The quality of a buffer depends on 2 factors 1. the buffer capacity and 2. the distance between the pH value and the pks value, you should work close to the pks. pKs1(Phosphoric Acid) 2,15. So it makes more sense add the phosphoric acid and then adjust pH with triethylamine. 

Setting the pH value takes time. Add small amounts of TEA gradually

When using this buffer, the regeneration of the system takes time, H3PO4 / TEA forms slimy deposits, flush well with MeOH when you're done.

I attached a Buffer Guide for additional information.

http://www.hplc.eu/Downloads/ACE_Guide_BufferSelection.pdf

Hello Mike, 

Thanks for insight!!!!!!

What i understand In case of TEAP buffer the molarity is actually referring to the  molarity of the species doing the buffering; in my case, pH 2.25, that would be phosphate (H3PO4 / H2PO4=) which is acting as buffering agent and it's molarity should be precisely adjusted to  1M in my case. 

First pKa value of phosphoric acid is 2.15 and  bringing the pH to 2.25 by adding TEA wouldn't disturb the molarity of phosphate ? or little change in molarity  is not that much important while doing HPLC analysis. ?

Wath´s type of sample and column you will use? 

The necessary accuracy also depends on your application. When working with ion exchangers, the salt concentration and the pH plays a much bigger role than if you want to separate neutral compounds on a C18.

I have Water XTerra RP C18 column, and my sample is metal complex having positive charge. 

I have no experience with the separation of metal complexes, is there a specific reason that you have to use the H3PO4 / TEA buffer? 

To your question concerning the accuracy of the buffer, if you add 1 mol of phosphoric acid and then adjust the pH value with TEA and fill up on the 1L market, you still have 1 mol/L of phosphoric acid in your solution, only the chemical equilibrium has shifted.