Quantitative comparison of performance of isothermal and temperature-programmed gas chromatography.

As a basic metric of separation for comparing isothermal and temperature-programmed GC (gas chromatography), we used the separation measure. S (defined elsewhere). We used this metric as both a measure of separation of any two peaks, and a measure of separation capacity of arbitrary intervals where peaks can potentially exist. We derived several formulae for calculation of S for any pair of peaks regardless of their shape and the distance from each other in isothermal and temperature-programmed GC. The formulae for isothermal GC can be viewed as generalizations of previously known expressions while, in the case of temperature-programmed GC, no equivalents for the new formulae were previously known from the literature. In all formulae for S. we identified similar key component-metrics (solute separability, intrinsic efficiency of separation, specific separation measure, separation power) that helped us to identify and better understand the key factors affecting the separation process. These metrics also facilitated the quantitative comparison of separation capacities and analysis times in isothermal and temperature-programmed GC. Some of these metrics can be useful beyond GC. In the case of GC, we have shown that, if the same complex mixture was analyzed by the same column, and the same separation requirements were used then isothermal analysis can separate more peaks than its temperature-programmed counterpart can. Unfortunately, this advantage comes at the cost of prohibitively longer isothermal analysis time. The latter is a well know fact. Here, however, we provided a quantitative comparison. In a specific example, we have shown that a single-ramp temperature program with a typical heating rate yields about 25% fewer peaks than the number of peaks available from isothermal analysis of the same mixture using the same column. However, that isothermal analysis would last 1000 times longer than its temperature-programmed counterpart. Using twice as longer column in the case of a temperature-programmed analysis, allows one to recover the 25% disadvantage in the number of separated peaks, while still retaining a 500-fold advantage in the speed of analysis.     

Disulfide‐Crosslinked Electrospun Poly(γ‐glutamic acid) Nonwovens as Reduction‐Responsive Scaffolds

Novel water‐insoluble, and reduction‐responsive nonwoven scaffolds were fabricated from γ‐PGA and tested in cell culture. An electrospinning method was developed to produce scaffolds of fibers with diameters of 0.05–0.5 µm. Crosslinking of the fibers with cystamine in the presence of EDC resulted in water‐insoluble γ‐PGA nonwovens with disulfide crosslinkages. These crosslinked fibers were easily decomposed under physiological conditions using L ‐cysteine, a biocompatible reductant. In vitro experiments with mouse L929 fibroblasts showed good adhesion onto γ‐PGA‐SS fiber matrices and excellent cell proliferation. These γ‐PGA‐SS nonwovens can be used as novel biocompatible and biodegradable scaffolds with reduction‐responsiveness for biomedical or tissue engineering applications.

     

Measurement of retention in comprehensive two-dimensional gas chromatography using flow modulation with methane dopant

Flow modulation of methane-doped carrier gas is used to visualize the second dimension hold-up time in GC × GC continuously throughout the run. This provides an internal reference of hold-up time and presents a straightforward means of examining retention in each dimension of GC × GC. Retention factors on similar and dissimilar column pairs are examined. Stationary phase bleed is shown to be retained by the second dimension column.     

Counterexamples to the Strong d -Step Conjecture for d≥5

A Dantzig figure is a triple (P,x,y) in which P is a simple d -polytope with precisely 2d facets, x and y are vertices of P , and each facet is incident to x or y but not both. The famous d -step conjecture of linear programming is equivalent to the claim that always # ^d P(x,y) ≥ 1 , where # ^d P(x,y) denotes the number of paths that connect x to y by using precisely d edges of P . The recently formulated strong d -step conjecture makes a still stronger claim—namely, that always # ^d P(x,y) ≥ 2 ^d-1 . It is shown here that the strong d -step conjecture holds for d ≤ 4 , but fails for d ≥ 5 . Received December 27, 1995, and in revised form April 8, 1996.     

On a conjecture of lindenstrauss

It is proved that eachn-dimensional centrally symmetric convex polyhedron admits a 2-dimensional central section having at least 2n vertices. Some other related results are obtained and some unsolved problems are mentioned. Research supported in part by the National Science Foundation, U.S.A. (NSF-GP-378).     

Generation of ion-bound solvent clusters as reactant ions in dopant-assisted APPI and APLI

We provide experimental and theoretical evidence that the primary ionization process in the dopant-assisted varieties of the atmospheric pressure ionization methods atmospheric pressure photoionization and atmospheric pressure laser ionization in typical liquid chromatography–mass spectrometry settings is—as suggested in the literature—dopant radical cation formation. However, instead of direct dopant radical cation–analyte interaction—the broadly accepted subsequent step in the reaction cascade leading to protonated analyte molecules—rapid thermal equilibration with ion source background water or liquid chromatography solvents through dopant ion–molecule cluster formation occurs. Fast intracluster chemistry then leads to almost instantaneous proton-bound water/solvent cluster generation. These clusters interact either directly with analytes by ligand switching or association reactions, respectively, or further downstream in the intermediate-pressure regions in the ion transfer stages of the mass spectrometer via electrical-field-driven collisional decomposition reactions finally leading to the predominantly observed bare protonated analyte molecules [M?+?H]⁺.