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.     

Flowing liquid-sheet jet for cavity ring-down absorption measurements

The use of a flowing liquid-sheet jet in cavity ring-down absorption measurements is described. A mechanical gear pump was used to pump solvent at low pressure through a circular orifice. The resulting cylindrical jet of solvent was fired at a flat surface. A flat sheet of liquid was formed in a small portion of the resulting spray, which was sufficiently stable to be positioned at the Brewster angle in a linear ring-down cavity setup operated with a pulsed laser. The path length through a sheet-jet of ethylene glycol was measured to be 23.2 +/- 0.6 mum. Malachite Green dye was used as an analyte to demonstrate a linear dynamic range of 12.6 dB (73.9 nM to 1.34 microM). The limit of detection for the system was determined to be alpha LOD = 0.0162 cm(-1), or 71 nM (at epsilon = 9.975 x 10(4) M(-1) cm(-1), 628 nm, 3 sigma). The technique is shown to have promise for analytical and spectroscopic measurements, for example, in studies of gas-liquid interfaces.     

Development of broadband cavity ring-down spectroscopy for biomedical diagnostics of liquid analytes

We present a spectrometer for sensitive absorption measurements in liquids across broad spectral bandwidths. The spectrometer combines the unique spectral properties of incoherent supercontinuum light sources with the advantages of cavity ring-down spectroscopy, which is a self-calibrating technique. A custom-built avalanche photodiode array is used for detection, permitting the simultaneous measurement of ring-down times for up to 64 different spectral components at nanosecond temporal resolution. The minimum detectable absorption coefficient was measured to be 3.2 × 10(-6) cm(-1) Hz(-1/2) at 527 nm. We show that the spectrometer is capable of recording spectral differences in trace levels of blood before and after hemolysis.     

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.     

偏振光腔衰荡技术测量单层SiO2薄膜特性

为了探究特定沉积工艺参数下,不同沉积角度对SiO2光学薄膜损耗及应力双折射的影响,本文采用一种高灵敏探测方法—偏振光腔衰荡技术表征单层SiO2光学薄膜.该技术基于测量光学谐振腔内偏振光来回反射累积后的衰荡时间特性及产生的相位差振荡频率,实现光学元件的光学损耗和残余应力的同点、同时绝对测量.实验对60°、70°和80°沉...     

Pulsed quantum cascade laser-based cavity ring-down spectroscopy for ammonia detection in breath

Breath analysis can be a valuable, noninvasive tool for the clinical diagnosis of a number of pathological conditions. The detection of ammonia in exhaled breath is of particular interest for it has been linked to kidney malfunction and peptic ulcers. Pulsed cavity ringdown spectroscopy in the mid-IR region has developed into a sensitive analytical technique for trace gas analysis. A gas analyzer based on a pulsed mid-IR quantum cascade laser operating near 970 cm(-1) has been developed for the detection of ammonia levels in breath. We report a sensitivity of approximately 50 parts per billion with a 20 s time resolution for ammonia detection in breath with this system. The challenges and possible solutions for the quantification of ammonia in human breath by the described technique are discussed.     

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.     

Ray-transfer-matrix model for accurate pulsed cavity ring-down measurement in the mismatching case

A theoretical model based on the ray-transfer matrix is developed for the pulsed cavity ring-down (CRD) technique to numerically investigate the influence of the geometric parameters of the pulsed-CRD arrangement on the CRD signal. By fitting the spatial distribution of the pulsed laser beam to that of the TEM(00) cavity mode, the geometric parameters are optimized to obtain perfect matching between the laser beam and the ring-down cavity. It is indicated by the numerical simulations that as long as the laser power exiting the ring-down cavity is fully collected, a single exponential-decay signal, identical to the perfectly-matched CRD signal, is obtained in the mismatching case to determine accurately the cavity decay time. Intensity fluctuations appear in the mismatched CRD signal if the laser power exiting the ring-down cavity is not fully collected. Both the conventional exponential decay fitting approach and a linear fitting procedure are employed to analyze these mismatched CRD signals and the latter is recommended to make an accurate pulsed-CRD measurement.     

