A contactless conductivity detector for capillary electrophoresis: effects of the detection cell geometry on the detector performance

The effect of the gap between the electrodes and of their width on the behavior of a capacitively wired contactless conductivity detector was studied. The results obtained have indicated that the detector response can be qualitatively described by a model based on the concept of the effective electrode width which is a complex parameter determined by the gap between the electrodes, the frequency of the input signal and the conductivity of the test solution. The detector sensitivity and the effect on the separation efficiency depend on the difference between the effective and geometric electrode widths. Higher detection sensitivities have been attained for detectors with wide electrodes operating at lower frequencies, however, better separation efficiencies have been achieved using detectors with narrow electrodes and higher operational frequencies. The noise increases with decreasing gap between the electrodes and increasing frequency, especially with detectors employing narrow electrodes.     

Lactone enols are stable in the gas phase but highly unstable in solution

2-Hydroxyoxol-2-ene (C(5)-1), the enol tautomer of gamma-butyrolactone, was generated in the gas phase as the first representative of the hitherto elusive class of lactone enols and shown by neutralization-reionization mass spectrometry to be remarkably stable as an isolated species. Ab initio calculations by QCISD(T)/6-311+G(3df,2p) provided the enthalpies of formation, proton affinities, and gas-phase basicities for gaseous lactone enols with four- (C(4)-1), five- (C(5)-1), and six-membered rings (C(6)-1). The acid-base properties of C(4)-C(6) lactones and enols and reference carboxylic acid enols CH(2)=C(OH)(2) (3) and CH(2)=C(OH)OCH(3) (4) were also calculated in aqueous solution. The C(4)-C(6) lactone enols show gas-phase proton affinities in the range of 933-944 kJ mol(-)(1) and acidities in the range of 1401-1458 kJ mol(-)(1). In aqueous solution, the lactone enols are 15-20 orders of magnitude more acidic than the corresponding lactones, with enol pK(a) values increasing from 5.6 (C(4)-1) to 14.5 (C(6)-1). Lactone enols are moderately weak bases in water with pK(BH) in the range of 3.9-8.1, whereas the lactones are extremely weak bases of pK(BH) in the range of -10.5 to -17.4. The acid-base properties of lactone enols point to their high reactivity in protic solvents and explain why no lactone enols have been detected thus far in solution studies.     

Nanoparticle-modified monolithic pipette tips for phosphopeptide enrichment

We have developed nanoparticle-modified monoliths in pipette tips for selective and efficient enrichment of phosphopeptides. The 5 μL monolithic beds were prepared by UV-initiated polymerization in 200 μL polypropylene pipette tips and either iron oxide or hydroxyapatite nanoparticles were used for monolith modification. Iron oxide nanoparticles were prepared by a co-precipitation method and stabilized by citrate ions. A stable coating of iron oxide nanoparticles on the pore surface of the monolith was obtained via multivalent electrostatic interactions of citrate ions on the surface of nanoparticles with a quaternary amine functionalized poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate) monolith. Hydroxyapatite nanoparticles were incorporated into the poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate) monolith by simply admixing them in the polymerization mixture followed by in situ polymerization. The nanoparticle-modified monoliths were compared with commercially available titanium dioxide pipette tips. Performance of the developed and commercially available sorbents was demonstrated with the efficient and selective enrichment of phosphopeptides from peptide mixtures of α-casein and β-casein digests followed by off-line MALDI/MS analysis.     

A personal computer-based data control interface for neutralization-reionization and tandem mass spectrometry

An improved data-acquisition and management system interfaces an IBM-compatible personal computer to a tandem mass spectrometer. In addition to increased spectral resolution and storage, the control system measures the absolute abundance (peak areas) of product ions, and also that of the precursor; these are capabilities critical for collisional activation, metastable ion decomposition and neutralization-reionization mass spectra. Improved experimental control features include multiple mass range and repetitive scan selection, spurious transient rejection and comparative spectral display.     

Stellan Hjertén's contribution to the development of monolithic stationary phases

This overview is presented to celebrate the birthday of one of the luminaries of the separation science and my friend - Stellan Hjertén. He made significant contributions to a variety of areas in separation science such as electrophoresis, LC, and CEC to name just a few. Since the scope of his work was enormous, this review will focus only on a single aspect of his scientific activities, the design and applications of monolithic materials. During the years starting from 1989, Stellan Hjertén published many excellent papers concerning the preparation of acrylamide chemistry-based monoliths and their use in both micro-HPLC and CEC. The following text details his works in the field.     

Experimental evidence for an inverse hydrogen migration in arginine radicals

Radicals formed by electron transfer to protonated arginine have been predicted by theory to undergo an inverse migration of the hydrogen atom from the C(alpha) position to the guanidine carbon atom. Experiments are reported here that confirm that a fraction of arginine and arginine amide radicals undergo such an inverse hydrogen migration. The rearranged arginine and arginine amide C(alpha) radicals are detected as stable anions after charge inversion by collisions with Cs atoms of precursor cations at 3 and 50 keV kinetic energies. RRKM calculations on the B3-PMP2/aug-cc-pVTZ potential energy surface indicate that arginine radicals undergo rapid rotations of the side chain to reach conformations suitable for C(alpha)-H transfer, which is calculated to be fast (k > 10(9) s(-1)) in radicals formed by electron transfer. By contrast, H-atom transfer from the guanidine group onto the carboxyl or amide C=O groups is >50 times slower than the C(alpha)-H atom migration. The guanidine group in arginine radicals is predicted to be a poor hydrogen-atom donor but a good H-atom acceptor and thus can be viewed as a radical trap. This property can explain the frequent observation of nondissociating cation radicals in electron capture and electron transfer mass spectra of arginine-containing peptides.     

Where does the electron go? Electron distribution and reactivity of peptide cation radicals formed by electron transfer in the gas phase

We report the first detailed analysis at correlated levels of ab initio theory of experimentally studied peptide cations undergoing charge reduction by collisional electron transfer and competitive dissociations by loss of H atoms, ammonia, and N-C alpha bond cleavage in the gas phase. Doubly protonated Gly-Lys, (GK + 2H) (2+), and Lys-Lys, (KK + 2H) (2+), are each calculated to exist as two major conformers in the gas phase. Electron transfer to conformers with an extended lysine chain triggers highly exothermic dissociation by loss of ammonia from the Gly residue, which occurs from the ground ( X ) electronic state of the cation radical. Loss of Lys ammonium H atoms is predicted to occur from the first excited ( A ) state of the charge-reduced ions. The X and A states are nearly degenerate and show extensive delocalization of unpaired electron density over spatially remote groups. This delocalization indicates that the captured electron cannot be assigned to reduce a particular charged group in the peptide cation and that superposition of remote local Rydberg-like orbitals plays a critical role in affecting the cation-radical reactivity. Electron attachment to ion conformers with carboxyl-solvated Lys ammonium groups results in spontaneous isomerization by proton-coupled electron transfer to the carboxyl group forming dihydroxymethyl radical intermediates. This directs the peptide dissociation toward NC alpha bond cleavage that can proceed by multiple mechanisms involving reversible proton migrations in the reactants or ion-molecule complexes. The experimentally observed formations of Lys z (+*) fragments from (GK + 2H) (2+) and Lys c (+) fragments from (KK + 2H) (2+) correlate with the product thermochemistry but are independent of charge distribution in the transition states for NC alpha bond cleavage. This emphasizes the role of ion-molecule complexes in affecting the charge distribution between backbone fragments produced upon electron transfer or capture.