Quantification of nitrite and nitrate in seawater by triethyloxonium tetrafluoroborate derivatization-headspace SPME GC-MS.

Triethyloxonium tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography-mass spectrometry (GC-MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO(2)). Isotopically enriched nitrite [(15)N] and nitrate [(15)N] are employed as internal standards and for quantification purposes. HS-GC-MS provided instrumental detection limits of 0.07 µM NO(2)(-) and 2 µM NO(3)(-). Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.

Structural characterization and agonist binding to human α4ß2 nicotinic receptors.

The Cys-loop receptor super-family of neurotransmitter-gated ion channels mediates fast synaptic transmission throughout the human nervous system. These receptors exhibit widely varying pharmacologies, yet their structural characterization has relied heavily on their homology with the naturally abundant muscle-type Torpedo nicotinic acetylcholine receptor. Here we examine for the first time the structure of a human α4ß2 neuronal nicotinic acetylcholine receptor. We show that human α4ß2 nicotinic receptors adopt a secondary/tertiary fold similar to that of the Torpedo nicotinic receptor with a large proportion of both α-helix and ß-sheet, but exhibit a substantially increased thermal stability. Both receptors bind agonist, but with different patterns of agonist recognition - particularly in the nature of the interactions between aromatic residues and the agonist quaternary amine functional group. By comparing α4ß2 and Torpedo receptors, we begin to delineate their structural similarities and differences.

Laser ablation single-collector inductively coupled plasma mass spectrometry for lead isotopic analysis to investigate evolution of the Bilbilis mint.

This work explores the performance of laser ablation-inductively coupled plasma mass spectrometry using different types of single-collector devices (sector field and time-of-flight instrumentation) for lead isotopic analysis of bronze coins, minted in the ancient city of Bilbilis. The aim of the study was achieving sufficient discrimination power to reveal similarities and differences for coins originating from different historical periods, and to obtain information on the possible source of the lead ores used in their production, while restricting the damage inflicted to the samples such that it is not visible to the naked eye. It was found that satisfactory results (RSD in the 0.15-0.30% range for (207)Pb/(206)Pb and (208)Pb/(206)Pb ratios) could be finally obtained, despite the noisy nature of signals generated upon ablation of the highly inhomogeneous coins, by means of a methodology based on: (a) selection of the line profiling ablation mode; (b) use of a dual pass spray chamber that permits the simultaneous introduction of a solution (containing thallium of known isotopic composition), thus resulting in a wet plasma that showed an increased robustness towards matrix effects and (c) detection using a TOF-ICPMS unit, which proved to be much better suited to deal with the transient signals obtained, while being also sufficiently sensitive to obtain good counting statistics, owing to the high lead level (average around 5%) present in the samples. Moreover, under these conditions, the simultaneous aspiration of the thallium spike permitted accurate correction for mass discrimination, such that it was not necessary to use external matrix-matched standards for calibration.

Phospholipase C activity affinity purifies with the Torpedo nicotinic acetylcholine receptor.

Nicotinic acetylcholine receptors mediate fast synaptic transmission by fluxing ions across the membrane in response to neurotransmitter binding. We show here that during affinity purification of the nicotinic acetylcholine receptor from Torpedo, phosphatidic acid, but not other anionic or zwitterionic phospholipids, is hydrolyzed to diacylglycerol. The phospholipase C activity elutes with the acetylcholine receptor and is inhibited by a lipid phosphate phosphohydrolase inhibitor, sodium vanadate, but not a phosphatidate phosphohydrolase inhibitor, N-ethylmaleimide. Further, the hydrolysis product of phosphatidic acid, diacylglycerol, enhances the functional capabilities of the acetylcholine receptor in the presence of anionic lipids. We conclude that a phospholipase C activity, which appears to be specific for phosphatidic acid, is associated with the nicotinic acetylcholine receptor. The acetylcholine receptor may directly or indirectly influence lipid metabolism in a manner that enhances its own function.

