Analytical applications of laser moding caused by intracavity absorption

A continuous CO₂ laser with a reflecting mirror can operate at several wavelengths simultaneously. If an organic vapor is introduced into a separate cavity in the laser optical path, the laser will sometimes mode rapidly causing some lasing lines to diminish to zero and others to become enhanced; this has been observed even for very low amounts (10^-5 g) of organic gases. Laser intercavity absorption spectroscopy depends on overlap of a vibrational—rotational line of a sample with a laser transition line. The absorption by the sample greatly affects the laser wavetrain at that particular wavelength and interferes with the lasing action. The technique is not based on Beer's law, and the detection limits observed are orders of magnitude better than those of conventional infrared absorption spectroscopy. Two laser systems were used and various organic gases were studied. When a totally reflecting mirror which permitted free moding was used, the detection limits found were 0.14 μg, 0.95 μg and 0.60 μg for vinyl chloride, propylene, and ethylene, respectively. When a grating was used as the rear cavity optics restricting the wavelengths of the laser lines, the detection limits were 140 μg, 94 μg, 63 μg and 0.26 μg for vinyl chloride, propylene. ethylene and ethyl chloride, respectively.     

Evaluation of different sample introduction approaches for the determination of boron in unalloyed steels by inductively coupled plasma mass spectrometry

An extended study of different sampling introduction approaches using inductively coupled plasma mass spectrometry (ICP-MS) is presented for the determination of boron in steel samples. The following systems for sample introduction were applied: direct sample solution nebulization by continuous nebulization (CN) using a cross-flow nebulizer and with flow injection (FI), applied to 0.1% (m/v) and 0.5% (m/v) sample solutions, respectively; FI after iron matrix extraction, using acetylacetone–chloroform, and isotopic dilution (ID) analysis as the calibration method; FI with on-line electrolytic matrix separation; and spark ablation (SA) and laser ablation (LA) as solid sampling techniques. External calibration with matrix-matching samples was used with CN, SA, and LA, and only acid solutions (without matrix matching) with FI methods. When FI was directly applied to a sample solution, the detection limit was of 0.15 μg g⁻¹, improving by a factor of 4 that was obtained from the CN measurements. Isotopic dilution analysis, after matrix removal by solvent extraction, made it possible to analyse boron with a detection limit of 0.02 μg g⁻¹ and, with the on-line electrolytic process, the detection limit was of 0.05 μg g⁻¹. The precision for concentrations above 10 times the detection limit was better than 2% for CN, as well as for FI methods. Spark and laser ablation sampling systems, avoiding digestion and sample preparation procedures, provided detection limits at the μg g⁻¹ levels, with RSD values better than 6% in both cases. Certified Reference Materials with B contents in the range 0.5–118 μg g⁻¹ were used for validation, finding a good agreement between certified and calculated values.     

Evaluation of the Performance of Small Diode Pumped UV Solid State (DPSS) Nd:YAG Lasers as New Radiation Sources for Atmospheric Pressure Laser Ionization Mass Spectrometry (APLI-MS)

The performance of a KrF^* bench top excimer laser and a compact diode pumped UV solid state (DPSS) Nd:YAG laser as photo-ionizing source in LC-APLI MS is compared. The commonly applied bench-top excimer laser, operating at 248 nm, provides power densities of the order of low MW/cm² on an illuminated area of 0.5 cm² (8 mJ/pulse, 5 ns pulse duration, beam waist area 0.5 cm², 3 MW/cm²). The DPSS laser, operating at 266 nm, provides higher power densities, however, on a two orders of magnitude smaller illuminated area (60 μJ/pulse, 1 ns pulse duration, beam waist area 2 × 10^–3 cm², 30 MW/cm²). In a common LC-APLI MS setup with direct infusion of a 10 nM pyrene solution, the DPSS laser yields a significantly smaller ion signal (0.9%) and signal to noise ratio (1.4%) compared with the excimer laser. With respect to the determined low detection limits (LODs) for PAHs of 0.1 fmol using an excimer laser, LODs in DPSS laser LC-APLI MS in the low pmol regime are expected. The advantages of the DPSS laser with respect to applicability (size, cost, simplicity) may render this light source the preferred one for APLI applications not focusing on ultimately high sensitivities. Furthermore, the impact of adjustable ion source parameters on the performance of both laser systems is discussed in terms of the spatial sensitivity distribution described by the distribution of ion acceptance (DIA) measurements. Perspectives concerning the impact on future APLI-MS applications are given.     

