Optimization of micro-Laser Induced Breakdown Spectroscopy analysis and signal processing

As a result of continuing instrumental development (Echelle spectrometer and ICCD detectors), micro-Laser Induced Breakdown Spectroscopy analysis may become an increasingly recognized analytical technique for determining elemental compositions of geologic materials. Best conditions of time resolution conditions (delay and time acquisition window) are estimated with respect to the collection geometry of optical plasma emission of our system. It turns out that the level of the Bremsstrahlung continuum emission is weak in the first tens of nanoseconds after the laser excitation pulse. The enlargement of the emission lines is identified in the first 100 ns but remains comparable to the spectral resolution of our system. Thus, results show that time-resolved conditions are not necessarily required to perform elemental analysis at the micrometric scale using LIBS, contrary to macro-LIBS. This suggests potential improvements of micro-LIBS analysis (sensitivity and spectral resolution) using non-intensified CCD connected with the laser pulse.     

Absolute characterization of laser-induced breakdown spectroscopy detection systems

A quantitative comparison of the performance of four different laser-induced breakdown spectroscopy detection systems is presented. The systems studied are an intensified photodiode array coupled with a Czerny–Turner spectrometer, an intensified CCD coupled with a Czerny–Turner spectrometer, an intensified CCD coupled to an Echelle spectrometer, and a prototype multichannel compact CCD spectrometer system. A simple theory of LIBS detection systems is introduced, and used to define noise-equivalent spectral radiance and noise-equivalent integrated spectral radiance for spectral detectors. A detailed characterization of cathode noise sources in the intensified systems is presented.     

A combined laser-induced breakdown and Raman spectroscopy Echelle system for elemental and molecular microanalysis

Raman and laser-induced breakdown spectroscopy is integrated into a single system for molecular and elemental microanalyses. Both analyses are performed on the same ~ 0.002 mm² sample spot allowing the assessment of sample heterogeneity on a micrometric scale through mapping and scanning. The core of the spectrometer system is a novel high resolution dual arm Echelle spectrograph utilized for both techniques. In contrast to scanning Raman spectroscopy systems, the Echelle–Raman spectrograph provides a high resolution spectrum in a broad spectral range of 200–6000 cm^? 1 without moving the dispersive element. The system displays comparable or better sensitivity and spectral resolution in comparison to a state-of-the-art scanning Raman microscope and allows short analysis times for both Raman and laser induced breakdown spectroscopy. The laser-induced breakdown spectroscopy performance of the system is characterized by ppm detection limits, high spectral resolving power (15,000), and broad spectral range (290–945 nm). The capability of the system is demonstrated with the mapping of heterogeneous mineral samples and layer by layer analysis of pigments revealing the advantages of combining the techniques in a single unified set-up.     

Space and time-resolved laser-induced breakdown spectroscopy using charge-coupled device detection

Space and time-resolved studies of laser induced plasmas in air at atmospheric pressure are presented. Photovoltaic solar cells have been used as samples. The second harmonic (532 nm) of a Nd : YAG laser at an irradiance of 18 × 10¹² W/cm² has been used. The precise focus of the beam allows a microanalysis at a 0.02 mm² surface area working in single-shot mode. The use of an intensified charge-coupled device (CCD) detector has allowed time-resolved studies in both imaging or spectroscopy modes. The two-dimensional capability of the CCD has enabled the study of atomic and ionic species distribution along the plume. Most data have been recorded using single-laser shot experiments. Spectral lines have been assigned to transitions in atomic components of the material under investigation in the neutral or ionic states of the corresponding atoms. Effects of delay in improving spectral resolution and some examples of spectral characterization of species as a function of its decay are shown.     

