From sea salt to seawater: a novel approach for the production of water CRMs.

Natural water certified reference materials (CRMs) are mostly available in a liquid form, and they are produced starting from suitable environmental samples. Many precautions are usually needed to avoid biological or physical degradation, including filtration, acidification, and sterilization. In this study, the drawbacks associated with liquid-based seawater CRMs were tackled by developing a salt-based seawater proxy for nutrients that could be reconstituted in water solution just before use. Phosphate, silicate, and nitrate were chosen as target analytes. Sea salt mimicking the composition of seawater was spiked with an aqueous solution of the analytes and homogenized using a high-energy planetary ball mill (uhom < 1.2%). The salt powder CRM SALT-1 ( https://doi.org/10.4224/crm.2022.salt-1 ) demonstrated good short- and long-term stability for nutrients. When the SALT-1 was reconstituted in water at the 4.0% w/w level, the resulting solution had similar properties with respect to typical seawater in terms of major constituents (± 20%), trace metals, density (1.023 g/mL), pH (8.8-9.0), and optical properties relevant to the photometric characterization. Phosphate and silicate were quantified by photometry (molybdenum blue method, batch mode), whereas nitrate was quantified by isotope dilution GC-MS (uchar < 1.2%). In the SALT-1 reconstituted seawater solution at the 4.0% w/w salt level, the nutrient amount concentration was w(phosphate, PO43-) = 1.615 ± 0.030 µmol/L, w(silicate as SiO2) = 8.89 ± 0.31 µmol/L, and w(nitrate, NO3-) = 18.98 ± 0.45 µmol/L at the 95% confidence (k = 2). Overall, the SALT-1 CRM exhibits similar nutrient profile and general analytical characteristics as the MOOS-3 CRM. However, the SALT-1 has much reduced preparation, storage, and distribution cost, likely much better long-term stability, and it could enable the production of lower cost and more accessible seawater reference materials.

Versatile derivatization for GC-MS and LC-MS: alkylation with trialkyloxonium tetrafluoroborates for inorganic anions, chemical warfare agent degradation products, organic acids, and proteomic analysis.

Analytical chemists resort to derivatization for improving the detection performance of certain categories of analytes. Within this context, alkylation reactions are regarded as an important asset for many methods based on GC-MS and LC-MS. Trialkyloxonium tetrafluoroborates (R[Formula: see text][BF4]-) are powerful alkylating agents with ionic liquid properties: they are nonvolatile salts soluble in water which are easier and safer to handle with respect to common alkylating agents like diazomethane. R[Formula: see text][BF4]- can perform the alkylation in both organic and aqueous media at pH conditions ranging from acidic to alkaline. Recent analytical applications of trialkyloxonium derivatizations include the high-precision determination of inorganic anions in complex matrices, the qualitative confirmation of chemical warfare agent degradation products in soils, the profiling of carboxylic acids in urine, and the detection of protein post-translational modifications induced by carbon dioxide. The common denominator for all methods presented can be found in the simplicity of the alkylation protocol which, in most of the cases, requires a single step addition of the reagent directly to the sample. Graphical Abstract Alkylation with trialkyloxonium salts for GC-MS and LC-MS analysis.

Application of direct analysis in real time to study chemical vapor generation mechanisms: reduction of dimethylarsinic(V) acid with aqueous NaBH4 under non-analytical conditions.

The aqueous-phase reaction of dimethylarsinc acid (DMAs(V)) with NaBH4 (THB) was studied under non-analytical conditions (1000 µg/mL As, 0.1 M HCl, 1% NaBH4) with the aim of identifying intermediates and reaction products. The use of direct analysis in real time (DART) with high-resolution mass spectrometry (HRMS), in combination with two different chemical vapor generation systems, allowed the identification of some species not detected by GC-MS such as Me2As-AsMe-AsMe2 and the arsonium species [Me3As-AsMe2]+ and [Me2As-AsMe2-AsMe2]+. Many other methylated species of arsenic containing up to four arsenic atoms have been observed. Unfortunately, the oxidation mechanism that took place in the DART source interfered with the identification of some of those species formed in solution following THB reduction. The species identified by DART-HRMS, together with those previously identified by GC-MS (Me2AsH, Me2AsOH, Me3As, Me3AsO, Me2AsAsMeH, Me2AsAsMe2, and Me2As-O-AsMe2)' enabled the formulation of hypotheses on the possible reaction pathways and revealed an aqueous-phase reactivity of DMAs(V) which could not be explained on the basis of current knowledge. Graphical Abstract.

Selective Gas Chromatography Mass Spectrometry Method for Ultratrace Detection of Selenocyanate.

