A Cryotrap Membrane Introduction Mass Spectrometry System for Analysis of Volatile Organic Compounds in Water at the Low Parts-per-Trillion Level.

Detection of volatile organic compounds (VOCs) in aqueous solution at low parts-per-trillion (ppt) levels is accomplished using a very simple and efficient on-line preconcentration cryotrap membrane introduction mass spectrometry (CT-MIMS) system. The conventional MIMS probe is modified so that the membrane interface is placed about 15 cm away from the ion source. A U-shaped trap tube is then inserted between the membrane interface and the ion source. Cryotrapping is performed with liquid nitrogen for 15 min, followed by fast heating at ∼15 °C s(-)(1), which thermally releases the condensed VOCs almost at once into the ion source region of a quadrupole mass spectrometer. By applying electron ionization and a selective ion monitoring scan mode, a very sharp and intense peak is obtained. The performance of the CT-MIMS system was compared with that of conventional MIMS, and after reaching the best conditions for the trapping and heating cycles, an improvement factor in signal intensity of about 100 was observed for a series of VOCs. The extraordinary sensitivity of CT-MIMS system allows VOCs to be detected at very low concentrations, detection limits being typically on the order of 10-20 ppt. The results also show excellent linearity and reproducibility for the system.

Multiple stage pentaquadrupole mass spectrometry for generation and characterization of gas-phase ionic species. The case of the PyC2H 5 (+·) isomers.

Eleven isomers with the PyC2H 5 (+·) composition, which include three conventional (1-3) and eight distonic radical cations (4-11), have been generated and in most cases successfully characterized in the gas phase via tandem-in-space multiple-stage pentaquadrupole MS(2) and MS(3) experiments. The three conventional radical cations, that is, the ionized ethylpyridines C2H5-C5H4N(+·) (1-3), were generated via direct 70-eV electron ionization of the neutrals, whereas sequences of chemical ionization and collision-induced dissociation (CID) or mass-selected ion-molecule reactions were used to generate the distonic ions H2C(·)-C5H4N(+)-CH3 (4-6), CH3-C5H4N(+)-CH 2 (·) (7-9), C5H5N(+)-CH2CH 2 (·) (10), and C5H5N(+)-CH(·)-CH3 (11). Unique features of the low-energy (15-eV) CID and ion-molecule reaction chemistry with the diradical oxygen molecule of the isomers were used for their structural characterization. All the ion-molecule reaction products of a mass-selected ion, each associated with its corresponding CID fragments, were collected in a single three-dimensional mass spectrum. Ab initio calculations at the ROMP2/6-31G(d, p)//6-31G(d, p)+ZPE level of theory were performed to estimate the energetics involved in interconversions within the PyC2H5 (+·) system, which provided theoretical support for facile 4⇌7 interconversion evidenced in both CID and ion-molecule reaction experiments. The ab initio spin densities for the a-distonic ions 4-9 and 11 were found to be largely on the methylene or methyne formal radical sites, which thus ruled out substantial odd-spin derealization throughout the neighboring pyridine ring. However, only 8 and 9 (and 10) react extensively with oxygen by radical coupling, hence high spin densities on the radical site of the distonic ions do not necessarily lead to radical coupling reaction with oxygen. The very typical "spatially separated" ab initio charge and spin densities of 4-11 were used to classify them as distonic ions, whereas 1-3 show, as expected, "localized" electronic structures characteristic of conventional radical ions.