Vibrational Spectroscopy-based Sensor Systems
Editor(s): Steven D. Christesen; Arthur J. Sedlacek III
Infrared cavity ringdown and integrated cavity output spectroscopy for trace species monitoring
Author(s): Joshua B. Paul; James J. Scherer; Anthony O'Keefe; Larry Lapson; Janelle R. Anderson; Claire F. Gmachl; Federico Capasso; Alfred Y. Cho
Although the ability of high finesse optical cavities to provide effective absorption path-lengths exceeding 10 km. has been known for quite some time, attempts to utilize this property for the purposes of high-resolution spectroscopy have often resulted in extremely complex experimental systems. Here, we demonstrate how off-axis optical paths through such cavities can be employed to produce relatively simple spectrometers capable of ultrasensitive absorption measurements. A proof-of-concept study using visible diode lasers has achieved a normalized absorption sensitivity of 1.8*10-10 cm-1Hz-1/2. Additionally, quantum cascade lasers have been employed to extend this method into the mid-infrared region, where sensitivities of 1.2*10-9 cm-1Hz-1/2 have been obtained.
Creation of 0.10-cm-1 resolution quantitative infrared spectral libraries for gas samples
Author(s): Steven W. Sharpe; Robert L. Sams; Timothy J. Johnson; Pamela M. Chu; George C. Rhoderick; Franklin R. Guenther
The National Institute of Standards and Technology (NIST) and the Pacific Northwest National Laboratory (PNNL) are independently creating quantitative, approximately 0.10 cm-1 resolution, infrared spectral libraries of vapor phase compounds. The NIST library will consist of approximately 100 vapor phase spectra of volatile hazardous air pollutants (HAPs) and suspected greenhouse gases. The PNNL library will consist of approximately 400 vapor phase spectra associated with DOE's remediation mission. A critical part of creating and validating any quantitative work involves independent verification based on inter-laboratory comparisons. The two laboratories use significantly different sample preparation and handling techniques. NIST uses gravimetric dilution and a continuous flowing sample while PNNL uses partial pressure dilution and a static sample. Agreement is generally found to be within the statistical uncertainties of the Beer's law fit and less than 3 percent of the total integrated band areas for the 4 chemicals used in this comparison. There does appear to be a small systematic difference between the PNNL and NIST data, however. Possible sources of the systematic difference will be discussed as well as technical details concerning the sample preparation and the procedures for overcoming instrumental artifacts.
Gas detection by cavity ringdown absorption with a fiber optic amplifier loop
Author(s): Kathryn Atherton; George Stewart; Brian Culshaw
Cavity ring-down is investigated as a technique to increase the sensitivity of optical fiber gas sensors. The ring-down cavity consists of an optical fiber loop containing a micro-optic cell and an erbium-doped fiber amplifier. The erbium fiber amplifier introduces gain into the cavity to increase the ring-down times and therefore the system sensitivity. This paper reports an investigation of the system sensitivity.
Using solid phase extraction media to increase the sensitivity of infrared and Raman spectroscopy
Author(s): Kenneth James Ewing
Solid phase extraction (SPE) media are usually used to collect samples from both the air and water for analysis by conventional analytical instruments such as the gas chromatograph or mass spectrometer. This approach requires that the analyte be sorbed onto the SPE media, then removed using either solvent extraction or thermal desorption of the vapor into an analytical instrument. Because the analyte is concentrated on the sorbent medium it is possible to acquire spectroscopic information of the analyte on the sorbent. This approach can be used to enhance the detection limits for both Raman and infrared spectroscopic techniques.
Infrared ATR: a probe for cellular activation
Author(s): Jerilyn A. Timlin; Laura E. Martin; M. Kathleen Alam; David M. Haaland; Kristen Garrison; C. Richard Lyons; Brian Hjelle
We employ infrared spectroscopy (IR) with attenuated total reflectance (ATR) as a sampling technique to monitor live and dried RAW cells (a murine macrophage cell line) during activation with g-interferon and lipopolysaccharide. By comparing the spectra of activated cells at various time points to the spectra of healthy control cells, we identify spectral bands associated with nucleic acids that are markers for the cell activation process. These spectral changes are slight and can be complicated with the normal metabolic changes that occur within cells. We will discuss the use of data pretreatment strategies to accurately correct for the contribution of the buffer to the live cell spectra. We find the standard background correction method inadequate for concentrated solutions of cells. Data presented shows the severe effect incorrect background subtraction has on the cell spectra. We report a more accurate correction for phosphate buffer spectral contribution using an interactive subtraction of the buffer spectrum. We will show classification of dried control and activated macrophage cell spectra using partial-least squares analysis with multiplicative scatter correction.
