High sensitivity sampling techniques

• Three analytical techniques used for trace metal analysis are: atomic absorption, atomic emission, and atomic fluorescence
• The sample is decomposed by intense heat into a cloud of hot gases containing free atoms and ions of the element of interest
• The lowest energy or ‘‘ground state’’ is the most stable electronic configuration of an atom
• If energy of the right magnitude is applied to an atom, the energy will be absorbed by the atom, and an outer electron will be promoted to a less stable configuration or ‘‘excited state’’
• The “excited state” is unstable, and the atom will immediately and spontaneously return to its ground state configuration
• The process of excitation and decay to the ground state is involved in all three fields of atomic spectroscopy namely, atomic absorption, atomic emission, and atomic fluorescence
• In atomic absorption spectrometry (AAS), light of a wavelength characteristic of the element of interest is shone through this atomic vapor
• Some of this light is then absorbed by the atoms of that element
• The amount of light that is absorbed by these atoms is then measured and used to determine the concentration of that element in the sample
• In optical emission spectrometry (OES), the sample is subjected to temperatures high enough to cause not only dissociation into atoms but to cause significant amounts of collisional excitation (and ionization) of the sample atoms to take place
• Once the atoms or ions are in their excited states, they can decay to lower states through thermal or radiative (emission) energy transitions
• The intensity of the light emitted at specific wavelengths is measured and used to determine the concentrations of the elements of interest
• In atomic fluorescence spectrometry (AFS), a light source is used to excite atoms only of the element of interest through radiative absorption transitions
• When these selectively excited atoms decay through radiative transitions to lower levels, their emission is measured to determine concentration
• The selective excitation of the AFS technique can lead to fewer spectral interferences than in OES
• For trace metal analyses at μg/L (ppb) and sub μg/L levels; more sensitive techniques are required
• For AA to occur, free ground state atoms are placed in a beam of light of a wavelength corresponding to an appropriate electron transition of the analyte
• Any sampling process used must address the process of creating ground state atoms and directing them to the spectrometer light path
• The sensitivity of the cold vapor technique is far greater than can be achieved by conventional flame AA
• All of the mercury in the sample solution placed in the reaction flask is chemically atomized and transported to the sample cell for measurement
• The detection limit for mercury by this cold vapor technique is approximately 0.02 mg/L
• Hydride generation sampling systems for atomic absorption bear some resemblances to cold vapor mercury systems
• Samples are reacted in an external system with a reducing agent, usually sodium borohydride
• Gaseous reaction products are then carried to a sampling cell in the light path of the AA spectrometer
• The major limitation to the hydride generation technique is that it is restricted primarily to the elements As, Pb, Sn, Bi, Sb, Te, Ge & Se