Michael J. Dale

Investigation of Porphyrins and Metalloporphyrins Using Two-Step Laser Mass Spectrometry

Laser desorption time-of-flight mass spectrometry with laser postionization at 193 nm has been used to study a series of selected porphyrins and metalloporphyrins. Molecular ions are predominantly formed under soft ionization conditions, with extensive fragmentation of the peripheral substituents being observed under hard ionization conditions. Cu-octaethylporphyrin is found to exhibit anomalous demetallation at this ionization wavelength. At high ionization laser fluences it is also possible to observe extensive fragmentation of the porphyrin macrocycle. Much of the observed fragmentation can be understood with reference to previous studies using electron impact (EI) ionization. The stability of the porphyrin nucleus allows the fragmentation of the side chains to be examined. As in earlier EI studies dehydration products are observed in the mass spectra of hematoporphyrin IX. These products are shown to be thermally induced during infrared laser desorption, rather than fragments arising from photoionisation. The flexibility of this two-step laser mass spectrometric approach has allowed the analysis of chlorophyll a directly from an organic matrix. The data obtained suggest that information concerning the interaction between adsorbate and substrate can be extracted.

Direct determination of polycyclic aromatic hydrocarbons in environmental matrices using laser desorption laser photoionization time-of-flight mass spectrometry

A series of environmental samples contaminated with polycyclic aromatic hydrocarbons (PAHs) were analysed using laser desorption laser photoionization time-of-flight mass spectrometry (L2TOFMS). Laser desorption removes the requirement for any sample pre-treatment and selective laser photoionization results in mass spectra containing signals corresponding principally to the contaminant PAH species. The methodology is based on the spatial and temporal separation of these evaporation and ionization steps. The instrument used is able to perform rapid, selective sample screening of contaminated soils, engine oil, coal soot and coal-tar creosote. The resulting fingerprint mass spectra indicate both the presence and composition of the PAHs in the matrices with instrumental detection limits in the sub-ppm range.

Birch reduction of C60—a new appraisal

Contrary to a previous report that Birch reduction of C60 affords C60H36 as the principal product, laser desorption-laser photoionisation time-of-flight (L2TOF), laser desorption Fourier transform ion cyclotron resonance (FTICR), and liquid secondary ion mass spectrometry (LSIMS) show collectively that a mixture of polyhydrofullerenes, containing C60H18 through to C60H36 with a skewed distribution centred on C60H32 is formed, the discrepancy in results arising from the thermal lability of this mixture of polyhydrofullerenes when subjected to the elevated temperatures (>250 °C) required for mass spectroscopic studies using direct-insertion heated probes.

Delayed ionization of laser desorbed C60 molecules following 193 nm photoexcitation

Delayed ionization of C60 on the microsecond timescale has been investigated following 193 nm excitation of infrared laser desorbed molecules. Biexponential decay of the delayed ion signal indicates the autoionization of two kinetically distinct superexcited states. The lifetimes of these states were determined to be 5 ± 1 μs and 0·8 ± 0·2 μs; their excitation energies are estimated to lie in the range 20–30 eV. These results are compared with the predictions of the Richardson-Dushman and Klots models of thermionic emission. The observation of a dependence of delayed ion intensity on desorption conditions is interpreted in terms of a thermally induced phase change during the desorption process.

DIRECT ANALYSIS OF POLYCYCLIC AROMATIC HYDROCARBONS IN CLOUD-WATER AEROSOL FILTRATES USING LASER DESORPTION MASS SPECTROMETRY

A novel technique for the rapid screening of polyaromatic hydrocarbons (PAHs) in atmospheric particulates is presented. Two-step laser desorption laser photoionisation time-of-flight mass spectrometry was used to assay for PAHs in cloud-water particulates collected near Peebles in southern Scotland. The particulates were examined in situ on their host filters removing the requirement for time-consuming sample extraction and separation. The mass spectra obtained from a single filter are shown. The principal components of these are assigned to PAH contaminants and phthalate contaminants. A distribution of PAHs was observed using 193 nm laser photoionisation. The mass spectrum obtained on using 248 nm laser photoionisation exhibits a different intensity distribution demonstrating an ionisation wavelength dependence. The utility of the technique for rapid screening of PAHs and other polyaromatic species is discussed.

Photoionisation and photodissociation of laser-vaporised metallotetraphenylporphyrins

Abstract The photoionisation and photodissociation processes induced by 193 and 266 nm excitation of a series of metallotetraphenylporphyrins (NiTPP, ZnTPP, CoTPP, CuTPP, and VOTPP) have been investigated, using laser-desorption laser-photoionisation time-of-flight mass spectrometry. At 193 nm, all of the molecules undergo molecular photoionisation followed by photofragmentation (class A behaviour). At 266 nm, ZnTPP and CuTPP exhibit class A behaviour, whereas NiTPP, CoTPP and VOTPP undergo neutral photodissociation followed by ionisation (class B behaviour). Neutral photodissociation results in clevage of the coordinated metal from the macrocycle, whereas photodissociation of the molecular ion leads to loss of phenyl substituents from the macrocycle without loss of the metal atom.

Laser-desorption mass spectrometry of [Ru6C(CO)17] and its derivatives: cluster aggregation in the gas phase

The ultraviolet-laser-desorption mass spectra of [Ru6C(CO)17], [Ru6C(CO)14(η6-C6H5Me)] and [Ru6C(CO)12(η6-C6H4Me 2)(µ-C6H7Me)] have been obtained. Positive-ion mass spectra showed abundant singly-charged species: peaks corresponding to the successive loss of carbonyl ligands from an otherwise intact parent cluster and high molecular-weight clusters formed from the aggregation of intact hexaruthenium cores.

Characterisation of ruthenium clusters carrying facial arene ligands using ultraviolet laser desorption mass spectrometry: aggregation of gas-phase monocharged anionic clusters from neutral molecular clusters

The ultraviolet laser desorption mass spectra of [Ru4(CO)9{µ3-η2:η2:η2-C6-C4(η2-CMeCH2)2-1,3}] and [Ru6C(CO)14(µ3-η2:η2:η2-C16H16)] show abundant singly-charged negative ions; peaks corresponding to successive loss of carbonyl ligands from an otherwise intact parent cluster along with a remarkable series of higher mass species formed in the source region of the mass spectrometer are seen.