Cadmium Assisted Photochemical Vapor Generation of Tungsten for Detection by Inductively Coupled Plasma Mass Spectrometry

Efficient Photochemical Vapor Generation from Low Concentration Formic Acid Media

Herein, we report on surprisingly efficient photochemical vapor generation (PVG) of Ru, Re, and especially Ir, achieved from very dilute HCOOH media employing a thin-film flow-through photoreactor operated in flow injection mode. In the absence of added metal ion sensitizers, efficiencies near 20% for Ir and approximately 0.06% for Ru and Re occur in a narrow range of HCOOH concentrations (around 0.01 M), significantly higher than previously reported from conventionally optimized HCOOH concentrations (1-20 M). A substantial enhancement in efficiency, to around 9 and 1.5%, could be realized for Ru and Re, respectively, when 0.005 M HCOONa served as the PVG medium. The addition of metal ion sensitizers (particularly Cd2+ and Co2+) to 0.01 M HCOOH significantly enhanced PVG efficiencies to 17, 2.2, and 81% for Ru, Re, and Ir, respectively. Possible mechanistic aspects occurring in dilute HCOOH media are discussed, wherein this phenomenon is attributed to the action of 185 nm radiation available in the thin-film flow-through photoreactor. An extended study of PVG of Fe, Co, Ni, As, Se, Mo, Rh, Te, W, and Bi from both dilute HCOOH and CH3COOH was undertaken, and several elements for which a similar phenomenon appears were identified (i.e., Co, As, Se, Te, and Bi). Although use of dilute HCOOH media is attractive for practical analytical applications employing PVG, it is less tolerant toward dissolved gases and interferents in the liquid phase due to the likely lower concentrations of free radicals and aquated electrons required for analyte ion reduction and product synthesis.

Sensitive Determination of Arsenic by Photochemical Vapor Generation with Inductively Coupled Plasma Mass Spectrometry: Synergistic Effect from Antimony and Cadmium

A novel method for the determination of trace arsenic (As) by photochemical vapor generation (PVG) with inductively coupled plasma mass spectrometry measurement was developed in this study. The synergistic effect from antimony (Sb) and cadmium (Cd) was found for the photochemical reduction of As for the first time. Effective photochemical reduction of As was obtained in the system containing 10% (v/v) acetic acid, 5.0 mg L-1 Sb(III), and 20.0 mg L-1 Cd(II) with 100 s UV irradiation. Analytical sensitivity of As(III) was comparable with that of As(V) under the tested conditions, making direct determination of total As feasible. Compared to the pneumatic nebulization method, analytical sensitivity of the developed method was enhanced about 50 folds. The PVG efficiency was estimated up to be 99 ± 3%. The limit of detection (LOD) (3σ) was found to be 2.1 ng L-1 for As, which was improved about 30-fold compared to that using direct sample introduction solution nebulization. Considering the sample dilution prior to analysis (usually one-fold), the LOD was actually enhanced about 15 folds. The relative standard deviations of seven replicate measurements of 1.0 μg L-1 As(III) and As (V) standard solutions were 2.3 and 2.9% for As(III) and As(V), respectively. The proposed method was successfully applied for the detection of As in certified reference materials of sediments (GBW07303a and GBW07305a), as well as three water samples. The mechanism of the PVG system was investigated by using gas chromatography mass spectrometry, electron paramagnetic resonance, and X-ray photoelectron spectroscopy. (CH3)3As along with (CH3)3Sb were synthesized under UV irradiation. Besides, volatile species of Cd were also found. The result obtained in this study is useful for developing efficient "sensitizers" in PVG and understanding the transformation of As in the presence of hydride/cold vapor forming elements in the photochemical process.

Chemical vapor generation of tungsten for atomic spectrometric determination: Homogeneous sensitizer and mechanism study

BACKGROUND

Inductively coupled plasma-mass spectrometry (ICP-MS) is one of the most powerful instrumental techniques for the determination of tungsten for its low detection limit and wide linear range, while it remains challenging since the analytical performance can be affected by complicated sample matrix. Chemical vapor generation (CVG) harbors the potential to be an alternative to conventional solution nebulization for sample introduction to reduce matrix effect. However, the CVG of tungsten was low in efficiency. It is clear that green and homogeneous enhancement for CVG of tungsten is desired and the mechanism is worth in-depth investigation.

