MCGV (Multichannel gradient valve) problem , Peak retention time shift

Hi ,

I face a Peak retention time shifting problem while using HPLC/DAD.

The peak retention time shifting was occurring for same sample under same condition , where difference appear between two injection and become greater between days.

I have try the method (Isocratic) on the different lines (A,B,C,D) and compare the results . It appear that there was a difference (shift in retention time about 2 mins)  in the result were line (A) is used in the method . Thus the problem might be in Line (A) . While there is little difference between other lines (B,C,D) concerning the retention time (+-0.2min.) this while using isocratic method.

It appear the line A in MCGV is giving a portion greater than that is specified .

While using a specific gradient method there was no great difference between lines (A,B,C,D) only a little shift (+-0.2 min.) and problem didn't appear at line A this might be due to concentrations of the gradient method.

I clean the MCGV lines using (Water and IPA) as recommended by Agilent , but there was no change in the results .

knowing that the MCGV is not old (about 2 yrs) and lines (A : water , B and C : organic , D : buffer) .

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