Laser Spectrometric Techniques in Analytical Atomic Spectrometry

Abstract

Laser light has a number of spectacular properties that make it useful for analytical spectrometry. One is that it has a high directionality (i.e. it looks like a real “beam”). This implies, among other things, that it can be focused down to micrometer‐sized spots. Another is that pulsed lasers can emit large amounts of light in very short pulses (often with a duration of 10 −9 − 10 −8 s). These two properties imply that laser light often can reach high irradiance (W m −2 ), which is of importance for a number of applications, not least when laser light is used for vaporization and/or atomization purposes of solid material. The most important attribute for spectroscopic applications, however, is that it often has a narrow frequency width (in the MHz–GHz range). This implies that laser light can induce one specific transition in one particular species at a time. The narrow frequency width is thus the basis for the high species selectivity that laser spectroscopic techniques possess. In addition, the combination of high irradiance and a narrow frequency width often gives laser light such staggering spectral irradiance (W m −2 Hz −1 ) that a significant fraction of the atoms under illumination can produce at least one detectable event during the interaction time (one or severalphotons or an ion–electron pair). This explains why laser spectroscopic techniques can benefit from high species sensitivity. The main purpose of this review article is to describe the theory and instrumentation for the field of laser spectroscopic techniques for analytical atomic spectrometry. This implies that techniques that use laser light for nonspectroscopic purposes, such as vaporization, and/or atomization purposes, e.g. laser ablation, laser‐induced plasma spectrometry, laser mass spectrometry and laser‐induced breakdown spectroscopy, will not be covered here. In addition, because atomic spectra in general consist of a few strong and narrow‐band transitions (whose widths often are comparable to those of the light from tunable laser systems and therefore seldom overlap with those from other atomic species) whereas those of molecules comprise a few broader, weaker and to a certain extent structured transition bands (which thus overlap more often with those of other molecular species), laser spectrometric techniques often show the highest sensitivity and selectivity when atomic species are detected. This is the main reason why this review focuses upon the use of laser spectrometric techniques for analytical atomic spectrometry. This implies, in turn, that laser‐based spectrometric techniques that predominantly detect molecules, e.g. Raman, thermal lensing and photoacoustic spectrometry, and spectrofluorometry, will not be discussed. This theory and instrumentation review focuses upon the most useful and versatile laser spectroscopic techniques for analytical atomic spectrometry. These techniques are based upon the concepts of fluorescence, ionization or absorption. Those based upon fluorescence are often referred to as either laser‐induced fluorescence (LIF) or laser‐excited atomic fluorescence spectrometry (LEAFS), whereas those based upon ionization are termed either laser‐enhanced ionization (LEI) or resonance ionization spectrometry (RIS). The working principles, instrumentation and present status of laser‐induced fluorescence/laser‐excited atomic fluorescence spectrometry (LIF/LEAFS) and LEI are covered in some detail in two separate sections, whereas RIS is covered more briefly. The reason for this is the broader versatility and applicability of LEI: it can be performed in a variety of atomizers, including those working under atmospheric pressure, whereas RIS has to be carried out in a vacuum and thus with significantly more complex instrumentation. However, because the laser instrumentation and the theoretical basis for all these techniques are quite similar, a short introduction to laser instrumentation and a theory section that outlines the common basic features of excitation of atoms by laser light precede the detailed descriptions. Those laser spectroscopic techniques that are based upon absorption and used for analytical atomic spectrometry are, nowadays, all performed in conjunction with some sort of modulation methodology (often wavelength‐modulation (WM)), frequently by the use of diode lasers owing to their rapid tunability. They are therefore often referred to as WM diode laser (atomic) absorption techniques and are covered in a separate section. Typical analytical qualities, e.g. limits of detection (LOD) and selectivity, as well as the typical strengths and limitations of each of these techniques, are given or discussed. It is concluded among other things that the most impressive performance of the LIF/LEAFS technique has been obtained together with the graphite furnace (GF) as atomizer, resulting in detection limits in the femtogram range and a very high selectivity for a large variety of elements. LEI, which has found its best use with the flame as atomizer, can provide LOD in the pg mL −1 range for many elements. It does not, however, show the same high selectivity as the LIF/LEAFS technique because it suffers from background effects when samples with high concentrations of easily ionized elements (EIEs) are analyzed. RIS combines a high sensitivity with an extraordinary selectivity, but instead has to pay the price of a more complex instrumentation. It was demonstrated earlier, for example, that the RIS technique is able to provide single atom detection (SAD). Because RIS in general is performed in a vacuum it can also provide good isotopic selectivity. Although not yet applied to a broad range of elements for analytical applications, it has shown impressive LOD (in the attogram range) and excellent isotopic selectivity (>10 9 ) under a few specific conditions. The wavelength‐modulation diode laser absorption spectrometry (WM‐DLAS) technique shows good promise for becoming a user‐friendly and widespread detection technique because it requires less complex instrumentation than most other laser spectroscopic techniques. Used with the GF as atomizer, LOD in the femtogram range have been achieved. The WM‐DLAS technique also has good ability to correct for various types of unstructured background absorption signals that might appear when samples with complex matrices are analyzed. One drawback, however, is that its applicability is still restricted owing to the limited availability of diode lasers that emit light in the visible and ultraviolet (UV) region. The WM‐DLAS technique, therefore, has been applied to only a limited number of elements so far. A common denominator for all these laser spectroscopic techniques is that they often have a large linear dynamic range (LDR). The LIF/LEAFS technique, for example, has demonstrated an LDR of 5–7 orders of magnitude. Finally, in addition to being used as sensitive and selective tools for analytical assessments, the laser spectroscopic techniques often show an excellent applicability to diagnostic studies (e.g. for the determination of processes such as atomization, diffusion, collision or ionization).