Determination of metals and metalloids in light and heavy crude oil by ICP-MS after digestion by microwave-induced combustion

A method for light and heavy crude oil digestion using microwave-induced combustion (MIC) in closed vessels is described for further determination of Ag, As, Ba, Bi, Ca, Cd, Cr, Fe, K, Mg, Li, Mn, Mo, Ni, Pb, Rb, Se, Sr, Tl, V, and Zn by inductively coupled plasma mass spectrometry (ICP-MS). Conventional microwave-assisted acid digestion (MW-AD) in pressurized vessels and analyte determination by inductively coupled plasma optical emission spectrometry (ICP OES) were also used for results comparison. For MIC procedure, samples were wrapped in polyethylene films and combusted using 20 bar of oxygen and 50 µl of 6 mol l^− 1 ammonium nitrate as aid for ignition. The concentration of nitric acid used as absorbing solution was evaluated (1, 2, 4, 7, 10 and 14 mol l^− 1) using an additional reflux step after combustion. Accuracy was evaluated for As, Ba, Ni, Se V, and Zn using certified reference material (CRM) with similar matrix composition and for Cr, Fe, K, Mg, Mn, and Mo by neutron activation analysis (NAA). Recovery tests were also performed for all the analytes by MIC and they were better than 97% using 2 mol l^− 1 nitric acid as absorbing solution (with reflux step). Agreement with certified values and NAA results was better than 95%. Using MIC it was possible to obtain lower limits of detection (LODs) by ICP-MS and also by ICP OES in comparison with those obtained by MW-AD. In spite of both sample preparation techniques were apparently suitable for crude oil digestion, MIC was preferable in view of the possibility of using diluted nitric acid as absorbing solution that is an important aspect to minimize interferences by ICP-MS and ICP OES. In order to avoid polyatomic interferences on ⁵²Cr and ⁵⁶Fe determinations by ICP-MS, a dynamic reaction cell with ammonia gas was used. Residual carbon content in digests obtained by MW-AD and MIC was 15% and < 1%, respectively. Using MIC the simultaneous digestion of 8 samples was possible in less than 30 min.     

Evaluation of sample preparation methods for elastomer digestion for further halogens determination

In this work, three sample preparation methods were evaluated for further halogen determination in elastomers containing high concentrations of carbon black. Samples of nitrile-butadiene rubber, styrene-butadiene rubber, and ethylene-propylene-diene monomer elastomers were decomposed using oxygen flask combustion and microwave-induced combustion (MIC) for further Br and Cl determination by ion chromatography (IC), inductively coupled plasma optical emission spectrometry (ICP OES), and inductively coupled plasma mass spectrometry (ICP-MS). Extraction assisted by microwave radiation in closed vessels was also evaluated using water or alkaline solution. Digestion by MIC was carried out using 50 mmol l⁻¹ (NH₄)₂CO₃ as the absorbing solution. The effect of the reflux step was also evaluated. Accuracy was evaluated using certified reference materials with polymeric matrix composition and by comparison of results using neutron activation analysis. Agreement for Br and Cl was better than 95% by MIC using 5 min of reflux, and no statistical difference was found using IC, ICP OES, and ICP-MS for determination of both analytes. For MIC, the relative standard deviation (RSD) was lower than 5%. Using extraction in closed vessels, a high amount of residues was observed, and recoveries were lower than 45% for both analytes. For oxygen flask combustion, the agreement was similar using MIC but RSD was higher (20%). The residual carbon content, an important parameter used to evaluate the digestion efficiency, was always below 1% for MIC. Using MIC, it was possible to digest elastomers with high efficiency, resulting in a single solution suitable for halogen determination by different techniques.     

