Matthew P. Nelson

Multivariate optical computation for predictive spectroscopy.

A novel optical approach to predicting chemical and physical properties based on principal component analysis (PCA) is proposed and evaluated using a data set from earlier work. In our approach, a regression vector produced by PCA is designed into the structure of a set of paired optical filters. Light passing through the paired filters produces an analog detector signal that is directly proportional to the chemical/physical property for which the regression vector was designed. This simple optical computational method for predictive spectroscopy is evaluated in several ways, using the example data for numeric simulation. First, we evaluate the sensitivity of the method to various types of spectroscopic errors commonly encountered and find the method to have the same susceptibilities toward error as standard methods. Second, we use propagation of errors to determine the effects of detector noise on the predictive power of the method, finding the optical computation approach to have a large multiplex advantage over conventional methods. Third, we use two different design approaches to the construction of the paired filter set for the example measurement to evaluate manufacturability, finding that adequate methods exist to design appropriate optical devices. Fourth, we numerically simulate the predictive errors introduced by design errors in the paired filters, finding that predictive errors are not increased over conventional methods. Fifth, we consider how the performance of the method is affected by light intensities that are not linearly related to chemical composition (as in transmission spectroscopy) and find that the method is only marginally affected. In summary, we conclude that many types of predictive measurements based on use of regression (or other) vectors and linear mathematics can be performed more rapidly, more effectly, and at considerably lower cost by the proposed optical computation method than by traditional dispersive or interferometric instrumentation. Although our simulations have used Raman experimental data, the method is equally applicable to Near-IR, UV-vis, IR, fluorescence, and other spectroscopies.

Waterborne Pathogen Detection Using Raman Spectroscopy

Raman spectroscopy is being evaluated as a candidate technology for waterborne pathogen detection. We have investigated the impact of key experimental and background interference parameters on the bacterial species level identification performance of Raman detection. These parameters include laser-induced photodamage threshold, composition of water matrix, and organism aging in water. The laser-induced photodamage may be minimized by operating a 532 nm continuous wave laser excitation at laser power densities below 2300 W/cm2 for Gram-positive Bacillus atrophaeus (formerly Bacillus globigii, BG) vegetative cells, 2800 W/cm2 for BG spores, and 3500 W/cm2 for Gram-negative E. coli (EC) organisms. In general, Bacillus spore microorganism preparations may be irradiated with higher laser power densities than the equivalent Bacillus vegetative preparations. In order to evaluate the impact of background interference and organism aging, we selected a biomaterials set comprising Gram-positive (anthrax simulants) organisms, Gram-negative (plague simulant) organisms, and proteins (toxin simulants) and constructed a Raman signature classifier that identifies at the species level. Subsequently, we evaluated the impact of tap water and storage time in water (aging) on the classifier performance when characterizing B. thuringiensis spores, BG spores, and EC cell preparations. In general, the measured Raman signatures of biological organisms exhibited minimal spectral variability with respect to the age of a resting suspension and water matrix composition. The observed signature variability did not substantially degrade discrimination performance at the genus and species levels. In addition, Raman chemical imaging spectroscopy was used to distinguish a mixture of BG spores and EC cells at the single cell level.

Fabrication and evaluation of a dimension-reduction fiberoptic system for chemical imaging applications

