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High-resolution EPR imaging of an ischemic rat heart
Representative 311C90 plasma profile after an oral dose of 2.5 mg indicating the LLOQs of both methods. (Adapted with permission from John Wiley & Sons.)
wells because there is less cross-well contaminationfromthe drain tips. For 311C90 and its desmethyl metabolite in human plasma, they were able to obtain a Mold decrease in sample preparation time, and LC/MS/MS provided a lower limit of quantitation (LLOQ) of 100 pg/mL They found that the batch processing approach to sample preparation works well in a high-throughput laboratory where dedicating one type of sample preparation system on-line with a mass spectrometer may cause scheduling problems. (Rapid Commun. Mass Spectrom. 1996,10,811-16.)
Thin membranes for FIA/MIMS Although membrane introduction MS (MIMS) systems are continually being improved, there is still a need for commercially available pervaporation membranes of appropriate material, selectivity, and thickness to improve performance and widen the scope of compounds that can be measured. However, decreasing the thickness means that more water is passing through the membrane, thereby decreasing the ratio of analyte to water in the permeate. In addition to causing dilution, too much water degrades the vacuum, which affects instrument performance. In ion trap analysis, some excess water is advantageous because chemical ionization can be performed using water as the reagent gas without sacrificing speed or detection limits. With the idea that thin membranes could facilitate ion trap analysis, R G. Cooks and colleagues at Purdue University and MIMS Technology investigated 464 A
been possible because of the rapid reduction of nitroxide to hydroxylamine. Jay L Zweier and co-workers at The Johns Hopkins Medical Institutions, the National Cancer Institute, and SynZyme Technologies have demonstrated that polynitroxyl-albumin can maintain the EPR signal intensity of the nitroxide 4-hydroxy-
Electron paramagnetic resonance imaging, the electronic-spin analog of NMR, shows great promise for biomedical imaging applications. (In order for a species to be EPR active, it must have at least one unpaired electron.) Stable nitroxyl radicals have been used as probes of biophysical and biochemical properties and as contrast agents for EPR imaging. Because the niFull (left) and cutaway high-resolution, three-dimensional EPR troxide EPR sigimages of an ischemic rat heart. nal reflects the local oxygen concentration and because nitroxides are 2,2,6,6-tetramethylpiperidine-l-oxyl in an preferentially reduced to a diamagnetic ischemic rat heart by reoxidizing the hyhydroxylamine derivative in oxygendroxylamine to nitroxide. They were able to depleted tissues, nitroxides can add meta- acquire high-resolution three-dimensional bolic information as they are providing EPR images of a rat heart during a 2.5-h image contrast. High-resolution images of period of prolonged cardiac ischemia. (Bioan ischemic heart have not previously chemistry 1996,35,7051-57.)
the selectivity and thickness of polydiits in the low part-per-billion range similar methylsiloxane-based membranes using to those obtained using a two-stage enon-line FIA/MIMS with electron impact in richment microporous membrane/jet sepan ion trap or triple quadrupole mass arator device. (Rapid Commun. Mass spectrometer. Spectrom. 1996,10, 751-56.) They used zeolite-filled, 10- to 50-um membranes and compared them in aqueous and organic (methanol) phases using a test mixture that consisted of methylene chloride, fra«s-l,2-dichloroethylene, benzene, toluene, chlorobenzene, carbon tetrachloride, and 1,1,2-trichloroethene. They observed no significant difference in the normal phase experiments, but in organic phase experiments, rise times and analysis times improved by a factor of two. This suggests that thin membranes may be useful in chemical and petrochemical on-line process monitoring in which sampling is done frequently. In ion trap experiments using wa- Performance of different types of membranes. Each colored bar represents one component of the test ter as the chemical ionization remixture. (Adapted with permission from John Wiley agent, they obtained detection lim- & Sons.)
Analytical Chemistry News & Features, August 1, 1996
