Time-resolved ultraviolet laser-induced breakdown spectroscopy for organic material analysis

Ultraviolet pulses (266 nm) delivered by a quadrupled Nd:YAG laser were used to analyze organic samples with laser-induced breakdown spectroscopy (LIBS). We present characteristics of the spectra obtained from organic samples with special attentions on the emissions of organic elements, O and N, and molecular bonds CN. The choice of these atomic or molecular species is justified on one hand, by the importance of these species to specify organic or biological materials; and on the other hand by the possible interferences with ambient air when laser ablation takes place in the atmosphere. Time-resolved LIBS was used to determine the time-evolution of line intensity emitted from these species. We demonstrate different kinetic behaviors corresponding to different origins of emitters: native atomic or molecular species directly vaporized from the sample or those generated through dissociation or recombination due to interaction between laser-induced plasma and air molecules. Our results show the ability of time-resolved UV-LIBS for detection and identification of native atomic or molecular species from an organic sample.     

Time-resolved fluorescence spectroscopy for illuminating complex systems

Over the years, the emissive characteristics (spectral, temporal, and polarization) of fluorophores have been widely used to probe a wide variety of systems. Fluorescence lifetime and rotational reorientation time measurements, in particular, offer a means to elucidate key details about complex systems. Further, because fluorescence occurs on the nanosecond (10⁻⁹ s) timescale, competing or perturbing kinetic processes like collisional quenching, solvent relaxation, energy transfer, and rotational reorientation can affect the fluorescence and hence be quantified. Thus, a carefully chosen and “placed” fluorophore can serve as an reporter on a wide range of nanosecond or faster events. This contribution is divided into three sections. The Theory section discusses time-resolved anisotropy and intensity decay kinetics (time and frequency domains), pump–probe spectroscopy, and up-conversion. The second section describes time-correlated single photon counting (TCSPC) and multifrequency phase-modulation fluorescence instruments. The final section is divided into subsections on the use of time-resolved fluorescence: (1) to study solvation dynamics, biochemical systems, polymer photophysics, and organized media; (2) as a tool in the separation sciences, microscopy, and sensing; and (3) coupled with multiphoton excitation strategies.     

Time-resolved plasma properties for double pulsed laser-induced breakdown spectroscopy of silicon

A systematic measurement of plasma properties (temperature, electron number density, pressure) was performed during LIBS of silicon with two nanosecond pulsed lasers operating at 1064 nm. The spectral characteristics of the plasmas were measured to determine the plasma properties as delay time between the laser pulses was changed from 0 to 10 ms. The plasma properties and crater dimensions increased abruptly from 100 to 200 ns. The crater depth increased from 2 to 10 μm (volume increased about 5 times) per pair of double pulses. Enhanced mass removal was indicative of a phase explosion mechanism. Spatial images of plasma emission were measured to study the dynamics of plasma expansion.     

Time-resolved fluorescence spectroscopy of hematoporphyrin derivative in micelles

Time-resolved fluorescence spectroscopy was performed on hematoporphyrin derivative in buffer at different concentrations of surfactants. New molecular species were detected, with fluorescence decay time constants of ≈0.7 and ≈2.7 ns and emission peaks at 630 and 675 nm, respectively. Deaggregation effects were observed, mainly in the presence of cationic micelles.     

Resonance fluorescence spectroscopy in laser-induced cavitation bubbles

Laser-induced breakdown spectroscopy (LIBS) in liquids using a double-pulse Q-switched Nd:YAG laser system has provided reliable results that give trace detection limits in water. Resonant laser excitation has been added to enhance detection sensitivity. A primary laser pulse (at 532 nm), transmitted via an optical fiber, induces a cavitation bubble and shockwave at a target immersed in a 10 mg l⁻¹–100 mg l⁻¹ indium (In) water suspension. The low-pressure rear of the shockwave induces bubble expansion and a resulting reduction in cavity pressure as it extends away from the target. Shortly before the maximum diameter is expected, a secondary laser pulse (also at 532 nm) is fed into the bubble in order to reduce quenching processes. The plasma field generated is then resonantly excited by a fiber-guided dye laser beam to increase detection selectivity. The resulting resonance fluorescence emission is optically detected and processed by an intensified optical multichannel analyzer system.      