The deposition of energy from reactions of laser excited atoms: Strontium 3P1 with hydrogen halides

The chemical reactions of laser excited Sr^*(³P₁) with HF and HCl are studied in a beam gas arrangement with LlF detection. This is the first experiment reported where both the specific electronic state of the reactant is laser prepared and the specific vibrational states of the products are laser detected. The results show that a large fraction of the available energy is released into vibration of SrX.     

Diode-laser atomic-absorption spectrometry by the double-beam—double-modulation technique

The limitations of absorption measurements in atomic-absorption spectrometry with tunable diode lasers are investigated. It is shown that the double modulation technique (diode-laser wavelength modulation and sample modulation) with detection at the sum or difference frequency suppresses spurious etalon effects, background absorption, residual diode-laser-amplitude modulation and the noise which accompanies these effects, and enables measurement of detection limits determined by the laser excess noise. Detection limits in absorption, defined as absorption equal to the root-mean square value of noise, as low as 1 × 10⁻⁶ AU (absorption units) were achieved for metastable Cl atoms in a modulated low-pressure microwave-induced plasma with a time constant of 1 s. In order to eliminate laser excess noise and signal variations due to changes of optical transmittance, a double-beam arrangement with logarithmic subtraction of sample and reference detector currents was developed. It enables suppression of variations of the laser radiation power outside the detection pass-band and the achievement of a detection limit of about 2 × 10⁻⁷ AU determined by shot noise only.     

Determination of Hydrogen and Oxygen in Chalcogenide Glasses Using a Tandem Laser Mass Reflectron

A procedure was developed for determining hydrogen and oxygen in As–Se and Se–S systems by laser mass spectrometry on a tandem laser mass reflectron. The procedure involves special (both preliminary and in the course of analysis) cleaning of the sample surface with a Q-switched laser. The performance characteristics of the proposed procedure were studied on certified standard reference materials of aluminum alloys, oxygen-doped As₂Se₃, and As₂S₃ with the known absorption at lines of hydrogen-containing impurities. The dependence of the analytical signal on the concentration of gas-forming impurities was linear in the concentration range 10^–5 to 10^–2 mass %. The relative random error of measurements was not worse than 0.21. The detection limit was 8 × 10^–6 mass %.     

Time discrimination in the laser fluorimetry and ultratrace determination of europium(III) and samarium(III) with 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione

Efficient application of time discrimination has succeeded in substantial reduction of the detection limit of laser fluorimetry. A nitrogen laser and a pulse-gated photon counting method are combined. The detection limits for europiuni(III) and samarium(III) with 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione in solution are 0.4 pg l^-1 and 0.3 ng l^-1, respectively. The results are attributed to effective removal of the short-lived unwanted signals. This technique makes possible the ultratrace determination of a fluorescent molecule in a mixture with others by making the most of the difference of their emission lifetimes. The europium(III) complex can be determined with high selectivity and sensitivity in a large excess of the samarium(III) complex     

Diode laser-based nonlinear degenerate four-wave mixing analytical spectrometry

Inexpensive low-power diode laser-based forward-scattering degenerate four-wave mixing is demonstrated as an unusually simple and sensitive nonlinear laser spectroscopic technique. This novel semiconductor laser-based nonlinear method offers many important advantages including ease of optical alignment, high wave-mixing efficiency, as well as compact and inexpensive design. A preliminary concentration detection limit of 7.7 × 10⁻⁸ M, and a mass detection limit of 12 × 10⁻¹⁸ mol inside a probe volume of 159 pl, corresponding to an absorbance detection limit of 6.9 × 10⁻⁵ are reported for rhodamine 800 using a single 10 mW laser diode as the excitation source.     

