I've done a very successful tonic batch recently.
Here's what I used:
2T cinchona bark - the chunky stuff, not the fine powder
1lb lemongrass - fresh from the korean market
dried peel of 1 temple orange - homemade... dried the peel of an orange I ate over the winter.
2 drops lime oil - NOW foods brand is what I used.
citric acid - standard food safe chemical grade... same stuff I use to clean my espresso machine.
Here's how I did it:
Part I - Dealing with the cinchona
In the past, I've dumped the cinchona bark in with the other stuff... and it made the tonic brown and woodsy and bark-flavored with weird floral notes. I did no want that. Noting the post above that mentions that quinine is much more soluble in alcohol than in water I decided to make use of that trait. This time I made cinchona tea... twice... and threw it out both times. Standard 5 minute steep with water near boiling. It was bitter and bark-y and weird like all of the boiled-in-water cinchona experiments before. I then soaked the pre-boiled cinchona in some cheap vodka for a half hour. That made for some slightly brown and very bitter quinine tincture. I've still got 20x the quinine tincture I used in this batch in a bottle.
Part II - Getting the citrus/sour/sweet thing balanced
I took the pound of lemongrass, 5 big long stalks and ran them through the slicing blade on the food processor. Then I took the heap of little woody circles and put them in a pot with 2 cups of sugar and the orange peel, and added water to cover. Heated to boiling. Added lime oil while it was boiling in hopes of it emulsifying some. Let it cool and filled a 750ml bottle with it. Didn't measure the citric acid I added... Just until it tasted "right" to me.
Part III - Getting it together
So I find that the quinine tincture goes a long long way. 2 or so tablespoons of it added to the 750 ml bottle seems about right for a good tonic-y bitterness.
Part IV - Using it
So far, I find that a 3/4 oz measure of the tonic syrup makes a good 10oz glass of gin and tonic when combined with a 1 1/2 oz measure of gin, and topped with seltzer and a squeeze of lime.
Moral of the story- boil your cinchona... then throw out the tea and extract the quinine you need with alcohol.