Photochemical Vapor Generation Research Articles

Cobalt and Copper Ions Synergistically Enhanced Photochemical Vapor Generation of Molybdenum: Mechanism Study and Analysis of Water Samples.

Herein, we report a discovery that photochemical vapor generation (PCVG) of molybdenum (Mo) can be synergistically enhanced dramatically by cobalt and copper ions sourced from acetates in the medium of formic acid, utilizing a flow-through reactor. The nature of this new PCVG was probed for the first time by an electron paramagnetic resonance (EPR) spin trapping technique for its possible reaction mechanism. Carboxyl and hydroxyl free radicals were verified during PCVG processes, and the results indicate that variations in the relative amounts and proportions of free radical species may account for the synergistic effect from concomitant Co2+ and Cu2+, as well as the generation of molybdenum hexacarbonyl from the UV-induced photolysis of formic acid. Under "dry" plasma conditions, the simultaneous spiking of Co2+ and Cu2+ to 20% (v/v) formic acid solutions and a 60 s irradiation time could give rise to a 15-fold enhancement in signal intensities, together with a blank-limited but still impressive limit of detection of 6 ng L-1 (6 ppt) by inductively coupled plasma mass spectrometry (ICP-MS). The accuracy and reliability of this methodology was validated by analysis of molybdenum in a seawater certified reference material, CASS-6, as well as two varieties of drinking water and seawater samples, with satisfactory spike recoveries (91-101%).

Reduction of Interferences Using Fe-Containing Metal-Organic Frameworks for Matrix Separation and Enhanced Photochemical Vapor Generation of Trace Bismuth.

Photochemical vapor generation (PVG) has emerged as a promising sample introduction method for atomic spectrometry in recent years. Despite its great success, a major impediment for the wide application of PVG is the interferences from the coexisting ions, especially transition-metal ions. In this work, iron and 1,3,5-benzenetricarboxylic (Fe-BTC), a Fe-containing metal-organic framework (MOF) material, was synthesized and first used as a platform integrating the sample matrix separation, preconcentration, and photocatalysis for the highly selective determination of elements by PVG. Bismuth was selected as a model analyte. Fe-BTC served not only as the photocatalyst for the PVG of Bi3+ but also as an efficient absorbent for the separation of analytes from the sample matrix. Compared with the previous PVG system, the performance of tolerating interferences toward coexisting ions of the proposed method has been greatly improved after using Fe-BTC-based matrix separation. Thus the excess of 10 mg L-1 of Co2+ and 100 mg L-1 of Cu2+, Ni2+, and Fe3+ caused no obvious interferences for 1 μg L-1 of Bi determination. Under the optimal conditions, the limit of detection (LOD, 3σ) of the developed method was 0.3 ng L-1 with the inductively coupled plasma-mass spectrometry (ICPMS) measurement, which could be lowered down to 0.04 ng L-1 after ten times of preconcentration with Fe-BTC prior to analysis. This method was successfully applied for the analysis of Bi in complicated sample matrices of soil (GBW07401), sediment (GBW07310), nickel-iron alloy (GBW01622), and nickel alloys (GBW01641) by the external calibration method.

Investigation of the conditions for selenium determination by photochemical vapor generation coupled to graphite furnace atomic absorption spectrometry