Validation of an isotope dilution, ICP-MS method based on internal mass bias correction for the determination of trace concentrations of Hg in sediment cores.

The development and validation of a method for the determination of mercury in sediments using a sector field inductively coupled plasma mass spectrometer (SF-ICP-MS) for detection is described. The utilization of isotope dilution (ID) calibration is shown to solve analytical problems related to matrix composition. Mass bias is corrected using an internal mass bias correction technique, validated against the traditional standard bracketing method. The overall analytical protocol is validated against NRCC PACS-2 marine sediment CRM. The estimated limit of detection is 12ng/g. The proposed procedure was applied to the analysis of a real sediment core sampled to a depth of 160m in Lake Como, where Hg concentrations ranged from 66 to 750ng/g.

Total selenium and selenomethionine in pharmaceutical yeast tablets: assessment of the state of the art of measurement capabilities through international intercomparison CCQM-P86.

Results of an international intercomparison study (CCQM-P86) to assess the analytical capabilities of national metrology institutes (NMIs) and selected expert laboratories worldwide to accurately quantitate the mass fraction of selenomethionine (SeMet) and total Se in pharmaceutical tablets of selenised-yeast supplements (produced by Pharma Nord, Denmark) are presented. The study, jointly coordinated by LGC Ltd., UK, and the Institute for National Measurement Standards, National Research Council of Canada (NRCC), was conducted under the auspices of the Comité Consultatif pour la Quantité de Matière (CCQM) Inorganic Analysis Working Group and involved 15 laboratories (from 12 countries), of which ten were NMIs. Apart from a protocol for determination of moisture content and the provision of the certified reference material (CRM) SELM-1 to be used as the quality control sample, no sample preparation/extraction method was prescribed. A variety of approaches was thus used, including single-step and multiple-step enzymatic hydrolysis, enzymatic probe sonication and hydrolysis with methanesulfonic acid for SeMet, as well as microwave-assisted acid digestion and enzymatic probe sonication for total Se. For total Se, detection techniques included inductively coupled plasma (ICP) mass spectrometry (MS) with external calibration, standard additions or isotope dilution MS (IDMS), inductively coupled plasma optical emission spectrometry , flame atomic absorption spectrometry and instrumental neutron activation analysis. For determination of SeMet in the tablets, five NMIs and three academic/institute laboratories (of a total of five) relied upon measurements using IDMS. For species-specific IDMS measurements, an isotopically enriched standard of SeMet (76Se-enriched SeMet) was made available. A novel aspect of this study relies on the approach used to distinguish any errors which arise during analysis of a SeMet calibration solution from those which occur during analysis of the matrix. To help those participants undertaking SeMet analysis to do this, a blind sample in the form of a standard solution of natural abundance SeMet in 0.1 M HCl (with an expected value of 956 mg kg(-1) SeMet) was provided. Both high-performance liquid chromatography (HPLC)-ICP-MS or gas chromatography (GC)-ICP-MS and GC-MS techniques were used for quantitation of SeMet. Several advances in analytical methods for determination of SeMet were identified, including the combined use of double IDMS with HPLC-ICP-MS following extraction with methanesulfonic acid and simplified two-step enzymatic hydrolysis with protease/lipase/driselase followed by HPLC-ICP-IDMS, both using a species-specific IDMS approach. Overall, satisfactory agreement amongst participants was achieved; results averaged 337.6 mg kg(-1) (n = 13, with a standard deviation of 9.7 mg kg(-1)) and 561.5 mg kg(-1) (n = 11, with a standard deviation of 44.3 mg kg(-1)) with median values of 337.6 and 575.0 mg kg(-1) for total Se and SeMet, respectively. Recovery of SeMet from SELM-1 averaged 95.0% (n = 9). The ability of NMIs and expert laboratories worldwide to deliver accurate results for total Se and SeMet in such materials (selensied-yeast tablets containing approximately 300 mg kg(-1) Se) with 10% expanded uncertainty was demonstrated. The problems addressed in achieving accurate quantitation of SeMet in this product are representative of those encountered with a wide range of organometallic species in a number of common matrices.

