High sensitive detection of Cd, Hg, Pb and Cr in rice based on LA-MPT-OES with optimized excitation regions by two-dimensional characterization of plasma plume.

Direct, rapid and highly sensitive detection of heavy metals in rice is essential to ensure food safety. In this research, a combination of laser ablation and microwave plasma torch optical emission spectrometry (LA-MPT-OES) was proposed. Based on the optimal observation positions, a high sensitivity and direct determination of Cd, Hg, Pb and Cr in rice were realized. The limits of detection (LOD) were 0.97, 0.12, 0.61 and 0.15 µg/kg, respectively, which were reduced by one order of magnitude compared to the optimal observation height. In addition, the LOD was reduced by one to two orders of magnitude compared with the techniques that require sample pre-treatment. Moreover, the results of the Certified Reference Materials and real samples were in agreement with the reference values with a relative error in the range of 0.28% âˆ¼ 14.16%. The results demonstrated that LA-MPT-OES could be a promising tool to detect heavy metals in rice.

Development of mass spectrometry imaging techniques and its latest applications.

Mass spectrometry imaging (MSI) is a novel molecular imaging technology that collects molecular information from the surface of samples in situ. The spatial distribution and relative content of various compounds can be visualized simultaneously with high spatial resolution. The prominent advantages of MSI promote the active development of ionization technology and its broader applications in diverse fields. This article first gives a brief introduction to the vital parts of the processes during MSI. On this basis, provides a comprehensive overview of the most relevant MS-based imaging techniques from their mechanisms, pros and cons, and applications. In addition, a critical issue in MSI, matrix effects is also discussed. Then, the representative applications of MSI in biological, forensic, and environmental fields in the past 5 years have been summarized, with a focus on various types of analytes (e.g., proteins, lipids, polymers, etc.) Finally, the challenges and further perspectives of MSI are proposed and concluded.

Elucidation of formation mechanism responsible for charge-transfer reagent ions in microwave induced plasma desorption ionization (MIPDI) source.

Charge transfer reagent ions NO+ and O2+ were major observed in microwave-induced plasma desorption ionization (MIPDI) source. Ions NO+ was prosperous in the discharge region while ions O2+ generated in the afterglow region. Fundamental aspects of conditions that controlled MIPDI were addressed here to help understand the specific role of atmospheric components for charge transfer reagent ions formation. In the presence of trace nitrogen in Ar-MIPDI plasma, the production of NO+ ions increased significantly, while ions N2+ were not observed. The controlled afterglow environment obtained by connecting the plasma source to the mass spectrometer using a quartz reaction chamber found that ions O2+ produced from the interaction between plasma-generated energy particles and oxygen molecules. With the aid of mass spectra and optical emissions spectra, the ionization capacity of Ar-MIPDI plasma was revealed. Both the discharge region and afterglow region are not energetic enough to ionize N2 molecules. Charge transfer reagent ions in the MIPDI source have vital ionization ability for aromatic compounds. A deeper understanding of fundamental aspects, especially of condition-controlled ionization in a MIPDI source, will be vital for future practical applications, particularly with regard to difficult to ionize (e.g., nonpolar) species.

Advances in pretreatment and analysis methods of aromatic hydrocarbons in soil.

Benzene compounds that are prevalent in the soil as organic pollutants mainly include BTEX (benzene, toluene, ethylbenzene, and three xylene isomers) and PAHs (polycyclic aromatic hydrocarbons). These pose a severe threat to many aspects of human health. Therefore, the accurate measurement of BTEX and PAHs concentrations in the soil is of great importance. The samples for analysis of BTEX and PAHs need to be suitable for the various detection methods after pretreatment, which include Soxhlet extraction, ultrasonic extraction, solid-phase microextraction, supercritical extraction, and needle trap. The detection techniques mainly consist of gas chromatography (GC), mass spectrometry (MS), and online sensors, and provide comprehensive information on contaminants in the soil. Their performance is evaluated in terms of sensitivity, selectivity, and recovery. Recently, there has been rapid progress in the pretreatment and analysis methods for the quantitative and qualitative analyses of BTEX and PAHs. Therefore, it is necessary to produce a timely and in-depth review of the emerging pretreatment and analysis methods, which is unfortunately absent from the recent literature. In this work, state-of-art extraction techniques and analytical methods have been summarized for the determination of BTEX and PAHs in soil, with a particular focus on the potential and limitations of the respective methods for different aromatic hydrocarbons. Accordingly, the paper will describe the basic methodological knowledge, as well as the recent advancement of pretreatment and analysis methods for samples containing BTEX and PAHs.

