Biomedical subjects

James P Murphy

Publications and source records attributed to James P Murphy.

2 recordsLinked to original sources

PubMed2004

Automated orthogonal control system for electrospray ionization.

Low-flow electrospray ionization is typically a purely electrostatic method, used without supporting sheath-gas nebulization. Complex spray morphology results from a large number of possible spray emission modes. Spray morphology may assume the optimal Taylor cone-jet spray mode under equilibrium conditions. When coupling to nanobore gradient elution chromatography, however, stability of the Taylor cone-jet spray mode is compromised by the gradient of mobile phase physiochemical properties. The common spray modes for aqueous/organic mobile phases were characterized using orthogonal (strobed illumination) transmitted light and (continuous illumination) scattered light imaging. Correlation of image sets from these complementary illumination methods provides the basis for spray mode identification using qualitative and quantitative image analysis. An automated feedback-controlled electrospray source was developed on a computer capable of controlling electrospray potential using an image-processing based algorithm for spray mode identification. The implementation of the feedback loop results in a system that is both self-starting and self-tuning for a specific spray mode or modes. Thus, changes in mobile phase composition and/or flow rate are compensated in real-time and the source is maintained in the cone-jet or pulsed cone-jet spray modes.

Angiotensins↗

PubMed2002

Effects of fragile ions on mass resolution and on isolation for tandem mass spectrometry in the quadrupole ion trap mass spectrometer.

In the quadrupole ion trap, it has been noted that factors other than an ion's mass and charge may affect its measured m/z, resulting in compound-dependent, or "chemical", mass shifts. We propose that ions can exhibit a chemical mass shift because they are "fragile" and may fragment during the application of resonance ejection during mass analysis; these effects were studied using ions that include protonated, deprotonated, and adduct ions of explosives, acylcarnitines, and macrolide antibiotics. Fragile ions affect mass resolution by causing broader peaks than nonfragile ions, especially at slower scan speeds, as the result of the application of resonance ejection. Fragile ions may also be fragmented by the application of the isolation waveform during selection of the parent ion for tandem mass spectrometry experiments, making it impossible to achieve unit isolation of a fragile ion. To obtain adequate isolation intensity, the isolation waveform notch width must be increased and the time period of isolation must be decreased. Fragile ions also require lower optimum collision energy to achieve efficient collision-induced dissociation. We have developed criteria for the determination of the degree of ion fragility based upon experimental results.

Anti-Bacterial Agents↗