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I have read conflicting things about the volatility and solubility of caffeine, and at what point during the tea brewing process caffeine is released.
When tea is used for multiple infusions, such as in Gongfu style brewing, do the different infusions have different amounts of caffeine?
This study specifically addresses the question regarding multiple infusions:
Results for all kinds of tea samples showed that the second tea infusion contained the highest contents of caffeine, catechins, and gallic acid when bag teas were steeped in 70°C water. It was different from that steeped at 85 and 100°C, the highest contents existed in the first infusion. These compounds decreased gradually in later infusions.
The paper measured 8 successive infusions on a variety of teas. You can see how caffeine content varied exactly in their data.
Here's an interesting light article that gives some data points that would let you construct your own curve for caffeine extraction per time (for green tea only):
http://www.tching.com/2009/10/thirty-second-decaffeination-method-or-myth/
Yes, they would all have different amounts of caffeine. It all depends on the kind of tea, the water temperature, how long the tea is brewing and how much water you use. If you want to make a reasonable comparison, you need to know the caffeine content as a percentage of the dry product's mass.