In this study instrumental parameters and sample preparation methods for Se determination using photochemical vapor generation (PVG) coupled to graphite furnace atomic absorption spectrometry (GF AAS) were investigated. Coconut water was selected as sample due to its nutritional importance and matrix complexity. The PVG was performed in a flow injection mode. The Se volatile species, generated in the UV reactor, were separated from the condensed phase in a gas-liquid separator and introduced automatically in the graphite furnace. These species were trapped on the integrated graphite platform surface coated with Ir as permanent modifier. The PVG conditions such as concentration of low molecular weight organic compound (LMWC), sample and carrier gas flow rate, effect of reagents frequently used in sample preparation (HNO3 and H2O2) were investigated. Formic acid 0.44 mol L−1 was selected as LMWC. The addition of HNO3 (14 mmol L−1) and H2O2 (145 mmol L−1) increased the sensitivity. Pre-reduction of Se (VI) to Se (IV) was performed on-line in a quartz spiral tube coated with a thin film of TiO2. The insertion of Ar flow rate (115 mL min−1) before the reactor inlet has improved the sensitivity, comparing with the introduction of Ar after the reactor. UV-assisted photo-oxidation of coconut water samples was employed as an alternative sample preparation to avoid the NO3−/NO2−, and H2O2 interference. The figures of merit were: sensitivity 0.0680 s ng−1, limits of detection and quantification 0.65 μg L−1 and 2.2 μg L−1, respectively, and characteristic mass of 62 pg. The PVG efficiency was estimated as 79% when comparing with conventional hydride generation. Furthermore, a mass balance study was accomplished in order to identify possible analyte losses in the PVG system, which the overall recovery was up to 85%. The results of recovery tests accomplished with real samples of coconut waters varied between 80 and 103% for Se (IV), Se (VI), and selenomethionine.

Photochemical Vapor Generation for Colorimetric Speciation of Inorganic Selenium.

Gold nanoparticles (AuNPs) are widely used as optical probes in colorimetric detection, thanks to their high molar extinction coefficient. However, sample matrixes of high salinity or strong acidity/alkalinity often break the electrostatic repulsion of AuNPs suspension, or/and the surface functionality of AuNPs, causing strong and unfavorable interferences. Photochemical vapor generation (PVG) is an efficient technique for the sample matrix separation. Besides, it possesses distinct features of green reducing reagent, reduced interferences from concomitant elements, and direct speciation by the assistance of photocatalyst. Herein, we developed a photochemical vapor generation (PVG) method for the green and direct speciation analysis of inorganic selenium (i.e., Se(IV) and Se(VI)), by colorimetric or visual monitoring of unmodified AuNPs. The generated Se species from PVG were directed into the AuNPs solution for a reaction to take place, which produced a specific new absorption band at 600 nm for detection. The experimental parameters, including the concentration of organic acid, the sample flow rate, the concentration of AuNPs, and the flow rate of carries gas, were optimized in detail. Under optimized conditions, the limits of detection (LOD) for Se(IV) and Se(VI) were 0.007 and 0.006 μg mL-1 by UV-vis detection, respectively. It is worth mentioning that 0.08 μg mL-1 Se can induce an obvious color change, which can be directly observed with the naked eye. Relative standard deviations (RSDs) of 4.5% and 4.3% were obtained from seven replicate measurements of 0.15 μg mL-1 Se(IV) and Se(VI) standard solution, respectively. The developed assay has been successfully applied for the speciation of Se in a dietary supplement sample and environmental water samples including lake water, seawater, simulated water reference materials, and tap water.

Determination of total inorganic arsenic in water samples by cadmium ion assisted photochemical vapor generation-atomic fluorescence spectrometry

A novel approach based on cadmium ions assisted photochemical vapor generation (PVG) combined with atomic fluorescence spectrometry (AFS) was proposed for sensitive and accurate detection of total inorganic arsenic. In this work, the prereduction step of As(V) to As(III) was avoided. An approximate 10-fold improvement in sensitivity was achieved for both As(III) and As(V) with the existence of 20 mg L−1 Cd2+ in the 30% (v/v) acetic acid solution for 60 s UV irradiation. Under the optimal experimental conditions, detection limits of 0.05 μg L−1 were obtained for As(III) and As(V). The relative standard deviations (RSD) of the 20 μg L−1 As(III)/As(V) standard solution from 11 times measurements were both within 2%. The accuracy of the proposed approach was verified by analyzing of several surface water samples using standard addition method, Yangtze river waters and Funan river waters, with recoveries from 94% to 105%, which were in good agreement with the results from ICPMS determination.