The recent interest in the determination of selenocyanate (SeCN-) in wastewater systems has spurred the development of analytical methods for its determination at the ultratrace level. Since most of the current procedures require complex and costly instrumental configurations, we have developed a simple and rapid gas chromatography tandem mass spectrometry (GC/MS/MS) method able to detect SeCN- in water samples with a LOD of 0.1 ng/g Se. A 1 mL volume of aqueous sample was buffered with sodium bicarbonate and treated with triethyloxonium tetrafluoroborate for conversion of the analyte into volatile EtSeCN. The derivatization yield was higher than 90%, and it could tolerate concentrations of chloride or sulfate up to 2%. The EtSeCN was extracted in chloroform and could be detected in electron ionization and also in negative chemical ionization mode with a further gain in signal-to-noise ratio by a factor of 2. The method was applied for the analysis of natural waters with quantitation of SeCN- in the low ng/g region. The Se13C15N- internal standard could be used for isotope dilution. Quantitative spike recoveries of 1 ng/g Se were obtained from seawater and river water, and 1 ng/g Se could be quantified within a standard uncertainty of 15%.

Application of direct analysis in real time to the study of chemical vapor generation mechanisms: identification of intermediate hydrolysis products of amine-boranes.

In order to elucidate controversial results emerging in chemical vapor generation (CVG) for trace element determination, we conducted a series of experiments devoted to the identification of intermediates formed by acid hydrolysis of amine-boranes. For the first time, direct analysis in real time coupled with high-resolution mass spectrometry (DART-Orbitrap) was applied for detection of this class of compounds. Mass spectra of both solid amine-boranes and their aqueous solutions (pH ~ 8, no hydrolysis) were acquired for understanding their ionization pathway. Mass spectra of aqueous solutions of t-BuNH2·BH3 and Me2NH·BH3 were acquired under conditions that are employed in CVG (0.017-4.0 mol L-1 HCl, 0.167-0.2 mol L-1 borane reagent). The results disclose a reactivity driven by pH of amine-boranes undergoing hydrolysis. At low acidity, the hydrolysis proceeds according to the currently accepted displacement mechanisms (i.e., R3N·BH3 + H3O+ → R3NH+ + H2OBH3). At higher acidity, N-tert-butyl, cyclotriborazane, and bis(dimethylamino)boronium were identified, for the first time, during the hydrolysis of t-BuNH2·BH3 and Me2NH·BH3, respectively. Formation of these intermediates was ascribed to a hydrolysis pathway starting with the ionization of the amine-borane, (i.e., R3N·BH3 + H3O+ → [(H2O)R3NBH2] + + H2). The new evidence explains the anomalous behavior observed in CVG by amine-borane derivatization, and updates the currently accepted mechanisms for the acid hydrolysis of amine-boranes. Graphical Abstract.

Certification of nitrate in spinach powder reference material SPIN-1 by high-precision isotope dilution GC-MS.

A high-precision exact-matching quadruple isotope dilution method (ID4MS) was employed for the quantitation of nitrate in an air-dried spinach powder Certified Reference Material (CRM). The analyte was extracted in hot water following addition of 15NO[Formula: see text] internal standard. The blend was then treated with sulfamic acid to remove nitrite and with triethyloxonium tetrafluoroborate to promote aqueous conversion of nitrate into volatile EtONO2. The derivative was analyzed by headspace GC-MS with 3-min elution time. The method performance was validated with a series of tests which demonstrated adequate selectivity and ruggedness. This method supported the development of novel SPIN-1 CRM giving a modest contribution to its uncertainty (uchar = 0.85%). With respect to previous attempts, the SPIN-1 was proven stable, homogeneous (uhom = 0.44%), and suitable for spinach monitoring under EU regulations. On dried basis, the nitrate content of SPIN-1 was found to be 22.53 ± 0.43 mg/g (Uc = 1.9%, k = 2). The material was also used in an inter-laboratory study where four laboratories employed a total of ten measurement methods. Graphical Abstract SPIN-1 Certified Reference Material for nitrate in spinach powder.

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.

Blank Correction in Isotope Dilution.

A novel method for compensation of the procedural blank in isotope dilution is presented. This method, entitled "blank-matching", copes with the blank through experimental design. Both sample and calibration solutions are exposed to the same amount of isotopic standard and same procedural blank. The identical treatment of sample and calibrators eliminates the need for subtracting the procedural blank from the result obtained by isotope dilution. A further advantage of the method is that quantitation of the analyte in the procedural blank is not required. Blank-matching is simple and fast to implement, and it permits direct determination of results without further corrections. This aspect has an important metrological outcome: blank-matching isotope dilution can be considered a primary method of analysis that does not involve the procedural blank as a potential source of bias.

Coordinate swapping in standard addition graphs for analytical chemistry: a simplified path for uncertainty calculation in linear and nonlinear plots.

Uncertainty of the result from the method of standard addition is often underestimated due to neglect of the covariance between the intercept and the slope. In order to simplify the data analysis from standard addition experiments, we propose x-y coordinate swapping in conventional linear regression. Unlike the ratio of the intercept and slope, which is the result of the traditional method of standard addition, the result of the inverse standard addition is obtained directly from the intercept of the swapped calibration line. Consequently, the uncertainty evaluation becomes markedly simpler. The method is also applicable to nonlinear curves, such as the quadratic model, without incurring any additional complexity.

Direct determination of dissolved phosphate and silicate in seawater by ion exclusion chromatography sector field inductively coupled plasma mass spectrometry.