RESULTS

Two green and homogeneous enhancement systems for CVG of tungsten were studied, including photochemical vapor generation (PVG) and hydride generation (HG) with sensitizers, Fe3+ and DDTC, respectively. Under optimal conditions, the limits of detection (LODs) were 0.02 μg L-1 for the PVG and 0.003 μg L-1 for the HG, respectively. For PVG, the Fe3+/Fe2+ cycling, free radical species, gaseous product, and the chemical speciation evolution of W in the PVG process were studied in detail. Photo-Fenton effect, generated reductive radical ·CO2-, gaseous product Fe(CO)5, and the mixed valence of W5+/W6+ in the PVG process were found to be crucial for the enhancement. As for HG, the complexation between W(VI) and DDTC might be conducive to the enhanced HG efficiency.

SIGNIFICANCE

This work not only in-depth expands the element scope of CVG, but also investigates the enhancement mechanisms experimentally, which might render a deep insight into the CVG processes and foreshadow new guidelines for screening green and efficient homogeneous sensitizers for CVGs of more elements in the future.

Detection of trace antimony by vanadium (IV) ion assisted photochemical vapor generation with inductively coupled plasma mass spectrometry measurement

In this work, a method for sensitive detection of trace antimony (Sb) was developed by inductively coupled plasma mass spectrometry (ICP MS) coupled with photochemical vapor generation (PVG). V(IV) ions were used as new "sensitizers" for improving the PVG efficiency of Sb. Factors influenced the PVG and the detection of Sb by ICP MS were investigated, including the type and concentration of low molecular weight organic acids, the UV irradiation time, the concentration of V(IV) ions, the air-liquid interface, the flow rate of Ar carrier gas, and interferences from co-existing ions. It was found that efficient reduction of Sb was obtained in the medium of 10% (v/v) formic acid (FA), 10% (v/v) acetic acid (AA), and 80 mg L^-1 of V(IV) with 100 s UV irradiation. Under the selected conditions, there was no significant difference in analytical sensitivity between Sb(III) and Sb(V). The limit of detection (LOD, 3σ) was 4.7 ng L^-1 for Sb with ICP MS measurement. Compared to traditional direct solution nebulization, the analytical sensitivity obtained in this work was enhanced about 19-fold. Relative standard deviations (RSDs, n = 7) were 1.9% and 2.3% for replicate measurement of 0.5 μg L^-1 Sb(III) and Sb(V) standard solutions, respectively. The proposed method was applied for the determination of trace Sb in water samples and two certified reference materials (CRMs) of sediments with satisfactory results. Moreover, the generated volatile species of Sb in this work was found to be (CH₃)₃Sb.

Determination of trace tellurium by photochemical vapor generation-atomic fluorescence spectrometry using bifunctional Co-MOF-74 for preconcentration and sensitization

The determination of trace tellurium in real samples with complicated matrix can be rather challenging due to the low abundance and interferences. Herein, we report a new method for the highly sensitive detection of Te(IV) by photochemical vapor generation-atomic fluorescence spectrometry (PVG-AFS), utilizing Co-MOF-74 as an adsorbent and a precursor of Co²⁺ ion sensitizer for preconcentration and enhanced PVG efficiency. The synthesized Co-MOF-74 can completely adsorb Te(IV) within 10 min in a wide pH range. Following filtration and re-suspension in a dilute solution of formic and acetic acid, the adsorbed Te(IV) was converted to volatile compounds under the UV irradiation and swept to AFS for detection. A limit of detection of 0.08 ng/mL for Te(IV) was obtained after a 50-fold preconcentration. The proposed method was used for analysis of various natural water samples for trace Te(IV), with satisfactory spike recoveries achieved.

Highly Efficient Photochemical Vapor Generation for Sensitive Determination of Iridium by Inductively Coupled Plasma Mass Spectrometry

Herein, we describe the highly efficient photochemical vapor generation (PVG) of a volatile species of Ir (presumably iridium tetracarbonyl hydride) for subsequent detection by inductively coupled plasma mass spectrometry (ICPMS). A thin-film flow-through photoreactor, operated in flow injection mode, provided high efficiency following optimization of identified key PVG parameters, notably, irradiation time, pH of the reaction medium, and the presence of metal sensitizers. For routine use and analytical application, PVG conditions comprising 4 M formic acid as the reaction medium, the presence of 10 mg L^-1 Co²⁺ and 25 mg L^-1 Cd²⁺ as added sensitizers, and an irradiation time of 29 s were chosen. An almost 90% overall PVG efficiency for both Ir³⁺ and Ir⁴⁺ oxidation states was accompanied by excellent repeatability of 1.0% (n = 15) of the peak area response from a 50 ng L^-1 Ir standard. Limits of detection ranged from 3 to 6 pg L^-1 (1.5-3 fg absolute), dependent on use of the ICPMS reaction/collision cell. Interferences from several transition metals and metalloids as well as inorganic acids and their anions were investigated, and outstanding tolerance toward chloride was found. Accuracy of the developed methodology was verified by analysis of NIST SRM 2556 (Used Auto Catalyst) following peroxide fusion for sample preparation. Practical application was further demonstrated by the direct analysis of spring water, river water, lake water, and two seawater samples with around 100% spike recovery and no sample preparation except the addition of formic acid and the sensitizers.

