Detection of separated analytes in subnanoliter volumes using coaxial thermal lensing

A collinear-beam thermal lens detector has been constructed and its properties were characterized. Its application to the high-performance liquid chromatography (HPLC) separation of a mixture of five anthraquinone dyes dissolved in water shows a linear response over 3.5 orders of magnitude and a detection limit that is subnanomolar in the dye concentrations. These results are compared with those obtained previously using cavity ring-down spectroscopy (CRDS) in a Brewster's angle flow cell (Bechtel, K. L.; Zare, R. N.; Kachanov, A. A.; Sanders, S. S.; Paldus, B. A. Anal. Chem. 2005, 77, 1177-1182). The peak-to-peak baseline noise of the thermal lensing detection is 3.5 x 10(-8) absorbance units (AU) with a path length of 200 microm, whereas the peak-to-peak baseline noise of CRDS detection is approximately 2 x 10(-7) AU with a path length of 300 microm. Both of these figures of merit should be compared to the peak-to-peak baseline noise of one of the best commercial UV-vis HPLC detection systems, which is approximately 5 x 10(-6) AU with a path length of 10 mm (1-s integration time). Therefore, the thermal lensing technique has a demonstrated sensitivity of subnanomolar detection that is approximately 140 times better than that of the best commercial UV-vis detector and approximately 5 times better than that of CRDS.     

Exploration of microwave plasma source cavity ring-down spectroscopy for elemental measurements

We are exploring sensitive techniques for elemental measurements using cavity ring-down spectroscopy (CRDS) combined with a compact microwave plasma source as an atomic absorption cell. The research work marries the high sensitivity of CRDS with a low-power microwave plasma source to develop a new instrument that yields high sensitivity and capability for elemental measurements. CRDS can provide orders of magnitude improvement in sensitivity over conventional absorption techniques. Additional benefit is gained from a compact microwave plasma source that possesses the advantages of low power and low-plasma gas flow rate, which are of benefit for atomic absorption measurements. A laboratory CRDS system consisting of a tunable dye laser is used in this work for developing a scientific base and demonstrating the feasibility of the technique. A laboratory-designed and -built sampling system for solution sample introduction is used for testing. The ring-down signals are monitored using a photomultiplier tube and recorded using a digital oscilloscope interfaced to a computer. Lead is chosen as a typical element for the system optimization and characterization. The effects of baseline noise from the plasma source are reported. A detection limit of 0.8 ppb (10(-)(10)) is obtained with such a device.     

Simultaneous and continuous multiple wavelength absorption spectroscopy on nanoliter volumes based on frequency-division multiplexing fiber-loop cavity ring-down spectroscopy

We demonstrate a method for measuring optical loss simultaneously at multiple wavelengths with cavity ring-down spectroscopy (CRD). Phase-shift CRD spectroscopy is used to obtain the absorption of a sample from the phase lag of intensity modulated light that is entering and exiting an optical cavity. We performed dual-wavelength detection by using two different laser light sources and frequency-division multiplexing. Each wavelength is modulated at a separate frequency, and a broadband detector records the total signal. This signal is then demodulated by lock-in amplifiers at the corresponding two frequencies allowing us to obtain the phase-shift and therefore the optical loss at several wavelengths simultaneously without the use of a dispersive element. In applying this method to fiber-loop cavity ring-down spectroscopy, we achieve detection at low micromolar concentrations in a 100 nL liquid volume. Measurements at two wavelengths (405 and 810 nm) were performed simultaneously on two dyes each absorbing at mainly one of the wavelengths. The respective concentrations could be quantified independently in pure samples as well as in mixtures. No crosstalk between the two channels was observed, and a minimal detectable absorbance of 0.02 cm(-1) was achieved at 405 nm.     