Atomabsorptionsspektrometrie mit verdampfung fester Proben durch laserstrahlung—II. Analytische anwendungen

The use of a pulsed neodymium laser permits the direct analysis of solids (metallic and non-metallic) that cannot be analyzed by flame or oven AAS without preparation of the specimen. Microgram quantities are vaporized and partly atomized by the focused laser radiation. AAS techniques are used to investigate the freely expanding vapour of the sample by space and time resolution. The analytical application of this procedure is demonstrated by the determination of the trace constituents Cu, Mn, Mg, Fe, Zn and Si in aluminium and of Cu and Mn in steel.The analytical capability of this method is compared to other AAS techniques. The relative detection limits of ～ 10^?6 g g^?1 for Cu, Mn and Mg in aluminium agree with results obtained on similar samples using flame AAS. The absolute detection limits of ～ 10^?10 to 10^?11 g for these elements approach the values obtained in furnace AAS. The precision is limited by fluctuations in the laser evaporation and in the expansion of the sample vapour.     

A Double‐Imprinted Diffraction‐Grating Sensor Based on a Virus‐Responsive Super‐Aptamer Hydrogel Derived from an Impure Extract

The detection of viruses is of interest for a number of fields including biomedicine, environmental science, and biosecurity. Of particular interest are methods that do not require expensive equipment or trained personnel, especially if the results can be read by the naked eye. A new “double imprinting” method was developed whereby a virus‐bioimprinted hydrogel is further micromolded into a diffraction grating sensor by using imprint‐lithography techniques to give a “Molecularly Imprinted Polymer Gel Laser Diffraction Sensor” (MIP‐GLaDiS). A simple laser transmission apparatus was used to measure diffraction, and the system can read by the naked eye to detect the Apple Stem Pitting Virus (ASPV) at concentrations as low as 10 ng mL⁻¹, thus setting the limit of detection of these hydrogels as low as other antigen‐binding methods such as ELISA or fluorescence‐tag systems.     

Direct determination of traces of lanthanide ions in aqueous solutions by laser-induced time-resolved spectrofluorimetry

The most convenient spectral regions for dye laser excitation are presented for seven lanthanide ions. Fluorescence lifetimes are given for 0.5 M sulphuric acid and 3 M potassium carbonate media. With the excimer-pumped dye laser, the detection limits are in the range 0.1 μg 1^?1 (Eu) to 50 μg 1^?1 (Gd). Examples of the resolution of mixtures are given.     

Pulsed microchip laser induced fluorescence for in situ tracer experiments

A compact system for remote and non intrusive in situ analysis of fluorescent tracers using a newly developed pulsed microchip laser coupled to fiber optics was used for in situ rhodamine determinations. By using a crystal doubling in front of the microchip Nd-YAG laser, it is possible to obtain 532 nm at 5 kHz with an energy of 0.6 μJ in a 0.5 ns pulse. Using fiber optics and a passive optode, it was possible to analyze remotely the fluorescence of rhodamine with a compact detection system (monochromator and photomultiplier). Limits of detection down to 10^–10–10^–11 mol/L can be reached depending on the rhodamine studied. Such a laser can be directly implanted in the optode avoiding laser losses when exciting in the U.V. Received: 30 July 1997 / Revised: 6 October 1997 / Accepted: 10 October 1997     

Pulsed microchip laser induced fluorescence for in situ tracer experiments

A compact system for remote and non intrusive in situ analysis of fluorescent tracers using a newly developed pulsed microchip laser coupled to fiber optics was used for in situ rhodamine determinations. By using a crystal doubling in front of the microchip Nd-YAG laser, it is possible to obtain 532 nm at 5 kHz with an energy of 0.6 μJ in a 0.5 ns pulse. Using fiber optics and a passive optode, it was possible to analyze remotely the fluorescence of rhodamine with a compact detection system (monochromator and photomultiplier). Limits of detection down to 10^–10–10^–11 mol/L can be reached depending on the rhodamine studied. Such a laser can be directly implanted in the optode avoiding laser losses when exciting in the U.V. Received: 30 July 1997 / Revised: 6 October 1997 / Accepted: 10 October 1997     

