Wei Guo

Quantitative Characterization of Gold Nanoparticles by Coupling Thin Layer Chromatography with Laser Ablation Inductively Coupled Plasma Mass Spectrometry

Metal nanoparticles (NPs) determination has recently attracted considerable attention because of the continuing boom of nanotechnology. In this study, a novel method for separation and quantitative characterization of NPs in aqueous suspension was established by coupling thin layer chromatography (TLC) with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Gold nanoparticles (AuNPs) of various sizes were used as the model system. It was demonstrated that TLC not only allowed separation of gold nanoparticles from ionic gold species by using acetyl acetone/butyl alcohol/triethylamine (6:3:1, v/v) as the mobile phase, but it also achieved the separation of differently sized gold nanoparticles (13, 34, and 47 nm) by using phosphate buffer (0.2 M, pH = 6.8), Triton X-114 (0.4%, w/v), and EDTA (10 mM) as the mobile phase. Various experimental parameters that affecting TLC separation of AuNPs, such as the pH of the phosphate buffer, the coating of AuNPs, the concentrations of EDTA and Triton X-114, were investigated and optimized. It was found that separations of AuNPs by TLC displayed size dependent retention behavior with good reproducibility, and the retardation factors (R(f) value) increased linearly with decreasing nanoparticle size. The analytical performance of the present method was evaluated under optimized conditions. The limits of detection were in the tens of pg range, and repeatability (RSD, n = 7) was 6.3%, 5.9%, and 8.3% for 30 ng of 13 nm AuNPs, 34 nm AuNPs, and 47 nm AuNPs, respectively. The developed TLC-LA-ICP-MS method has also been applied to the analysis of spiked AuNPs in lake water, river water, and tap water samples.

Soil monitoring of arsenic by methanol addition DRC ICP-MS after boiling aqua regia extraction

Trace levels of arsenic (As) in soil samples can be determined by conventional ICP-QMS with closed vessel digestion, but always suffer from interference of ions originating from the complicated matrix, mainly including 40Ca35Cl+, 40Ar35Cl+, 39K36Ar+, 150Nd2+, 150Eu2+ and 150Sm2+etc. In this study, 75As+, the mass for general detection, was effectively changed to 75As16O+ which could be detected at m/z 91 by reaction with oxygen in a Dynamic Reaction Cell (DRC). For the new interference of 91Zr+ on 75As16O+, boiling aqua regia extraction was used to remove most of the zirconium in the sample preparation procedure, and the residual 91Zr+ was oxidized to 91Zr16O+ by oxygen in the DRC. In addition, the signal intensity of 75As16O was improved 4-fold by addition of 4% methanol into the sample solution. The signal to background ratio (SBR) of As was obviously improved from 0.18 to 205 (the certified arsenic concentration is 1 μg L−1), and a limit of quantification (LOQ, 10σ) of 0.1 ng g−1 was obtained. The proposed method was successfully applied to analyze 20 soil samples collected from pig farms in livestock farming processes, and shows its great potential for trace As determination in environmental and geological samples.

Selective Identification of Organic Iodine Compounds Using Liquid Chromatography–High Resolution Mass Spectrometry

A method to selectively and sensitively detect organic iodine compounds and identify their structures has been developed using liquid chromatography-high resolution mass spectrometry (LC-HRMS). Using extracted ion chromatograms of product ions (iodine ion) collected on a rapid scanning quadrupole orbitrap mass spectrometer, the retention times of the unknown organic iodine compounds were determined, and the structural information were acquired according to the MS/MS experiments and the matching with reference standards. We have demonstrated the application of this method by identifying unknown organic iodine compounds in seaweed. A total of 28 possible organic iodine peaks were discovered, among them, the accurate mass and element composition of the corresponding precursor ions were identified for 12 peaks, and molecular structures were confirmed for 4 peaks, which were 3-iodo-L-tyrosine, 3,5-diiodo-L-tyrosine, 4-iodophenol, and 2-iodobenzoic acid. This method is expected to lead to the future discovery of new organic iodine compounds via LC-HRMS in different environmental samples, which is crucial for understanding the iodine biogeochemical cycling.

