Sensitive detection of ammonia and ethylene with a pulsed quantum cascade laser using intra and interpulse spectroscopic techniques

Spectroscopic concentration measurements of ammonia and ethylene were done with a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm⁻¹. An astigmatic Herriot cell with 150 m path length was employed, and we compare the results from experiments using inter- and intrapulse techniques, respectively. The measurements include the detection of ammonia in breath with these methodologies. In the interpulse technique, the laser was excited with short current pulses (5–10 ns), and the pulse amplitude was modulated with an external current ramp resulting in a ∼0.3 cm⁻¹ frequency scan. A standard amplitude demodulation technique was implemented for extracting the absorption line, thus avoiding the need for a fast digitizer or a gated integrator. In the intrapulse technique, a linear frequency down-chirp is used for sweeping across the absorption line. A 200 ns long current pulse was used for these measurements which resulted in a spectral window of ∼1.74 cm⁻¹ during the down-chirp. The use of a room temperature mercury-cadmium-telluride detector resulted in a completely cryogen free spectrometer. We demonstrate detection limits of ∼3 ppb for ammonia and ∼5 ppb for ethylene with less than 10 s averaging time with the intrapulse method and ∼4 ppb for ammonia and ∼7 ppb for ethylene with the interpulse technique with an integration time of ∼5 s.     

Breath ammonia detection based on tunable fiber laser photoacoustic spectroscopy

A new detection method for ammonia in high concentration of CO₂ and H₂O is reported, which uses a wavelength modulated photoacoustic spectrometer based on a near-infrared tunable erbium-doped fiber laser in combination with an optical fiber amplifier. The multi-wavelength (1522.44 nm, 1522.94 nm and 1545.05 nm) photoacoustic signal measurement is established to detect multi-spectrum signal in samples. The problem of ammonia detection in high concentration of CO₂ and H₂O is resolved at atmospheric pressure. The minimum detection limit of 16 ppb (signal-to-noise ratio = 1) in simulated breath samples (5.3% CO₂ and 6.2% H₂O (100% relative humidity at 37°C)) is achieved.     

Open-path trace gas detection of ammonia based on cavity-enhanced absorption spectroscopy

A compact open-path optical ammonia detector is developed. A tunable external-cavity diode laser operating at 1.5 μm is used to probe absorptions of ammonia via the cavity-enhanced absorption (CEA) technique. The detector is tested in a climate chamber. The sensitivity and linearity of this system are studied for ammonia and water at atmospheric pressure. A cluster of closely spaced rovibrational overtone and combination band transitions, observed as one broad absorption feature, is used for the detection of ammonia. On these molecular transitions a detection limit of 100 ppb (1 s) is determined. The ammonia measurements are calibrated independently with a chemiluminescence monitor. Compared to other optical open-path detection methods in the 1–2 μm region, the present result shows an improved sensitivity for contactless ammonia detection by over one order of magnitude. Using the same set-up, a detection limit of 100 ppm (1 s) is determined for the detection of water at atmospheric pressure. Received: 19 January 2000 / Revised version: 6 March 2000 / Published online: 7 June 2000     

High precision determinations of NH3 concentration by means of diode laser spectrometry at 2.005 μm

This paper reports on the development of a compact analyzer for ammonia monitoring in air, based on a distributed feedback diode laser at 2.005 μm. A dual-beam long-path technique was combined with wavelength modulation detection of absorption in order to reach a detection limit of about 25 ppb, with a 0.2-Hz equivalent noise bandwidth. Retrieval of NH₃ concentration was accomplished through a careful spectra analysis procedure based on the formalism of Fourier expansion of the 2nd harmonic signals, also taking into account residual-amplitude-modulation effects. The system was tested on certified gas-mixtures and revealed a good performance in terms of accuracy and reproducibility. Particularly, the short-term precision was found to be about 1 ‰, for NH₃ mixing ratios of 10 ppm. Finally, the possibility to make use of the analyzer for measurements of ammonia fluxes from soils is discussed. PACS 42.55.Px; 42.62.Fi; 82.80.Gk     

Photoacoustic detection of nitric oxide with a Helmholtz resonant quantum cascade laser sensor

Nitric oxide detection has been performed with a Helmholtz resonant photoacoustic sensor previously developed in Reims. The cell was coupled with a continuous wave distributed feedback quantum cascade laser working at cryogenic temperature in continuous wave mode in the 5.4 μm region. A detection limit of 20 ppb of nitric oxide in nitrogen with a laser power of 3 mW can be predicted. Possible applications and improvements are discussed.     

