The various energy regions of the electromagnetic spectrum are typically defined by wavelength, or energy. The analytical chemist is interested in interactions of matter with the various energies of radiation since these form the basis of many useful types of spectrographic analysis. These interactions range from changes in spin of subatomic particles at the low energy end of the spectrum (radio waves) to nuclear events at the high energy end. The energy utilized in X-ray fluorescence (XRF) spectroscopy is sufficient to penetrate outer electron shells, but not energetic enough to cause atomic transitions. The unique atomic event induced in the XRF process is the loss of inner shell electrons. An element ionized in this way, leaving a vacancy in an inner electron shell, is highly unstable and quickly reverts to a more stable form by transfer of an outer shell electron to fill the inner shell vacancy. This transition across an energy gap from outer to inner shells within an atom is accompanied by fluorescence emission with an energy unique to the atomic structure of the element. The sketch of the atom shown above illustrates this excitation and emission process.
Detection of the emitted fluorescence following the proper excitation conditions forms the basis of the XRF measurement. The energy of an emission peak in an XRF spectrum provides unambiguous identification of an element in a sample. Then, as in other forms of spectroscopy, a relationship between emission intensity and the amount of an element present is established using standards which span a concentration range of interest. In this way, both qualitative and quantitative information may be gathered via the XRF technique.
XRF at the ppb level and below
As an analytical tool, XRF offers many features which make it an attractive choice for on-line applications. The method is non-destructive and requires little or no sample preparation. No reagents are needed and no waste is generated. Until recently, on-line applications were limited to samples with analyte concentrations in the ppm range or higher. This limitation excluded most metals of interest in power plant streams where corrosion products and other metals are typically in the ppb range or below.
Detora has lowered the sensitivity barrier by using a concentrating mechanism where analytes are accumulated on filters, for particulate materials, or ion exchange media, for dissolved metals. In this way, the detection limit refers to a mass amount of analyte rather than a concentration. And, since the accumulated mass depends on sample flow rate and collection time, either of these operating parameters may be increased, within practical limits, to push the detection limit lower.