OK. Triethylamine has a pKa of 10.78 at 25 deg C.
Your pH of 2.8 is very far away.
You are not preparing a buffer. You are not preparing a buffer. A buffer is not what you're preparing, and this thing, that you're preparing, is not a buffer.
Sorry, I felt I had to be crystal clear. Feel free to call it a buffer if you feel like it, I always do. I'm just defending you from someone pedantic.
You need pH 2.8? No problem -- weigh or measure more TEA than you will need, stir it in water (it won't dissolve), then add 85% phosphoric acid until it dissolves and is the correct pH, then add water until the volume gives you 1 M TEAP. You will have stock solution, that you can dilute further for 0.05 M.
You will not have a buffer. (Did I mention that? Humph, sounds like I heard it somewhere before.) You will have a solution of dilute phosphoric acid at 0.1 M TEAP. If this is what your protocol calls for, this is what they mean. Edit: I'm just reiterating what Borek: said here.
Please be certain you're not mis-reading your protocol. This low pH will quickly strip the bonded phase off of a silica column. Your concentration is quite low -- are you doing mass spec detection? Because even faint traces of stripped off bonded phase will wreak havoc with the mass spec. Unless you're not using a bonded to silica column.