Biomedical subjects

Hans-Peter Loock

Publications and source records attributed to Hans-Peter Loock.

5 recordsLinked to original sources

PubMed2006

Capillary electrophoresis absorption detection using fiber-loop ring-down spectroscopy.

The application of phase-shift, fiber-loop, ring-down spectroscopy (PS-FLRDS) as an on-line detector for capillary electrophoresis (CE) of biomolecules is demonstrated. CE was conducted using a custom-designed capillary/fiber interface coupled to an absorption detector, which is based on the ring-down of an optical signal in a closed fiber waveguide loop. The ring-down times were obtained by measuring the phase difference between intensity modulated light entering and exiting the fiber loop. The incorporation of a microlens to enhance transmission through the sample gap led to an improvement of the sensitivity by up to 80% compared to the square-cut fiber and a reduction in the detection limit. The performance of the PS-FLRDS absorption technique as an online detector was characterized by flow injection through a capillary. Good repeatability and linear response were obtained, and the detection limit using the lensed fiber/capillary interface system was determined to be alpha(min) = 1.6 cm(-1) for an absorption path of approximately 30 microm. PS-FLRDS coupled to CE was also applied to the analysis of human serum albumin (HSA) by using a NIR dye as a noncovalent label. The excess free dye and the dye/protein complex were resolved. The labeling coefficient was determined to be approximately 6, and good repeatability of peak areas (RSD = 8.7%) was obtained for the analysis of HSA. Furthermore, an excellent linear response (R2 > 0.99) was obtained between the peak areas and concentrations of HSA. The detection limit of labeled HSA was determined to be 1.67 microM.

Electrophoresis, Capillary↗

PubMed2006

Photoelectron imaging following 2 + 1 multiphoton excitation of HBr.

The photodissociation and photoionization dynamics of HBr via low-n Rydberg and ion-pair states was studied by using 2 + 1 REMPI spectroscopy and velocity map imaging of photoelectrons. Two-photon excitation at about 9.4-10 eV was used to prepare rotationally selected excited states. Following absorption of the third photon the unperturbed F (1)Delta(2) and i (3)Delta(2) states ionize directly into the ground vibrational state of the molecular ion according to the Franck-Condon principle and upon preservation of the ion core. In case of the V (1)Sigma(+)(0(+)) ion-pair state and the perturbed E (1)Sigma(+)(0(+)), g (3)Sigma(-)(0(+)), and H (1)Sigma(+)(0(+)) Rydberg states the absorption of the third photon additionally results in a long vibrational progression of HBr(+) in the X (2)Pi state as well as formation of electronically excited atomic photofragments. The vibrational excitation of the molecular ion is explained by autoionization of repulsive superexcited states into the ground state of the molecular ion. In contrast to HCl, the perturbed Rydberg states of HBr show strong participation of the direct ionization process, with ionic core preservation.

Algorithms↗

PubMed2004

Two-photon state selection and angular momentum polarization probed by velocity map imaging: application to H atom photofragment angular distributions from the photodissociation of two-photon state selected HCl and HBr.

A formalism for calculating the angular momentum polarization of an atom or a molecule following two-photon excitation of a J-selected state is presented. This formalism is used to interpret the H atom photofragment angular distributions from single-photon dissociation of two-photon rovibronically state selected HCl and HBr prepared via a Q-branch transition. By comparison of the angular distributions measured using the velocity map imaging technique with the theoretical model it is shown that single-photon dissociation of two-photon prepared states can be used for pathway identification, allowing for the identification of the virtual state symmetry in the two-photon absorption and/or the symmetry of the dissociative state. It is also shown that under conditions of excitation with circularly polarized light, or for excitation via non-Q-branch transitions with linearly polarized light the angular momentum polarization is independent of the dynamics of the two-photon transition and analytically computable.

Journal Article↗

PubMed2004

Phase-shift fiber-loop ring-down spectroscopy.

Fiber-loop ring-down spectroscopy (FLRDS) is a recently developed absorption spectroscopic technique suitable for very small liquid samples. It is based on measurements of the optical decay constant of laser intensity in a loop made of optical waveguide material. This decay constant changes as small liquid samples containing absorbing species are introduced into the loop. In this report, it is demonstrated that one can also obtain the optical decay constant using a continuous wave laser beam that is intensity modulated and then coupled into an optical fiber loop. The inherent exponential decay in the fiber loop introduces a phase shift of the light emitted from the loop with respect to the pumping beam. By measuring this phase shift, one can readily determine the concentration of the analyte introduced between the two fiber ends and a model is established to describe the relationship. It is demonstrated that this technique, dubbed phase-shift fiber-loop ring-down spectroscopy (PS-FLRDS), is well suited as an absorption detector for any flow system in which the optical absorption path is limited by the instrument architecture. By measuring the phase angle as a function of concentration of 1,1'-diethyl-4,4'-dicarbocyanine iodide in dimethyl sulfoxide, the detection limit was determined as approximately 6 microM for a 30-40-microm absorption path. A temporal resolution of approximately 100 ms was demonstrated by a rapid displacement of the solutions between the two fiber ends. Proof-of-principle use of the PS-FLRDS detection in capillary flow systems using a commercial four-way microcross established that the alignment of the fiber and the capillary can be made simple and effective, while retaining both a low detection limit and a fast response.

Journal Article↗

PubMed2004

Superexcited state reconstruction of HCl using photoelectron and photoion imaging.

The velocity-map imaging technique was used to record photoelectron and photofragment ion images of HCl following two-photon excitation of the E Sigma(+)(0+), V 1Sigma(+)(0+) (nu=9,10,11) states and subsequent ionization. The images allowed us to determine the branching ratios between autoionization and dissociation channels for the different intermediate states. These branching ratios can be explained on the basis of intermediate state electron configurations, since the configuration largely prohibits direct ionization in a one-electron process, and competition between autoionization and dissociation into H* (n=2)+Cl and H+Cl*(4s,4p,3d) is observed. From a fit to the vibrationally resolved photoelectron spectrum of HCl+ it is apparent that a single superexcited state acts as a gateway to autoionization and dissociation into H+Cl*(4s). Potential reconstruction of the superexcited state to autoionization was undertaken and from a comparison of different autoionization models it appears most likely that the gateway state is a purely repulsive and low-n Rydberg state with a (4Pi) ion core.

Journal Article↗