Cavity ring-down spectroscopy as a detector for liquid chromatography

We have demonstrated the use of cavity ring-down spectroscopy (CRDS) as a detector for high performance liquid chromatography (HPLC). For this use, we have designed and implemented a Brewster's angle flow cell such that cavity ring-down spectroscopy can be performed on microliter volumes of liquids. The system exhibits a linear dynamic range of 3 orders of magnitude (30 nM to 30 microM quinalizarin at 470 nm) for static measurements and 2 orders of magnitude (0.5 microM to 50 microM) for HPLC measurements. For the static measurements, the baseline noise is 2.8 x 10(-6) AU rms and 1.0 x 10(-5) AU peak-to-peak, and for the HPLC separations, it is 3.2 x 10(-6) AU rms and 1.3 x 10(-5) AU peak-to-peak. The baseline noise is determined after the data are smoothed by an 11-point boxcar average. The peak areas detected from HPLC separations are reproducible to within 2-3%. The HPLC mass detection limit for a molecule with epsilon = 9 x 10(3) M(-1) cm(-1) in a 300-microm path length cell (illuminated volume, 0.5 microL) is reported as 2.5 x 10(-8) g/mL. These results were obtained using a simple pulsed CRDS system and are comparable to, if not better than, a high-quality commercial UV-vis absorption detector for the same path length.     

Moving beyond traditional UV-visible absorption detection: cavity ring-down spectroscopy for HPLC

We describe the use of liquid-phase continuous-wave cavity ring-down spectroscopy for the detection of an HPLC separation. This technique builds on earlier work by Snyder and Zare using pulsed laser sources and improves upon commercially available UV-visible detectors by a factor of up to 50. The system employs a compact doubled-diode single-mode continuous-wave laser operating at 488 nm and a previously described Brewster's-angle flow cell. Ring-down time constants as long as 5.8 micros were observed with liquid samples in a 0.3-mm path length cell. The baseline noise during an HPLC separation was only 2 x 10(-7) absorbance units (AU) peak to peak, as compared to 1 x 10(-5) AU for a state-of-the-art commercial UV-visible detector.     

Top notch design for fiber-loop cavity ring-down spectroscopy

Fiber-loop cavity ring-down spectroscopy (CRDS) is a highly sensitive spectroscopic absorption technique which has shown considerable promise for the analysis of small-volume liquid samples. We have developed a new light coupling method for fiber-loop CRDS, which overcomes two disadvantages of the technique: low efficiency light coupling into the cavity and high loss per pass. The coupler is based on a 45° reflective notch polished between 10 and 30 μm into the core of a large-core-diameter (365 μm) optical fiber, and allows for nearly 100% light coupling into the cavity, with a low loss per pass (<4%). The coupler has the additional advantage that the input and output light is spatially separated on opposite sides of the fiber. The detection sensitivity of a fiber-loop CRD spectrometer employing the new coupling method is established from ring-down measurements on aqueous rhodamine 6G (Rh6G) at 532 nm. The results are compared with data obtained using the same light source and detector, but a conventional bend-coupled small-core-diameter (50 μm) optical fiber loop. With our new coupler, a detection limit of 0.11 cm(-1) is found, which corresponds to detection of 0.93 μM Rh6G in a volume of only 19 nL. This is an improvement of over an order of magnitude on our bend-coupled small-core optical fiber results, in which a detection limit of 5.3 cm(-1) was found, corresponding to a detection of 43 μM Rh6G in a volume of 20 pL.     

Detection of sputtered metals with cavity ring-down spectroscopy

We report on use of cavity ring-down spectroscopy (CRDS) as a means to detect and quantify ion sputtering of refractory metal species. CRDS measurements are made with a neodymium:YAG-pumped optical parametric oscillator laser system in the 375-400 nm region. CRDS sputtering measurements are presented for argon ions incident on iron, aluminum, molybdenum, and titanium. The measurements are based on absorption from fine-structure levels of the electronic ground-state multiplets. For each species, characteristic spectra are provided, the dependence of sputtered particle number density on the beam current is examined, measured densities are compared with a sputter model, and detection limits are determined. For iron, aluminum, and titanium we probe multiple fine-structure levels within the ground-state multiplet and obtain information on their relative populations.     