Determination of tributyltin in marine sediment: Comité Consultatif pour la Quantité de Matière (CCQM) pilot study P-18 international intercomparison.

The capabilities of National Metrology Institutes (NMIs-those which are members of the Comité Consultatif pour la Quantité de Matière (CCQM)of the CIPM) and selected outside "expert" laboratories to quantitate (C(4)H(9))(3)Sn(+) (TBT) in a prepared marine sediment were assessed. This exercise was sanctioned by the 7th CCQM meeting, April 4-6, 2001, as an activity of the Inorganic Analysis Working Group and was jointly piloted by the Institute for National Measurement Standards of the National Research Council of Canada (NRC) and the Laboratory of the Government Chemist (LGC), UK. A total of 11 laboratories submitted results (7 NMIs, and 4 external labs). Two external laboratories utilized a standard calibration approach based on a natural abundance TBT standard, whereas all NMIs relied upon isotope dilution mass spectrometry for quantitation. For this purpose, a species specific (117)Sn-enriched TBT standard was supplied by the LGC. No sample preparation methodology was prescribed by the piloting laboratories and, by consequence, a variety of approaches was adopted by the participants, including mechanical shaking, sonication, accelerated solvent extraction, microwave assisted extraction and heating in combination with Grignard derivatization, ethylation and direct sampling. Detection techniques included ICP-MS (with GC and HPLC sample introduction), GC-MS, GC-AED and GC-FPD. Recovery of TBT from a control standard (NRCC CRM PACS-2 marine sediment) averaged 93.5+/-2.4% ( n=14). Results for the pilot material averaged 0.680+/-0.015 micro mol kg(-1) ( n=14; 80.7+/-1.8 micro g kg(-1)) with a median value of 0.676 micro mol kg(-1). Overall, performance was substantially better than state-of-the-art expectations and the satisfactory agreement amongst participants permitted scheduling of a follow-up Key comparison for TBT (K-28), a Pilot intercomparison for DBT (P-43), and certification of the test sediment for TBT content and its release as a new Certified Reference Material (HIPA-1) with a TBT content of 0.679+/-0.089 micro mol kg(-1) (expanded uncertainty, k=2, as Sn) (80.5+/-10.6 micro g kg(-1)).

Expanding the scope of chemical vapor generation for noble and transition metals.

A modified Bergener parallel path nebulizer was used for the generation of volatile metal species of Pt, Co, Ag, Cu, Rh, Pd, Au, Ni, Ir, Ti and Mn through reaction with tetrahydroborate (III). Several physicochemical factors were identified which impact on their yield which, at present, is estimated to range from 0.02-2% absolute.

Determination of total mercury in biological tissues by flow injection cold vapour generation atomic absorption spectrometry following tetramethylammonium hydroxide digestion.

A simple, rapid and reliable method was developed for the determination of total mercury in biological samples. Samples were solubilized using tetramethylammonium hydroxide (TMAH). The organically bound mercury was cleaved and converted to inorganic mercury by on-line addition of KMnO4. The decomposed mercury together with inorganic mercury originally present in samples was determined by flow injection cold vapour atomic absorption spectrometry after reduction to elemental mercury vapour using NaBH4. A sample throughput of 100 measurements per hour was achieved after a 30 min dissolution with TMAH. The relative standard deviation for 20 micrograms l-1 Hg was 1.3% (n = 11) and the limit of detection was 0.1 microgram l-1 (3 sigma). The proposed method was validated by the analysis of a suite of certified marine biological reference materials, DORM-2 (dogfish muscle), DOLT-2 (dogfish liver) and TORT-2 (lobster hepatopancreas), with calibration against simple HgII standards.

Immobilization of 8-hydroxyquinoline onto silicone tubing for the determination of trace elements in seawater using flow injection ICP-MS.

