Instrumentation - Atomic Absorption Spectroscopy

Instrumentation

Atomic absorption spectrophotometers (Figure 10.37) are designed using either the single-beam or double-beam optics described earlier for molecular absorption spec- trophotometers (see Figures 10.25 and 10.26). There are, however, several impor- tant differences that are considered in this section.

Atomization 

The most important difference between a spectrophotometer for atomic absorption and one for molecular absorption is the need to convert the ana- lyte into a free atom. The process of converting an analyte in solid, liquid, or solu- tion 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 atom- ized using other methods.

Flame Atomizers 

In flame atomization the sample is first converted into a fine mist consisting of small droplets of solution. This is accomplished using a nebulizer assembly similar to that shown in the inset to Figure 10.38. The sample is aspirated into a spray chamber by passing a high-pressure stream consisting of one or more combustion gases, past the end of a capillary tube immersed in the sample. The im- pact of the sample with the glass impact bead produces an aerosol mist. 

The aerosol mist mixes with the combustion gases in the spray chamber before passing to the burner where the flame’s thermal energy desolvates the aerosol mist to a dry aerosol of small, solid particles. Subsequently, thermal energy volatilizes the particles, pro- ducing a vapor consisting of molecular species, ionic species, and free atoms.

Thermal energy in flame atomization is provided by the combustion of a fuel–oxidant mixture. Common fuels and oxidants and their normal temperature ranges are listed in Table 10.9. Of these, the air–acetylene and nitrous oxide-acetylene flames are used most frequently. Normally, the fuel and oxidant are mixed in an ap- proximately stoichiometric ratio; however, a fuel-rich mixture may be desirable for atoms that are easily oxidized. The most common design for the burner is the slot burner shown in Figure 10.38. This burner provides a long path length for monitoring absorbance and a stable flame.

The burner is mounted on an adjustable stage that allows the entire burner as- sembly to move horizontally and vertically. Horizontal adjustment is necessary to ensure that the flame is aligned with the instrument’s optical path. Vertical adjust- ments are needed to adjust the height within the flame from which absorbance is monitored. 

This is important because two competing processes affect the concen- tration of free atoms in the flame. An increased residence time in the flame results in a greater atomization efficiency; thus, the production of free atoms increases with height. On the other hand, longer residence times may lead to the formation of metal oxides that absorb at a wavelength different from that of the atom. For easily oxidized metals, such as Cr, the concentration of free atoms is greatest just above the burner head. For metals, such as Ag, which are difficult to oxidize, the concen- tration of free atoms increases steadily with height (Figure 10.39). Other atoms show concentration profiles that maximize at a characteristic height.

The most common means for introducing samples into a flame atomizer is continuous aspiration, in which the sample is continuously passed through the burner while monitoring the absorbance. Continuous aspiration is sample- intensive, typically requiring 2–5 mL of sample. Flame microsampling provides a means for introducing a discrete sample of fixed volume and is useful when the volume of sample is limited or when the sample’s matrix is incompatible with the flame atomizer. For example, the continuous aspiration of a sample contain- ing a high concentration of dissolved solids, such as sea water, may result in the build-up of solid deposits on the burner head. These deposits partially obstruct the flame, lowering the absorbance. Flame microsampling is accomplished using a micropipet to place 50–250 μL of sample in a Teflon funnel connected to the nebulizer, or by dipping the nebulizer tubing into the sample for a short time. Dip sampling is usually accomplished with an automatic sampler. The signal for flame microsampling is a transitory peak whose height or area is proportional to the amount of analyte that is injected.

The principal advantage of flame atomization is the reproducibility with which the sample is introduced into the spectrophotometer. A significant disadvantage to flame atomizers is that the efficiency of atomization may be quite poor. This may occur for two reasons. First, the majority of the aerosol mist produced during nebu- lization consists of droplets that are too large to be carried to the flame by the com- bustion gases. Consequently, as much as 95% of the sample never reaches the flame. A second reason for poor atomization efficiency is that the large volume of combus- tion gases significantly dilutes the sample. Together, these contributions to the effi- ciency of atomization reduce sensitivity since the analyte’s concentration in the flame may be only 2.5 x 10–6 of that in solution.

Electrothermal Atomizers 

A significant improvement in sensitivity is achieved by using resistive heating in place of a flame. A typical electrothermal atomizer, also known as a graphite furnace, consists of a cylindrical graphite tube approximately 1-3 cm in length, and 3-8 mm in diameter (Figure 10.40).

The graphite tube is housed in an assembly that seals the ends of the tube with optically transparent win- dows. The assembly also allows for the passage of a continuous stream of inert gas, protecting the graphite tube from oxidation, and removing the gaseous products produced during atomization. A power supply is used to pass a current through the graphite tube, resulting in resistive heating.

Samples between 5 and 50 μL are injected into the graphite tube through a small-diameter hole located at the top of the tube. Atomization is achieved in three stages. In the first stage the sample is dried using a current that raises the tempera- ture of the graphite tube to about 110 °C. Desolvation leaves the sample as a solid residue. In the second stage, which is called ashing, the temperature is increased to 350–1200 °C. At these temperatures, any organic material in the sample is con- verted to CO₂ and H₂O, and volatile inorganic materials are vaporized. These gases are removed by the inert gas flow. In the final stage the sample is atomized by rapidly increasing the temperature to 2000–3000 °C. The result is a transient ab- sorbance peak whose height or area is proportional to the absolute amount of ana- lyte injected into the graphite tube. The three stages are complete in approximately 45–90 s, with most of this time used for drying and ashing the sample.

Electrothermal atomization provides a significant improvement in sensitivity by trapping the gaseous analyte in the small volume of the graphite tube. The ana- lyte’s concentration in the resulting vapor phase may be as much as 1000 times greater than that produced by flame atomization. The improvement in sensitivity, and the resulting improvement in detection limits, is offset by a significant decrease in precision. Atomization efficiency is strongly influenced by the sample’s contact with the graphite tube, which is difficult to control reproducibly.

Miscellaneous Atomization Methods 

A few elements may be atomized by a chemi- cal reaction that produces a volatile product. Elements such as As, Se, Sb, Bi, Ge, Sn, Te, and Pb form volatile hydrides when reacted with NaBH₄ in acid. An inert gas carries the volatile hydrides to either a flame or to a heated quartz observation tube situated in the optical path. Mercury is determined by the cold-vapor method in which it is reduced to elemental mercury with SnCl₂. The volatile Hg is carried by an inert gas to an unheated observation tube situated in the instrument’s optical path.