Matrix-assisted diffusion-ordered spectroscopy: application of surfactant solutions to the resolution of isomer spectra.

The component spectra of a mixture of isomers with nearly identical diffusion coefficients cannot normally be distinguished in a standard diffusion-ordered spectroscopy (DOSY) experiment but can often be easily resolved using matrix-assisted DOSY, in which diffusion behaviour is manipulated by the addition of a co-solute such as a surfactant. Relatively little is currently known about the conditions required for such a separation, for example, how the choice between normal and reverse micelles affects separation or how the isomer structures themselves affect the resolution. The aim of this study was to explore the application of sodium dodecyl sulfate (SDS) normal micelles in aqueous solution and sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) aggregates in chloroform, at a range of concentrations, to the diffusion resolution of some simple model sets of isomers such as monomethoxyphenols and short chain alcohols. It is shown that SDS micelles offer better resolution where these isomers differ in the position of a hydroxyl group, whereas AOT aggregates are more effective for isomers differing in the position of a methyl group. For both the normal SDS micelles and the less well-defined AOT aggregates, differences in the resolution of the isomers can in part be rationalised in terms of differing degrees of hydrophobicity, amphiphilicity and steric effects.

True chemical shift correlation maps: a TOCSY experiment with pure shifts in both dimensions.

Signal resolution in (1)H NMR is limited primarily by multiplet structure. Recent advances in pure shift NMR, in which the effects of homonuclear couplings are suppressed, have allowed this limitation to be circumvented in 1D NMR, gaining almost an order of magnitude in spectral resolution. Here for the first time an experiment is demonstrated that suppresses multiplet structure in both domains of a homonuclear two-dimensional spectrum. The principle is demonstrated for the TOCSY experiment, generating a chemical shift correlation map in which a single peak is seen for each coupled relationship, but the principle is general and readily extensible to other homonuclear correlation experiments. Such spectra greatly simplify manual spectral analysis and should be well-suited to automated methods for structure elucidation.

Matrix-assisted diffusion-ordered spectroscopy: mixture resolution by NMR using SDS micelles.

Diffusion-ordered spectroscopy (DOSY) is a powerful technique for mixture analysis, but in its basic form it cannot separate the component spectra for species with very similar diffusion coefficients. It has been recently demonstrated that the component spectra of a mixture of isomers with nearly identical diffusion coefficients (the three dihydroxybenzenes) can be resolved using matrix-assisted DOSY (MAD), in which diffusion is perturbed by the addition of a co-solute such as a surfactant [R. Evans, S. Haiber, M. Nilsson, G. A. Morris, Anal. Chem. 2009, 81, 4548-4550]. However, little is known about the conditions required for such a separation, for example, the concentrations and concentration ratios of surfactant and solutes. The aim of this study was to explore the concentration range over which matrix-assisted DOSY using the surfactant SDS can achieve diffusion resolution of a simple model set of isomers, the monomethoxyphenols. The results show that the separation is remarkably robust with respect to both the concentrations and the concentration ratios of surfactant and solutes, supporting the idea that MAD may become a valuable tool for mixture analysis.

Isomer resolution by micelle-assisted diffusion-ordered spectroscopy.

Diffusion-ordered NMR spectroscopy ("DOSY") is a useful tool for the identification of mixture components. In its basic form it relies on simple differences in hydrodynamic radius to distinguish between different species. This can be very effective where species have significantly different molecular sizes, but generally fails for isomeric species. The use of surfactant co-solutes can allow isomeric species to be distinguished by virtue of their different degrees of interaction with micelles or reversed micelles. The use of micelle-assisted DOSY to resolve the NMR spectra of isomers is illustrated for the case of the three dihydroxybenzenes (catechol, resorcinol, and hydroquinone) in aqueous solution containing sodium dodecyl sulfate micelles, and in chloroform solution containing AOT reversed micelles.

LC-MS study to reduce ion suppression and to identify N-lactoylguanosine 5'-monophosphate in bonito: a new umami molecule?

In this study a specific taste modulating flavor ingredient, N-lactoylguanosine 5'-monophosphate (N-lactoyl GMP), was determined in bonito (Japanese, Katsuobushi, dried fermented skipjack) and in powdered bonito using liquid chromatography-electrospray ionization (+) mass spectrometry-mass spectrometry (LC-ESI(+)-MS/MS) with a methanol/ammonium acetate or formate gradient. Furthermore, the influence of ion suppression due to sample matrix effect was investigated and was found to substantially influence the total MS response of N-lactoyl GMP; by adjusting the LC conditions the response could be approximately 5-fold-enhanced. The N-lactoyl GMP concentrations in different types of bonito products were between 0.2 and 2.4 microg/g.

Identification of N-gluconyl [corrected] ethanolamine in wine by negative electrospray ionization with post-column chloride attachment and accurate mass determination on a triple-quadrupole mass spectrometer.

In this study a specific taste modulating flavour-ingredient, N-gluconyl [corrected] ethanolamine, was determined in two Beerenauslese wines using two different LC-MS techniques. For a first screening LC-MS(2) on an ion-trap mass spectrometer with negative electrospray ionization (ESI(-)) was applied. Sensitivity (and selectivity) was successfully increased approx. 10-fold by post-column addition of chloroform to form [M+Cl](-) species. In a second step LC-MS(2) on a triple-quadrupole mass spectrometer in accurate mass mode confirmed the presence of N-gluconyl [corrected] ethanolamine in wine. The application of the right MS(2) transitions for an unambiguous identification is discussed. N-Gluconyl [corrected] ethanolamine concentrations in the wines were found to be 1.1 and 4.0 microg/l.

Analysis of functional groups on the surface of plasma-treated carbon nanofibers.

Plasma chemically modified carbon nanofibers were characterized by X-ray photoelectron spectroscopy with regard to the content of carbon, oxygen, and nitrogen and the contribution of carboxylic groups or ester, carbonyl and hydroxylic groups or ether on the surface. Unfortunately, X-ray photoelectron spectroscopy only provides an average value of the first 10 to 15 molecular layers. For comparison, depth profiles were measured and wet chemical methods were applied to estimate the thickness of the functionalized layer and the distribution of oxygen-containing functional groups within the near-surface layers. The results indicate that the fiber surface is covered by a monomolecular oxygen-containing layer and that plasma treatment allows a complete oxygen functionalization of the uppermost surface layer. The best conditions for plasma treatment found within the set of parameters applied to generate complete functionalization are: plasma gas O(2)/Ar ratio 1:1, gas pressure 1-1.5 hPa, plasma power 80 W, treatment time >or= 5 min. Additionally, three quick and easy methods are presented to estimate the efficiency of plasma treatment with regard to surface functionalization: pyrolysis, contact angle measurements, and light permeability measurements of aqueous carbon nanofiber suspensions.