A unified approach to mechanistic aspects of photochemical vapor generation

A general model of redox reactions for PVG is proposed, driven by reducing radicals (e(aq)−, H˙, R˙ and CO2˙−) arising from concurrent VUV photolysis of carboxylic acids to yield the nascent elemental species which are subsequently attacked by H˙, H3C˙, H+and CO to yield volatile products.

Analytical monitoring of selenium nanoparticles green synthesis using photochemical vapor generation coupled with MIP-OES and UV–Vis spectrophotometry

Three synthesis routes of selenium nanoparticles (SeNPs), using glucose, ascorbic acid and yeast Saccharomyces boulardii, alternatively, as green chemistry reductants for ionic Se, have been successfully monitored using both continuous photochemical vapor generation (PCVG) coupled with MIP-OES and UV–Vis spectrophotometry. PCVG-MIP-OES allows for fast and selective determination of unreacted selenium(IV) oxyanion in the presence of SeNPs and matrix components, enabling accurate calculation of the yield of the SeNPs synthesis. For the analysis, these analyte ions were converted into volatile species by UV irradiation of their aqueous solution in the presence of 15% (v/v) acetic acid. Severe matrix effects were observed from organic matrix components leading to relatively high limit of detection for Se(IV) by PCVG-MIP-OES (0.52 mg L−1). Moreover, both the particle size in the range from 20 to 240 nm and particle number concentration up to 5 × 1017 SeNPs per mL were assessed based on UV–Vis spectra of SeNPs dispersions. The proposed monitoring protocol meets green chemistry requirements and allows for fast and selective determination of the yield of SeNPs synthesis.

Enhanced Photochemical Vapor Generation for the Determination of Bismuth by Inductively Coupled Plasma Mass Spectrometry.

An enhanced photochemical vapor generation (PVG) sample introduction procedure is developed for the determination of trace Bi with inductively coupled plasma mass spectrometry (ICP MS) by the addition of iron. Gas chromatography mass spectrometry (GC-MS) reveals that (CH3)3Bi is the major component of the volatile Bi species formed in the presence of 20% (v/v) acetic acid, 5% (v/v) formic acid, and 60 μg mL-1 Fe3+ under UV irradiation. The addition of Fe3+ not only largely increases the PVG efficiency of Bi3+ but also accelerates the reaction kinetics of photochemical reduction of Bi3+. The analytical sensitivity was enhanced 30-fold using PVG for sample introduction compared to that for direct solution nebulization detection by ICP MS detection. Furthermore, the proposed method shows much better tolerance of interference from Cu2+ and Ni2+ than that from conventional hydride generation (HG). Under the optimized conditions, a detection limit of 0.3 ng L-1 was obtained for Bi by ICP MS determination. The relative standard deviation (RSD) was 2.5% for seven replicate measurements of 0.5 ng mL-1 Bi3+ standard solution. The proposed method has been successfully applied for the determination of Bi in environmental samples, including water samples, and certified reference material of soil (GSS-1) and sediments (GSD-5a and GSD-10) with satisfying results.

Efficient Photochemical Vapor Generation of Molybdenum for ICPMS Detection.

Photochemical vapor generation (PVG) of Mo was accomplished using a 19 W high-efficiency flow-through photoreactor operating in a flow injection mode using 30-50% (w/v) formic acid as a reaction medium. The generated volatile product (most probably molybdenum hexacarbonyl) was directed by an argon carrier gas to a plastic gas-liquid separator and introduced into the spray chamber of an inductively coupled plasma mass spectrometer for detection. Particular attention was paid to the determination of overall PVG efficiency relative to that for liquid nebulization. Utilizing a sample flow rate of 1.25 mL min-1, corresponding to an irradiation time of 38 s, PVG efficiencies in the range 46-66% were achieved. The efficiency could be further enhanced by the presence of mg L-1 added Fe3+ ions. A limit of detection of 1.2 ng L-1 and precision of 3% (RSD) at 250 ng L-1 were achieved. Interferences from inorganic anions likely to be encountered during analytical application to real samples (NO3-, Cl-, SO42-, NO2-, and ClO4-) were investigated in detail. The accuracy and applicability of this sensitive methodology was successfully verified by analysis of fresh water Standard Reference Material NIST 1643e, two seawater Certified Reference Materials (NASS-7 and CASS-6), and by analysis of two samples of commercial dietary supplements solubilized in formic acid.