Giant enhancement of spectral emissions from molecules adsorbed on fractal/microcavity composite media
Author(s): Robert L. Armstrong; Won-Tae Kim; Viktor A. Podolskiy; Vladimir M. Shalaev; Vladimir P. Drachev; Vladimir P. Safonov
We describe the characteristics of a novel optical material developed in our laboratory, a fractal/microcavity composite. Both components of this composite exhibit resonance behavior whereby the amplitudes of spectral emissions, generated by molecules either adsorbed onto the composite or located remotely from it, are significantly enhanced. In the composite, the individual enhancement factors combine multiplicatively with the result that spectral emissions are enhanced by an extremely large factor. In particular, the extremely large enhancement factors facilitate the generation of nonlinear optical processes in the composite, which may be exploited in the fabrication of ultra-sensitive detectors.
Assembly of gold nanostructured films templated by colloidal crystals and use in surface-enhanced Raman spectroscopy
Author(s): Peter M. Tessier; Kate Ong; Steven D. Christesen; Abraham M. Lenhoff; Eric W. Kaler; Orlin D. Velev
Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive technique for quantifying trace amounts of analyte adsorbed at a roughened metal surface. Many techniques, including electrochemical etching and e-beam lithography, have been used previously to produce roughened metallic surfaces. In this work we demonstrate how novel gold nanostructured films, which are simply fabricated using gold nanoparticles and latex microspheres, can be used as highly sensitive SERS substrates. The gold films are templated by 3D colloidal crystals and display long ranged ordered regions. Since the films are porous on two length scales and, therefore, possess a high surface area, we have investigated their SERS activity using sodium cyanide as a model compound. We have integrated these substrates into a flow chamber and demonstrated the quantitative detection of sodium cyanide form 5 to 500 ppb. Our results also reveal that cyanide detection can be significantly enhanced by lowering the pH after cyanide adsorption, likely indicating a conformational change of the bound cyanide. This study illustrates how novel materials formed by simple wet chemistry techniques can be used in practical devices for the detection of chemical agents, and, more generally, illustrates how material design and spectroscopic evaluation can be rapidly integrated.
Field-portable AOTF-based monitor technology for environmental sensing
Author(s): Brian M. Cullum; Joel Mobley; Alan L. Wintenberg; R. A. Maples; David L. Stokes; Tuan Vo-Dinh
The development of a field-portable Raman instrument for environmental analyses is described. It is based on the use of a near-infrared frequency-stabilized diode laser for excitation, an acousto-optic tunable filter for wavelength selection, and an avalanche photodiode for detection. Evaluation of this instrument for the monitoring of environmentally important species will be discussed as well as its ability to be operated in room light without significantly increasing the background signal. In addition, we will also describe a baseline removal procedure based on second derivative approach that simplifies and increases the accuracy of the instrument' s automated identification algorithm
In-line infrared sensors covering the midinfrared from 2 to 14 micrometers (5000 to 700 cm-1)
Author(s): Paul A. Wilks
The wide spread use of FT-IR spectrometers has created many potential applications for MIR spectroscopy in quality control and process monitoring. Because of the complexity, cost and environmental requirements of FT-IR spectrometers, it is usually impractical to move them into the field or along side the process stream. Portable MIR instruments such as the Wilks Infracal Filtometers are filling the need for field measurements. For in-line sensing, ATR probes combined with optical fibers or light pipes are being installed to transmit absorption data to remotely located FT-IR instruments. While such installations have performed satisfactorily in some applications, optical fibers are limited in energy transmission efficiency and in the MIR are extremely expensive. A new type of MIR sensor has been developed by Wilks Enterprise, Inc. called the IR Plug, that makes the absorption measurement directly in stream, transmitting digital concentration information to a remote PC or display panel. The Wilks IR Plug eliminates the need for optical fibers, light pipes and the FT-IR spectrometer itself.
