Did you properly blank it? If your solution in the blank was more absorbant than the suspending medium you would get a negative value.
Thankyou very much can you please tell me at what wavelength i can find optical density of chlorella vulgaris as in different research paper different wavelength has been suggested like 500nm,530,430 etc
You can try for four wavelength for finding lambda max between 540,600,620,660,680 nm ...
Hi,
You need to set a reference (=blank) at every wavelength you are interested in. We usually fill pure culture medium in the cuvette and run a "reference spectrum". Then we measure the algae suspension. If the photometer is not able to scan a reference spectrum, you have to blank each wavelength (= only medium is set to zero) immediately before you are measuring the suspension.
Optical density is usually done at wavelengths with minimum intereference of pigments ( you are not intereseted in the pigment content, but in the particles). This is either at 550 nm (green gap of chlorophyll) or at around 750nm, where pigments are no longer absorbing.
Optical density depends on the particle size, so it is a RELATIVE UNIT! Same biomass, but different particle size will result in different optical densities!
All the best
Michael
660 nm is the best wavelength for algae growth study as it measures chlorophyll fluorescence. However, if you are getting -ve absorbance, Then chlorophyll must have degraded in your sample may be due to ageing, stress or chemical application. you can take your OD at 540 nm or it would be better if you study growth at 750 nm, as at 750 there will be complete reflectance.
you scan from 400 to 700 against blank.preferably reagent blank and then from spectrum select maximum wavenumber.
I mentioned in my paper it is due to impurities probably, as illustrated in the figures, after a while (elimination of impurities), Acid Yellow 17 (a dye) with negative removal efficiencies had positive removal efficiencies.
Article A comparative study on removal of four types of Acid Azo dye...
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