An improved electrochemical sensor based on triton X-100 functionalized SnO2 nanoparticles for ultrasensitive determination of cadmium

The drastic increases in the concentration of heavy metals ions in the environment have become a serious concern for a number of years. Heavy metals pose serious impacts on human and aquatic life and cause severe health hazards. Amongst heavy metals, cadmium is known for its lethal effects on human health as it easily reacts with enzymes and creates free radicals in the biological system that causes carcinogenicity and other serious diseases. Thus, to tackle this challenge, TX-100 SnO₂ nanoparticles based chemically modified sensor is introduced to assess the quantity of Cd⁺² in the water system. The engineered SnO₂ nanoparticles were electrochemically characterized through cyclic voltammetry and electrochemical impedance spectroscopy to ensure the better charge transfer kinetics and electrocatalytic properties of fabricated sensors. Under the optimized conditions e.g., scan rate 80 mV/s, PBS electrolyte pH 7, and potential window (-0.2 to -1.4 V), the engineered TX-100/SnO₂/GCE-based sensor manifested a phenomenal response for cadmium ions in water media. The LOD and LOQ of developed TX-100/SnO₂/GCE were calculated in the nanomolar range as 0.0084 nM and 0.27 nM. The recovery values of the proposed method for Cd⁺² were found in an acceptable limit that witnesses the effectiveness of the fabricated sensor. Moreover, the excellent stability and anti-interference behavior of the sensor highlights its dynamic profile to be commercially utilized for the determination of Cd⁺² ions in water bodies.

Photochemical vapor generation for germanium: synergistic effect from cobalt/chloride ions and air-liquid interfaces

Photochemical vapor generation (PVG) of germanium (Ge) was first reported in this work. The synergistic effect from cobalt/chloride ions and air-liquid interfaces was found for the PVG of Ge. No obvious signal response was observed from the standard solution of Ge in 10% (v/v) formic acids (FAs) under UV irradiation. The addition of 300 mg L^-1 of Co²⁺ and 30 mmol L^-1 of Cl^- resulted in enhanced photochemical reduction for Ge, and the introduction of air-liquid interfaces proceeding and succeeding the sample solution caused another 4.6 folds of enhancement in signal response of Ge. Under the selected condition, the limit of detection (LOD, 3σ, n = 11) was obtained to be 0.008 ng mL^-1 with inductively coupled plasma mass spectrometry (ICP MS) measurement. A good precision, expressed as a relative standard deviation (RSD, n = 7) of 2.0%, was found from replicated measurements of 2 ng mL^-1 of Ge. The generation efficiency was found to be no better than 9 ± 2%. The PVG mechanism of Ge was investigated in this work. The new finding is useful for understanding the principle of PVG, and further exploring the analytical and environmental application of PVG.

An Ultrasensitive and Selective Determination of Cadmium Ions at ppt Level Using an Enzymic Membrane with Colorimetric and Electrochemical Detection

Cadmium ions (Cd²⁺) are extremely toxic heavy metal pollutants found in the environment, and which endanger human health. Therefore, it is critical to develop a sensitive and simple method for rapidly detecting Cd²⁺ in water samples. Herein, an enzymic membrane was developed based on an easy and rapid immobilization method of horseradish peroxidase (HRP), for determination of Cd²⁺ in drinking water. Hence, for the first time, an enzymic membrane was applied for the detection of Cd²⁺ without being pretreated. In the first format, the inhibition of horseradish peroxidase was performed using a colorimetric microplate reader. Under optimal conditions, the achieved limit of detection was 20 ppt. In addition, an electrochemical biosensor was developed, by combining the enzymic membrane with screen printed electrodes, which showed a linear calibration range between 0.02–100 ppb (R² = 0.990) and a detection limit of 50 ppt. The use of this enzymic membrane proved to be advantageous when reversible inhibitors such as the copper ion (Cu²⁺) were present in water samples, as Cu²⁺ can interfere with Cd²⁺ and cause erroneous results. In order to alleviate this problem, a medium exchange procedure was used to eliminate Cu²⁺, by washing and leaving only cadmium ions as an irreversible inhibitor for identification. The use of this membrane proved to be a simple and rapid method of immobilizing HRP with a covalent bond.