Detection of mercury in water by laser-induced breakdown spectroscopy with sample pre-concentration

A method based on pre-concentration, using electrolysis, has been proposed and used for the improvement of detection sensitivity of mercury ions at ultra-trace level of concentration in an aqueous matrix by laser-induced breakdown spectroscopy (LIBS). The experimental evaluation of this method was carried out on mercury chloride (HgCl₂) in triple-distilled water for a concentration range of the element of 0.01–10 mg l⁻¹. A pre-concentration factor of ∼180 was obtained for LIBS detection of mercury emission line at 253.65 nm. The limit of detection (LoD) of ∼0.011 mg l⁻¹ (or ∼11 ppb) for mercury in the water sample was achieved.     

Ammonia trace measurements at ppb level based on near-IR photoacoustic spectroscopy

A photoacoustic sensor using a laser diode emitting near 1532 nm in combination with an erbium-doped fibre amplifier has been developed for ammonia trace gas analysis at atmospheric pressure. NH₃ concentration measurements down to 6 ppb and a noise-equivalent detection limit below 3 ppb in dry air are demonstrated. Two wavelength-modulation schemes with 1f and 2f detection using a lock-in amplifier were investigated and compared to maximise the signal-to-noise ratio. A quantitative analysis of CO₂ and H₂O interference with NH₃ is presented. Typical concentrations present in ambient air of 400 ppm CO₂ and 1.15% H₂O (50% relative humidity at 20 °C) result in a NH₃ equivalent concentration of 36 ppb and 100 ppb, respectively. PACS 42.62.Fi; 43.35.Ud; 42.55.Px     

Tunable fiber laser based photoacoustic spectrometer for breath ammonia analysis during hemodialysis1

A photoacoustic (PA) spectrometer based on a near-IR tunable fiber laser is developed and used for breath ammonia analysis. We successfully measured the breath ammonia level variation of six patients with end-stage renal disease while they were undergoing hemodialysis in the hospital. The measurement results showed that the initial concentration level of the breath ammonia were from 1600 to 2200 ppb before dialysis treatment, the levels decreased to 200–600 ppb in the end stage of dialysis, which close to the levels of healthy persons. Further improvement and applications of this PA spectrometer are discussed.     

A ppb level sensitive sensor for atmospheric methane detection

A high sensitivity sensor, combining a multipass cell and wavelength modulation spectroscopy in the near infrared spectral region was designed and implemented for trace gas detection. The effective length of the multipass cell was about 290 meters. The developed spectroscopic technique demonstrates an improved sensitivity of methane in ambient air and a relatively short detection time compared to previously reported sensors. Home-built electronics and software were employed for diode laser frequency modulation, signal lock-in detection and processing. A dual beam scheme and a balanced photo-detector were implemented to suppress the intensity modulation and noise for better detection sensitivity. The performance of the sensor was evaluated in a series of measurements ranging from three hours to two days. The average methane concentration measured in ambient air was 2.01 ppm with a relative error of ± 2.5%. With Allan deviation analysis, it was found that the methane detection limit of 1.2 ppb was achieved in 650 s. The developed sensor is compact and portable, and thus it is well suited for field measurements of methane and other trace gases.     

Faraday rotation spectrometer with sub-second response time for?detection of nitric oxide using a cw DFB quantum cascade laser at?5.33 μm

A Faraday modulation spectrometer for sensitive and fast detection of nitric oxide at 5.33 μm utilizing a room temperature continuous wave distributed feedback quantum cascade laser and a Peltier cooled MCT detector is presented. The magnetic field was modulated at 7.4 kHz whereas the laser wavelength was scanned at 20 Hz across the most favorable rotational-vibrational transition for FAMOS, Q _3/2(3/2), at 5.33 μm. Using a 15 cm optical path and lineshape fitting, the spectrometer provides a detection limit of 4.5 ppb for a response time of 1 s. An Allan variance analysis demonstrates that the system has an excellent stability, up to several hours of operation.     

Photoacoustic spectroscopy for trace gas detection with cryogenic and room-temperature continuous-wave quantum cascade lasers

The main characteristics that a sensor must possess for trace gas detection and pollution monitoring are high sensitivity, high selectivity and the capability to perform in situ measurements. The photacoustic Helmholtz sensor developed in Reims, used in conjunction with powerful Quantum Cascade Lasers (QCLs), fulfils all these requirements. The best cell response is # 1200 V W⁻¹ cm and the corresponding ultimate sensitivity is j 3.3 × 10⁻¹⁰ W cm⁻¹¹ Hz^−11/2. This efficient sensor is used with mid-infrared QCLs from Alpes Lasers to reach the strong fundamental absorption bands of some atmospheric gases. A first cryogenic QCL emitting at 7.9 μm demonstrates the detection of methane in air with a detection limit of 3 ppb. A detection limit of 20 ppb of NO in air is demonstrated using another cryogenic QCL emitting in the 5.4 μm region. Real in-situ measurements can be achieved only with room-temperature QCLs. A room-temperature QCL emitting in the 7.9 μm region demonstrates the simultaneous detection of methane and nitrous oxide in air (17 and 7 ppb detection limit, respectively). All these reliable measurements allow the estimated detection limit for various atmospheric gases using quantum cascade lasers to be obtained. Each gas absorbing in the infrared may be detected at a detection limit in the ppb or low-ppb range.     

