Chromatography Forum

Hi all
I am trying to work out the total carrier gas flow used in my GC methods. I have a 7890 with two inlets, each method using one inlet only while the other inlet is “idle”. Front inlet is connected to a FID, Back one to a MSD. Each inlet is configured in split or splitless depending on the method.

I am trying to get my head around the purge to split flow, the gas saver flow..and am confused! So just looking for someone to double check that I have this right. Here is an example of the total gas flow calculated for a method where the front inlet is used for sampling in Split mode whereas the back inlet is not used:

Inlet Front
Injection Split
Split ratio 20:1
Column flow ml/min 1.5
Column+Split flow ml/min 31.5
Septum purge ml/min 3
Total ml/min for inlet 34.5

Inlet Back
Injection Splitless
Column flow ml/min 1
Septum purge ml/min 3
Purge to split ml/min (1 min) 20
Total ml/min for inlet 24

Total ml/min for both inlets 58.5

After 2 minutes, the gas saver leads to:

Inlet Front
Injection Split
Column flow ml/min 1.5
Gas saver ml/min 20
Septum purge ml/min 3
Total ml/min for inlet 24.5

Inlet Back
Injection Splitless
Column flow ml/min 1
Gas saver ml/min 20
Septum purge ml/min 3
Total ml/min for inlet 24

Total ml/min for both inlets 48.5


Does this look ok?
Also, what is best for an inlet that is not used (back one in my example)? Do I really need a septum purge and a purge to split vent?
Thanks for the help

You should be able to to take the split vent flow after the gas saver kicks on to near zero. That would get your total flow down near 5 ml.

I've routinely set the unused column flow to 1 ml with no split vent flow. I've never tried turning off the septum purge. You could, but the big deal is to get the split vent turned off.

I worked in a lab that had a number of dual channel instruments that were run at 100/1 split. If people didn't use the gas saver, we wasted carrier like you wouldn't believe.

The total will be the column flow + split flow + septum purge, so adding them together gives the total.

If you set the rear to splitless with a 1min valve timing you will only have the splitless flow for one minute, then it will go to the 20ml/min after 1 minute.

The second example is both after the gas saver has activated, which should be the total flow after two minutes into a run and it will stay at that until the next run prepares to inject, so if the sequence finishes it will stay at that flow indefinitely.

I think gas saver is limited to a flow of 15ml/min, so if you want less than that you have to set to lower flow and disable it. I normally set the unused column to lowest flow and a split of 5:1 or 10:1 and no gas saver, and the column flow is normally about 0.5ml/min (for 0.25ID and smaller columns).

You can make a method where both front and back columns have setting like this and load that method before starting a sequence, and once the sequence is finished it will revert to that method as a gas saving standby method until you begin a new sequence. You don't have to load the active method from a sequence before starting a sequence, the sequence will automatically load what is requested. If you put the last line of the sequence as your standby method but you have a different one loaded before starting the sequence it will still revert to that method after the standby method has finished, so easier to just load it before starting a sequence and not worry about putting one as a last line item.

I will make a new method with lower gas flows for the end of my sequence. I can see now how I could reduce my flows in both split and splitless.
Thanks to you both for the replies-that helped.