Vanadium Species-Assisted Photochemical Vapor Generation for Direct Detection of Trace Tellurium with Inductively Coupled Plasma Mass Spectrometry

Photochemical vapor generation (PVG) is emerging as an alternative sample introduction method in the field of atomic spectrometry. The addition of transition metals can largely improve the PVG yields of elements with the enhancement of 1.4 to 30 000-fold, based on previous reports. In this work, the use of vanadium species as novel "sensitizers" in PVG was first reported, tellurium (Te) was selected as the target. The efficient photochemical reduction of Te was observed in the presence of 9% (v/v) formic acid (FA), 20%(v/v) acetic acid (AA), and 40 mg L^-1 of V(V) (existing as VO₃^-) with the conversion efficiency of 87 ± 3%. Under the selected conditions, there was no significant difference in analytical sensitivity between Te(IV) and Te(VI), making the direct detection of total Te possible. The limit of detection (LOD, 3σ) was 2.9 ng L^-1 for Te with inductively coupled plasma mass spectrometry (ICP MS) measurement. Good precisions of 2.3% and 2.2% (relative standard deviations, RSD) for seven times replicate measurement of 0.5 μg L^-1 Te(IV) and Te(VI) standard solutions were obtained. The sensitivity was enhanced about 55-fold compared to that using traditional direct solution nebulization. The method was applied for the determination of trace Te in three water samples and two certified reference materials of sediment with satisfactory results. The possible mechanism was investigated. The generation of volatile vanadium along with (CH₃)₂Te was found in PVG for the first time. The new findings in this work will be helpful for exploration of efficient "sensitizers" in PVG and further expanding the scope of elements amenable to PVG as well.

Simultaneous Detection of Ruthenium and Osmium by Photochemical Vapor Generation-Inductively Coupled Plasma-Mass Spectrometry

Efficient simultaneous photochemical vapor generation (PVG) of ruthenium (Ru) and osmium (Os) in the medium of formic acid was demonstrated. A flow-through photoreactor hyphenated to an inductively coupled plasma-mass spectrometer (ICP-MS) was used for the PVG and subsequent detection of the two elements. A similar synergistic enhancement from cobalt and cadmium ions on the PVG efficiency of both Ru and Os was discovered. Following the critical evaluation of the impacts of various transition metal ions, the concentrations of formic acid, cobalt, and cadmium ions, the flow rate of carrier gas, and the UV irradiation time, impressive limits of detection (LODs) of 5 and 0.5 ng L^-1 were achieved for Ru and Os, respectively. The accuracy of the proposed PVG-ICP-MS method was validated by the analysis of several water samples with desirable spike recoveries obtained. Furthermore, the volatile compounds of Ru were directed and cryogenically trapped in acetonitrile and generation of carbonyls of Ru was verified by high-resolution electrospray ionization-mass spectra (ESI-MS).

Ultrasensitive Detection of Ruthenium by Coupling Cobalt and Cadmium Ion-Assisted Photochemical Vapor Generation to Inductively Coupled Plasma Mass Spectrometry

An extremely sensitive methodology for the determination of Ru was developed by coupling photochemical vapor generation (PVG) analyte introduction with inductively coupled plasma mass spectrometry (ICPMS). PVG was undertaken with a thin-film flow-through photoreactor in a medium comprising 8 M formic acid in the presence of 10 mg L^-1 Co²⁺ and 25 mg L^-1 Cd²⁺. The volatile product (presumably ruthenium pentacarbonyl) was generated in a flow injection mode, yielding an overall efficiency of 29% at a sample flow rate of 1.4 mL min^-1. The presence of both Co²⁺ and Cd²⁺ sensitizers enhanced PVG efficiency by 3,200-fold, permitting a 31 s irradiation time. Although enhanced efficiency (≈40%) could be obtained with increased Co²⁺ concentration, this was not suitable for routine use due to co-generation of cobalt carbonyl. Excellent repeatability (<2.5%) and reproducibility (4%) were achieved for 200 ng L^-1 Ru³⁺. Limits of detection ranged from 20 to 42 pg L^-1 (10-21 fg absolute) depending on the measured isotope and operational mode of the ICPMS reaction/collision cell. Interferences from inorganic acids and their anions, several transition metals, and metalloids were investigated. Practical application of the methodology was demonstrated by the analysis of seven water samples of various matrix complexities (well water, spring water, contaminated water, and seawater).