Space and time-resolved laser-induced breakdown spectroscopy using charge-coupled device detection

Space and time-resolved studies of laser induced plasmas in air at atmospheric pressure are presented. Photovoltaic solar cells have been used as samples. The second harmonic (532 nm) of a Nd : YAG laser at an irradiance of 18 x 10(12) W/cm(2) has been used. The precise focus of the beam allows a microanalysis at a 0.02 mm(2) surface area working in single-shot mode. The use of an intensified charge-coupled device (CCD) detector has allowed time-resolved studies in both imaging or spectroscopy modes. The two-dimensional capability of the CCD has enabled the study of atomic and ionic species distribution along the plume. Most data have been recorded using single-laser shot experiments. Spectral lines have been assigned to transitions in atomic components of the material under investigation in the neutral or ionic states of the corresponding atoms. Effects of delay in improving spectral resolution and some examples of spectral characterization of species as a function of its decay are shown.     

飞秒时间分辨荧光非共线光参量放大光谱技术

时间分辨荧光光谱技术是研究激发态弛豫、能量传递以及电荷转移等光化学过程的重要且直接的工具．飞秒时间分辨荧光非共线光参量放大光谱技术是一种新发展的具有高时间分辨率、宽探测带宽、高增益的时间分辨光谱技术．本文对该技术的基本原理与工作特性、系统配置、荧光收集和会聚、数据采集模式、时问分辨光谱数据处理等进行了系统阐述．最后简单介绍了飞秒时间分辨荧光非共线光参量放大光谱技术在物理、生物和化学领域的3个典型应用,并以此展现该技术的高时间分辨率、宽光谱探测及高增益优势．     

A combined experimental and theoretical study on photoinduced intramolecular charge transfer in trans-ethyl p-(dimethylamino)cinamate

Intramolecular charge transfer (ICT) behavior of trans-ethyl p-(dimethylamino)cinamate (EDAC) in various solvents has been studied by steady-state absorption and emission, picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption experiments as well as time-dependent density functional theory (TDDFT). Large fluorescence spectral shift in more polar solvents indicates an efficient charge transfer from the donor site to the acceptor moiety in the excited state compared to the ground state. The energy for 0,0 transition (ν_0,0) for EDAC shows very good linear correlation with static solvent dielectric property. The relaxation dynamics of EDAC in the excited state can be effectively described by a “three state” model where, the locally excited (LE) state converts into the ICT state within 350 ± 100 fs. A combination of solvent reorganization and intramolecular vibrational relaxation within 0.5–6 ps populates the relaxed ICT state which undergoes fluorescence decay within few tens to hundreds of picoseconds.     

Investigation of plasmas produced by laser ablation using single and double pulses for food analysis demonstrated by probing potato skins

We report on investigations of plasmas produced by laser ablation of fresh potatoes using infrared nanosecond laser radiation. A twin laser system consisting of two Nd:YAG oscillators was used to generate single or double pulses of adjustable interpulse delay. The potatoes were irradiated under ambient air with moderate pulse energies of about 10 mJ. The expansion dynamics of the ablation plume was characterized using fast imaging with a gated camera. In addition, time-resolved optical emission spectroscopy was applied to study the spectral line emission of the various plasma species. The electron density was deduced from Stark broadening, and the plasma temperature was inferred from the relative emission intensities of spectral lines. The relative concentrations of metals were estimated from the comparison of the measured emission spectra to the spectral radiance computed for a plasma in local thermal equilibrium. It is shown that the plasma produced by double pulses has a larger volume and a lower density. These properties lead to an increase of the signal-to-noise ratio by a factor of 2 and thus to an improved measurement sensitivity.     

Investigation on native vesicles containing the nicotinic acetylcholine receptor using FTIR-spectroscopy

The ligand gated ion channel nicotinic acetylcholine receptor is responsible for the electrochemical signal transduction in nerve cells and at the motor endplates. In the recent years the structure of the channel has emerged to a resolution of 4.6 Å [J. Mol. Biol. 288 (1999) 765]. We have used ATR–FTIR and SEIRA spectroscopy to investigate the extramembraneous structure of the receptor. The adsorption of nicotinic acetylcholine receptor rich vesicles on the surface of Ag-cluster leads to the detection of high content of helical structure in the extra membranous parts of the receptor. Spectra indicate a β-sheet structure perpendicular to the crystal plane.     