UV-visible absorption spectra of silicon CVD intermediates

Transient absorption in 394-435 nm wavelength range following 193 nm photolysis of disilane has been measured by using cavity ring-down spectroscopy (CRDS). A broad and continuum absorption band was observed. Time profiles of the absorption measured at several wavelengths were similar and found to have at least two components. The decaying part of the absorption can be attributed to Si(H₂)Si based on the kinetic consideration and available information from the literature. The absorption was also measured in the hot wire CVD (HW-CVD) of SiH₄. A broad and continuum band was observed.     

温室气体浓度监测的光腔衰荡光谱研究进展

全球气候变化给人类生活带来的影响受到世界各国的普遍关注,温室气体是影响和改变全球气候的关键因素之一,限制和降低温室气体排放量成为人类发展的重要议题.温室气体大多都在10"(每百万个气体分子中所含该种气体分子的个数)级别,且气体分子结构差异大,因此传统方法很难获得较高的精度,而光腔衰荡光谱法是能解决该难题的关键技术之一....     

Miniaturized cavity ring-down detection in a liquid flow cell

A novel method for applying cavity ring-down spectroscopy in the liquid phase, compatible with LC analyses, is presented. The core of the setup is a home-built cavity ring-down flow cell (cell volume 12 microL) that is constructed using a silicon rubber spacer, which is clamped leak-tight between two high-reflectivity mirrors. The mirrors are in direct contact with the liquid flow, which provides for a small path length and short ring-down times. Inside the cavity there are no windows, reflection losses, or Brewster angles to be considered. Due to the small size of the presented cavity geometry, the setup can be implemented in conventional-size LC apparatuses. With a flow injection setup, a detection limit of 2.5 nM was obtained for Crystal Violet in ethanol, and the linear dynamic range of the system is at least 2 orders of magnitude. The method has the potential to become a powerful alternative for commercial LC UV/visible absorbance detectors.     

Numerical analysis of beam propagation in pulsed cavity ring-down spectroscopy

A numerical simulation of pulsed cavity ring-down spectroscopy (CRDS) is developed with the commercially available software package GENERAL LASER ANALYSIS AND DESIGN. The model is verified through a series of numerical experiments. Several issues of concern in CRDS are investigated, including spatial resolution, misalignment, non-Gaussian beam input, and the effect of flames inside a ring-down cavity. Suggestions for the design of pulsed CRDS instruments are provided.     

Development of alternative plasma sources for cavity ring-down measurements of mercury

We have been exploring innovative technologies for elemental and hyperfine structure measurements using cavity ring-down spectroscopy (CRDS) combined with various plasma sources. A laboratory CRDS system utilizing a tunable dye laser is employed in this work to demonstrate the feasibility of the technology. An in-house fabricated sampling system is used to generate aerosols from solution samples and introduce the aerosols into the plasma source. The ring-down signals are monitored using a photomultiplier tube and recorded using a digital oscilloscope interfaced to a computer. Several microwave plasma discharge devices are tested for mercury CRDS measurement. Various discharge tubes have been designed and tested to reduce background interference and increase the sample path length while still controlling turbulence generated from the plasma gas flow. Significant background reduction has been achieved with the implementation of the newly designed tube-shaped plasma devices, which has resulted in a detection limit of 0.4 ng/mL for mercury with the plasma source CRDS. The calibration curves obtained in this work readily show that linearity over 2 orders of magnitude can be obtained with plasma-CRDS for mercury detection. In this work, the hyperfine structure of mercury at the experimental plasma temperatures is clearly identified. We expect that plasma source cavity ring-down spectroscopy will provide enhanced capabilities for elemental and isotopic measurements.     

