The Analytical Flame

The flame excitation source of a flame emission spectrometer must fulfill several requirements if it is to be satisfactory. These include the ability to (1) evaporate a liquid droplet sample, (2) vaporize the sample, (3) decompose the compound(s) in the evaporated sample, and (4) spectrally excite the ground state atoms. These processes must occur at a steady rate to achieve a steady emission signal. 

Temperatures of Commonly Used Fuel—Oxidant Combinations in Analytical Flames 

The processes listed above occur in an environment that includes fuel and oxidant molecules and the products of the reactions between the fuel and the oxidant. The temperature of the flame, which is primarily responsible for the occurrence of these processes, is determined by several factors, including (1) type of fuel and oxidant, (2) the fuel-to-oxidant ratio, (3) type of solvent, (4) amount of solvent entering the flame, (5) type of burner, and (6) the region in the flame that is focused onto the entrance slit of the spectral isolation unit. 

A variety of burners and fuel-oxidant combinations have been used to produce the analytical flame. Early usage was commonly natural gas and air. This combination, frequently used with a Meeker burner, produced relatively low temperatures and low excitation energies, leading to simple spectra of easily excited elements, so that its usefulness was limited primarily to the alkali metals. The successful development of flame methods for alkali 

FIGURE 9-3. Flame profile showing areas of maximum emission intensities of various elements. [From B. E. Buell, Use of Organic Solvents in Limited Area Flame Spectrometry, Anal. Chem., 34, 636 (1962). Used by permission of the American Chemical Society.] 

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Two improvements to the FPD design have been made in recent years. First, the addition of an oxygen-rich burner upstream of the FPD oxidizes hydrocarbons to CO and C02 and thereby eliminates the hydrocarbon interference within the analytical flame [105, 106], This increases the sensitivity of the detector to approximately 20 pg S s"1 for a nonlinearized FPD [102], In a second improvement, the PFPD was developed to significantly reduce the background and increase the sensitivity to all detectable atoms [107], This detector is now commercially... 

The FID uses a continuous quartz braid to transport the column effluent (sprayed onto the braid through a 0.1-mm orifice) through a solvent removal zone and into the analytical and cleaning flames. The FID flows were as follows 140 ml/min H2 and 400 ml/min air for the analytical flame 300 ml/min H2 and 150 ml/min 02 for the cleaning flame. The oven temperature control was set to slightly less than mid-range, which yields a block temperature of approximately 140°C. 

This is by far the most frequently encountered interference in AAS. Basically, a chemical interference can be defined as anything that prevents or suppresses the formation of ground state atoms in the flame. A common example is the interference produced by aluminium, silicon and phosphorus in the determination of magnesium, calcium, strontium, barium and many other metals. This is due to the formation of aluminates, silicates and phosphates which, in many instances, are refractory in the analytical flame being used. 

Flame temperatures and flame composition have an influence on interferences that may cause erroneous readings to occur. This aspect of the analytical flame will be detailed in a later section of this chapter. 

Atomic absorption spectroscopy instruments place a sample in a high temperature flame that yields atomic species and passes selected, element specific, illumination through the flame to detect what wavelengths of light the sample atoms absorb. Either acetylene or nitrous oxide fuels the analytical flame. This process again demands that solid samples be digested (dissolved in an acid or fused with a salt) and dissolved to form a solution that can be aspirated or sprayed into the instrument s flame, while protecting the sample material from contamination or adulteration. 

Atomization The most important difference between a spectrophotometer for atomic absorption and one for molecular absorption is the need to convert the analyte into a free atom. The process of converting an analyte in solid, liquid, or solution form to a free gaseous atom is called atomization. In most cases the sample containing the analyte undergoes some form of sample preparation that leaves the analyte in an organic or aqueous solution. For this reason, only the introduction of solution samples is considered in this text. Two general methods of atomization are used flame atomization and electrothermal atomization. A few elements are atomized using other methods. 

Ionization interferences occur when thermal energy from the flame or electrothermal atomizer is sufficient to ionize the analyte... 

