Volatile chelates

Other fluorinated derivatives of acetylacetone are trifluoroacetylacetone (CF3COCH2COCH3) and hexafluoroacetylacetone (CF3COCH2COCF3), which form stable volatile chelates with aluminium, beryllium, chromium(III) and a number of other metal ions. These reagents have consequently been used for the solvent extraction of such metal ions, with subsequent separation and analysis by gas chromatography [see Section 9.2(2)]. 

Gas chromatographic analysis of metals as volatile chelates. D. N. Sokolov, Russ. Chem. Rev. (Engl. Transl.), 1977,46,388-393 (78). 

Belcher and his colleagues examined bis chelates of R1CSCH2COR2 and found them to be mainly monomeric.95 Hexafluorothioacetylacetone formed a number of volatile chelates (63 M = Cd, Cu, Fe11, Ni, Pb, Pt and Zn) with quite low melting points ranging from 108 °C (Cd) to as little as 54.5 °C (Zn), and many satisfactory separations are reported. Monothiotrifluoro-acetylacetone has been used to determine Ni down to 10-u g.97... 

The most important feature of the chromium compounds, as well as of the other derivatives of early transition 3difference between the derivatives of different oxidation states (T able 12.18). The alkoxides of chromium (II and HI) are in the majority the insoluble and non-volatile polymers. The most important exclusion from this rule appears to be the volatile chelate Cr(OCMe2CH2OMe)3 complex, and in the future, possibly, the number of the representatives of this family will increase. The derivatives of chromium (IV and VI) are monomers highly soluble in organic solvents. The lack of volatility for Cr(VI) compounds in contrast to those of Cr(IV) is caused apparently by the high electronegativity of the central atom, leading to thermal destruction and not to evaporation. 

Belcher, R., Majer, R. J., Perry, R., Stephen, W. I. Volatile chelates of alkali metals. Anal. Chim. Acta 45, 305 (1969). 

Blood, serum Sample after wet digestion converted to a volatile chelate usually with fluorinated acetylacetone GC/ECD 0.03 pg 0.5 pg 1.0 ng No data Fishbein 1984... 

Sakamoto et al. [626] demonstrated the application of the microwave plasma detector to the analysis of trace amounts of aluminium and copper in zinc by GC in the form of their volatile chelates. A column packed with 0.5% of SE-30 on glass beads was used at 140°C. With the same chelates as above, the detection limit was 1 ng for copper and 0.5 ng for aluminium. 

Beryllium was analysed by GC as a volatile chelate the most frequently of all elements. A rapid micro-analytical procedure for the determination of beryllium in biological fluids was developed and published by Black and Sievers [627], who devoted their attention to urine, blood, liver homogenates and plant extracts. The sample in ajsealed glass ampoule was treated directly with trifluoroacetylacetone, by which means the losses of the sample or possible contamination that occur in conventional ashing procedures were eliminated. 

Two methods that have been used in tracer studies with are neutron activation analysis CNAA) and EIMS, mass spectrometry of a volatile chelate with electron impact ionization C15>19,20). Neither technique can match TIMS in accuracy or precision, but both may offer greater sample throughput. Until more precise techniques become more generally available, research utilizing stable metal isotopes will remain limited to the conditions imposed by the analytical facilities at hand. As will be shown below, research with stable magnesium isotopes to date, has had to be tailored to the limitations of available analytical techniques ... 

Since EIMS is suitable only for relatively volatile materials, most metals must be converted to volatile chelates before they can be analysed by this technique. The magnesium chelate found most amenable to EIMS is the diketonate Mg(2,2, 6,6 -tetra-methyl-3,5-heptanedione)2 (Mg(THD>2) (19). The chelate can be formed in aqueous solutions at pH >9 in the presence of excess THD (16). Extraction into ethyl ether provided a simple method for separating the chelate from excess THD which otherwise interferes in the MS analysis of Mg(THD)2 The ether layer was transferred to a 10 ml glass tube and allowed to stand at room temperature. 

The volatile chelates Ba(dpm)2, Ba(ptfa)2 and Ba(hfod)2 were studied by gas-phase UPS and characterized by thermal analysis, IR spectra and vacuum sublimation. In the UPS of these chelates obtained at >200°C, ionization bands of p and p orbitals were identified at 8-10 eV. At ca 280 °C thermal decomposition began. The use of UPS to study Ba diketonates is complicated by the formation of ohgomers in the gas phase. ... 

In situ gas phase preparation directly on the carrier (carbon or graphit) either from TCB and a volatile chelate such as metal (Fe, Mn, Fe/Cu, Fe/Sn) dipivaloylmethanes at 520-670 k249,250-252"255 or from PMDA and FeCl2 in a flow of NH3 gas248. Loading is 10-50%. 

A useful method of increasing substantially the stability of derivatives during chromatographic separation that has been applied in the analysis of volatile chelates of a number of elements can be mentioned. 

It is also important to point out the possibility of the preliminary separation of metals by the GC of their volatile chelates [28]. 

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. 

Aggarwal et al. recently implemented methods for the determination of isotope ratios and concentrations of different elements in biological specimens. Thermally stable and volatile chelates were identified for a number of trace elements Ni [43], Cr [45], Pt [46], Cu [33], Se [47], Co [48], Cd [49], Pb [44], Hg [50], and Te [51]. Memory effect was evaluated and found to be insignificant. Precision and accuracy in the measurement of isotope ratios was evaluated. Finally, the isotope dilution GC-MS method was validated by analyzing NIST reference materials (e.g., urine, semm), by comparing the results with ETAAS and by using the standard addition approach. 

Another /S-diketone that reacts with Am(m) to produce a volatile chelate complex is PD with all its terminal H atoms replaced by fluorine 1,1,1,5,5,5-hexafluoropentane-2,4-dione(HFPD) (Fig. 20.14). In this case a molecular anion, [Am(HFPD)4]", is formed and the cation is Cs [459]. 

Increasing interest in volatile chelating agents and derived complexes for use in the separation of metals by distillation or gas chromatography, oltratrace analysis of metals, vapour deposition work, solvent extraction applications, etc., has assured a steady flow of publications centred around fluorinated / -diketones. The new polyfluorinated chelating agents (29),... 

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. 

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