Rugged high-speed rotary imaging Fourier transform spectrometer for industrial use
Author(s): Winthrop Wadsworth; Jens-Peter Dybwad
A new rugged rotary Fourier Transform Spectrometer (FTS) has been developed. It can be used for environmental remote sensing and monitoring of chemical processes. Both single pixel and mosaic imaging configurations have been built and tested. The continuous rotary scan of the 'Turbo FT' allows operation without the laser reference of a conventional FTS, and it has been demonstrated to deliver 30 to 360 spectral scans per second and 1 cm-1 resolution, with excellent lineshape. A new 'space frame' version of the interferometer, with excellent mechanical and thermal stability, was field tested in both airborne and ground systems during the year 2000, with good results. The interferometer for this instrument is palm sized, and weighs 20 oz. It is totally sealed from the environment, and can be mounted, with its drive electronics, into a temperature stabilized enclosure for outside remote sensing applications For industrial applications, it can be used on-line or off-line, in conjunction with fiber optics, to measure and/or control multiple process lines. With the appropriate optics and detector set, the wavelength range can be adjusted from 1.0 to 25.0 micrometers. The resolution is variable from 1 to 8 cm-1. Various processors, data acquisition boards, and software have been used in the development, including the Labview package from National Instruments. Custom software for acquisition, display, and storage is being developed in summer 2001. The data acquisition system can be tailored for the speed and number of pixels required for the application. The current commercially available hardware being used can support up to 16 pixels at up to 100 scans per second.
Chemical imaging cameras: the first steps
Author(s): Richard A. Crocombe; Ellen V. Miseo; Norman A. Wright; Kevin J. Beltis; Alan S. Louie
This paper describes the use of a fast readout MCT focal plane array for spectrochemical imaging.
Short-range noncontact detection of surface contamination using Raman lidar
Author(s): Arthur J. Sedlacek III; Mark D. Ray; N. Scott Higdon; Dale A. Richter
The application of a novel mini-Raman Lidar to the standoff detection and identification of chemical spills is discussed. The new chemical sensor combines the spectral fingerprintign of solar-blind UV Raman spectroscopy with the principles of lidar to open a new venue of short-range, non- contact detection and identification of unknown substances on surfaces. In addition to discussing experimental result collected with a 'proof-of-principle' system, a next generation system, currently under development, is also presented.
Data analysis strategies for passive multispectral and hyperspectral infrared remote sensors
Author(s): Gary W. Small; Lin Zhang
The detection of airborne chemicals is a key capability in a variety of environmental monitoring scenarios. For these applications, passive IR remote sensors collect IR emissions form natural and man-made sources such as the radiant emission from the earth or emissions from the stacks of a chemical plant. Chemical compounds absorb or emit IR energy at characteristic wavelengths, and the profile of these absorption or emission signatures can be used to identify a chemical and to estimate the amount present. Passive IR remote sensors can be implemented in either imaging or non- imaging configurations and can be constructed to acquire IR emission data in either multispectral or hyperspectral modes. Implementing these measurements successfully requires the construction of rugged, portable instruments and the development of computer processing techniques that allow the automated analysis of the large quantities of data acquired by these sensors. The research presented here describes the development of novel signal processing and pattern recognition methodology for application to multispectral imaging data and to non-imaging data acquired with a hyperspectral instrument. Remote sensing data were collected with these instruments mounted on an aircraft platform. Data acquired at an industrial site are used to demonstrate the characteristics of each sensor and the data analysis methodology.
FT-IR interferogram signal processing for uniform-time sampling
Author(s): Christopher J. Manning; Roger J. Combs
A new approach to Fourier transform-infrared (FT-IR) signal acquisition and processing has been developed recently. This approach relies on digital signal processing (DSP) and enables the use of sigma-delta analog-to-digital converters (ADCs). Data are simultaneously oversampled from both the infrared (IR) and reference laser channels at uniform time intervals. This approach contrasts sharply with the traditional method, in which reference channel zero-crossings are used to trigger ADC sampling of the IR channel at equal increments of optical path difference (OPD). In one embodiment of the new approach, data from the laser channel are processed to values of mirror position. These data can be used to interpolate the IR signal. The uniform time sampling method shifts instrument complexity from hardware to software, while improving the spectral accuracy by fully correcting the effects of optical velocity variation. The correction is sufficient even with a sinusoidal variation in the optical retardation rate. This study demonstrates the approach using the Lomb-Scargle periodogram.
