How to partition free amino acids from proteins in plant extract

[Tony Larson]

Hi,
    I am trying to quantitatively recover free amino acids away from
total soluble proteins in a plant leaf extract.  The extract is made of
5-100 mg frozen leaf material homogenized in 1 mL of 80% ethanol and
then centrifuged to remove particulates.  The amino acids in the
extracts will ultimately be quantified by either HPLC of PITC
derivatives, or GCMS of MTBSTFA derivatives.
    I have heard of a method where such extracts are passed through a
C18 SPE column to adsorb the proteins and recover the free amino
acids in the eluate.  If this works, it would be an attractive
alternative to using expensive and time-consuming low-molecular-weight
cut-off centrifugation or protein precipitation methods.  However, I
can't find a protocol for such a method using C18 SPE columns (i.e.
rinse, load, elution conditions).  Can anyone help?

thanks
Tony