Method Development for the Determination of Total Fluorine in Foods by Tandem Inductively Coupled Plasma Mass Spectrometry with a Mass-Shift Strategy

A method for total F determination in food and tea samples based on a mass-shift strategy using tandem inductively coupled plasma quadrupole mass spectrometry (ICP-MS-MS) was developed. This method consists of four steps: (1) conversion of the hardly ionized F atoms to BaF+ via ICP, (2) use of the first quadrupole (Q1, set at 157) to ensure only the m/z 157 ions (i.e., 138Ba19F+, 157Gd+, and 138Ba18OH+) enter the reaction cell (RC), (3) shifting 138Ba19F+ to a new mass 138Ba19F(14NH3)3+ by reacting with NH3 in RC to avoid the interfering ions (i.e., 157Gd+ and 138Ba18OH+), and (4) passing interference-free 138Ba19F(14NH3)3+ to the second quadrupole (Q2, set at 208) for detection by the MS detector. The mass-shift process of the target F (in ICP and RC) expected to follow the path: F + 138Ba+ → 157BaF+ + 3NH3 → 208BaF(NH3)3+, while the reaction pathway of dominant 157Gd+ in RC proposed to 157Gd+ + NH3 → 157Gd14N1H+ + nNH3 → 157Gd14N1H(14N1H3)n+ (n = 0-5). Under the optimized setting of tandem MS (Q1 → Q2 = 157 → 208) and RC reaction gas flow rate (NH3/He = 10:90, 8.0 mL min-1), the background equivalent concentrations (BECs) and limits of detection (LODs) were 0.021 and 0.022 μg mL-1, respectively. The calibration curve was linear in the range between 0.1 and 10 μg mL-1, with a correlation coefficient of R2 = 0.9999. The results obtained for 14 different food-related standard reference materials (SRMs) were in good agreement with the certified values on a 95% confidence level. The proposed method was then employed to evaluate the F contents of 13 branded tea samples. The total F concentrations ranged from 39.2 to 93.2 μg g-1. The tea infusions contained F between 23.5 and 85.4 μg g-1, with an extraction efficiency of 56.0-91.6%, and the water-soluble F contents of a Puer brick tea were 58.7, 21.4, 3.82, and 1.41 μg g-1 for filtrates 1, 2, 3, and 4, respectively.

Thioarsenate Formation Coupled with Anaerobic Arsenite Oxidation by a Sulfate-Reducing Bacterium Isolated from a Hot Spring

Thioarsenates are common arsenic species in sulfidic geothermal waters, yet little is known about their biogeochemical traits. In the present study, a novel sulfate-reducing bacterial strain Desulfotomaculum TC-1 was isolated from a sulfidic hot spring in Tengchong geothermal area, Yunnan Province, China. The arxA gene, encoding anaerobic arsenite oxidase, was successfully amplified from the genome of strain TC-1, indicating it has a potential ability to oxidize arsenite under anaerobic condition. In anaerobic arsenite oxidation experiments inoculated with strain TC-1, a small amount of arsenate was detected in the beginning but became undetectable over longer time. Thioarsenates (AsO4-xSx2- with x = 1–4) formed with mono-, di- and tri-thioarsenates being dominant forms. Tetrathioarsenate was only detectable at the end of the experiment. These results suggest that thermophilic microbes might be involved in the formation of thioarsenates and provide a possible explanation for the widespread distribution of thioarsenates in terrestrial geothermal environments.