Continuous internal response calibration for FT-IR spectrometry
Author(s): Roger J. Combs; Christopher J. Manning
Calibration of Fourier transform infrared (FT-IR) spectrometer response is crucial to quantitative spectroradiometric measurements. The use of light emitting diodes (LEDs) as probes of detector channel response is further demonstrated. Detector channel response functions significantly impact spectrometer performance. LED modulation bandwidths, some extending well into the megaHertz (MHz) range, are more than fast enough for characterization of FT-IR detector channel responses. A variety of optical probe signals can be generated using LEDs driven by waveform generators, lock-in amplifiers or digital signal processors. Accurate determination of the phase and gain responses of both the IR signal and laser reference channels is straightforward. With appropriate modulation of the IR LEDs, channel response is measurable on a scan-by-scan basis, perhaps even to the point of accurately determining detector saturation in real time.
Application of multivariate optical computing to near-infrared imaging
Author(s): Michael L. Myrick; Olusola O. Soyemi; Fred Haibach; Lixia Zhang; Ashley Greer; Hongli Li; Ryan Priore; Maria V. Schiza; J. R. Farr
Rapid quantitative imaging of chemical species is an important tool for investigating heterogenous mixtures, such as laminated plastics, biological samples and vapor plumes. Using traditional spectroscopic methods requires difficult computations on very large data sets. By embedding a spectral pattern that corresponds to a target analyte in an interference filter in a beamsplitter arrangement; the chemical information in an image can be obtained rapidly and with a minimal amount of computation. A candidate filter design that is tolerant to the angles present in an imaging arrangement is evaluated in near-infrared spectral region for an organic analyte and an interferent.
Surface-enhanced Raman as a water monitor for warfare agents
Author(s): Kevin M. Spencer; James M. Sylvia; Susan L. Clauson; James A. Janni
The threat of chemical warfare agents being released upon civilian and military personnel continues to escalate. One aspect of chemical preparedness is to analyze and protect the portable water supply for the military. Chemical nerve, blister, and choking agents, as well as biological threats must all be analyzed and low limits of detection must be verified. For chemical agents, this generally means detection down to the low ppb levels. Surface-Enhanced Raman Spectroscopy (SERS) is a spectroscopic technique that can detect trace levels of contaminants directly in the aqueous environment. In this paper, results are presented on the use of SERS to detect chemical and biological agent simulants with an end goal of creating a Joint Service Agent Water Monitor. Detection of cyanide, 2-chloroethyl ethyl sulfide, phosphonates, Gram-positive and Gram-negative bacteria using SERS has been performed and is discussed herein. Aspects of transferring laboratory results to an unattended field instrument are also discussed.
Chemical agent identification by surface-enhanced Raman spectroscopy
Author(s): Stuart Farquharson; Paul Maksymiuk; Kate Ong; Steven D. Christesen
The recent distribution of anthrax through the U.S. postal system and the subsequent infection and death of several postal and national media employees, amplifies the need for methods to rapidly detect, identify, and quantify this and other chemical and biological warfare agents. The U.S. military has also identified water supplies as a likely method of warfare agent deployment and is funding the development of a Joint Service Agent Water Monitor. In an effort to aid military personnel and the public at large, we are developing a portable analyzer capable of identifying and quantifying chemical agents rapidly, either 'on-demand' or continuously.
