Alfacalcidol low concentration detection by HPLC 0.5 ng

[raghdaa]

i have problem in alfacalcidol dissolution sample detection .
1- when dissoluted at dissolsion media which consist of water and tween not detected but only detected when it was dissolved in methanol (the method of its determenation is needed precolumn concentrated and then eiluted methanol .HPLC ,i have doesn't consist of precolumen dose any one know another way to detected it by HPLC ?
Note :Its concentration is very low 0.5 nanogram in dissolution media

[tom jupille]

First of all, "0.5 ng" is an amount, not a concentration. So, what is the concentration and what is the volume injected?

Second, if an established method call for the use of a preconcentrator, then it is probably necessary (that is not the kind of thing that people do for fun).The best advice I can give you is "get the tools you need to run the method properly".