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Using the density of the liquid, calculate the volume of liquid and use a syringe.
Weigh an empty syringe, fill the syringe, and weigh again.
If the liquid is in a flask, and you only need a fraction of it, try this: if you have 90 mg in the flask, and you need 30 mg, dissolve the material in 3 mL of solvent, and remove 1 mL. You can remove the solvent to check the weight. This method is more accurate than you might think.
For small amounts of viscous liquids, weigh an empty pipette, dip the tip in the substance and weigh again. This is not a generally useful method, but good sometimes.
Isolating oils after workup or a column:
Use a heart-shaped flask (it has a pointy end). During rotoevaporation, the oil will become concentrated in the point, making it easy to dissolve and remove by syringe or pipette.
Oils can be distilled or purified by column chromatography. The choice of method will depend on factors like reaction scale and boiling point of the oil.
Preparing NMR samples of oils:
Dip a glass pipette into the oil until you have a column 1/2 inch high.
Dissolve the sample in 0.75 mL of the NMR solvent.