Biomedical subjects

L Nyholm

Publications and source records attributed to L Nyholm.

7 recordsLinked to original sources

PubMed2001

Evaluations of the stability of sheathless electrospray ionization mass spectrometry emitters using electrochemical techniques.

The processes that cause the failure of sheathless electrospray ionization (ESI) emitters, based on different kinds of gold coatings on fused-silica capillaries, are described and explained. The methods chosen for this study include electrochemical methods, ICPMS analysis of the electrolytes used, SEM studies, and electrospray experiments. Generally, the failure occurs by loss of the conductive coating. It is shown that emitters with sputter-coated gold lose their coatings because of mechanical stress caused by the gas evolution accompanying water oxidation or reduction. Emitters with gold coatings on top of adhesion layers of chromium and nickel alloy withstand this mechanical stress and have excellent durability when operating as cathodes. When operating as anodes, the adhesion layer is electrochemically dissolved through the gold film, and the gold film then flakes off. It is shown that the conductive coating behaves as a cathode even in the positive electrospray mode when the magnitude of a superimposed reductive electrophoretic current exceeds that of the oxidative electrospray current. Fairy-dust coatings developed in our laboratory (see Barnidge, D. R.; etal.Anal. Chem. 1999, 71, 4115-4118,) bygluing gold dust onto the emitter, are unaffected by the mechanical stress due to gas evolution. When oxidized, the fairy-dust coatings show an increased surface roughness and decreased conductivities due to the formation of gold oxide. The resistance of this oxide layer is however negligible in comparison with that of the gas phase in ESI. Furthermore, since no flaking and only negligible electrochemical etching of gold was found, practically unlimited emitter lifetimes may be achieved with fairy-dust coatings.

Journal Article↗

PubMed2001

Elimination of high-voltage field effects in end column electrochemical detection in capillary electrophoresis by use of on-chip microband electrodes.

The influence of the separation voltage on end column electrochemical detection (EC) in capillary electrophoresis (CE) has been investigated using an electrochemical detector chip based on an array of microband electrodes. It is shown, both theoretically and experimentally, that the effect of the CE electric field on the detection can be practically eliminated, without using a decoupler, by positioning the reference electrode sufficiently close to the working electrode. In the present study, this was demonstrated by using an experimental setup in which neighboring microband electrodes on a chip, positioned 30 microns from the end of the CE capillary, were used as working and reference electrodes, respectively. The short distance (i.e., 10 microns) between the working and reference electrode ensured that both of the electrodes were very similarly affected by the presence of the CE electric field. With this experimental setup, no significant influence of the CE voltage on the peak potentials for gold oxide reduction could be seen for CE voltages up to +30 kV. The detector noise level was also found to be reduced.

Journal Article↗

PubMed2001

A simple and robust conductive graphite coating for sheathless electrospray emitters used in capillary electrophoresis/mass spectrometry.

A graphite-polyimide mixture was used as a conductive coating for sheathless electrospray emitters. The coating procedure described is simple and inexpensive compared to previously described methods. An investigation of the stability of the conductive coating carried out by electrochemical methods revealed good performances during oxidative stress. In addition, no decrease in emitter performance was seen during continuous electrospray in the positive electrospray mode for two weeks. Fast capillary electrophoresis with attomole sensitivity demonstrated the excellent performance of the described sheathless interface when used in conjunction with an orthogonal time-of-flight mass spectrometer. The overall simplicity, stability and low cost of this type of sheathless emitter make the described approach highly suitable for any on-column coupling of low flow rate separation techniques to a mass spectrometer.

