For primer from 20-35 in length, standard desalted grade is good enough for qPCR application. Since the coupling efficiency per base is approximately 99%, a quick way to calculate the total number of full length species in the oligo mixture is to raise the percentile by the total number of bases in the primer. Estimate purity of 18 bases primer will be 0.99^18 = 83%. As long as primer purity is >85%, it is good to use for qPCR application. HPLC or PAGE purification is only recommended for very long primers.