Chromatography Forum

The procedure degassing water for GPC analysis where I work is to pump on a 4L jug with a mechanical pump (with a dry ice acetone cold trap in the line) until it stops bubbling. Then the degassed water is added to an HPLC solvent reservoir and sparged with He for 15-30 minutes. After that, the solvent reservoir is sealed and pressurized with He.

This seems a little bit excessive to me (and kind of hard on the mechanical pumps - we've ruined two so far). I have never read of anyone else degassing water in this fashion. Is He sparging likley to be sufficient for removing dissolved gases from water for GPC analysis (with RI detection)? Are there any possible long-term effects that may arise if I change the degassing method?

I've been told that it's done that way because it's always been done that way, but I am looking for a better justification.

He sparging should be adequate. He will displace pretty much all of the air. Just give it a low flow for a long time or set it to a very low flow after an initial sparge period of several minutes at a higher flow. You might also consider a vacuum degasser. They're pretty good, especially if fed a previously sparged eluent.

We use a sonicator, in addition to Helium sparging for purified water to be used as MP.

Alfred.

We use an air-driven vacuum pump from PIAB. They use a 3-stage Venturi-effect and have no moving mechanical parts. A water aspirator would work equally well. If you insist on a mechanical pump, the KNF diaphram pumps are quite tolerant of vapors of all kinds, and can be easily serviced by the user. Sonication really speeds things up, but is not strictly necessary. Together, it only takes about 60 seconds. The use of He prevents re-gassing, and some pumps work better with a bit of pressure on the reservoir.

We have been through this before in even more detail than here, especially regarding He, which is also a gas and can produce some trouble.

QZ

what is the pressure you are working at?
generally in GPC you will have very low pressure (not more then 30 bars)
then you can have a tendency of the gases going out of solution especialy for aqeous ones.
if you are only doing conventional GPC without viscosimeter then it might not hurt you too bad to simply use He if you have an online vacuum degasser in the pump.
if you have a viscosimeter in the system then you must degass via vacuum because even using He; will give you very noisy information. have a vacuum degaser online in the pump; it is a must for more complex then conventionnal GPC if you ever have to do it.

unmgvar, very interesting observation on He causing noise in a viscosity detector. Do you have a reference on this or is this your own observation?

People tend not to believe me that a UV detector clearly and strongly shows He induced absorbance (actually it must be scattering) as well as noise when compared to a vaccuum degassed phase. One usually doesn´t see this, because the UV detector needs to be zeroed before starting work with it. It is, strangely, hardly recognized that He chops off a sizeable chunk of the available linear detection range of a UV detector.

HW Mueller

my reference is the "guys from Viscotek". they build the machines.
they can very easily show that He causes high noise levels for a viscosity detector and for a light scatering detector as well.
for them most GPC solutions, mainly the water based one, must be vacuum degassed.

Thanks, while the viscosity detector was surprising to me the scattering detector corroberates what I see in UV: scattering..... Good to know.