Total gas flow calculation

[Julieke]

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

[larkl]

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.

[James_Ball]

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.

[Julieke]

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.