Detection systems amplification

The true energy scale of the y-spectrum in units of keV usually cannot be derived directly from the pulse height spectrum because the overall amplification of the detection system is not known. Therefore, the y-lines eventually have to be identified by trial and error when a new system is set up by checking for the occurrence of the Mossbauer effect. 

The ABC detection system has been shown to be more sensitive than most other detection system (5,6), primarily because of the large size of the preformed ABC complexes, which result in amplification of the signals. Alternative detection systems for immunohistochemical analysis include the peroxidase-antiperoxidase (PAP) (1) and the alkaline phosphatase-antialkaline phosphatase (APAAP) systems (7) (see Chapter 24). These approaches are conceptually and technically similar, and will not be discussed here. 

The Fido technology is currently under evaluation for use by U.S. military forces. The Fido X and Fido XT are available as commercial off-the-shelf (COTS) items. Consequently, the technology is adequately mature for commercial deployment. However, as a platform technology, the AFP sensor and Fido detection system support broad application to meet explosives detection needs. Further, Nomadics has incorporated the amplification features of AFP into other sensor mechanisms aimed at the detection of analytes that are not explosives related, including other chemicals and compounds of interest in the biomedical and food safety fields. Thus, while the technology is mature enough for commercialization, its potential is far from fully exploited. 

Figure 11.21 Catalytic amplification of the DNA detection system. Adapted from Ref. 75 with permission.

Hatanaka Y, Imaoka Y, Torisu K, et al. A simplified, sensitive immunohistochemical detection system employing signal amplification based on fluorescyl-tyramide/antifluorescein antibody reaction its application to pathologic testing and research. Appl. Immunohistochem. Mol. Morphol. 2008 16 87-93. 

Indirect detection does require more steps, but oftentimes yields amplified signals relative to direct methods because layering of bridging molecules may increase the number of detector molecules per probe molecule. It is probably this bridging/amplification technique that has allowed current enzyme detection systems to approach the sensitivity of radiolabeled systems. The use of these indirect methods reduces steric problems that might arise from having enzyme molecules directly bound to probe molecules. 

While enzymes may be covalently attached directly to primary probe molecules, as noted above for reasons of reagent versatility, steric factors, and potential signal amplification, indirect detection systems appear to be the more popular. Consequently, enzyme-probe conjugates are typically complexes of a desired enzyme marker and a secondary level probe that is, a probe molecule that can specifically identify a primary level probe molecule, such as an alkaline phosphatase-streptavidin conjugate can identify a biotinylated nucleic acid probe by virtue of the binding affinity between streptavidin and biotin. Other examples of enzyme-probe systems are given in the preceding section on direct and indirect detection systems. 

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