Gas leak detection by diode laser absorption spectrometry

Diode laser atomic absorption measurements of argon traces in low-pressure discharges were carried out to detect and measure gas leaks in a test chamber. Helium flows as a carrier gas through the test chamber and the discharge. In the case of a leak, air and thus also its natural content of argon is mixed to the helium gas-flow through the chamber. The argon content of the mixed gas flow through the discharge is determined by wavelength modulation diode laser atomic absorption spectrometry. The resulting absorption signal is a measure for the existing leak-rate. For barometric pressure of ambient air lowest detectable leak rates are typically 10⁻⁶ mbar l s⁻¹. By application of pure Ar with pressures above 1 bar detectable leak rates can in practice be lower than 10⁻⁸ mbar l s⁻¹.     

Determination of molecular line parameters for acrolein (C(3)H(4)O) using infrared tunable diode laser absorption spectroscopy

Acrolein (C(3)H(4)O) molecular line parameters, including infrared (IR) absorption positions, strengths, and nitrogen broadened half-widths, must be determined since they are not included in the high resolution transmission (HITRAN) molecular absorption database of spectral lines. These parameters are required for developing a quantitative analytical method for measuring acrolein in a single puff of cigarette smoke using tunable diode laser absorption spectroscopy (TDLAS). The task is complex since acrolein has many highly overlapping infrared absorption lines in the room temperature spectrum and the cigarette smoke matrix contains thousands of compounds. This work describes the procedure for estimating the molecular line parameters for these overlapping absorption lines in the wavenumber range (958.7-958.9 cm(-1)) using quantitative reference spectra taken with the infrared lead-salt TDLAS instrument at different pressures and concentrations. The nitrogen broadened half-width for acrolein is 0.0937 cm(-1)atm(-1) and to our knowledge, is the first time it has been reported in the literature.     

Wavelength modulation diode laser atomic absorption spectrometry in analytical flames

The potential of wavelength modulation laser atomic absorption spectrometry in analytical flames is demonstrated by the measurement of titanium, cesium and chromium applying fundamental and frequency doubled radiation of commercially available semiconductor diode lasers. In dependence on the radiation power, absorbances of the order of 10⁻⁴–10⁻⁶ are measured, which reveal very low detection limits even when weak absorption lines are used.     

Isotope selective element analysis by diode laser atomic absorption spectrometry

The analytical figures of merit of isotope selective diode laser atomic absorption spectrometry (DLAAS) in low-pressure graphite furnaces are given for lithium and rubidium. While⁶Li and⁷Li were measured by Doppler-limited as well as by Doppler-free absorption spectroscopy of the 670.79 nm resonance line, Doppler-free saturation spectroscopy was applied for analysis of the⁸⁵Rb and⁸⁷Rb D2 resonance line at 780.03 nm. Three different modulation techniques were applied and compared: (i) intensity modulation, (ii) wavelength modulation, and (iii) a combination of intensity and wavelength modulation.     

A compact NIR fiber-optic diode laser spectrometer for CO and CO(2): analysis of observed 2f wavelength modulation spectroscopy line shapes

A compact fiber-optic diode laser spectrometer for the measurement of CO and CO(2) gas concentrations in the near infrared around 1580 nm is described. By use of a balanced receiver to suppress diode laser intensity noise a sensitivity of 6.4 x 10(-7) at 1 Hz system bandwidth was achieved. At a reduced pressure of 80 hPa this equals to a detection limit of 5.1 ppm CO and 9.1 ppm CO(2) with 1m absorption path length. The observed line shapes of the 2f wavelength modulation spectroscopy (WMS) scheme are analyzed theoretically and experimentally. Accurate measurements of magnitude and phase of the diode laser frequency and intensity modulation responses were found critically for modeling the observed line shapes. In situ measurements of gas dissociation processes inside of a medium-power carbon dioxide laser are presented as an application example.     

Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-atomic absorption spectrometry

A diode laser was used for the selective detection of ²³⁵U and ²³⁸U in a laser-induced plasma ignited by a Nd:YAG laser beam focused onto uranium oxide samples. The diode laser was sequentially tuned to the absorption lines of both isotopes (682.6736 nm for ²³⁵U, and 682.6913 nm for ²³⁸U). The absorption was measured on a pulse-to-pulse basis; the transient absorption peak was used as an analytical signal. Three samples were used with the relative abundance of the minor isotope ²³⁵U of 0.204%, 0.407% and 0.714%. Optimal conditions for the detection of the minor isotope were obtained at a distance of ∼3 mm from the sample surface, an argon pressure of ∼3 kPa and for 7.5 mJ pulse energy of the Nd:YAG laser. Absorption in the wing of the broadened line of the ²³⁸U isotope was found to be the main source of background for the measurement of the absorption of the minor isotope. The limit of detection of the minor isotope, evaluated on the basis of the 3σ criteria was estimated to be 100 μg g⁻¹. At the optimal conditions for the detection of the minor isotope optical thick conditions in the line centre of the main isotope were observed. Therefore, the isotope ratio measurements were performed by rationing the intensity of the net absorption signal measured in the line centre of the minor isotope and the absorption signal measured in the wing of the main isotope. This strategy was checked by determination of the isotope ratios for the two samples with depleted ²³⁵U concentration using the sample with the natural isotope composition (0.714%) as a standard. The accuracy and precision for this measurement strategy was evaluated to approximately 10%.     

Analysis of chlorine in polymers by laser sampling and diode laser atomic absorption spectrometry

The combination of Laser Ablation and Diode Laser Atomic Absorption Spectrometry in a low-pressure Microwave Induced Plasma (MIP) is presented as a method for analysis of solid samples. The capability of this technique is demonstrated by the analysis of chlorine in polymer matrices compared with the major constituents carbon and hydrogen. The detection limit of chlorine was found to be approximately 10 pg/shot or 85 μg/g applying eight laser shots. Furthermore, the reproduction of the stoichiometry of different polyvinylchloride samples verified the absence of severe fractionation effects during the ablation and transport into the MIP.     

Measurement of Cr(III)/Cr(VI) species by wavelength modulation diode laser flame atomic absorption spectrometry

High-performance flow atomic spectrometry of Cr(III) and Cr(VI) species by high-performance liquid chromatography (HPLC) separation, hydraulic high-pressure nebulization (HHPN) for sample introduction and wavelength modulation-laser atomic absorption spectrometry (WM-LAAS) with diode lasers in an analytical flame is reported. 3σ detection limits of 0.5 and 1 ng ml⁻¹ have been obtained for Cr(VI) in deionized and drinking water, respectively. Due to relatively high blank levels, the corresponding detection limits of Cr(III) are higher than for Cr(VI), i.e. 1.6 ng ml⁻¹ in deionized water and 5 ng ml⁻¹ in drinking water.     

Element-selective detection in liquid and gas chromatography by diode laser absorption spectrometry

An element-selective detector for chromatography based on atomic absorption spectrometry with semiconductor diode lasers is described. The analytical utility of the technique is demonstrated by speciation examples of HPLC and GC employing analytical flames and plasmas to atomize.     

Isotope selective analysis of CO(2) with tunable diode laser (TDL) spectroscopy in the NIR

The performance of a home-built tunable diode laser (TDL) spectrometer, aimed at multi-line detection of carbon dioxide, has been evaluated and optimized. In the regime of the (30(0)1)(III) <-- (000) band of (12)CO(2) around 1.6 microm, the dominating isotope species (12)CO(2), (13)CO(2), and (12)C(18)O(16)O were detected simultaneously without interference by water vapor. Detection limits in the range of few ppmv were obtained for each species utilizing wavelength modulation (WM) spectroscopy with balanced detection in a long-path absorption cell set-up. High sensitivity in conjunction with high precision -- typically +/-1 (per thousand) and +/-6 (per thousand) for 3% and 0.7% of CO(2), respectively -- renders this experimental approach a promising analytical concept for isotope-ratio determination of carbon dioxide in soil and breath gas. For a moderate (12)CO(2) line, the pressure dependence of the line profile was characterized in detail, to account for pressure effects on sensitive measurements.     

