Organometallic tracer

The mid-1980s saw the development of the first three non-isotopic immunoassays to use organometallic tracers that actually came to fruition. They are based on three different analytical techniques atomic absorption, electrochemistry, and Fourier-transform infrared spectrocopy (FT-IR) [99-101], Cais himself did not bring to completion an actual immunoassay by atomic absorption, although he tested the technique with this view in mind [102], Brossier was the first to validate... 

Scheme 1.11 Representative organometallic tracers used in metalloimmunoassays.

Since the subject of the immunoassay may not be familiar to readers whose interests are in the field of organometallic chemistry, we will begin this chapter with a brief overview of the prindple behind the immunoassay method before describing the synthesis of organometallic tracers and reviewing the various detection methods that have facilitated the creation of novel immunoassays. 

Fig. 8.13 Principle of a homogeneous electrochemical immunoassay using a Naflon coated electrode as the detector for a cationic organometallic tracer.

The use of fluorescent labels has been very successful for inorganic complexes such as the europium chelates, which are used today in the Delfia commercial system. In contrast, fluorescence has rarely been employed in the area of organometallic tracers, and only the immunoassay developed by Lakowicz uses such a tracer [88]. This is a homogeneous competitive immunoassay, the tracer being the complex (Re-L) -HSA, 48 obtained as shown in Scheme 8.20 and the detection method fluorescence polarization (FP). This compound 48 displays highly polarized emission (with a maximum polarization near 0.4 and maximum anisotropy near 0.3) in the absence of rotational diffusion and a long average lifetime (2.7 ps) when bound to proteins in air-equilibrated aqueous solution. 

The only respect in which the hot atom chemistry of organometallic compounds has so far been applied to other fields of study is in the area of isotope enrichment. Much of this has been done for isolation of radioactive nuclides from other radioactive species for the purpose of nuclear chemical study, or for the preparation of high specific activity radioactive tracers. Some examples of these applications have been given in Table II. The most serious difficulty with preparation of carrier-free tracers by this method is that of radiolysis of the target compound, which can be severe under conditions suited to commercial isotope production, so that the radiolysis products dilute the enriched isotopes. A balance can be struck in some cases, however, between high yield and high specific activity (19, 7J),... 

Consequent to the work of many and employing such techniques as structure variation, isotopic tracers, and stereochemistry, a large number of different adsorbed hydro bon firagments have been identified as key intermediates in various reactions of hydrocarbons. Correlation of these spedes with similar polynuclear organometallic spedes has been of interest. However, the author feels that mechanistic understanding has lagged behind some other aspects of catalysis. 

In our lab, R.T.Ds are experimentally determined for the 3 phases by pulse injections of radioactive tracers in the real medium gaseous Ar or Kr liquid organometallic complexes, or halides solid neutron-irradiated metal catalysts. 

Given a method of preparing Mo organometallic compounds, the p decay transformation of Mo to Tc could be studied. The decay of Mo to Tc yields a nuclide with much lower recoil energy than that formed in the molybdenum (n, y ) process. However, this decay produces a cascade of Auger electrons see Auger Spectroscopy) which can cause bond disruption. These studies are difficult, because the technetium-99m product is produced at radiochemical tracer levels. Macroscopic quantities of products are not available for spectroscopic characterization. 

More universal is the method of Cp determination using selective tracers such as carbon monoxide and carbon dioxide which interact only with active metal = polymer bondsUsing tagged CO and CO2 as quenching agents systematic data have been accumulated so far on the influence of the composition of catalysts (titanium chlorides with various organometallic compounds) and polymerization conditions on Cp and kp values for the polymerization of ethylene and propylene IS)... 

An approximate order of response for the halogens is F < Cl < Br < I. Applications include trace analysis of environmental samples for chlorinated solvents, plasticisers, chlorofluorocarbon gases, pesticides and herbicides (DDT, 7BHC/lindane, aldrin), SFg tracer gas in flue gases and mine atmospheres, organometallics (lead tetra-alkyls), polynuclear aromatic carcinogens, NO and SO2 in chimney-stack gases. 

In the great majority of examples of MIA, the organometaUic tracers used are molecules derived from the analyte to be assayed (structures shown in Table 8.1) to which an organometallic moiety has been attached. The analyte is normally a hapten (cf. definition above). 

Tables. The analytes, their tracers, their organometallic units (OM unit) and the analytical methods used for their quantification. 

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