Water for Atomic Absorption

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Application Overview

Atomic absorption (AA) is one of the oldest and most well established of the analytical methods, with roots tracing back to the very early observations in which the presence of specific salts in a chemical sample imparted characteristic colors to a luminous flame. AA is perhaps the most prominent and widely used of the family of methods employed for elemental analysis. As the name implies, it uses the absorption of light to measure the concentration of gas-phase atoms. The samples are usually liquids or solids, so the analyte atoms or ions must be vaporized in a flame or graphite furnace. If flame is used to desolvate and vaporize the analyte, the technique is called Flame Atomic Absorption Spectroscopy (FAA). If a graphite furnace is used, the technique is known as Graphite Furnace Absorption Spectroscopy (GFAA), also called electrothermal atomic absorption spectroscopy. Numerous methods from the United States Environmental Protection Agency (EPA) for the determination of metals use AA. Examples are shown in Table 1.

Method Number
Atomic Absorption Method
Cobalt 219.1 Flame atomic absorption (FAA)
219.2 Graphite furnace atomic absorption (GFAA)
Lead 239.1 FAA
239.2 GFAA
Zinc 289.1 FAA
289.2 GFAA
Copper 220.1 FAA
220.2 GFAA
Iridium 235.1 FAA
235.2 GFAA
Table 1: Examples of EPA methods that employ atomic absorption methods

The environmental field is not the only area where AA is commonly used. In clinical analysis, it is used in analyzing metals in biological fluids such as blood and urine. In some pharmaceutical manufacturing processes, trace amounts of a metal catalyst used in the process are sometimes present in the final product. AA is used to determine the amount of catalyst present in that final product. In the manufacturing industry, many raw materials are examined and AA is widely used to check that the major elements are present and that toxic impurities are lower than specified. For instance in concrete, where calcium is a major constituent, the level of lead should be low because it is toxic. In mining, AA is used to determine the amount of metals such as gold in rocks, to see whether it is worth mining.

Figure 1 is a simplified schematic of a basic atomic absorption system. The light source is usually a hollow-cathode lamp of the element that is being measured. When a ground state atom absorbs energy in the form of light of a specific wavelength, it is promoted to a higher energy level. The amount of energy absorbed at this wavelength is proportional to the number of atoms of a particular element. AA requires that the analyte atoms be in the gas phase, so the ions or atoms in a sample must undergo desolvation and vaporization in a high-temperature source such as a flame (FAA) or graphite furnace (GFAA). The main purpose of the monochromator in the AA instrument is to isolate the absorption line from background light due to interferences. Simple dedicated AA instruments often replace the monochromator with a bandpass interference filter. Photomultiplier tubes are the most common detectors for AA.
GFAA has several advantages over FAA. The burner-nebulizer system in FAA is a relatively inefficient sampling device. Only a small fraction of the sample reaches the flame, and the atomized sample passes quickly through the light path. The graphite furnace is a much more efficient atomizer because all of the analyte is atomize and the atoms are retained within the light path, hence significantly improving sensitivity and detection limits. Additionally, GFAA can directly accept very small absolute quantities of sample.

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