Optical chromatography involves loosely focusing a laser beam into a fluid flowing opposite to the
direction of laser propagation. When microscopic particles in the flow path encounter this beam they
are optically trapped along the beam and are pushed upstream by the radiation pressure from the laser
focal point to rest at a position where the optical and fluid drag forces on the particle balance.
Because optical and fluid forces are sensitive to differences in the physical and chemical properties of
a particle, fine separations are possible. A laser beam which completely fills a fluid channel has been
operated as an optically tunable filter for the separation of polymeric/colloidal and biological
samples. We demonstrate here how this technique coupled with an advanced microfluidic platform
can be used as both a coarse and fine method to fractionate particles in an injected sample. The
microfluidic network allows for a monodisperse mixed particle sample of polystyrene and
poly(methyl methacrylate) to be injected, hydrodynamically focused and completely separated. To
test the limit of separation, a mixed polystyrene sample containing two particles varying in size by
less than 0.2% was run in the system. The analysis of the resulting separation sets the framework for
continued work to perform ultra-fine separations.
Laser separation of particles is achieved using forces resulting from the momentum exchange
between particles and photons constituting the laser radiation. Particles can experience different
optical forces depending on their size and/or optical properties, such as refractive index. Thus,
particles can move at different speeds in the presence of an optical force, leading to spatial
separations. Several studies for aqueous suspension of particles have been reported in the past. In
this paper, we present extensive analysis for optical forces on non-absorbing aerosol particles.
We used a loosely focused Gaussian 1064 nm laser to simultaneously hold and deflect particles
entrained in flow perpendicular to their direction of travel. The gradient force is used to hold the
particles against the viscous drag for a short period of time. The scattering force simultaneously
pushes the particles during this period. Theoretical calculations are used to simulate particle
trajectories and to determine the net deflection: a measure of the ability to separate. We invented a novel method for aerosol generation and delivery to the flow cell. Particle motion was imaged using a high speed camera working at 3000+ frames per second with a viewing area up to a few millimeters. An 8W near-infrared 1064 nm laser was used to provide the optical force to the particles. Theoretical predictions were corroborated with measurements using polystyrene latex particles of 20 micron diameter. We measured particle deflections up to about 1500 microns. Such large deflections represent a new milestone for optical chromatography in the gas phase.
A variation on the typical optical chromatography system was used to measure optical force
differentials of complex micro-particles that have been assembled or fabricated using bead
chemistries, bio-molecule tethers, or biological bead coatings. A number of bio-inspired particle
types have been created to help elucidate the origin of optical force differentials that are known
or suspected in biological systems such as bacterial cells / spores, and mammalian cells. A
number of optical force measurements will be presented for a variety of micro-fabricated
particles and the results and capabilities discussed.
Optical Chromatography involves the elegant combination of opposing optical and
fluid drag forces on colloidal samples within microfluidic environments to both
measure analytical differences and fractionate injected samples. Particles that
encounter the focused laser beam are trapped axially along the beam and are pushed
upstream from the laser focal point to rest at a point where the optical and fluid
forces on the particle balance. In our recent devices particles are pushed into a
region of lower microfluidic flow, where they can be retained and fractionated.
Because optical and fluid forces are sensitive to differences in the physical and
chemical properties of a sample, differences between samples and thus separations
are possible. An optical chromatography beam focused to completely fill a fluid
channel is operated as an optically tunable filter for the separation of
polymeric/colloidal and biological samples. We demonstrate this technique coupled
with an advanced microfluidic platform and show how it can be used as an effective
method to fractionate particles in an injected multi-component sample. Our
advanced microfluidic design accommodates three lasers simultaneously to
effectively create a sequential cascade optical chromatographic separation system.
In our experiments, microscopic polymer and glass spheres in microfluidic devices
are manipulated using pressure generated by a high power laser beam. The effect of
the laser on the particles and the manipulations are imaged using a microscope lens
connect to a CCD camera. Differential forces on particles of varying physical and
chemical composition have been measured. The goal is to measure the optical forces
on chemically different particles and catalog the associated chemical and physical
differences to understand which properties and mechanisms result in force
differentials. The aim is to better understand the range of optical separations that may
be possible and the extent to which the instrument can differentiate between similar
microspheres in terms of size and/or chemical composition.
Optical Chromatography, used for particle separation, involves loosely focusing a laser into a fluid flowing opposite the direction of laser propagation. When microscopic particles in the flow path encounter this beam they are trapped axially along the beam and are pushed upstream from the laser focal point to rest at a point where the optical and fluid forces on the particle balance. Because optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, fine separations are possible. An optical chromatography beam which completely fills a fluid channel can operate as an optically tunable filter for the separation of polymeric/colloidal and biological samples. We will show how the technique can be used to separate injected samples containing large numbers of colloids. The power of optical chromatographic separations will be illustrated through the combination with other analytical techniques.