Off-axis cavity ringdown spectroscopy: application to atmospheric nitrate radical detection

Atmospheric nitrate radicals (NO3) are detected using off-axis cavity ringdown spectroscopy (CRDS) for the first time to our knowledge with a room-temperature continuous-wave (cw) diode laser operating near 662 nm. A prototype instrument was constructed that achieved a 1sigma absorption sensitivity of 5 x 10(-10) cm(-1) Hz(-1/2), corresponding to a 1.4 part per trillion by volume 2sigma detection limit in 4.6 s at 80 degrees C. This sensitivity is a significant improvement over a recent implementation of off-axis cavity-enhanced absorption spectroscopy and comparable to that of the most advanced cw CRDS and pulsed CRDS applications for atmospheric detection of NO3. A comparison of measurements of ambient air in Fairbanks, Alaska, recorded with the off-axis CRDS instrument and a previously characterized conventional cw CRDS instrument showed good agreement (R2 = 0.97).     

Optical properties of dispersed aerosols in the near ultraviolet (355 nm): measurement approach and initial data

An aerosol albedometer combining cavity ring-down spectroscopy (CRDS) with integrating sphere nephelometry was developed for use at λ = 355 nm. The instrument measures extinction and scattering coefficients of dispersed particulate matter in the near ultraviolet (UV) spectral region. Several samples have been analyzed, including: ammonium sulfate, secondary organic aerosols (SOA) resulting from the ozonolysis of α-pinene and photooxidation of toluene, redispersed soil dust samples, biomass burning aerosols, and ambient aerosols. When particle size and number density were experimentally controlled, extinction coefficients and scattering coefficients were found to have a linear relationship with particle number concentration, in good agreement with light scattering theory. For ammonium sulfate and pinene samples, extinction cross sections for size-selected (D(p) = 300 nm) samples were within the range of 1.65-2.60 × 10(-9) cm(2) with the largest value corresponding to ammonium sulfate and the lowest value for pinene SOA. The scattering cross sections of pinene and ammonium sulfate aerosols were indistinguishable from the extinction cross sections, indicating that these particle types had minimal light absorption at 355 nm. However, soil dusts and biomass burning aerosols showed significant absorption with single scatter albedo (SSA) between 0.74 and 0.84. Ambient aerosols also had transient absorption at 355 nm that correlated well with a particle-soot absorption photometer (PSAP) measuring visible light absorption.     

Trace moisture detection using continuous-wave cavity ring-down spectroscopy

We have developed an instrument to measure trace concentrations of small hydride species in gases using continuous-wave cavity ring-down spectroscopy with near-infrared diode laser excitation. An rms baseline equivalent absorbance of 9.2 x 10(-11) cm(-1)/square root(n) is found, where n is the number of ring-down transients. When the 1396.376-nm absorption line of water is used, this corresponds to a noise equivalent moisture concentration in nitrogen gas of 68 pptv/square root(n). Water vapor concentration is detected over a range extending from 3 to 1000 ppbv and found to depend linearly on the concentration as determined by a calibrated commercial moisture sensor.     

A new radiation detector made of multi-walled carbon nanotubes

We report first results on a microstrip radiation detector made of multi-walled carbon nanotubes directly grown on a sapphire substrate between platinum electrodes. Signals induced in nanotubes by pulsed laser beams have been collected and analysed at three different wavelengths, 355, 532 and 1064 nm. Improvement of sensitivity towards UV wavelengths has been observed reflecting the shape of the absorbance spectrum of material. Signals clearly indicate the production of charges and the subsequent transport in the direction of the applied electric field. Measurement of the charge amount collected at different drain voltages as well as the detection efficiency are reported for each of the three wavelengths exploited.     

Detection of vapors of explosives and explosive-related compounds by ultraviolet cavity ringdown spectroscopy

The detection of vapors of dinitrobenzenes and dinitrotoluenes by UV cavity ringdown spectroscopy (CRDS) was investigated. Absorption cross sections at 248 nm were estimated by measurements on saturated vapors and compared with solution-phase values. The computed subparts per 10(9) detection sensitivity with no effort at preconcentration was demonstrated through measurements on diluted flows. The factors affecting measurements on 1 atm total pressure were considered, and it was demonstrated that Rayleigh scattering by air will reduce the detection sensitivity by 5%-10%. The UV absorption spectra of these compounds are broad, resulting in a relatively poor selectivity for single-wavelength measurements with the UV CRDS technique.     