Optimization of substrates for surface-enhanced Raman spectroscopy of bacteria
Author(s): Nicholas F. Fell Jr.; Alicia G. B. Smith; Melissa Vellone; Augustus W. Fountain III
The use of surface-enhanced Raman spectroscopy (SERS) has recently seen a revitalization of interest with advances in surface coating technologies and other related areas. Recent reports have indicated that enhancement factors of up to 14 orders of magnitude can be achieved, providing the sensitivity requisite to trace level detection of target analytes. Due to the short range of the SERS effect, the interference of background materials may be reduced if the target analyte can be selectively brought near the SERS surface. SERS also holds the promise of providing the ability to determine the identity of bacterial species through recognition of the unique spectrum of a given species. The first major hurdle to its application to this problem is the design and optimization of appropriate surfaces for SERS of bacteria. This is complicated by the negative surface charge of the metal surface and the bacterium that results in a repulsive force that must be overcome. Our efforts have focused on selection of the best SERS substrate for this purpose. We are examining four potential SERS substrates: Au colloids in suspension with the bacteria, Au colloids immobilized on a surface, electrochemically roughened Au surfaces, and Ag periodic particle arrays provided by Prof. Richard van Duyne.
SERS of whole-cell bacteria and trace levels of biological molecules
Author(s): Andrew A. Guzelian; James M. Sylvia; James A. Janni; Susan L. Clauson; Kevin M. Spencer
Through its several orders of magnitude signal enhancement over normal Raman, surface-enhanced Raman spectroscopy (SERS) provides an opportunity to extend the benefits of vibrational spectroscopy to trace level detection. SERS in particular holds great potential for biological sensing due to the weak Raman bands of water and the reduction in fluorescence backgrounds from interactions of the analyte with the metal SERS substrate. This work examines the trace level detection of biological molecules and oligomers such as amino acids, peptides, and oligonucleotides as well as the detection of whole cell bacteria. The SERS substrates employed are electrochemically roughened gold. The biological molecules show well-resolved and intense bands that are an effective spectral signature; these bands also persist in corresponding oligomeric compounds. Spectra from whole cell bacteria have been obtained for several species, including gram-positive and gram-negative strains. Viable and nonviable cells have also been examined and significant spectral differences are observed. The results show the potential for using SERS as an analytical tool for the identification of biological molecules and microorganisms with applications in biological agent detection, food and water monitoring, and the search for signs of extraterrestrial life.
UV Raman detection of micro-organisms and their toxins in fish tissue
Author(s): Wilfred H. Nelson; Jay F. Sperry; Paul Hargraves; Eugene G. Hanlon; Ramachandra R. Dasari; Michael S. Feld
It has been determined that domoic acid (DA) can be quantitated from homogenized shellfish tissue by means of resonance Raman spectra excited by 251 nm light. Detection limits have been found to be substantially below the 20 micrograms DA per grain tissue deemed by regulators to be unfit for human consumption. Clam tissue obtained from a supermarket has been prepared for analysis by direct homogenization for 2 minutes in a Waring blender. The homogenized samples were placed in a flow system and subjected to a 5-10 mw 251 nm excitation. Back-scattering collected for 20-30 seconds provided sufficient information for analysis. The method is extremely simple to use since the DA produces a single intense peak at 1652 cm-1. Because relatively-weak protein, nucleic acid and lipid spectra are excited from the tissue, background interference is surprisingly low. Bacteria can be detected using the same approach, but sensitivities are much lower primarily due to spectral interference from tissue nucleic acids.
Surface enhanced Raman detection of CW agents in water using gold sol gel substrates
Author(s): W. Ranjith Premasiri; Richard H. Clarke; M. Edward Womble
The development of a water analysis system capable of detecting both inanimate trace chemical contaminants and viable microbial contaminants has long been a project of interest to our group. The capability of detecting both chemical and biological agent sources in a single device configuration would clearly add to the value of such a product. In the present work, we describe results with chemical warfare agents from our efforts to produce a Raman system for the detection of both chemical and biological warfare agents in water. We utilize laser Raman light scattering and employ Surface Enhanced Raman Spectroscopy (SERS)on solid state gold sol-gel detectors combined with fiber optic collection of the enhanced light signal in the sampling system to augment the normally low intensity Raman Scattering signal from trace materials.
