Modern electronic vacuum degassing
modules are a great convenience. Changing Helium gas cylinders, monitoring gas pressures
and levels over time have been replaced in many labs with the sound of small gurgling
vacuum pumps. While the convenience gained through the use of these devices can be great,
there are some important downsides to this technology. One of them will be mentioned here.
Total Channel Volume: The volume
of liquid contained in the tubing and vacuum chamber of EACH vacuum degassing
channel can be enormous compared to the tiny volume often found in systems which utilized
Helium sparging alone. When the system is turned off, this solvent often sits stagnant
over night, allowing air to bleed in, and must be flushed out when the system is
re-initialized for use the following day. *If you just switched bottles of mobile phase,
then the old mobile phase is still inside the system and needs to be completely flushed
out before use or you will have some very strange chromatographic results in the coming
weeks! This process takes time and should be performed on every channel in your system,
even if you are not going to use them all. How much volume you ask ? Well, let’s look
at two common examples of vacuum degassers we find at client sites (but this applies to
any manufacturer’s system, just look up the spec’s or measure it on your own).
- Agilent/HP Brand Degassing Module, Model G1322A; Each vacuum channel chamber has
a volume of about 12 ml. The interconnecting tubing (solvent frit to bottle head, bottle
head to degasser and degasser to pump inlet) can add another 10 – 15 mls more. In
this example, we recommend that you flow at least 30 mls of the appropriate solvent
through the line before use. This will insure that the channel is primed with fresh mobile
phase. That step is for EACH channel so it is best to flush them one-at-a-time, in
sequential order, to keep track of them.
- Agilent/HP Brand Micro Degassing Module, Model G1379A: Each vacuum channel chamber has a volume of about 1
ml. The interconnecting tubing (solvent frit to bottle head, bottle head to degasser and
degasser to pump inlet) can add another 10 – 15 mls more. In this example, we
recommend that you flow at least 20 mls of the appropriate solvent through the line before
use. This will insure that the channel is primed with fresh mobile phase. That step is for
EACH channel so it is best to flush them one-at-a-time, in sequential order, to keep track
of them.
NOTE: Some early models of HPLC
degassing systems had individual chamber volumes of greater than 30 mls each ! When you add the total tubing volume to
that you end up with a flush volume of 40 to 50 mls per channel.
To save time performing this
“flush” step, you should take advantage of the highest available flow rate
setting of your pump. With the prime purge valve open (so the solvent goes to waste or if
appropriate, to a separate collection container for re-use), set the pump flow rate to 10
ml/min and you will be able to flush out a single channel in two or three minutes. Repeat
for each of the other channels and then adjust the flow rate back down to your normal
equilibration flow rate before closing the prime purge valve again. Our lab performs this
flushing procedure at the start of each day to insure the columns receive fresh solvent.
This procedure should also be performed any time a mobile phase bottle is changed out to a
different bottle or type of mobile phase too.
One last reminder. Please keep in mind
that running liquid at 10 ml/min during the flush phase in most degassing systems will not
result in very efficient degassing as degassing efficiency is a function of flow rate
(surface area of liquid exposed over time). You will still need to wait a period of time
until the new solvent is fully degassed once again, but at least you will know it is free
of any older mobile phase. *Remember, everything you do before you start an analysis
contributes to the final result. > Bill Letter 05/29/10.