Direct monitoring of absorption in solution by cavity ring-down spectroscopy

Cavity ring-down spectroscopy is applied to the liquid phase by placing the target solution directly into the optical cavity. We demonstrate that solutions in the cavity can be stirred and more importantly monitored in a flow. We report a minimum detectable absorption of 10(-6) cm(-1) for a range of organic solvents. This detection limit corresponds to picomolar concentrations for strong absorbers.     

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.     

Isotopic measurements of uranium using inductively coupled plasma cavity ringdown spectroscopy

Inductively coupled plasma cavity ringdown spectroscopy (ICP-CRDS) is applied to isotopic measurements of uranium. We have successfully obtained the isotopic-resolved spectra of uranium at three different atomic/ionic transition lines, 286.57, 358.49, and 409.01 nm. Of the three lines, the largest isotope shift of approximately 9 pm was measured at the 286.57 ionic line. Isotopic-resolved spectra were recorded in ratio of 1:1 (235U/238U, 2.5 micrograms/mL) and at the natural abundance ratio of 0.714% (235U/238U, 1.25 micrograms/mL 235U). The smallest measurable isotope shift of approximately 3 pm was determined for the 409.01 nm ion spectral line. Detection limits (DL) were obtained under optimized ICP operating conditions to be in the range of 70-150 ng/mL, except for the 238U component of the 286.57 nm line (300 ng/mL). This latter result was determined to be due to a strong, previously unreported, absorption interference from the argon plasma. The 235U isotope component (DL 70 ng/mL) was found to be unaffected. This work demonstrates the applicability of ICP-CRDS for uranium isotopic measurements. The potential of development of a field-deployable, on-line uranium isotope monitor using plasma-CRDS is discussed.     

A compact diode laser cavity ring-down spectrometer for atmospheric measurements of NO3 and N2O5 with automated zeroing and calibration

A compact rack-mounted cavity ring-down spectrometer (CRDS) for simultaneous measurements of the nocturnal nitrogen oxides NO(3) and N(2)O(5) in ambient air is described. The instrument uses a red diode laser to quantify mixing ratios of NO(3) (at its absorption maximum at 662 nm) and of N(2)O(5) following its thermal dissociation to NO(3) in a second detection channel. The spectrometer is equipped with an automated zeroing and calibration setup to determine effective NO(3) absorption cross-sections and NO(3) and N(2)O(5) inlet transmission efficiencies. The instrument response was calibrated using simultaneous measurements of NO(2), generated by thermal dissociation of N(2)O(5) and/or by titration of NO(3) with excess NO, using blue diode laser CRDS at 405 nm. When measuring ambient air, the (2σ, 10 s) precision of the red diode CRDS varied between 5 and 8 parts-per-trillion by volume (pptv), which sufficed to quantify N(2)O(5) concentrations under moderately polluted conditions. Sample N(2)O(5) measurements made on a rooftop on the University of Calgary campus in August 2010 are presented. A maximum N(2)O(5) mixing ratio of 130 pptv was observed, corresponding to a steady-state lifetime of less than 50 min. The NO(3) mixing ratios were below the detection limit, consistent with their predicted values based on equilibrium calculations. During the measurement period, the instrument response for N(2)O(5) was 70% of the theoretical maximum, rationalized by a slight mismatch of the laser diode output with the NO(3) absorption line and a N(2)O(5) inlet transmission efficiency less than unity. Advantages and limitations of the instrument's compact design are discussed.     

Use of broadband, continuous-wave diode lasers in cavity ring-down spectroscopy for liquid samples

Cavity ring-down spectroscopy (CRDS) is an extremely sensitive absorption technique that has been applied primarily to gas samples, which are characterized by having narrow absorption features. Recently, CRDS has also been applied to liquid samples, which have broad absorption features. The use of small inexpensive diode lasers as light sources for liquid samples is demonstrated. The low cost coupled with the ease and technical straightforwardness of application gives this technique wide appeal.     

