2D-LC User Guide 43
2 Concepts of 2D-LC
Active Solvent Modulation (ASM)
Example for calculation of split ratio and ASM factor.
A longer capillary results in higher backpressure and therefore lower flow
compared to a short capillary.
Example:
If the back pressure of the capillaries between ports 7 and 3 (2D-LC valve to
sample loop and back) is twice as high as the back pressure of the ASM capillary
between ports 9 and 6, twice as much solvent will run through the ASM capillary.
This will dilute
1
D solvent in the sample loop by a factor of about 3, which is
called the ASM factor.
In the recommended configuration with the ASM capillary kit (see note above)
one can simplify the formulae for the calculation of split ratio and ASM factor as
follows:
Usage of the ASM capillary kit results in the following situation:
• The capillaries in ASM branch and transfer branch have the same inner
diameter.
• The two transfer capillaries are equally long.
• The difference between ID
loop
= 0.35 mm and ID
capillaries
= 0.12 mm is large.
Therefore the backpressure of the loops is negligible (this is, because the
radius enters the Hagen-Poiseuille-Equation with the power of 4).
• Solvent composition and their viscosity in the parallel flowpaths are not
predictable.
l
ASM
= Length of ASM capillary
l
tc1,2
= Length of transfer capillary 1 or 2
The ASM factor calculated by the software should not be considered to be a fix
number but as a guiding value which is subject to method development.
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