GPC help

Record: 473

Category: Column Problem
Product: SB
Sample:

Question

Answer
I have found that the pressure change was due to adding Acetonitrile to the mobile phase. It is strange, should adding CAN lowered the system pressure? In my HPLC (C18 column), system pressure decreased when the ACN increased.

I have another problem, please take a look at the below graph, I modified polymer A to get polymer B, B should have lower MW(10% difference). I am getting A (Mw=23.8K, Mn=13.5 K) and B (Mw=23.5K, Mn=16.3K), but I expect B should have a lower Mw like 21K.

I have tried below mobile phases:

1) 0.05M NaNO3 in water
2) 0.1 M NaNO3 in water
3) 10% ACETONITRILE and 0.05 MNaNO3

But theresults are always similar, could you advise anything I can do? The columns are 2X 804 and 1X802.5

 
 
Additional Comments
About the pressure of organic/aqueous mixture, it is no surprise that 10% ACN solvent provides higher pressure.
Below is an example from C18 column, but depending on the percentage of the ACN, the viscosity of the solvent can actually be higher than pure water/ACN.

Regarding the separation of 23.8K and 21K compounds, I’m sorry to say but I don’t think <3K differences can be separated using SEC columns. SEC is simple and robust, but it’s its limit… http://shodex.net/index.php?seitenid=1&applic=1050 http://shodex.net/index.php?seitenid=1&applic=1052 http://shodex.net/index.php?seitenid=1&applic=1042

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