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Determinative Techniques to Measure Organics and Inorganics
Published in Paul R. Loconto, Trace Environmental Quantitative Analysis, 2020
By separating all or most all the VOCs and/or the SVOCs present in the headspace or trapped on Tenax® or present in a liquid solvent extract from either LLE, SPE, or other trace sample prep techniques using a WCOT GC column and then letting these analytes enter the ion source of the MSD! Figure 4.68 depicts exactly what happens when a sample vapor containing a neutral molecule M subjected to a 70 eV impact. A highly energetic molecular ion whose symbol is M+· (the symbol denotes the loss of an electron while leaving an unpaired electron) is temporarily produced in the ion source. A uni-molecular decomposition occurs, often called molecular-ion fragmentation. This leads to one or more lower-molecular-weight fragments of well-defined mass-to-charge ratios, denoted by the symbol m/z.
Crude Oil and Asphaltene Characterization
Published in Francisco M. Vargas, Mohammad Tavakkoli, Asphaltene Deposition, 2018
R. Doherty *, S. Rezaee *, S. Enayat, M. Tavakkoli, F. M. Vargas
In addition to VPO and GPC methods, mass spectroscopy is also one of the most commonly used methods for measuring molecular weight. This method is mainly based on the volatilization/ionization of the sample. This is a powerful method to measure the mass-to-charge ratio (m/z) of ions to identify and quantify molecules in simple and complex mixtures. In this method, the sample (asphaltene) is vaporized and then ionized by an ion source, which creates molecular ions. These ions will be deflected by electric and magnetic fields. Then, the deflected ions will hit a detector of ions. Based on the strength of the magnetic field, different ions (different m/z) will be detected by the ion detector at different times. The computer connected to the mass spectrometer analyzes the data from the detector and produces a plot of m/z (on the x-axis) against the relative abundance (on the y-axis) of the ions. Assuming that the charge of the ions is +1 (one electron is lost), it is possible to find the weight percentage of each of the ion fractions based on the relative abundance on the y-axis) (Coelho and Franco 2013).
A Survey of New Directions Being Explored, and Potential New Applications
Published in Marcos Dantus, Femtosecond Laser Shaping, 2017
I discussed the use of shaped laser pulses for the standoff detection of explosives in Chapter 12. Here, I discuss further analytical applications of these laser sources. The ability of femtosecond lasers to excite rotations, vibrations and electronic states makes them ideal for spectroscopic applications. In addition, femtosecond pulses can easily ionize any material. The ions that are formed can then be analyzed by their mass-to-charge ratio for identification. Analytical chemistry, which depends on a wide variety of spectrometers, gives forensic science and drug discovery a solid foundation. Shaped femtosecond laser pulses can be used for carrying out most of the different spectroscopic modalities including gene and protein sequencing. The future of shaped laser pulses in analytical chemistry applications will depend on the development of robust and relatively inexpensive laser sources that include the pulse shaper. These automated sources will then be integrated into multidimensional analytical robots that can be used for widely different tasks such as analyzing a crime scene, for the development of new drugs, for identifying pathogens, and even for space explorations.
Probing the nature of soil organic matter
Published in Critical Reviews in Environmental Science and Technology, 2022
Zhe (Han) Weng, Johannes Lehmann, Lukas Van Zwieten, Stephen Joseph, Braulio S. Archanjo, Bruce Cowie, Lars Thomsen, Mark J. Tobin, Jitraporn Vongsvivut, Annaleise Klein, Casey L. Doolette, Helen Hou, Carsten W. Mueller, Enzo Lombi, Peter M. Kopittke
Technical background: Nanoscale secondary ion mass spectrometry (NanoSIMS) is an analytical technique that provides information of the microscale (∼50–100 nm spatial resolution) elemental and isotopic composition of a material (Herrmann et al., 2007; Hoppe et al., 2013; Mueller et al., 2013). A primary ion beam (either Cs+ or O−) is accelerated onto the sample surface which releases secondary ion particles. These ions are separated according to their mass to charge ratio in a sector mass spectrometer. The primary ion beam can be focused to a spot of sample to achieve a lateral resolution of up to 50 nm, with scanned area typically between 5 × 5 μm up to 30 × 30 μm (Mueller et al., 2012; Steffens et al., 2017).