A method is described for the direct determination of dissolved phosphate and silicate in seawater using ion exclusion chromatography (IEC) coupled with sector field inductively coupled plasma mass spectrometry (SF-ICPMS). Dissolved silicate was determined by double isotope dilution using a (29)Si spike, whereas one point gravimetric standard addition with internal standard of the same (29)Si spike was employed to quantitate dissolved phosphate. Medium resolution was used for all measurements in order to resolve polyatomic interferences on Si and P isotopes. Concentrations of 1.670 ± 0.008 and 30.20 ± 0.09 µM (SD, n = 6) with precisions of 0.47 and 0.31% for the dissolved phosphate and silicate, respectively, were obtained in National Research Council Canada certified reference material MOOS-3 seawater, in good agreement with certified values of 1.60 ± 0.15 and 30.5 ± 0.8 µM (U, k = 2), respectively. The reported method is a rapid (10 min per run), simple, and accurate online technique that requires no sample pretreatment. Moreover, this procedure achieves <0.5% precision (at above analyte concentrations) and method detection limits of 0.006 and 0.004 µM (0.18 as P and 0.11 ng g(-1) as Si), respectively, using a of 100 µL injection of seawater. The proposed technique is robust and well-suited for the determination of dissolved phosphate and silicate in seawater.

Novel ethyl-derivatization approach for the determination of fluoride by headspace gas chromatography/mass spectrometry.

We report a novel derivatization chemistry for determination of fluoride based on the batch reaction of fluoride ions with triethyloxonium tetrachloroferrate(III) in a closed vessel to yield fluoroethane. Gaseous fluoroethane was readily separated from the matrix, sampled from the headspace, and determined by gas chromatography/mass spectrometry. The method was validated using rainwater certified reference material (IRMM CA408) and subsequently applied to the determination of fluoride in various matrixes, including tap water, seawater, and urine. An instrumental limit of detection of 3.2 µg/L with a linear range up to 50 mg/L was achieved. The proposed derivatization is a one-step reaction, requires no organic solvents, and is safe, as the derivatizing agent is nonvolatile. Determination of fluoride is affected by common fluoride-complexing agents, such as Al(III) and Fe(III). The effect of large amounts of these interferences was studied, and the adverse effect of these ions was eliminated by use of the method of standard additions.

Reduction of measurement uncertainty by experimental design in high-order (double, triple, and quadruple) isotope dilution mass spectrometry: application to GC-MS measurement of bromide.

Since its introduction a century ago, isotope dilution analysis has played a central role in developments of analytical chemistry. This method has witnessed many elaborations and developments over the years. To date, we have single, double, and even triple isotope dilution methods. In this manuscript, we summarize the conceptual aspects of isotope dilution methods and introduce the quadruple dilution and the concept of exact matching triple and quadruple dilutions. The comparison of isotope dilution methods is performed by determination of bromide ions in groundwater using novel ethyl-derivatization chemistry in conjunction with GC/MS. We show that the benefits of higher-order isotope dilution methods are countered with a greater need for careful experimental design of the isotopic blends. Just as for ID(2)MS, ID(3)MS and ID(4)MS perform best when the isotope ratio of one sample/spike blend is matched with that of a standard/spike blend (exact matching).

Negative chemical ionization GC/MS determination of nitrite and nitrate in seawater using exact matching double spike isotope dilution and derivatization with triethyloxonium tetrafluoroborate.

The alkylation of nitrite and nitrate by triethyloxonium tetrafluoroborate allows determination of their ethyl esters by headspace gas chromatography/mass spectrometry (GC/MS). In the present study, significant improvement in analytical performance is achieved using negative chemical ionization providing detection limits of 150 ng/L for NO(2)(-) and 600 ng/L for NO(3)(-), an order of magnitude better than those achieved using electron impact ionization. The derivatization procedure was optimized and alkaline conditions adopted to minimize conversion of nitrite to nitrate (determined to be 0.07% at 100 mg/L NO(2)(-)) and to avoid the exchange of oxygen between the analytes and the solvent (water). Quantitation entails use of isotopically enriched standards (N(18)O(2)(-) and (15)NO(3)(-)), which also permits monitoring of potential conversion from nitrite to nitrate during the analysis (double spike isotope dilution).

Condensation cascades and methylgroup transfer reactions during the formation of arsane, methyl- and dimethylarsane by aqueous borohydride and (methyl) arsenates.

The formation of volatile products during the reaction of As(III: ), As(V: ), MeAsO(OH)(2), and Me(2)AsO(OH) with aqueous NaBH(4) has been investigated, and the formation of arsanes, diarsanes, and triarsanes has been detected. The presence of triarsanes is reported here for the first time. Diarsanes and triarsanes are likely formed in condensation cascade reactions, whereas trimethylarsane arises via the transfer of a methyl group. The formation of volatile by-products is considerably reduced by increasing the acidity of the medium and the concentration of NaBH(4) or by the addition of thiols, such as cysteine. A reaction scheme is proposed which reconciles the evidence reported herein and elsewhere in the literature that is valid for both analytical (trace analysis) and non-analytical reaction conditions.