Plate readers

Plate readers, also known as microplate readers or microplate photometers, are instruments which are used to detect biological, chemical or physical events of samples in microtiter plates. They are widely used in research, drug discovery, bioassay validation, quality control and manufacturing processes in the pharmaceutical and biotechnological industry and academic organizations. Sample reactions can be assayed in 6-1536 well format microtiter plates. The most common microplate format used in academic research laboratories or clinical diagnostic laboratories is 96-well (8 by 12 matrix) with a typical reaction volume between 100 and 200 µL per well. Higher density microplates (384- or 1536-well microplates) are typically used for screening applications, when throughput (number of samples per day processed) and assay cost per sample become critical parameters, with a typical assay volume between 5 and 50 µL per well. Common detection modes for microplate assays are absorbance, fluorescence intensity, luminescence, time-resolved fluorescence, and fluorescence polarization.

Absorbance detection has been available in microplate readers for more than 3 decades, and is used for assays such as ELISA assays, protein and nucleic acid quantification or enzyme activity assays (i.e. in the MTT assay for cell viability). A light source illuminates the sample using a specific wavelength (selected by an optical filter, or a monochromator), and a light detector located on the other side of the well measures how much of the initial (100%) light is transmitted through the sample: the amount of transmitted light will typically be related to the concentration of the molecule of interest. Several conventional colorimetric analyses have been miniaturized to function quantitatively in a plate reader, with performance suitable for research purposes. Examples of analyses converted to plate reader methods include several for ammonium, nitrate, nitrite,urea, iron(II), and orthophosphate. More recent colorimetric chemistries have been developed directly for use in plate readers.

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