isotope dilution from many other analytical methods is the inherent possibility for a nonlinear theoretical response curve. Most implementations of isotope dilution calibration today either eliminate the nonlinearity by employing internal standards with markedly different molecular weight or they employ empirical polynomial fits. Here we show that the exact curvature of any isotope dilution curve can be obtained from three-parameter rational function, y = f(q) = (a0 + a1q)/(1 + a2q), known as the Padé[1,1] approximant. The use of this function allows eliminating an unnecessary source of error in isotope dilution analysis when faced with nonlinear calibration curves. In addition, fitting with Padé model can be done using linear least squares.

Direct Determination of Trace Antimony in Natural Waters by Photochemical Vapor Generation ICPMS: Method Optimization and Comparison of Quantitation Strategies.

A novel and sensitive approach for the accurate determination of antimony (Sb) in natural waters is described using photochemical vapor generation (PVG) coupled with inductively coupled plasma mass spectrometry (ICPMS) for detection. Utilizing a unique flow-through photochemical reactor capable of subjecting the samples to deep-UV (185 nm) radiation, generation efficiency was found to be independent of whether Sb(III), Sb(V), or organometallic species [trimethyltantimony(V)dibromine, TMSb(V)] were present, eliminating the shortcoming of Sb species depended sensitivity encountered during direct solution nebulization by ICPMS. Furthermore, the potentially severe matrix effect from seawater was efficiently eliminated by using a mixture of 5% (v/v) formic and 15% acetic acids (v/v) as the photochemical reductant, making direct determination of Sb in seawater feasible. The proposed method provides a 15-fold improvement in sensitivity over direct solution nebulization. A method detection limit of 0.0006 ng g(-1) based on external calibration was obtained (0.0002 ng g(-1) for isotope dilution), yielding a 15-fold improvement over that for direct solution nebulization. Accuracy is demonstrated by analysis of two water certified reference materials (CRMs, e.g., SLRS-6 and NIST 1640a) with satisfying results. In addition, spike recoveries of 100.6 ± 5.5% and 100.8 ± 3.8% (standard deviation, n = 3) were obtained for NASS-6 and CASS-5 seawater CRMs, respectively, since no certified values for Sb has been established for these materials. The performance of several calibration strategies, including double isotope dilution (ID), multiple and single-point gravimetric standard additions with internal standardization, as well as multiple and single-point gravimetric standard additions alone was examined. High precision of determination of Sb in four natural water samples (0.51-1.4%) was realized based on ID calibration, whereas one-point gravimetric standard addition calibration with internal standardization provided precisions of 1.6% and 3.3% at 0.22 and 0.44 ng g(-1) levels, respectively, in seawater.