Photophysics of the interaction between a fluorescein derivative and Ficoll

Ficoll has been widely used as a crowding agent to mimic intracellular media because it is believed to be noninteracting and is composed of mixed sizes such that smaller and larger diffusing solutes can be studied. Due to the interest that the fluorescent dye 9-[1-(2-methyl-4-methoxyphenyl)]-6-hydroxy-3H-xanthen-3-one (TG-II) as a fluorometric probe of phosphate ions in intracellular media could generate, we describe the spectral characteristics of the system TG-II-Ficoll in aqueous solution by means of absorption spectroscopy, steady-state fluorescence, time-resolved fluorescence, time-resolved emission spectroscopy, and fluorescence lifetime correlation spectroscopy. The spectral characteristics found are consistent with the formation of an adsorption complex on the surface of Ficoll, probably due to hydrogen bonding between TG-II and Ficoll. In addition, the diffusion coefficient calculated for the association was similar to the diffusion coefficient previously recovered for Ficoll in the same experimental conditions. Therefore, our overall data clearly demonstrate that Ficoll is not an inert crowding agent when in the presence of fluorescein derivative dyes.     

Chemometrics: the issues of measurement and modelling

In this paper, two spectral data sets have been used to illustrate the importance of maintaining chemical information whilst generating predictive multivariate calibration models. The first data set is based on 26 duplicate UV/VIS spectra for four meal ions (Fe, Ni, Co, Cu) present at varying concentrations in aqueous solution. Spectra were collected across the range 180–800 nm at a resolution of 3.5 nm generating 211 data points for each sample. Calibration was carried out using multiple linear regression (MLR) and a K-matrix approach to demonstrate the advantages the latter method has in describing real spectral features. In addition, the limitation of MLR in accommodating noise and spectral overlap in the data is also illustrated. The second data set based on NIR spectroscopy, was generated using a four-level 2 factor Factorial design strategy and consisted of two additives present at a range of concentrations in an aqueous caustic system, with the spectra being collected over the range 10,000–3000 cm⁻¹. Whilst a conventional partial least squares (PLS) model was applied to the data, it was through the use of variable selection (VS) prior to PLS and the application of weighted ridge regression (WRR) techniques that the need to develop chemometric methodology which intuitively reflected chemical information has been demonstrated. The results will also illustrate how a poorly designed experimental design protocol and missing data can limit the performance of the calibration models generated. The aims of this paper are not to prescribe ideal calibration methodology but rather to demonstrate the relevance of selecting multivariate calibration methodology that relates more to the chem rather than just the metrics in chemometrics.     

Elemental analysis of steel scrap metals and minerals by laser-induced breakdown spectroscopy

The atomic emission of laser-induced plasma on steel samples has been studied for quantitative elemental analysis. The plasma has been created with 8 ns wide pulses using the second-harmonic from a Q-switched Nd:YAG laser, in air at atmospheric pressure. The plasma emission is detected with temporal resolution, using an Echelle spectrometer of wide spectral range (300–900 nm) combined with an intensified charge coupled device camera. A plasma temperature of 7800 ± 400 K is determined using the Boltzmann plot method, from spectra obtained under optimized experimental conditions.As an example of an industrial application the concentration of copper in scrap metals is studied, which is an important factor to determine the quality of the samples to recycle. Cu concentrations down to 200 ppm can be detected. Another application of the laser-induced plasma spectroscopy method is the measurement of the nickel and copper concentrations in an iron-containing sample of reduced magma from the 1870s expedition to western Greenland by Adolf Erik Nordenskiöld. Different spectral lines of nickel are used for calibration, and their results are compared.     