Capillary electrophoresis of ANTS labelled oligosaccharide ladders and complex carbohydrates

The detection limits of the ANTS (8-aminonaphthalene-1,3,6-trisulfonic acid) label and ANTS maltose as a model carbohydrate conjugate were investigated with on-column UV and laser induced fluorescence detection. Under capillary electrophoresis conditions, the concentration and mass detection limits were found to be 5×10^–7 mol/l or 8 femtomole with UV and 5×10^–8 mol/l or 400 attomole with laser induced fluorescence detection, respectively. Including the derivatization reaction, the best concentration detection limit increases to 1×10^–6 mol/l carbohydrate. A model calculation shows that these detection levels are still insufficient to match those of current protein sequencing protocols.Derivatization conditions for dextran and polygalacturonic acid ladders are described with subsequent fast separation in a capillary electrophoresis system under acidic pH buffer conditions. Up to 30 oligomers could be separated in less than 10 min. The application of ANTS labelled carbohydrate analysis in the food industry is demonstrated with the carbohydrate fraction of sweets and the kinetic monitoring of the hydrolysis of polygalacturonic acid.The described ANTS derivitization protocol works with as little as 5 g carbohydrate as demonstrated with a complex oligosaccharide labelled in a reaction volume as little as 2 l. To demonstrate the applicability of this approach to complex carbohydrate analysis, an oligosaccharide mixture derived from human Immunoglobuline G was labelled and separated within 5 min. Separation efficiency and speed are superior to state-of-the-art chromatographic methods. Both electrophoretic and chromatographic methods are complementary because of their different separation mechanism. The implications of using capillary electrophoresis with laser induced fluorescence and appropriate labelling strategies for structural and compositional analysis of complex carbohydrates are discussed.Dedicated to Professor Dr. Dieter Klockow on the occasion of his 60th birthday     

Development of routines for simultaneous in situ chemical composition and stable Si isotope ratio analysis by femtosecond laser ablation inductively coupled plasma mass spectrometry

Stable metal (e.g. Li, Mg, Ca, Fe, Cu, Zn, and Mo) and metalloid (B, Si, Ge) isotope ratio systems have emerged as geochemical tracers to fingerprint distinct physicochemical reactions. These systems are relevant to many Earth Science questions. The benefit of in situ microscale analysis using laser ablation (LA) over bulk sample analysis is to use the spatial context of different phases in the solid sample to disclose the processes that govern their chemical and isotopic compositions. However, there is a lack of in situ analytical routines to obtain a samples' stable isotope ratio together with its chemical composition. Here, we evaluate two novel analytical routines for the simultaneous determination of the chemical and Si stable isotope composition (δ³⁰Si) on the micrometre scale in geological samples. In both routines, multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) is combined with femtosecond-LA, where stable isotope ratios are corrected for mass bias using standard-sample-bracketing with matrix-independent calibration. The first method is based on laser ablation split stream (LASS), where the laser aerosol is split and introduced simultaneously into both the MC-ICP-MS and a quadrupole ICP-MS. The second method is based on optical emission spectroscopy using direct observation of the MC-ICP-MS plasma (LA-MC-ICP-MS|OES). Both methods are evaluated using international geological reference materials. Accurate and precise Si isotope ratios were obtained with an uncertainty typically better than 0.23‰, 2SD, δ³⁰Si. With both methods major element concentrations (e.g., Na, Al, Si, Mg, Ca) can be simultaneously determined. However, LASS-ICP-MS is superior over LA-MC-ICP-MS|OES, which is limited by its lower sensitivity. Moreover, LASS-ICP-MS offers trace element analysis down to the μg g⁻¹-range for more than 28 elements due to lower limits of detection, and with typical uncertainties better than 15%. For in situ simultaneous stable isotope measurement and chemical composition analysis LASS-ICP-MS in combination with MC-ICP-MS is the method of choice.     