Determination of cadmium and lead in cetacean Dolphinidae tissue from the coast of Bahia state in Brazil by GFAAS

In the present study, cadmium and lead in the muscle, lung, liver and kidney of dolphins (Sotalia guianensis and Stenella clymene) of the Bahia coast in the northwest of Brazil were determined by graphite furnace atomic absorption spectrometry. Samples were digested using a diluted oxidant mixture (HNO₃ + H₂O₂) with a microwave heating program performed in five steps. The optimized temperatures and chemical modifier for the pyrolysis and atomization were 700 °C, 1400 °C and Pd plus Mg for Cd, and 900 °C, 1800 °C and NH₄H₂PO₄ for Pb, respectively. Characteristic masses and limits of detections (n = 20, 3σ) for Cd and Pb were 1.6 and 9.0 pg and 0.82 ng g^− 1 and 0.50 ng g^− 1, respectively. Repeatability ranged from 0.87 to 8.22% for Cd and 4.31 to 8.09% for Pb. The found concentrations presented no statistical differences at the 95% confidence level when compared with the ICP OES methods. Addition and recovery tests were also performed and the results ranged between 87 and 112% for both elements. Samples of cetacean Dolphinidae (S.guianensis and S.clymene) were analyzed, and the higher concentrations ranged from 0.09 to 46.2 µg g^− 1 for Cd and 0.04 to 0.47 µg g^− 1 for Pb in liver, and from 0.133 to 277 µg g^− 1 for Cd in the kidney.     

Characterization of a microwave microstrip helium plasma with gas-phase sample introduction for the optical emission spectrometric determination of bromine,chlorine, sulfur and carbon using a miniaturized optical fiber spectrometer

Continuous flow generation of Br₂, Cl₂ and H₂S coupled to a low-power 2.45 GHz microwave microstrip He plasma exiting from a capillary gas channel in a micro-fabricated sapphire wafer with microstrip lines has been used for the optical emission spectrometric determination of Br, Cl and S using a miniaturized optical fiber CCD spectrometer. Under optimized conditions, detection limits (3σ) of 330, 190 and 220 μg l^− 1 for Br, Cl and S, respectively, under the use of the Br II 478.5 nm, Cl I 439.0 nm and S I 469.0 nm lines were obtained and the calibration curves were found to be linear over 2 orders of magnitude. In addition, when introducing CO₂ and using the rotational line of the CN molecular band at 385.7 nm the detection limit for C was 4.6 μg l^− 1. The procedure developed was found to be free from interferences from a number of metal cations and non-metal anions. Only the presence of CO₃²⁻ and CN⁻ was found to cause severe spectral interferences as strong CN and C₂ molecular bands occurred as a result of an introduction of co-generated CO₂ and HCN into the plasma. With the procedure described Br, Cl and S could be determined at a concentration level of 10–30 mg l^− 1 with accuracy and precision better than 2%.     

Characterization and determination of 28 elements in fly ashes collected in a thermal power plant in Argentina using different instrumental techniques

Different techniques were selected for comprehensive characterization of seven samples of fly ashes collected from the electrostatic precipitator of the San Nicolás thermal power plant (Buenos Aires, Argentina). Particle size was measured using laser based particle size analyzer. X-ray diffraction powder (XRD) analysis and scanning electron microscopy (SEM) were used to characterize the mineral phase present in the matrix consisting basically of aluminosilicates and large amounts of amorphous material. The predominant crystalline phases were mullite and quartz. Major and minors elements (Al, Ca, Cl, Fe, K, Mg, Na, S, Si and Ti) were detected by energy dispersive X-ray analysis (EDAX). Trace elements (As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Se, V and Zn) content was quantified by inductively coupled plasma optical emission spectrometry (ICP OES). Different acid mixtures and digestion procedures were compared for subsequent ICP OES measurements of the dissolved samples. The digestion procedures used were: i) a mixture of FH + HNO₃ + HClO₄ (open system digestion); ii) a mixture of FH + HNO₃ (MW-assisted digestion); iii) a mixture of HF and aqua regia (MW-assisted digestion). Instrumental neutron activation analysis (INAA) was employed for the determination of As, Ba, Co, Cr, Ce, Cs, Eu, Fe, Gd, Hf, La, Lu, Rb, Sb, Sc, Sm, Ta, Tb, Th, U and Yb. The validation of the procedure was performed by the analysis of two certified materials namely, i) NIST 1633b, coal fly ash and ii) GBW07105, rock. Mean elements content spanned from 41870 μg g^− 1 for Fe to 1.14 μg g^− 1 for Lu. The study showed that Fe (41870 μg g^− 1) ? V (1137 μg g^− 1) > Ni (269 μg g^− 1) > Mn (169 μg g^− 1) are the main components. An enrichment, with respect to crustal average, in many elements was observed especially for As, V and Sb that deserve particular interest from the environmental and human health point of view.     