Breaking the nanosecond barrier in FTIR time-resolved spectroscopy

The state of the art in broad spectral bandwidth infrared time-resolved spectroscopy (IRTRS) is reviewed, with particular regard to time resolution in the nanosecond and sub-nanosecond regime. While step-scan Fourier transform infrared (S²FTIR) has been successful in pushing the time resolution of IRTRS to sub-microsecond limits, and is, in principle, applicable for monitoring time-dependent phenomena on any time scale, a practical limit for S²FTIR is currently about 1 ns, due to the limitations of parts of the instrument other than the interferometer itself. For the particular case of IRTRS of transient photo-excited states illustrated here and other photo-excitation studies, it is proposed that the most effective way to breach the nanosecond barrier and to push the time resolution limit of IRTRS to 10 ps, or even lower, while still maintaining the spectral bandwidth and resolution and the multiplex and throughput advantages of interferometry, is to turn to constant velocity, continuous-scan (CS) FTIR, in the pump–probe asynchronous sampling mode. In the method described, the pump is provided by the picosecond UV pulse of an electron storage ring-powered free electron laser and the infrared probe is the picosecond `white light' synchrotron pulse from the same storage ring. The design specifications of this system are 10 ps time resolution with 3 cm⁻¹ spectral resolution.     

An EPR/ENDOR study of the asymmetric hydrogen bond between the quinone electron acceptor and the protein backbone in Photosystem I

Hydrogen bonding to the photoaccumulated secondary acceptor radical anion A₁^√− in photosystem (PS) I has been studied using pulsed Q-band ENDOR spectroscopy. With deuterated quinone in protonated PS I particles it is demonstrated that the observed radical anion has only one hydrogen-bond hyperfine coupling (hfc) tensor with tensor components above the 2 MHz range. Below 2 MHz the protein matrix protons dominate and a second weak H-bond could not be detected. The spectral resolution of pulsed Q-band ENDOR is critically required to separate the signals of the H-bond proton from those of the primary chlorophyll acceptor, A₀^√−, which cannot be avoided to be formed to some extent in the photoaccumulation procedure. The determined H-bond hfc tensor of A₁^√− is found to be close to axial symmetry with a small isotropic component, as expected from a predominantly dipolar electron–proton spin interaction in a hydrogen-bond. The principal tensor components are A=(+)7.7, MHz A=(−)4.9 MHz, A_iso=(−)0.7 MHz. The magnitude of the dipolar tensor corresponds to an unusually short H-bond which can be estimated from the point-dipole approximation (1.5±0.1 Å). Based on previous studies with A- and B-branch specific site-directed mutants of the A₁ site of PS I and the chosen photoaccumulation protocol, the observed A₁^√− radical anion can be assigned to the Q_K–A site of the A-branch. The observed H-bond hfc tensor is compared to those determined for related quinone radical anions observed in frozen protic solution as well as in the Q_A site of type II bacterial reaction centers.     

The detection of carbon monoxide by cavity enhanced absorption spectroscopy with a DFB diode laser

It has been proven that cavity enhanced absorption spectroscopy is a high sensitive spectral technique. The aim of our study was to apply this spectral technique to the detection of carbon monoxide with a narrow line width tunable DFB diode laser and high Q factor optical cavity. Absorption signals were extracted from a measurement recording the average of 20 highest light intensities that leak out of the cavity. The absorption spectrum of CO centered at 6354.18 cm⁻¹ was recorded; the experiment results indicate that cavity enhanced absorption spectroscopy could produce accurate high resolution spectrum. A detection sensitivity about 5.687 × 10⁻⁷ cm⁻¹ has been achieved in a 45 cm-long cell.     

Spectral line selection for time-resolved investigations of laser-induced plasmas by an iterative Boltzmann plot method

The aim of this work is to provide a procedure to determine time-resolved electron temperatures with minimized relative errors by the Boltzmann plot method. The applied procedure consists of two parts, a systematic theoretical spectral line selection and an iterative Boltzmann plot algorithm. After a pre-selection of an appropriate non-disturbed or overlapped set of spectral lines of a particular atomic or ionic species Boltzmann plots are generated using experimentally recorded data for every time window and laser pulse energy of interest. Spectral lines with the highest average deviations from the regression function are assumed as being not representative for the considered ensemble of spectral lines and are therefore discarded gradually until a threshold value for the coefficient of determination is exceeded. Laser-induced breakdown spectroscopy (LIBS) is applied for time-resolved and spatially integrated investigations of plasmas on 1.1750 C75 steel alloy samples with laser pulse energies ranging between 200 µJ and 2 mJ. For the specific chemical composition of these samples a selection of atomic and ionic Fe spectral lines has been carried out. In spite of the fact that only laser pulse energies in the low millijoule regime are applied the final sets of spectral lines comprise in total 61 Fe I and 12 Fe II emission lines. By applying this method electron temperatures can be determined with averaged relative errors of down to 1.8% for Fe I and 4.4% for Fe II emission lines.     

