High-precision ethanol measurement by mid-IR laser absorption spectroscopy for metrological applications.

We report on the development and validation of a compact laser instrument using mid-IR direct absorption spectroscopy (DAS) for high-precision measurements of ethanol in breath-like air mixtures. Leveraging the intermittent continuous wave (iCW) driving for conventional narrow-band distributed feedback (DFB) quantum cascade laser (QCL) emitting around 9.3 µm and using a 25 m path length multiple-pass absorption cell at reduced pressure, a precision of 9 ppb (amount fraction, nmol mol-1) at 60 s integration time is achieved even in the presence of 5% of H2O and CO2. Thus, the instrument is well suitable for metrological studies to investigate observed, but yet unquantified, discrepancies between different breath alcohol reference-generation methods. The approach can be generalized and applied for other organic molecules in a wide range of applications.

Laser driving and data processing concept for mobile trace gas sensing: Design and implementation.

High precision mobile sensing of multi-species gases is greatly demanded in a wide range of applications. Although quantum cascade laser absorption spectroscopy demonstrates excellent field-deployment capabilities for gas sensing, the implementation of this measurement technique into sensor-like portable instrumentation still remains challenging. In this paper, two crucial elements, the laser driving and data acquisition electronics, are addressed. Therefore, we exploit the benefits of the time-division multiplexed intermittent continuous wave driving concept and the real-time signal pre-processing capabilities of a commercial System-on-Chip (SoC, Red Pitaya). We describe a re-designed current driver that offers a universal solution for operating a wide range of multi-wavelength quantum cascade laser device types and allows stacking for the purpose of multiple laser configurations. Its adaptation to the various driving situations is enabled by numerous field programmable gate array (FPGA) functionalities that were developed on the SoC, such as flexible generation of a large variety of synchronized trigger signals and digital inputs/outputs (DIOs). The same SoC is used to sample the spectroscopic signal at rates up to 125 MS/s with 14-bit resolution. Additional FPGA functionalities were implemented to enable on-board averaging of consecutive spectral scans in real-time, resulting in optimized memory bandwidth and hardware resource utilisation and autonomous system operation. Thus, we demonstrate how a cost-effective, compact, and commercial SoC can successfully be adapted to obtain a fully operational research-grade laser spectrometer. The overall system performance was examined in a spectroscopic setup by analyzing low pressure absorption features of CO2 at 4.3 µm.

Cryogenic Spectroscopy and Quantum Molecular Dynamics Determine the Structure of Cyclic Intermediates Involved in Peptide Sequence Scrambling.

Collision-induced dissociation (CID) is a key technique used in mass spectrometry-based peptide sequencing. Collisionally activated peptides undergo statistical dissociation, forming a series of backbone fragment ions that reflect their amino acid (AA) sequence. Some of these fragments may experience a "head-to-tail" cyclization, which after proton migration, can lead to the cyclic structure opening in a different place than the initially formed bond. This process leads to AA sequence scrambling that may hinder sequencing of the initial peptide. Here we combine cryogenic ion spectroscopy and ab initio molecular dynamics simulations to isolate and characterize the precise structures of key intermediates in the scrambling process. The most stable peptide fragments show intriguing symmetric cyclic structures in which the proton is situated on a C2 symmetry axis and forms exceptionally short H-bonds (1.20 Å) with two backbone oxygens. Other nonsymmetric cyclic structures also exist, one of which is protonated on the amide nitrogen, where ring opening is likely to occur.

Infrared multiple photon dissociation of chloromethyltrifluorosilane.

Infrared multiphoton absorption and dissociation of chloromethyltrifluorosilane molecules under the action of pulsed transversely excited atmospheric pressure CO2 laser were experimentally studied. Dissociation products were analyzed. The dissociation proceeds via chlorine atom transfer from carbon to silicone. High degrees of silicon isotope separation were achieved. The presence of alpha-chlorine atom in a silicon organic compound brings about a significant improvement in multiple photon dissociation characteristics and an essential increase in isotopic selectivity.