Atmospheric Pressure Chemical Ionization Conditions Research Articles

Determination of bromethalin in commercial rodenticides found in consumer product samples by HPLC-UV-vis spectrophotometry and HPLC-negative-ion APCI-MS.

A small amount of green particulate material is encountered in a consumer complaint sample. The green particulates in the sample are identified as a bromethalin-containing rodenticide using high-performance liquid chromatographic (HPLC)-UV-vis spectrophotometric and HPLC-negative-ion atmospheric pressure chemical ionization (APCI)-mass spectrometric (MS) approaches, which are commonly used for the detection and confirmation of bromethalin in grain-based rodenticides. The selective and sensitive nature of the MS detector makes it possible to determine bromethalin without extensive sample cleanup and preconcentration. The estimated detection limit with the UV-vis detector is 500 pg of bromethalin injected into the column. The extensive fragmentation of the bromethalin molecule under APCI conditions provides sufficient structural information for positive identification.

Liquid chromatography/electron capture atmospheric pressure chemical ionization/mass spectrometry: analysis of pentafluorobenzyl derivatives of biomolecules and drugs in the attomole range.

The corona discharge used to generate positive and negative ions under conventional atmospheric pressure chemical ionization conditions also provides a source of gas-phase electrons. This is thought to occur by displacement of electrons from the nitrogen sheath gas. Therefore, suitable analytes can undergo electron capture in the gas phase in a manner similar to that observed for gas chromatography/electron capture negative chemical ionization/mass spectrometry. This technique, which has been named electron capture atmospheric pressure chemical ionization/mass spectrometry, provided an increase in sensitivity of 2 orders of magnitude when compared with conventional atmospheric pressure chemical ionization methodology. It is a simple procedure to tag many biomolecules and drugs with an electron-capturing group such as the pentafluorobenzyl moiety before analysis. Pentafluorobenzyl derivatives have previously been used as electron capturing derivatives because they undergo dissociative electron capture in the gas phase to generate negative ions through the loss of a pentafluorobenzyl radical. A similar process was found to occur under electron capture atmospheric pressure chemical ionization conditions. By monitoring the negative ions that were formed, it was possible to obtain attomole sensitivity for pentafluorobenzyl derivatives of a representative steroid, steroid metabolite, prostaglandin, thromboxane, amino acid, and DNA-adduct.

Mechanistic and kinetic aspects of chemical ionization mass spectrometry of polynuclear aromatic hydrocarbons and their halogen-substituted analogues using oxidizing reagents: A gas chromatographic-mass spectrometric and Fourier transform mass spectrometric study

The mass spectral behavior of some substituted polynuclear aromatic hydrocarbons (PAHs) was investigated by using low-pressure chemical ionization conditions with a Fourier transform ion cyclotron resonance spectrometer (FT-MS) and by using gas chromatography-mass spectrometry (GC-MS) under higher pressure chemical ionization conditions. For the reactions of O −· with substituted PAHs, several reaction pathways are observed, including hydrogen atom transfer, hydrogen atom displacement, nucleophilic aromatic substitution, ring cleavage, and abstraction of H + 2 to generate (M - 2H) −. Bimolecular rate constants and product branching ratios for the reactions of O −· with benzene and a variety of substituted naphthalenes were obtained using FT-MS. The rate constants vary from (2.1 ± 0.4) × 10 −10 cm 3 molecule −1 s −1 for naphthalene to (2.1 ± 0.4) × 10 −9 cm 3 molecule −1 s −1 for 1 − chloronaphthalene, corresponding to reaction efficiencies of 8% and 60%, respectively. In contrast to previous pulsed radiolysis studies on the reactions of O −· with aromatic compounds in solution, O −· is observed to be a powerful gas-phase nucleophile. Nucleophilic substitution reactions are observed even in the absence of activating groups. Competition between this and other reaction pathways is related to the leaving-group abilities and inductive effects of the various substituents. Observation of reactions with the chlorinated naphthalenes implies limits on the heats of formation for the corresponding carbanions of Δ H f(1-ClC 10H − 6) < 46 ± 5 kcal mol −1 and Δ H f(2-ClC 10H − 6) < 49 ± 5 kcal mol −1 (1 kcal=4.184 kJ). Although the isomeric PAHs often give very similar mass spectra and their chromatographic peaks are not well resolved, differences in their GC-MS behavior can be used to distinguish anthracene and phenanthrene. For the unsubstituted PAHs reaction pathways are observed in agreement with recent studies and mass-selected FT-MS suggests which reaction pathways are due to ion-molecule reactions. Low ion intensities for many of the oxidation products relative to products formed by electron-transfer reactions of the PAHs suggest oxidation may occur by wall-catalyzed processes, in agreement with recent studies by Stemmler and Buchanan. The positional isomers of chloronaphthalene give clearly different mass spectra under both N 2O and N 2OO 2 chemical ionization conditions whereas the chloroanthracenes are not readily distinguished using GC-MS.

Low pressure ion/molecule clustering reactions towards cationic dimers

Molecular ionic dimers of water, hydrochloric acid, ethane, cyanogen and acetonitrile are produced via a well-controlled successive two-step molecule exchange reaction from noble gas ionic dimers in a low pressure Fourier transform ion cyclotron resonance (FT-ICR) cell. To this end, the ionic dimers of argon, krypton and xenon have been generated under high pressure chemical ionization conditions in an external ion source located outside the influence of the ICR magnetic field and subsequently injected into the FT-ICR cell, where they have been allowed to react with gaseous water, hydrochloric acid, ethane, cyanogen and acetonitrile in the absence of stabilizing collisions at very low pressure. Endothermic electron transfer accompanying these molecule exchange reactions suggests that possibly higher bound electronic states of the studied noble gas ionic dimers may be involved. The generated ionic dimers of water, hydrochloric acid, ethane, cyanogen and acetonitrile in the reaction with their neutral monomers all exhibit chemical behaviour which is identical to that of their ionic monomers, suggesting that at least the reactive structure of the above-mentioned ionic dimers resembles that of the corresponding solvated ionized monomer.

Formation de l'ion enolate [CH 3COCHCOCH 3] − par condensation en phase gazeuse

The formation of [CH 3COCHCOCH 3] − enolate anion by a condensation reaction between acetone and its enolate anion under high pressure chemical ionization conditions is described.