Volatile chelates trace metal analysis

Gas chiomatograplty of rare-earth j8-diketonates is not only of interest for the separation of mixtures of rare earths. The method can also be used for trace metal analysis. First, formation of the volatile chelates is achieved by extraction of the rare-earth ions with the )8-diketonate ligands from an aqueous solution into an immiscible organic solvent Alternatively, direct reaction between the ligand and the metal ions in the aqneons solntion can be used (in the absence of a solvent). After elntion, the volatile rare-earth j3-diketonate complexes can be detected by conventional detectors such as the flame ionization detector (FID) or the thermal condnctivity detector (TCD). Because the [R(tfac)3] and the R(fod)3] complexes contain electronegative fluorine atoms, they can be selectively detected at low concentrations by an electron capture detector (ECD) (Sievers and Sadlowski, 1978). Bnigett and Fritz (1972, 1973) studied the separation and quantitative determination of rare earths by gas chromatography of [R(thd)3(dbso)3] complexes. 

In many cases, nonvolatile substances are converted into more volatile derivatives prior to mass-spectral analysis. Examples include trimethylsilyl derivatives of alcohols or molecules containing sugar groups, ester derivatives of acids, and volatile chelates of trace-metal ions. Usually, a suitable volatile derivative can be synthesized by well established and relatively simple procedures. 

GC has been used extensively for the separation and determination of volatile organic molecules, and most aspects of this application area are fully documented in monographs on this technique. In the inorganic trace analysis area, however, fewer species possess the required volatility, and applications tend to be limited to the separation of volatile species of low molecular weight (such as methyl derivatives of As, Se, Sn, Hg) and the separation of semi-volatile organo-metals, metal halides, metal hydrides, metal carbonyls and metal chelates. For organo-metal species, the type of detection system required varies with the nature of the analyte, and the options include electron capture detection, flame photometric detection (sometimes ICP), AAS and MS. 

In inorganic chemistry, mixtures of metal ions in solution can be analyzed by electron-impact mass spectrometry. First the metal ions are complexed with an organic ligand (usually various substituted acetylacetonates) to form volatile metal chelates. If many metal ions are anticipated, the mixture is separated by GC and the separated fractions identified by mass spectrometry. Simple mixtures can be analyzed directly using the mass spectrometer. Because of the high sensitivity of mass spectrometry, trace analysis is possible. 

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