Effective N2 capture by aryl cations at ambient temperature and pressure.

Here, we show that molecular N2 was efficiently captured by organic arylium cations in a well-defined manner at ambient pressure and temperature, which was monitored by on-line mass spectrometry analysis. A kinetic picture was proposed to disclose the principle of the ion-molecule reaction behavior for exclusive aryldiazonium production. The observation has an implication for direct nitrogen fixation into an organic framework via the intermediacy of such cationic species.

Synergetic effect of laser and micro-fabricated glow discharge plasma in a new ion source for ambient mass spectrometry.

A new ambient ionization technique named laser ablation micro-fabricated glow discharge plasma (LA-MFGDP) was developed for mass spectrometry in this study. This technique used low energy laser for sample ablation and ionized sample aerosol with MFGDP in sequence. The combination of laser ablation and MFGDP exhibited a synergetic effect that significantly improved the performance of MFGDP. Experimental results showed that MFGDP dominated the ionization process while laser played the role of desorption in LA-MFGDP. [M+H]+ and M+ proved that proton transfer reactions and charge transfer reactions were involved in the ionization process, respectively, indicating that the ionization character was the same as MFGDP. LA-MFGDP could analyze less volatile samples that were unable to be detected by MFGDP because laser significantly improved the ionization capability of MFGDP. Strong ion signals were obtained by LA-MFGDP with low sample consumption. The limits of detection (LODs) of LA-MFGDP was as low as three orders of magnitude than that of MFGDP, which demonstrated that LA-MFGDP possessed an outstanding advantage in detecting trace substances. LA-MFGDP was successfully applied to detect pharmaceutical tablets without any pretreatment. Benefited from the excellent performance, LA-MFGDP offers great potential in broadening the application of ambient mass spectrometry.

Methylation in combination with temperature programming enables rapid identification of polysaccharides by ambient micro-fabrication glow discharge plasma (MFGDP) desorption ionization mass spectrometry.

Obtaining the fingerprints of polysaccharides was known to be notoriously difficult by ambient mass spectrometry due to their resistance for desorption and ionization. Ambient mass spectrometry technology has recently been recognized as a quick analysis tool for obtaining fingerprints, which is attributed to its characteristics of no sample pretreatment and easy operation under atmospheric pressure. However, it still remains a challenge for accurate identification of the fingerprints of macromolecular polysaccharides by ambient micro-fabrication glow discharge plasma (MFGDP). In this study, a simplified methylation method was introduced to realize rapid analysis of polysaccharide mixture by MFGDP with the assistance of a temperature-programmed system (TPS). At the optimal temperature of TPS, oligosaccharides, plant polysaccharides and polysaccharide mixtures were all well characterized by TPS-MFGDP. In the proposed method, the characteristic [M + NH4] + adduct ions of oligosaccharides ions of oligosaccharides that are difficult for other ambient mass spectrometric methods were abundantly produced, making it possible to simultaneously identify a mixture of five or more polysaccharides. In addition, a supervised classification model which based on MS spectra, was used to classify seven typical hypoglycemic polysaccharides with excellent sensitivity, specificity and accuracy, indicating the good classification performance of the RF model constructed. Thus, the proposed mass spectroscopic method provides a cost effective, accurate and high throughput tool for identification and classification of polysaccharides, which is beneficial for studying the biological activity of polysaccharides.

Efficient degradation of Fipronil in water by microwave-induced argon plasma: Mechanism and degradation pathways.

