Atomic absorbance spectrometry using flame

Metals contained in samples are determined by a wide variety of analytical methods. Bulk metals, such as copper in brass or iron in steel, can be analyzed readily by chemical methods such as gravimetry or electrochemistry. However, many metal determinations are for smaller, or trace, quantities. These are determined by various spectroscopic or chromatographic methods, such as atomic absorbance spectrometry using flame (FAAS) or graphite furnace (GFAAS) atomization, atomic emission spectrometry (AES), inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS), x-ray fluorescence (XRF), and ion chromatography (IC). 

Atomic absorption spectrometry (AA). This is a standard laboratory analytical tool for metals. The metal is extracted into a solution and then vaporized in a flame. A light beam with a wavelength absorbed by the metal of interest passes through the vaporized sample for example, to measure zinc, a zinc resonance lamp can be used so that the emission and absorbing wavelengths are perfectly matched. The absorption of the light by the sample is measured and Beer s law is applied to quantify the amount present. 

Radiation absorbed by atoms under conditions used in atomic absorption spectrometry may be re-emitted as fluorescence. The fluorescent radiation is characteristic of the atoms which have absorbed the primary radiation and is emitted 1n all directions. It may be monitored in any direction other than in a direct line with radiation from the hollow-cathode lamp which ensures that tha detector will not respond to the primury absorption process nor to unabsorbed radiation from the lamp. The intensity of fluorescent emission is directly proportional to the concentration of the absorbing atoms but it is diminished by collisions between excited atoms and other species within the flame, a process known as quenching. Nitrogen and hydrocarbons enhance quenching, and flames incorporating either should be avoided or their effect modified by dilution with argon. 

Spectrometric methods require a prior sampling preparation containing a separation step. The separation step is necessary especially to eliminate interference. Nonspectral interferences in flame atomic absorption spectrometry can be overcome by using a calibration model.221 The model uses two independent variables for analyte quantification (the amount of the sample and the amount of analyte added) the measured absorbance is the dependent variable. To control the matrix interferences without prior knowledge of the matrix composition, it is necessary to carry out nine calibration points to obtain accurate analytical information. This confers high reliability of the analytical information for determination of trace elements in complex matrices. 

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. 

The procedure is strictly analogous to that used for absorbance measurements in UV and visible molecular spectrometry (p. 355). To avoid interference from emission by excited atoms in the flame and from random background emission by the flame, the output of the lamp is modulated, usually at 50 Hz, and the detection system tuned to the same frequency. Alternatively, a mechanical chopper which physically interrupts the radiation beam, can be used to simulate modulation of the lamp output. 

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. 

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