Chromatography Forum

Just a few quick questions...

1 - I was told by someone to never let a GC column go "dry"; that is to always have a flow of the carrier gas (in my case, helium) unto the column since otherwise, it would cause damage to the column (HP-1).

But isn't this only if the oven is on? Doesn't oxygen attack the silica only when it is heated? If the oven is off, the column should be safe whether there's a flow of helium or not, no?

2 - Overnight, we put our GC's in a "sleep mode" that was set by the Agilent rep. It sets the inlet mode to splitless, with a column flow of 3.0 mL/min and the GasSaver option is on and set to 20 mL/min at 2.00 mins - therefore giving a total flow of 25.8 mL/min. The "purge Flow to Split Vent" is set to 10.0 mL/min at 1.00 min so if we disable the GasSaver option, the total flow falls to 15.9 mL/min.

My question is, are such high flows necessary? We shut down the oven, detector and inlet heating at night. In order to save gas, wouldn't it be more logical to bring the "purge flow to split vent" to 0 mL/min and set the column flow to something like 1.5 mL/min...? Or is this a bad idea...?


Thanks,

Roxanne.

Roxanne,

If the GC has been set up to be in splitless mode, then the only flow will be the column flow of 3 ml and septum purge flow of about 2.5ml. The 'Purge flow to vent" would only be active in splitless mode if you started a run.

I will leave it to the column experts to explain the effects of oxygen on the column phase.

Gasman

We've done pretty well with DB-1, DB-5, and SP-2330 type columns with no carrier flow at ambient temperatures for years. For a PEG-type column, I'd leave some carrier flowing at all times.

Consumer Products Guy is right in every aspect.

All temperatures under 40°C without carrier gas shouldn't damage the column, which is especially true for unpolar phases.

PEG/WAX phases are extremely sensitive to oxygen. Even the storage at ambient temperatures for several months makes an extensive heat-out necessary. I've just experienced this with a fresh PEG column.

At 40 C or below non-polar phases will be safe with no carrier gas flow in that they will not be attacked by atmospheric oxygen. But the most likely place that oxygen will get into the colum if the gas flow is off is at the detector end - where it is at about 240 C. The inlet end is also probably above 200 C so if you really want to save a few cents worth of gas (do the calculation of cost per ml) then you have to turn off the inlet and detector heaters as well.

To save (a little bit of) money and still have gas flow rig a two-way valve that changes the carrier from helium to nitrogen at night. Then curse the first time that you forget to switch it back again the next morning !

Even better, switch to hydrogen as carrier gas with the appropriate precautions, then you save money and get faster analyses at the same time.

Peter

Thanks for the input.

If the GC has been set up to be in splitless mode, then the only flow will be the column flow of 3 ml and septum purge flow of about 2.5ml. The 'Purge flow to vent" would only be active in splitless mode if you started a run.

Well, that's what I would have thought, but the "Total Flow" (under "Actual" reads at 25.8 mL/min - which is what confuses me...



Roxanne.

Well, that's what I would have thought, but the "Total Flow" (under "Actual" reads at 25.8 mL/min - which is what confuses me...

Roxanne, you definitely waste helium. Decrease the column flow to 0.5 mL/min - will be sufficient to protect stationary phase film.

Turn off gas saver. You may leave inlet in splitless mode, or change to split mode 1:1.

Wojtek

The acquisition method you are using is set up as:

septum flow: 2.9 ml/min
column flow: 3.0 ml/min
split flow: 10 ml/min (on @ 1 min)
gas gaser flow: 20 ml/min (on @ 2 min)

In the first min of the run, total flow is: 2.9 ml/min + 3 ml/min = 5.9 ml/min

Between 1 to 2 min, total flow is: 2.9 ml/min + 3 ml/Min + 10 ml/min = 15.9 ml/min

After 2 min to the start of next run (this also includes the time you are not making any injections), total flow is: 2.9 ml/min + 3 ml/min + 20 ml/min = 25.9 ml/min

Now it should be easy to figure out how to minimize the total flow.

Roxanne,

As far as I can remember, the Agilent Capillary inlet works as follows.

When the inlet has been set up for splitless mode, in standby mode it will actually be in split mode. This means that the gas coming into the inlet is FLOW controlled, and this is what you set up for total flow. Shortly before you inject in splitless mode, the inlet will switch over to forward PRESSURE control, which gives a better control of the column head pressure. When the splitless mode switches on the purge flow, it in fact reverts back to a split inlet.

This is complicated to understand, and is perhaps the reason that it is not so explained in the manual. If you look at page 11 of the 6890 Inlets manual it explains how gas saver works in split mode and in splitless mode.

During standby there is basically no difference for split or splitless.

Gasman

Thanks for all the help!