A General Overview of Atomic Spectrometric Techniques

ALFREDO SANZ-MEDEL, ROSARIO PEREIRO AND JOSÉ MANUEL COSTA-FERNÁNDEZ

Department of Physical and Analytical Chemistry, University of Oviedo, Oviedo, Spain

1.1 Introduction: Basis of Analytical Atomic Spectrometric Techniques

Analytical atomic spectrometry comprises a considerable number of techniques based on distinct principles, with different performance characteristics and hence with varied application scopes, but in all cases providing elemental chemical information about the composition of samples. Figure 1.1 shows that these techniques can be classified into three main groups according to the type of particle detected: optical spectrometry, where the intensity of either non-absorbed photons (absorption) or emitted photons (emission and fluorescence) is detected as a function of photon energy (in most cases, plotted against wave-length); mass spectrometry (MS), where the number of atomic ions is determined as a function of their mass-to-charge ratio; and electron spectroscopy, where the number of electrons ejected from a given sample is measured according to their kinetic energy, which is directly related to the bonding energy of the corresponding electron in a given atom.

X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are the two main techniques based on electron spectroscopy. In XPS, a source of photons in the X-ray energy range is used to irradiate the sample. Superficial atoms emit electrons (called photoelectrons) after the direct transfer of energy from the photon to a core-level electron (see Figure 1.2a). Photoelectrons are subsequently separated according to their kinetic energy and counted. The kinetic energy will depend on the energy of the original X-ray photons (the irradiating photon source should be monochromatic) and also on the atomic and, in some cases, the molecular environment from which they come. This, in turn, provides important information about oxidation states and chemical bonds as the stronger the binding to the atom, the lower is the photoelectron kinetic energy.

In an Auger process, the kinetic energy of the emitted electron does not depend on the energy of the excitation source. AES consists of a two-step process: first, the sample is irradiated with an electron beam (or, less commonly, with X-rays), which expels an inner electron (e-1). In a second step, the relaxation of the excited ion takes place through the fall of a more external electron (e-2) to fill the 'hole', and then a third electron (e-Auger) uses the energy released in that movement to exit the atom (Figure 1.2b). XPS and AES are considered powerful techniques for surface analysis, with good depth and lateral resolution. However, due to their narrow range of applications in qualitative studies and the scarcity of quantitative analyses, they will not be considered further in this chapter.

The aim of this chapter is, therefore, to introduce briefly the most common quantitative atomic techniques based on both optical and mass spectrometric detection. The main emphasis will be given to conceptual explanations in order to stress the advantages and disadvantages of each technique, the increase in the complexity of the data they generate and how this can be addressed. References to chemometric tools presented in the following chapters will be given.

For these techniques, a dissolved sample is usually employed in the analysis to form a liquid spray which is delivered to an atomiser (e.g. a flame or electrically generated plasma). Concerning optical spectrometry, techniques based on photon absorption, photon emission and fluorescence will be described (Section 1.2), while for mass spectrometry (MS) particular attention will be paid to the use of an inductively coupled plasma (ICP) as the atomisation/ionisation source (Section 1.3). The use of on-line coupled systems to the above liquid analysis techniques such as flow injection manifolds and chromatographic systems will be dealt with in Section 1.4 because they have become commonplace in most laboratories, opening up new opportunities for sample handling and pretreatment and also to obtain element-specific molecular information.

Finally, direct solid analysis by optical and mass spectrometry will be presented in Section 1.5. This alternative is becoming more appealing nowadays and implemented in laboratories because of the many advantages brought about by eliminating the need to dissolve the sample. Techniques based on the use of atomiser/excitation/ionisation sources such as sparks, lasers and glow discharges will be briefly described in that section.

1.2 Atomic Optical Spectrometry

Routine inorganic elemental analysis is carried out nowadays mainly by atomic spectrometric techniques based on the measurement of the energy of photons. The most frequently used photons for analytical atomic spectrometry extend from the ultraviolet (UV: 190–390 nm) to the visible (Vis: 390–750 nm) regions. Here the analyte must be in the form of atoms in the gas phase so that the photons interact easily with valence electrons. It is worth noting that techniques based on the measurement of X-rays emitted after excitation of the sample with X-rays (i.e. X-ray fluorescence, XRF) or with energetic electrons (electron-probe X-ray micro-analysis, EPXMA) yield elemental information directly from solid samples, but they will not be explained here; instead, they will be briefly treated in Section 1.5.

The measurement of analytes in the form of gaseous atoms provides atomic spectra. Such spectra are simpler to interpret than molecular spectra (since atoms cannot rotate or vibrate as molecules do, only electronic transitions can take place when energy is absorbed). Atomic spectra consist of very narrow peaks (e.g. a few picometres bandwidth) providing two types of crucial analytical information: the observed wavelength (or frequency or photon energy), which allows for qualitative analysis, and the measurement of the peak height or area at a given frequency, which provides quantitative information about the particular element sought. The relative simplicity of such atomic spectra and the fairly straightforward qualitative and quantitative information have led to the enormous practical importance of atomic optical spectrometry for inorganic elemental analysis. However, it should be stressed again that to obtain such spectra the analytes must be converted into atoms...