Time-resolved laser-induced fluorescence with high-performance liquid chromatography for analysis of polycyclic aromatic hydrocarbon mixtures

Time-resolved fluorescence is used to enhance the selectivity for polycyclic aromatic hydrocarbon mixtures by high-performance liquid chromatography. A pulsed laser is used as the excitation source. Only fluorophores with long fluorescence lifetimes (e.g. fluoranthene) are monitored if the delay between excitation and detection is sufficiently long. Limits of detection are of the order of 1–10 pg (5–50 fmol). The advantages are illustrated with a sample from the condensed steam distillate following a coal gasification experiment.     

Time-resolved total internal reflection fluorescence spectroscopy of polymer films

Using total internal reflection, the possibility of a subnanosecond fluorescence spectroscopy for elucidating photophysical and photochemical processes of polymer surface is demonstrated. The thickness which can be studied under the present experimental conditions is of the order of 0.01 μm.     

Time-resolved spectroscopy of the human forearm.

For spectroscopic purposes, the forearm is a conveniently large object to be investigated because consistent oxygenation and blood volume changes can be obtained. Human forearm spectral properties were investigated using picosecond near-IR laser spectroscopy. The behaviour of the temporal point spread function in resting conditions and during ischaemia, venous occlusion and exercise is reported. The effect of path length inaccuracy on muscle oxygen consumption, obtained by combining spectral data with the path length, is discussed.     

Time-resolved fluorescence spectroscopy reveals functional differences of cationic polymer-DNA complexes

Cationic polymers bind DNA and form compacted nanoparticulates (i.e., polyplexes). Polyplexes augment DNA delivery into the cells as a nonviral method of gene therapy. DNA packing and release are the key factors in polyplex-mediated gene delivery, but they are poorly understood due to the lack of physical methods of investigation. We used time-resolved fluorescence spectroscopy to study poly(ethylenimine) (PEI) and poly(L-lysine) (PLL) polyplexes. Analysis of fluorescence lifetimes and time-resolved spectra revealed that DNA exists in several different states in PEI polyplexes and only in one tightly bound state in PLL polyplexes. The observed difference in the nature of the polyplexes may explain why PEI releases DNA more easily than PLL even though both polycations condense DNA effectively. The present method utilizing time-resolved fluorescence spectroscopy gives information on the specific interactions between DNA and the cationic polymers in the polyplexes. This kind of information is very important in the development of biologically effective nonviral systems for DNA delivery.     

Time-resolved and steady-state fluorescence spectroscopy of eumelanin and indolic polymers

Eumelanin plays a variety of important physiological roles in human skin. However, its structure and fundamental properties still remain poorly understood. Although the absorbance of eumelanin is broad and reveals little about its structure, a variety of techniques have revealed the presence of a disordered array of chromophores within the melanin compound. In order to examine the fluorescence decay dynamics of these chromophores, time-resolved spectroscopy was applied to solutions of synthetic eumelanin and a melanin-like polymer of N-methyl,5-hydroxy,6-methoxyindole (N-Me-5H6MI). Solutions were excited with 80 fs laser pulses at 355, 370, 390 and 400 nm, and decay time courses were acquired at 20 nm intervals between 400 and 600 nm for each excitation wavelength. Decay profiles for both eumelanin and the polymer exhibited a characteristic multiexponential behavior with decay times between 0.5 and 15 ns, although steady-state spectra for the polymer exhibited only two peaks. The long-decay component in the polymer showed a significant decrease in both amplitude (30-5%) and decay time (14-6 ns) with increasing emission wavelength. In contrast, the amplitude and decay time in melanin increased slightly (10-15% and 7-10 ns, respectively) from 400 to 520 nm emission, at which point they leveled off. These trends were consistent for all excitation wavelengths. These results suggest that the multiexponential behavior of melanin fluorescence is characteristic of each oligomer within the eumelanin compound, and is consistent with the assertion that the diversity of constituents within eumelanin provides it with a robustness in spectral properties.     

