[minimal]

I have noticed after I pour myself some tea, that as it cools and sits there is a thin oil-like film covering the top of the tea.  I do not add sugar, nor milk/cream. 
I know it's not the caffeine because there is not so much caffeine that it would not be soluble even at room temp, even though I am adding about two tea bags per 400ml water.  I doubt it is the theobromine because it looks structurally too similar to caffeine to have a much different solubility curve.
Perhaps it is some sort of lipid, but then I'm wondering why it was soluble in the water in the first place, even close to 100C.

Thanks for the help.

edit: Do you think it is http://en.wikipedia.org/wiki/Kaempferol?

[Borek]

Use filtered water.

[Arkcon]

It's very possible that it is simply residual oil or detergent from when the cup was last washed or used.  Or it may be one of many natural oils extracted from the tea leaves by the hot water.

*[EDIT]*
Like Borek: says, if you're really serious about tea analysis, you should use distilled water, scrupulously clean lab glassware, and not touch the tea bag with your fingers.  An LC-MS would also make short work of the problem.