Abstract. This work demonstrates a method to prepare homogeneous distributions of analytes to improve data reproducibility in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). Natural-air drying processes normally result in unwanted heterogeneous spatial distributions of analytes in MALDI crystals and make quantitative analysis difficult. This study demonstrates that inducing Marangoni flows within drying droplets can significantly reduce the heterogeneity problem. The Marangoni flows are accelerated by changing substrate temperatures to create temperature gradients across droplets. Such hydrodynamic flows are analyzed semi-empirically. Using imaging mass spectrometry, changes of heterogeneity of molecules with the change of substrate temperature during drying processes are demonstrated. The observed heterogeneities of the biomolecules reduce as predicted Marangoni velocities increase. In comparison to conventional methods, drying droplets on a 5°C substrate while keeping the surroundings at ambient conditions typically reduces the heterogeneity of biomolecular ions by 65%-80%. The observation suggests that decreasing substrate temperature during droplet drying processes is a simple and effective means to reduce analyte heterogeneity for quantitative applications.

This work presents the unique features of a novel configuration of a synchronized dual-polarity time-of-flight mass spectrometer for comprehensive surface imaging. Mass spectrometry imaging of surface samples covering positive and negative ion modes is difficult due to rapid signal depletion. This limitation is overcome here by dual-polarity time-of-flight mass spectrometry (DP-TOFMS) via two separate TOF mass analyzers that are installed above a sample surface. The new instrument eliminates the polarity bias characteristic of most mass spectrometers, which is important for the analysis of samples with diverse physical and chemical properties. The experimental results show for the first time that the spatial distribution of positive and negative ions of various photolabile samples can be distinguished, including pigments and conventional matrix-assisted laser desorption/ionization samples. The different positive and negative ion distributions suggest that accurate quantitative information can only be obtained when the entire sample region is examined by DP-TOFMS, which was unfeasible in the past. Such a comprehensive diagnostic method is essential for the molecular imaging of trace compositions in delicate biological tissues, as demonstrated here with a Phyllanthus urinaria leaf that only produced ion signals in the first examination and not in the subsequent measurements.

A miniature matrix-assisted laser desorption/ionization linear time-of-flight mass spectrometer suitable for the high mass-to-charge (m/z) region is described. The instrument size is roughly 1/50 that of regular instruments, and detailed...

Web forums become the means of online communication and information sharing sources for the learning about health care and related treatment knowledge. By adopting web crawlers and natural language processing techniques, the automatic identification approach of the concerned HIV-related messages is proposed to facilitate the health authorities and social support groups in instant counseling. The proposed supervised GA/k-means for classification approach can help construct an effective identification and classification model with acceptable classification performance accompanied with its full flexibility to develop different fitness functions in accordance with the need of different requirements. Furthermore, with the aid of correspondence analysis, the most frequently used terms in concerned HIV-related messages are identified and focus on risky sexual behavior whereas unconcerned messages are those who of worried well.

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