Time-resolved spectroscopy of long-path fluorescence and scattering for measuring total absorption of fluids

Optical methods are appropriate tools to detect organic micro-pollutants in fluids. A new technique is introduced which uses the decay of interaction processes like fluorescence and elastically scattered radiation by a fluid. Principally two different parameters are determined:

1. (i) the decay-time of the conventional interaction τ_C, which occurs at relatively short path-lengths of the incidence beam in the fluid, and

2. (ii) the decay-time τ_MP of the multi-path-saturation interaction originating at long path-lengths, e.g. in multi-path-reflection cuvettes, where the incidence beam is fully absorbed by the fluid.

A relation between the decay-time and the absorption coefficient of a fluid is theoretically derived. A simple preliminary experiment is performed considering distilled water polluted with non-fluorescent azobenzene and fluorescent quinine-sulphate. A nitrogen laser has been used to generate the fluorescence and scattering signals. The reciprocal value of the difference between the decay-time of the multi-path and conventional signals, 1/(τ_MP − τ_C), yields the total absorption coefficient directly. In comparison to the conventional absorption technique the decay-time method is characterized by a higher sensitivity.     

Time-resolved fluorescence spectroscopy of polystyrene and poly(1-vinylnaphthalene)

An apparatus is described which enables the rapid recording of time-resolved emission spectra on a nanosecond time scale. The instrument permits detailed study of the time-dependent photo-physical processes occurring in many polymer systems. The technique has been used to investigate the kinetics of excimer formation in methylene chloride solutions of polystyrene and poly(1-vinyl-naphthalene). Unambiguous evidence for excimer dissociation is observed in poly(1-vinylnaphthalene) but the process appears unimportant in solutions of polystyrene.     

Time-resolved fluorescence spectroscopy and imaging of proteinaceous binders used in paintings

The differentiation of proteins commonly found as binding media in paintings is presented based on spectrally resolved and time-resolved laser-induced fluorescence (LIF) and total emission spectroscopy. Proteins from eggs and animal glue were analysed with pulsed laser excitation at 248 nm (KrF excimer) and 355 nm (third harmonic of Nd:YAG) for spectrally resolved measurements, and at 337 nm (N₂) and 405 nm (N₂ pumped dye laser) for spectrally resolved lifetime measurements and fluorescence lifetime imaging (FLIM). Total emission spectra of binding media are used for the interpretation of LIF spectra. Time-resolved techniques become decisive with excitation at longer wavelengths as fluorescence lifetime permits the discrimination amongst binding media, despite minimal spectral differences; spectrally resolved measurements of fluorescence lifetime have maximum differences between the binding media examined using excitation at 337 nm, with maximum observed fluorescence at 410 nm. FLIM, which measures the average lifetime of the emissions detected, can also differentiate between media, is non-invasive and is potentially advantageous for the analysis of paintings. Figure The fluorescence of solid ox glue extracted from collagen can be visualised in this Total Fluorescence Spectrum; three different peaks from multiple fluorophores are present and allow the discrimination between collagen- and non-collagen proteinaceous binding media found in paintings     

Sensitive and selective spectrochemical analysis of metallic samples: the combination of laser-induced breakdown spectroscopy and laser-induced fluorescence spectroscopy