A novel analytical system involving hydride generation and gold-coated W-coil trapping atomic absorption spectrometry for selenium determination at ng l level

A novel analytical technique was developed where gaseous hydrogen selenide formed by sodium tetrahydroborate reduction is transported to and trapped on a resistively heated gold-coated W-coil atom trap for in situ preconcentration. Gold coating on W-coil was prepared by using an organic solution of Au. The atom trap is held at 165 °C during the collection stage and is heated up to 675 °C for revolatilization; analyte species formed are transported to an externally heated quartz T-tube where the atomization takes place and the transient signal is obtained. The carrier gas consisted of 112.5 ml min^− 1 Ar with 75 ml min^− 1 H₂ during the collection step and 112.5 ml min^− 1 Ar with 450 ml min^− 1 H₂ in the revolatilization step. The half width of the transient signal obtained is less than 0.5 s. The RSD for the measurements was found to be 3.9% (n = 11) for 0.10 µg l^− 1 Se using peak height measurements.     

Evaluation of liquid chromatography inductively coupled plasma mass spectrometry for arsenic speciation in water from industrial treatment of shale

This work describes an arsenic speciation analysis in aqueous effluent from a shale industrial plant using liquid chromatography coupled to inductively coupled plasma mass spectrometry (LC–ICP–MS). Arsenic species have been separated through an anion-exchange column and several parameters investigated, such as retention time, pH, flow rate and concentration of the mobile phase (ammonium carbonate), chloride interference and column conditioning time. The best conditions have been found by fixing the pH of the mobile phase at 8.7. Keeping the mobile phase flow rate at 1.5 ml min^− 1, arsenic species were separated by varying the concentration of the mobile phase and the time of elution, as follow: 1.5 mmol l^− 1 for 10 min, 12 mmol l^− 1 for 10 min and 20 mmol l^− 1 for 10 min, respectively. Up to 13 As species present in the samples were separated under these conditions and the following species could be identified and quantified: arsenite [As(III)], dimethylarsinic acid (DMA), monomethylarsonic acid (MMA) and arsenate [As(V)]. The limits of detection of the LC–ICP–MS method were 0.02, 0.06, 0.04 and 0.10 μg l^− 1 of As(III), DMA, MMA, and As(V), respectively. The concentration of these species in the samples were from 3.7 to 6.4 μg l^− 1, 6.9 to 13.2 μg l^− 1, 100 to 142 μg l^− 1 and 808 to 1363 μg l^− 1 for As(III), DMA, MMA and As(V), respectively. The accuracy, evaluated by recovery tests, varied from 94 to 105% and the precision, evaluated by the relative standard deviation was typically lower than 10%.     

Determination of Se in biological samples by axial view inductively coupled plasma optical emission spectrometry after digestion with aqua regia and on-line chemical vapor generation

A simple and fast method for the determination of Se in biological samples, including food, by axial view inductively coupled plasma optical emission spectrometry using on-line chemical vapor generation (CVG–ICP OES) is proposed. The concentrations of HCl and NaBH₄, used in the chemical vapor generation were optimized by factorial analysis. Six certified materials (non-fat milk powder, lobster hepatopancreas, human hair, whole egg powder, oyster tissue, and lyophilised pig kidney) were treated with 10 mL of aqua regia in a microwave system under reflux for 15 min followed by additional 15 min in an ultrasonic bath. The solutions were transferred to a 100 mL volumetric flask and the final volume was made up with water. The Se was determined directly in these solutions by CVG–ICP OES, using the analytical line at 196.026 nm. Calibration against aqueous standards in 10% v/v aqua regia in the concentration range of 0.5–10.0 µg L^− 1 Se(IV) was used for the analysis. The quantification limit, considering a 0.5 g sample weight in a final volume of 100 mL^− 1 was 0.10 µg g^− 1. The obtained concentration values were in agreement with the total certified concentrations, according to the t-test for a 95% confidence level.     

