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ANALYTICAL CURRENTS
Nonthrombogenic sensors
adhesion—are significantly lower than those of "NO-free" sensors. The NO-releasing compound used in the experiments was an A^A^-dimethylhexanediamine nitric oxide (DMHD/N2Oj) adduct, a diazeniumdiolate species that spontaneously generates NO in aqueous solutions. The compound was incorporated as a fine disper-
sion into either polyurethane or high molecular weight poly (vinyl chloride) membranes used to construct ion-selective electrodes for H+ and K+. Release of NO was confirmed by One of the "Holy Grails" of analytical chemmeasuring the formation of nitrite by flowistry is the development of implantable injection analysis. The electrodes with or chemical sensors that continuously meawithout the diazeniumdiolate compound sure physiologically important ions (such were tested against platelet-rich sheep as H+ and K+) and gases (such as C0 2 and 0 2 ) in blood. So far, plasma. The devices dischemists have had limited played comparable analytisuccess because these cal performance, but the devices can be thromboNO-releasing sensors had genic and cause vasoconfewer adhered platelets. striction of arteries. Also, the platelets were not activated, as measured by Mark Meyerhoff and the lack of pseudopod exhis colleagues at the Unitensions. Although the poversity of Michigan attential toxicity of this aptacked this problem by proach needs to be investiincorporating a nitric gated the authors calculate oxide-releasing compound that the NO released from into their sensors. NO is a an implanted catheter made potent platelet antiaggregafrom these new films would tion and vasodilation agent be far less than the 1 mol or They report that when low more of NO produced endolevels of NO are emitted, genouslv each dav in huthe thrombogenic properScanning electron micrographs of sensing membranes (A) without and (B) with mans (l.Am Chem Soc ties of the sensor as meaDMHD/N202, showing the difference in platelet adhesion. Top images 1997 119 2321-22) sured by in vitro platelet represent a five-fold increase in magnification of the boxed area on the bottom.
Measuring the carbo load of proteins
A 300-fold increase in sensitivity. (A) MALDI mass spectrum of the underivatized oligosaccharide A2F (disialylated, galactosylated, ,iantennary, core substituted with fucose). (B) MALDI mass spectrum of derivatized A2F. (Adapted with permission from the National Academy of Sciences.)
Many proteins are more than just an amino acid sequence, and a complete characterization of any protein would ideally include the structure of any posttranslational modifications that might affect its behavior. Unfortunately, the structural elucidation of carbohydrates, a moiety frequently found in modified proteins, often proves daunting. Brian T. Chait and co-workers at Rockefeller University and the Scripps Research Institute increased the sensitivity of MS measurements of oligosaccharides by ligating them to an aminooxyacetyl form of a basic peptide. The resulting glycopeptide was digested by an exoglycosidase array, and the digestion products were analyzed by MALDI-MS.
For the glycopeptides, the strongest ion signals were observed using saturated a-cyano-4-hydroxycinnamic acid matrix. The underivatized oligosaccharides were ionized most effectively with a matrix that was a mixture of 2,5-dihydroxybenzoic acid and 1-hydroxy isocarbostyril. The sensitivity of die mass spectrometric measurement increased anywhere from 50- to 1000-fold for derivatized relative to underivatized oligosaccharides because the saccharides were easier to ionize and the mass was in a range better suited for MALDI-MS analysis. Sensitivities for the derivatized oligosaccharides are in the low femtomole range This derivatization method represents an improvement over conventional oligosaccharide sequencing because it provides higher resolution, tolerates oligosaccharide heterogeneity, and can be performed without complete digestion. (Proc. Natl. Acad. Sci. USA 1997, 94, 1629-33)
Analytical Chemistry News & Features, May 1, 1197 277 A
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Schematic of the experimental apparatus. (Adapted with permission from MacMillan Magazines.)