A rapid and simple on-line method is described for the preconcentration of Mn, Co, Ni, Cu, Zn, Cd and Pb from sea water using 8-hydroxyquinoline immobilized onto silicone tubing (Sil-8-HQ) via the Mannich reaction. Recoveries between 35 and 95% and limits of detection in the ppt range were obtained using a 2 m long Sil-8-HQ tube with a sample flow rate of 1.0 ml min(-1). A tube could be subjected to sample loading and elution cycles over 200 times. The capacity was 1.5 and 1.3 mug cm(-2) for Cu and Mn, respectively. Cu, Cd, Co, Pb, Mn, Zn and Ni were determined in coastal and open ocean seawater using flow injection inductively coupled plasma mass spectrometry (FI-ICP-MS). Good agreement with certified values for the certified reference materials NASS-4 and CASS-3 was demonstrated when quantitation was undertaken by the method of additions.

Atomic absorption by free atoms in solution following chemical reduction from the ionic state.

The initial stages of chemical reduction of metallic ions in solution must proceed through the formation of single atoms, followed by the rapid formation of clusters with other atoms until continued coalescence produces observable precipitates or evidence of stable sol formation. Dilute solutions of sodium borohydride (0.0025%) merged with microgram per milliliter concentration solutions of Ag, Pd, and Cu in a custom spectrophotometric flow cell produce transient background-corrected atomic absorption signals during the initial few seconds of the reaction. High-resolution atomic absorption with both a hollow cathode line source and a continuum source reveals that the absorption line profile of the absorbing species is extremely narrow (11 pm for Ag) and not influenced by the aqueous environment. Such selective absorption of radiation suggests that free atoms are being observed in aqueous solution.

Determination of cadmium in environmental samples by hydride generation with in situ concentration and atomic absorption detection.

A volatile Cd species (presumed to be the hydride) was generated from aqueous solutions by merging sample and tetrahydroborate reductant in a continuous-flow system. The gaseous analyte was transferred onto the inner wall of a graphite tube furnace for in situ preconcentration at 200 degrees C. Calibration was achieved via the method of standard additions. An absolute detection limit (3 sigma blank) of 10 pg was obtained using KBH4 as reductant. The precision of the determination was 12% (RSD) for a Cd concentration of 0.2 ng ml-1 using KBH4. The method was successfully applied to the determination of Cd in several certified environmental reference materials (soil, sediment, sea-water, biological samples) following pressure digestion of the samples by microwave heating in a mixture of acids.

Minimization of transition metal interferences with hydride generation techniques.

A new hydride generator has been characterized for use with the acid-NaBH(4) hydride generation systems based on the insertion of a capillary tube into the sample introduction channel of a standard Meinhard nebulizer. The acidic sample and the tetrahydroborate solution are mixed at a merge point 1.5 cm from the end of the nebulizer orifice. Nebulization of the reaction solutions into a 0.7 mL tubular "spray chamber" follows a very short mixing time (less than 0.012 s) of the reagents. This approach permits 10 000 µg/mL Ni(2+) or Cr(3+) to be present in the sample solution without producing any interferences. Additionally, in the presence of Fe(3+) added as a "releasing agent", 5000 µg/mL Co(2+) or 160 µg/mL Cu(2+) can also be tolerated without interference. An 80 ± 2% generation efficiency is attained for the test element selenium. A detection limit of 6 µg/L (3σ(b)) is achieved with ICP-AES detection. Precision of replicate measurements at the 12 µg/L level varies from 5 to 12% relative standard deviation.

Furnace atomization plasma ionization mass spectrometry.

