Atomic absorption spectroscopy is a form of absorption spectroscopy for which the analyte of interest is a metal, usually as part of an organic compound or an ionic salt. A solution of the sample to be tested is aspirated through a nebulizer, with the resulting mist passing through a flame. The thermal energy provided by the flame converts the analyte into gaseous atoms or elemental ions. A hollow cathode lamp provides a source of ultraviolet light at a wavelength absorbed by the analyte of interest. The UV beam is aimed through the flame, and the absorbance is measured. The concentration of the sample can be determined by comparing its absorbance to that of a standard solution or solutions with known concentrations.
The sensitivity of atomic absorption spectroscopy for a particular metal is dependent upon many variables. These include the efficiency of the nebulization process, the temperature of the flame used, and any interferences present in the sample matrix or resulting from the analysis conditions. The nebulization process is generally controlled by adjusting the rate of sample aspiration and the choice of either a flow spoiler or impact bead to limit the amount of the sample stream introduced into the flame. The most common fuels used are natural gas, hydrogen, and acetylene. Common oxidants include oxygen, air, and nitrous oxide. An acetylene/air flame has a temperature of 2300oC and would typically be used to measure things like calcium or zinc. An acetylene/nitrous oxide flame has a temperature of about 2700oC and would typically be used for measuring magnesium. The most common interferences fall into the categories of chemical and ionization interferences. A chemical interference occurs when the analyte of interest reacts with something in the sample matrix to form a less volatile compound. One way of dealing with this is to add something more reactive than the analyte of interest to the solution, such as lanthanum to eliminate interactions between calcium and phosphate. At very high temperatures a high concentration of free electrons are present in the flame, and can cause ionization interferences. Potassium is often added to a sample matrix to suppress this type of interference.
Although atomic absorption spectroscopy is a relatively inexpensive way to measure metals, it is not sensitive enough for all applications. Also, it is sometimes desirable to measure more than one analyte at a time. For these situations inductively coupled plasma is generally used in preference to spectroscopic methods utilizing flames.