The application of laser radiation as a method for the manipulation of microscopic particle suspensions for biological, thermodynamic and microfluidic interests has brought about a revolution in micro-scale research across many different scientific disciplines. It has been shown that a diffraction limited focused laser can be used to trap microscopic particles whose refractive index is greater than their surrounding solvent. Termed Optical Trapping, work in this arena has yielded, new methods, techniques and applications that have flourished, and applications of this technology to areas of research involving microscopic systems for analysis, detection, separation and concentration have blossomed.
A related technique, Optical Chromatography, used for particle separation involves loosely focusing a laser into a fluid flowing opposite to the direction of laser propagation. When microscopic particles in the flow path encounter this beam they are trapped axially along the beam and are pushed upstream from the laser focal point to rest at a point where the optical and fluid forces on the particle balance. Because optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, fine separations are possible.
Recently, this method has been used to separate spores of different Bacillus species based on their optical and fluidic properties. We will describe how an optical chromatography beam directed into a tailored flow environment housed in a glass flowcell, has been adapted to operate as an optically tunable filter for the concentration or bioenrichment of colloidal and biological samples. Application of these methods and further design of fluidic and optical environments will allow for more specific identification, concentration and separation of many more microscopic particle and biological suspensions.
Since the discovery that laser radiation could be used to impart momentum to microscopic entities, a wide variety of methods, devices, and applications have followed. The most well known method for harnessing optical pressure, optical trapping, involves the highly convergent diffraction limited focusing of a laser for optical manipulation. A lesser known method, termed optical chromatography, involves mildly focusing a laser propagating directly against a fluid flow. When microscopic particles in the flow path encounter this beam they are trapped axially along the beam and are pushed upstream from the focal point to rest at a point where the optical pressure and fluid forces on the particle are equal. Because the optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, a new level of sensitivity, discrimination, and control is possible. This method has recently been used to separate colloidal particles differing only in refractive index. Flow environments designed and fabricated in poly(dimethylsiloxane) (PDMS) using a modification to the method termed Soft Lithography have been adapted to accommodate a loosely focused laser beam entering through a sidewall of the PDMS microfluidic system. Optical chromatographic separations, active sorting, and other particulate mixture manipulations within these customized PDMS microfluidic channel systems are being investigated. The application of these methods and the design of fluidic and optical environments will allow for the very specific separation, identification, and manipulation of a multitude of microscopic particle suspensions.
Laser trapping has been used for the manipulation of a wide variety of microscopic inorganic, metallic, polymeric, and biological particles, including cells, bacteria, and viruses. A more recently developed technique, optical chromatography, uses a lightly focused laser beam introduced into a counter-propagating fluid containing the particles to be trapped. Particle trapping and separation occurs through a balance of fluid drag forces and optical forces. The optical pressure that the laser exerts on a particle depends in part on its size, shape, and refractive index (chemical composition). Particles with a larger refractive index experience greater optical pressure and hence move farther upstream. Particles move against the fluid flow until reaching an equilibrium position where the fluid and optical forces are balanced. This position in relation to the focal point is termed the retention distance. Several important samples of biological origin have been separated and studied, including bacterial cells, spores, and pollens. The utility and prospects for optical chromatography are greatly expanded and the stage is set for many other optical characterization applications.
12 An optical fiber probe designed for use with molecularly imprinted polymers (MIP) is detailed and evaluated with respect to fluorescence sensitivity enhancements. MIPs are polymeric materials that are selective towards a molecule included in the mixture during the formation of the polymer. MIPs allow the selective analysis of a single analyte contained in a complex mixture through the interactions of the analyte with analyte-specific cavities formed within the polymer matrix. The optical design involves a dual optical fiber probe coupled to a transparent capillary for enhanced fluorescence excitation and emission collection. The inner capillary surface is coated with the MIP to select the analyte of choice while the capillary serves to enhance the fluorescence signal obtained. The selectivity and sensitivity of the MIP probe toward anthracene in water is discussed.
The outputs from a physics-based modeler of magnetometry data have been successfully used with a probabilistic neural network (PNN) to discriminate UXO from scrap. Model outputs from one location at a site were used to train a PNN model, which could correctly discriminate UXO from scrap at a different location of the same site. Data from one site location, the Badlands Bombing Range, Bull's Eye 2 (BBR 2), was used to predict targets detected at a different location at the site, Badlands Bombing Range, Bull's Eye 1 (BBR 1) containing different types of items. The UXO detection rate obtained for this analysis was 93 percent with a false alarm rate of only 28 percent. The possibility of discriminant individual UXO types within the context of a coarser two- class problem was demonstrated. The utility of weighting the sum of squared errors in cross-validation optimization of the (sigma) parameter has been demonstrated as a method of improving the classification of UXO versus scrap.