Sensitized extraction spectrophotometric determination of Hg(II) with dithizone after its flotation as ion-associate using iodide and ferroin

This paper describes a simple and highly selective method for the separation, preconcentration and spectrophotometric determination of extremely low concentration of mercury. The method is based on the flotation of an ion-associate of HgI₄²⁻ and ferroin between aqueous and n-heptane interface at pH 5. The ion-associate was then separated and treated with ammonia and dithizone solutions to extract only the mercury chelate with CH₂Cl₂. The measurement is feasible when the volume of the water sample containing Hg(II) was varied over 50-800 ml. Beer's law was obeyed over the concentration range of 8 × 10⁻⁹ to 1.6 × 10⁻⁷ mol l⁻¹ with an apparent molar absorptivity of 6.53 × 10⁶ l mol⁻¹ cm⁻¹ for a 500 ml aliquot of the water sample. The detection limit (n = 7) was 5.0 × 10⁻¹⁰ mol l⁻¹ and the R.S.D. (n = 5) for 8.0 × 10⁻⁷ mol l⁻¹ of Hg(II) was 3.7%. A notable advantage of the method is that the determination of Hg(II) is free from the interference of almost all cations and anions found in the environmental and waste water samples. The determination of Hg(II) in tap, synthetic sea water and human hair samples was carried out by the present method and cold vapor atomic absorption spectrometry (CV-AAS). The results were satisfactorily comparable so that the applicability of the proposed method was confirmed to the real samples.     

Iodine determination in food by inductively coupled plasma mass spectrometry after digestion by microwave-induced combustion

Iodine determination in food samples was performed by inductively coupled plasma mass spectrometry (ICP-MS) after digestion by microwave-induced combustion (MIC). Sample masses up to 500 mg of bovine liver, corn starch, milk powder, or wheat flour were completely combusted using the MIC system. Ammonium nitrate (6 mol l⁻¹ solution, 50 μl) was used as an aid for ignition and vessels were charged with 15 bar of O₂. The use of H₂O, 0.9 mmol l⁻¹ H₂O₂, 10 to 50 mmol l⁻¹ (NH₄)₂CO₃ and 56 mmol l⁻¹ tetramethylammonium hydroxide was investigated as absorbing solutions, as well as the suitability of performing a reflux step after the combustion process. Digestion of food samples by pressurized microwave-assisted acid digestion, microwave-assisted extraction and conventional extraction of iodine in alkaline solution were also evaluated. Iodine recoveries higher than 99% were obtained using MIC and 50 mmol l⁻¹ (NH₄)₂CO₃ or 56 mmol l⁻¹ tetramethylammonium hydroxide as absorbing solution and with 5 min for the reflux step. Accuracy was evaluated using certified reference materials (bovine muscle, corn bran, and milk powder) and agreement better than 97% was obtained. The limit of quantification by MIC and further ICP-MS determination was 0.002 μg g⁻¹. Blanks were always low and no memory effects were observed. Digestion by MIC allowed the processing of up to eight samples by each run in 25 min with high efficiency of digestion (residual carbon content lower than 1%) providing a suitable medium for further iodine determination by ICP-MS.     

A multicommuted flow system for sequential spectrophotometric determination of hydrosoluble vitamins in pharmaceutical preparations