Determination of molecular line parameters for acrolein (C3H4O) using infrared tunable diode laser absorption spectroscopy

Acrolein (C₃H₄O) molecular line parameters, including infrared (IR) absorption positions, strengths, and nitrogen broadened half-widths, must be determined since they are not included in the high resolution transmission (HITRAN) molecular absorption database of spectral lines. These parameters are required for developing a quantitative analytical method for measuring acrolein in a single puff of cigarette smoke using tunable diode laser absorption spectroscopy (TDLAS). The task is complex since acrolein has many highly overlapping infrared absorption lines in the room temperature spectrum and the cigarette smoke matrix contains thousands of compounds. This work describes the procedure for estimating the molecular line parameters for these overlapping absorption lines in the wavenumber range (958.7–958.9 cm⁻¹) using quantitative reference spectra taken with the infrared lead-salt TDLAS instrument at different pressures and concentrations. The nitrogen broadened half-width for acrolein is 0.0937 cm⁻¹ atm⁻¹ and to our knowledge, is the first time it has been reported in the literature.     

Characterization of an element selective GC-plasma detector based on diode laser atomic absorption spectrometry

An element-selective GC-plasma detector consisting of a modulated low-pressure microwave-induced plasma and a laser detection system based on Wavelength Modulation Diode Laser Atomic Absorption Spectroscopy (WM-DLAAS) was investigated. The influence on the plasma conditions and the fragmentation capability of different types of organic compounds, such as alkanes, alkenes, alkynes and aromatics, was studied by measurements of the element ratios of C, H and Cl, and the population density of the metastable 1s₅ level of Kr added as a trace to the plasma gas (He or Ar). Deviations from the expected stoichiometrical ratios were found to be insignificant. Therefore, calibration and determination of the sum formula of analyte species is possible if an internal standard is used. Furthermore, the correlation between the metastable density in the microwave-induced plasma and the total dissociation energy of the analytes was studied.     

Determination of traces of Ni in Li(2)CO(3)/Na(2)CO(3) melts by graphite furnace atomic absorption spectrometry

A graphite furnace atomic absorption method is described for the determination of traces of Ni in 52 mole percent (mol.%) Li₂CO₃ and 48 mol.% Na₂CO₃ melts after dissolution of the sample in dilute nitric acid. Maximum pyrolysis and optimum atomisation temperatures for the analyte were determined in the presence of the Li and Na matrix constituents. Pre-pyrolysed ascorbic acid (typical amount of 5 μg) has been used as chemical modifier for effectively suppressing the chemical and spectral interferences of the Li/Na matrix, thus that the analysis can be conducted using acid-matched standard solutions. The results of the Ni analysis in synthetic sample solutions by calibration graph against acid-matched standards well agree with those obtained by the method of standard additions. Recoveries ranged from 99 to 101% and the relative standard deviation is around 3% at the 20 μg L⁻¹ level. Moreover, the use of the chemical modifier leads to an improvement of the lifetime of the graphite tube. The Ni detection limit (3σ) in Li/Na carbonate melts for the proposed method is similar to that obtained in aqueous solution, i.e. 5 × 10⁻⁸ g analyte per gram of (Li_0.52Na_0.48) ₂CO₃ melt. This method is successfully applied to the determination of nickel in real carbonate melt samples.     

Rapid accurate analysis of metal (oxide)-on-silica catalysts by atomic absorption spectrometry

The catalysts, which contain 10–60% copper, chromium, nickel and silicon, are decomposed in sealed Teflon-lined vessels and analyzed by atomic absorption spectrometry. Matrix matching and bracketing standards are applied. The RSD of a single determination is about 1% for all components.     

Quantitative element analysis by isotope dilution in diode laser graphite furnace atomic absorption spectrometry

The paper reviews the application of the isotope dilution technique in optical atomic absorption spectrometry by use of a low-pressure graphite tube furnace as atomizer and diode lasers as radiation sources. The principles and the methodology to obtain accurate quantitative results despite of the occurrence of interferences are presented. The successful application of different Doppler-limited and Doppler-free spectrometric techniques is also presented. The perspectives but also the limitations of this new method are discussed.     

