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NMR spectroscopy has been coupled on-line to capillary isotachophoresis (cITP) to enhance structural analyses of dilute charged species through separation and sample concentration. Microcoils, the most mass-sensitive NMR probes available, provide optimal detection for cITP/NMR. To evaluate the utility of cITP/NMR for natural product analysis, a homogenate of the hypobranchial gland from the marine snail Calliostoma canaliculatum containing a cationic neurotoxin (1, a disulfide-bonded dimer of 6-bromo-2-mercaptotryptamine) was studied. For comparison, hypobranchial gland homogenate was also examined by diffusion-NMR, an alternative approach for NMR mixture analysis. cITP/NMR concentrated the neurotoxin by almost 20-fold and isolated it from some of the other components present in the matrix. However, a minor component, likely a precursor or degradant, co-migrated with compound 1. Diffusion-NMR also did not resolve the two, indicating that the compounds possessed similar diffusion coefficients and electrophoretic mobilities. The strengths and limitations of the two approaches for NMR mixture analysis are discussed.

Predicting the final neuropeptide products from neuropeptides genes has been problematic because of the large number of enzymes responsible for their processing. The basic processing of 22 Aplysia californica prohormones representing 750 cleavage sites have been analyzed and statistically modeled using binary logistic regression analyses. Two models are presented that predict cleavage probabilities at basic residues based on prohormone sequence. The complex model has a correct classification rate of 97%, a sensitivity of 97%, and a specificity of 96% when tested on the Aplysia dataset. Keywords: proteolytic processing • neuropeptides • MALDI MS • prediction algorithms

Molecular transport through nanoporous nuclear-track-etched membranes was investigated with fluorescent probes by manipulating applied electric field polarity, pore size, membrane surface functionality, pH, and the ionic strength. Three forces contribute to analyte transport through membranes:  ion migration, electroosmosis, and diffusion. Diffusion dominates under field-free conditions with surface hydrophobicity controlling solvent access to the nanochannels and hence the magnitude of transport by diffusion. In low ionic strength solutions (μ ~ 10 mM), electroosmosis dominates transport when the membranes are biased, and the charge state of the surface determines the direction of flow. At high ionic strength (μ ~ 1 M), ion migration dominates in hydrophobic membranes, and diffusion is controlling in hydrophilic membranes. The magnitude and polarity of the interior surface charge is controlled by surface functionality and displays the largest impact on molecular transport. The analyte can migrate in opposite directions under the same applied electric field by modifying either membrane surface charge or solution ionic strength. Transport can be fine-tuned by adjusting pH under low ionic strength conditions in either type of membrane. Increasing the surface charge density, σs, enhances the mobile counterion concentration, increasing the electroosmotically driven flux. Comparisons of behavior under different conditions are understood by reference to the product, κa, of the inverse Debye length, κ, and the pore diameter, a.

This article is the first in a two-part series describing the operation, characteristics, and application of a new class of solid-state multichannel UV-visible detectors. In this paper, charge transfer devices (CTDs) are described. Detector characteristics pertinent to spectroscopic application—including quantum efficiency, read noise, dark count rate, and available formats—are emphasized. Unique capabilities, such as the ability to nondestructively read out the detector array and the ability to alter the effective detector element size by a process called binning, are described. CTDs with peak quantum efficiencies over 80% and significant responsivity over the wavelength range of 0.1 nm to 1100 nm are discussed. Exceptionally low dark count rates, which allow integration times of up to many hours and read noises more than two orders of magnitude lower than those read by commercially available PDA detectors, contribute to the outstanding performance offered by these detectors.

This is the second article in a two-part series describing the operation, performance characteristics, and spectroscopic application of charge transfer devices (CTDs) in analytical chemistry. The first article in the series describes the new generation of integrating multichannel detectors, the charge injection device (CID), and the charge-coupled device (CCD). The first article also discusses the spectroscopically pertinent characteristics of these detectors and presents performance data for representative devices. This article covers three major topics related to the optimum use of integrating detectors in analytical spectroscopy. The advantages of employing integrating multichannel detectors in analytical spectroscopy, rather than a single detector in a wavelength scanning system or an interferometer, are discussed. Included are detector read noise considerations which have not been considered in previous performance comparisons. When one is employing an integrating detector in luminescence, absorption, and emission applications, achievable sensitivity is dependent on differing detector parameters. In the first case, quantum efficiency and read noise are of the greatest importance, whereas in the later two cases, dynamic range is most significant. The calculation of minimum detectable analyte signal for these three techniques illustrates the differences between integrating detectors and detectors which produce a photocurrent. This discussion also illustrates the great sensitivity that can be achieved with a modern CTD detector. Factors pertaining to the optical design of spectrometers which efficiently use CTDs are presented, along with examples of linear and two-dimensional dispersive polychromators employing CTDs. Low-light-level imaging and a nonconventional method of using a CCD for rapid scanning spectrophotometry are also discussed.