Airborne passive FT-IR spectrometry
Author(s): Robert T. Kroutil; Roger J. Combs; Robert B. Knapp; Gary W. Small
Rapid airborne identification and quantification of vapor hazards is an environmentally important capability for a variety of open-air scenarios. This study demonstrates the use of a commercially available passive Fourier transform IR (FT-IR) spectrometer to detect, identify, and quantify ammonia and ethanol vapor signatures depending on the appropriate signal processing strategy. The signal- processing strategy removes the need for a representative background spectrum and it consists of three steps to extract the spectral information associated with the target vapor. The first step is optimal interferogram segment selection which depends on the bandwidth of the target spectral feature. The second step applies the statistically signicant finite impulse responses matrix filter to the optimal interferogram segment to attenuate spectral interferences. The third step quantifies the FIRM filter results with a discriminant analysis. The signal processing results prove that low-altitude airborne passive FT-IR spectrometry allows rapid quantitative detection of ammonia and ethanol vapor generated plumes. This effort also documents the direct interferogram analysis of data from the fast scanning airborne passive FT-IR spectrometer.
Infrared imaging of skin lesions
Author(s): Laura M. McIntosh; James R. Mansfield; Michael Jackson; A. Neil Crowson; Henry H. Mantsch
IR spectroscopy produces spectra in which detailed information concerning chemical structure is inherent. Numerous studies have demonstrated that the most useful IR methods for analysis of biological tissues are microscopic image-based techniques in which fine-scaled spatial and high-quality spectral information is integrated. Unlike traditional visible microscopic methods, the contrast in IR imaging is gained by differences in spectra and the spatial heterogeneity of biochemical components, not by the addition of stains. In order for IR imaging to be more broadly accepted, non-subjective data processing methods are being developed to extract the most out of the large spectral images that are acquired. This paper demonstrates data processing techniques that have been extremely useful in the analysis of normal and abnormal skin. Analysis of skin specimens is of particular clinical importance due to the difficulty in rendering a differential diagnosis. Unstained frozen skin sections were mapped using an IR microscope. Functional group mapping, clustering routines and linear discriminant analysis were used to process the data. Functional group mapping and clustering routines were useful in the initial interpretation of images and to research for trends in uncharacterized spectral images. LDA was useful for differentiating normal from abnormal tissue once a well- defined training spectral set was established. Representative spectroscopic images are shown that demonstrate the power of IR imaging.
SERS detection of the nuclear weapons explosive triaminotrinitrobenzene
Author(s): James A. Janni; James M. Sylvia; Susan L. Clauson; Kevin M. Spencer
The sensitivity demonstrated for other nitro-aromatic explosives through SERS has been applied to triaminotrinitro-benzene (TATB). Gas phase and solution phase in strong acids and bases, as well as organic solvents SERS spectra have been collected. For each method of TATB sample introduction on electrochemically roughened gold substrates or gold colloids, different bands and sensitivities were observed. These bands likely result from the three possible adsorption sites in the molecule and its reaction with the gold surface. In some cases, the SERS spectra closely overlapped the carbonaceous background and indicate TATB degradation. Although the mechanisms of the reaction of TATB with the surface are not understood, important aspects of optimized TATB SERS detection have been observed. Para-nitroaniline (p-NA) was also studied due to its similarity with TATB and its greater solubility in water.
Detection of soil surface contaminants by infrared reflection spectroscopy
Author(s): Thomas A. Blake; Paul L. Gassman
A benchtop Fourier transform infrared spectrometer and specular reflection accessory have been used to record reflection spectra of several chemical compounds coating a loamy, psamment soil. Unpolarized reflection spectra were recorded between 4600 and 500 cm-1 with a resolution of 4 cm-1 for the compounds dimethyl methylphosphonate, trimethyl phosphate, methylphosphonic acid, 2,2'-thiodiethanol, diazinon, diesel fuel, and ammonium nitrate mixed separately into soil samples with concentrations of 10, 5, and 1 mg of analyte per gram of soil. The soil reflection spectra are compared to liquid or solid transmission spectra of the pure compound and frequency shifts and relative intensity changes for the absorption features are noted. As an example of detection sensitivity, we have estimated that we can detect 300 nanograms of dimethyl methylphosphonate on the surface layer of soil in the focal spot (15 mm2) of the reflection accessory using one of DMMP's C - H stretch modes. The signal-to-noise (peak-to-peak) of this spectral feature under these circumstances would be 3/1. We also estimate that the flux of infrared photons reaching the soil is 1014 photons/sec/cm-1. We have recorded polarization-resolved reflection spectra of the uncoated soil and coarse and fine grained quartz sands as a function of angle of incidence and reflection and have determined the degree of polarization for light reflected off of these materials at frequencies associated with volume scattering and surface scattering features. As might be expected, the volume scattering features show a significant depolarization of the light - degree of polarization after reflection is < 20% - and the surface scattering features retain a much higher degree of polarization upon reflection, > 75%. We have also recorded polarization resolved spectra of tributyl phosphate on the soil and found significant differences between the s- and p-polarized spectra. This fact could be used to employ polarization modulation detection to improve detection sensitivity.
