Dear Kazimierz,
Just courious if there is a specific reason for using HILIC chromatography for your application. All your compounds are very hydrophilic but most of them have been already been analysed with reversed phase LC and UV or amperometric detection. Furthermore, ganciclovir and acyclovir have been analysed simultaneously with RPLC in human plasma/serum.
Ribavirin has also been analysed by RPLC (although not in mixture with the above).
Does your compounds co-elute, or do you have problems of matrix interference when you are analysing these compounds by RPLC? If one of the above apply, or if you are using MS (which you do not) then I would use HILIC for these compounds -I can understand that due to the high polarity of these compounds you will need high aqueous mobile phases for their RPLC separation which are not the best conditions for high sensitivity with the MS-. Otherwise, as you are using UV, a RPLC seems more straightforward to me.
Here are some references about the use of RPLC for your compounds:
Title: A simple and simultaneous determination of acyclovir and ganciclovir in human plasma by high-performance liquid chromatography
Author(s): Teshima D, Otsubo K, Yoshida T, Itoh Y, Oishi R
Source: BIOMEDICAL CHROMATOGRAPHY 17, 8, 500-503 DEC 2003
Title: Liquid chromatographic method for the determination of ganciclovir and/or acyclovir in human plasma using pulsed amperometric detection
Author(s): Kishino S, Takekuma Y, Sugawara M, Shimamura T, Furukawa H, Todo S, Miyazaki K
Source: JOURNAL OF CHROMATOGRAPHY B-ANALYTICAL TECHNOLOGIES IN THE BIOMEDICAL AND LIFE SCIENCES 780 (2): 289-294 NOV 25 2002
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Kostas