On the product selectivity in the electrochemical reductive cleavage of 2-phenoxyacetophenone, a lignin model compound
Published in Green Chemistry Letters and Reviews, 2022
Marcia Gabriely A. da Cruz, Bruno V. M. Rodrigues, Andjelka Ristic, Serhiy Budnyk, Shoubhik Das, Adam Slabon
GC-MS samples were dissolved in a methanol–chloroform mixture (volumetric ratio 3:7, dilution factor of 1:1000). Sample solutions were then introduced into a LTQ Orbitrap XL hybrid tandem high-resolution mass spectrometer from Thermo Fisher Scientific (Bremen, Germany) by direct infusion applying a flow rate of 5 µL/min. The instrument was fitted with electrospray ionization (ESI) ion source and operated in positive or negative ion mode respectively. Nitrogen was used as sheath gas. Helium was used both as buffer and collision gas in the linear ion trap section where lower energy collision-induced dissociation (CID) was performed. For the identification of chemical structures by tandem MS, product ions were generated in the linear ion trap via CID and detected by the high-resolution orbitrap section of the instrument at a resolution of 60,000 (full width at half maximum, FWHM). The mass measurements were acquired with a mass accuracy of 5 ppm or better. Data processing and interpretation were done using the software tools Xcalibur version 2.0.7 and Mass Frontier version 6.0 from Thermo Fisher Scientific (Bremen, Germany). m/z describes the mass-to-charge ratio of the detected ions. As all negatively charged ions analyzed were single charged species, m/z also referred to the monoisotopic molecular masses of the detected ions. It is noted that ionization capability of chemical compounds is dependent on the chemical structure. Thus, for spectrum interpretation, especially of the different depolymerization routes and model products, the intensities detected can only be compared between the same species found in the analyzed samples obtained by the similar procedure but not between different species.
Study on the Pyrolysis Kinetics of Corn and Qualification of Pyrolysis Products
Published in Combustion Science and Technology, 2023
Heng Yu, Congxue Yao, Yifan Zhou, Jingwen Wang, Wenru Zeng, Lei Song, Xiaowei Mu, Yuan Hu
In our study, we use TG-GC-MS for more accurate product determination. 9–15 mg of the sample was subjected to pyrolysis under a nitrogen atmosphere by a thermogravimetric analyzer, and the generated gas was passed through a six-way valve to be detected and analyzed by gas chromatography and mass spectrometry, and the sample gas was protected at 280°C. The thermogravimetric analyzer is model TA Q50. The GC-MS model is the Trace 1300 and ISQ QD produced by Thermo Scientific. The column used in this article is Trace TR-5 MS (30 m × 0.25 mm × 0.25 μm). In each test, the temperature of the column oven was increased from 50 to 280°C (at 10 °C/min), maintained for 25 min and the scanning range of the mass spectrum was 50–500 amu in order to avoid the interference from the release of small molecular gases such as H2O (M = 18), CO2 (M = 44), and CH3OH (M = 32). In order to achieve the desired temperature quickly and without imposing too heavy load on the thermogravimetric analyzer, the temperature of TGA was increased from 20°C to 800°C (at 80°C /min) under nitrogen atmosphere. The pyrolysis gas produced from TGA enters GC-MS at the temperature of the maximum weight loss rate. When a mixed sample of multiple components enters the chromatographic column of a gas chromatograph, the adsorption capacity of the adsorbent for each component is different, so the running speed of each component is different. Finally, each component separates in the chromatographic column and enters the mass spectrometer. Different components will be broken into ion fragments after entering the mass spectrometer, and the components can be distinguished according to the mass-to-charge ratio distribution of ion fragments detected by mass spectrometry. A blank test was performed before and after each test, and there were no peaks in the spectrum to ensure no interference from other impurities. All test results were repeated twice or more to ensure repeatability.