M.A. Bolshov

A comparison of nanosecond and femtosecond laser-induced plasma spectroscopy of brass samples

The ablation of brass samples in argon shield gas by 170 fs and 6 ns laser pulses has been studied by optical emission spectroscopy of the evolving plasmas. Differences observed in the temporal behavior of the spectral line intensities are explained by the shielding effect of the Ar plasma for ns-pulses and the free expansion of the plasma of the ablated material in case of fs-pulses. Brass with different ZnrCu ratios were used as samples. Different types of crater formation mechanisms in the case of ns- and fs-pulses were observed. At 40 mbar argon pressure the thresholds of ablation were found to be ; 0.1 and ; 1.5 J cm y2 for fs- and ns-pulses, respectively. With an internal standardization of zinc to copper it is possible to correct for differences in the ablation rates and to obtain linear calibration curves. For optimum experimental conditions, narrower confidence intervals for the determination of unknown concentrations were found in case of fs-pulses. Within the range of the laser intensities used, no dependence of the ZnrCu line intensity ratio on the number of laser pulses applied to the same ablation spot was observed, neither for fs- nor for ns-pulses, which is interpreted as the absence of fractional vaporization. Q 2000 Elsevier Science B.V. All rights reserved.

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

Abstract A diode laser is used for the selective excitation of 235 U and 238 U in a laser-induced plasma applying Nd:YAG laser pulses to UO 2 samples. The diode laser is rapidly scanned immediately following each laser sampling and the resonance atomic fluorescence spectrum for both isotopes is obtained on a pulse-to-pulse basis. Time-integrated measurements, with the diode laser fixed at either isotope, were also made. Optimum signal-to-noise was obtained at a distance of 0.8 cm from the sample surface, a pressure of 0.9 mbar and a Nd:YAG laser pulse energy of 0.5 mJ (880 MW cm −2 ). Three samples with 0.204, 0.407 and 0.714% 235 U were measured. For example, for the UO 2 pellet with the natural uranium isotopic composition (99.281% 238 U and 0.714% 235 U), the accuracy and precision were 7% and 5% (460 shots), respectively, limited by the continuum emission background from the laser-induced plasma.

Atomic fluorescence spectrometry with laser excitation

Abstract A laser atomic fluorescence spectrometry for the detection of trace concentrations of the elements is described. The detection limits for Pb, Fe, Na, Pt, Ir, Eu, Cu, Ag, Co and Mn in aqueous solutions obtained at present are the best ones for the rapid spectral analytical methods. The analytical potentials of the laser spectrometer are exemplified by the analysis of real samples of different chemical composition.

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

Abstract A diode laser was used for the selective detection of 235 U and 238 U 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 235 U, and 682.6913 nm for 238 U). 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 235 U 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 238 U 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 235 U 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%.

Direct ultratrace determination of cadmium in Antarctic and Greenland snow and ice by laser atomic fluorescence spectrometry

Abstract Results of the first direct measurements of Cd in Antarctic and Greenland ancient ice and recent snow at ultratrace levels by ultrasensitive laser-excited atomic fluorescence spectrometry with electrothermal atomization are presented. Ultratrace concentration Cd standards allowed calibration of the spectrometer down to the 0.1 pg Cd g −1 level. The limit of detection was found to be as low as about 0.01 pg Cd g −1 using 50-μl sample volumes, which corresponds to a minimum detected Cd absolute mass of about 0.5 fg. Various Antarctic and Greenland samples were then analysed without any preliminary preconcentration step.

