UV resonance raman spectroscopic detection of nitrate and nitrite in wastewater treatment processes

The 204- and 229-nm excited UV resonance Raman spectra of wastewater solutions containing sodium nitrite and nitrate were measured in the concentration range 7 microM to 3.5 mM (0.1-50 ppm nitrogen). The other chemical species present in wastewater do not interfere with Raman measurements of NO2-/NO3- bands. We observe detection limits of < 14 microM (< 200 ppb) for both NO2- and NO3-. UV resonance Raman spectroscopy appears to be an excellent tool for on-line monitoring of NO2-/NO3- in wastewater for the real-time control of water treatment plants.     

Application of 266-nm and 355-nm Nd:YAG laser radiation for the investigation of fuel-rich sooting hydrocarbon flames by raman scattering

We describe the use of linear Raman scattering for the investigation of fuel-rich sooting flames. In comparison, the frequency-tripled and -quadrupled fundamental wavelengths of a Nd:YAG laser have been used as an excitation source for study of the applicability of these laser wavelengths for analysis of sooting flames. The results obtained show that, for the investigation of strongly sooting flames, 266-nm excitation is better than 355-nm excitation. Although the entire fluorescence intensity of polycyclic aromatic hydrocarbons (PAHs) decreases with rising excitation wavelength, there is increased interference with the Raman signals by displacement of the spectral region of the Raman signals toward the fluorescence maximum of the laser-induced fluorescence emissions. Besides the broadband signals of PAHs, narrowband emissions of laser-produced C2 occur in the spectra of sooting flames and affect the Raman signals. These C2 emission bands are completely depolarized and can be separated by polarization-resolved detection. A comparison of the laser-induced fluorescence emissions of an ethylene flame with those of a methane flame shows the same spectral features, but the intensity of the emissions is larger by a factor of 5 for the ethylene fuel. Using 266-nm radiation for Raman signal excitation makes possible measurements in the ethylene flame also.     

Studies on stress-induced changes at the subcellular level by Raman microspectroscopic mapping

Raman microspectroscopic mapping enables one to study the chemical composition and molecular structure of subcellular components in individual cells without the need for labeling. Lung fibroblast cells were prepared under normal conditions and under stress, which was induced by 24 h of exposure to glyoxal. Raman microspectroscopic maps were recorded from fixed cells with 785-nm excitation and with 1-microm step width. Cluster analysis was applied to generate pseudocolor images of the cell morphology. Raman maps revealed that the cell nucleus shrinks in stressed cells, called pyknosis, which refers to an early stage of apoptosis. The intensity of nucleic acid bands decreased in cluster-averaged Raman spectra of the nucleus and cytoplasm, which is consistent with degradation and conformational changes of DNA and RNA. During a later stage of apoptosis, Raman maps indicate a rounding of cells, a further intensity decrease of nucleic acids bands, fragmentation of the nucleus, disappearance of lipid bodies, and formation of blisters at the cell surface. Whereas the peripheral membrane of the undisturbed cell is composed of lipids and cholesterol, the blisters have a higher protein content with nucleic acids incorporated. The results demonstrate that Raman spectroscopic mapping might become a powerful tool in cell biology for single cell analysis.     

Temperature measurements from first-negative N(2)(+) spectra produced by laser-induced multiphoton ionization and optical breakdown of nitrogen

A 248-nm excimer laser was used to produce ionized nitrogen by the process of multiphoton excitation in gaseous nitrogen at room temperature. First-negative N(2)(+) emission spectra were analyzed to yield rotational temperatures of typically 600 to 1200 K. Rotational Raman scattering of H(2) in gaseous mixtures of N(2) and H(2) was used to determine if laser heating of the gas produced the observed increase in temperature, but the room temperature value of 295 K was inferred from the H(2) Raman data. Therefore the use of N(2)(+) spectra produced by multiphoton excitation at 248 nm does not appear to be acceptable for air-temperature diagnostics. N(2)(+) emission spectra were also recorded subsequent to optical breakdown in air induced by Nd:YAG 1064-nm radiation, and temperatures were determined to be greater than 5000 K in the decaying plasma.     

