Hi there,
You have to apply a 109 dilution factor(10-9 dilution). For accuracy purpose and for limiting the volume, serial dillution should be used. I would go for repeating four times 10-2 dilutions (10µL in 1mL final volume) and then 10-1 dilution (for instance, 10µL in 100µL but depending on the volume needed at the end).
It is a 1:109 dilution, so it must be done in stages unless you want to make an enormous volume of it (1 µl diluted into 1000 liters).
One way to do it is to make 3 serial 1:1000 dilutions. For example, take 2 µl (with a suitable pipettor designed for such small volumes) and add it to 1998 µl of dilution buffer (1:1000 = 1 µg/ml), then take 2 µl of that and do the same thing (1:106 = 1 ng/ml). Then take 2 µl of that and do the same again (1:109 = 1 pg/ml). The last step can be scaled up if you need more of the solution.
1 pg/ml is very dilute. For a protein with molecular weight 50,000, for example, that corresponds to 2 x 10-14 M (20 fM). There is a good chance that the protein could be lost to surface adsorption without a carrier substance. Also, some proteins are unstable when extremely dilute.
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