Analytical signal conversion

The generation of analytical signals is a complex process that takes place in several steps. Methods of instrumental analysis often need five steps, namely (1) the genesis, (2) the appearance, (3) the detection and conversion, (4) the registration, and (5) the presentation of signals see Fig. 3.3. 

A unique property of LC/API/MS is the extent to which the analyte signal is affected by the sample matrix or the existence of co-eluting analytes. This property can have a profound influence on sensitivity and assay reproducibility. Because of matrix-ion suppression, it is not possible to estimate extraction recovery by comparison of the signal from a neat sample to an extracted sample. This is because the reduction in signal represents the combined effects of recovery and ion suppression. As first shown by Buhrman et al., quantitative assessment of extraction efficiency is made by spiking the neat sample into an extracted blank and comparison of the result to a similar sample spiked before extraction [120]. Conversely, the extent of ion suppression is obtained by the comparison of the signals for a neat unextracted sample to the same neat solution spiked into an extracted matrix blank. 

PAA Photon activation analysis involves irradiation with high energy photons (produced by conversion of electron energy into bremsstrahlung serving as the source of the photons). Photons emitted in the delayed decay are detected and used as the analytical signal. 

By definition, an interference effect occurs when the analytical signal is changed by the sample matrix compared with the reference or calibration standard, typically an acidified aqueous solution. This article is only concerned with nonspectral interferences in ET-AAS spectral interferences are considered elsewhere. It has been demonstrated in ET-AAS that the atomization efficiency (conversion of analyte to free atoms) is 100% for the majority of elements in simple solutions, which means that, in most cases, only negative nonspectral interferences can occur, i.e., the signal can only be reduced by the presence of... 

The necessity to know the properties of the conversion factor between the analyte amount and the analytical signal magnitude follows not only from the requirement of correct interpretation of the results of the measuring system but also from trace-ability requirements, i.e., demonstration of the chain of conversion of rough data into the analyte amount traceability - property of the result of a measurement or the value of a standard whereby it can be related with a stated uncertainty, to stated references, usually national or international standards (i.e., through an unbroken chain of comparisons) (lUPAC, 1998). 

Thus, In the microenvironment, calibration standards have different requirements than standards that are used to relate the Instrument signal to compound concentration. Standards used to calibrate the response of a mlcrospectrofluorometer for day-to-day or Instrument-to-lnstrument comparisons must not photobleach. Conversely, a standard used to quantify the concentration of an analyte In a sample must photobleach In a manner Identical to that of the sample. 

With the multitude of transducer possibilities in terms of electrode material, electrode number, and cell design, it becomes important to be able to evaluate the performance of an LCEC system in some consistent and meaningful maimer. Two frequently confused and misused terms for evaluation of LCEC systems are sensitivity and detection limit . Sensitivity refers to the ratio of output signal to input analyte amount generally expressed for LCEC as peak current per injected equivalents (nA/neq or nA/nmol). It can also be useful to define the sensitivity in terms of peak area per injected equivalents (coulombs/neq) so that the detector conversion efficiency is obvious. Sensitivity thus refers to the slope of the calibration curve. 

Fig. 31 (A) Principle of a sandwich immunoassay using FDA particulate labels. The analyte is first immobilized by the capture antibody preadsorbed on the solid phase (a) and then exposed to antibody-coated microparticle labels (b). Every microparticle contains 108 FDA molecules. High signal amplification is achieved after solubilisation, release, and conversion of the precursor FDA into fluorescein molecules by the addition of DMSO and NaOH (c). (B) Calibration curves of IgG-FDA microcrystal labels with increasing surface coverage of detector antibody (a-d) compared with direct FITC-labeled detector antibody (e). The fluorescence signals increase with increasing IgG concentration. FDA microcrystals with a high IgG surface coverage (c,d) perform better than those with lower surface coverage (a,b). (Reprinted with permission from [189]. Copyright 2002 American Chemical Society)...

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