A multicommuted flow system is proposed for spectrophotometric determination of hydrosoluble vitamins (ascorbic acid, thiamine, riboflavine and pyridoxine) in pharmaceutical preparations. The flow manifold was designed with computer-controlled three-way solenoid valves for independent handling of sample and reagent solutions and a multi-channel spectrophotometer was employed for signal measurements. Periodic re-calibration as well as the standard addition method was implemented by using a single reference solution. Linear responses (r=0.999) were obtained for 0.500-10.0 mg l⁻¹ ascorbic acid, 2.00-50.0 mg l⁻¹ thiamine, 5.00-50.0 mg l⁻¹ riboflavine and 0.500-8.00 mg l⁻¹ pyridoxine. Detection limits were estimated as 0.08 mg l⁻¹ (0.5 μmol l⁻¹) ascorbic acid, 0.8 mg l⁻¹ (2 μmol l⁻¹) thiamine, 0.2 mg l⁻¹ (0.5 μmol l⁻¹) riboflavine and 0.1 mg l⁻¹ (0.9 μmol l⁻¹) pyridoxine at 99.7% confidence level. A mean sampling rate of 60 determinations per hour was achieved and coefficients of variation of 1% (n=20) were estimated for all species. The mean reagent consumption was 25-fold lower in relation to flow-based procedures with continuous reagent addition. Average recoveries between 95.6 and 100% were obtained for commercial pharmaceutical preparations. Results agreed with those obtained by reference methods at 95% confidence level. The flow system is suitable for application in quality control processes and in dissolution studies of vitamin tablets.     

Thermal modified Kaolinite as useful material for separation and preconcentration of trace amounts of manganese ions

This work assesses for the first time the potential of natural Kaolinite as adsorptive material for preconcentration of metal traces. Manganese is quantitatively retained by 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) on thermal modified Kaolinite by column method in pH range of 8.5-10.0 at flow rate of 2 ml min⁻¹. Manganese was removed from column with 5.0 ml of H₂SO₄ 4 mol l⁻¹ and determined by flame atomic absorption spectrometric at 279.5 nm. In this case, 0.l μg of manganese can be concentrated from 800 ml of aqueous sample (where concentration is as low as 0.125 μg l⁻¹). Detection limit is 4.3 μg l⁻¹ (3 δ_bl m⁻¹) and analytical curve is linear in the 0.02-10 mg l⁻¹ in final solution with correlation coefficient 0.9997 and relative standard deviation for eight replicate determination of 5 μg of manganese in final solution is 0.71%. The interference of a large number of anions and cations has been studied in detail to optimize the conditions and method was successfully applied for determination of manganese in complex materials.     

Determination of myo-inositol hexakisphosphate (phytate) in urine by inductively coupled plasma atomic emission spectrometry

Myo-inositol hexakisphosphate (phytate) is a substance present in urine with an important role in preventing calcium renal calculi development. In spite of this, the use of urinary phytate levels on stone-formers’ evaluation and treatment is still notably restricted as a consequence of the enormous difficulty to analyze this substance in urine. In this paper, a simple procedure for routinary urinary phytate determination based on phosphorus determination through inductively coupled plasma atomic emission spectrometry is described.The method only requires a previous separation of phytate from other components by column anion exchange chromatography. The working linear range used was 0-2 mg l⁻¹ phosphorus (0-7 mg l⁻¹ phytate). The limit of detection was 64 μg l⁻¹ of phytate and the limit of quantification was 213 μg l⁻¹. The relative standard deviation (R.S.D.) for 1.35 mg l⁻¹ phytate was 2.4%. Different urine samples were analyzed using an alternative analytical methodology based on gas chromatography (GC)/mass detection used for inositol determination (phytate was previously hydrolyzed), resulting both methods comparable using as criterion to assess statistical significance P<0.05.     

Synergistic flotation of U(VI)-alizarin complex with some diamines followed by spectrophotometric determination of U(VI) using 4,4′-diaminophenylmethane

In this work, synergistic flotation of U(VI)-alizarin complex at the presence of some diamine compounds was firstly investigated by the spectrophotometric method. The flotation process was carried out on aliquots of 100 ml of U(VI) solutions containing alizarin and the diamine at pH of 5.00 using n-heptane. The floated layer was then dissolved in acetonitrile and its absorbance was measured. Since the synergistic effect of 4,4′-diaminophenylmethane (dapm) was much more than the others, it was used for the determination of U(VI) by this method. Beer's law was obeyed (λ_max = 591 nm) in the range of 5 × 10⁻⁷ to 1 × 10⁻⁵ mol l⁻¹ with the apparent molar absorptivity of 1.12 × 10⁶ l mol⁻¹ cm⁻¹, and a correlation coefficient of 0.9991. The detection limit (n = 7) was 1 × 10⁻⁷ mol l⁻¹, and the R.S.D. (n = 7) obtained for 4 × 10⁻⁶ mol l⁻¹ of U(VI) was 2.23%. Except that only a few analogous cations, which could be masked by EDTA, no interference was observed at the presence of various conventional ions, even at high concentrations. The reliability and applicability of the method were confirmed using some geological standard reference materials and spiked synthetic water samples, respectively.     

