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Detection — Analytical
Published in Lorris G. Cockerham, Barbara S. Shane, Basic Environmental Toxicology, 2019
Christine A. Purser, Arthur S. Hume
The most sophisticated ionizing detector for GC, as well as HPLC, is the mass spectrometer (MS). MS is so sensitive that only a few nanograms (and sometimes picograms) of analyte are needed to obtain characteristic information about its structure and molecular weight. Since newer, more easily operated, and cheaper versions have been marketed, the MS is vital in the analysis of environmental contaminants. Two types of MS commonly used today in environmental laboratories are the quadrupole MS and the ion trap MS. Both instruments use either electron impact (EI) or chemical ionization (CI) to fragment compounds to obtain a mass spectrum. In the electron impact mode (hard ionization), the compounds entering the source are bombarded by electrons at a constant electron energy (usually 70 eV). Bonds are cleaved depending upon their bond energies, which results in the formation of positive and negative ion fragments. Fragmentation of compounds is reproducible if conditions such as electron energy, source temperature, and vacuum pressure are kept constant, thus producing a mass spectrum unique to that compound. The mass spectrum is sometimes referred to as the chemical “fingerprint” of a compound. Chemical ionization is a softer means of ionization. A gas such as methane, ammonia, or butane is introduced into the source where it is ionized by electrons. Gaseous ions, which contain much less energy than electrons, bombard the compound to produce relatively large fragments. This type of ionization is more useful in molecular weight determinations than electron impact.
Basics Of Gas Chromatography Mass Spectrometry System
Published in Raquel Cumeras, Xavier Correig, Volatile organic compound analysis in biomedical diagnosis applications, 2018
William Hon Kit Cheung, Raquel Cumeras
However, in some instances electron impact ionization may be too excessive and cause the molecular ion to be present in low abundance or even absent, resulting in inaccurate chemical matching and chemical formula generation. Chemical ionization is a softer form of ionization (x < 10 eV), which uses reagent gas such as isobutene, ammonia, methane or carbon dioxide to create a cloud of charge ion to transfer the charge to the analyte (charge transfer reaction). CI does not induce excesses fragmentation due to the low collisional energy used and can be used determine the molecular weight of the parent compound for structure elucidation. CI can be applied in either positive or negative mode polarity; most small scale molecules are able to form positive ions, however, in order to generate a negative ion, the molecule must be able to generate a stable negative charge such as those containing acidic functional groups or halogenated compounds.
Sampling and Laboratory Analysis for Solvent Stabilizers
Published in Thomas K.G. Mohr, William H. DiGuiseppi, Janet K. Anderson, James W. Hatton, Jeremy Bishop, Barrie Selcoe, William B. Kappleman, Environmental Investigation and Remediation, 2020
Thomas K.G. Mohr, Jeremy Bishop
Chemical ionization uses an ionization gas such as methane, butane, or carbon dioxide mixed with the sample stream. The ionization gas absorbs the initial ionizing electron and transfers the energy to the sample molecule. Chemical ionization creates a lower energy state than electron-impact ionization and thus provides less fragmentation and a larger abundance of molecular ions (McMaster and McMaster, 1998).
Association between fluoride exposure and blood pressure in children and adolescents aged 6 to19 years in the United States: NHANES, 2013–2016
Published in International Journal of Environmental Health Research, 2023
Meng Guo, Francis-Kojo Afrim, Zhiyuan Li, Na Li, Xiaoli Fu, Limin Ding, Zichen Feng, Shuo Yang, Hui Huang, Fangfang Yu, Guoyu Zhou, Yue Ba
General demographic characteristics, including age, sex (boys and girls), race/ethnicity (Mexican American, non-Hispanic white, non-Hispanic black, and other races), and familiesʻ poverty income ratios (PIR) were collected by at-home interviews. PIR was calculated by dividing the annual family income by the poverty threshold determined annually by the US Department of Health and Human Services. Poverty status was classified as PIR <1 or ≥ 1 (Park et al. 2019). Body mass index (BMI) was calculated by dividing weight by the square of height in meters (kg/m2). Serum cotinine was determined by an isotope-dilution high performance liquid chromatography/atmospheric pressure chemical ionization tandem mass spectrometry. Daily intake of calorie, sodium, calcium, and potassium exposure, which were also potential dietary risk factors of hypertension (Hermansen 2000), were obtained from the first 24-hour dietary recall interview. For each participant, calorie and nutrient intake were calculated by multiplying the frequency that each food item was reported by the calorie or nutrient content of the specified portion size. Nutritional values of all the dietary items and beverages were provided by the United States Department of Agriculture ‘s Food and Nutrient Database for Dietary Studies which regularly updates each cycle and supplies the nutrient profiles for every food and beverage reported in NHANES.
Polycyclic aromatic hydrocarbons (PAHs): Updated aspects of their determination, kinetics in the human body, and toxicity
Published in Journal of Toxicology and Environmental Health, Part B, 2023
Fernando Barbosa, Bruno A. Rocha, Marília C. O. Souza, Mariana Z. Bocato, Lara F. Azevedo, Joseph A. Adeyemi, Anthony Santana, Andres D. Campiglia
LC-MS/MS methods have also been developed for quantifying PAHs (Avagyan et al. 2015; Cai et al. 2009; Lien, Chen, and Wu 2007; Lung and Liu 2015; Raponi et al. 2017; Wang et al. 2017; Wolkenstein 2019). Liquid chromatography is required when molecules are not easily volatilized and when it is necessary to simultaneously quantify polar- and nonpolar molecules. In addition, LC-MS presents a reliable alternative for HMW PAH quantification when fluorescence detection is not possiblePlaza-Bolaños, Frenich, and Martínez Vidal (2010). HPLC is a reliable method for separating isomers due to the use of specific columns and organic additives; however, quantifying these compounds by sequential mass spectrometry is difficult due to the PAH structure, which is minimally ionized under mild ionization sources such as electrospray (ESI). Further, ESI is incompatible with the normal phase using flammable organic solvents such as hexane because there is a risk of explosion despite these solvents offering the best degree of solubility for these PAHs (Cai et al. 2009). Therefore, other ionization sources need to be used, such as atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) (Lien, Chen, and Wu 2007; Lung and Liu 2015; Wolkenstein 2019). PAH analysis by LC-MS/MS is not common in research labs, and these methods are scarce in the literature.
On the Basic Extraction Properties of a Phenyl Trifluoromethyl Sulfone-Based GANEX System Containing CyMe4-BTBP and TBP
Published in Solvent Extraction and Ion Exchange, 2018
Jenny Halleröd, Christian Ekberg, Thea Authen, Laura Bertolo, Mu Lin, Bohumír Grüner, Jaroslav Švehla, Christoph Wagner, Andreas Geist, Petra Panak, Emma Aneheim
Mass spectrometry (Thermo-Finnigan Fleet Ion Trap Instrument) with atmospheric pressure chemical ionization (APCI) in positive mode was used to identify the decomposition products in the samples after irradiation. All the samples and their corresponding reference solution were stored in a freezer at −33°C, both before and after analyses. Then, 25 L of each sample was diluted to a volume of 1.0 mL by adding acetonitrile. Infusion from the syringe into an ion source was used. Experimental conditions for the MS interface were as follows: flow rate from a syringe infusion pump, 10 L/min; sheath gas flow, 20 L/min; auxiliary gas flow, 9 L/min; source voltage, 4.17 kV; vaporizer temperature, 400°C; capillary temperature, 250°C; capillary voltage, 3 V; mass range from 50 to 1000 mass units; and [M + H] peaks were observed.