Uniform elemental analysis Although surface techniques such as secondary-ion MS can detect elements at ppb levels, they are typically semiquantitative. Chun He and colleagues from SRI International described a surface technique that provides highly uniform, quantitative, and sensitive analysis, even when the bulk material contains elements with different ionization potentials. The method uses a steady-state 5-keV argon ion beam to sputter material from a substrate's surface and a high-intensity 2.3-eV (532-nm) laser to induce multiphoton ionization of the sputtered atoms. The photoions were then measured by TOFMS. Steady-state sputtering, defined as a pulse duration of > 1 us, avoids preferential sputtering and ensures sampling over all velocity components of the sputtered material. The photoionizing laser beam passes parallel to, and < 1 mm above, the sample surface and subtends a large solid angle. Quantification is based on flux, which is equal to the number density of atoms and molecules times the velocity. The authors demonstrated the method by analyzing GaAs and SiC single crystals. (Nature 1997,385, 797-99)
Electrons get cool negative ions are simultaneously held in the Penning trap, the analytes can Electron-impact ionization MS is usually the method of choice for detecting be detected by FT-ICRMS in the and identifying environmentally impor- same trap. tant volatile compounds. However, The Florida group used a 3-T magEI-MS leads to extensive fragmentanetic field in these experiments and tion, which reduces the molecular ion formed negative ions of polycyclic aroabundance and makes it difficult to matic hydrocarbons with electron affindistinguish between primary and fragities as low as 0.45 eV (pyrene). ment ions in complex mixtures. For Among the advantages of this apmolecules with high electron affinities, proach, say the authors, is that it negative-ion or electron-capture MS avoids the use of the reagent gases (EC-MS) is an alternative approach needed in other EC-MS approaches, that offers selective and sensitive dethe cold electrons also cool the negative ions in the trap by sympathetic tection even for small amounts of cooling, and the negative ions form compounds. One drawback is that EC-MS requires low-energy electrons, only near the center of the trap and produce a compact ion packet that opwhich will attach themselves to t?astimizes the FT-ICR experiment. (J. Am. phase neutral molecules Chem. Soc. 1997 119 2267-72) Alan G. Marshall and colleagues at Florida State University introduced a new approach for producing these lowenergy electrons by using the Penning ion trap of an FT-ICRMS instrument. Electrons emitted from a hot filament (~ 2 eV) are injected into the trap and cooled to room temperature in about 1 s by classical radiative emission. The coldtrapped electrons can then attach to neutrals with elecSchematic of the dual cubic Penning trap used tron affinities of < 0.5 eV. for producing cool electrons; gas-phase neutrals Because the electrons and are generated from a heated solids probe.
ing just mat with insulin released from single pancreatic (3TC3 cells. A single pancreatic cell was hydrodynamically injected into the end of a capillary. Digitonin, which produces pores in the cell membrane by dissolving cholesterol, was then injected into the capillary, and insulin was released. After 15 min, the insulin was separated from the cell either by electrophoresis (at 3 kV) or hydrodyMonitoring cellular namic flow. After the two zones were spatially separated, ,he cell was lysed bb inchanges creasing the voltage to 30 kV, releasing Changes in cellular content reveal more the residual insulin. The two insulin zones about a cell's interaction with its environment than can simple quantitative measure- could then be quantified separately with a 100-amol detection limit. ments of intracellular components. The question is how to measure both the seThe authors also determined the creted and residual portions with a single amount of insulin released by digitonin in assay. Edward S. Yeung and Wei Tong of an off-column method that involved incubatthe Ames Laboratory-USDOE at Iowa State ing and spinning downtihesamples. The University have developed a CE laser-inoff-column method consistently yielded duced native fluorescence method for dohigher percentages of insulin. The authors 278 A
Analytical Chemistry News & Features, May 1, 1997
believe that the spinning process caused additional intracellular insulin to leak. (J. Chromatogr.B1997,689,321-25)
Schematic of on-column monitoring of insulin release from a single cell. (Adapted with permission from Elsevier Science.)
