An overview of organic molecule soft ionization using vacuum ultraviolet laser radiation

Abstract

The utility of coherent vacuum ultraviolet (VUV) single-photon ionization (SPI) combined with time-of-flight mass spectrometry (TOF-MS) for organic molecule detection by parent mass is explored in this short review. Nonresonant tripling in phase-matched Xe-Ar gas mixtures was used to generate photons at a fixed energy of 10.5 eV. Representative organic molecules with different functional groups were examined, including aliphatic and aromatic alkanes, alkenes, alkynes, alkanols, ethers, amines, aldehydes, ketones, carboxylic acids, and esters. In almost every case, the intensity of the resultant parent molecular ion peak detected by TOF-MS was found to be superior to that obtained using 70 eV electron impact (EI), and comparable to that obtained with 12 eV EI. In those instances when fragmentation reactions did occur, the resultant ions were similar to those found using EI but with significantly reduced mass spectral intensities. It was still possible to establish one dominant fragmentation pathway that could be used for molecular identification even if the parent molecular ion was not the strongest feature in the spectrum, for example, in the case of alcohols, alcohol clusters, and alcohol-ether adducts. Several of the fragment ions were metastably broadened. Not surprisingly, their known appearance energies or estimated reaction enthalpies were very similar to the fixed photon energy used. The success of using VUV for organic molecule soft ionization is attributed to the low photon energy that removes predominantly a π-or non-bonding electron from the functionalized species. As most organic compounds have ionization potentials in the 10.5 eV region, this approach is expected to be near universal. Résumé :Dans cette courte revue, on examine l'utilité d'une ionisation par un photon unique dans l'ultraviolet lointain cohérent combinée avec un spectromètre de masse à temps d'envol (SM-TDE) pour la détection de molécules organiques à l'aide de la masse parente. Le triplage non résonant de mélanges de gaz Xe-Ar en phase a été utilisé pour gé-nérer des photons d'énergie fixe de 10,5 eV. Des molécules organiques représentatives portant divers groupes fonctionnels ont été examinées, y compris des alcanes, des alcènes, des alcynes, des alcools, des éthers, des amines, des aldéhydes, des cétones, des acides carboxyliques et des esters aliphatiques et aromatiques. Dans pratiquement tous les cas, il a été trouvé que l'intensité du pic de l'ion moléculaire parent qui en résulte telle que détectée par SM-TDE est supérieure à celle obtenue en utilisant un impact électronique (IE) de 70 eV, et qu'elle est comparable à celle obtenue avec un IE de 12 eV. Dans les cas où des réactions de fragmentation se sont produites, les ions qui en résultent sont semblables à ceux observés avec un IE, mais les intensités des masses spectrales sont de beaucoup réduites. Il est toujours possible d'établir une voie de fragmentation dominante qui pourrait être utilisée pour l'identification molécu-laire, même si l'ion moléculaire parent n'est pas l...

Geometries and energetics of methanol–ethanol clusters: a VUV laser/time-of-flight mass spectrometry and density functional theory study

“…The neutral mixed cluster binding energies were decomposed into the terms representing the many-body interactions (see eqs. [5][6][7][8]) to understand the size-dependence trend found here for the incremental binding energy. Table 3. shows the average binding energies and their components calculated by DFT (without ZPE corrections) for the alcohol clusters ranging from the dimer to the pentamer.…”

“…< n E > are the n-body terms defined in eqs. [5][6][7][8]. E c corresponds to the non-additive terms.…”

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