Fipronil and its metabolites are potentially harmful to the ecological environment and have chronic neurotoxic effects, which makes it to be classified as class C carcinogens. Fipronil has been banned from agricultural use in China since 2009, but its residue remains in the environment. Therefore, an efficient and economical method is urgently needed to degrade fipronil residues in the environment. Herein, the degradation of fipronil in water solution using argon microwave-induced plasma (MIP) system was studied and a plausible reaction pathway was proposed in combination with Density Functional Theory (DFT) calculations. The degradation of fipronil by MIP system was optimized in terms of input power, plasma-sample distance, initial concentration and gas flow rate. After short time MIP treatment with an input power of 150 W, as high as 85.62% degradation efficiency was achieved for the fipronil at concentration of 20 mg·L--1 under the optimized conditions, and the corresponding energy efficiency was 1334.8 mg·kwh-1. Optical emission spectrometry (OES) was employed to characterize the distribution and intensity of OH, H and O species which play key roles in the degradation of fipronil by plasma, and it revealed that the degradation reaction mainly occurs at gas-liquid interface where the highest intensity of OH, H and O species was observed. High resolution mass spectrometric analysis in combination quantum chemical calculations indicate that a wide diversity of reaction processes occurred for fipronil degradation under MIP treatment, involving oxidation or reduction, nitro reduction, oxidative dichlorination, reductive dichlorination, hydration, dehydration and thiourea to urea. The possible degradation mechanism and pathways were proposed based on the degrading species identified by high resolution Mass Spectrometry (HRMS) and the thermodynamic profiles.

Interpretation of Ionization Mechanism Responsible for Reagent Ion and Analyte Formation in Microwave-Induced Plasma Desorption Ionization Mass Spectrometry.

Ambient desorption/ionization (ADI) sources coupled to mass spectrometer have gained increasing interest in the field of analytical chemistry for its fast and direct analysis of samples. Among many ADI sources, plasma-based ADI sources are an important branch. Despite its extensive use in mass spectrometry analysis, the ionization mechanism of these sources still remain uncertain. The study on ionization mechanism is of great significance to optimize the design of ion sources and to improve ionization efficiency. In this study, targeted research on a better understanding of afterglow distance effects on ionization process was conducted. Based on the quantified signal expression of reagent ions in mass spectrum, the concept that optimal atmospheric analysis distance of plasma ADI source is defined for the first time. From the perspective of mutual restriction effect between atmospheric components, the formation progress of reagent ions was visually revealed in detail, which involved the initial step of forming precursor reagent ions, the clusters reaction for increasing production of reagent ions, and the matrix effect results in reagent ion depletion. The formation mechanism of reagent ions further clarified the explicit reason for abundant reagent ions generated at an optimal distance. Most importantly, the analyte analysis results verified the significant impact of appropriate distance on ionization efficiency in afterglow region. It was confirmed that the quantity and type of reagent ions intimately influenced the status of analyte ions in mass spectrum.

Development of microwave plasma proton transfer reaction mass spectrometry (MWP-PTR-MS) for on-line monitoring of volatile organic compounds: Design, characterization and performance evaluation.

Proton transfer reaction mass spectrometry (PTR-MS) is a revolutionary on-line VOCs monitoring method. In this study, a new microwave plasma-based proton transfer reaction mass spectrometry (MWP-PTR-MS) is developed. The MWP consists of a surfatron type resonator and a cooperative ion extraction device to achieve high efficiency production of hydronium ions. Non-local thermodynamic equilibrium water plasma is generated in a quartz tube at 200 Pa with 2.45 GHz microwave. Characters of MWP, such as emission spectroscopy, input power, dirty ion suppressing and ion extraction, are explored systematically. With the same test platform, MWP has a 7-fold increment of hydronium ion production rate than glow discharge source. The counts rate of hydronium ion is 8 × 106 cps with abundance of NO+ and O2+ is less than 0.4% and 5% respectively. The performance of MWP-PTR-MS is validated by analysis of several chemicals and demonstration application is conducted. The instrument showed good linearity above 99% in the range of 4.5 × 10-11 to 4.5 × 10-9 mol/L. Benefited from the character of surface wave and 2.45 GHz microwave, MWP has high potential in improving the performance and reliability of current PTR-MS.

A mechanism study of positive ionization processes in flowing atmospheric-pressure afterglow (FAPA) ambient ion source with controlled plasma and ambient conditions.