A novel system for rapid chemical imaging is described and evaluated. The system operates via single-frame spectroscopic chemical imaging with high spectroscopic resolution using a second-generation dimension-reduction fiberoptic array. Images are focused onto a rectangular array of square close-packed 25 μm cross-sectional f/2 optical fibers that are drawn into a linear distal array with serpentine ordering. The distal end is then imaged with an f/2 spectrograph equipped with a holographic grating and a gated intensified charge-coupled device (ICCD) camera for analysis. Software is used to extract the spatial/spectral information contained in a single ICCD image and deconvolute it into wavelength-specific univariate reconstructed images or position-specific spectra that span an 86 nm wavelength space using our present grating. A description of the fabrication of the dimension-reduction array is given as well as a zero-order reconstruction of a binary target and single-wavelength image reconstructions of a laser-induced plasma. The system is evaluated for spatial and spectral resolution, throughput, image brightness, resolving power, depth of focus, and channel cross talk. Treatment of the spectroscopic data obtained from the ICCD images for use in potential chemical imaging applications is discussed.A novel system for rapid chemical imaging is described and evaluated. The system operates via single-frame spectroscopic chemical imaging with high spectroscopic resolution using a second-generation dimension-reduction fiberoptic array. Images are focused onto a rectangular array of square close-packed 25 μm cross-sectional f/2 optical fibers that are drawn into a linear distal array with serpentine ordering. The distal end is then imaged with an f/2 spectrograph equipped with a holographic grating and a gated intensified charge-coupled device (ICCD) camera for analysis. Software is used to extract the spatial/spectral information contained in a single ICCD image and deconvolute it into wavelength-specific univariate reconstructed images or position-specific spectra that span an 86 nm wavelength space using our present grating. A description of the fabrication of the dimension-reduction array is given as well as a zero-order reconstruction of a binary target and single-wavelength image reconstructions of ...

Single-Shot Multiwavelength Imaging of Laser Plumes

A novel optical approach to single-shot chemical imaging with high spectroscopic resolution is described with the use of a prototype dimension-reduction fiber-optic array. Images are focused onto a 30 × 20 array of hexagonally packed 250 μm o.d. f/2 optical fibers that are drawn into a 600 × 1 distal array with specific ordering. The 600 × 1 side of the array is imaged with an f/2 spectrograph equipped with a holographic grating and a charge-coupled device (CCD) camera for spectral analysis. Software is used to extract the spatial/spectral information contained in the CCD images and de-convolute them into wavelength-specific reconstructed images or position-specific spectra that span a 190 nm wavelength space. “White light” zero-order images and first-order spectroscopic images of laser plumes have been reconstructed to illustrate proof-of-principle. Index Headings: Fiber optics; Chemical imaging; Spectroscopic imaging; Charged-coupled device (CCD); Laser-induced breakdown spectroscopy (LIBS).

Use of a 2D to 1D Dimension Reduction Fiber-Optic Array for Multiwavelength Imaging Sensors

A dimension reduction fiber-optic array is used to measure the response of a stacked-layer, image-guide CO2/O2 sensor, simultaneously at several different wavelengths. Two different image-guide CO2/O2 sensor configurations are described: a stacked-layer sensor, where luminescence indicators for CO2 and O2 are uniformly coated on the tip of the sensor; and a side-by-side coated sensor where the two indicators are coated on different halves of the fiber tip. It is shown that a single image-guide measurement, made by using the dimension reduction array, can be used to generate response plots, intensity profiles, and reconstructed images at different luminescence wavelengths. The spatial resolution of an image guide sensor is limited by the number of fibers used to construct the dimension reduction array.

Multivariate optical computation for predictive spectroscopy

A novel optical approach to predicting chemical and physical properties based on principal component analysis (PCA) is proposed and evaluated using a data set from earlier work. In our approach, a regression vector produced by PCA is designed into the structure of a set of paired optical filters. Light passing through the paired filters produces an analog detector signal directly proportional to the chemical/physical property for which the regression vector was designed. This simple optical computational method for predictive spectroscopy is evaluated in several ways, using the example data for numeric simulation. First, we evaluate the sensitivity of the method to various types of spectroscopy errors commonly encountered, and find the method to have the same susceptibilities toward error as standard methods. Second, we use propagation of errors to determine the effects of detector noise on the predictive power of the method, finding the optical computation approach to have a large multiplex advantage over conventional methods. Third, we use two different design approaches to the construction of the paired filter set for the example measurement to evaluate manufacturability, finding that adequate methods exist to design appropriate optical devices. Fourth, we numerically simulate the predictive errors introduced by design errors in the paired filters, finding that predictive errors are not increased over conventional methods. Fifth, we consider how the performance of the method is affected by light intensities that are not linearly related to chemical composition, and find that the method is only marginally affected. In summary, we conclude that many types of predictive measurements based upon use of regression vectors and linear mathematics can be performed more rapidly, more effectively, and at considerably lower cost by the proposed optical computation method than by traditional dispersive or interferometric instrumentation. Although our simulations have used Raman experimental data, the method is equally applicable to NIR, UV-Vis, IR, fluorescence and other spectroscopies.

