Reading at 600 nm is the traditional wavelength for cell growth. However, because the absorbance noted at 600 nm is due to light scattering and because not all plate readers are equipped with a 600 nm capability, a scan of the contents of H-12 was performed to see what effect various wavelengths had on the results. The contents were transferred to a Corning 3635 UV transmissible microplate to determine if 600 nm featured a unique peak, if other wavelengths could be used, and if pathlength correction to 1 cm could be used if necessary. As noted in Figure 4, there is nothing unique regarding the 600 nm wavelength used as compared to other wavelengths. When other wavelengths are used a very similar shaped curve is observed. Theoretically, a whole variety of wavelengths could be used to measure cell growth. Note that at lower wavelengths, (e.g. 340 nm) other reagents, such as the microplate itself and dissolved solutes also absorb light and these wavelengths are not suitable for measuring light scattering. Reading at 600 nm is still the traditional wavelength however, and the one cited in the literature.
In order to directly compare the results of bacterial growth curves in microplates to those obtained in spectrophotometer cuvettes a correction for pathlength is required. Automatic BioTek pathlength correction depends on the solution having an isolated absorbance peak for the diluent. In the case of aqueous solutions, water has a peak at 977 nm. A reading at 900 nm is used to subtract nonspecific absorbance effects such as plastic. Unfortunately, this method of pathlength correction cannot be used because the actual measurement of bacterial growth is based on light scatter rather than true absorbance. As demonstrated previously, these experiments scatter light all wavelengths including 977 nm and 900 nm, which affect the pathlength correction calculations. At 977 nm, with 200 µL in the well, the absorbance of 200 µL of aqueous solution should be around 0.100 above the absorbance at 900 nm. Instead, the respective absorbance values are 0.883 and 0.870 for 977 and 900 nm respectively, a difference of 0.013 which indicates other reagents are absorbing and or scattering light at those wavelengths, a condition that negates the use of that method of pathlength correction. A more effective method of pathlength correction involves the determination of the pathlength of a specific volume of fluid using water or absorbing colored dye prior to running a bacterial growth curves. This calculated pathlength could then be used as a constant in a mathematical transformation to correct each well to a 1-cm pathlength value.
Reading at 600 nm is the traditional wavelength for cell growth. However, because the absorbance noted at 600 nm is due to light scattering and because not all plate readers are equipped with a 600 nm capability, a scan of the contents of H-12 was performed to see what effect various wavelengths had on the results. The contents were transferred to a Corning 3635 UV transmissible microplate to determine if 600 nm featured a unique peak, if other wavelengths could be used, and if pathlength correction to 1 cm could be used if necessary. As noted in Figure 4, there is nothing unique regarding the 600 nm wavelength used as compared to other wavelengths. When other wavelengths are used a very similar shaped curve is observed. Theoretically, a whole variety of wavelengths could be used to measure cell growth. Note that at lower wavelengths, (e.g. 340 nm) other reagents, such as the microplate itself and dissolved solutes also absorb light and these wavelengths are not suitable for measuring light scattering. Reading at 600 nm is still the traditional wavelength however, and the one cited in the literature.In order to directly compare the results of bacterial growth curves in microplates to those obtained in spectrophotometer cuvettes a correction for pathlength is required. Automatic BioTek pathlength correction depends on the solution having an isolated absorbance peak for the diluent. In the case of aqueous solutions, water has a peak at 977 nm. A reading at 900 nm is used to subtract nonspecific absorbance effects such as plastic. Unfortunately, this method of pathlength correction cannot be used because the actual measurement of bacterial growth is based on light scatter rather than true absorbance. As demonstrated previously, these experiments scatter light all wavelengths including 977 nm and 900 nm, which affect the pathlength correction calculations. At 977 nm, with 200 µL in the well, the absorbance of 200 µL of aqueous solution should be around 0.100 above the absorbance at 900 nm. Instead, the respective absorbance values are 0.883 and 0.870 for 977 and 900 nm respectively, a difference of 0.013 which indicates other reagents are absorbing and or scattering light at those wavelengths, a condition that negates the use of that method of pathlength correction. A more effective method of pathlength correction involves the determination of the pathlength of a specific volume of fluid using water or absorbing colored dye prior to running a bacterial growth curves. This calculated pathlength could then be used as a constant in a mathematical transformation to correct each well to a 1-cm pathlength value.
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