Nuclear methods of analysis applied to quantitation in laser microprobe mass spectrometry

Transmission laser microprobe mass spectrometry (LMMS) uses a Nd:YAG laser to vaporize and ionize a small volume of material from a thin section of a sample. The ions are resolved by a time of flight mass spectrometer. LMMS allows a spatial resolution of ∼1 μm and detection limits in the μg/g range. A major problem in LMMS is that it is currently considered to be only semi-quantitative. In the absence of a satisfactory theoretical model, quantitation in the analysis of biological materials is best accomplished by use of a standard. The authors have investigated the use of plastics doped with organo-metallic compounds (crown-ether complexes) for this purpose. INAA has been used to determine the concentration of Hg in doped polymerized plastics. Thin sections (0.5 μm) were then used to establish a LMMS detection limit of ∼46 μg/g for Hg. The appearance energy for Hg was determined to be ∼60 μJ. A PIGE procedure is also being developed for the determination of Li in doped plastic embedding media for use as an internal standard in the analysis of biological tissue for Al. Lithium and Al have similar ionization potentials and should behave comparably in LMMS