ask for help on seperation of cyclosporin a

[jonathonswq]

i wanted to seperate cyclosporin A from c18 column. i have tried ammoniu acetate and ammonium bicarbonate as buffer. the problem was that the retention time was 7-8min when i used acetonitril/water(90:10), and the peakwidth was 1.2min.
it seems the performance was too poor. some paper about detection of this compound shows acetonitrile/water(75:25), however, it doesn't work to me.
anyone can give me some advice?
thanks for your comment.

[tom jupille]

Jonathan, we are not mind readers.

Before anyone can make a useful suggestion, we need details about what you are doing:

First: what (specific) column did you use? and what (specific) column was used in the original paper you referred to? All C18 columns are not equivalent, and there is no reason to expect two different C18 columns to perform optimally at the same % organic.

Next, what were your dimensions and flow rates? Your retention time may be too high, too low, or just right depending on these parameters.

Was the peak symmetrical or did it tail or have other shape problems?

How old is your column?

What temperature?

[jonathonswq]

some details about my problem.
zorbax xdb c18, 2.0*10mm,3.5um, and the column was bought about 2 months ago. judging from the pressure of the column, there was not any problem with my column. by the way, i tried another compound, which showed good retention on the column.
flow rate 0.2ml/min, 40°C. i can not understand why the peakwidth of this compound was 1.2min. it seems too poor for 3.5um column.
used acetonitrile-water 85:15 as mobile phase.