Atomic absorption spectrophotometry (AAS) is one of the principal instrumen­tal techniques for elemental analysis (Fe, Mn, Zn, Cu and some heavy metals) in most soil science laborato­ries. In AAS, atoms excited in an oxyacetylene flame absorb light at wavelengths characteristic of each element. Light absorption is proportional to the concentration of atoms in a light path. The most commonly used radiation source is a hollow cathode lamp, consisting of a tungsten anode and a cylindrical cathode sealed in a glass tube filled with neon or argon gas. Though both single-beam and double-beam instruments are available, double- beam instruments are preferred because they minimize effects of lamp emission variations, detector sensitivity, and electronic gain. Acetylene (C2H2) as fuel and air or nitrous oxide (N20) as oxidant is commonly used. An air-Q-12 flame (2100­-2400°C) is used for elements (such as calcium, magnesium) that do not form refractory compounds arid have low ionization potentials; an N20-C2H2 flame (2600-2800°C) is used for elements (such as aluminum) forming refractory com­pounds. More than 60 elements can be determined by AAS. Besides detection limits, factors such as matrix or interference effects are major influences on the viability of particular analyses. Problems caused by the formation of stable compounds or compounds of low volatility by the element of interest in combination with some anion can be over­come by the addition of an excess of a "competing cation" or a "releasing agent". For example, lanthanum (1000 mg/1) as chloride can be added in the determination of calcium and magnesium to remove the potential interference due to aluminum and phosphorus.