XRF               

        X-rays were first discovered in 1895 by the physicist Wilhelm Conrad Roentgen by accident, he noticed that while he was using a high voltage cathode ray tube that was enclosed in a black box it caused a barium-platinocyanide screen to emit fluorescent light. His discovery went unnoticed for a number of years until 1913 when Mosley showed the relationship between atomic number and the reciprocal of a element's wavelength for each of their respective spectral emission lines using his concept.  Mosley also created the first attempted X-ray spectrometer but it proved to be inefficient since it used electrons that were lost through heat for its energy source.  The Bragg brothers also developed an x-ray spectrometer with the use of a slit and collminator but could not solve the efficiently problem.  An XRF spectrometer was developed by Friedman and Birks in 1948, their device was built with a diffractometer and used a Geiger counter for detection of the x-rays.  This method proved to be very sensitive for most of the atomic number range.  Karl Manne Georg Siegbahn of Sweden who won the Nobel Prize in 1924 (pictured right).  He used electrons of high energy for excitation and measured the x-ray wavelengths of the elements.  Siegbahn's work to improve air pumps and x-ray tubes increased the radiation intensity.He continually increased the accuracy of his measurements through crystal gratings and spectographs.  These studies lead to the discovery of the x-radiations of elements.  The precision of his instruments and measurements lead to the complete documentation of the energy and radiation conditions in the electron shells of atoms.  To read a complete history of all his work please visit http://nobelprize.org/nobel_prizes/physics/laureates/1924/siegbahn-bio.html

        X-Ray Fluorescence is initiated by the bombarding a material with high energy x-rays or gamma rays and exciting the electrons of the material.  XRF is primarily used for chemical and elemental analysis and is efficient in identifying metals like lead.  The term X-ray comes the energy used to bombard and excite the material and the term fluorescence refers to the emission of lower energy radiation after higher energy radiation has been absorbed. 
        The X-rays have enough energy to force the electron to surpass its ionization energy and actually eject the electron from the atoms in a 'photoelectric effect',  the resulting instability forces other electrons to fall into the gaps and thus release energy.  The energy is released as a photon with a specific energy relating to the difference in energy of the orbitals.  This emitted radiation is characteristic of the specific atoms present in the material.  The wavelength of the  emitted radiation can be calculated using Plank's Law.  The inner K and L shells are most typically involved, on the x-ray spectrum the peaks are labeled as K,L,M,N to show the shell where the electron was originally located. 
Images from http://www.amptek.com/xrf.html.

      
One type of XRF spectrometer is the wavelength dispersive spectrometers that use a diffraction crystal to focus the specific wavelengths on to the detector.  The wavelength range is adjusted by changing the angle of the x-rays hitting the surface of the crystal.    The x-rays are then passed from the crystal on to the solid surface detector.  The angle of the x-ray, the analytical crystal and detector must all be contained in precise measurements of each other, this is referred to as the Rowland Circle.   The crystal is tangent to the circle and the slit for the x-rays and the detector are points on the circle.  The circle is pictured to the left from http://serc.carleton.edu/research_education/geochemsheets/wds.html.

One other type of XRF Spectrometer is the Energy Dispersive spectrometer which focuses all of the emitted x-rays directly onto an energy analyzing detector.  These are not as sensitive as its wavelength dispersive partner and also have a lower resolution.

        Generally the XRF method is non destructive to the subject being analyzed.  The most accurate readings are achieved when the material is ground down and formed into a uniform sample, please refer to the Sample Preparation page for the methods that the toys are prepared for analysis.  This extensive testing protocol is used by industry when toys are applying for safety ratings so that their products can be marketed in the United States and abroad.  However, the XRF method is very versatile since it operates with xray that can penetrate most any substance.  The XRF method can be none destructive to the material and therefore is great for use in the field.  It is also very efficient and very cost effective.  Agencies use portable XRF scanners that use gamma rays as the energy source (Pictured to the right) to voluntarily test toys at community events and health fairs.  These XRF scanners can also be rented rather inexpensively to test not only children's toys but surface paints, soil, minerals and any other household application.  Accuracy has improved with these devices, a major concern was the control of how deep the x-rays penetrated the sample.  This is an issue when testing a material for surface paint since the substrate which may be lead free would influence and lower the percentage results of the surface contaminants.