1 23 Analytical and Bioanalytical Chemistry ISSN 1618-2642 Volume 401 Number 7 Anal Bioanal Chem (2011) 401:2187-2193 DOI 10.1007/s00216-011-5303-x Quantification of total and free concentrations of R- and S-warfarin in human plasma by ultrafiltration and LC- MS/MS Berit Packert Jensen, Paul Ken Leong Chin & Evan James Begg