Buffered Eluents; Eluent Viscosity; Eluent Degassing - Waters 515 HPLC Operator's Manual

Some eluents are immiscible with eluents at both ends of the lipophilicity
scale. These eluents are assigned two M numbers:
• The first number, always smaller than 16, indicates the degree of
miscibility with highly lipophilic eluents.
• The second number, always larger than 16, indicates the degree of
miscibility with hydrophilic eluents. A large difference between the two
M numbers indicates a limited range of miscibility.
For example, some fluorocarbons are immiscible with all the standard eluents
and have M numbers of 0 and 32. Two liquids that each have two M numbers
are usually miscible with each other.

Buffered eluents

After you use a buffer, flush the buffer from the pump with at least 10 mL of
HPLC-grade water. For shutdowns lasting more than one day, flush the pump
with 10% methanol-water to prevent growth of microorganisms.

Eluent viscosity

Generally, viscosity is not important when you operate your HPLC system
with a single eluent or under low pressure. However when you run a gradient,
the viscosity changes that occur as the eluents are mixed in different
proportions can result in pressure changes during the run. For example, a 1:1
mixture of water and methanol produces twice the pressure of either water or
methanol alone.
If the extent to which the pressure changes affect the analysis is not known,
monitor the pressure during the run using the controller chart output
provided for this purpose (select %A or %B).

Eluent degassing

Eluent difficulties account for most liquid chromatographic problems.
Degassing eluents is one of the most effective measures to eliminate these
problems. Degassing provides:
• Stable baselines and enhanced sensitivity
• Reproducible retention times for eluting peaks
Buffered eluents D-5