Atomic emission spectroscopy plasma sources

The use of a plasma as an atomisation source for emission spectroscopy has been developed largely in the last 20 years. As a result, the scope of atomic emission spectroscopy has been considerably enhanced by the application of plasma techniques. 

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 X-ray diffraction (XRD) patterns were obtained by Philips X pert Pro X-ray diffractometer equipped with a Cu-K source at 40 kV and 40 mA. The crystalline sizes of R particles were calculated from Scherrer s equation [15]. Transmission electron microscopy (TEM) images were obtained using the G2 FE-TEM Tecnai microscope at an accelerating voltage of 200 kV. The content of platinum and carbon in the sample was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, RF source Jobin Yvon 2301, 40.68 MHz). 

The following ionization sources are used mainly in inorganic (atomic) MS, where the elemental composition of the sample is desired. The glow discharge (GD) and spark sources are used for solid samples, while the inductively coupled plasma (ICP) is used for solutions. All three sources are also used as atomic emission spectroscopy sources they are described in more detail with diagrams in Chapter 7. 

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. 

Plasma sources have been used for element-specific detection in gas chromatography. However, the plasma source most widely employed for gas chromatographic detection is the MIP, which has been the only plasma source that developed into a commercial gas chromatographic detector. Since the operating principle of the MIP is atomic emission spectroscopy, the detector has been termed atomic emission detector (AED). 

Figure 6.36 shows a variety of gas-phase techniques that have been used to synthesize 0-D nanoparticles. Radio frequency plasma sources have long been used for quantitative analysis by atomizing component species in liquid or solid samples - a technique referred to as inductively-coupled plasma atomic emission spectroscopy (ICP-AES). The extreme energy of an ICP may also be exploited to vaporize precursor sources to afford the growth of nanoparticles (Figure 6.36a). In this system, the nanoparticle size/morphology would be mostly controlled by the concentration of precursor in the plasma, and the rate of cooling - a function of its distance from the plasma source.

Atomic emission spectroscopy is one of the most useful and commonly used techniques for analyses of metals and nonmetals providing rapid, sensitive results for analytes in a wide variety of sample matrices. Elements in a sample are excited during their residence in an analytical plasma, and the light emitted from these excited atoms and ions is then collected, separated and detected to produce an emission spectrum. The instrumental components which comprise an atomic emission system include (1) an excitation source, (2) a spectrometer, (3) a detector, and (4) some form of signal and data processing. The methods discussed will include (1) sample introduction, (2) line selection, and (3) spectral interferences and correction techniques. 

Sample introduction into the plasma is a critical part of the analytical process in atomic emission spectroscopy (AES). Since the ICP is the most commonly used source, the sample introduction schemes described below will focus more on it than the other sources mentioned previously. Sample is carried into the plasma at the head of a torch by an inert gas, typically argon, flowing in the centre tube at 0.3-1.5 L min". The sample may be an aerosol, a thermally or spark generated vapour, or a fine powder. Other approaches may also be taken to facilitate the way the analyte reaches the plasma. These procedures include hydride generation and electrothermal vaporization. 

Elemental analysis by atomic emission and mass spectrometry with inductively coupled plasmas is shown by Houk to be the techniques of choice for the analysis of rare earth materials. In this chapter the instrumentation and principles of use of inductively coupled plasmas themselves are outlined as well as their function as a source for atomic emission spectroscopy and mass spectrometry. A number of important applications for which these techniques are eminently suited are also discussed. 

The development of many alternative plasma sources has led to a resurgence of analytical atomic emission spectroscopy in recent years. The major plasma emission sources used for gas chromatographic detection have been the microwave-induced helium plasma, under atmospheric or reduced pressure (MIP), and the DC argon plasma (DCP). The inductively coupled argon plasma (ICP) has been used much less for GC than as an HPLC detector [4]. 

An inductively coupled plasma (ICP) is a very high temperature, up to 8,000K, excitation source that efficiently desolvates, vaporizes, excites, and ionizes atoms. ICP sources are used to excite atoms for atomic emission spectroscopy and to ionize atoms for mass spectrometry. Inductively coupled... 

Use of glow-discharge and the related, but geometrically distinct, hoUow-cathode sources involves plasma-induced sputtering and excitation (93). Such sources are commonly employed as sources of resonance-line emission in atomic absorption spectroscopy. The analyte is vaporized in a flame at 2000—3400 K. Absorption of the plasma source light in the flame indicates the presence and amount of specific elements (86). 

Two colorimetric methods are recommended for boron analysis. One is the curcumin method, where the sample is acidified and evaporated after addition of curcumin reagent. A red product called rosocyanine remains it is dissolved in 95 wt % ethanol and measured photometrically. Nitrate concentrations >20 mg/L interfere with this method. Another colorimetric method is based upon the reaction between boron and carminic acid in concentrated sulfuric acid to form a bluish-red or blue product. Boron concentrations can also be deterrnined by atomic absorption spectroscopy with a nitrous oxide—acetjiene flame or graphite furnace. Atomic emission with an argon plasma source can also be used for boron measurement. 

---