RESUMEN
The recent progress made in developing laser-induced breakdown spectroscopy (LIBS) has transformed LIBS from an elemental analysis technique to one that can be applied for the reagentless analysis of molecularly complex biological materials or clinical specimens. Rapid advances in the LIBS technology have spawned a growing number of recently published articles in peer-reviewed journals which have consistently demonstrated the capability of LIBS to rapidly detect, biochemically characterize and analyse, and/or accurately identify various biological, biomedical or clinical samples. These analyses are inherently real-time, require no sample preparation, and offer high sensitivity and specificity. This overview of the biomedical applications of LIBS is meant to summarize the research that has been performed to date, as well as to suggest to health care providers several possible specific future applications which, if successfully implemented, would be significantly beneficial to humankind.
Asunto(s)
Rayos Láser , Análisis Espectral/instrumentación , Análisis Espectral/métodos , Bacterias/química , Tecnología Biomédica , Humanos , Técnicas Microbiológicas , Diente/químicaRESUMEN
A sensor for the rapid (10-ms response time) measurement of vapors from the hydrocarbon-based fuels JP-8, DF-2, and gasoline is described. The sensor is based on a previously reported laser-mixing technique that uses two tunable diode lasers emitting in the near-infrared spectral region [Appl. Opt. 39, 5006 (2000)] to measure concentrations of gases that have unstructured absorption spectra. The fiber-mixed laser beam consists of two wavelengths: one that is absorbed by the fuel vapor and one that is not absorbed. Sinusoidally modulating the power of the two lasers at the same frequency but 180 degrees out of phase allows a sinusoidal signal to be generated at the detector (when the target gas is present in the line of sight). The signal amplitude, measured by use of standard phase-sensitive detection techniques, is proportional to the fuel-vapor concentration. Limits of detection at room temperature are reported for the vapors of the three fuels studied. Improvements to be incorporated into the next generation of the sensor are discussed.
RESUMEN
We describe the development and characterization of a near-infrared diode-laser-based sensor to measure the vapor from trace gases having unstructured absorption spectra. The technique uses two equal amplitude-modulated laser beams, with the modulation of the two lasers differing in phase by 180 deg. One of the laser beams is at a wavelength absorbed by the gas [for these experiments, vapor is from pyridine (C(5)H(5)N)], and the second laser beam is at a wavelength at which no absorption occurs. The two laser beams are launched onto near-coincident paths by graded-index lens-tipped optical fibers. The mixed laser beam signal is detected by use of a single photodiode and is demodulated with standard phase-sensitive detection. Data are presented for the detection and measurement of vapor from pyridine (C(5)H(5)N) by use of the mixed laser technique. The discussion focuses on experimental determination of whether a compound exhibits unstructured absorption spectra (referred to here as a broadband absorber) and methods used to maximize sensitivity.
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Laser-induced breakdown spectroscopy is evaluated as a means of detecting the fire suppressants CF(3)Br, C(3)F(7)H, and CF(4) and the refrigerant C(2)F(4)H(2). The feasibility of employing laser-induced breakdown spectroscopy for time- and space-resolved measurement of these agents during use, storage, and recharge is discussed. Data are presented that demonstrate the conditions necessary for optimal detection of these chemicals.
RESUMEN
We have employed tunable diode laser absorption spectroscopy (TDLAS) to characterize low-pressure premixed CH(4)/O(2)/Ar flames inhibited with Halon 1301 (CF(3)Br) and the candidate Halon alternative compounds FE-13 (CF(3)H) and HFC-125 (C(2)F(5)H). This work is part of a larger program designed to help identify replacement fire-suppression compounds for the currently used Halon 1301. We have used CO two-line thermometry to profile the temperature in low-pressure laminar flames and have determined concentration profiles for a large number of flame species, including reactive intermediates. To date, we have detected 12 flame species by using TDLAS in our laboratory and report on seven of them here: CH(4), H(2)O, CO, CF(2)O, CF(2)H(2), CF(3)H, and CF(4). To the best of our knowledge, this is the first time the last four species have been observed in flame by the use of TDLAS. Our data are important for validating the detailed kinetic mechanisms of chemical flame inhibition. Our results indicate that TDLAS is a versatile and powerful diagnostic technique for studying combustion processes.
RESUMEN
Tomographic analysis is used to provide a correction to low-pressure stoichiometric premixed CH(4)/O(2) flame temperatures measured with tunable diode laser absorption spectroscopy employing CO two-line thermometry. It is shown that flame temperatures measured with line-of-sight-based two-line thermometry are always too low and that the correction to the observed temperature is a nonlinear function of the height above the burner surface. It is also shown that, at a given height in the flame, a constant temperature across the flame does not imply that vibrational populations are constant and that, at low pressures (<20 Torr), the flame spreads radially beyond the burner diameter and so may no longer be approximated by a one-dimensional model.
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This is the introduction to the Applied Optics feature issue on Laser Applications to Chemical Analysis III. The issue is an outgrowth of the Topical Meeting on Laser Applications to Chemical Analysis III that was held in January 1992.
