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Andreas Haase

Publications and source records attributed to Andreas Haase.

3 recordsLinked to original sources
PubMed2006

Optimizing UV laser focus profiles for improved MALDI performance.

Matrix assisted laser desorption/ionization (MALDI) applications, such as proteomics, genomics, clinical profiling and MALDI imaging, have created a growing demand for faster instrumentation. Since the commonly used nitrogen lasers have throughput and life span limitations, diode-pumped solid-state lasers are an alternative. Unfortunately this type of laser shows clear performance limitations in MALDI in terms of sensitivity, resolution and ease of use, for applications such as thin-layer sample preparations, acceptance of various matrices (e.g. DHB for glycopeptides) and MALDI imaging. While it is obvious that the MALDI process has some dependence on the characteristics of the laser used, it is unclear which features are the most critical in determining laser performance for MALDI. In this paper we show, for the first time, that a spatially structured laser beam profile in lieu of a Gaussian profile is of striking importance. This result enabled us to design diode-pumped Nd : YAG lasers that on various critical applications perform as well for MALDI as the nitrogen lasers and in some respects even better. The modulation of the beam profile appears to be a new parameter for optimizing the MALDI process. In addition, the results trigger new questions directing us to a better understanding of the MALDI process.

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PubMed2005

Activation and inactivation kinetics of a Ca2+-activated Cl- current: photolytic Ca2+ concentration and voltage jump experiments.

The activation kinetics of the endogenous Ca(2+)-activated Cl(-) current (I (Cl,Ca)) from Xenopus oocytes was investigated in excised "giant" membrane patches with voltage and Ca(2+) concentration jumps performed by the photolytic cleavage of the chelator DM-nitrophen. Currents generated by photolytic Ca(2+) concentration jumps begin with a lag phase followed by an exponential rising phase. Both phases show little voltage dependence but are Ca(2+)-dependent. The lag phase decreases from about 10 ms after a small Ca(2+) concentration jump (0.1 microM) to less than 1 ms after a saturating concentration jump (55 microM). The rate constant of the rising phase is half-maximal at about 5 microM. At saturating Ca(2+) concentrations, the rate constant is 400 to 500 s(-1). The Ca(2+) dependence of the stationary current can be described by the Hill equation with n=2.3 and K (0.5)=0.5 microM. The amplitude of the stationary current decreases after the excision of the membrane patch with t (1/2) approximately 5 min (run-down). The activation kinetics of the current elicited by a Ca(2+) concentration jump is not affected by the run-down phenomenon. At low Ca(2+) concentration (0.3 microM), voltage jumps induce a slowly activating current with voltage-independent time-course. Activation is preceded by an initial transient of about 1-ms duration. At saturating Ca(2+) levels (1 mM), the initial transient decays to a stationary current. The transient can be explained by a voltage-dependent inactivation process. The experimental data reported here can be described by a linear five-state reaction model with two sequential voltage-dependent Ca(2+)-binding steps, followed by a voltage-independent rate-limiting transition to the open and a voltage-dependent transition to a closed, inactivated state.

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