Application of matrix-assisted laser desorption/ionization to on-line aerosol time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization (MALDI) mass spectra were obtained from single biological aerosol particles using an aerosol time-of-flight mass spectrometer (ATOFMS). The inlet to the ATOFMS was coupled with an evaporation/condensation flow cell that allowed the aerosol to be coated with matrix material as the sampled stream entered the spectrometer. Mass spectra were generated from aerosol composed either of gramicidin-S or erythromycin, two small biological molecules, or from aerosolised spores of Bacillus subtilis var niger. Three different matrices were used: 3-nitrobenzyl alcohol, picolinic acid and sinapinic acid. A spectrum of gramicidin-S was generated from approximately 250 attomoles of material using a molar ratio of 3-nitrobenzyl alcohol to analyte of approximately 20:1. A single peak, located at 1224 Da, was obtained from the bacterial spores. The washing liquid and extract solution from the spores were analyzed using electrospray mass spectrometry and subsequent MS/MS product ion experiments. This independent analysis suggests that the measured species represents part of the B. subtilis peptidoglycan. The on-line addition of matrix allows quasi-real-time chemical analysis of individual, aerodynamically sized particles, with an overall system residence time of less than 5 seconds. These results suggest that a MALDI-ATOFMS can provide nearly real-time identification of biological aerosols. Copyright 2000 John Wiley & Sons, Ltd.     

Application of the Floquet theory to multiple quantum NMR of dipolar-coupled multi-spin systems under magic angle spinning

A new analytical Liouville-space representation of the time-propagator under magic angle spinning (MAS) is introduced using the formalized quantum Floquet theory. This approach has the advantage that it is applicable to the analysis of any type of NMR experiment where MAS is combined with multiple-pulse excitation. General relationships describing the spectral parameters in multiple-quantum (MQ) MAS spectra are derived in this representation. Their use is illustrated with an application to double-quantum (DQ) NMR spectra of dipolar-coupled multi-spin systems. Corresponding to the separation of the MAS time-propagator into a rotor modulated and a dephasing component, two distinct mechanisms for DQ excitation are identified. One of them exploits the rotor-modulated component to excite DQ coherences through dipolar-recoupling techniques, which are familiar for spin pairs. Analytical expressions of the integral intensities and linewidths in the resulting DQ sideband pattern are derived in the form of power series expansions of the inverse rotor frequency, of which coefficients depend on structural parameters. In a multi-spin system they can most reliably be extracted in the fast spinning regime. The other mechanism exploits the dephasing component, which is characteristic to multi-spin systems only. This is shown to give rise to DQ coherences by free evolution at full rotor periods. The possibility to exploit it for selective excitation of higher order MQ coherences is discussed. In either case, the dephasing component also leads to residual broadening. The main results of the theoretical developments are demonstrated experimentally on adamantane.     

A mathematical model of aging-related and cortisol induced hippocampal dysfunction

Background  

The hippocampus is essential for declarative memory synthesis and is a core pathological substrate for Alzheimer's disease (AD), the most common aging-related dementing disease. Acute increases in plasma cortisol are associated with transient hippocampal inhibition and retrograde amnesia, while chronic cortisol elevation is associated with hippocampal atrophy. Thus, cortisol levels could be monitored and managed in older people, to decrease their risk of AD type hippocampal dysfunction. We generated an in silicomodel of the chronic effects of elevated plasma cortisol on hippocampal activity and atrophy, using the systems biology mark-up language (SBML). We further challenged the model with biologically based interventions to ascertain if cortisol associated hippocampal dysfunction could be abrogated.     

Spatial profiles of electron density in the inductively coupled plasma

Spatially resolved, radial electron number density (n_e) profiles have been measured at rf power settings of 1.00, 1.25, 1.50, 1.75 and 2.00 kW, and vertical heights of 4, 8, 12, 16 and 20 mm above the load coil. These profiles have been condensed and presented as electron density contour plots for each input power. The precision of the method was evaluated by doing ten replicate measurements of electron density. The relative standard deviation varied between 2 and 10 % with the maximum at the plasma centre. Electron density was measured with and without the presence of the easily ionizable element—Cs, and no significant difference was observed.     

An evaluation of ion-atom emission intensity ratios and local thermodynamic equilibrium in an argon inductively coupled plasma

Spatially resolved, radial electron density (n_e) profiles have been measured at rf power settings of 0.75, 1.25 and 1.75 kW, and vertical heights of 4,8,12,16,20 and 24 mm above the load coil. These measured values of n_e have been used to construct a theoretical local thermal equilibrium (LTE) framework. Ion-atom emission intensity ratios for Mg and Cd calculated from this framework have been compared to experimentally measured values. The measured ion-atom emission intensity ratios are found to be (ess than calculated LTE ratios. This suggests that the aerosol channel of the inductively coupled plasma can be characterized as an ionizing plasma.     

Analyte ionization in the inductively coupled plasma

Spatially resolved ion-atom emission intensity ratios for Sr, Ca, Mg, Cd and Zn have been measured at rf power settings of 1.00, 1.25, 1.50, 1.75 and 2.0 kW at a vertical height of 16 mm above the load coil. Measured values of electron density have been used to construct a theoretical local thermal equilibrium (LTE) framework, and ion-atom emission intensity ratios calculated from this framework have been compared to experimentally measured values. The measured ion-atom emission intensity ratios were found to be within an order of magnitude of these calculated LTE ratios.The experimental degree of ionization for these five elements was determined for the various rf input powers. These values have been compared to the analagous LTE values. Both degree of ionization and departure from LTE were found to be strongly correlated with the ionization potential of the element.The radial spatial dependence of the degree of ionization for Cd at an rf power of 1.25 kW has been measured for aerosol flow rates of 0.6, 0.8 and 1.21 m⁻¹ for vertical heights of 4, 8, 12, 16 and 20 mm above the load coil. The spatial distribution of electron number density was measured at an rf power of 1.25 kW and at aerosol flow rates of 0.6, 0.8 and 1.21 m⁻¹ and a correlation between degree of ionization and electron density identified. Finally the relative concentration of Cd ions has been calculated from ion spatial emission profiles and plasma operating conditions which produce a maximum in the ion density identified.     

Excitation temperature and electron density in the inductively coupled plasma—aqueous vs organic solvent introduction

Using a photodiode array spectrometer, spatially resolved Fe I, excitation temperatures and electron densities have been measured for an ICP with both water and xylene solution introduction. The excitation temperature is lower in the ICP with organic aerosol than in the ICP with aqueous aerosol at a fixed power and height. For an ICP with organic aerosol the input power must be increased by approx. 0.5 kW to simulate the temperatures reached in an aqueous ICP.