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ArF-laser-produced microplasmas in CO, CO(2), methanol, and chloroform are studied by time-resolved emission measurements of the plasma decay. Electron densities are deduced from Stark broadening of the line profiles of atomic H, C, O, and Cl. Plasma ionization and excitation temperatures are determined from measurements of relative populations of ionic and neutral species produced in the plasmas. A discussion of the thermodynamic equilibrium status of ArF-laser microplasmas is presented. In general, the ArF-laser-produced microplasma environment is found to be similar in all the gases studied, in terms of both temperature (15,000-20,000 K) and electron density (10(17) cm(-3)-10(18) cm(-3)), despite the considerable differences observed in the breakdown thresholds and relative energies deposited in the various gases.
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A recently developed research apparatus for characterization of low-pressure premixed flames has been developed and was used to characterize the C2H4/N2O/Ar flame at 20 torr. This instrument incorporates several diagnostic techniques in one apparatus so that individual techniques can be quantitatively compared and the usable detection range (both in terms of resolution and species detection) expanded. Results discussed in this report include mass analysis by triple quadrupole mass spectrometer and temperature measurement by thermocouple. Concentration profiles in the one-dimensional flame include CO, N2, and C2H4, at nominal m/z 28 as well as CO2 and N2O at m/z 44.
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Characteristic laser-produced microplasma emissions from various simple carbon-containing vapors entrained in a He carrier gas have been observed and compared. A focused ArF (193-nm) excimer laser is used to induce microplasmas with modest pulse energies (15 mJ or less) in the effluent region of a gas chromatography capillary column. Strong atomic (C, H, O, Cl, and F) as well as molecular (C(2), CH, and CCI) emissions are observed with very high SNRs. A plasma emission survey indicates that different classes of molecule show unique spectra which make it relatively easy to distinguish one chemical class from another. These results suggest that a laser microplasma gas chromatography detector (LM-GCD) should offer additional discrimination/resolution for unknown sample gas mixture analysis. In addition, the LM-GCD exhibits a significant advantage over certain other GC detectors, like the widely used flame ionization detector, by readily detecting nonresponsive gases such as CO, CO(2), CCl(4) and Freons.
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Some of the papers presented at the OSA Topical Meeting on Laser Applications to Chemical Analysis (January 1987) have been grouped together, and this preface introduces them.
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Excimer lasers are currently being utilized as a means of photofragmentation and fragment excitation for chemical detection purposes. In the case of small hydrocarbons, this phenomenon is generally not well characterized and is poorly understood. Experiments aimed at a better understanding of the interaction of simple carbon-containing molecules with the ArF (193-nm) excimer laser and at exploring the potential analytical applications of this process are described. Specifically, carbon atoms were generated by multiphoton photolysis of CO, CH(4), C(2)H(2), C(3)H(8), CH(3)OH, and CH(3)COCH(3) using the ArF laser. Their presence was detected by two sensitive methods, laser-induced fluorescence (LIF) and resonance ionization emission spectroscopy (RIES), both of which take advantage of the coincident overlap between the ArF laser and the(1)D(2) ? (1)P(1)(0) transition at 193.1 nm with emission detection at 247.9 nm. The RIES method detects single photons resulting from the photolytically produced carbon ion recombination and relaxation processes. An enhancement in the RIES signal was observed when a second tunable laser pulse operating at 247.9 nm followed the ArF laser pulse. Both methods not only offer sensitive detection of the photolytic precursor molecules but also require only relatively simple experimental apparatus. Detection levels for the precursor molelcule considerably lower than 10(11)/cc for LIF and 10(12)/cc for RIES can be estimated based on the observed rates of signal production.
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A Nd:YAG-pumped dye-laser system was used to two-photon excite oxygen atoms at 225.6 nm in an atmosphericpressure CH(4)-N(2)O-N(2) flame. Subsequent emission at 844.7 nm from the directly populated state as well as a stronger emission at 777.5 nm that was due to the O(3p(3)P ? 3p(5)P) collisional-energy transfer process was monitored. Two-photon-resonant oxygen-atom and hydrogen-atom (656.3-nm) emissions were also observed in the absence of a flame. Closer examination revealed that the tightly focused probe beam was producing these atoms by promoting multiphoton photolysis of the oxidizer as well as of the fuel molecules. Thus this type of laser-diagnostic probe is potentially quite intrusive, depending on the combustion region that is probed as well as on the laser energies used.
RESUMEN
Double-resonance laser emission at 283.3 and 600.2 nm was used to excite gas-phase atomic lead present in an electrothermal atomizer. Buffer-gas-induced collisional energy transfer occurs to states as much as 6000 cm(-1) below the highest optically pumped level. Single photons emitted from these states, whose wavelengths are as short as 202.2 nm, were detected. This study illustrates the utility and versatility of the saturated double-resonance technique for ultrasensitive and specific atomic detection (detection limits in the 10(-15)-g range) as well as its potential in atomic-spectroscopy and excited-state-dynamics studies.
