Plasma emission spectroscopy excitation sources

Besides flame AA and graphite furnace AA, there is a third atomic spectroscopic technique that enjoys widespread use. It is called inductively coupled plasma spectroscopy. Unlike flame AA and graphite furnace AA, the ICP technique measures the emissions from an atomization/ionization/excitation source rather than the absorption of a light beam passing through an atomizer. 

In 1960, emission spectroscopy was displaced by the arrival of atomic absorption spectroscopy but experienced a revival in 1970 with the appearance of modem excitation sources, such as Inductively Coupled Plasma (ICP), the development of electronic detection methods and microprocessor technology. 

In atomic emission spectroscopy, the radiation source is the sample itself. The energy for excitation of analyte atoms is supplied by a plasma, a flame, an oven, or an electric arc or spark. The signal is the measured intensity of the source at the wavelength of interest. In atomic absorption spectroscopy, the radiation source is usually a line source such as a hollow cathode lamp, and the signal is the absorbance. The latter is calculated from the radiant power of the source and the resulting power after the radiation has passed through the atomized sample. 

The major disadvantage of arc/spark emission spectroscopy is the instability of the excitation source. This problem can be virtually eliminated by the use of a plasma torch. The most common commercially available method uses an inductively coupled plasma (ICP), which is also called RF plasma, to excite the sample (13-19). The resulting spectrometers (Fig. 4) can simultaneously measure up to 60 elements with high sensitivity and an extraordinarily wide linear dynamic range. 

With the exception of better optical resolution needed, the basic instrument used for atomic emission is very similar to that used for atomic absorption with the difference that no primary light source is used for atomic emission. One of the most critical components for this technique is the atomisation source because it must also provide sufficient energy to excite the atoms as well as atomise them. The earliest energy sources for excitation were simple flames, but these often lacked sufficient thermal energy to be truly effective sources. The development in 1963 and the introduction in 1970 of the first commercial inductively coupled plasma (ICP) as a source for atomic emission dramatically changed the use and the utility of emission spectroscopy (Thompson Walsh 1983). 

This chapter deals with optical atomic, emission spectrometry (AES). Generally, the atomizers listed in Table 8-1 not only convert the component of samples to atoms or elementary ions but, in the process, excite a fraction of these species to higher electronic stales.. 4, the excited species rapidly relax back to lower states, ultraviolet and visible line spectra arise that are useful for qualitative ant quantitative elemental analysis. Plasma sources have become, the most important and most widely used sources for AES. These devices, including the popular inductively coupled plasma source, are discussedfirst in this chapter. Then, emission spectroscopy based on electric arc and electric spark atomization and excitation is described. Historically, arc and spark sources were quite important in emission spectrometry, and they still have important applications for the determination of some metallic elements. Finally several miscellaneous atomic emission source.s, including jlanies, glow discharges, and lasers are presented. 

In general, detection limits with the KiP source are contparable to or better thait other atomic spectral procedures. Table lO-.f compares detection limits for Several of these methods. Note that more elements can be detected at levels of 10 ppb or less with plasma excitation than with other emission or absttrplion melhods. As we shall see in Chapter 11, the ICP coupled with mass spectrontetrie detection improves detection limits by two to live orders of magnitude for many elements and is thus strong competition for ICP optical emission spectroscopy. 

A number of electrical excitation-sources are available for emission spectroscopy. In most commercial spectrochemical instruments, more than one excitation source is contained in a single power-supply cabinet a typical combination may include a spark, a direct-current arc, and an alternating-current arc. A list of the various electrical excitation-sources, some of their characteristics, their approximate cost and the types of samples generally required is given in Table 11.1. Because of the actual or potential widespread use in emission spectroscopy, only the arc, spark, and inductively coupled plasma discharges will be described here in detail. 

Classical excitation sources in atomic emission spectroscopy do not meet these requirements. Flame, arc, and spark all suffer from poor stability, low reproducibility, and substantial matrix effects. However, the modem plasma excitation sources, especially ICPs, come very close to the specification of an ideal AES source. 

Minute amounts of sample material ablated with the focused radiation of a pulsed laser are transported into an independent excitation source, e.g., inductively coupled plasma (ICP) for further atomization, excitation, or ionization. The detection of target atoms after laser ablation (LA) is performed by hyphenated techniques using optical emission or mass spectrometry LA-ICP-OES laser ablation-lCP-optical emission spectroscopy LA-ICP-MS laser ablation-l CP-mass spectrometry... 

---