Effect of additives on cadmium chemical vapor generation and reliable quantification of generation efficiency

Chemical vapor generation (CVG) of cadmium was optimized based on response from atomic absorption spectrometry (AAS) with a heated quartz tube atomizer (QTA). Effect of several modifiers on analytical performance was studied. These additives were: inorganic salts of Cr³⁺, Ti⁴⁺ and Co²⁺ and their on-line synthesized complexes with KCN and thiourea, respectively. The use of these additives resulted in sensitivity enhancement, better repeatability and correspondingly in improvement of overall CVG efficiency. The latter was quantified by two independent approaches: a) by means of ^115mCd radioactive indicator, b) from comparison of sensitivities obtained with conventional solution nebulization and with CVG, both coupled simultaneously to inductively coupled plasma mass spectrometry. Both approaches provided comparable results. The highest efficiency, between 60 and 70%, was reached in the presence of Cr³⁺/KCN and Ti⁴⁺/KCN while 19% was achieved in Co²⁺/ascorbic acid/thiourea environment. Highly irreproducible results with low CVG efficiency ranging from 2.5 to 15% were reached in the absence of any additives. The generated cadmium species were identified to be mostly free atoms regardless of the additives presence or their absence. Cr³⁺/KCN environment was selected as the most robust for CVG of Cd reaching sensitivity of 6.6 s ng^-1 Cd and limit of detection of 60 pg mL^-1 Cd (9 pg Cd absolute) with detection by QTA-AAS.

Impact of Gas-Liquid Interface on Photochemical Vapor Generation

Interfacial effect has attracted increasing interest as the inherent asymmetric environment of a gas-liquid interface leads to different chemical and physical properties between this region and the bulk phase, resulting in enhanced chemical processes, specific reactions, and mass transfer at the interface. Photochemical vapor generation (PVG) is regarded as a simple and green sample introduction method in atomic spectrometry. However, the photochemical behavior of elements with the interface is not known. Herein, we report the PVG of elements at the gas-liquid interface along with a possible mechanism investigated for the first time. Enhancement and/or suppression effects from the gas-liquid interface were observed on the PVG of 17 elements, which was correlated with the properties of analytes and the generated intermediate substances/products of PVG and the applied conditions. Enhancement from 1.1- to 7.3-fold in analytical sensitivity was found for 12 elements in the system with gas-liquid interface(s) compared to the results obtained in previous reports of PVG using traditional flow injection with inductively coupled plasma mass spectrometry measurement. The introduction of gas-liquid interface(s) and the resultant elevated temperature inside the PVG reactor likely facilitated the generation of radicals, the subsequent radical-based reactions, and the separation/transport/detection of volatile species of elements. In contrast, intermediate substances/products generated in PVG with poor thermostability will readily decompose at elevated temperatures, leading to a decreased signal response of analytes. The finding is helpful to understand the transport of elements under UV irradiation in the environment and has potential for analysis of trace elements in environmental and biological samples.

Preconcentration with Chlorella vulgaris combined with energy dispersive X-ray fluorescence spectrometry for rapid determination of Cd in water

Freshwater green algae Chlorella vulgaris was selected as an adsorbent, and a simple, rapid, economical and environmentally friendly method for the detection of heavy metal Cd in water samples based on preconcentration with C. vulgaris combined with energy dispersive X-ray fluorescence (EDXRF) spectrometry was proposed.  Chlorella vulgaris could directly and rapidly adsorb Cd²⁺ without any pretreatment, and the maximum adsorption efficiency could be obtained when the contact time was 1 min with an optimal pH of 10. The obtained Cd-enriched thin samples after preconcentration with C. vulgaris by suction filtration of reaction solution had very good uniformity, which could be directly measured by EDXRF spectrometry, and the net integral fluorescence intensity of Cd Kα characteristic peak had a very good linear relationship with the initial concentration of Cd in the range of 0.703-74.957 µg ml^-1 with a correlation coefficient of 0.9979. When the Cd thin samples with a Cd-enriched region of 15.1 mm in diameter were formed by the developed preconcentration method with suction filtration of 10 ml reaction solution, the detection limit of this method was 0.0654 µg ml^-1, which was lower than the maximum allowable discharge concentration of Cd in various industrial wastewaters. The proposed method was simple to operate, and could effectively remove the influence of matrix effect of water samples and effectively improve the sensitivity and stability of EDXRF spectrometry directly detecting heavy metals in water samples, which was successfully applied to detect Cd in real water samples with satisfactory results, and the recoveries ranged from 94.80% to 116.94%. Moreover, this method can be applied to the rapid detection and early warning of excessive Cd in discharged industrial wastewaters. This work will provide a methodological basis for the development of rapid and online monitoring technology and instrument of heavy metal pollutants in water.