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An assay (analysis) is never an isolated process, as it must be accompanied with pre- and post-analytic procedures. Both the communication order (the request to perform an assay plus related information) and the handling of the specimen itself (the collecting, documenting, transporting, and processing done before beginning the assay) are pre-analytic steps.
Such assays are called homogeneous immunoassays, or less frequently non-separation immunoassays. The use of a calibrator is often employed in immunoassays. Calibrators are solutions that are known to contain the analyte in question, and the concentration of that analyte is generally known.
Molecular diagnostics uses in vitro biological assays such as PCR-ELISA or Fluorescence in situ hybridization. [19] [20] The assay detects a molecule, often in low concentrations, that is a marker of disease or risk in a sample taken from a patient. Preservation of the sample before analysis is critical. Manual handling should be minimised. [21]
Direct assay In a direct assay, the stimulus applied to the subject is specific and directly measurable, and the response to that stimulus is recorded. The variable of interest is the specific stimulus required to produce a response of interest (ex. death of the subject). [6] [12] Indirect assay
There are different definitions within laboratory quality control, wherein "analytical sensitivity" is defined as the smallest amount of substance in a sample that can accurately be measured by an assay (synonymously to detection limit), and "analytical specificity" is defined as the ability of an assay to measure one particular organism or ...
A normal experiment may involve 1–10 mL solution with an analyte concentration between 1 and 10 mmol/L. More advanced voltammetric techniques can work with microliter volumes and down to nanomolar concentrations. Chemically modified electrodes are employed for the analysis of organic and inorganic samples.
The first instrumental analysis was flame emissive spectrometry developed by Robert Bunsen and Gustav Kirchhoff who discovered rubidium (Rb) and caesium (Cs) in 1860. [4] Most of the major developments in analytical chemistry took place after 1900. During this period, instrumental analysis became progressively dominant in the field.
Fluorometric assays use a difference in the fluorescence of substrate from product to measure the enzyme reaction. These assays are in general much more sensitive than spectrophotometric assays, but can suffer from interference caused by impurities and the instability of many fluorescent compounds when exposed to light.