D Solvent Considerations
300 July 29, 2013, 715003794 Rev. B
• If the difference is 17 or greater, the liquids are immiscible or their
critical solution temperature is above 75 °C.
Some solvents prove immiscible with solvents at both ends of the lipophilicity
scale. These solvents receive a dual M-number:
• The first number, always lower than 16, indicates the degree of
miscibility with highly lipophilic solvents.
• The second number applies to the opposite end of the scale. A large
difference between these two numbers indicates a limited range of
miscibility.
For example, some fluorocarbons are immiscible with all the standard
solvents and have M-numbers of 0 and 32. Two liquids with dual M-numbers
are usually miscible with each other.
A liquid is classified in the M-number system by testing for miscibility with a
sequence of standard solvents. A correction term of 15 units is then either
added or subtracted from the cutoff point for miscibility.
Buffered solvents
When using a buffer, use a good quality reagent and filter it through a
0.45-µm filter.
Do not leave the buffer stored in the system after use. Flush all flow pathways
with HPLC-quality water before shutting down the system and leave distilled
water in the system (flush with 90% HPLC-quality water/10% methanol for
shutdowns scheduled to be more than one day). Flush using a minimum of
45 mL.
Head height
Position the solvent reservoirs at a level above the separations module or on
top of the separations module (with adequate spill protection).
Solvent viscosity
Generally, viscosity is not important when you are operating with a single
solvent or under low pressure. However when you are running a gradient, the
viscosity changes that occur as the solvents are mixed in different proportions
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