Is LSD purity a concern? Whats the possibility its laced with harmful substances?
As it has already been mentioned, blotters can contain an active substance other than LSD, not as a combo, meaning the blotter may contain no LSD at all and only have another substance.
I would say that the substances most commonly misrepresented as LSD are members of the DOx family, DOB and DOM are the most popular but there's more in that family that could be laid on blotter.
Also NBOMes are a concern as not only the effects can be much stronger than LSD, but they can also cause serious medical emergencies such as seizures and death (it seems that the cause of death is blood clots causing embolisms).
Even when the substance is relatively benign, there are risks associated with the different pharmacokinetics of the drugs: in instance, a user convinced to have taken LSD but that has really ingested DOM or DOB, will notice that he's only feeling weak effects 1 hour after ingestion of the blotter(s).
He may then think that the blotters are dosed with less LSD than what was promised and take more to compensate.
The issue with that is that is that DOM and DOB will come up very slowly compared to LSD and the effect grow in intensity for at least 90 minutes after ingestion.
So the user will soon feel the effects of the first blotter, and then later the second blotter will also kick in, probably/potentially producing effects of an intensity that the user did not anticipate, with all the problems this can cause.
DOM specifically lasts longer than LSD on a low dose, and when the dose is increased up to 10mg (typically, a blotter is 4 to 5mg), it can last 24 hours or even more, which obviously has the potential to create even more issues like having to either go to work still tripping or having to call in sick at the last minute, being unable to drive home from a location at the time you had initially planned, the trip taking a dark turn when sleep deprivation sets in etc.
As far as blotters containing actual LSD, the most common contaminant is Iso-LSD one of the 2 LSD isomers created during synthesis.
4 LSD isomers are theoretically possible, but only 2 form during synthesis from d-lysergic acid, which is the only precursor I have heard of for making LSD.
The 2 isomers formed are (+)-LSD and (+)-iso-LSD, and only (+)-LSD is active and it is colloquially called LSD.
During synthesis, the chemist is supposed to separate (+)-LSD from (+)-iso-LSD using a process called chromatography followed by a crystallisation, which needs to repeated twice to achieve the ideal >98% purity.
If this is not done at all or not done with enough skill, the final product will be a mix of LSD and iso-LSD.
This means that if the product is 75% LSD and 25% iso-LSD, a blotted laid with 110ug will contain only 82.5ug of LSD, and a blotter laid with 250ug only 187.5ug and so on.
This obviously explains the difference in potency between blotters laid with the same amount of LSD bought from different vendors.
Obviously, vendors also lie about the potency, but purity is also a concern.
Some people also claim that while iso-LSD is inactive on its own (especially at the dosages typically found in LSD blotters), it will change the experience, diminishing it with a negative body load and causing the user to be less focused/clear-minded.
I have personally experienced LSD trips of both types, but not having had the blotters tested I am not able to say if when I experienced body load and confusion I had taken a blotter containing pure (+)-LSD or if it was contaminated by (+)-iso-LSD.
Some people call blotters that don't cause the desired effects "bad acid" which is basically an umbrella term for "I have taken this blotter and I had an experience that wasn't what I expected it to be", which includes also bad experiences with pure (+)-LSD.