Karl X. Yang

Laser excited atomic fluorescence spectrometry — a review☆

Abstract This review focuses on the development of new instruments, and new applications of laser excited atomic fluorescence spectrometry, LEAFS, in recent years since the last published reviews. Such developments include solid-state tunable lasers, deep UV tunable lasers, the use of charge coupled detectors (CCDs), and the applications of LEAFS for trace metal determination in various samples. The advent of diode lasers with their now somewhat improved range of wavelengths and power output, provides opportunities for research and applications in LEAFS. The further development of the coupling of second and third harmonic crystals to pulsed diode lasers shows promise for compact and robust instrumentation. There have been no recent instrumental developments that might provide more isotopic selectivity beyond the elements like uranium where the spectral isotope splitting is greater than most elements, but laser diodes could provide this due to their potential to provide an output with very narrow spectral bandwidth. The advent of optical parametric oscillator-based lasers has enabled LEAFS to be much more practical then in the past when dye lasers were used. This should be the harbinger of more applications of LEAFS to complex real sample analyses that can not be done by other techniques for reasons of sensitivity or selectivity. Array detectors provide an additional degree of freedom by provision of more spectral information more rapidly, which should aid the study of complex samples that might produce complex background problems. The recent literature indicates that the sensitivity, selectivity and ease of method development of LEAFS is well-established, and that there are no substantial analytical disadvantages to the technique beyond the instrumental limitations associated with the single element at a time mode of operation and the complexity of the laser systems. Laser technology continues to develop rapidly, which heralds a bright future for LEAFS.

Resonant Laser Ablation of Metals Detected by Atomic Emission in a Microwave Plasma and by Inductively Coupled Plasma Mass Spectrometry

It has been shown that an increase in sensitivity and selectivity of detection of an analyte can be achieved by tuning the ablation laser wavelength to match that of a resonant gas-phase transition of that analyte. This has been termed resonant laser ablation (RLA). For a pulsed tunable nanosecond laser, the data presented here illustrate the resonant enhancement effect in pure copper and aluminum samples, chromium oxide thin films, and for trace molybdenum in stainless steel samples, and indicate two main characteristics of the RLA phenomenon. The first is that there is an increase in the number of atoms ablated from the surface. The second is that the bandwidth of the wavelength dependence of the ablation is on the order of 1 nm. The effect was found to be virtually identical whether the atoms were detected by use of a microwave-induced plasma with atomic emission detection, by an inductively coupled plasma with mass spectrometric detection, or by observation of the number of laser pulses required to penetrate through thin films. The data indicate that a distinct ablation laser wavelength dependence exists, probably initiated via resonant radiation trapping, and accompanied by collisional broadening. Desorption contributions through radiation trapping are substantiated by changes in crater morphology as a function of wavelength and by the relatively broad linewidth of the ablation laser wavelength scans, compared to gas-phase excitation spectra. Also, other experiments with thin films demonstrate the existence of a distinct laser–material interaction and suggest that a combination of desorption induced by electronic transition (DIET) with resonant radiation trapping could assist in the enhancement of desorption yields. These results were obtained by a detailed inspection of the effect of the wavelength of the ablation laser over a narrow range of energy densities that lie between the threshold of laser-induced desorption of species and the usual analytical ablation regime. Normal ablation employs high-power lasers in an attempt to create a vapor plume without selective vaporization, and with a stoichiometry that accurately represents the stoichiometry of species in the solid sample. RLA, as a method of selective vaporization, appears to provide an opportunity to exploit selective vaporization in new ways.

Laser-excited atomic fluorescence spectrometry in a graphite furnace with an optical parametric oscillator laser for sequential multi-element determination of cadmium, cobalt, lead, manganese and thallium in Buffalo river sediment

It is demonstrated, for the first time, that solid-state lasers based on optical parametric oscillation (OPO) allow relatively rapid sequential multi-element analysis of samples by laser-excited atomic fluorescence spectrometry (LEAFS) in a graphite furnace. These lasers are tunable by facile computer keyboard control over the wavelength region 220–2000 nm. A method is described for the sequential multi-element determination of cadmium, cobalt, lead, manganese and thallium in a river sediment standard reference material (NIST SRM 2704) by graphite furnace LEAFS. With a slew rate of 0.125 nm s–1, the OPO laser could be tuned to cover the wavelength range needed for these elements, from 228 to 304 nm, in 15 min. This allowed each element to be determined sequentially with the analysis time determined primarily by the slow heating cycle of the furnace rather than the laser wavelength tuning. Detection limits in the multi-element mode were 545, 111, 28, 445 and 24 fg for cadmium, cobalt, lead, manganese and thallium, respectively, limited primarily by the low repetition rate of the laser (10 Hz). The multi-element detection limits were within a factor of 2–4 of those in the single element mode. Higher excitation energies, by a factor of 2–5, were required to optically saturate the transitions of the analytes in the sediment sample solution compared with aqueous standards. By use of several aliquots of one sample solution, and simple aqueous calibration, it was possible to analyze the sample, accurately, for the five elements over a concentration range between 1 ng ml–1 for thallium and 460 ng ml–1for manganese. Different dilutions were not necessary owing to the long calibration range of the technique. The high sensitivity of LEAFS allowed sufficient initial dilution to remove an interference on thallium that is normally irresolvable by atomic absorption measurements of the same sample in the same graphite furnace.