Prototype surface acoustic wave chemical sensor systems are described, which can detect and identify toxic vapors in real-time at trace concentrations. To operate autonomously for long periods, without failure, requires a thorough understanding of the hardware and software requirements of the sensor system. The SAWCAD and SAWRHINO prototypes, which implement several improvements to the hardware, over previously developed systems, are described. Software for vapor detection and neural network identification are also discussed. Preliminary results from two new software enhancements are described. Improved chemical discrimination occurs when the response slopes are incorporated into the analysis of the SAW ambient data. The generalized rank annihilation method is shown to be a powerful tool for extracting pure component analyte signatures from trap and purge gas solid chromatographic SAW data.
A multi-channel fiber optic laser induced fluorescence (LIF) detection system has been developed for flow injection analysis (FIA). It has been applied to the detection of standard PAH mixtures and real world samples. The instrument uses a fiber optic array to deliver Raman shifter generated excitation beams (260 nm - 340 nm) to samples flowing in a fused silica capillary. An identical fluorescence collection fiber optic array was positioned perpendicular to the excitation array to collect fluorescence generated by the sample at each excitation wavelength. A spectrograph and charge coupled device (CCD) were used to spectrally discriminate and detect the fluorescence delivered by each collection fiber. Several samples were investigated including standard PAH mixtures and a fuel oil sample. Identification and quantitation were accomplished using rank annihilation factor analysis (RAFA) using pure single component standards and the EEMs of mixtures measured during FIA of the unknown samples. For all components in the standard PAH mixtures, quantitative estimates of concentration were within a factor of 2.5 from the known values with some of the components being determined to within a factor of 1.3. For the complex fuel oil sample, 6 out of 8 components were correctly identified.
Fiber optic probes employing single channel laser excitation and fluorescence collection have been seeing increasing use for remote sensing applications. However, multi-channel systems offer enhanced capacity for qualitative and quantitative determination of analytes. We describe a system which employs simultaneous delivery of laser excitation wavelengths arising from stimulated Raman scattering. Separate fluorescence responses for each excitation channel are imaged through a spectrograph onto a CCD array detector. Each channel has a dedicated fiber optic pair to deliver and collect light. REsults will be presented which evaluate the capabilities of this type of spectrometer for determination of organic contaminant mixtures in various sample matrices.
A laser-induced fluorescence (LIF) excitation-emission matrix (EEM) probe has been developed for subsurface monitoring of fluorescent organic contaminants. The fourth harmonic of a flashlamp-pumped Nd:YAG laser (at 266 nm) is used to pump a Raman shifter. Up to ten laser beams (in the wavelength region of 258 to 379 nm) from the raman shifter are launched into optical fibers that conduct the light to the probe near the tip of the cone penetrometer. The fluorescence emission is excited through ten separate sapphire windows and collected by ten collection fibers that conduct the fluorescence to a spectrograph/CCD detection system. This probe allows real-time collection of LIF-EEMs of contaminants adsorbed on solids or dissolved in groundwater. LIF-EEMs provide a substantial amount of spectral information that can be used to determine the composition and quantity of contaminants in soils. The system was tested and calibrated in the laboratory. Spectra of different organic contaminants were measured in aqueous solutions, in organic solvents, and in different types of soils.
A laser-induced fluorescence (LIF) excitation-emission matrix (EEM) probe has been developed in the laboratory, and installed and tested in a cone penetrometer. The laser excitation system uses the fourth harmonic of a flashlamp-pumped Nd:YAG laser (at 266 nm) to pump a Raman shifter. Up to ten laser beams (in the wavelength region of 257 to 400 nm) from the Raman shifter are launched into optical fibers that are connected to the optical fibers of the cone penetrometer probe through standard connectors. In the probe head, the laser radiation is focused onto the outer surface of sapphire windows that are in contact with the soils. The fluorescence emission is collected by ten collection fibers that take the fluorescence to a detection system consisting of a spectrograph and a CCD detector. This probe allows real- time collection of LIF-EEMs of pollutants adsorbed on solids or dissolved in groundwater. LIF-EEMs provide a substantial amount of spectral information that can be used to determine the composition and quantity of pollutants in soils. This probe can be used to measure POL (petroleum, oil, lubricants), PAH (polycyclic aromatic hydrocarbons), and other fluorescent pollutants. The LIF-EEM instrument has been developed in the laboratory, and installed in a cone penetrometer truck for a field test at Hill Air Force Base, Utah. The experience of the test is discussed.
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