Generation of 224-nm radiation by stimulated Raman scattering of ArF excimer laser radiation in a mixture of H2 and D2

Raman shifting of tunable ArF excimer laser radiation in a mixture of H(2) and D(2) produces tunable radiation in the 224-nm region as a result of Stokes shifting the frequency of the fundamental radiation (193 nm) once in both H(2) and D(2). At a total pressure of 25 bars, a 19% H(2) in D(2) mixture is found to provide a maximum conversion efficiency (2.5%) to the 224-nm range. Both fundamental and 224-nm radiation were used to record laser-induced fluorescence excitation spectra of nitric oxide produced in an oxyacetylene flame. From the excitation spectra, we determined the tuning range of the 224-nm radiation to be 270 cm(-1) with a linewidth of 0.9 cm(-1), which is similar to the fundamental laser radiation. We derived the exact Raman shift of the generated radiation by comparing both excitation spectra which was found to be 7142.3(5) cm(-1).     

Improved multiple-pass Raman spectrometer

An improved Raman gain spectrometer for flame measurements of gas temperature and species concentrations is described. This instrument uses a multiple-pass optical cell to enhance the incident light intensity in the measurement volume. The Raman signal is 83 times larger than from a single pass, and the Raman signal-to-noise ratio (SNR) in room-temperature air of 153 is an improvement over that from a single-pass cell by a factor of 9.3 when the cell is operated with 100 passes and the signal is integrated over 20 laser shots. The SNR improvement with the multipass cell is even higher for flame measurements at atmospheric pressure, because detector readout noise is more significant for single-pass measurements when the gas density is lower. Raman scattering is collected and dispersed in a spectrograph with a transmission grating and recorded with a fast gated CCD array detector to help eliminate flame interferences. The instrument is used to record spontaneous Raman spectra from N(2), CO(2), O(2), and CO in a methane-air flame. Curve fits of the recorded Raman spectra to detailed simulations of nitrogen spectra are used to determine the flame temperature from the shapes of the spectral signatures and from the ratio of the total intensities of the Stokes and anti-Stokes signals. The temperatures measured are in good agreement with radiation-corrected thermocouple measurements for a range of equivalence ratios.     

Validation of a rotational coherent anti-Stokes Raman spectroscopy model for carbon dioxide using high-resolution detection in the temperature range 294-1143 K

Experiments were performed in the temperature range of 294-1143 K in pure CO(2) using high-resolution rotational coherent anti-Stokes Raman spectroscopy (CARS), in the dual-broadband approach. Experimental single-shot spectra were recorded with high spectral resolution using a single-mode Nd:YAG laser and a relay imaging lens system on the exit of a 1 m spectrometer. A theoretical rotational CARS model for CO(2) was developed for evaluation of the experimental spectra. The evaluated mean temperatures of the recorded single-shot dual-broadband rotational coherent anti-Stokes Raman spectroscopy (DB-RCARS) spectra using this model showed good agreement with thermocouple temperatures, and the relative standard deviation of evaluated single-shot temperatures was generally 2-3%. Simultaneous thermometry and relative CO(2)/N(2)-concentration measurements were demonstrated in the product gas of premixed laminar CO/air flames at atmospheric pressure. Although the model proved to be accurate for thermometry up to 1143 K, limitations were observed at flame temperatures where temperatures were overestimated and relative CO(2)/N(2) concentrations were underestimated. Potential sources for these discrepancies are discussed.     