Only Ku-band microwave absorption by Fe3O4/ferrocenyl-CuPc hybrid nanospheres

A novel kind of hybrid nanospheres made of Fe₃O₄ and ferrocenyl-CuPc (FCP) was prepared via effective solvothermal method and performed microwave absorptivity only in Ku-band with minimum reflection loss of −25 dB at 16.0 GHz corresponding to absorbing about 99.7% content of microwave. Scanning electron microscopy images indicated that the nanospheres with uniform particle size distribution have the average diameter of 135 nm. Due to the synergistic reaction between magnetic ferrocenyl-CuPc and Fe₃O₄, the hybrid nanospheres showed novel electromagnetic properties. The real part of complex permittivity of hybrid nanospheres remains stable in the range of 0.5–12.0 GHz and has a large fluctuation at 16.5 GHz. Moreover, the dielectric loss of hybrid nanospheres also appeared a sharp peak at 16.3 GHz with the value of 2.7. The specific gravity of hybrid nanospheres is about 2.08. On the basis of these results, the novel hybrids are believed to have potential applications in the microwave absorbing area in Ku-band.     

Synthesis,characterization, and photocatalytic properties of core/shell mesoporous silica nanospheres supporting nanocrystalline titania

The facile bulk synthesis of silica nanospheres makes them an attractive support for the transport of chemical compounds such as nanocrystalline titanium dioxide. In this contribution we present a promising route for the synthesis of mesoporous silica nanospheres (m-SiO₂) with diameter in range 200 nm, which are ideal supports for nanocrystalline titanium dioxide (TiO₂). The detailed microscopic and spectroscopic characterizations of core/shell structure (m-SiO₂/TiO₂) were conducted. Moreover, the photocatalytic potential of the nanostructures was investigated via phenol decomposition and hydrogen generation. A clear enhancement of photoactivity in both reactions as compared to commercial TiO₂-Degussa P25 catalyst is detected.     

CO2 isotope sensor using a broadband infrared source, a spectrally narrow 4.4 μm quantum cascade detector, and a Fourier spectrometer

We report a prototype CO₂ gas sensor based on a simple blackbody infrared source and a spectrally narrow quantum cascade detector (QCD). The detector absorption spectrum is centered at 2260 cm⁻¹ (4.4 μm) and has a full width at half maximum of 200 cm⁻¹ (25 meV). It covers strong absorption bands of two spectrally overlapping CO₂ isotopomers, namely the P-branch of ¹²CO₂ and the R-branch of ¹³CO₂. Acquisition of the spectral information and data treatment were performed in a Fourier transform infrared (FTIR) spectrometer. By flushing its sample compartment either with nitrogen, dry fresh air, ambient air, or human breath, we were able to determine CO₂ concentrations corresponding to the different gas mixtures. A detection limit of 500 ppb was obtained in these experiments.     

A Novel Captopril Chemiluminescence System for Determination of Copper(II) in Human Hair and Cereal Flours

A novel chemiluminescence (CL) reaction, captopril–H₂O₂, for determination of Cu(II) at nanogram per milliliter level in batch-type system has been described. The method relies on the catalytic effect of Cu(II) on the oxidation of captopril with hydrogen peroxide in alkaline medium. The optimization step was performed using univariate methodology and the factors studied were: pH and concentrations of the utilized reagents. Under the optimum conditions, calibration plot was linear in the range of 0.1–2.0 ppm. Limit of detection was 30 ppb and relative standard deviation for five replicate determinations of 0.8 ppm Cu(II) was 1.89%. The proposed method was successfully applied to the determination of Cu(II) in human scalp hair and cereals, rice and wheat, flour with satisfactory selectivity and sensitivity. The results were validated by comparison with a standard method (FAAS). The possible mechanism of the new CL reaction has also been discussed.     

