I've been playing around with making tonic water syrup this year. Working from a lot of recipes floating around, I've settled (for now, anyway) on one that I really like:
GG's G&T Tonic Syrup
Mix together in a small bowl
2 1/4 tsp Chinchoa bark
1 1/2 tblsp Everclear 151
Combine, cover, leave overnight
The next day combine:
2 1/2 cups water
1 1/2 cups sugar
20 Cardamon pods crushed
6 juniper berries
1/2 tsp corriander seed
4 lemon grass stalks chopped
1 tblsp citric acid (I've used up to 50% more and it definitely has a more acid zing, but I prefer the 1 tbs.)
zest and juice of one orange
zest and juice of 2 large Meyer lemons
zest and juice of 2 limes
Add the Chincoa-alcohol slurry.
Simmer covered for 45 min
Strain through coarse sieve, fine sieve then through a coffee filter. There's a lot of crud in here, it's hard to strain.
Mix with seltzer 1:4 or, more to the point, with gin 1:1:4.