Detection of nano-oxidation sites on the surface of hemoglobin crystals using tip-enhanced Raman scattering

Hemoglobin nanocrystals were analyzed with tip-enhanced Raman scattering (TERS), surface-enhanced resonance Raman scattering (SERRS) and conventional resonance Raman scattering (RRS) using 532 nm excitation. The extremely high spatial resolution of TERS enables selective enhancement of heme, protein, and amino acid bands from the crystal surface not observed in the SERRS or RRS spectra. Two bands appearing at 1378 and 1355 cm(-1) assigned to the ferric and ferrous oxidation state marker bands, respectively, were observed in both TERS and SERRS spectra but not in the RRS spectrum of the bulk sample. The results indicate that nanoscale oxidation changes are occurring at the hemoglobin crystal surface. These changes could be explained by oxygen exchange at the crystal surface and demonstrate the potential of the TERS technique to obtain structural information not possible with conventional Raman microscopy.     

A portable Raman acoustic levitation spectroscopic system for the identification and environmental monitoring of algal cells

We report the coupling of a portable Raman spectrometer to an acoustic levitation device to enable environmental monitoring and the potential taxonomic identification of microalgae. Spectra of living cells were recorded at 785 nm using a fiber-optic probe coupled to a portable Raman spectrometer. The spectra exhibit an excellent signal-to-noise ratio and clearly show bands from chlorophyll a and beta-carotene. Spectra of levitated photobleached microalgae clearly show a reduction in chlorophyll a concentration relative to beta-carotene after 10 min of exposure to a quartz halogen lamp. Spectra recorded from levitated nitrogen-limited cells also show a significant reduction in bands associated with chlorophyll a, as compared to nitrogen-replete cells. To investigate the diagnostic capability of the technique, four species of microalgae were analyzed. Good quality spectra of all four species were obtained showing varying ratios of beta-carotene to chlorophyll. The combination of an acoustic levitation device and a portable Raman spectrometer shows potential as a taxonomic and environmental monitoring tool with direct application to field studies in remote environments.     

Characterization of the growth of sol-gel derived Ba---Ti---O thin films by laser Raman spectroscopy

The isothermal growth kinetics of sol-gel derived BaTi₅O₁₁ thin films has been studied by laser Raman spectroscopy. Thin films of the precursors were spin coated on silicon wafers with platinum or gold overlayers. The films were then annealed isothermally, and Raman spectra were recorded in situ. The time-dependent intensity changes of the Raman bands could be analyzed in terms of Avrami models. The interaction of the BaTi₅O₁₁ film with the platinum or gold overlayers during heat treatment apparently increased the Raman intensity by a factor of about 100.     

New vibrational bands in nitrogen laser emission spectra

Nitrogen laser emission spectra was analyzed using a 0.5-m Jarrell-Ash monochromator. In addition to the (0,0) 337.1-nm band, the laser spectra were found to contain (0,1) 357.69-nm, (2,4) 371.05-nm, (1,0) 315.93-nm, (0,2) 380.49-nm, (0,3) 405.94-nm, (1,3) 375.54-nm, (1,4) 399.84-nm, and (1,2) 353.67-nm bands belonging to the second positive system of the N2 molecule. A new transition from one of the mixed vibrational levels of C and C' states of the N2 molecule was observed in the laser spectra at 331.83 nm with a relative intensity of approximately 29%. Two bands, at 340.85 and 303.49 nm, belonging to the beta system of the NO molecule were also observed. The intensity variation of the prominent bands observed was studied with respect to change in operating pressure and voltage for deriving the optimum conditions for emission at these wavelengths. Variations in the intensity of the bands were also studied with change in distance between the laser head and the monochromator; it was observed that an amplification effect had taken place at 331.83 and 340.85 nm, in addition to the (0,0) 337.1-nm band, in the nitrogen laser discharge.     

