Negative ion electrospray mass spectrometry of nucleotides: ionization from water solution with SF6 discharge suppression

The total current and selected ion currents from the electrospray ionization (ES1) of 10^?5 M solutions of cocaine hydrochloride and deoxycytidine monophosphate (dCMP) monosodium salt in methanol and water solvents were compared in positive and negative ion modes, respectively, without and with SF₆, gas as a discharge suppressant. The ESI onset voltages (V_on), were the same for the positive and negative ion modes. The V_on, for methanol was much lower than that for water and in agreement with the equation of D. P. H. Smith, who attributes the difference to the higher surface tension of water. The onset of electric discharge (V_dis) without SF₆, occurred at lower capillary voltages for the negative relative to the positive ion mode for methanol; but V_dis is much higher than V_on for methanol, and discharges do not interfere with ESI operation. For water, V_on ≈ V_dis in the absence of SF₆, in the negative ion mode, and ESI operation is impossible without SF₆, discharge suppression. The discharge problem in the positive ion mode is less severe, but SF₆, is still very useful. A dynamic range of 10 ^?7–10^?5 M was obtained by selected ion monitoring of [dCMP - H]^? at 4.5 and 20 μL/min. flows. Subpicomole detection limits for the nucleotide salt were obtained under these conditions.     

Electrospray mass spectrometry of methanol and water solutions suppression of electric discharge with SF6 gas

An equation by D. P. H. Smith predicts the capillary voltage required for the onset of electrospray (ES). For different solvents the voltage increases with the square root of the surface tension. Water requires a potential that is 1.8 times higher than that for methanol. This is verified experimentally. The higher potential required for water leads to ES in the presence of corona electric discharge. For low total ES plus corona currents, the electrosprayed analyte ion intensity is not adversely affected by the presence of discharge. At high total currents, there is a large decrease of analyte sensitivity. The sensitivity decrease is probably due to adverse space charge effect at high currents. The discharge can be suppressed by adding sulfur hexafluoride to the ambient gas. Both sensitivity and signal stability are improved. However, the sensitivity still remains lower by a factor of ≈ 4 relative to that observed with methanol. This is attributed to lower efficiency of gas-phase ion formation from charged water, relative to methanol, droplets.     

An electropneumatic‐heated nebulizer for enhancing spray ionization in PhotoSpray atmospheric pressure photoionization sources for liquid chromatography/mass spectrometry

We introduce a novel electropneumatic‐heated nebulizer (EPn‐HN), incorporating an electrified internal pneumatic nebulizer, to enhance the yield of sprayed ions from PhotoSpray? atmospheric pressure photoionization (APPI) sources for liquid chromatography/mass spectrometry (LC/MS). Spray ionization from the pneumatic‐heated nebulizers used in APPI sources provides a supplemental, complementary ionization method to be used for involatile and thermally labile compounds, otherwise intractable to APPI. Details of the construction and operation of the EPn‐HN device are provided. The performance of the EPn‐HN is demonstrated using two model compounds: substance P, a peptide used as a standard in studies of ion fragmentation mechanisms, and aztreonam, a thermally labile antibiotic. At the optimum voltage for spray ionization, improvements in sensitivity of two orders of magnitude are obtained relative to when the sprayer is grounded, the conventional case. Since both substance P and aztreonam cannot be detected using the APPI method alone, the results demonstrate how spray ionization from the EPn‐HN may be used to extend the range of compounds amenable to PhotoSpray sources. Copyright © 2009 John Wiley & Sons, Ltd.     

A sliding interface method for unsteady unstructured flow simulations

The objective of this work is to develop a sliding interface method for simulations involving relative grid motion that is fast and efficient and involves no grid deformation, remeshing, or hole cutting. The method is implemented into a parallel, node‐centred finite volume, unstructured viscous flow solver. The rotational motion is accomplished by rigidly rotating the subdomain representing the moving component. At the subdomain interface boundary, the faces along the interface are extruded into the adjacent subdomain to create new volume elements forming a one‐cell overlap. These new volume elements are used to compute a flux across the subdomain interface. An interface flux is computed independently for each subdomain. The values of the solution variables and other quantities for the nodes created by the extrusion process are determined by linear interpolation. The extrusion is done so that the interpolation will maintain information as localized as possible. The grid on the interface surface is arbitrary. The boundary between the two subdomains is completely independent from one another; meaning that they do not have to connect in a one‐to‐one manner and no symmetry or pattern restrictions are placed on the grid. A variety of numerical simulations were performed on model problems and large‐scale applications to examine conservation of the interface flux. Overall solution errors were found to be comparable to that for fully connected and fully conservative simulations. Excellent agreement is obtained with theoretical results and results from other solution methodologies. Copyright © 2006 John Wiley & Sons, Ltd.     

A spatial study of electron density and analyte emission in a d.c. argon plasma

Spatially resolved electron density measurements have been performed on a three-electrode d.c. plasma using a linear photodiode array based spectrometer. The electron density values measured are between 1× 10¹⁵ and 1 × 10¹⁶ cm^?3 depending on spatial position. The spatial distribution of Ca I (422.7 nm) and Ca II (393.4 nm) emission has also been measured and the Ca II-Ca I emission intensity ratio evaluated. Using the n_e values measured, an analagous LTE ratio has been calculated and this has been compared to the experimental values. Measured ratios are found to be from 28 to 100 times less than LTE ratios. Some possible sources leading to these infrathermal ratios are discussed.     

Direct determination of polycyclic aromatic hydrocarbons in solid matrices using laser desorption/laser photoionization ion trap mass spectrometry

The development and characterization of a new instrument for solid sampling which couples IR laser desorption followed by UV laser photo-ionization and analysis using an ion trap mass spectrometer has been investigated. For calibration, a new type of solid sample preparation involving activated charcoal as the solid substrate was used. This solid sample provided a steady signal for several thousand laser shots, which allowed optimization of the experimental procedure. It was found that both the IR and UV intensity and the delay between them play an important role in both the magnitude and type of signals observed. A method of gas phase accumulation with multiple laser shots was examined. Finally, this technique was demonstrated to be effective in providing direct qualitative information for N.I.S.T. SRM 1944 river sediment sample with no sample pre-treatment.     

The development of direct multicolour fluorescence cross-correlation spectroscopy: towards a new tool for tracking complex biomolecular events in real-time

Direct three-colour fluorescence cross-correlation spectroscopy can reveal interactions between three fluorescently labelled biomolecules, giving insight toward the complex events that constitute signal transduction pathways. Here we provide the optical and theoretical basis for this technology and demonstrate its ability to detect specific biological associations between nanoparticle-labelled DNA molecules.     

A fast mesh deformation method using explicit interpolation

A novel mesh deformation algorithm for unstructured polyhedral meshes is developed utilizing a tree-code optimization of a simple direct interpolation method. The algorithm is shown to provide mesh quality that is competitive with radial basis function based methods, with markedly better performance in preserving boundary layer orthogonality in viscous meshes. The parallelization of the algorithm is described, and the algorithm cost is demonstrated to be O(n log n). The parallel implementation was used to deform meshes of 100 million nodes on nearly 200 processors demonstrating that the method scales to large mesh sizes. Results are provided for a simulation of a high Reynolds number fluid–structure interaction case using this technique.