Combining preconcentration of air samples with cavity ring-down spectroscopy for detection of trace volatile organic compounds in the atmosphere

Quantitative detection of small volatile organic compounds in ambient air is demonstrated using a combination of continuous wave cavity ring-down spectroscopy (cw-CRDS) and the preconcentration of air samples with an adsorbent trap. The trap consists of a zeolite molecular sieve, selected for efficient trapping of the test compounds ethene (ethylene) and ethyne (acetylene). Upon heating of the trap, these organic compounds desorb into a small-volume ring-down cavity, and absolute concentrations are measured by CRDS at 6150.30 cm(-1) (ethene) and 6512.99 cm(-1) (ethyne) without the need for calibration. The efficiency of the trapping and desorption was tested using commercial standard gas mixtures and shown to be 100% in the case of ethene, whereas some ethyne is retained under the current operating conditions. Samples of indoor and outdoor air were analyzed for ethene content, and measurements were made of mixing ratios as low as 6 ppbv. Removal of water vapor and CO(2) from the air samples prior to trapping was unnecessary, and the selectivity of the trapping, desorption, and spectroscopic detection steps eliminates the need for gas chromatographic separation prior to analysis. With anticipated improvements to the design, measurements of these and other trace atmospheric constituents should be possible on time scales of a few minutes.     

Detection of trace water vapor in high-purity phosphine using cavity ring-down spectroscopy

The presence of trace water vapor in process gases such as phosphine, used for compound semiconductor epitaxial growth, can negatively affect the optical and electrical properties of the final device. Therefore, sensitive H2O measurement techniques are required to monitor precursor purity and detect unacceptable contamination levels. A commercial cavity ring-down spectrometer that monitors an H2O absorption line at a wavelength of 1392.53 nm was investigated for service in high purity PH3. Spectral parameters such as the line shape of water vapor in the presence of PH3 as well as background features due to PH3 were measured at different pressures and incorporated into the data analysis software for accurate moisture readings. Test concentrations generated with a diffusion vialbased H2O source and dilution manifold were used to verify instrument accuracy, sensitivity, linearity, and response time. H2O readings at 13.2 kPa corresponded well to added concentrations (slope=0.990+/-0.01) and were linear in the tested range (0-52.7 nmol mol-1). The analyzer was sensitive to changes in H2O concentration of 1.3 nmol mol-1 based on 3sigma of the calibration curve intercept for a weighted linear fit. Local PH3 absorption features that could not be distinguished from the H2O line were present in the purified PH3 spectra and resulted in an additional systematic uncertainty of 9.0 nmol mol-1. Equilibration to changing H2O levels at a flow rate of 80 std cm3 min-1 PH3 occurred in 10-30 minutes. The results indicate that cavity ring-down spectroscopy (CRDS) at 1392.53 nm may be useful for applications such as on-line monitoring (and dry-down) of phosphine gas delivery lines or the quality control of cylinder sources.     

Sensitive Spectroscopic Analysis of Biomarkers in Exhaled Breath

We have developed a novel optical setup which is based on a high finesse cavity and absorption laser spectroscopy in the near-IR spectral region. In pilot experiments, spectrally resolved absorption measurements of biomarkers in exhaled breath, such as methane and acetone, were carried out using cavity ring-down spectroscopy (CRDS). With a 172-cm-long cavity, an efficient optical path of ~?132 km was achieved. The CRDS technique is well suited for such measurements due to its high sensitivity and good spectral resolution. The detection limits for methane of ~?8 ppbv and acetone of ~?2.1 ppbv with spectral sampling of 0.005 cm^?1 were achieved, which allowed to analyze multicomponent gas mixtures and to observe absorption peaks of ¹²CH₄ and ¹³CH₄. Further improvements of the technique have the potential to realize diagnostics of health conditions based on a multicomponent analysis of breath samples.