Role of lasers in analytical atomicspectroscopy: where, when and why

Abstract

Nowadays, a laser source that can be tuned to a particular atomic or molecular transition is a standard tool in many laboratories and traditionally, this source has been identified with the dye laser. Yet, many laser systems tend to be too complex and expensive to acquire, so that their deployment in routine applications has always been, and still remains, rather limited. In addition, the field of analytical spectrometry seems to be more need-driven than technique-driven. Laser and associated methods, in particular fluorescence and ionisation with excimer-pumped dye lasers, and absorption with diode lasers, are the only methods capable of performing sophisticated diagnostics in most commonly used atomic and molecular reservoirs such as flames, plasmas and graphite furnaces. Indeed, lasers have been put to work in these atomisers to derive highly resolved (spatial and temporal) maps of essential physical parameters such as temperature, electron density, atomisation and ionisation efficiency, and to study complex excitation–ionisation mechanisms, line profiles and matrix interferences. From the stricter analytical point of view, despite the fact that the use of lasers has led to the ultimate performance in terms of detection limits (single atom and molecule), other techniques, such as ICP-MS are superior in terms of versatility, due to their multielement capabilities. It is therefore natural to ask the question of who would need lasers in analytical atomic spectrometry. In this context, there are some areas in which the use of laser techniques will still be the best choice, either because it is the ‘absolute’ limit of detection that matters, as for example in laser ablation and surface studies (microanalysis) or because they can provide the ultimate best in terms of selectivity and sensitivity for typical elements and applications (e.g., the determination of radionuclides), or finally because they can provide data much faster than other conventional methods (e.g., real time monitoring of selected metals and pollutants in the atmosphere). Several of these applications will be presented here, in particular, those that better respond to the three questions expressed in the title, i.e., where, when and why lasers would be the only choice. Some examples will be taken from on-going activities on the use of laser photofragmentation, fluorescence and plasma emission in the characterisation of atmospheric aerosols. The development of new, all solid state laser sources, continuously tuneable from the low UV to the IR is growing at an ever increasing rate due to the continuous search and discovery of new lasing materials and to new pumping and frequency conversion techniques. Diode-pumped, solid state lasers offer the most potential for increased commercial acceptance, new vibronic lasers extend their operating wavelength range and optical parametric oscillators are becoming serious candidates for replacing standard pulsed excimer-pumped dye lasers. Last but not least, semiconductor diode lasers are used as alternative sources to HCLs in atomic absorption measurements. A ‘universal’ laser spectrometer, based on these developments and capable of responding to all essential analytical and spectroscopic requirements and applications, is foreseen on the horizon. With such a system, all remaining hurdles that have limited the use of lasers to special situations where ultrahigh sensitivity is critical and cost and complexity are secondary criteria, should be surmounted. Finally, brief mention will be made of how lasers have been used to make an atomic system transparent at selected frequencies (electromagnetically induced transparency), making laser action possible without population inversion. This outcome can be exploited to push practical laser operation into the vacuum UV and X-ray regions.