For plasma: denaturation with deuterated internal standard (MeOH-Acetonitril). After centrifugating dilute with formic acid(2%).

My current laboratory analyzes this drug by GC/MS!

http://www.clinchem.org/content/44/12/2506.full.pdf

Unfortunately we do not have GC/MS ALL WHAT WE HAVE LC/MSMS SO, IS IT POSSIBLE TO USE SAME METHODOLOGY RUNNING ON LC/MSMS HAVE ANY CLUE? 

The GC/MS method will not work for LC/MS the following are several articles I collected on LC/MS of Busulfan:

http://link.springer.com/article/10.1365%2Fs10337-009-1352-0#/page-1

http://pdf.medrang.co.kr/Kjlm/2014/034/Kjlm034-01-02.pdf

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3724260/

http://link.springer.com/protocol/10.1007%2F978-1-60761-459-3_12

Which one to use depends on your matrix, the HPLC columns and instrumentation you have available.