Development and initial characterization of FAPES as an ion source for elemental mass spectrometry is presented. The source is configured with an integrated contact cuvette in which a 40 MHz He plasma is sustained at atmospheric pressure. A differentially pumped interface consisting of a stainless steel sampling tube (0.5 mm i.d.) and tandem skimmer cone serves to sample the plasma from the open end of the cuvette. Mass spectra, characterizing plasma species with the furnace at ambient and elevated temperatures, are relatively structureless and show no evidence of He(+), He(2)(+), or carbonaceous ions. Transient ion signals, generated during the atomization of a number of analytes introduced in solution form, reveal that the plasma contains sufficiently energetic species to ionize elements having ionization potentials as high as 10.45 eV (iodine). Significant ionization does not occur in the absence of the plasma, nor in the extraction interface. Using Pb as a test element, acceptable isotopic abundances can be obtained, with an estimated absolute limit of detection of 10 pg (2 ng/mL relative). Future directions are discussed.

Determination of total chromium in Marine Sediment Reference Material BCSS-1.

The determination of Cr in National Research Council of Canada Marine Sediment Reference Material BCSS-1 is addressed. Mixed acid digestions utilizing HF, HClO(4) and HNO(3) were investigated. Single microwave assisted digestions in closed vessels at medium pressures (8 bar) were inappropriate (80% recovery). Double digestion at moderate pressure and digestions at high pressure (70 bar) or lengthy open-beaker hot plate dissolutions resulted in 93+/-5% recovery of certified Cr content. Flame atomic absorption and inductively coupled plasma atomic emission spectrometry were used for quantitation. In all cases, the method of standard additions was necessary to eliminate the approximately 10% enhancement in re- sponce relative to unmatched standards. Compared to a certified content of 123+/-7 microg/g (mean and standard deviation), values of 116+/-6 were obtained using an open beaker digestion; 114+/-5 using a double microwave digestion at 8 bar; 113+/-2 for microwave digestion at high pressure and 111+/-4 at high pressure (81 bar) with triple microwave digestion in a Parr bomb. No acid dissolution procedure is adequate for Cr in this sample although recovery of total Cr is complete from National Institute of Standards and Technology Buffalo River Sediment (SRM 2704).

The graphite furnace and its role in atomic spectroscopy.

The diversity of applications of the graphite furnace is extraordinary, encompassing the fields of physics, thermochemistry, spectroscopy and analytical chemistry. In this respect, the graphite furnace has been used on a continuous basis as a research tool for nearly a century. Following its introduction as an atomization source for atomic absorption spectrometry by L'vov in 1959, its role in atomic spectrometry expanded considerably to encompass analytical applications in emision, fluorescence, absorption and mass spectrometry. In addition to its conspicuous use as an atomization source in these areas, it is frequently employed as a vaporizer when used in the format of combined and tandem sources with other instrumentation. The unique physico-chemical micro-environment which can be attained within the graphite furnace has also been used to advantage in a number of investigations, including the determination of gas- and solid-phase diffusion coefficients of high-temperature metal vapours, the heats of sublimation of refractory metals, fundamental optical constants and the measurement of the heats of desorption of adatoms from high-temperature surfaces. The range of such applications remains to be more fully explored. The attractive features of this source, viz., the high atomization/vaporization efficiency, comparatively long atomic vapour residence times, controllable chemical and thermal environment and its ability to handle high dissolved solids content samples (

Determination of copper in environmental matrices following vapor generation.

Copper was determined in environmental matrices following generation, separation, and atomization of a volatile species formed by the merging of an acidified sample solution with an aqueous sodium tetrahydroborate solution at room temperature. The copper species, as yet unidentified, was phase separated in a conventional gas-liquid separator and directed via a stream of Ar carrier gas to an inductively coupled plasma atomic emission detection system. Optimum conditions for generation were investigated. The efficiency of generation/transfer was estimated to be 50%, and no interference from the presence of 1000 mg/L concentration of As, Cd, Co, Ni, Fe, Cr, Mn, Pb, Se, and Zn concomitants was evident. Simple aqueous standards were used for calibration purposes, and good agreement was obtained with certified values in the analysis of National Research Council of Canada marine sediment BCSS-1 and lobster hepatopancreas tissue TORT-1.