Highly sensitive catalytic determination of molybdenum

A novel, highly sensitive, selective, and simple kinetic method was developed for the determination of Mo(VI) based on its catalytic effect on the oxidation of 1-amino-2-naphthol-4-sulfonic acid (ANSA) with H₂O₂. The reaction was followed spectrophotometrically by tracing the oxidized product at 465 nm after 30 min of mixing the reagents. The optimum reaction conditions were: 10 mmol l⁻¹ ANSA, 50 mmol l⁻¹ H₂O₂, 100 mmol l⁻¹ acetate buffer of pH 5.0 ± 0.05 and at 40 °C. Addition of 200 μg ml⁻¹ diethylenetriaminepentaacetic acid (DTPA) conferred high selectivity for the proposed method. Following the recommended procedure, Mo(VI) could be determined with a linear calibration graph up to 2.5 ng ml⁻¹ and a detection limit, based on the 3S_b-criterion, of 0.027 ng ml⁻¹. The unique sensitivity and selectivity of the implemented method allowed its direct application to the determination of Mo(VI) in natural and industrial waste water. The method was validated by comparison with the standard ETAAS method. Moreover, published catalytic-spectrophotometric methods for the determination of molybdenum were reviewed.     

Two rapid and sensitive automated methods for the determination of nitrite and nitrate in soil samples

Spectrophotometric flow injection methods were developed for the individual determination of nitrite or nitrate, and for the simultaneous determination of nitrite and nitrate, in soil samples. Nitrite was determined directly using a modified version of the Griess-Ilosvay diazo-coupling reaction, measuring at 543 nm the absorbance of the azo-dye complex formed. Simultaneous nitrite and nitrate determinations were based on on-line nitrate reduction in a micro column containing copperised cadmium. A single chromogenic reagent containing all the necessary reactants was used in both methods. For determinations, the chemical and instrumental variables were optimised by univariate analysis and simplex chemometric method. The optimised conditions gave a linear calibration range between 0.05 and 1.6 µg m L^− 1 for N-NO₂⁻ and between 0.05 and 7.0 µg m L^− 1 for N-NO₃⁻. The detection limits for nitrite and nitrate were 22 µg L^− 1 and 44 µg L^− 1 respectively. The proposed methods allowed up to 35-40 samples per hour to be analysed with good precision. The simultaneous method was successfully used for the determination of nitrite and nitrate in soil samples (the results obtained were validated against those obtained by reference methods). The proposed methods are simpler and faster than conventional methods and could be routinely used in environmental monitoring laboratories.     

A piezoelectric immunosensor for the determination of pesticide residues and metabolites in fruit juices

A quartz crystal microbalance (QCM) immunosensor was developed for the determination of the insecticide carbaryl and 3,5,6-trichloro-2-pyridinol (TCP), the main metabolite of the insecticide chlorpyrifos and of the herbicide triclopyr. The detection was based on a competitive conjugate-immobilized immunoassay format using monoclonal antibodies (MAbs). Hapten conjugates were covalently immobilized, via thioctic acid self-assembled monolayer (SAM), onto the gold electrode sensitive surface of the quartz crystal. This covalent immobilization allowed the reusability of the modified electrode surface for at least one hundred and fifty assays without significant loss of sensitivity. The piezoimmunosensor showed detection limits (analyte concentrations producing 10% inhibition of the maximum signal) of 11 and 7 μg l⁻¹ for carbaryl and TCP, respectively. The sensitivity attained (I₅₀ value) was around 30 μg l⁻¹ for both compounds. Linear working ranges were 15-53 μg l⁻¹ for carbaryl and 13-83 μg l⁻¹ for TCP. Each complete assay cycle took 20 min. The good sensitivity, specificity, and reusability achieved, together with the short response time, allowed the application of this immunosensor to the determination of carbaryl and TCP in fruits and vegetables at European regulatory levels, with high precision and accuracy.     