In order to improve on analytical selectivity and sensitivity, the technique of laser-induced fluorescence spectroscopy (LIFS) was combined with laser-induced breakdown spectroscopy (LIBS). The main thrust of this investigation was to address analytical scenarios in which the measurement site may be difficult to access. Hence, a remote LIBS+LIFS arrangement was set up, and the experiments were carried out on samples surrounded by air at atmospheric pressure, rather than in a controlled buffer gas environment at reduced pressure. Representative for proof of principle, the detection of aluminium, chromium, iron and silicon at trace level concentrations was pursued. These elements are of importance in numerous chemical, medical and industrial applications, and they exhibit suitable resonance transitions, accessible by radiation from a pulsed Ti:sapphire laser system (its 2nd and 3rd harmonic outputs). All investigated elements have an energy level structure in which the laser-excited level is a member of a group of closely-spaced energy levels; thus, this allowed for easy off-resonant fluorescence detection (collisional energy transfer processes). Since numerous of the relevant transition wavelengths are within a narrow spectral interval, this opens the possibility for multi-element analysis; this was demonstrated here for Cr and Fe which were accessed by rapidly changing the tuneable laser wavelength.     

Applicability of UV laser-induced solid-state fluorescence spectroscopy for characterization of solid dosage forms

High production output of solid pharmaceutical formulations requires fast methods to ensure their quality. Likewise, fast analytical procedures are required in forensic sciences, for example at customs, to substantiate an initial suspicion. We here present the design and the optimization of an instrumental setup for rapid and non-invasive characterization of tablets by laser-induced fluorescence spectroscopy (with a UV-laser (λ _ex?=?266 nm) as excitation source) in reflection geometry. The setup was first validated with regard to repeatability, bleaching phenomena, and sensitivity. The effect on the spectra by the physical and chemical properties of the samples, e.g. their hardness, homogeneity, chemical composition, and granule grain size of the uncompressed material, using a series of tablets, manufactured in accordance with design of experiments, was investigated. Investigation of tablets with regard to homogeneity, especially, is extremely important in pharmaceutical production processes. We demonstrate that multiplicative scatter correction is an appropriate tool for data preprocessing of fluorescence spectra. Tablets with different physical and chemical characteristics can be discriminated well from their fluorescence spectra by subjecting the results to principal component analysis.     

Novel techniques for plasma diagnostics: Electron paramagnetic resonance and laser-induced fluorescence spectroscopy

Diagnostic techniques for low-pressure, cold plasmas have mostly been limited to emission and mass spectrometry. Herein, the techniques of gas-phase electron paramagnetic resonance and laser-induced fluorescence spectroscopy are briefly reviewed. Particular attention is paid to their attributes which make them good candidates for plasma diagnostic tools. It is found that gas-phase electron paramagnetic resonance can be used to determine and monitor the absolute concentration of a number of important plasma species, e.g., free radicals and atoms. Laser-induced fluorescence can also monitor, with even more sensitivity, but perhaps not so well absolutely, the concentrations of many plasma species, e.g., free radicals, metastable excited states, and molecular ions.     

基于SAO-VMD-FFT的激光诱导荧光光谱信号信噪比提升方法

针对基于激光诱导荧光光谱法(LIFs)的便携式场地地下水检测技术装备在原位检测地下水中典型重金属时,光传感信号含噪声严重,影响后续定量分析准确度的问题,提出了一种雪消融优化器(SAO)寻优算法优化的变分模态分解(VMD)结合快速傅里叶变换(FFT)的LIFs光传感信号信噪比提升算法。利用SAO算法对VMD分解关键参数分解模态个数K和惩罚因子α进行优化,实现含噪信号的自适应最佳VMD分解,得到一系列本征模态分量。进一步结合FFT算法寻找频率突变点从而筛选出相关模态。累计相关模态得到信噪比提升后的信号。通过仿真和实测信号分析,结果表明与EMD、WTD和S-G等常用算法相比,本文算法能更好地去除LIFs信号中的噪声分量,保留有用信息,验证了该方法的可行性和有效性,可为后续利用LIFs传感信号原位定量分析地下水中重金属元素含量提供支持。