Atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES) were until recently the most widely used techniques for trace metal analysis, reflecting its ease of use and relative freedom from interferences. Although now superseded in many laboratories by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), flame atomic absorption spectrometry still is a very valid option for many applications. For instance AAS is still widely used in environmental and clinical chemistry laboratories for analysis of various metals, although this technique is capable of analyzing many elements (both metals and non-metals), including trace elements that can be transformed into atomic form after vaporization. Although many elements can be measured by atomic absorption, in clinical laboratories, lead, zinc, copper, and trace elements are the most commonly measured in blood.
- The sample is applied (environmental water, whole blood, serum, urine, etc.) to the sample cup.
- Liquid solvent is evaporated and the dry sample is vaporized to a gas or droplets.
- Components of the gaseous sample are converted into free atoms; this can be achieved in either a flame or flameless manner using a graphite chamber that can be heated after application of the sample.
- A hollow cathode lamp containing an inert gas like argon or neon at a very low pressure is used as a light source. Inside the lamp is a metal cathode that contains the same metal as the analyte of analysis. For example, for copper analysis a hollow copper cathode lamp is needed. For analysis of lead, a hollow lead cathode lamp is required.
- Atoms in the ground state then absorb a part of the light emitted by the hollow cathode lamp and are boosted into the excited state. Therefore, a part of the light beam is absorbed and results in a net decrease in the intensity of the beam that arrives at the detector. By application of the principles of Beer’s Law, the concentration of the analyte of interest can be measured.
Because atoms for most elements are not in the vapor state at room temperature, flame or heat must be applied to the sample to produce droplets or vapor, and the molecular bonds must be broken to produce atoms of the element for further analysis. An exception is mercury because mercury vapor can be formed at room temperature. Therefore, only cold vapor atomic absorption can be used for analysis of mercury.