Miniature Chemical Sensor combining Molecular Recognition with Evanescent Wave Cavity Ring-Down Spectroscopy

To address the chemical sensing needs of DOE, a new class of chemical sensors is being developed that enables qualitative and quantitative, remote, real-time, optical diagnostics of chemical species in hazardous gas, liquid, and semi-solid phases by employing evanescent wave cavity ring-down spectroscopy (EW-CRDS). The feasibility and sensitivity of EW-CRDS was demonstrated previously under Project No.60231. The objective of this project is to enhance the selectivity and range of application of EW-CRDS. Selectivity is achieved spectroscopically by using vibrational overtones in the near infrared and chemically by using modified surfaces to encourage selective adsorption of analyte while preventing non-selective adsorption. The range of application is expanded by extending EW-CRDS to liquids and by combining EW-CRDS with the unique optical properties of nanoparticles

Miniature Chemical Sensor Combining Molecular Recognition with Evanescent Wave Cavity Ring-Down Spectroscopy

To address the chemical sensing needs of DOE, a new class of chemical sensors is being developed that enables qualitative and quantitative, remote, real-time, optical diagnostics of chemical species in hazardous gas, liquid, and semi-solid phases by employing evanescent wave cavity ringdown spectroscopy (EW-CRDS). The sensitivity of EW-CRDS was demonstrated previously under Project No.60231. The objective of this project is to enhance the range of application and selectivity of the technique by combining EW-CRDS with refractive-index-sensitive nanoparticle optics, molecular recognition (MR) chemistry, and by utilizing the polarization-dependence of EW-CRDS. Research Progress and Implication

Miniature Chemical Sensor combining Molecular Recognition with Evanescent Wave Cavity Ring-Down Spectroscopy

To address the chemical sensing needs of DOE, a new class of chemical sensors is being developed that enables qualitative and quantitative, remote, real-time, optical diagnostics of chemical species in hazardous gas, liquid, and semi-solid phases by employing evanescent wave cavity ring-down spectroscopy (EW-CRDS). The feasibility and sensitivity of EW-CRDS was demonstrated previously under Project No.60231. The objective of this project is to enhance the selectivity and domain of application of EW-CRDS. Selectivity is enhanced by using molecular recognition (MR) chemistry and polarized ''fingerprint'' near-IR spectroscopy, while the domain of application is expanded by combining EW-CRDS with the unique optical properties of nanoparticles and by extending the technique to liquids

Submultiple Data Collection to Explore Spectroscopic Instrument Instabilities Shows that Much of the “Noise” is not Stochastic

As has long been understood, the noise on a spectrometric signal can be reduced by averaging over time, and the averaged noise is expected to decrease as <i>t</i>^1/2, the square root of the data collection time. However, with contemporary capability for fast data collection and storage, we can retain and access a great deal more information about a signal train than just its average over time. During the same collection time, we can record the signal averaged over much shorter, equal, fixed periods. This is, then, the set of signals over submultiples of the total collection time. With a sufficiently large set of submultiples, the distribution of the signal’s fluctuations over the submultiple periods of the data stream can be acquired at each wavelength (or frequency). From the autocorrelations of submultiple sets, we find only some fraction of these fluctuations consist of stochastic noise. Part of the fluctuations are what we call “fast drift”, which is defined as drift over a time shorter than the complete measurement period of the average spectrum. In effect, what is usually assumed to be stochastic noise has a significant component of fast drift due to changes of conditions in the spectroscopic system. In addition, we show that the extreme values of the fluctuation of the signals are usually not balanced (equal magnitudes, equal probabilities) on either side of the mean or median without an inconveniently long measurement time; the data is almost inevitably biased. In other words, the unbalanced data is collected in an unbalanced manner around the mean, and so the median provides a better measure of the true spectrum. As is shown here, by using the medians of these distributions, the signal-to-noise of the spectrum can be increased and sampling bias reduced. The effect of this submultiple median data treatment is demonstrated for infrared, circular dichroism, and Raman spectrometry

Stray Light Correction in the Optical Spectroscopy of Crystals

It has long been known in spectroscopy that light not passing through a sample, but reaching the detector (i.e., stray light), results in a distortion of the spectrum known as absorption flattening. In spectroscopy with crystals, one must either include such stray light or take steps to exclude it. In the former case, the derived spectra are not accurate. In the latter case, a significant amount of the crystal must be masked off and excluded. In this paper, we describe a method that allows use of the entire crystal by correcting the distorted spectrum