Single-Frame Chemical Imaging: Dimension Reduction Fiber-Optic Array Improvements and Application to Laser-Induced Breakdown Spectroscopy

A single-frame approach to chemical imaging with high spectroscopic resolution is described that makes use of a second-generation dimension-reduction fiber-optic array. Laser-induced plume images are focused onto a 17 × 32 rectangular array of square close-packed 25 μm cross-sectional f/2 optical fibers that are drawn into a 544 × 1 distal array with serpentine ordering. The 544 × 1 side of the array is imaged with an f/2 spectrograph equipped with a holographic grating and a gated intensified charge-coupled device (ICCD) camera for spectral analysis. Software is used to extract the spatial/spectral information contained in the ICCD images and de-convolute them into wavelength-specific univariate reconstructed images or position-specific spectra that span an 86 nm wavelength space. Temporal resolution is provided by imaging sequential laser plumes with varying time delays after each laser pulse on the gated intensifier.

Evaluation of a high-throughput liquid crystal tunable filter for Raman chemical imaging of threat materials

Liquid crystal tunable filters (LCTF) have been used in systems developed for Raman Chemical Imaging spectroscopy of chemical, biological and explosives threat materials. However, an ongoing challenge in detecting trace levels of materials is the limited throughput provided by previous generation LCTFs. In this article, we describe a new class of birefringent LCTFs based on a Multi-Conjugate Filter design that provides high throughput over an extended wavelength range (440 nm-750 nm). The spectral resolution, tuning accuracy, out-of-band rejection efficiency have been evaluated and are demonstrated on a Raman chemical imaging microscope platform. Detection of trace threat particulate matter in the presence of complex background with improved overall detection performance is demonstrated.

Raman chemical imaging using flexible fiberscope technology

Raman chemical imaging microscopy has been proven to be a powerful methodology for analyzing a wide range of solid state materials. For biomedical applications, Raman chemical imaging has been shown to be effective in assessing clinical samples including breast tissue lesions and arterial plaques. With Raman chemical imaging systems based on microscopes, materials can be analyzed with molecular specificity, without labor intensive sample preparation or the use of dyes and stains at diffraction limited spatial resolution (< 250 nm). However, microscopes cannot readily be used to perform in vivo measurements. With the recent development of flexible fiberscope technology, Raman chemical imaging can be applied within remote and confined environments and the potential exists for in vivo use. This manuscript provides the first description of novel Raman chemical imaging fiberscope technology, including data analysis strategies for extracting information from Raman chemical imaging data sets.

Thermodynamic characterization of separation phenomena at the silica/polymer interface within glass-reinforced composites using adsorption chromatography. Part I

Glass-reinforced thermoset epoxies do not exhibit their theoretical mechanical performance because the interface between the glass and the polymer is weak. It is well known that glass surfaces can separate compounds differing in polarity because the silica surface is acidic and highly polar. Adsorption chromatography on silica stationary phases can be used to quantitatively evaluate the thermodynamics of separation phenomena. A variety of surface modifications are commonly used to qualitatively enhance the bonding between silica and the organic matrix; however, this report provides baseline data on the interactions between unmodified silica surfaces and a resin-hardener pair typical of those used in commercial glass-reinforced polymer composites. The adsorption equilibria of the polyfunctional amine, Jeffamine T-403, and the bifunctional epoxide. DER332, were quantitatively compared on an unmodified silica liquid chromatography column. It was determined that the competition between these premix components...