Heated Slurry Sampling for the Determination of Cadmium in Food by Electrothermal Atomic Absorption Spectrometry

A heating procedure is reported with slurry sampling electrothermal atomic absorption spectrometry to improve the accuracy of cadmium determination in food. In comparison to conventional slurry sampling, the heating significantly increased cadmium recovery and improved the precision. For the optimized procedure, 25–250 mg of food were treated with 2% HNO3 and 1% H2O2 with heating at 120°C for 20 min, followed by the addition of 50 µL of 10% Triton X-100, and homogenization in an ultrasonic bath prior to analysis. Tungsten and rhodium were employed as a permanent modifier with optimum pyrolysis and atomization temperatures of 500°C and 1500°C. Calibration with aqueous standards resulted in good agreement between certified or information values and measured results at the 95% confidence level. A characteristic mass of 0.8 ± 0.1 pg and a detection limit of 0.7 ng g−1 for a 2% slurry were obtained. The method was employed for the direct determination of cadmium in food certified reference materials.

High-Resolution Trace Element Profiles in Calcium Carbonate Shells using LA-ICP-MS: An Environmental Proxy and Biogeochemical Cycling?

In this study we have used 193 ArF excimer laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to produce a high resolution (32 mum) shells record of Mn, Zn, Sr, Ba, Mg, Cr, Cu, As, Cd, Co and Pb for the Periglypta reticulate and Tridacna gigas mussel shells from the Yalong bay, Hainan Island of China. Our synthesized CaCO3 co-precipitation multi-elements discs as the solid matrix- matched standards for calibration, and 42 bCa was selected as the internal standard to compensate the draft of signal. The results also demonstrated our co-precipitation matrix matched calibration method was more appropriate than the commercial NIST610/612 glass standards calibration method. Validation of the matrix- matched LA- ICP-MS technique indicated that the method provides accurate and reproducible (RSD<10%) analysis of trace elements in shells (0.01~3000 mug/g). The detection limits of this method for these elements were obtained in the range of 0.103 to 0.709 mug /g with correlation coefficients (R) better than 0.993. The proposed method was successfully applied to determine the trace elements in the two shells. The high-resolution trace element profiles could be as an environmental proxy and biogeochemical cycling for the area in which the fish or mussel lives.

Determination of Trace Pb in Environmental Samples by GF-AAS after Microsphere Phase Separation Extraction

A novel method is described for preconcentration and separation of trace Pb in environmental samples such as water, soil, plant and sediment using a microsphere phase separation extraction. After complexation with 2-[5-Bromo-2-pyridylazo]-5-diethylamino-phenol (5-Br-PADAP) in alkalescent medium, the analytes are extracted to 1-octanol in the present of co-solvent alcohol, and then adding water to change the phase ratio, the contained 5-Br-PADAP-Pb microsphere phase (200 muL) separates out from the homogeneous solution, later the microsphere introduced into the graphite furnace (GF) and detected by atomic absorption spectrometry (AAS). The graphite furnace apparatus conditions such as pyrolysis or atomization temperature and the chemical variables affecting the separation phase were optimized. Detection limit (3 sigma) of 0.05 mug/L, along with enrichment factors of 35 for Pb was achieved. The develop method was applied to the determination of trace lead in a series environmental reference materials. All of the results were found to be in reasonable agreement with the certified values. This simple method shows a great potential for determination other trace metals in environmental samples.

Identification of Organic Iodine Compounds and their Transformation Products in Edible Iodized Salt using Liquid Chromatography-High Resolution Mass Spectrometry

The consumption of edible iodized salt is a key strategy to control and eliminate iodine deficiency disorders worldwide. We herein report the identification of the organic iodine compounds present in different edible iodized salt products using liquid chromatography combined with high resolution mass spectrometry. A total of 38 organic iodine compounds and their transformation products (TPs) were identified in seaweed iodine salt from China. Our experiments confirmed that the TPs were generated by the replacement of I atoms from organic iodine compounds with Cl atoms. Furthermore, the organic iodine compound contents in 4 seaweed iodine salt samples obtained from different manufacturers were measured, with significant differences in content being observed. We expect that the identification of organic iodine compounds in salt will be important for estimating the validity and safety of edible iodized salt products.