Formation and reactions of negative ions relevant to chemical ionization mass spectrometry. I. Cl mass spectra of organic compounds produced by F ⁻ reactions

Abstract

A systematic study of the negative-ion chemical ionization mass spectra produced by the reaction of F− with a wide variety of organic compounds has been accomplished. A time-of-flight mass spectrometer fitted with a modified high pressure ion source was employed for these experiments. The F− reagent ion was generated from CF3H or NF3, typically at an ion source pressure of 100 μm. In pure NF3, F− is the major ion formed and constitutes more than 90% of the total ion intensity. While F− is also the major primary ion formed in pure CF3H, it undergoes rapid ion-molecule reactions at elevated source pressures, yielding (HF)nF− (n = 1−3) ions, which makes CF3H less suitable as a chemical ionization reagent gas. Among the organic compounds investigated were carboxylic acids, ketones, aldehydes, esters, alcohols, phenols, halides, nitriles, nitrobenzene, ethers, amines and hydrocarbons. An intense (M − 1)− ion was observed in the F− chemical ionization mass spectra of carboxylic acids, ketones, aldehydes and phenols. Alcohols yield only (M + F)− ions upon reaction with F−. A weaker (M + F)− ion was also detected in the F− chemical ionization spectra of carboxylic acids, aldehydes, ketones and nitriles. The F− chemical ionization mass spectra of esters, halides, nitriles, nitrobenzene and ethers are characterized primarily by the ions, RCOO−, X−, CN−, NO2−, and OR−, respectively. In addition, esters show a very weak (M − 1)− ion (except formates). In the F− chemical ionization spectra of some aliphatic alkanes and o-xylene, a very weak (M + F)− ion was observed. Amines and aliphatic alkenes exhibit only insignificant fragment ions under similar conditions, while aromatic hydrocarbons, such as benzene and toluene are not reactive at all with the F− ion. The mechanisms of the various reactions mentioned are discussed, and several experimental complications are noted. In still other studies, the effects of varying several experimental parameters, including source pressure, relative proportions of the reagent and analyte, and other ion source parameters, on the observed chemical ionization mass spectra were also investigated. In a mixture of NF3 and n-butanol, for example, the ratio of the intensities of the ions characteristic of the alcohol to that of the (HF)nF− ion was found to decrease with increasing sample pressure, with increasing NF3 pressure, and with increasing electron energy. No significant effects on the spectra were observed to result from variation of the source repeller field or the source temperature. The addition of argon to the source as a potential moderator did not alter the F− chemical ionization spectrum significantly, but the use of oxygen appears to inhibit formation of the (HF)nF− cluster ion. The advantages of using F− as a chemical ionization reagent are discussed, and comparisons are made with other reagent ions.

Mass spectral fragmentation of the intravenous anesthetic propofol and structurally related phenols

“…Phenols are known to readily form negative ions in the gas phase, and there are several reports on their negative ion chemical ionization mass spectra [12][13][14]. One report on the fragmentation of phenolic anions in the gas phase is restricted to phenol only [12].…”

“…Thus, the loss of a methyl radical from the isopropyl side chain was expected to dominate the product ion spectrum of propofol. [12][13][14], which have a methyl as well as an ethyl group in the ␤ position, the most prominent product ion was formed by the loss of m/z 30, which may correspond to a loss of ethane rather than methane.…”

Exploring the adaptability and robustness of the central carbon metabolism of Mycoplasma pneumoniae

“…In humans, phosphatidylcholine (lecithin) is used as main nutritional source for one carbon metabolism (CH 3 ) 49 One carbon metabolism and its relation to nucleotide synthesis in cancer cells has been extensively studied 48 . It has been argued that in human cancer cells, glycolysis can produce enough energy for growth by diverting its flux to other metabolic pathways including one-carbon metabolism.…”

“…Two annotation techniques were used to measure metabolite abundance, [M+F] - ion and [M-H] - ion detection 48 . Some differences are present in the correlations between individual metabolites in the [M+F] - ion and [M-H] - data, however, clustering of samples for both detection techniques is highly similar.…”