Flame atomic absorption spectrometry nebulizers

Detection limits are presented for 61 elements by ten analytical determinative methods FAAS flame atomic absorption spectrometry ETAAS electrothermal atomization atomic absorption spectrometry HGAAS hydride generation atomic absorption spectrometry including CVAAS cold vapor atomic absorption spectrometry for Hg ICPAES(PN) inductively coupled plasma atomic emission spectrometry utilizing a pneumatic nebulizer ICPAES(USN) inductively coupled plasma atomic emission spectrometry utilizing an ultrasonic nebulizer ICPMS inductively coupled plasma mass spectrometry Voltammetry TXRF total reflection X-ray fluorescence spectrometry INAA instrumental activation neutron analysis RNAA radiochemical separation neutron activation analysis also defined in list of acronyms. 

Figure 4.1. Single-line FIA manifold for the determination of metal ions by flame atomic absorption spectrometry (AA). The sample (5) is injected into a carrier stream of diluted acid (5 X 0 M sulfuric acid), propelled forward by pump P, and transported to the nebulizer of the AA system, the distance between the injection valve and the AA instrument being reduced as much as possible (length 20 cm) in order to secure limited dispersion of the injected sample.

Flame atomic absorption was until recently the most widely used techniques for trace metal analysis, reflecting its ease of use and relative freedom from interferences. Although now superceded in many laboratories by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry, flame atomic absorption spectrometry still is a very valid option for many applications. The sample, usually in solution, is sprayed into the flame following the generation of an aerosol by means of a nebulizer. The theory of atomic absorption spectrometry (AAS) and details of the basic instrumentation required are described in a previous article. This article briefly reviews the nature of the flames employed in AAS, the specific requirements of the instrumentation for use with flame AAS, and the atomization processes that take place within the flame. An overview is given of possible interferences and various modifications that may provide some practical advantage over conventional flame cells. Finally, a number of application notes for common matrices are given. 

Detection systems for speciation have commonly consisted of atomic spectrometry instrumentation. One of the earliest techniques employed was flame atomic absorption spectrometry (EAAS). Sample is introduced into a flame using a pneumatic nebulizer system. The light source for atomic absorption is a low pressure (a few Torr) hollow cathode lamp (HCL) that includes a ceramic cylinder cathode coated with the pure metal or a compound of the analyte. Application of 150-300 V across the electrodes produces a plasma that results in a narrow atomic emission line that is absorbed by analyte atoms in the flame. EAAS instrumentation is relatively inexpensive and easily interfaced to chromatography systems. However, HCL-EAAS is characterized by relatively poor sensitivity that has limited its use for practical speciation analysis. 

Fig. 82. Burner-nebulizer assembly for flame atomic absorption spectrometry, a Burner head with mixing chamber b nebulizer c impactor bead d impact surfaces e nebulizer socket. (Courtesy of Bodenseewerk PerkinElmer, Oberlingen.)...

Figure 32. Burner-nebulizer assembly for flame atomic absorption spectrometry...

As noted earlier, USNs have been employed for sample insertion into atomic spectrometers suoh as flame atomio absorption spectrometry (FAAS) [9,10], electrothermal atomic absorption speotrometry (ETAAS) [11], atomic fluorescence spectrometry (AFS) [12,13], induotively ooupled plasma-atomic emission spectrometry (ICP-AES) [14,15], inductively coupled plasma-mass spectrometry (ICP-MS) [16,17] and microwave induced plasma-atomic emission spectrometry (MIP-AES) [18,19]. Most of the applications of ultrasonic nebulization (USNn) involve plasma-based detectors, the high sensitivity, selectivity, precision, resolution and throughput have fostered their implementation in routine laboratories despite their high cost [4]. 

Direct nebulization of an aqueous or organic phase containing extracted analytes has been widely used in flame atomic absorption spectroscopy [69-72], inductively coupled plasma atomic emission spectrometry [73-76], microwave induced plasma atomic emission spectrometry [77-80] and atomic fluorescence spectrometry [81], as well as to interface a separation step to a spectrometric detection [82-85]. 

Work with slurries requires that the slurries are first nebulized and behave just as solutions with respect to the sample introduction into the aerosol. From electron probe micrographs of aerosol particles sampled on Nuclepore filters under isokinetic conditions, it was found that at nebulizer gas flows of 3 L/min, being typical of plasma spectrometry but far below those for flame atomic absorption, particles with a diameter of up to 15 pm can be found in the aerosol (Fig. 44) [117]. This would imply that powders with a grain size of up to about 15 pm could still be nebulized as could a solution. This, however, is not true as the mass distribution in the case of powders may be quite different in the slurry and in the aerosol, as shown for the case of SiC (Fig. 45) [118]. The nebulization limitations for the case of slurry nebulization thus must be investigated from case to case and leads to certain types of restrictions. 

In addition, for speciation coupling of flow injection analysis and column chromatography with flame AAS and also a direct coupling of HPLC with flame AAS, as is possible with high-pressure nebulization, are most powerful. Here the Cr line in the visible region can be used, which makes the application of diode laser atomic absorption spectrometry possible [325]. This has been shown recently by the example of the determination of methylcyclopentadienyl manganese tricarbonyl. 

The nebulization rates achieved with the spray and nebulizer systems used in ICP spectrometry are much slower than those used in flame atomic absorption. The transport efficiency of the sample introduction systems is less than 3% in ICP spectrometry, whereas that in flame atomic absorption is about 15% or less. 

In the premix burner, the sample, in solution form, is first aspirated into a nebulizer where it forms an aerosol or spray. An impact bead or flow spoiler is used to break the droplets from the nebulizer into even smaller droplets. Larger droplets coalesce on the sides of the spray chamber and drain away. Smaller droplets and vapor are swept into the base of the flame in the form of a cloud. An important feature of this burner is that only a small portion (about 5%) of the aspirated sample reaches the flame. The droplets that reach the flame are, however, very small and easily decomposed. This results in an efficient atomization of the sample in the flame. The high atomization efficiency leads to increased emission intensity and increased analytical sensitivity compared with other burner designs. The process that occurs in the burner assembly and flame is outlined in Table 7.2. This process is identical to the atomization process for atomic absorption spectrometry (AAS), but now, we want the atoms to progress beyond ground-state free atoms to the excited state. 

Conventional pneumatic nebulizers typically consume sample solution at the rate of ca. 5-8 ml min-1. Thus generally, when flame spectrometry is used on a routine basis, 2-5 ml of sample solution is used per determination. However it is possible to employ much smaller volumes of sample solution.16 Figure 3, for example shows typical atomic absorption signals for the nebulization of 0.01, 0.02, and 0.05 ml of a 1 mg l-1 standard solution, as recorded on a storage oscilloscope, compared with the signal from continuous nebulization. It is clear that only about 0.04 ml of solution is required to obtain the maximum absorbance signal. 

In the pneumatic nebulizers used in atomic spectrometry, the liquid flow is usually of the order of 1 to a few mL/min and the full efficiency of the nebulizer (a few %) can actually be realized at gas flows of 2 L/min. However, even with gas flows below 2 L/min, droplet diameters as low as about 10 pm and injection velocities below about 10 m/ s are obtained. Pneumatic nebulization can be realized with a number of types of nebulizers. For flame emission and atomic absorption as well as for plasma spectrometry, they include concentric nebulizers, so-called cross-flow nebulizers, Babington nebulizers and fritted-disk nebulizers (Fig. 40) [95]. 

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