We've been monitoring these issues for years and have taken the advice of professional chemists and pharmacists.
The individual vaper should buy a glycerine product marked 'Glycerine, USP' (USA) or 'Glycerine, BP' (UK) or 'Glycerine, EU' (in the EU). Assuming the product is not counterfeit, it will be the best grade available to the retail buyer. The label heading must be exactly this without any variation. The product may also be described in the smaller print as glycerol, glycerin, pharmaceutical grade, food grade, kosher, vegetable glycerine, etc. (as the small print is irrelevant).
This is not the same as the absolute best grade, which is only available in practice to manufacturers, due to MOQ issues. Because we have to give advice to both ecig manufacturers and retail buyers, our advice sometimes appears confusing, and I apologise for that.
The absolute best grade of glycerine is 100% synthetic because it is almost pure and the contaminant is likely to be water. We only know of one product of this type, Dow Optim. It is available in 18 litre jugs and up, and is the best possible base material for manufacturers.
Other suitable types for manufacturers are any glycerine product with a provable pharmaceutical license for inhalation, as used for example in asthma inhalers. However it has recently become necessary to order tests for a specific contaminant that may be present: see below.
The reason why Glycerine, USP is not the best choice for a manufacturer
For some time now the FDA has been warning about a specific type of contamination in glycerine products which has appeared in all grades including USP. This is called jatropha. The carcinogenic phorbol esters of the jatropha plant are found in biodiesel byproduct glycerine.
Because USP glycerine can be produced from any of the multiple feedstocks used to produce glycerine retail products, some USP glycerine will contain biodiesel byproduct glycerine (which of course qualifies as 'vegetable glycerine' as it is processed from vegetable sources) and may contain jatropha toxins. Currently there is no known way to exclude jatropha toxins from normal glycerine. The only solution is to use 100% synthetic glycerine, which is why it should be used by manufacturers of e-liquid products unless they have access to guaranteed pure products supported by analytical proof THAT SPECIFICALLY INCLUDES DOCUMENTED TESTING for jatropha toxins.
A synthetic glycerine such as Dow Optim is the best solution for manufacturing. Retail customers could conceivably form a club to buy a minimum size jug of 18 litres.
We hope this explains the current situation regarding the purity of glycerine products, and especially the retail purchase vs manufacturing issues.