Progress in laser excited atomic fluorescence spectrometry

Abstract Laser excited atomic fluorescence spectrometry, LEAFS, is a high sensitivity, high selectivity, atomic spectrometric technique. This paper reviews progress in LEAFS over the last several years, with emphasis on new lasers, new detectors, modifications to atomizers, and multi-element measurement approaches. Applications of LEAFS to real sample analyses are also highlighted.

Electrothermal atomizer laser-excited atomic fluorescence spectrometry for the determination of phosphorus in polymers by direct solid analysis and dissolution

Electrothermal atomizer laser-excited atomic fluorescence spectrometry (ETA-LEAFS) with direct solid analysis was used to measure phosphorus in polymers that consisted mainly of poly(ethylene terephthalate). The ETA-LEAFS detection limit for phosphorus was 8 pg, and the linear dynamic range extended over approximately five orders of magnitude. Electrothermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) were used to validate quantitative results obtained. A novel dissolution method that utilized trifluoroacetic acid and toluene was used for validation purposes. Of the three techniques investigated, only ETA-LEAFS, with either direct solid analysis or dissolution, could measure the phosphorus content of all the samples over the range 2–3000 µg g–1; ETAAS and ICP-AES were limited by a relatively poor linear dynamic range and/or poor detection limit. The relative standard deviation for the ETA-LEAFS analyses by dissolution and direct solid analysis was in the range 5–11% and 11–12%, respectively. A general comparison was made between the use of ETA-LEAFS and alternative techniques for the direct solid analysis of polymers.

Digital video clips for improved pedagogy and illustration of scientific research — with illustrative video clips on atomic spectrometry

Abstract This article is an electronic publication in Spectrochimica Acta Electronica (SAE), a section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by an electronic archive, stored on the CD-ROM accompanying this issue. The archive contains video clips. The main article discusses the scientific aspects of the subject and explains the purpose of the video files. Short, 15–30 s, digital video clips are easily controllable at the computer keyboard, which gives a speaker the ability to show fine details through the use of slow motion. Also, they are easily accessed from the computer hard drive for rapid extemporaneous presentation. In addition, they are easily transferred to the Internet for dissemination. From a pedagogical point of view, the act of making a video clip by a student allows for development of powers of observation, while the availability of the technology to make digital video clips gives a teacher the flexibility to demonstrate scientific concepts that would otherwise have to be done as ‘live’ demonstrations, with all the likely attendant misadventures. Our experience with digital video clips has been through their use in computer-based presentations by undergraduate and graduate students in analytical chemistry classes, and by high school and middle school teachers and their students in a variety of science and non-science classes. In physics teaching laboratories, we have used the hardware to capture digital video clips of dynamic processes, such as projectiles and pendulums, for later mathematical analysis.

Determination of Tin in Nickel-based Alloys by Electrothermal Laser-excited Atomic Fluorescence With Confirmation of Accuracy by Inductively Coupled Plasma Mass Spectrometry and Atomic Absorption Spectrometry

The determination of tin in nickel-based alloys by laser-excited atomic fluorescence in a graphite furnace was investigated. The concentrations of tin in four nickel-based alloys from Pratt & Whitney Aircraft were determined. The nickel in the alloys was found to behave as a permanent chemical modifier that remained in the graphite tube during analyses. The use of a mixture of hydrofluoric, hydrochloric and nitric acids proved to be the most efficient dissolution method. In order to confirm the accuracy of the results, Zeeman-effect background corrected ETAAS and ICP-MS methods were also used. The results from these different methods were compared by use of the Student’s t -test. The laser-excited atomic fluorescence method was found to be as accurate as the other techniques. The precisions of the techniques varied from 4 to 12% RSD. For the dissolution of 100 mg of nickel alloy in 100 ml of aqueous solution, the effective atomic fluorescence detection limit in the original nickel alloy samples was 1.5 ng g -1 . The atomic fluorescence method was simple to develop and did not need a sophisticated background correction technique to perform the analyses.