One-Dimensional, Time-Resolved Raman Measurements in a Sooting Flame made with 355-nm Excitation

Single-shot vibrational Raman measurements were performed along an 11-mm-long line crossing the reaction zone in a premixed, fuel-rich (phi = 10), laminar methane-air flame by use of a frequency-tripled Nd:YAG laser with a 355-nm emission wavelength. This laser source seems to have advantages relative to KrF excimer lasers as well as to Nd:YAG lasers at 532 nm for hydrocarbon combustion diagnostics. The Raman emissions of all major species (N(2), O(2), CH(4), H(2), CO(2), H(2)O) were detected simultaneously with a spatial resolution of 0.4 mm. By integration over selected spectral intervals, the mole fractions of all species and subsequently the local gas temperatures have been obtained. A comparison of the temperatures that were found with results from filtered Rayleigh experiments showed good agreement, indicating the success of what are to the best of our knowledge the first one-dimensional single-shot Raman measurements in a sooting hydrocarbon flame.     

Anti-Stokes Raman spectrometry with 1064-nm excitation: an effective instrumental approach for field detection of explosives

Anti-Stokes Raman spectra of 28 explosive materials were obtained with 1064-nm excitation using fiber-optic sampling and a dispersive spectrograph equipped with a charge-coupled device (CCD) array detector. By using a silicon CCD detector, anti-Stokes features could clearly be observed for the majority of samples from -250 to -1650 cm(-1). Using the fiber-optic probe, spectra were routinely obtained from samples positioned up to twelve meters from the spectrograph within 240 s. The utility of an anti-Stokes correction routine is demonstrated, which routine allowed anti-Stokes spectra measured with 1064-nm excitation to be successfully searched and identified against libraries of Stokes spectra obtained using a Fourier transform (FT) Raman system equipped with a 1064-nm Nd:YAG laser.     

Steady-state and transient ultraviolet resonance Raman spectrometer for the 193-270 nm spectral region

We describe a state-of-the-art tunable ultraviolet (UV) Raman spectrometer for the 193-270 nm spectral region. This instrument allows for steady-state and transient UV Raman measurements. We utilize a 5 kHz Ti-sapphire continuously tunable laser (approximately 20 ns pulse width) between 193 nm and 240 nm for steady-state measurements. For transient Raman measurements we utilize one Coherent Infinity YAG laser to generate nanosecond infrared (IR) pump laser pulses to generate a temperature jump (T-jump) and a second Coherent Infinity YAG laser that is frequency tripled and Raman shifted into the deep UV (204 nm) for transient UV Raman excitation. Numerous other UV excitation frequencies can be utilized for selective excitation of chromophoric groups for transient Raman measurements. We constructed a subtractive dispersion double monochromator to minimize stray light. We utilize a new charge-coupled device (CCD) camera that responds efficiently to UV light, as opposed to the previous CCD and photodiode detectors, which required intensifiers for detecting UV light. For the T-jump measurements we use a second camera to simultaneously acquire the Raman spectra of the water stretching bands (2500-4000 cm(-1)) whose band-shape and frequency report the sample temperature.     

Pure rotational coherent anti-Stokes Raman spectroscopy in mixtures of CO and N2

We present a model for quantitative measurements in binary mixtures of nitrogen and carbon monoxide by the use of dual-broadband rotational coherent anti-Stokes Raman spectroscopy. The model has been compared with experimental rotational coherent anti-Stokes Raman scattering spectra recorded within the temperature range of 294-702 K. Temperatures and concentrations were evaluated by spectral fits using libraries of theoretically calculated spectra. The relative error of the temperature measurements was 1-2%, and the absolute error of the CO concentration measurements was <0.5% for temperatures < or =600 K. For higher temperatures, the gas composition was not chemically stable, and we observed a conversion of CO to CO2. The influence of important spectroscopic parameters such as the anisotropic polarizability and Raman line-broadening coefficients are discussed in terms of concentration measurements. In particular, it is shown that the CO concentration measurement was more accurate if N2-CO and CO-N2 line-broadening coefficients were included in the calculation. The applicability of the model for quantitative flame measurements is demonstrated by measuring CO concentrations in ethylene/air flames.     