Mercury Speciation in Fish by High-Performance Liquid Chromatography (HPLC) and Post-Column Ultraviolet (UV)-Photochemical Vapor Generation (PVG): Comparison of Conventional Line-Source and High-Resolution Continuum Source (HR-CS) Atomic Absorption Spectrometry (AAS)

An ultraviolet-photochemical generator (UV-PVG) capable of post-column on-line transformation of both organic and inorganic mercury species to cold vapor (Hg0) with subsequent detection by quartz tube-atomic absorption spectrometry (QT-AAS) was developed. Mercury(II), methylmercury(I), ethylmercury(I), and phenylmercury(I) were successfully detected after separation by reversed-phase high-performance liquid chromatography (RP-HPLC). Two types of AAS detectors were compared. The first was a commonly used line-source instrument while the second was a high-resolution continuum source (HR-CS) AAS. The latter provided better limits of detection: 0.47 µg L−1 for Hg(II), 0.84 µg L−1 for methylmercury(I), 0.80 µg L−1 for ethylmercury(I), and 2.0 µg L−1 for phenylmercury(I). The repeatability at 30 μg L−1 was 3.6%, 4.1%, 6.2%, and 4.5% for these species (n = 10). These figures of merit were comparable with those reported for more sensitive atomic fluorescence spectrometry. Nine sample extraction procedures were investigated. Extraction by tetramethylammonium hydroxide and HCl at 75 °C was selected as the only method compatible with the proposed separation and detection steps providing high extraction efficiency and no changes in mercury speciation. The applicability of the proposed high-performance liquid chromatography–ultraviolet-photochemical vapor generation–quartz tube-atomic absorption spectrometry method was demonstrated using fish samples and certified reference materials (CRM) DOLT-4 (dogfish liver) and ERM-CE464 (tuna fish). The results were comparable to those obtained by a reference method based on L-cysteine extraction and high-performance liquid chromatography–inductively coupled plasma-mass spectrometry (HPLC–ICP-MS) determination.

Eco-scale non-chromatographic method for mercury speciation in fish using formic acid extraction and UV–Vis photochemical vapor generation capacitively coupled plasma microtorch optical emission spectrometry

An eco-scale method based on UV–Vis photochemical vapor generation and detection in a capacitively coupled plasma microtorch by optical emission measurement with a low resolution microspectrometer was developed for Hg speciation as Hg2+ and CH3Hg+ in seafood. Our proposal provides a simpler, greener analytical method in terms of sample preparation and derivatization, and use of small-scale instrumentation. Coupling of UV–Vis photo-reactor with plasma microtorch is a very promising approach to develop eco-scale methods for Hg speciation in seafood with major advantages to classical methods. The most distinctive characteristic of the method is the use of a single reagent, formic acid, for the extraction of Hg species from solid sample and as medium for the photo-induced cold vapor generation. An amount of 0.2 g lyophilized sample was subjected to ultrasound assisted extraction in 10 ml HCOOH 98–100% at 50 °C for 3 h then on line UV–Vis photo-induced derivatization in 0.6 mol l−1 HCOOH was achieved. Total Hg and Hg2+ species were quantified against Hg2+ external standard solutions by measuring emission at 253.652 nm in a low power/low Ar consumption (15 W/100 ml min−1) plasma source with Maya2000 Pro (Ocean Optics) microspectrometer. Under UV exposure both Hg2+ and CH3Hg+ species were converted with same efficiency to cold vapor, while natural room light (Vis) achieved selectively derivatization of Hg2+. Concentration of CH3Hg+ was obtained by subtracting Hg2+ amount from total. The proposed method was compared with Thermal Decomposition Atomic Absorption Spectrometry eco-scale method developed for the determination of total Hg from solid sample and CH3Hg+ according to the procedure recommended by the European Commission. The limits of detection/quantification provided by the proposed method were 9/27 μg kg−1 total Hg and 4.8/14.4 μg kg−1 Hg2+. Study of accuracy against certified reference materials gave recovery of 99 ± 6% total Hg, 99 ± 9% Hg2+ and 99 ± 10% CH3Hg+, while precisions assessed from measurements on real sample were 2.6–10.2%, 2.0–13.4% and 5.3–14.5% respectively, similar to those in the reference method.