Infrared diode laser absorption spectroscopy of C(2)H(2) and C(2)H(6) in capacitively coupled methane RF discharges

Low-temperature RF discharges with methane as feed gas are widely used for the deposition of hydrogenated films. The film properties depend strongly on the chemical composition and therefore two of the main stable products in this kind of discharge, namely ethane (C(2)H(6)) and acetylene (C(2)H(2)), have been measured for the understanding of the reaction kinetics in the plasma. An absorption spectrometer has been built up for the investigation of the concentrations of these as a function of the input power and the flow rate. The time scales for reaching steady state after the discharge is switched on and the depletion time scale after the plasma is switched off have been determined. Assuming the recombination of CH(3) molecules to be the only production mechanism for C(2)H(6) and using a simplified rate equation, the measured densities of C(2)H(6) can be reproduced very well by analytical fitting curves.     

Element selective detection of molecular species applying chromatographic techniques and diode laser atomic absorption spectrometry

Tunable diode laser atomic absorption spectroscopy (DLAAS) combined with separation techniques and atomization in plasmas and flames is presented as a powerful method for analysis of molecular species. The analytical figures of merit of the technique are demonstrated by the measurement of Cr(VI) and Mn compounds, as well as molecular species including halogen atoms, hydrogen, carbon and sulfur.     

Measurements of line strengths in the 2nu1 band of the HO2 radical using laser photolysis/continuous wave cavity ring-down spectroscopy (cw-CRDS)

Absolute absorption cross sections of the absorption spectrum of the 2nu1 band of the HO2 radical in the near-IR region were measured by continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled to laser photolysis in the wavelength range 6604-6696 cm(-1) with a resolution better than 0.003 cm(-1). Absolute absorption cross sections were obtained by measuring the decay of the HO2 self-reaction, and they are given for the 100 most intense lines. The most important absorption feature in this wavelength range was found at 6638.20 cm(-1), exhibiting an absorption cross section of sigma = 2.72 x 10(-19) cm2 at 50 Torr He. Using this absorption line, we obtain a detection limit for the HO2 radical at 50 Torr of 6.5 x 10(10) cm(-3).     

Tunable diode laser absorption spectroscopy study of CH(3)CH(2)ODD(2)O binary condensation in a supersonic Laval nozzle

We have developed a dual-beam tunable diode laser absorption spectroscopy system to follow the cocondensation of water and ethanol in a supersonic Laval nozzle. We determine the D(2)O monomer concentration in the vapor phase by fitting a Voigt profile to the measured line shape but had to develop a calibration scheme to evaluate the C(2)H(5)OD monomer concentration. To measure the temperature of the gas, we seed the flow with CH(4) and measure two absorption lines with different lower state energies. These data give a far more detailed picture of binary condensation than axially resolved pressure measurements. In particular, we observe that the C(2)H(5)OD monomer starts to be depleted from the gas phase well before D(2)O begins to condense.     

Infrared diode laser spectroscopy of jet-cooled NiCO, Ni(CO)3 (13CO), and Ni(CO3(C 18O)

Gas phase infrared spectroscopic investigations of the CO vibration of jet-cooled NiCO, Ni(CO)3(13CO), and Ni(CO)3(C18O) are reported. The spectra were obtained using a recently assembled pulsed-discharge slit-jet IR diode laser spectrometer. The rotationally resolved spectrum of NiCO was collected as it was formed in the discharge, while the spectra of Ni(CO)3(13CO) and Ni(CO)3(C18O) were recorded as they were destroyed. For NiCO, band origins of 2010.692 89(34) and 2010.645 28(23) cm(-1) were measured, along with values of B0=0.151 094(7) and 0.149 597(6) cm(-1) and B(1)=0.150 244(7) and 0.148 742(6) cm(-1) for 58NiCO and 60NiCO, respectively. The B0 values for these isotopologs were used to determine the two bond lengths in NiCO, giving r0 (Ni-C)=1.641(40) A and r0 (C-O)=1.193(53) A, in agreement with recent microwave measurements. The constants determined for Ni(CO)3(13CO) were upsilon0=2022.075 753(95) cm(-1), B"=0.034 736(2) cm(-1), and B'=0.034 688(2) cm(-1). For Ni(CO)3(C18O), upsilon0=2021.936 83(18) cm(-1), B"=0.033 764(4) cm(-1), and B'=0.033 710(4) cm(-1) were obtained. From these rotational constants, bond lengths of r0 (Ni-C)=1.839+/-0.007 A and r0 (C-O)=1.121+/-0.010 A were obtained. These values are discussed in relation to the bond lengths measured by electron and x-ray diffraction methods.