I caught copepods before the spring bloom in the Southern ocean and measured length and sinking speed of their bodies. Both were only weakly correlated, so I had the idea to measure the body density in relation to body length to find out why. Does anyone have an idea how to measure body density of copepods?
Maybe you can measure the water displacement of copepods with no adherent water. Filter them with a fine sieve and remove the adherent water in an airflow. Determine the mass of a certain number of copepods with a precise scale. If you put them in e.g. in a small test tube with water, where the water level is precisely labeled the copepods will displace the water and the water level rises. By means of a micropipette, you determine the volume of the volume increase of water (which corresponds with the volume of the copepods). To do so you tip with the pipette tip into the water and screw the volume adjustment until you have decreased the water level back to the mark. (You do not need the piston of the pipette for this operation, just move the volume adjustment). The volume is the volume indicated on the volume readout of the pipette after removing the water minus the starting value on the volume readout (e.g. you initial read out: 20 µl, after removal of the displaced volume 55 µl: the volume of your objects is about 35 µl). To determine the density you just need to divide the volume by the mass. Hope this will work.
Hi, Svenja, I translated for you selection from good russian site
http://chem21.info/info/902015/
that help you to prepare the linear gradient dilution. Methods of forming density gradients is now well developed. By layering solutions with a pipette to obtain a series of discrete layers with progressively decreasing density where you should put yours objects. The density distribution becomes smooth or after standing of the gradient for some time, or after a light stirring with wire. The gradual smoothing of the gradient when standing is a long process, but this way you can easily obtain any nonlinear gradients. Apparently, the most common device for the formation of the density gradient — two connected cylinder with the contained equal volumes of a solution of different density. The contents of the cylinder with a denser solution is continuously stirred and slowly is issued in a centrifuge tube. If both cylinders have the same dimensions, then appear a linear density gradient.
Most often for the formation of density gradients used sucrose, with properties corresponding to the requirements.
Solutions for the formation of a density gradient of sucrose a) 0.8 M sucrose, 50 mM sodium chloride, 50 mM Tris-HC1 buffer at pH=7,8, 2 mM magnesium chloride b) 1.0 M sucrose, 50 mM sodium chloride, 50 mM Tris-HC1 buffer at pH=7,8, 2 mm magnesium chloride) 1.5 M sucrose, 50 mM sodium chloride, 50 mM Tris-HC1 buffer at pH=7,8, 2 mM magnesium chloride.
Andrey
ratio of mass and volume can be considered. Volume through image analyser
Thank you all for your answers, they were really helpful! Nickolai Shadrin Abhijit Mitra Andriy Kovalchuk
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