Determination of carbon dioxide in gaseous samples by gas diffusion-flow injection

A gas diffusion-flow injection system was developed for the determination of carbon dioxide in gaseous samples. The calibration was based on the use of either gaseous carbon dioxide or aqueous sodium carbonate standards. Gaseous carbon dioxide samples and gaseous or aqueous standards were injected directly into a donor stream of 1.0×10⁻⁴ M H₂SO₄. In the gas diffusion unit, carbon dioxide diffused through a PTFE membrane into an acceptor stream containing a mixed acid/base indicator. The absorbance of the acceptor stream was monitored spectrophotometrically at 554 nm. The calibration plot was linear over the range of 5.00×10² to 1.27×10⁴ μl l⁻¹ with a sample throughput of 28 h⁻¹ and 3.2% R.S.D. ([CO₂]=2.37×10³ μl l⁻¹, n=12). The detection limit was determined as 2.50×10² μl l⁻¹. The flow system was successfully applied to the analysis of several natural gaseous samples and the headspace of milk containers during storage. The flow injection results were found to be statistically indistinguishable at the 95% confidence level from those obtained by gas chromatography using thermal conductivity detection.     

Solvent extraction flow-injection manifold for the simultaneous spectrophotometric determination of free cyanide and thiocyanate ions based upon on-line masking of cyanides by formaldehyde

This study reports a sensitive solvent extraction flow-injection (FI) method for the simultaneous spectrophotometric determination of free cyanide and thiocyanate in human saliva and pralidoxime solutions. Cyanide and thiocyanate form colored (λ_max=540 nm) ternary complexes with copper and 2,2′-dipyridyl-2-quinolylhydrazone (DPQH) that are extractable into chloroform. The determination of thiocyanates in the presence of cyanides is accomplished after on-line masking of the latter with formaldehyde through a binary inlet static mixer (BISM). Total thiocyanates and cyanides are determined in a second run, without the use of the masking agent. The proposed method allows the determination of the analytes in the range of 0-4 mg l⁻¹ thiocyanates and 0-3 mg l⁻¹ cyanides, with the 3σ detection limits being 0.007 and 0.004 mg l⁻¹, respectively. The precision of the method (s_r<1.0% at 1 mg l⁻¹ CN⁻ or SCN⁻, n=12 in both cases) and the sampling rates were quite satisfactory (60 injections per hour). The method was applied to the analysis of human saliva and pralidoxime solutions and gave recoveries in the range of 98.0-102.2% for both analytes whereas the mean relative error was e_r=1.7%.     

Microwave-induced combustion of crude oil for further rare earth elements determination by USN–ICP-MS

A procedure for light and heavy crude oils digestion by microwave-induced combustion (MIC) is proposed for the first time for further rare earth elements (REE) determination by inductively coupled plasma mass spectrometry (ICP-MS) equipped with an ultrasonic nebulizer (USN). Samples of crude oil (API density of 10.8–23.5, up to 250 mg) were inserted in polycarbonate capsules and combusted using 20 bar of oxygen and 50 μL of 6 mol L⁻¹ ammonium nitrate as igniter. Nitric acid solutions (1–14.4 mol L⁻¹) were evaluated for analyte absorption and a reflux step was applied after combustion (5 min of microwave irradiation at 1400 W) in order to achieve better analyte recoveries. Accuracy was evaluated using a spiked sample and also by comparison of results obtained by microwave-assisted digestion combined to ultraviolet radiation (MW–UV) and by neutron activation analysis (NAA). Using 3 mol L⁻¹ HNO₃, quantitative recoveries (better than 97%) were obtained for all analytes. Blank values were always negligible. Agreement was higher than 96% for La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y by comparison of results with those obtained by MW–UV and by NAA (only for La, Ce, Nd, Sm, and Yb). Residual carbon content in digests using MIC was always below 1%. As an advantage over conventional procedures for crude oil digestion, using MIC, it was possible to use diluted acid as absorbing solution, obtaining better limits of detection and avoiding interferences in REE determination by USN–ICP-MS.