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.

Effect of variable ultraviolet wavelength and intensity on photochemical vapor generation of trace selenium detected by atomic fluorescence spectrometry

Selenium was taken as an example to investigate the effect of variable ultraviolet (UV) wavelength and intensity on its photochemical vapor generation (photo-CVG) process. A broadband laser-driven light source (LDLS) and a UV lamp were utilized as irradiation sources for the photo-CVG of trace selenium which was sensitively detected by atomic fluorescence spectrometry (AFS). Five filters with bandpass at different wavelength (254 nm, 285 nm, 315 nm, 365 nm and 395 nm) were used to filtrate the light from the LDLS and the UV lamp. The signal change of Se(VI) by photo-CVG-AFS was monitored when a TiO2-coated quartz coil was irradiated by variable wavelength and intensity. The experimental results indicated: (1) the efficiency of Se(VI) photo-CVG varied with irradiation wavelength; (2) a synergistic effect existed for the photo-CVG, and the efficiency of the full spectrum was thus better or close to the sum of all wavelengths; and (3) the efficiency of Se(VI) photo-CVG was also affected by the UV intensity.

Speciation of nitrite, nitrate and p-nitrophenol by photochemical vapor generation of NO using High-Resolution Continuum Source Molecular Absorption Spectrometry

This paper presents a new speciation method of n-compounds in food samples using the technique of High-Resolution Continuum Source Molecular Absorption Spectrometry (HR-CS MAS). The nitrite ion was determined in a quartz cell of the HR-CS MAS by generating NO steam. Speciation methods of nitrite and nitrate were studied, using the reduction of nitrate to nitrite. The most efficient alternative was the irradiation of ultraviolet light (UV) in the samples containing nitrate in an ammonium bufferand EDTA. Therefore,the nitrate concentration in the samples, using the reduction process to nitrite,was added to the previous nitrite concentration and was specified by subtracting the initial concentration to the final one. The p-nitrophenol was determined by its degradation throughUV in the presence of H2O2, generating, among other products, nitrite, which was determined through the reduction of nitrite to NO steam. After that, samples of natural water and canned foods were analyzed. The R2, LOD (mg L−1), LOQ (mg L−1) and m0 values were, respectively 0.990; 0.047,0.16 and 0.25 μg, for nitrate; 0.990; 0.090 and 0.30 and 0.52 μg, for nitrate, and 0.996; 0.034, 0.11 and 0.29 μg, for p-nitrophenol.

Photochemical Vapor Generation of Tellurium: Synergistic Effect from Ferric Ion and Nano-TiO2.

Photochemical vapor generation (PVG) is emerging as a promising analytical tool for Te determination, thanks to its efficient matrix separation, and simple and green procedure. However, the low PVG generation efficiency of Te is the bottleneck for its wide application in environmental samples containing trace Te. Herein, we reported a high efficient PVG for Te determination by synergistic effect of ferric ion and nano-TiO2. The analytical sensitivity was enhanced approximately 15-fold for Te(IV) in the presence of both ferric ions and nano-TiO2, comparing to conventional PVG. Besides, the use of nano-TiO2 can provide Te(VI) and Te(IV) an equal and high PVG efficiency in the presence of ferric ions, owned to the high photocatalytic performance of TiO2 under short-wavelength UV irradiation (254 and 185 nm). Under the optimized experimental conditions, a detection limit of 1.0 ng L-1 was obtained. The precision of replicate measurements was 2.3% (RSD, n = 7) at 0.5 μg L-1 for Te(IV). The methodology was validated by successful determination of Te in surface waters and two standard reference sediment samples. To our best knowledge, this is the first report of the synergistic enhancement of transitional metal ions and nano-TiO2 in PVG, which possesses potential for highly sensitive determination of vapor-forming elements.

