Flame emission multielement analysis

The inductively coupled plasma13 shown at the beginning of the chapter is twice as hot as a combustion flame (Figure 21-11). The high temperature, stability, and relatively inert Ar environment in the plasma eliminate much of the interference encountered with flames. Simultaneous multielement analysis, described in Section 21 1. is routine for inductively coupled plasma atomic emission spectroscopy, which has replaced flame atomic absorption. The plasma instrument costs more to purchase and operate than a flame instrument. 

The advantage of ICP is that the emissions are of such intensity that it is usually more sensitive than flame AA (but less sensitive than graphite furnace AA). In addition, the concentration range over which the emission intensity is linear is broader. These two advantages, coupled with the possibility of simultaneous multielement analysis offered by the direct reader polychromator design, make ICP a very powerful technique. The only real disadvantage is that the instruments are more expensive. See Workplace Scene 9.3. 

Each spectroscopic method has a characteristic application. For example, flame photometry is still applicable to the direct determination of Ca and Sr, and to the determination of Li, Rb, Cs and Ba after preconcentration with ion-exchange resin. Fluorimetry provides better sensitivities for Al, Be, Ga and U, although it suffers from severe interference effects. Emission spectrometry, X-ray fluorescence spectrometry and neutron activation analysis allow multielement analysis of solid samples with pretty good sensitivity and precision, and have commonly been applied to the analysis of marine organisms and sediments. Recently, inductively-coupled plasma (ICP)... 

Before the 1960 s, the analysis of toxic elements in airborne materials employed separations and colorimetric determination for single-element problems, or spectrographic methods for multielement, multisample studies. Variable matrices in most aerosols sampled had prevented sensitive, but interference-prone, flame-emission methods from attaining much usage. The increased concern over the environmental effects of toxic elements in the late 1960 s resulted in a need for greater sensitivity and ease of operation in measurements of these elements. The many laboratories with increased responsibilities found AAS most useful because of its accuracy, sensitivity, and relative lack of matrix effects, plus the low cost of the equipment. 

A variety of techniques are available for the determination of trace elements in crude oils, and chemical methods of analysis have been summarized by Milner and McCoy. Most geochemical studies of metals in petroleum have used emission spectrography of petroleum ashes because of the multielement nature of the method. " In recent years techniques such as polarograplWfcolorimetric analysis, X-r fluorescence, ESR, flame atomic absorption, and flameless atomic absorption have been used for the analysis of crude oils. Many of the techniques require preconcentration of the metals, usually by ashing techniques and consequently involve the risk of loss of volatile compounds or contamination by reagents. For many elements at very low concentrations (<1 ug/g) the risk of contamination is very high. Most of the applications cited above involve the determination of one or a few specific elements and are not suitable for multielement analysis. 

FIGURE 9-14. Multielement flame emission spectrum from 3886 to 4086 A. [From K. W. Busch, N. G. Howell, and G. H. Morrison, The Vidicon Tube as a Detector for Multielement Flame Spectrometric Analysis, Anal. Chem., 46, 575 (1974). Used by permission of the American Chemical Society.]... 

Emission of UV/VIS radiation Flame photometry, Auger electron spectroscopy (AES) Qualitative and quantitative multielement analysis... 

Why are atomic emission methods with an ICP source better suited for multielement analysis than arc flame atomic absorption methods ... 

Analytical Techniques Atomic absorption spectrometry, 158, 117 multielement atomic absorption methods of analysis, 158, 145 ion microscopy in biology and medicine, 158, 157 flame atomic emission spectrometry, 158, 180 inductively coupled plasma-emission spectrometry, 158, 190 inductively coupled plasma-mass spectrometry, 158, 205 atomic fluorescence spectrometry, 158, 222 electrochemical methods of analysis, 158, 243 neutron activation analysis, 158, 267. 

Flame and electrothermal techniques Both atomic absorption and emission have been used, with the former most widely applied to the analysis of minor soil components. Detection limits can often be very similar to the concentration found in extraction solutions of natural soils. While the sampling procedure is easily automated, AAS, it is inefficient (both time and sample volume) for routine multielement... 

Atomic absorption spectrometry is commonly used to measure a wide range of elements as shown in Table 2. Such techniques as flame, graphite furnace, hydride generation, and cold vapor are employed. Measurements are made separately for each element of interest in turn to achieve a complete analysis these techniques are relatively slow to use. More sensitive, but also more expensive, multielement analytical techniques such as inductively coupled plasma-atomic emission spectrometry and inductively coupled plasma-mass spectrometry can be used if lower (pgl and below) detection limits are required. These detectors can also be coupled with separation systems if speciation data, e.g., Cr(III) and Cr(VI), are needed. 

The roots of ICP-MS began in the mid-1960s with the advent of a technique called inductively couple plasma—atomic emission spectrometry (ICP-AES). For decades, prior to this, atomic emission spectrometry (flame, direct current-arc, and controUed-waveform spark) was the predominant method used for elemental analysis. The work of Greenfield et al. (1964) and work done essentially simultaneously by Wendt and Fassel (1965) introduced an emission spectrometric technique that provided high sensitivity trace element analysis with a multielement detection capability. This technique is still widely used today and can be studied in publications by Boumans (1987) and Montaser and Golightly (1992). 

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