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Xu Z.-C.,Peking University | Pan D.,Peking University | Zhuang W.,Peking University | Zhuang W.,National Institute of MetrologyBeijing | Chen J.-B.,Peking University
Chinese Physics Letters

The experimental scheme of 633nm and 1359nm good-bad cavity dual-wavelength active optical frequency standard is proposed, where He-Ne 633nm and Cs 1359nm stimulated emissions are working at good-cavity and bad-cavity regimes, respectively. The cavity length is stabilized by locking the 633nm output frequency to a super-cavity with the Pound-Drever-Hall (PDH) technique. The frequency stability of 1359nm bad-cavity stimulated emission output is then expected to be further improved by at least 1 order of magnitude than the 633nm PDH system due to the suppressed cavity pulling effect of active optical clock, and the quantum limited linewidth of 1359nm output is estimated to be 72.5 mHz. © 2015 Chinese Physical Society and IOP Publishing Ltd. Source

Wei Z.,Tsinghua University | Xiong X.,Tsinghua University | Xiong X.,National Institute of MetrologyBeijing | Guo C.,Tsinghua University | And 12 more authors.
Analytical Chemistry

We had developed pulsed direct current electrospray ionization mass spectrometry (pulsed-dc-ESI-MS) for systematically profiling and determining components in small volume sample. Pulsed-dc-ESI utilized constant high voltage to induce the generation of single polarity pulsed electrospray remotely. This method had significantly boosted the sample economy, so as to obtain several minutes MS signal duration from merely picoliter volume sample. The elongated MS signal duration enable us to collect abundant MS2 information on interested components in a small volume sample for systematical analysis. This method had been successfully applied for single cell metabolomics analysis. We had obtained 2-D profile of metabolites (including exact mass and MS2 data) from single plant and mammalian cell, concerning 1034 components and 656 components for Allium cepa and HeLa cells, respectively. Further identification had found 162 compounds and 28 different modification groups of 141 saccharides in a single Allium cepa cell, indicating pulsed-dc-ESI a powerful tool for small volume sample systematical analysis. © 2015 American Chemical Society. Source

Gong X.,Tsinghua University | Gong X.,National Institute of MetrologyBeijing | Xiong X.,National Institute of MetrologyBeijing | Wang S.,Tsinghua University | And 4 more authors.
Analytical Chemistry

Sensitive detection of biomolecules in small-volume samples by mass spectrometry is, in many cases, challenging because of the use of buffers to maintain the biological activities of proteins and cells. Here, we report a highly effective desalting method for picoliter samples. It was based on the spontaneous separation of biomolecules from salts during crystallization of the salts. After desalting, the biomolecules were deposited in the tip of the quartz pipet because of the evaporation of the solvent. Subsequent detection of the separated biomolecules was achieved using solvent assisted electric field induced desorption/ionization (SAEFIDI) coupled with mass spectrometry. It allowed for direct desorption/ionization of the biomolecules in situ from the tip of the pipet. The organic component in the assistant solvent inhibited the desorption/ionization of salts, thus assured successful detection of biomolecules. Proteins and peptides down to 50 amol were successfully detected using our method even if there were 3 × 105 folds more amount of salts in the sample. The concentration and ion species of the salts had little influence on the detection results. © 2015 American Chemical Society. Source

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