Diacetyl is the word you are looking for, and I wouldn't flat-out declare it's the culprit of your coughing, which could well be due to some other reason (dehydration) or a combination of factors. But, diacetyl is a reported carcinogen found in buttery or custardy flavorings, which
are trace ingredients in a lot more juices than you might think. Due to customer concerns, several vendors began eliminating it from their juice or flavoring lines some time ago, but if you are concerned you must either research a lot, or simply ask whether it's an ingredient. Another substance in some flavorings is acrolein, which apparently breaks down into diacetyl when heated. Some people are very worried about consuming these even in tiny amounts, but others (I'm one) don't stress over it particularly. I don't use much of it anyway.
Please, be very careful with 100mg nic base. I have been DIYing for well over a year and the strongest I've used is 60mg. I started off with 36mg base until I got the hang of using measuring equipment, etc. If you are new to mixing your own liquids, I promise - sooner or later you WILL spill something! Accidents can happen no matter how careful you are. As for the ebay testing kit, people who know more than I do have posted recently that they're not necessarily accurate. (I don't test myself, I just use one of the excellent e-juice calculators to mix the nic level I want.)
I suspect you haven't yet found the DIY section of the forum, where there is a TON more information. Do check it out, here's a link:
DIY E-Liquid - there are lots of other DIY threads on ECF also (just search the forum for "DIY"). You will gain a lot of learning and insights if you do a lot of reading. (Caution though, such reading is likely to make you want to order more and more stuff! LOL)
One Stop DIY Shop is great (as are others), but I have particularly enjoyed the Flavor Store. I was very excited when they opened shop because I wanted a large
variety of flavorings to try, instead of the much-larger size bottles that most vendors sell. A little flavoring goes a long way, especially if you are still learning what you like (or on a budget). They sell 1-dram bottles from several different manufacturers and Chris is helpful and responsive, if there is a flavor you want in larger size, just email him to discuss. Here's their forum link:
http://www.e-cigarette-forum.com/fo...174159-flavor-store-new-flavors-requests.html
I mix right into dropper bottles. Leave some airspace at the top and then shake very well, warm it in some hot water (I now set the bottle over the pilot light of my gas stove for about 5 minutes), shake well again. Some flavors are good immediately, others benefit from a few days, or even a month, of steeping in a cool dark place to let the ingredients blend.
One more thing - WRITE DOWN your recipes! I use 3x5 cards and make a lot of notes. There's nothing worse than coming up with a great mix and not being able to reproduce it again.
Although time-consuming, it's really a lot of fun to mix your own e-juice, I have enjoyed the learning experience (and playing mad scientist!). There are SO many recipes and experiments to try, good luck with it!