Development of a cavity ringdown laser absorption spectrometer for detection of trace levels of mercury

A potential new laser-based air pollution measurement technique, capable of measuring ultralow concentrations of urban air toxins in the field and in real time, is examined. Cavity ringdown laser absorption spectroscopy (CRLAS) holds promise as an air pollution monitor because it is a highly sensitive species detection technique that uses either pulsed or continuous tunable laser sources. The sensitivity results from an extremely long absorption path length and the fact that the quantity measured, the cavity decay time, is unaffected by fluctuations in the laser source. In laboratory experiments, we reach detection limits for mercury of the order of 0.50 parts per trillion. We developed a CRLAS system in our laboratory and measured Hg with the system, investigating issues such as background interference. We report experimental results for mercury detection limits, the dynamic range of the sensor, detection of Hg in an absorbing background of ozone and SO(2), and detection of a mercury-containing compound (HgCl(2) in this case).     

Evanescent-wave cavity ring-down spectroscopy for enhanced detection of surface binding under flow injection analysis conditions

The feasibility of liquid-phase evanescent-wave cavity ring-down spectroscopy (EW-CRDS) for surface-binding studies under flow-injection analysis (FIA) conditions is demonstrated. The EW-CRDS setup consists of an anti-reflection coated Dove prism inside a linear cavity (with standard or super-polishing of the total internal reflective (TIR) surface). A teflon spacer with an elliptical hole clamped on this surface acts as a 20 muL sized flow cell. The baseline noise of this system is of the order of 10(-4) absorbance units; the baseline remains stable over a prolonged time and the prism surface does not become contaminated during repeated injections of the reversibly adsorbing test dyes Crystal Violet (CV) and Direct Red 10 (DR10). At typical FIA or liquid chromatography (LC) flow rates, the system has sufficient specificity to discriminate between species with different surface affinities. For CV a much stronger decrease in ring-down time is observed than calculated based on its bulk concentration and the effective depth probed by the evanescent wave, indicating binding of this positively charged dye to the negatively charged prism surface. The amount of adsorption can be influenced by adjusting the flow rate or the eluent composition. At a flow rate of 0.5 mL/min, an enrichment factor of 60 was calculated for CV; for the poorly adsorbing dye DR10 it is 5. Super-polishing of the already polished TIR surface works counter-productively. The adsorbing dye Crystal Violet has a detection limit of 3 muM for the standard polished surface; less binding occurs on the super-polished surface and the detection limit is 5 muM. Possible applications of EW-CRDS for studying surface binding or the development of bio-assays are discussed.     

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.     

High-extinction-ratio resonant cavity polarizer for quantum-optics measurements

The use of a high-finesse Fabry-Perot ring cavity with an odd number of reflections as a high-extinction-ratio resonant polarizer is shown. Experimental results from quantum-noise measurements using resonant cavities as spatial and spectral filters and precision polarizers are presented.     

Effects of linear birefringence and polarization-dependent loss of supermirrors in cavity ring-down spectroscopy

In cavity ring-down spectroscopy (CRDS), residual or stress-induced birefringence (10(-7)-10(-6) rad) of supermirrors will lift the polarization degeneracy of TEM(00) modes and generate two new polarization eigenstates in the cavity with small resonant frequency splitting (approximately 0.1 kHz); the new eigenstates are nearly linearly polarized. When both modes are excited simultaneously, the intracavity polarization state will evolve as the energy decays in the cavity. Without polarization analysis, such mode beating would not be observable. However, real supermirrors have a linear polarization-dependent loss (dichroism) that leads to a change in the loss rate as the polarization state evolves and thus to deviation from the expected single-exponential decay. We develop a model for the evolution of the intracavity polarization state and intensity for a cavity with both birefringence and polarization-dependent loss in the mirrors. We demonstrate, experimentally, that these parameters (both magnitudes and directions) can be extracted from a series of measurements of the cavity decay and depolarization of the transmitted light.