Impact of environmental oxygen on nanoparticle formation and agglomeration in aluminum laser ablation plumes.

The role of ambient oxygen gas (O2) on molecular and nanoparticle formation and agglomeration was studied in laser ablation plumes. As a lab-scale surrogate to a high explosion detonation event, nanosecond laser ablation of an aluminum alloy (AA6061) target was performed in atmospheric pressure conditions. Optical emission spectroscopy and two mass spectrometry techniques were used to monitor the early to late stages of plasma generation to track the evolution of atoms, molecules, clusters, nanoparticles, and agglomerates. The experiments were performed under atmospheric pressure air, atmospheric pressure nitrogen, and 20% and 5% O2 (balance N2), the latter specifically with in situ mass spectrometry. Electron microscopy was performed ex situ to identify crystal structure and elemental distributions in individual nanoparticles. We find that the presence of ≈20% O2 leads to strong AlO emission, whereas in a flowing N2 environment (with trace O2), AlN and strong, unreacted Al emissions are present. In situ mass spectrometry reveals that as O2 availability increases, Al oxide cluster size increases. Nanoparticle agglomerates formed in air are found to be larger than those formed under N2 gas. High-resolution transmission electron microscopy demonstrates that Al2O3 and AlN nanoparticle agglomerates are formed in both environments; indicating that the presence of trace O2 can lead to Al2O3 nanoparticle formation. The present results highlight that the availability of O2 in the ambient gas significantly impacts spectral signatures, cluster size, and nanoparticle agglomeration behavior. These results are relevant to understanding debris formation in an explosion event, and interpreting data from forensic investigations.

A design for a compact time-of-flight mass spectrometer.

The design of a prototype, compact time-of-flight (TOF) mass spectrometer is described. The system primarily consists of an ion acceleration∕focusing∕steering assembly (AFSA), an 8 cm field-free region, a 4 cm dual-stage reflectron, and a miniature microchannel plate detector. Consequently, the resulting flight length of the system is 12 cm. The system has been designed with the capability to sample directly from atmosphere at ambient pressures. This is accomplished through the use of an electrodynamic ion funnel, housed in an intermediate-vacuum chamber that is coupled to the inlet of the TOF chamber. TOF spectra were obtained using noble gases (Ar, Kr, and Xe) as test chemicals. These measured flight times were used to probe the performance of the instrument. A temporal resolution (t(flight)∕Δt) of approximately 125, acquired using (129)Xe(+), has been measured for the system.

Probing shapes of bichromophoric metal-organic complexes using ion mobility mass spectrometry.

Ion mobility mass spectrometry (IM-MS) was used to probe the structures of several metal complexes carrying pendant chromophores. The three complexes investigated were the copper(II) complex Cu(DAC)2+ (DAC = 1,8-bis(9-methylanthracyl)cyclam, cyclam = 1,4,8,11-tetraazacyclotetradecane), the N-nitrosylated ligand DAC-NO, and the Roussin's red salt ester (mu-S,mu-S')-protoporphyrin-IX-bis(2-thioethyl ester)tetranitrosyldiiron (PPIX-RSE). From the IM-MS data coupled with theoretical calculations, it was found that [Cu(II)(DAC - H)]+ exists as a single conformer, with one anthracenyl group above the cyclam and the other below, similar to the crystal structure of Cu(II)(DAC)2+. The metal-free N-nitrosylated ligand (DAC-NO + H)+ has two conformations: one family of structures has one anthracenyl group above the cyclam and one below, while the other has both anthracenyl groups on the same side of the cyclam. These observations are consistent with 1H NMR data for the neutral DAC-NO complex that indicate the presence of two geometric isomers in solution. The third species, PPIX-RSE, has a porphyrin chromophore covalently linked to an Fe2S2(NO)4 cluster for use as a precursor for the photochemical delivery of nitric oxide in single- and two-photon excitation processes. Ion mobility indicates the presence of two (PPIX-RSE + H)+ conformations, consistent with the previous interpretation of the bimodal fluorescence lifetime decay seen for PPIX-RSE. DFT structures, in good agreement with the IM-MS cross sections, indicate two "bent" conformations with the planes of the porphyrin and Fe2S2 rings at different angles with respect to each other.

Probing the structure of gas-phase metallic clusters via ligation energetics: sequential addition of C2H4 to Agm+ (m=3-7).

A new method that utilizes ligation to probe geometries of clusters in the gas-phase has been developed. This technique bases structural assignments on thermodynamic quantities obtained for sequential ligand additions to a bare cluster. The information is obtained from temperature-dependent equilibrium data. This method is also sensitive to changes in bare cluster conformations that occur as a result of ligand addition, and the results can be fine-tuned by choosing ligands that appropriately adjust cluster-ligand binding energies.

Structural analysis of metal interactions with the dinucleotide duplex, dCG x dCG, using ion mobility mass spectrometry.

The metal binding properties of the dinucleotide duplex, dCG x dCG, were analyzed in the gas phase with ion mobility mass spectrometry. Both MALDI and ESI were used to generate [M(dCG x dCG)]+ complexes. The collision cross section of each complex was measured in helium using ion mobility based methods and compared to calculated cross sections of theoretical structures. When metal cations classified as hard acids were combined with dCG x dCG, the [M(dCG x dCG)]+ complex organized into a globular structure. However, when soft acid metal cations were examined, a structure was observed where the two C-G base pairs were Watson-Crick bound.