UV resonance Raman detection and quantitation of domoic acid in phytoplankton

Cultures of the phytoplankton diatom, Pseudonitzschia multiseries, have been harvested under controlled growth conditions ranging from late logarithmic to late stationary phase (17-58 days). The amount of domoic acid (DA) present in the growth media and in the homogenized cells has been determined by HPLC. Defined samples of media, homogenized cells, whole cells, and whole cells in media have been laser excited at 251 nm for the purpose of selectively exciting intense UV resonance Raman spectra from DA in the samples. Neither media nor cell component spectra from algae seriously interfere with DA spectra. The spectral cross sections for the dominant 1652-cm-1 mode of DA have been determined for 242-, 251-, and 257-nm excitation. Maximum sensitivities are achieved with 251-nm excitation because cross sections for DA are a maximum, and interference from other algal components becomes very small. DA concentrations that have been determined with 251-nm excitation by resonance Raman methods correlate closely with values determined independently with HPLC, especially at higher DA concentrations. The UV resonance Raman analysis of DA in phytoplankton algae is shown to be very sensitive and quantitative as well as rapid and nonintrusive.     

Diode laser sensor for measurements of CO, CO(2), and CH(4) in combustion flows

A diode laser sensor has been applied to monitor CO, CO(2), and CH(4) in combustion gases with absorption spectroscopy and fast extraction-sampling techniques. Survey spectra of the CO 3nu band (R branch) and the 2nu(1) + 2nu(2)(0) + nu(3) CO(2) band (R branch) near 6350 cm(-1) and H(2)O lines from the nu(1) + 2nu(2) and 2nu(2) + nu(3) bands in the spectral region from 6345 to 6660 cm(-1) were recorded and compared with calculated spectra (from the HITRAN 96 database) to select optimum transitions for species detection. Species concentrations above a laminar, premixed, methane-air flame were determined from measured absorption in a fast-flow multipass absorption cell containing probe-sampled combustion gases; good agreement was found with calculated chemical equilibrium values.     

Diode-Laser Absorption Measurements of CO(2) Near 2.0 mum at Elevated Temperatures

A diode-laser sensor system based on absorption spectroscopy techniques has been developed for nonintrusive measurements of CO(2) in high-temperature environments. Survey spectra of the CO(2) (20 degrees 1,04 degrees 1)(I)-00 degrees 0 and (20 degrees 1,04 degrees 1)(II)-00 degrees 0 bands between 1.966 and 2.035 mum (4915-5085 cm(-1)) were recorded at temperatures between 296 and 1425 K in a heated static cell and compared with calculated spectra (by using the HITRAN 96/HITEMP database) to find candidate transitions for CO(2) detection. High-resolution measurements of the CO(2) R(56) line shape [(20 degrees 1,04 degrees 1)(II)-00 degrees 0 band] were used to determine the transition line strength, the self-broadening half-width, and the coefficient of temperature dependence of the self-broadening half-width. The results represent what are believed to be the first measurements of CO(2) absorption near 2.0 mum with room-temperature diode lasers. Potential applications of the diode-laser sensor system include in situ combustion measurements and environmental monitoring.     

Simultaneous vibrational and pure rotational coherent anti-stokes Raman spectroscopy for temperature and multispecies concentration measurements demonstrated in sooting flames

The potential of measuring temperature and multiple species concentrations (N2, O2, CO) by use of combined vibrational coherent anti-Stokes Raman spectroscopy (CARS) and pure rotational CARS has been investigated. This was achieved with only one Nd:YAG laser and one dye laser together with a single spectrograph and CCD camera. From measurements in premixed sooting C2H4-air flames it was possible to evaluate temperatures from both vibrational CARS and rotational CARS spectra, O2 concentration from the rotational CARS spectra, and CO concentration from the vibrational CARS spectra. Quantitative results from premixed sooting C2H4-air flames are presented, and the uncertainties in the results as well as the possibility of extending the combined CARS technique for probing of additional species are discussed.