Microchip micellar electrokinetic chromatography coupled with electrochemical detection for analysis of synthetic oestrogen mimicking compounds

A miniaturised analytical system for separating and detecting a range of steroidal oestrogens, based on the coupling of a micromachined capillary electrophoresis chip with glassy-carbon electrode amperometric detector, is described. Factors influencing the on-chip separation utilising the technique micellar electrokinetic chromatography (MEKC) and detection processes are optimised. The addition of modifiers such as organic solvents and surfactants improve separation and resolution of these hydrophobic compounds. Using a borate running buffer (5 mM, pH 11) with 20% methanol and SDS (20 mM) and a separation voltage of 2000 V, baseline resolution is observed for 16-keto-17β-oestradiol, oestriol, 11β-hydroxyoestradiol, oestrone, and β-oestradiol in 420 s with limits of detection 16–84 μM. The implications for on-site environmental analysis are discussed.     

Laser-induced breakdown spectroscopy at a water/gas interface: A study of bath gas-dependent molecular species

Single-pulse laser-induced breakdown spectroscopy has been performed on the surface of a bulk water sample in an air, argon, and nitrogen gas environment to investigate emissions from hydrogen-containing molecules. A microplasma was formed at the gas/liquid interface by focusing a Nd:YAG laser beam operating at 1064 nm onto the surface of an ultra-pure water sample. A broadband Echelle spectrometer with a time-gated intensified charge-coupled device was used to analyze the plasma at various delay times (1.0–40.0 μs) and for incident laser pulse energies ranging from 20–200 mJ. In this configuration, the dominant atomic spectral features at short delay times are the hydrogen H-alpha and H-beta emission lines at 656 and 486 nm, respectively, as well as emissions from atomic oxygen liberated from the water and air and nitrogen emission lines from the air bath gas. For delay times exceeding approximately 8 μs the emission from molecular species (particularly OH and NH) created after the ablation process dominates the spectrum. Molecular emissions are found to be much less sensitive to variations in pulse energy and exhibit a temporal decay an order of magnitude slower than the atomic emission. The dependence of both atomic hydrogen and OH emission on the bath gas above the surface of the water was studied by performing the experiment at standard pressure in an atmospheric purge box. Electron densities calculated from the Stark broadening of the H-beta and H-gamma lines and plasma excitation temperatures calculated from the ratio of H-beta to H-gamma emission were measured for ablation in the three bath gases.     

Application of light guides on the PLASMAQUANT-110 ICP spectrometer — Analytical capabilities and analytical performance

Summary On a high-aperture, high-resolution Echelle spectrometer individual fibre light guides are used to transmit the spectral light from the focal plane to the detectors. For this purpose, 132 light guides are arranged behind an exit slit mask. These light guides transmit the light of the spectral lines of 70 elements. The application of light guides provides numerous advantages for analytical program changes and extensions and enables fast survey analyses. Thoroughly chosen fibre types and precise arrangement of light guides are the prerequisites for a good analytical performance which — beside reproducibilities of lines and background as well as detection limits — is exemplified by the spectral resolution of boron and iron lines as well as of the iron triplet at 310 nm.     

Concept for a single-shot mid-infrared spectrometer using synchrotron radiation

Several attempts have been made to extend time-resolved mid infrared spectroscopy to higher time resolution. Such methods are either limited to specific samples that are cyclic and therefore allow the reaction under investigation to be repeated multiple times in the same manner, or they lack spectral resolution or sufficient signal-to noise ratio. Here, we report on a single-shot spectrometer concept which overcomes the aforementioned limitations utilizing fast linear detector arrays and highly brilliant infrared synchrotron radiation. The spectrometer may find applications, beside others, for the investigation of irreversible cascades of structural alterations in proteins.