Absorption linewidth inversion with wavelength modulation spectroscopy

For absorption linewidth inversion with wavelength modulation spectroscopy（WMS）, an optimized WMS spectral line fitting method was demonstrated to infer absorption linewidth effectively, and the analytical expressions for relationships between Lorentzian linewidth and the separations of first harmonic peak-to-valley and second harmonic zero-crossing were deduced. The transition of CO_2 centered at 4991.25 cm^（-1） was used to verify the optimized spectral fitting method and the analytical expressions. Results showed that the optimized spectra fitting method was able to infer absorption accurately and compute more than 10 times faster than the commonly used numerical fitting procedure. The second harmonic zero-crossing separation method calculated an even 6 orders faster than the spectra fitting without losing any accuracy for Lorentzian dominated cases. Additionally, linewidth calculated through second harmonic zero-crossing was preferred for much smaller error than the first harmonic peak-to-valley separation method. The presented analytical expressions can also be used in on-line optical sensing applications, electron paramagnetic resonance, and further theoretical characterization of absorption lineshape.

Spectroscopy system based on a single quantum cascade laser for simultaneous detection of CO and CO_2

A quantum cascade laser（QCL） based system for simultaneous detection of CO and CO_2 is developed.The QCL can scan over two neighboring CO（2055.40 cm^（-1）） and CO_2（2055.16 cm^（-1）） lines with a single current scan.The wavelength modulation spectroscopy（ f = 20 k Hz） is utilized to enhance the signal-to-noise ratio.A white cell with an effective optical path length of 74 m is used.The calibration of the sensor is performed and minimum detection limits of 1.3 ppb（1 × 10^（-9））for CO and 0.44 ppm（1 × 10^（-6）） for CO_2 are achieved.

Tuning plasmon absorption of unmodified silver nanoplates for sensitive and selective detection of copper ions by introduction of ascorbate

Silver nanoplates as novel optical sensors for Cu^2＋ detection have been demonstrated.Silver nanoplates are synthesized via previous H_2O_2-NaBH_4 cyclic oxidation-reduction reactions.With introduction of ascorbate as mild reductants,Cu^2＋ ions are reduced into Cu~＋ and the Cu^＋ is further reduced to Cu,which is deposited on the surface of the silver nanoplates.The deposition of the Cu on the surface of the silver nanoplates allows a significant red-shift of their plasmon absorption.Therefore,trace Cu^2＋ can be detected.The shift of the plasmon absorption wavelength of silver nanoplates is proportional to the Cu^2＋concentration over a range of 40-340 μmol L^（-1） with a limit of detection of 9.0 μmol L^（-1）.Moreover,such silver nanoplate-based optical sensors provide good selectivity for Cu^2＋ detection,and most other metal ions do not disturb its detection.Moreover,the practicality of the proposed sensor was tested.This Cu^2＋assay is advantageous in its simplicity,selectivity,and cost-effectiveness.