Changes in cadmium concentrations in Antarctic ice and snow during the past 155,000 years

Changes in Cd concentrations in Antarctic ice and snow during the last full climatic cycle (the past 155,000 yrs) have been investigated by analysing various sections of the Dome C and Vostok deep Antarctic ice cores and several blocks of recent Antarctic snow. Each sample was mechanically decontaminated using ultraclean procedures and then analysed for Cd by the new ultrasensitive laser excited atomic fluorescence technique. Cd concentrations are found to have been highly variable in ancient Antarctic ice and therefore in the past pristine south polar atmosphere during the last climatic cycle, the highest values being observed during the cold terminal stages of the last and next to last ice ages. Concentrations measured in recent Antarctic snows are comparable with those in Antarctic Holocene ice several thousand years old, which suggests that the anthropogenic influence is probably still negligible for this heavy metal in the south polar atmosphere. For some of the samples, measured Cd concentrations can be simply accounted for by rock and soil dust and volcanic emissions, while for others there is a significant excess over the contributions from these two sources.

Elemental analysis by diode laser spectroscopy

Abstract After about 25 years of development of laser analytical spectroscopy, laser spectrochemical instruments at present work efficiently only in research laboratories. There are practically no commercial laser spectroscopic instruments for routine elemental analysis. The only laser-based instruments are Raman spectrometers with stable cw laser sources for molecular analysis and pulsed lasers with fixed wavelengths which are used for laser ablation (LA) – laser sampling of solid materials. The main difficulties of laser spectroscopic techniques are: relatively high cost, insufficient reliability, and the necessity of qualified personal. Semiconductor laser diodes are mass produced for compact disc players, laser printers, optical data storage systems and telecommunication equipment. A number of these laser diodes have excellent spectroscopic properties, which make them attractive sources for spectrochemical analysis. Presently, laser analytical instruments based on laser diodes have the highest potential for transfer from research laboratories to routine practice. Recently, the excellent analytical capabilities of diode laser spectrometry in the detection of low concentrations of analytes have been well documented. Recent advances in diode lasers and analytical trends are discussed below.

Automated laser excited atomic fluorescence spectrometer for determination of trace concentrations of elements

Abstract A new version of a computer-controlled laser excited atomic fluorescence spectrometer LAFAS-1 is described. The laser part of the spectrometer consists of a dye laser pumped by an excimer laser. Electrothermal atomization in an argon atmosphere and under vacuum conditions may be employed. The spectrometer was tested by acidified standard solutions prepared in the Grenoble clean laboratory. These standards were based on ultrapure water. A limit of detection for Pb of 0.18 pg/ml was achieved. For a 20 μl aliquot volume this value gives an absolute limit of detection of 4fg. A Pb content of 0.28 pg/ml in the Grenoble ultrapure water was measured by LAFAS-1 with a confidence interval of 0.05 pg/ml. This value is in excellent agreement with that obtained previously by isotope dilution mass spectrometry: 0.27 pg/ml.

The use of a dye laser for the detection of sub-picogram amounts of lead and iron by atomic fluorescence spectrometry

Abstract The frequency-doubled radiation of a dye laser excited by the second harmonic radiation of a YAG:Nd 3+ laser has been used as light source in graphite-tube atomic fluorescence analysis. Detection limits and linear ranges of the analytical curves for two elements, Pb and Fe, were studied. Direct-line fluorescence at 373.5 mn for Fe and at 405.8 nm for Pb was observed. The limits of detection, obtained using linear extrapolation to the background level, were 25 pg/ml for Fe and 2.5 pg/ml for Pb. This corresponds to an absolute detection limit of 0.75 pg for Fe and 0.075 pg for Pb. In the case of Pb, saturation was reached at an intensity of the exciting radiation of about 20 kW/cm 2 , but in the case of Fe, linearity was maintained up to 300 kW/cm 2 .

Variations in heavy metal concentrations in fresh Greenland snow from January to August 1989

Abstract Concentrations of lead, cadmium, copper and zinc have been measured in a variety of samples of fresh or slightly aged snow collected at Dye 3, south Greenland, on a precipitation event basis from January to August 1989. Measured concentrations are found to be very variable from one snowfall to another, with high concentration peaks occurring in April and June. The four metals are shown to be mainly derived from anthropogenic sources, with the exception of Cu and Zn for some of the samples. The data obtained for several snow events are further discussed using 5 days backward air mass trajectories together with data for various other chemical species.