Spectroscopy is an analytical method which, by quantifying discrete wavlengths of radiation from the substances being analyzed, can give qualitative or quantitative information about the substances.  In general, there are two kinds of radiation which can be measured by spectroscopy; emission methods measure an increase in energy which happens when the subject study is exposed to thermal or electrical energy.  In this case, the electrons in the outer shells of the atoms being studied are excited by a beam of energy (from either a visible or ultra-violet wavelength lamp), and the energy produced is then measured by a detector.

Atomic absorption methods measure the amount of energy (in the form of photons of light, and thus a change in the wavelength) absorbed by the sample.  Specifically, a detector measures the wavelengths of light transmitted by the sample (the "after" wavelengths), and compares them to the wavelengths which originally passed through the sample (the "before" wavelengths).  A signal processor then integrates the changes in wavelength, which appear in the readout as peaks of energy absorption at discrete wavelengths.

Any atom has its own distinct pattern of wavelengths at which it will absorb energy, due to the unique configuration of electrons in  its outer shell. This allows for the qualitative analysis of a pure sample.

In order to tell how much of a known element is present in a sample, one must first establish a basis for comparison using known quantities.  This is done by producing a calibration curve.  For this process, a known wavelength is selected, and only energy emitted at that wavelength will be measured by the detector.  However, as the concentration of the target atom in the sample increases, absorbtion will also increase proportionally.  Thus, one runs a series of known concentrations of some compound, and records the corresponding degree of absorbance, which is an inverse percentage of light transmitted.  A straight line can then be drawn between all of the known points.  From this line, one can then extrapolate the concentration of the substance under investigation from its absorbance.
 
 

adapted from Analytical Chemistry: an introduction by Douglas Skoog, Donald West, and
F. James Holler.  6th edition, Suderns College Publishers, 1994.