Clogging the aspirator and burner assembly decreases the rate of aspiration, decreasing the analyte s concentration in the flame. The result is a decrease in the signal and the introduction of a determinate error. 

Flame Ionization Detector Combustion of an organic compound in an Hz/air flame results in a flame rich in electrons and ions. If a potential of approximately 300 V is applied across the flame, a small current of roughly 10 -10 A develops. When amplified, this current provides a useful analytical signal. This is the basis of the popular flame ionization detector (FID), a schematic of which is shown in Figure 12.22. 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

Figure 19.7 shows a typical construction of a concentric-tube nebulizer. The sample (analyte) solution is placed in the innermost of two concentric capillary tubes and a flow of argon is forced down the annular space between the two tubes. As it emerges, the fast-flowing gas stream causes a partial vacuum at the end of the inner tube (Figure 19.4), and the sample solution lifts out (Figure 19.5). Where the emerging solution meets the fast-flowing gas, it is broken into an aerosol (Figure 19.7), which is swept along with the gas and eventually reaches the plasma flame. Uptake of sample solution is commonly a few milliliters per minute.

The thermospray device produces a wide dispersion of droplet sizes and transfers much of sample solution in unit time to the plasma flame. Therefore, it is essential to remove as great a proportion of the bigger droplets and solvent as possible to avoid compromising the flame performance. Consequently, the thermospray device usually requires both spray and desolvation chambers, especially for analyte solutions in organic solvents. 

Having removed the larger droplets, it may remain only to encourage natural evaporation of solvent from the remaining small droplets by use of a desolvation chamber. In this chamber, the droplets are heated to temperatures up to about 150 C, often through use of infrared heaters. The extra heat causes rapid desolvation of the droplets, which frequently dry out completely to leave the analyte as small particles that are swept by the argon flow into the flame. 

Having assisted desolvation in this way, the carrier gas then carries solvent vapor produced in the initial nebulization with more produced in the desolvation chamber. The relatively large amounts of solvent may be too much for the plasma flame, causing instability in its performance and, sometimes, putting out the flame completely. Therefore, the desolvation chamber usually contains a second section placed after the heating section. In this second part of the desolvation chamber, the carrier gas and entrained vapor are strongly cooled to temperatures of about 0 to -10 C. Much of the vapor condenses out onto the walls of the cooled section and is allowed to drain away. Since this drainage consists only of solvent and not analyte solution, it is normally directed to waste. 

For mass spectrometric ionization and introduction into a plasma flame, the analyte needs to be separated from most of the accompanying solvent. One way to accomplish this separation is to break the solution down into small droplets using a nebulizer. 

The aerosol is swept to the torch in a stream of argon gas. During passage from the nebulizer to the plasma flame, the droplets rapidly become smaller, as solvent evaporates, and evenmally become very small. In many cases, almost all of the solvent evaporates to leave dry particulate matter of residual analyte. 

To assist in the deposition of these larger droplets, nebulizer inlet systems frequently incorporate a spray chamber sited immediately after the nebulizer and before the desolvation chamber. Any liquid deposited in the spray chamber is wasted analyte solution, which can be run off to waste or recycled. A nebulizer inlet may consist of (a) only a nebulizer, (b) a nebulizer and a spray chamber, or (c) a nebulizer, a spray chamber, and a desolvation chamber. Whichever arrangement is used, the object is to transfer analyte to the plasma flame in as fine a particulate consistency as possible, with as high an efficiency as possible. 

The transfer efficiencies of analyte solution from the nebulizer to the plasma flame depend on nebulizer design and vary widely from about 5-20% up to nearly 100%. 

For mass spectrometric analysis of an analyte solution using a plasma torch, it is necessary to break down the solution into a fine droplet form that can be swept into the flame by a stream of argon gas. On the way to the flame, the droplets become even smaller and can eventually lose all solvent to leave dry analyte particulate matter. This fine residual matter can be fragmented and ionized in the plasma flame without disturbing its operation. 

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