It depends on the presence or absence of aromatic amino acids in your proteins and the wavelength of the HPLC detection (280 nm for proteins with aromatic amino acids or 210 nm for peptide bonds in any proteins). Usually, proteins having some aromatic amino acids (Trp, Phe, Tyr) demonstrate higher absorbance sensititivity and can be detected at lower concentrations). It also depends on the purity (quality) of your organic elution buffer (because it serves as a background absorbance that needs to be subtracted from you protein absorbance peak reading). It also depends on the diameter of the column you are using (usually, the less diameter the higher sensitivity).

Detection limit depend on detector, which are you using. To increase detection limit you can increase injection volume, which can be up to 1% from column volume. 

Dear Ashish,

It depends on what flow rate and column inner diameter you are employing.   If you want to use your existing set up, then, changing wavelength detection from 280 to eg,, 214nm will enhance detection, as suggested by Viktor Butnev above.  However, at 214nm, you will also start seeing absorbance/detection of your mobile phase, especially organic phase.  So, the baseline (background) in absence of any protein will start to be detected. 

What is the Molecular weight range of the proteins you are trying to detect?  It is important to make sure you are eluting these proteins from your column...

Hediye.