Laser ionization of H2S and ion-molecule reactions of H3S+ in laser-based ion mobility spectrometry and drift cell time-of-flight mass spectrometry

The detection of hydrogen sulfide (H₂S) by 2?+?1 resonance-enhanced multi-photon ionization (REMPI) and the application of H₂S as a laser dopant for the detection of polar compounds in laser ion mobility (IM) spectrometry at atmospheric pressure were investigated. Underlying ionization mechanisms were elucidated by additional studies employing a drift cell interfaced to a time-of-flight mass spectrometer. Depending on the pressure, the primary ions H₂S⁺, HS⁺, S⁺, and secondary ions, such as H₃S⁺, were observed. The 2?+?1 REMPI spectrum of H₂S near λ?=?302.5 nm was recorded at atmospheric pressure. Furthermore, the limit of detection and the linear range were established. In the second part of the work, H₂S was investigated as an H₂O analogous laser dopant for the ionization of polar substances by proton transfer. H₂S exhibits a proton affinity (PA) similar to that of H₂O, but a significantly lower ionization energy facilitating laser ionization. Ion-molecule reactions (IMR) of H₃S⁺ with a variety of polar substances with PA between 754.6 and 841.6 kJ/mol were investigated. Representatives of different compound classes, including alcohols, ketones, esters, and nitroaromatics were analyzed. The IM spectra resulting from IMR of H₃S⁺ and H₃O⁺ with these substances are similar in structure, i.e., protonated monomer and dimer ion peaks are found depending on the analyte concentration.     

Rapid analysis of aromatic contaminants in water samples by means of laser ionization mass spectrometry

Three different approaches to laser ionization mass spectrometric analysis of aromatic compounds in water samples are described and their performances are compared. Whereas the first two methods are based on direct laser desorption and subsequent laser ionization of either frozen or adsorbed samples in a time-of-flight mass analyzer, the third performs laser ionization in a quadrupole ion-trap into which the sample is transferred from a GC injector via a short piece of capillary tubing. For the laser-desorption method a detection limit in the 100 µg L^–1 range was determined for fluorene in frozen samples. The easier to handle analysis of adsorbed samples yielded sensitivities which were lower by about two orders of magnitude. As both direct techniques do not reach the sensitivity required for ultra trace analysis in water a preconcentration step in form of solid-phase microextraction was added before measurement using the laser ionization quadrupole ion-trap mass spectrometer. Sensitivity in the desired ng L^–1 range was easily achieved.     

Analytical atomic spectroscopy using ion trap devices

Investigations using ion trap devices for analytical atomic spectroscopy purposes have focused on the use of an inductively coupled plasma (ICP) ion source with ion trap mass spectrometric (ITMS) detection. Initial studies were conducted with an instrument assembled by simply appending an ion trap as the detector to a fairly conventional ICP/MS instrument, i.e. leaving an intermediate linear quadrupole between the plasma source and the ion trap. The principal advantages found with this system include the destruction of nearly all problematic and typical ICP/MS polyatomic ions (e.g., ArH⁺, ArO⁺, ArCl⁺, Ar₂⁺, etc) and a dramatic reduction of the primary plasma source ion, Ar⁺. These results prompted the development of a second-generation plasma source ion trap instrument in which direct coupling of the ICP and ion trap has been effected (i.e. no intermediate linear quadrupole); the same performance benefits have been largely preserved. Initial operation of this instrument is described, characterized, and compared to the originally described ICP/ITMS and conventional ICP/MS systems. In addition, experiments aimed at improving ICP/ITMS sensitivity and selectivity using broadband resonance excitation techniques are described. Finally, the potential for laser optical detection of trapped ions for analytical purposes is speculated upon.     

Infrared laser heterodyne systems

This paper gives a review of mid-infrared laser heterodyne systems. The advantages of using heterodyne detection are described and the different techniques corresponding to the choice of the local oscillator are presented. A thorough discussion of the gas laser heterodyne systems is followed by the presentation of tunable diode laser systems capabilities. Many experiments in the mid-infrared region are reviewed covering the astronomical and atmospheric, ground-based and airborne investigations as well as the laboratory measurements.     

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%.