Concentrates with high content of nicotine are made from high purity components of food.
The concentrates can only be used for self-production of liquids for electronic cigarettes. Calculate the required concentration you can use the programs:
E-Liquid Calculator Apple: here.
E-Liquid Calculator for Android: here.
Web Calculator to calculate a mixture of prepared liquid and its value: here.

WARNING! Nicotine is a poison that you use it at your own risk! You should not use the concentrate, while not dilute it to secure consistency. Store in a dark, cool place away from food, inaccessible to children. In case of contact with eyes or skin, wash with warm soapy water.