Synthesis of Ni<Subscript>80</Subscript>Fe<Subscript>20</Subscript> and Co nanodot arrays by self-assembling of polystyrene nanospheres: magnetic and microstructural properties

Array of dots have been designed by assembling a monolayer of polystyrene nanospheres (PN) on sputtered thin films having Ni₈₀Fe₂₀ and Co composition with different thickness, ranging in the interval 20 ÷ 80 nm. Subsequently the films are nanopatterned using the nanospheres as a mask during sputter etching with Ar⁺ ions. A Reactive Ion Etching (RIE) process before sputter etching is used to control the final diameter of the magnetic dots that thus can be tailored as desired (typically ranging in the interval 250 ÷ 400 nm depending on the PN starting diameter). In addition, electron beam lithography has been exploited to obtain arrays of dots in Ni₈₀Fe₂₀ thin films having approximately the same mean size and dot distance as in self-assembled samples. All films have been routinely characterized by SEM and AFM microscopy to evaluate the microstructure. Magnetic domain patterns at magnetic remanence and in the demagnetised state have been imaged by MFM microscopy technique. Room-temperature hysteresis properties have been measured by an alternating gradient force magnetometer. In general, the magnetization process in all patterned films has been observed to have features typical of a vortex whose nucleation field depends on sample thickness and mean dot dimension. A comparison between magnetic arrays of Ni₈₀Fe₂₀ dots prepared by self-assembling of polystyrene nanospheres and electron beam lithography is presented to rule out the role of microstructure (i.e., order, size, and mutual distance of the magnetic dots) on magnetic properties.     

Tunable diode laser spectroscopy of ethylene oxide near 1693 nm

We describe the application of the vertical-cavity surface-emitting laser (VCSEL) to absorption spectroscopy of the ethylene oxide (EO) Q-branch near 1693 nm. The VCSEL was electrically scanned over spectral intervals of up to ∼13 cm⁻¹ at a 1 kHz repetition rate. A methane absorption line at 5903.3 cm⁻¹ and Fabry–Pérot etalon with a free spectral range of 0.4 cm⁻¹ were used to calibrate frequency scale. The EO was mixed with ambient air, and total gas pressure and mixing ratios were varied from a few mbar to 1 bar and from ∼10² to 10⁵ parts per million, respectively. A rapid roll off of EO absorbance at 5903.7 cm⁻¹ was observed at gas pressures below ∼0.5 bar. A linear dependence of EO peak absorbance on mixing ratio was found at a total gas pressure of 1 bar. We conclude that monolithic VCSELs operating near 1693 nm could be used in EO sensors with a detection limit in the ppb range.     

Electron microscopy investigations of the organization of cerium oxide nanocrystallites and polymers developed in polyvinylpyrrolidone-assisted polyol synthesis process

The microstructures and the organization mechanism of cerium oxide (ceria) nanospheres were investigated by transmission electron microscopy. The ceria nanospheres with the diameter of 50–150 nm were produced by the polyol synthesis method with cerium nitrate precursor and with polyvinylpyrrolidone (PVP) used as a protecting agent. Dose-limited observations performed by energy-filtering transmission electron microscopy revealed that the ceria nanospheres were the aggregated products consisting of ceria nanocrystallites ~3 nm in size and PVP-derived polymer products. It was found that the ceria nanocrystallites were oriented within the nanospheres by high-resolution transmission electron microscopy and selected-area electron diffraction. Selective adsorption of PVP on the crystal facets of the ceria nanocrystallites was suggested, and the aggregation of the PVP-adsorbed ceria through cross-linking reaction of PVP causes the crystal orientation.     

Influence of ammonia on the morphologies and enhanced photocatalytic activity of TiO2 micro/nanospheres

TiO₂ micro/nanospheres were synthesized by a combination process contains hydrolysis of titanium tetra-n-butyl in mixed solution of anhydrous ethanol/ammonia and the subsequent calcination under 550 °C for 7 h. The pH values of the mixed solution were tuned to be 10.4, 11.0 and 11.6, respectively, by adding different amounts of ammonia. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to characterize the morphologies and the crystallinity. X-ray diffraction (XRD) patterns indicated that pH value of the precursors has an important effect on the crystal phase composition. UV-vis diffuse reflectance spectrum was applied to characterize the optical properties of samples. Degradation of methylene blue under the irradiation of 300 W Hg lamp confirmed the enhanced photocatalytic activity of TiO₂ micro/nanospheres. In addition, the formation mechanism was proposed.     

Synthesis and Characterization of Acylated Polycaprolactone (PCL) Nanospheres and Investigation of Their Influence on Aggregation of Amyloid Proteins

Acylated polycaprolactone (PCL) nanospheres were fabricated and employed to interact with amyloid-β-(25–35) peptides (Aβ_25–35), "peptide 11 of the 40 peptide full amyloid-β". The nanospheres were characterized by scanning electron microscopy (SEM), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and dynamiclightscattering (DLS), all of which confirmed that the acylated polycaprolactone (Ac-PCL) nanospheres were successfully fabricated. The effect of the nanospheres on the aggregation of Aβ_25–35 peptides was investigated by thioflavin T fluorescence measurements. The result showed that, without nanospheres, the Aβ_25–35 peptides aggregated gradually from monomers and oligomers to long fibrils with increasing incubation time. In comparison, the nanospheres were effective in interfering with fibrillogenesis and aggregation of amyloid-β. We suggest this study may contribute to the development of new therapeutic strategies against amyloid-related disorders.