EXPRESS: Landmark Publications in Analytical Atomic Spectrometry: Fundamentals and Instrumentation Development.

The almost-two-centuries history of spectrochemical analysis has generated a body of literature so vast that it has become nearly intractable for experts, much less for those wishing to enter the field. Authoritative, focused reviews help to address this problem but become so granular that the overall directions of the field are lost. This broader perspective can be provided partially by general overviews but then the thinking, experimental details, theoretical underpinnings and instrumental innovations of the original work must be sacrificed. In the present compilation, this dilemma is overcome by assembling the most impactful publications in the area of analytical atomic spectrometry. Each entry was proposed by at least one current expert in the field and supported by a narrative that justifies its inclusion. The entries were then assembled into a coherent sequence and returned to contributors for a round-robin review.

Detecting Temporal Changes of Self-Absorption in a Laser-Induced Copper Plasma from Time-Resolved Photomultiplier Signal Emission Profiles.

This work reports an investigation on the feasibility of using a photomultiplier tube (PMT) to follow the time evolution of self-absorption of copper resonance transitions at 324.7 nm and 327.4 nm. The plasma was obtained by focusing a Nd:YAG laser, operated at 1064 nm, on a series of aluminum alloy standard disks containing different copper concentrations. The results described have been obtained at different times and with different set-ups. These set-ups consisted of a Paschen-Runge polychromator, a LIBS 2000 spectrometer, and a spectrometer equipped with both an intensified charge-coupled device (ICCD) and PMT. Both PMT signals and time-resolved spectra were obtained and the ratio of the two Cu resonant lines was calculated, compared, and discussed. By selecting different delay times and integration gates of the PMT signals, the self-absorption effect of the Cu resonant lines was found to be changing, implying that, by careful selection of the integration window of PMT signals, the self-absorption may be minimized, thus improving the calibration linearity of the technique.

Laser spark ignition of premixed methane-air mixtures: parameter measurements and determination of key factors for ultimate ignition results.

In this study, we present an experimental investigation of the parameters of the laser spark ignition of premixed methane-air mixtures and the determination of the key factors for the ultimate ignition result. Ignition is achieved in a mesh honeycomb burner using the 1064 nm output of a neodymium-doped yttrium aluminum garnet (Nd : YAG) laser. All pertinent laser ignition parameters, including the minimum ignition energy, the ignition time and blow out time, and the effects that the variation of experimental conditions, such as the spark energy, ignition position, equivalence ratio (ER), and flow rate, have on these parameters have been addressed systematically. To identify the key factors for the ultimate result of laser ignition, several parameters of the ignition processes are measured simultaneously, with an emphasis given to the temporal behavior of the hydroxyl (OH) radicals in relation to the data regarding the spark energy and the local ER. A clear finding of the study is that successful ignition events are always related to higher OH radical photon emissions, considered to be proportional to the concentration level of the OH radicals present, thus indicating a direct link between the OH level at early times (on a microsecond scale) and the ultimate result of laser ignition. Two-dimensional correlation plots of the spark energy, local ER, and OH radical photon count at early times with the ultimate results of laser ignition indicate that the spark energy and local ER do not play a critical role in determining the success or failure of the ignition and that the OH concentration in the early time range is the key factor in determining the final fate of laser ignition. Finally, on the basis of the results obtained here and in the existing literature, some considerations of the mechanism of laser ignition are presented.

Photofragmentation of nitro-based explosives with chemiluminescence detection.

A simple, fast, reliable, sensitive and potentially portable explosive detection device was developed employing laser photofragmentation (PF) followed by heterogeneous chemiluminescence (CL) detection. The PF process involves the release of NO(x(x = 1,2)) moieties from explosive compounds such as TNT, RDX, and PETN through a stepwise excitation-dissociation process using a 193 nm ArF laser. The NO(x(x = 1,2)) produced upon PF is subsequently detected by its CL reaction with basic luminol solution. The intensity of the CL signal was detected by a thermoelectrically cooled photomultiplier tube with high quantum efficiency and negligible dark current counts. The system was able to detect trace amounts of explosives in various forms in real time under ambient conditions. Detection limits of 3 ppbv for PETN, 2 ppbv for RDX, and 34 ppbv for TNT were obtained. It was also demonstrated that the presence of PETN residue within the range of 61 to 186 ng/cm(2) can be detected at a given signal-to-background ratio of 10 using a few microjoules of laser energy. The technique also demonstrated its potential for the direct analysis of trace explosive in soil. An LOD range of 0.5-4.3 ppm for PETN was established, which is comparable to currently available techniques.

Single particle fluorescence: a simple experimental approach to evaluate coincidence effects.

Real-time characterization of the chemical and physical properties of individual aerosol particles is an important issue in environmental studies. A well-established way of accomplishing this task relies on the use of laser-induced fluorescence or laser ionization mass spectrometry. We describe here a simple approach aimed at experimentally verifying that single particles are indeed addressed. The approach has been tested with a system consisting of a series of aerodynamic lenses to form a beam of dye-doped particles aerosolized from a solution of known concentration with a medical nebulizer. Two independent spectral detection channels simultaneously measure the fluorescence signals generated in two different spectral regions by the passage of a mixture of two dye-doped particles through a focused laser beam in a vacuum chamber. Coincidence effects, arising from the simultaneous observation of both fluorescence emissions, can then be directly observed. Both dual-color fluorescence and pulse height distribution have been analyzed. As expected, the probability of single- or multiple-particle interaction strongly depends on the particle flux in the chamber, which is related to the concentration of particles in the nebulized solution. In our case, to achieve a two-particle coincidence smaller than 10%, a particle concentration lower than 1.2x10(5) particles/mL is required. Moreover, it was found that the experimental observations are in agreement with a simple mathematical model based on Poisson statistics. Although the results obtained refer to particle concentrations in solution, our approach can equally be applicable to experiments involving direct air sampling, provided that the number density of particles in air can be measured a priori, e.g., with a particle counter.

Monitoring atmospheric particulate matter through cavity ring-down spectroscopy.

Cavity ring-down spectroscopy was explored as a means to measure atmospheric optical extinction. Ambient air was sampled through a window on the campus of the University of Florida and transported to a ring-down cell fashioned from standard stainless steel vacuum components. When a copper vapor laser operating at 10 kHz is employed, this arrangement allowed for nearly continuous monitoring of atmospheric extinction at 510 and 578 nm. We have characterized the system performance in terms of detection limit and dynamic range and also monitored a change in atmospheric extinction during a nearby wildfire and fireworks exhibition. The sensitivity and compatibility with automation of the technique renders it useful as a laboratory-based measurement of airborne particulate matter.