Although plasma based ambient desorption/ionization (ADI) sources have been widely used for direct analysis of complex samples, mass spectrometric imaging, high throughput screening etc., the ionization mechanism of plasma-based ADI remains a mystery by now. In this report, a targeted study was conducted aiming at a better understanding of the ionization processes of plasma-based ambient desorption ionization source. As a representative of ambient desorption ionization source, an FAPA source was used and modified as a test platform to control the plasma discharge parameters and ambient ionization environment such as discharge gases, environmental gases and sampling conditions. Based on the ionization results from different ambient ionization conditions, a new mechanism was proposed to reveal the nature of regent ion production of FAPA. At the same time, the effect of buffer gas was investigated. For the first time, the multi-clustered hydronium ions formed by the massive water vapor in the air were explored to clarify reasons for the occurrence of selective ionization and the factors affecting ionization efficiency in such complex events. In addition, the formation of molecular ions and relevant reagent ions was speculated based on experimental observations.

Rapidly monitoring the quality of flavoring essence based on microwave-induced plasma ionization mass spectrometry and multivariate statistical analysis.

Microwave-induced plasma ionization mass spectrometry (MIPI-MS) combined with multivariate statistical analysis was first applied to rapidly monitor the quality of tobacco flavoring essence. A small isolation and reaction chamber was set up between the ion source and the injection port of mass spectrometer to effectively eliminate the interference of external environment in the process of analyzing samples. The improved experimental apparatus (MIPI-MS) could achieve online and high-throughput analysis, with minimal sample preparation steps. Further, two types of tobacco flavoring essences with the similar appearance and physicochemical parameters were employed to verify the usability of the promising method in the field of quality monitoring. Firstly, the mass spectral fingerprint of each essence was established by the improved MIPI-MS method within 2 min. Then, two multivariate statistical processes were carried out to analyze mass spectral data. The similarity results indicated that the thresholds of tobacco flavoring essences from different batches were 1.512 and 2.638, respectively. The first three principal components of the established PLS-DA described 93.6% of the total variability, and provided a visualized comparison for the two types of flavoring materials. Finally, the adulterated samples were successfully distinguished by employing the two multivariate statistical processes.

A Novel Microwave-Induced Plasma Ionization Source for Ion Mobility Spectrometry.

This work demonstrates the application of a novel microwave induced plasma ionization (MIPI) source to ion mobility spectrometry (IMS). The MIPI source, called Surfatron, is composed of a copper cavity and a hollow quartz discharge tube. The ion mobility spectrum of synthetics air has a main peak with reduced mobility of 2.14 cm2V-1s-1 for positive ion mode and 2.29 cm2V-1s-1 for negative ion mode. The relative standard deviations (RSD) are 0.7% and 1.2% for positive and negative ion mode, respectively. The total ion current measured was more than 3.5 nA, which is much higher than that of the conventional 63Ni source. This indicates that a better signal-to-noise ratio (SNR) can be acquired from the MIPI source. The SNR was 110 in the analysis of 500 pptv methyl tert-butyl ether (MTBE), resulting in the limit of detection (SNR = 3) of 14 pptv. The linear range covers close to 2.5 orders of magnitude in the detection of triethylamine with a concentration range from 500 pptv to 80 ppbv. Finally, this new MIPI-IMS was used to detect some volatile organic compounds, which demonstrated that the MIPI-IMS has great potential in monitoring pollutants in air.

Fiber Optic Surface Plasmon Resonance-Based Biosensor Technique: Fabrication, Advancement, and Application.

Fiber optic-based biosensors with surface plasmon resonance (SPR) technology are advanced label-free optical biosensing methods. They have brought tremendous progress in the sensing of various chemical and biological species. This review summarizes four sensing configurations (prism, grating, waveguide, and fiber optic) with two ways, attenuated total reflection (ATR) and diffraction, to excite the surface plasmons. Meanwhile, the designs of different probes (U-bent, tapered, and other probes) are also described. Finally, four major types of biosensors, immunosensor, DNA biosensor, enzyme biosensor, and living cell biosensor, are discussed in detail for their sensing principles and applications. Future prospects of fiber optic-based SPR sensor technology are discussed.