Microphotonic sensors for the rapid detection of the presence of explosive gas mixtures
Author(s): Kevin L. McNesby; Andrzej W. Miziolek
A first generation, microphotonic sensor for rapid (10 ms response time) measurement of vapors from the hydrocarbon-based fuels JP-8, DF-2, and gasoline has been developed at the U.S. Army Research Laboratory. This sensor is based upon a previously reported laser mixing technique that uses two tunable diode lasers emitting in the near-infrared spectral region to measure concentrations of gases having unstructured absorption spectra. The fiber-mixed laser beam consists of two wavelengths, one of which is absorbed by the fuel vapor, and one of which is not absorbed. By sinusoidally modulating the power of the two lasers at the same frequency but 180 degrees out of phase, a sinusoidal signal is generated at the detector (when the target gas is present in the line of sight). The signal amplitude, measured using standard phase sensitive detection techniques, is proportional to fuel vapor concentration. A second generation sensor, designed to measure the full envelope of the first overtone C-H vibrations in middle distillate fuels is currently being developed. Both sensors are described. Limits of detection using the first generation sensor are reported for vapors of the three fuels studied.
Ground-based passive FT-IR spectrometry
Author(s): Robert B. Knapp; Roger J. Combs; Robert T. Kroutil
Absorbance and transmittance spectra were acquired with ground-based passive FT-IR spectrometry for industrial stack evaluations and open-air controlled vapor generation experiments. The industrial stack effluents of sulfur dioxide and nitrous oxide were detected from a coal-burning power plant and an acid plant, respectively, with both MWIR and LWIR passive sensors. The controlled open-air experiments relied on only a LWIR sensor. These experiments produced plumes of methanol and ethanol at three and four elevated plume temperatures, respectively. Various vapor concentration pathlength produces of both ethanol and methanol were generated and gravimetrically monitored in the range from 0 to 300 ppm-m. The associated absorbance values for these concentration pathlength products were found to obey Beer's Law for each elevate stack temperature of 125, 150, 175, and 200 degrees C.
Nonintrusive Raman analysis of chemical agents
Author(s): Steven D. Christesen; Michael M. Carrabba; Job M. Bello
Portable fiber-optic Raman systems are being used to analyze chemical agents and other toxic chemicals in sealed glass containers. These containers include ampoules and bottles that are contents of chemical agent identification sets (CAIS) developed for use in training military personnel in chemical agent identification, safe handling, and decontamination. Real-time nonintrusive analysis of these sets is required so that the items containing chemical agents can be identified for proper disposal. This paper details the laboratory measurement of Raman spectra of chemical agents, Raman scattering cross sections of chemical agents, and the analysis of CAIS items in the field.
Enhanced Raman spectroscopy in the deep ultraviolet: custom-designed optics for continuously tuneable excitation
Author(s): Roland K. Appel; Chris Dyer; Anita C. Jones; Andy Bell; John Lockwood
A prototype optical system has been constructed that is chromatically-corrected from the deep ultraviolet (UV) to the far red (200-700nm), facilitating reliable and straightforward sample positioning, as required for quantitative resonance Raman spectroscopy (RRS). The collection side is fully achromatic, whilst the illumination side requires minimal user intervention. Results are presented that demonstrate the axial and spatial imaging performance of the instrument. Spectra illustrate the application of RRS for selective enhancement of analytes in low concentration. Variations in enhancement factor and spectral signature as a function of excitation wavelength are demonstrated. The results illustrate the need for well-characterized achromatic optics when carrying out quantitative investigation using a tuneable UV source laser. A UV-sensitive video-rate CCD is also incorporated into the optical scheme, enabling limited operation as a deep UV microscope. An imaging resolution of approximately 7 micrometers has been demonstrated.