We describe direct imaging of the densities of helium metastable atoms in the afterglow of a helium dielectric-barrier discharge (He-DBD) using collisionally assisted laser-induced fluorescence (LIF). For the conditions tested, comparison of fluorescence images of a He-DBD with analogous maps of emission from highly excited helium atoms revealed that helium metastable atom densities did not correlate well with emission from the plasma. Fluorescence images also showed that helium metastable atom densities increased substantially when a glass slide was placed 10.0 mm from the discharge capillary in a geometry typical for desorption-ionization experiments. We also studied the effect hydrogen has on the helium metastable atom densities. The hydrogen severely quenched the metastable state leaving it virtually undetectable. Emission was quenched as well, but to a lesser extent. The addition of 1% H(2) to the helium in the source provided nearly a factor of 2 improvement in the sensitivity of the signal for coumarin 47 when the plasma was used to ionize the dye under ambient conditions, despite the quenching of the helium metastable atom population.

We present mass spectrometric data demonstrating the effect that hydrogen has on a helium-based dielectric-barrier discharge (DBD) atmospheric-pressure plasma jet used as an ambient desorption/ionization (ADI) source. The addition of 0.9 % hydrogen to the helium support gas in a 35-W plasma jet increased signals for a range of test analytes, with enhancement factors of up to 68, without proportional increases in background levels. The changes in signal levels result from a combination of changes in the desorption kinetics from the surface and increased ion production in the gas phase. The enhancement in ADI-MS performance despite the quenching of key plasma species reported in earlier studies suggests that ionization with a H2/He plasma jet is the result of an alternate mechanism involving the direct generation of ionized hydrogen.

We present a mathematical description of the S/N ratio in a fluorescence-based protein detector for capillary electrophoresis that uses a pulsed UV laser at 266 nm as an excitation source. The model accounts for photobleaching, detector volume, laser repetition rate, and analyte flow rate. We have experimentally characterized such a system, and present a comparison of the experimental data with the predictions of the model. Using the model, the system was optimized for test analytes tryptophan, tyrosine, BSA, and conalbumin, producing detection limits (3σ) of 0.67 nM, 5.7 nM, 0.9 nM, and 1.5 nM, respectively. Based on the photobleaching data, a photobleaching cross section of 1.4×10 −18 cm 2 at 266 nm was calculated for tryptophan.

The effect of phospholipid, 1,2-Dipalmitoyl-sn-glycero-3-Phosphocholine (DPPC) on the spectroscopy of the cyanine dye, 1-ethyl-1’-octadecyl-2,2’-cyanine iodide (PIC-18), has been investigated using UV-Vis spectroscopy. Vesicles of DPPC containing PIC-18 in the molar ratio of 1:3 (dye/phospholipids) were prepared in aqueous solution. J-aggregates of PIC-18 were detected in the bilayer wall of the vesicles. When an aqueous solution of mixed PIC-18/DPPC vesicles is treated with excess DPPC vesicles that are prepared separately, the dye molecules in the mixed vesicles underwent a rapid (aggregate)n' n(monomer) equilibrium as the appearance of only one isosbestic point in the absorbance of the dye indicates. The equilibrium constant was calculated at room temperature (Keq = 6.7x10-2). An aggregation number of 4 was calculated for the dye in the bilayer vesicles.