In order to purchase vape juice you must understand the basic principles that govern the ingredients.
(1) VG/PG Ratio
(3) Nicotine Concentration.
Vegetable Glycerin and Propylene Glycol are the prominent ingredients in vape juice. Simply put, VG is a thicker liquid and leads to heavier clouds. PG on the other hand is a thinner liquid that binds to flavour molecules and leads to lighter vapour when exhaled.
The most common VG/PG ratios are 70VG/30PG and 50VG/50PG although you can purchase vape juices that cover the full range of VG/PG ratios but these are less common.
The volume of all nicotine-containing vape juices are 10ml. Liquids that do not contain nicotine can come in any volume but are generally limited to 50ml and 100ml.
Nicotine concentration is heavily tied to the volume of the juice purchased. Generally speaking, if you would like to purchase juices that are between 0mg and 6mg you will be able to find many options in the larger 50ml and 100ml bottles. However, if you are interested in purchasing juices that are over 6mg, you will have to stick to 10ml e-Liquids that come pre-mixed with nicotine and are verified as legally compliant by the Tobacco Products Directive.