23.9 How do I degas chromatographic solvents?
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
23.9 How do I degas chromatographic solvents?
One major problem with pressurising chromatography systems using liquid
solvents is that pressure reductions can cause dissolved gases to come out
of solution. The two locations where this occurs are the suction side of the
pump ( which is not self-priming, consequently a gas bubble can sit in the
pump and flow is reduced ), and at the column outlet ( where the bubbles
then pass through the detector causing spurious signals). Note that the
problem is usually restricted to solvents that have relatively high gas
solubilities - usually involving an aqueous component, especially if a
gradient is involved where the water/organic solvent ratio is changing.
As water usually has a higher dissolved gas content, then a gradient
programme may cause the gases to come out of solution as the mobile phase
components mix.
There are three traditional strategies used to remove problem dissolved
gases from chromatographic eluants. Often they are used in combination to
lower the dissolved gases.
a. Subject the solvent to vacuum for 5-10 mins. to remove the gases.
b. Subject the solvent to ultrasonics for 10-15 mins. to remove the gases.
c. Sparge the solvent with a gas that has a very low solubility compared
to the oxygen and nitrogen from the atmosphere. Helium is the preferred
choice - 5 minutes of gentle bubbling from a 7um sinter is usually
sufficient, although maintaining a positive He pressure is even better.
Note that most aqueous-based solvents usually have to be degassed every
24 hours. Also remember that solubility of gases increases as temperature
decreases, so ensure eluants are at instrument temperature prior to
degassing. Helium is preferred as the degassing solvent because it has
relatively low solubility in water, and the solubility is less affected by
temperature.
The following data is from Kaye and Laby, 13th edition, and the units are
the number of cm3 of gas at 0C and 760 mmHg which dissolve in 1 cm3 of water
at the temperature stated ( when the gas is at 760 mmHg pressure and in
equilibrium with the water ).
Temp.(C) 0 10 20 30 40 50 60 Helium 0.0098 0.0091 0.0086 0.0084 0.0084 0.0086 0.0090 Hydrogen 0.0214 0.0195 0.0182 0.0170 0.0164 0.0161 0.0160 Nitrogen 0.0230 0.0185 0.0152 0.0133 0.0119 0.0108 0.0100 Oxygen 0.047 0.037 0.030 0.026 0.022 0.020 0.019 Argon 0.054 0.041 0.032 0.028 0.025 0.024 0.023 CO2 1.676 1.163 0.848 0.652 0.518 0.424 0.360
I've no explanation for the aberrant trend for helium at higher temperatures,
but I assume it's real - but it's irrelevant for HPLC solvents that are
usually stored at ambient temperature. Points to note - the lower solubility
of helium over the range of concern, *and* the lower rate of change of
decreasing solubility with increasing temperature. There is heat generated
in the compression of the solvent, along with friction in HPLC pump heads
and, more importantly, HPLC columns are often heated - thus the solvent
could outgas and form bubbles in UV detector cells that are at ambient.
By using helium, there is less chance of that happening. For example, if the
temperature increased from 10C to 40C, the undissolved gas volume would be
0.0007 cm3 for helium, and 0.0066 cm3 for nitrogen.
Modern HPLCs are sold with a "solvent degassing module" that removes
undissolved gases in the solvent automatically. These usually consist of
a tube made from gas-permeable membrane that passes through a vacuum
chamber.
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