Copper Ion Assisted Photochemical Vapor Generation of Chlorine for Its Sensitive Determination by Sector Field Inductively Coupled Plasma Mass Spectrometry.

A novel, reliable, and sensitive approach for the determination of chlorine by sector field inductively coupled plasma mass spectrometry (SF-ICPMS) using photochemical vapor generation for sample introduction is presented. Methyl chloride is generated from different chlorine species in a flow-through photochemical reactor using a 1% solution of acetic acid containing 7.5 μg g-1 of Cu2+. The volatile product is directed by an argon carrier gas to a gas-liquid separator and introduced into the instrument. A sample flow rate at 1.7 mL min-1 and a 45 s irradiation time provided a 74-fold enhancement in sensitivity compared to conventional nebulization. A blank-limited detection limit of 0.5 ng g-1 for chloride, suitable for quantitation at trace levels, was achieved. The proposed method was validated by analysis of two certified reference materials, NIST SRM 1568b rice flour and SRM 1571 orchard leaves, with satisfactory results, as well as three varieties of bottled water, achieving spike recoveries between 101% and 105%.

Headspace Solid-Phase Microextraction Coupled to Miniaturized Microplasma Optical Emission Spectrometry for Detection of Mercury and Lead.

Despite increased interest in microplasma atomic spectrometry over the past several years, its applications to field analysis of real samples still remain limited due to its low sensitivity. In this work, headspace solid-phase microextraction (HS-SPME) using PDMS/DVB fiber was used as a sample introduction method to improve the sensitivity of a miniature (6.0 cm length × 5.0 cm width × 2.8 cm height) low power (<10 W) microplasma point discharge optical emission spectrometer (PD-OES) for the determination of Hg and Pb after their derivatization with sodium tetraethylborate (NaBEt4). Owing to its advantages of combining sampling, separation, and preconcentration of analytes from sample matrices into a single step, HS-SPME not only simplifies sample pretreatment and the experimental setup but also eliminates interferences from matrices and water vapor while significantly improving analytical performance. In comparison to photochemical vapor generation PD-OES, limits of detection for Hg and Pb were enhanced by at least 100-fold, providing great potential for field analysis of these and other trace elements in real samples. Under optimal conditions, the limits of detection for Hg and Pb were 0.001 and 0.003 μg L-1, respectively, with relative standard deviations (RSDs) better than 2.1% and 4.5% at a concentration of 1 μg L-1. The accuracy of the HS-SPME-PD-OES system was validated by analysis of three Certified Reference Materials, including DORM-4 (Hg, 412 ± 36 μg kg-1; Pb, 404 ± 62 μg kg-1), TORT-3 (Hg, 292 ± 22 μg kg-1; Pb, 225 ± 18 μg kg-1) and SRM1568b (Hg, 5.53 ± 0.58 μg kg-1), providing analytical results in excellent agreement with certified values at the 95% confidence level. This method was also successfully used for the analysis of several real rice and water samples with satisfactory results (90-130% recoveries of added spikes).

UV photochemical vapor generation–nitrogen microwave induced plasma optical emission spectrometric determination of nickel

Photochemical vapor generation was utilized for gaseous sample introduction into a nitrogen microwave induced plasma optical emission spectrometer in order to enhance the analytical performance.

Evaluation of Mercury Concentration in Biodiesel Samples by Photochemical Vapor Generation

Evaluation of Mercury Concentration in Biodiesel Samples by Photochemical Vapor Generation

A highly sensitive eco-scale method for mercury determination in water and food using photochemical vapor generation and miniaturized instrumentation for capacitively coupled plasma microtorch optical emission spectrometry

A sensitive method with a green profile for Hg determination in food and water was developed.