Biomedical subjects

O J Schmitz

Publications and source records attributed to O J Schmitz.

6 recordsLinked to original sources

PubMed2005

Atmospheric-pressure laser ionization: a novel ionization method for liquid chromatography/mass spectrometry.

We report on the development of a new laser-ionization (LI) source operating at atmospheric pressure (AP) for liquid chromatography/mass spectrometry (LC/MS) applications. APLI is introduced as a powerful addition to existing AP ionization techniques, in particular atmospheric-pressure chemical ionization (APCI), electrospray ionization (ESI), and atmospheric pressure photoionization (APPI). Replacing the one-step VUV approach in APPI with step-wise two-photon ionization strongly enhances the selectivity of the ionization process. Furthermore, the photon flux during an ionization event is drastically increased over that of APPI, leading to very low detection limits. In addition, the APLI mechanism generally operates primarily directly on the analyte. This allows for very efficient ionization even of non-polar compounds such as polycyclic aromatic hydrocarbons (PAHs). The APLI source was characterized with a MicroMass Q-Tof Ultima II analyzer. Both the effluent of an HPLC column containing a number of PAHs (benzo[a]pyrene, fluoranthene, anthracene, fluorene) and samples from direct syringe injection were analyzed with respect to selectivity and sensitivity of the overall system. The liquid phase was vaporized by a conventional APCI inlet (AP probe) with the corona needle removed. Ionization was performed through selective resonance-enhanced multi-photon ionization schemes using a high-repetition-rate fixed-frequency excimer laser operating at 248 nm. Detection limits well within the low-fmol regime are readily obtained for various aromatic hydrocarbons that exhibit long-lived electronic states at the energy level of the first photon. Only molecular ions are generated at the low laser fluxes employed ( approximately 1 MW/cm(2)). The design and performance of the laser-ionization source are presented along with results of the analysis of aromatic hydrocarbons.

Atmospheric Pressure↗

PubMed2001

Decrease in concentration of free catechins in tea over time determined by micellar electrokinetic chromatography.

Cancer chemoprevention is a new and important medical science and much interest has been focused on catechins, not only for their antioxidant activity, but also because of their known antimutagenic and antitumorigenic properties. Green tea and black tea, which are among the most popular beverages consumed worldwide, contain many different catechins. Due to the instability of catechins in solutions with neutral or basic pH values the concentrations of catechins in tea decrease in time. In this presentation we used micellar electrokinetic chromatography to determine the real concentration of catechins between 0 and 60 min after the tea was brewed.

Catechin↗

PubMed2000

Synthesis of fluorescently labeled alkylated DNA adduct standards and separation by capillary electrophoresis.

DNA adducts are regarded as individual internal dosimeters for the exposure to chemical carcinogens. To date, the most sensitive method for DNA adduct analysis is the radioactive 32P-postlabeling method, which allows the detection of one adduct in 10(10) unmodified nucleotides in microg amounts of DNA. However, this technique suffers from disadvantages such as working with radioactive phosphorus and time-consuming chromatographic separation procedures. In addition, the simultaneous detection of adducts from different classes of carcinogens in a DNA sample is difficult. In order to overcome these drawbacks, we are developing a new detection method, comprising fluorescence labeling of DNA adducts, capillary electrophoretic (CE) separation, and on-line detection by monitoring laser-induced fluorescence (LIF). So far, we have evaluated the separation power and the detection limit of CE with fluorescently labeled standard compounds such as unmodified nucleotides or alkylated thymidines. For this purpose, we developed a universal method for labeling 5'-OH-mononucleosid-3'-dicyanoethyl-phosphates with fluorescent dyes based on the phosphoramidite technology for DNA synthesis. The separation of N3-methylated, N3-, O2- and O4-butylated thymidines from the unmodified nucleotide within a few minutes recommends CE-LIF as a powerful method for DNA adduct analysis.

Alkylation↗

PubMed2000

Analysis of catechins and caffeine in tea extracts by micellar electrokinetic chromatography.

Cancer chemotherapy is a new and important medical science and much interest has been focused on catechins, not only for their antioxidant activity, but also because of their known antimutagenic and antitumorigenic properties. Green tea and black tea, which are among the most popular beverages consumed worldwide, contain many different catechins. We developed an analytical method capable of separating six different catechins and caffeine in tea by micellar electrokinetic chromatography in only 20 min without extensive sample preparation. Furthermore, we compared the amount of catechins and caffeine in several teas and different preparation modes.

Caffeine↗

PubMed1997

Experimental evidence for a behavior-mediated trophic cascade in a terrestrial food chain.

Predators of herbivorous animals can affect plant populations by altering herbivore density, behavior, or both. To test whether the indirect effect of predators on plants arises from density or behavioral responses in a herbivore population, we experimentally examined the dynamics of terrestrial food chains comprised of old field plants, leaf-chewing grasshoppers, and spider predators in Northeast Connecticut. To separate the effects of predators on herbivore density from the effects on herbivore behavior, we created two classes of spiders: (i) risk spiders that had their feeding mouth parts glued to render them incapable of killing prey and (ii) predator spiders that remained unmanipulated. We found that the effect of predators on plants resulted from predator-induced changes in herbivore behavior (shifts in activity time and diet selection) rather than from predator-induced changes in grasshopper density. Neither predator nor risk spiders had a significant effect on grasshopper density relative to a control. This demonstrates that the behavioral response of prey to predators can have a strong impact on the dynamics of terrestrial food chains. The results make a compelling case to examine behavioral as well as density effects in theoretical and empirical research on food chain dynamics.

Journal Article↗

PubMed1994

Resource edibility and trophic exploitation in an old-field food web.

I tested a food web model that predicts how environmental productivity (nutrient supply) and top carnivores should mediate interactions among herbivores, edible plants, and plants that are resistant to herbivory because they possess anti-herbivore defenses. Feeding trials with the dominant grasshopper herbivore at the study site confirmed that certain plant species were resistant to herbivory because of protection by pubescent leaves and stems. Experimental food webs with various numbers of trophic levels composed of edible and resistant plants, grasshoppers, and hunting spiders were assembled in enclosure cages. I randomly assigned half of the cages to a nutrient-enrichment treatment and half remained as a control. Nutrient supply directly enhanced primary productivity and plant and herbivore biomass. Experimentally changing spider abundance caused a classic "trophic cascade" in which herbivore biomass increased and edible plant biomass decreased. Resistant plant biomass increased. These results matched predictions of the model with one exception. A trophic cascade was not observed under enriched conditions. The study nevertheless shows that a simple model attempting to explain heterogeneous interactions in food webs may give considerable insight into the dynamics of natural systems.

Journal Article↗