Journal Article↗

PubMed2000

Characterization of the atmospheric pressure ionization mass spectrometric process obtained using a fused-silica emitter with the high voltage applied upstream

The atmospheric pressure ionization process obtained when a mixture of methanol and water (90:10, v/v) also containing 50 microM sodium hydroxide is dispersed from a fused-silica emitter was studied. A combination of a high electric field and a nebulizer gas with the high voltage applied upstream in the liquid flow was utilized to facilitate the spray process. By comparing the dependences of the spray current and ion signals on the spray potential, it was found that electrical corona discharges were obtained for potentials higher than about 2.6 kV, which resulted in a mixed electrospray and chemical ionization process. By introducing vapour from a solvent, such as benzene or toluene, with a low ionization energy into the nebulizing gas, it was found that the appearance of the corresponding molecular ion was correlated with a change in the slope of the spray current-potential curve. This indicates that the breakpoints in the spray current-potential curves observed were correlated with the onsets of corona discharges. It was shown that the mixed ionization process gives rise to increased amounts of protonated solvent molecules and assists in the formation of sodiated adduct ions from an uncharged fatty acid methyl ester.

Journal Article↗

PubMed1999

End-column amperometric detection in capillary electrophoresis: influence of separation-related parameters on the observed half-wave potential for dopamine and catechol.

Capillary electrophoresis (CE) was coupled to a micro-electrode-based end-column amperometric detector. The influences of separation voltage, CE buffer concentration, and capillary-to-electrode distance on the observed hydrodynamic voltammetry of dopamine and catechol were studied using a separation capillary with an i.d. of 25 microns. It was found that an increased CE voltage, increased buffer concentration, or decreased capillary-to-electrode distance resulted in a positive shift of the observed half-wave potentials for both dopamine and catechol. At a constant separation current of 1.6 microA, the observed half-wave potential was found to increase with applied separation voltage. Furthermore, when experiments were carried out with a platinum quasi-reference electrode instead of a Ag/AgCl reference electrode, similar shifts in half-wave potential were observed. These results indicate that the observed shifts are an effect of the separation voltage rather than the separation current or a change in the reference potential. The characteristics of end-column detection with and without a fracture decoupler were compared. It was found that the effects of separation voltage, CE buffer concentration, and capillary-to-electrode distance were minimized by the use of a decoupling device. The observed half-wave potentials for dopamine and catechol were more positive when a CE capillary without a decoupler was employed compared to when a decoupler was used. Additionally, using the fracture decoupler, the observed half-wave potentials for both dopamine and catechol were approximately the same as when no CE voltage was applied (i.e., when the hydrodynamic voltammograms were recorded under flow injection conditions).

Catechols↗

PubMed1983

Immunochemical partial identity between two independently identified and isolated major allergens from Alternaria alternata (Alt-I and Ag 1).

Partially purified preparations of Alt-I, the main allergenic fraction of Alternaria alternata isolated by Yunginger, and of Ag 1, shown in crossed radioimmunoelectrophoresis (CRIE) to be the dominating major allergen of A. alternata (Løwenstein, Nyholm), were compared by tandem crossed immunoelectrophoresis (CIE), RAST inhibition, and the CRIE-related technique, single radial radioimmunodiffusion (SRRID). The two allergen preparations showed reaction of identity in tandem-CIE and indistinguishable specific IgE binding in CRIE and SRRID, regardless of antibodies and serum pools used. In RAST inhibition, the relative potencies of the allergen preparations and of the crude extracts correlated well with their Alt-I/Ag 1 content as estimated by rocket immunoelectrophoresis. Moreover, all inhibition curves were parallel, confirming identical IgE binding by Alt-I and Ag 1 with the serum pools used. A second preparation of Alt-I, isolated from another strain of Alternaria, showed reaction of partial identity with Ag 1 in tandem-CIE, indicating that different variants of Alt-I (Ag 1) may exist in different strains of A. alternata.

Allergens↗

PubMedDate not supplied

Benchmarking: a case report.

In mid 1993, administrators and physicians at Bristol Medical Center teams up with HCIA to perform clinical pathway analysis on five diagnosis related groups. The major goal of this project was to establish a partnership between the hospital administration and the medical staff to meet or beat existing benchmarks.

Critical Pathways↗