Chemical ionization reagent gases used

Membranes can be classified as porous and nonporous based on the structure or as flat sheet and hollow fiber based on the geometry. Membranes used in pervaporation and gas permeation are typically hydrophobic, nonporous silicone (polydimethylsiloxane or PDMS) membranes. Organic compounds in water dissolve into the membrane and get extracted, while the aqueous matrix passes unextracted. The use of mircoporous membrane (made of polypropylene, cellulose, or Teflon) in pervaporation has also been reported, but this membrane allows the passage of large quantities of water. Usually, water has to be removed before it enters the analytical instrument, except when it is used as a chemical ionization reagent gas in MS [50], It has been reported that permeation is faster across a composite membrane, which has a thin (e.g., 1 pm) siloxane film deposited on a layer of microporous polypropylene [61],... 

A Hewlett-Packard (Palo Alto, CA, USA) Model 5988A TSP LC-MS quadrupole mass spectrometer and a Hewlett-Packard Model 59970C instrument for data acquisition and processing were employed, The TSP temperatures were stem 100 fiC, tip 178 9C, vapour 194 C and ion source 296 aC with the filament on. In all the experiments the filament-on mode (ionization by an electron beam emitted from a heated filament) was used. In this mode of operation conventional positive and negative ohemioal ionization can be carried out by using the vaporised mobile phase as the chemical ionization reagent gas (4). 

Christopher, R. K. and Duffield, A. M. (1980) Use of vinyl methyl ether as a chemical ionization reagent gas for gas chromatographic chemical ionization mass spectrometric discrimination between cyclopropanoid and monoenoic fatty acid methyl esters. Biomed. Mass Spectrom., 1 (10), 429-32. 

Computer mass range up to m/z 1450. Ion source temperature, 250 C, accelerating voltage 3 kV, resolution M/dM 1000 (10% valley). The electron energy for El was 70 eV the Cl (chemical ionization) reagent gas was generally isobutane, but methane was also used. 

Figure 3.2 Piocesses occurring in chemical ionization mass spectrometry using methane as the reagent gas.

Resistance to physical shocks and vibration required careful attention to selection of rugged components and to securing electrical and vacuum systems, wiring, connectors, components, and boards. Chemical ionization (Cl) was used for the first time in a fieldable military detector because of the advent of rugged turbomolecular pumps capable of handling the gas load from the Cl reagent. 

Production of Ions. Several methods are used (11 by bombardment with electrons from a heated filament (2 by application of a strong electrostatic field (field ionization, field desorption) Ot by reaction with an ionized reagent gas (chemical ionization) (4 by direct emission of ions from a solid sample that is deposited on a heated filament (surface ionization) (SI by vaporization from a crucible and subsequent electron bombardment (e.g.. Knudsen cell for high-lcmperalure sludies id solids and (6) by radio-frequency spark bomhardmenl of sample fur parts-per-biliion (ppb) elemental analysis of solids as encountered in metallurgical, semiconductor, ceramics, and geological studies. Ions also are produced by photoion izution and laser ionizalion. 

It is possible to introduce a sample directly into the chemical ionization source on a tungsten or rhenium wire. A drop of sample in solution is applied to the wire, the solvent is allowed to evaporate, and the sample inserted into the Cl source. The sample molecules are desorbed by passing a current through the wire, causing it to heat. The analyte molecules then ionize by interaction with the reagent gas ions as has been described. This technique is called desorption chemical ionization and is used for nonvolatile compounds. 

The most common use of batch inlets is to introduce a controlled flow of compounds for calibrating the mass scale. Frequently used calibration compounds include perfluorotributylamine (FC-43, heptacosa) and perfluorokerosene (PFK) both are effective with electron ionization (El) but give limited responses in chemical ionization (Cl). If used for calibration in Cl, or to provide lock masses (Section 3.1.1), the concentration of the reagent gas must be reduced. This reducation somewhat compromises the effectiveness of the Cl process. 

Bose AK, Fujiwara H, Pramanik BN, Lazaro E, Spillert CR. Mass spectral studies. VIII. Some aspects of chemical ionization mass spectroscopy using ammonia as reagent gas A valuable technique for biomedical and natural products studies. Anal Biochem 1978 89 284—291. 

Chemical ionization (Q) source is very similar to the El source but the beam of electrons is used to create a plasma of ionized reagent gas (e.g., isobutane, methane, ammonia). 

Mancel,V., Sellier,N., Lesage, D.,Fournier, F.,Tabet, I-C. (2004) Gas phase enantiomeric distinction of (R)- and (S)-aromatic hydroxy esters by negative ion chemical ionization mass spectrometry using a chiral reagent gas. Int. 7. Mass Spectrom.,237,185-195. 

Most modern mass spectrometers are designed so that electron-impact ionization and chemical ionization can be carried out interchangeably. Such sources are called EI-Cl sources. In chemical ionization, gaseous atoms of the sample (from either a batch inlet or a heated probe) are ionized by collision with ions produced by electron bombardment of an excess of a reagent gas. Usually, positive ions are used, but negative ion chemical ionization is occasionally used with analytes that contain very electronegative atoms. Chemical ionization is probably the second-most common procedure for producing ions for mass spectrometry. ... 

Another potential analytic application of gas-phase metal-ion chemistry was suggested by Preiser and coworkers but apparently as yet with no practical consequences. Pattern recognition methods were used to evaluate and demonstrate the efficacy of the rare earth metal ions Sc, Y" ", and La" " as chemical ionization reagents for molecular MS a large number of organic molecules were examined by FTICR-MS, comprising six classes alkanes, alkenes, ketones, aldehydes, ethers, and alcohols (Forbes et al., 1986a,b, 1987). 

Chemical ionization mass spectra. Several of the metabolites described above, particularly the di- and trihydroxy compounds did not produce molecular ions. Chemical ionization mass spectra using isobutane as the reagent gas were therefore used in an attempt to confirm the molecular weight of these compounds. The spectra of a number of reference cannabinoids were recorded and in... 

In chemical ionization-mass spectrometry (CI-MS), the sample molecules are combined with a stream of ionized reagent gas that is present in great excess relative to the sample. When the sample molecules collide with the preionized reagent gas, some of the sample molecules are ionized by various mechanisms, including proton transfer, electron nansfer, and adduct formation. Almost any readily available gas or highly volatile liquid can be used as a reagent gas for CI-MS. 

Molecular ion Chemical ionization using ammonia as reagent gas establishes the molecular weights of sugar acetates. 

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